Theory and Design of Automotive Engine [UandiStar.org]

7/29/2019 Theory and Design of Automotive Engine [UandiStar.org]

1/127

THEORY AND DESIGN

OF

AUTOMOTIVE ENGINES

I Introduction

1 General - Historical development of automobiles, Types of power plant, Principle of engine operation,

Classification of engines.2. Two stroke & four stroke engines; Principles of engine operation (SI & CI), Scavenging - systems,theoretical processes, parameters, relative merits & demerits; Port timing diagrams, port design.Relative merits & demerits compared to petrol & diesel engines, scavenging pumps.

II Engine components

Classification/types, function, materials, construction details, design and manufacturing processes ofthe following engine components

3. Cylinders and liners - design, cylinder wear and corrosion, details of water jacket, dry and wet liners,Cylinder head - design;

4. Piston, piston rings, piston pin - design - stress analysis, methods of manufacture, compensation of

thermal expansion in pistons, heat treatment, piston ring selection, limits of fit for pins5. Connecting rod - design, effects of whipping, bearing materials, lubrication6. Crank shaft - design, firing order, balancing and torsional vibration analysis, vibration dampers,

bearings,. Lubrication7. Flywheel - design; Camshaft - drives of cams, materials, Types (only descriptive)8. Valve and valve mechanism - design, types of valve operating mechanisms, valve springs, guides,

push rods, rocker arms, tappets, valve timing diagrams9. Crank Case- Design of crank case, oil sumps and cooling features10. Manifolds-construction and design of inlet and exhaust manifolds.

TEXT BOOKS:I. High Speed Engines - P .M.Heldt, Oxford & IBH , 19652. Auto Design - R.B Gupta, Satya Prakashan, New Delhi 1999

REFERENCE BOOKS:I.A course in I.c. Engine - Mathur & Sharma, Dhanput Rai & Sons, Delhi, 19942.Automobile Engineering VoU & II - Kirpal Singh, Standard publications, New Delhi, 19723. Modem Petrol Engine ~ A.W.Judge, B.I. Publications. 19834. I.c. Engine - Maleev &Litchy, McGrawHill5. I.C.Engines - H.B.Keshwani, Standard Pub New Delhi., 19826. Fundamentals of I.C.Engines - J.B.Heywood

7. Machine design exercises - S.N.Trikha, Khanna publications, Delhi8. Automotive mechanics - N.K.Giri, Khanna publications,Delhi9. Automotive mechanics - William H. Crouse, Tata Mc,Graw Hill Publications Co. New Delhi10. I.C.Engines and Air Pollution - B.P.Obel'rlntext harper & Roni Pub, New york )

THEORY AND DESIGN OF AUTOMOTIVE ENGINES

w.UandiStar.org

www.UandiStar.org


2/127

Theory and Design of Automotive EnginesCHAPTER - 1

HISTORY

Automobiles through the Years - Since they originated in the late 1800s, automobiles havechanged and developed in response to consumer wishes, economic conditions, and advancingtechnology. The first gas-powered vehicles looked like horse buggies with engines mounted underneath

because this was the style to which people were accustomed. By 1910, however, features like the front-

mounted engine were already established, giving the automobile a look that was all its own. As publicdemand for cars increased, the vehicles became more stylized. The classic cars of the 1920s and 1930sepitomize the sleek, individually designed luxury cars called the classic cars. During the 1940s and1950s, automobiles generally became larger until the advent of the compact car, which immediately

became a popular alternative. The gasoline crisis is reflected in the fuel efficient cars made in the 1970sand 1980s. Current designs continue to reflect economy awareness, although many different marketsexist.

The history of the automobile actually began about 4,000 years ago when the first wheel wasused for transportation in India.

In the early 15th century the Portuguese arrived in China and the interaction of the two cultures

led to a variety of new technologies, including the creation of a wheel that turned under its own power.By the 1600s small steam-powered engine models had been developed, but it was another centurybefore a full-sized engine-powered vehicle was created.

In 1769 French Army officer Captain Nicolas-Joseph Cugnot built what has been called the firstautomobile. Cugnots three-wheeled, steam-powered vehicle carried four persons. Designed to moveartillery pieces, it had a top speed of a little more than 3.2 km/h (2 mph) and had to stop every 20minutes to build up a fresh head of steam.

Cugnot Steam Tractor-the first self-propelled road vehicle, thus,the earliest automobile. Powered by steam,the three-wheeled tractor- invented in 1769by Nicolas-Joseph Cugnot. designed tocarry artillery, but similar vehicles soonfound many other uses in industry.

As early as 1801, successfulbut very heavy steam automobileswere introduced in England. Laws

barred them from public roads andforced their owners to run them like

trains on private tracks.In 1802 a steam-powered coach designed by British engineer Richard Trevithick journeyed more than160 km (100 mi) from Cornwall to London. Steam power caught the attention of other vehicle builders.In 1804 American inventor Oliver Evans built a steam-powered vehicle in Chicago, Illinois. Frenchengineer Onsiphore Pecqueur built one in 1828.

British inventor Walter Handcock built a series of steam carriages in the mid-1830s that wereused for the first omnibus service in London.

By the mid-1800s England had an extensive network of steam coach lines. Horse-drawnstagecoach companies and the new railroad companies pressured the British Parliament to approveheavy tolls on steam-powered road vehicles. The tolls quickly drove the steam coach operators out of

business.During the early 20th century steam cars were popular in the United States. Most famous was

the Stanley Steamer, built by American twin brothers Freelan and Francis Stanley. A Stanley Steamerestablished a world land speed record in 1906 of 205.44 km/h (121.573 mph). Manufacturers produced

about 125 models of steam-powered automobiles, including the Stanley, until 1932.

2

w.UandiStar.org

www.UandiStar.org


3/127

Theory and Design of Automotive EnginesInternal-Combustion Engine

Development of lighter steam cars during the 19th century coincided with major developmentsin engines that ran on gasoline or other fuels. Because the newer engines burned fuel in cylinders insidethe engine, they were called internal-combustion engines.

In 1860 French inventor Jean-Joseph-tienne Lenoir patented a one-cylinder engine that usedkerosene for fuel. Two years later, a vehicle powered by Lenoirs engine reached a top speed of about6.4 km/h (about 4 mph).

In 1864 Austrian inventor Siegfried Marcus built and drove a carriage propelled by a two-cylinder gasoline engine.

American George Brayton patented an internal-combustion engine that was displayed at the1876 Centennial Exhibition in Philadelphia, Pennsylvania.In 1876 German engineer Nikolaus August Otto built a four-stroke gas engine, the most direct ancestorto todays automobile engines. In a four-stroke engine the pistons move down to draw fuel vapor intothe cylinder during stroke one; in stroke two, the pistons move up to compress the vapor; in stroke threethe vapor explodes and the hot gases push the pistons down the cylinders; and in stroke four the pistonsmove up to push exhaust gases out of the cylinders. Engines with two or more cylinders are designed socombustion occurs in one cylinder after the other instead of in all at once. Two-stroke engines

accomplish the same steps, but less efficiently and with more exhaust emissions.Automobile manufacturing began in earnest in Europe by the late 1880s.German engineer Gottlieb Daimler and German inventor Wilhelm Maybach mounted a gasoline-

powered engine onto a bicycle, creating a motorcycle, in 1885.In 1887 they manufactured their first car, which included a steering tiller and a four-speed

gearbox. Another German engineer, Karl Benz, produced his first gasoline car in 1886.

Early CarThe first practical car, built by German engineer Karl Benz in 1885,initiated the era of automobile manufacturing. Benz madeimprovements to the internal combustion engine and invented thedifferential drive and other automotive components. The company Benzfounded grew into one of the largest automobile manufacturers inGermany.

In 1890 Daimler and Maybach started a successful car manufacturing company, The DaimlerMotor Company, which eventually merged with Benzs manufacturing firm in 1926 to create Daimler-Benz. The joint company makes cars today under the Mercedes-Benz nameplate.

3

w.UandiStar.org

www.UandiStar.org


4/127

Theory and Design of Automotive EnginesIn France, a company called Panhard-Levassor began making cars in 1894 using Daimlers

patents. Instead of installing the engine under the seats, as other car designers had done, the companyintroduced the design of a front-mounted engine under the hood. Panhard-Levassor also introduced, aclutch and gears, and separate construction of the chassis, or underlying structure of the car, and the car

body. The companys first model was a gasoline-powered buggy steered by a tiller.French bicycle manufacturer Armand Peugeot saw the Panhard-Levassor car and designed an

automobile using a similar Daimler engine. In 1891 this first Peugeot automobile paced a 1,046-km(650-mi) professional bicycle race between Paris and Brest.

Other French automobile manufacturers opened shop in the late 1800s, including Renault.In Italy, Fiat (Fabbrica Italiana Automobili di Torino) began building cars in 1899.

4

w.UandiStar.org

www.UandiStar.org


5/127

Theory and Design of Automotive EnginesAmerican automobile builders were not far behind. Brothers Charles Edgar Duryea and James

Frank Duryea built several gas-powered vehicles between 1893 and 1895. The first Duryea, a one-cylinder, four-horsepower model, looked much like a Panhard-Levassor model.

Horseless CarriageThe originalhorselesscarriage wasintroduced in1893 bybrothersCharles andFrankDuryea. ItwasAmericasfirst internal-combustion

motor car,and it was

followed by Henry Fords first experimental car that same year.

In 1893 American industrialist Henry Ford built an internal-combustion engine from plans hesaw in a magazine. In 1896 he used an engine to power a vehicle mounted on bicycle wheels andsteered by a tiller.Early Electric Cars

For a few decades in the 1800s, electric engines enjoyed great popularity because they were

quiet and ran at slow speeds that were less likely to scare horses and people. By 1899 an electric cardesigned and driven by Belgian inventor Camille Jenatzy set a record of 105.8810 km/h (65.79 mph).Early electric cars featured a large bank of storage batteries under the hood. Heavy cables connected the

batteries to a motor between the front and rear axles. Most electric cars had top speeds of 48 km/h (30mph), but could go only 80 km (50 mi) before their batteries needed recharging. Electric automobileswere manufactured in quantity in the United States until 1930.

Automobiles in the 20th century

For many years after the introduction of automobiles, three kinds of power sources were incommon use: steam engines, gasoline engines, and electric motors.In 1900 more than 2,300 automobiles were registered in New York City; Boston, Massachusetts; and

Chicago, Illinois. Of these, 1,170 were steam cars, 800 were electric cars, and only 400 were gasolinecars. Gasoline-powered engines eventually became the nearly universal choice for automobiles becausethey allowed longer trips and faster speeds than engines powered by steam or electricity.

Improvements in the operating and riding qualities of gasoline automobiles developed quicklyafter 1900. The 1902 Locomobile was the first American car with a four-cylinder, water-cooled, front-mounted gasoline engine, very similar in design to most cars today. Built-in baggage compartmentsappeared in 1906, along with weather resistant tops and side curtains. An electric self-starter wasintroduced in 1911 to replace the hand crank used to start the engine turning. Electric headlights wereintroduced at about the same time.

Most automobiles at the turn of the 20th century appeared more or less like horseless carriages.In 1906 gasoline-powered cars were produced that had a style all their own. In these new models, a

hood covered the front-mounted engine. Two kerosene or acetylene lamps mounted to the front servedas headlights. Cars had fenders that covered the wheels and step-up platforms called running boards,which helped passengers, get in and out of the vehicle. The passenger compartment was behind the

5

w.UandiStar.org

www.UandiStar.org


6/127

Theory and Design of Automotive Enginesengine. Although drivers of horse-drawn vehicles usually sat on the right, automotive steering wheelswere on the left in the United States.

6

w.UandiStar.org

www.UandiStar.org


7/127

Theory and Design of Automotive EnginesIn 1903 Henry Ford incorporated the Ford Motor Company, which introduced its first

automobile, the Model A, in that same year. It closely resembled the 1903 Cadillac, which was hardlysurprising since Ford had designed cars the previous year for the Cadillac Motor Car Company. Fordscompany rolled out new car models each year, and each model was named with a letter of the alphabet.By 1907, when models R and S appeared, Fords share of the domestic automobile market had soared to35 percent.

Ford Model T

A Ford Model T rolls off the assembly l ine. Between 1908 and 1927,Ford built 15 million Model Ts.

Fords famous Model T debuted in 1908 but was called a 1909 Ford. Ford built 17,771 ModelTs and offered nine body styles. Popularly known as the Tin Lizzy, the Model T became one of the

biggest-selling automobiles of all time. Ford sold more than 15 million before stopping production ofthe model in 1927. The companys innovative assembly-line method of building the cars was widelyadopted in the automobile industry.

Silver Ghost

7

w.UandiStar.org

www.UandiStar.org


8/127

Theory and Design of Automotive EnginesOne of the highest-rated early luxury automobiles, the 1909 Rolls-Royce Silver Ghosts features included a quiet 6-cylinder engine,leather interior, folding windscreens and hood, and an aluminum body.Generally driven only by chauffeurs, the emphasis of the luxury car wason comfort and style rather than speed.

By 1920 more than 8 million Americans owned cars. Major reasons for the surge in automobileownership were Fords Model T, the assembly-line method of building it, and the affordability of carsfor the ordinary wage earner.

8

w.UandiStar.org

www.UandiStar.org


9/127

Theory and Design of Automotive EnginesImprovements in engine-powered cars during the 1920s contributed to their popularity:

synchromesh transmissions for easier gear shifting; four-wheel hydraulic brake systems; improvedcarburetors; shatterproof glass; balloon tires; heaters; and mechanically operated windshield wipers.

PhaetonCars of the 1920s exhibited design refinements such as balloon tires,pressed-steel wheels, and four-wheel brakes. Although assembly lines(which originated with Henry Ford in 1908) continued to bring the priceof automobiles down, many cars in this time were one-of-a-kind vintagemodels, made to individual specifications. The 1929 Graham Paige DCPhaeton shown here featured an 8-cylinder engine and an aluminumbody.

From 1930 to 1937, automobile engines and bodies became large and luxurious. Many 12- and16-cylinder cars were built. Independent front suspension, which made the big cars more comfortable,appeared in 1933. Also introduced during the 1930s were stronger, more reliable braking systems, andhigher-compression engines, which developed more horsepower. Mercedes introduced the worlds firstdiesel car in 1936.

Automobiles on both sides of the Atlantic were styled with gracious proportions, long hoods,and pontoon-shaped fenders. Creative artistry merged with industrial design to produce appealing,

aerodynamic automobiles.

9

w.UandiStar.org

www.UandiStar.org


10/127

Theory and Design of Automotive Engines

De Luxe SedanThe roomy interior and rear-hinged back door of this 1937 Pontiac DeLuxe sedan represent a move toward a car more suited to the needs of

families. With these consumers in mind, cars were designed to beconvenient, reliable, and relatively inexpensive. Vehicles in the 1930swere generally less boxy and more streamlined than theirpredecessors.

Some of the first vehicles to fully incorporate the fender into the bodywork came along just afterWorld War II, but the majority of designs still had separate fenders with pontoon shapes holdingheadlight assemblies. Three companies, Ford, Nash, and Hudson Motor Car Company, offered postwardesigns that merged fenders into the bodywork. The 1949 Ford was a landmark in this respect, and itsnew styling was so well accepted the car continued in production virtually unchanged for three years,selling more than 3 million. During the 1940s, sealed-beam headlights, tubeless tires, and the automatictransmission were introduced.

Two schools of styling emerged in the 1950s, one on each side of the Atlantic. The Europeanscontinued to produce small, light cars weighing less than 1,300 kg (2,800 lb). European sports cars ofthat era featured hand-fashioned aluminum bodies over a steel chassis and framework.

10

w.UandiStar.org

www.UandiStar.org


11/127


StudebakerT

his

1940 Studebaker Champion two-door sedan was designed byRaymond Loewy and built by Studebaker craftsmen. Featuresemerging in the 1940s include automatic transmission, sealed-beamheadlights, and tubeless tires.

In America, automobile designers borrowed features for their cars that were normally found onaircraft and ships, including tailfins and portholes. Automobiles were produced that had more space,more power, and smoother riding capability. Introduction of power steering and power brakes made

bigger cars easier to handle. The Buick Motor Car Company, Olds Motor Vehicle Company(Oldsmobile), Cadillac Automobile Company, and Ford all built enormous cars, some weighing asmuch as 2,495 kg (5,500 lb). The first import by German manufacturer Volkswagen AG, advertised as

the Beetle, arrived in the United States in 1949. Only two were sold that year, but American consumerssoon began buying the Beetle and other small imports by the thousands.

VW BeetleThe

Volkswagen Beetle dominated the market for several years, during which fewmodifications were made on the original design. Volkswagens name means carfor the people, and the car served at least two important consumer needs. Therear-mounted engine and small, rounded, buglike shape of the European carrepresented an appealing combination of look and economy that remainedpopular for more than four decades.

That prompted a downsizing of some American-made vehicles. The first American car called acompact was the Nash Rambler. Introduced in 1950, it did not attract buyers on a large scale until 1958.

11

w.UandiStar.org

www.UandiStar.org


12/127

Theory and Design of Automotive EnginesMore compacts, smaller in overall size than a standard car but with virtually the same interior bodydimensions, emerged from the factories of many major manufacturers. The first Japanese imports, 16compact trucks, arrived in the United States in 1956.

In the 1950s new automotive features were introduced, including air conditioning andelectrically operated car windows and seat adjusters. Manufacturers changed from the 6-volt to the 12-volt ignition system, which gave better engine performance and more reliable operation of the growingnumber of electrical accessories.

By 1960 sales of foreign and domestic compacts accounted for about one-third of all passengercars sold in the United States. American cars were built smaller, but with increased engine size andhorsepower. Heating and ventilating systems became standard equipment on even the least expensivemodels. Automatic transmissions, power brakes, and power steering became widespread. Stylingsometimes prevailed over practicalitysome cars were built in which the engines had to be lifted toallow simple service operations, like changing the spark plugs. Back seats were designed with nolegroom.

12

w.UandiStar.org

www.UandiStar.org


13/127


GullwingPowerful high-performance carssuch as this 1957 Mercedes-Benz300SL were built on compact and

stylized lines. Also called theGullwing because its doors openupward into the shape of a gullswings, the 300SL was capable of230 kmh (144 mph), its on-roadperformance matching its racingcapacity.

El Dorado

This 1957 Cadillac El Dorado convertible epitomizes the large cars of theAmerican Dream era. Tail fins are an example of a trend in car design.Although the feature did little for the performance of the vehicle, consumersloved the look, and demanded fins of increasing size until the 1960s.

MustangMore than 100,000 Ford Mustangs sold duringfirst four months the model was on the marketin 1964, making it Fords best early salessuccess since the introduction of the Model T. Avehicle from the muscle car category, the

Mustangs popular characteristics included asmall, fast design, excellent handling, apowerful engine, and a distinctive look.

13

w.UandiStar.org

www.UandiStar.org


14/127


In the 1970s American manufacturers continued to offer smaller, lighter models in addition tothe bigger sedans that led their product lines, but Japanese and European compacts continued to sellwell. Catalytic converters were introduced to help reduce exhaust emissions.

Digital speedometers and electronic prompts to service parts of the vehicle appeared in the1980s. Japanese manufacturers opened plants in the United States. At the same time, sporty cars andfamily minivans surged in popularity.

Advances in automobile technology in the 1980s included better engine control and the use ofinnovative types of fuel. In 1981 Bayerische Motoren Werke AG (BMW) introduced an on-boardcomputer to monitor engine performance. A solar-powered vehicle, SunRaycer, traveled 3,000 km(1,864 mi) in Australia in six days.

MR-2 Turbo

Modern cars like the Japanese 1992 MR-2 Turbo T-bar Toyota are generallylight, aerodynamically shaped, and compact. Japanese imports changed theautomobile industry significantly. The generally reliable, inexpensive carsincreased competition between manufacturers dramatically, to the benefit ofconsumers.

New technologies

Gas-Electric Hybrids

The Toyota Prius,a four-seat hybrid electric vehicle (HEV), was the first HEV to bemarketed when Toyota introduced it in Japan in 1997.

The Honda Insight,a two-seat HEV, followed in 1999 when it was sold in both Japan andthe United States. The Prius had its U.S. debut in 2000.

14

w.UandiStar.org

www.UandiStar.org


15/127


Gas-Electric Hybrids The Toyota Prius, a four-seat hybrid electric vehicle (HEV), was the firstHEV to be marketed when Toyota introduced it in Japan in 1997. The Honda Insight, a two-seat HEV,followed in 1999 when it was sold in both Japan and the United States. The Prius had its U.S. debut in2000.

Pollution-control laws adopted at the beginning of the 1990s in some of the United States and inEurope called for automobiles that produced better gas mileage with lower emissions. In 1996 GeneralMotors became the first to begin selling an all-electric car, the EV1, to California buyers. The all-electric cars introduced so far have been limited by low range, long recharges, and weak consumerinterest.

Engines that run on hydrogen have been tested. Hydrogen combustion produces only a trace ofharmful emissions, no carbon dioxide, and a water-vapor by-product. However, technical problemsrelated to the gass density and flammability remains to be solved.

Diesel engines burn fuel more efficiently, and produce fewer pollutants, but they are noisy.

Popular in trucks and heavy vehicles, diesel engines are only a small portion of the automobile market.A redesigned, quieter diesel engine introduced by Volkswagen in 1996 may pave the way for morediesels, and less pollution, in passenger cars.

While some developers searched for additional alternatives, others investigated ways to combineelectricity with liquid fuels to produce low-emissions power systems. Two automobiles with suchhybrid engines, the Toyota Prius and the Honda Insight, became available in the late 1990s. Prius hitautomobile showrooms in Japan in 1997, selling 30,000 models in its first two years of production. ThePrius became available for sale in North America in 2000. The Honda Insight debuted in North Americain late 1999. Both vehicles, known as hybrid electric vehicles (HEVs), promised to double the fuelefficiency of conventional gasoline-powered cars while significantly reducing toxic emissions.

Computer control of automobile systems increased dramatically during the 1990s. The central

processing unit (CPU) in modern engines manages overall engine performance. Microprocessorsregulating other systems share data with the CPU. Computers manage fuel and air mixture ratios,ignition timing, and exhaust-emission levels. They adjust the antilock braking and traction controlsystems. In many models, computers also control the air conditioning and heating, the sound system,and the information displayed in the vehicles dashboard.

Expanded use of computer technology, development of stronger and lighter materials, andresearch on pollution control will produce better, smarter automobiles.

In the 1980s the notion that a car would talk to its driver was science fiction; by the 1990s ithad become reality.

Onboard navigation was one of the new automotive technologies in the 1990s. By using thesatellite-aided global positioning system (GPS), a computer in the automobile can pinpoint the vehicles

location within a few meters. The onboard navigation system uses an electronic compass, digitizedmaps, and a display screen showing where the vehicle is relative to the destination the driver wants toreach. After being told the destination, the computer locates it and directs the driver to it, offeringalternative routes if needed.

Some cars now come equipped with GPS locator beacons, enabling a GPS system operator tolocate the vehicle, map its location, and if necessary, direct repair or emergency workers to the scene.Cars equipped with computers and cellular telephones can link to the Internet to obtain constantlyupdated traffic reports, weather information, route directions, and other data. Future built-in computersystems may be used to automatically obtain business information over the Internet and manage

personal affairs while the vehicles owner is driving.During the 1980s and 1990s, manufacturers trimmed 450 kg (1,000 lb) from the weight of the

typical car by making cars smaller. Less weight, coupled with more efficient engines, doubled the gasmileage obtained by the average new car between 1974 and 1995. Further reductions in vehicle size are

15

w.UandiStar.org

www.UandiStar.org


16/127

Theory and Design of Automotive Enginesnot practical, so the emphasis has shifted to using lighter materials, such as plastics, aluminum alloys,and carbon composites, in the engine and the rest of the vehicle.

Looking ahead, engineers are devising ways to reduce driver errors and poor driving habits.Systems already exist in some locales to prevent intoxicated drivers from starting their vehicles. Thetechnology may be expanded to new vehicles. Anti-collision systems with sensors and warning signalsare being developed. In some, the cars brakes automatically slow the vehicle if it is following anothervehicle too closely. New infrared sensors or radar systems may warn drivers when another vehicle is intheir blind spot.

Catalytic converters work only when they are warm, so most of the pollution they emit occurs inthe first few minutes of operation. Engineers are working on ways to keep the converters warm forlonger periods between drives, or heat the converters more rapidly.

16

w.UandiStar.org

www.UandiStar.org


17/127

Theory and Design of Automotive EnginesTypes of power plant

An engine is a device which transforms one form of energy into another form. However, whiletransforming energy from one form to another, the efficiency of conversion plays an important role.

Normally, most of the engines convert thermal energy into mechanical work and therefore they arecalled 'heat engines'.

Heat engine is a device which transforms the chemical energy of a fuel into thermal energy andutilizes this thermal energy to perform useful work. Thus, thermal energy is converted to mechanicalenergy in a heat engine.

Heat engines can be broadly classified into two categories:(i) Internal Combustion Engines (IC Engines) (ii) External Combustion Engines (EC Engines)

Table 1.1 Classification of heat engines

Engines whether Internal Combustion or External Combustion are of two types, viz.,(i) Rotary engines (ii) Reciprocating engines

Of the various types of heat engines, the most widely used ones are the reciprocating internalcombustion engine, the gas turbine and the steam turbine. The steam engine is rarely used nowadays.The reciprocating internal combustion engine enjoys some advantages over the steam turbine due to theabsence of heat exchangers in the passage of the working fluid (boilers and condensers in steam turbine

plant). This results in a considerable mechanical simplicity and improved power plant efficiency of theinternal combustion engine.

Fig.1.1 Classification of heat engines

17

w.UandiStar.org

www.UandiStar.org


18/127

Theory and Design of Automotive EnginesAnother advantage of the reciprocating internal combustion engine over the other two types is

that all its components work at an average temperature which is much below the maximum temperatureof the working fluid in the cycle. This is because the high temperature of the working fluid in the cycle

persists only for a very small fraction of the cycle time. Therefore, very high working fluid temperaturescan be employed resulting in higher thermal efficiency.

Further, in internal combustion engines, higher thermal efficiency can be obtained withmoderate maximum working pressure of the fluid in the cycle, and therefore, the weight of power ratiois less than that of the steam turbine plant. Also, it has been possible to develop reciprocating internalcombustion engines of very small power output (power output of even a fraction of a kilowatt) withreasonable thermal efficiency and cost.

The main disadvantage of this type of engine is the problem of vibration caused by thereciprocating components. Also, it is not possible to use a variety of fuels in these engines. Only liquidor gaseous fuels of given specification can be efficiently used. These fuels are relatively moreexpensive.

Considering all the above factors the reciprocating internal combustion engines have been foundsuitable for use in automobiles, motor-cycles and scooters, power boats, ships, slow speed aircraft,locomotives and power units of relatively small output.

External Combustion and Internal Combustion EnginesExternal combustion engines are those in which combustion takes place outside the enginewhereas in internal combustion engines combustion takes place within the engine. For example, in asteam engine or a steam turbine, the heat generated due to the combustion of fuel is employed togenerate high pressure steam which is used as the working fluid in a reciprocating engine or a turbine.In case of gasoline or diesel engines, the products of combustion generated by the combustion of fueland air within the cylinder form the working fluid.

Principle of engine operation (4 stroke & 2 stroke operating cycles)

In reciprocating engines, the piston moves back and forth in acylinder and transmits power through a connecting rod and crank

mechanism to the drive shaft as shown in Fig1.2. The steady rotation ofthe crank produces a cyclical piston motion. The piston comes to rest atthe top center (TC) crank position and bottom-center (BC) [These crank

positions are also referred to as top-dead-center (TDC) and bottom-dead-center (BDC)] crank position when the cylinder volume is a minimum ormaximum, respectively. The minimum cylinder volume is called theclearance volume.

The volume swept out by the piston, the difference between themaximum or total volume Vt and the clearance volume, is called thedisplaced or swept volume Vd. The ratio of maximum volume to minimumvolume is the compression ratio rc.Typical values of rcare 8 to 12 for SI

engines and 12 to 24 for CI engines.

Fig 1.2

Basic geometry of the reciprocating

internal combustion engine.Vc, Vd, and Vt, indicate clearance,displaced, and total cylinder volumes.

18

w.UandiStar.org

www.UandiStar.org


19/127


Fig.1.3 :-The f our-stroke operating cycle.

The majority of reciprocating engines operate on what is known as the four-stroke cycle. Eachcylinder requires four strokes of its piston-two revolutions of the crankshaft-to complete the sequence ofevents which produces one power stroke. Both SI and CI engines use this cycle which comprises1. An intake stroke, which starts with the piston at TC and ends with the piston at BC, which drawsfresh mixture into the cylinder. To increase the mass inducted, the inlet valve opens shortly before thestroke starts and closes after it ends.2.A compression stroke, when both valves are closed and the mixture inside the cylinder is compressedto a small fraction of its initial volume. Toward the end of the compression stroke, combustion isinitiated and the cylinder pressure rises more rapidly.3.A power stroke, or expansion stroke, which starts with the piston at TC and ends at BC as the high-

temperature, high-pressure, gases push the piston down and force the crank to rotate. About five timesas much work is done on the piston during the power stroke as the piston had to do during compression.As the piston approaches BC the exhaust valve opens to initiate the exhaust process and drop thecylinder pressure to close to the exhaust pressure.4 An exhaust stroke, where the remaining burned gases exit the cylinder: first, because the cylinder

pressure may be substantially higher than the exhaust pressure; then as they are swept out by the pistonas it moves toward TC. As the piston approaches TC the inlet valve opens. Just after TC the exhaustvalve closes and the cycle starts again.

Though often called the Otto cycle after its inventor, Nicolaus Otto, who built the first engineoperating on these principles in 1876, the more descriptive four-stroke nomenclature is preferred.

The four-stroke cycle requires, for each engine cylinder, two crankshaft revolutions for each

power stroke.To obtain a higher power output from a given engine size, and a simpler valve design, the two-

stroke cycle was developed. The two-stroke cycle is applicable to both SI and CI engines.

Figure 1.4 shows one of the simplest types of two-stroke engine designs. Ports in the cylinderliner opened and closed by the piston motion, control the exhaust and inlet flows while the piston isclose to BC. The two strokes are:

A compression stroke, which starts by closing the inlet and exhaust ports, and then compressesthe cylinder contents and draws fresh charge into the crankcase. As the piston approaches TC,combustion is initiated.

19

w.UandiStar.org

www.UandiStar.org


20/127


Fig.1.4 The two-stroke operating cycle.

A crankcase-scavenged engine

A power or expansion stroke, similar to that in the four-stroke cycle until the piston approachesBC, when first the exhaust ports and then the intake ports are uncovered. Most of the burnt gases exit

the cylinder in an exhaust blow down process. When the inlet ports are uncovered, the fresh chargewhich has been compressed in the crankcase flows into the cylinder.The piston and the ports are generally shaped to deflect the incoming charge from flowing directly intothe exhaust ports and to achieve effective scavenging of the residual gases.

Each engine cycle with one power stroke is completed in one crankshaft revolution. However, itis difficult to fill completely the displaced volume with fresh charge, and some of the fresh mixtureflows directly out of the cylinder during the scavenging process. The example shown is a cross-

scavengeddesign; other approaches use loop-scavengingoruniflow systems

20

w.UandiStar.org

www.UandiStar.org


21/127

Theory and Design of Automotive EnginesEngine classifications

Fig.1.5IC engine classification

There are many different types of internal combustion engines. They can be classified by:

1. Application.

Automobile, truck, locomotive, light aircraft, marine, portable power system, power generation

2 Basic engine designs

Reciprocating engines (inturn subdivided by arrangement ofcylinders: e.g., in-line, V, radial,opposed-ref, fig1.6.), rotary

engines (Wankel and othergeometries)

Fig1.6.Engine Classification by Cylinder Arrangements

21

w.UandiStar.org

www.UandiStar.org


22/127

(C)

Theory and Design of Automotive Engines3. Working cycle.

Four-stroke cycle: naturally aspirated (admitting atmospheric air), supercharged (admitting pre-compressed fresh mixture), and turbocharged (admitting fresh mixture compressed in a compressordriven by an exhaust turbine),Two-stroke cycle: crankcase scavenged, supercharged, and turbocharged,Constant volume heat addition cycle engine or Otto cycle engine -SI engine or Gasoline engine,Constant pressure heat addition cycle engine or Diesel cycle engine-CI engine orDiesel engine.

4 Valve or port design and location.

Overhead (or I-head) valves, under head(or L-head) valves, rotary valves, cross-scavenged porting (inlet and exhaust ports onopposite sides of cylinder at one end), loop-scavenged porting (inlet and exhaust ports onsame side of cylinder at one end), through- oruni-flow scavenged (inlet and exhaust ports or

valves at different ends of cylinder)

Fig1.7classification of SI engineby port/ valve location

(a)Cross, (b) Loop, (c) Uniflow Scavenging

22

w.UandiStar.org

www.UandiStar.org


23/127

Theory and Design of Automotive Engines5. Fuel

Gasoline (or petrol), fuel oil (or diesel fuel), natural gas, liquid petroleum gas, alcohols(methanol, ethanol), hydrogen, dual fuel

6. Method of mixture preparation.

Carburetion, fuel injection into the intake ports or intake manifold, fuel injection into the enginecylinder

7. Method of ignition

Spark ignition (in conventional engines where the mixture is uniform and in stratified-chargeengines where the mixture is non-uniform), compression ignition (in conventional diesels, as well asignition in gas engines by pilot injection of fuel oil)

8. Combustion chamber design.

Open chamber (many designs: e.g., disc, wedge, hemisphere, bowl-in-piston), divided chamber(small and large auxiliary chambers; many designs: e.g., swirl chambers, pre-chambers)

9. Method of load control.Throttling of fuel and air flow together so mixture composition is essentially unchanged, controlof fuel flow alone, a combination of these

10. Method of cooling.

Water cooled, air cooled, un-cooled (other than by natural convection and radiation)

. All these distinctions are important and they illustrate the breadth of engine designs availablefrom a fundamental point of view. The method of ignition has been selected as the primary classifyingfeature. From the method of ignition-spark-ignition or compression-ignition-follow the importantcharacteristics of the fuel used, method of mixture preparation, combustion chamber design, method of

load control, details of the combustion process, engine emissions, and operating characteristics. Some ofthe other classifications are used as subcategories within this basic classification. The engine operatingcycle--four-stroke or two-stroke--is next in importance.

23

w.UandiStar.org

www.UandiStar.org


24/127


Table 1.2

24

w.UandiStar.org

www.UandiStar.org


25/127

Theory and Design of Automotive EnginesTable 1.3 Engine characteristics Emphasized by Type of Service

References:1. Microsoft Encarta2. Fundamentals of IC Engines By J B Heywood3. Theory & Practice in IC Engines By C F Taylor

4. I C Engines By M L Mathur & RP Sharma5. I C Engines By Ganesan

25

w.UandiStar.org

www.UandiStar.org


26/127

Theory and Design of Automotive EnginesCHAPTER 2

FOUR-STROKE CYCLE S-I ENGINE - PRINCIPLE OF OPERATION

Fig: cross section of a SI Engine

In Four-stroke cycle engine, the cycle of operation is completed in four-strokes of the piston ortwo revolutions of the crankshaft. Each stroke consists of 180, of crankshaft rotation and hence a cycleconsists of 720of crankshaft rotation. The series of operations of an ideal four-stroke. SI engine are asfollows (see Fig.2.1 & 2.2)1. Suction stroke

Suction stroke 0-1 starts when the piston is at top dead centre and about to move downwards.The inlet valve is open at this time and the exhaust valve is closed. Due to the suction created by themotion of the piston towards bottom dead centre, the charge consisting of fresh air mixed with the fuel

is drawn into the cylinder. At the end of the suction stroke the inlet valve closes.2. Compression stroke.The fresh charge taken into the cylinder during suction stroke is compressed by the return stroke

of the piston 1-2. During this stroke both inlet and exhaust valves remain closed. The air whichoccupied the whole cylinder volume is now compressed into clearance volume. Just before the end ofthe compression stroke the mixture is ignited with the help of an electric spark between the electrodes ofthe spark plug located in combustion chamber wall. Burning takes place when the piston is almost at topdead centre. During the burning process the chemical energy of the fuel is converted into sensibleenergy, producing a temperature rise of about 2000C, and the pressure is also considerably increased.3. Expansion or power stroke.

Due to high pressure the burnt gases force the piston towards bottom dead centre, stroke 3-4,

and both the inlet and exhaust valves remaining closed. Thus power is obtained during this stroke. Bothpressure and temperature decrease during expansion.4. Exhaust stroke.

At the end of the expansion stroke the exhaust valve opens, the inlet valve remaining closed, andthe piston is moving from bottom dead centre to top dead centre sweeps out the burnt gases from thecylinder, stroke 4-0. The exhaust valve closes at the end of the exhaust stroke and some 'residual' gasesremain in the cylinder.

Each cylinder of a four-stroke engine completes the above four operations in two enginerevolutions. One revolution of the crankshaft occurs during the suction and compression strokes, andsecond revolution during the power and exhaust strokes. Thus for one complete cycle, there is only one

power stroke while the crankshaft turns by two revolutions. Most of the spark-ignition internal

combustion engines are of the four-stroke type. They are most popular for passenger cars and smallaircraft applications.

26

w.UandiStar.org

www.UandiStar.org


27/127


Fig.2.1-The four-stroke spark-ignition (SI) engine cycle (Otto cycle or constant volume cycle)

Fig.2.2-Ideal and actual indicator diagrams for four-stroke SI engine

27

w.UandiStar.org

www.UandiStar.org


28/127


Fig. 2.3 Four-stroke petrol engine valve timing diagram in relation to the pressure volume diagram

Actual Valve Timing Of Four-Stroke Petrol Engine.

Valve timing is the regulation of the points in the cycle at which the valves are set to open andclose. As described above in the ideal cycle inlet and exhaust valves open and close at dead centres, butin actual cycles they open or close before or after dead centres as explained below. There are twofactors, one mechanical and other dynamic, for the actual valve timing to be different from thetheoretical valve timing.

(a) Mechanical factor.

The poppet valves of the reciprocating engines are opened and closed by cam mechanisms. Theclearance between cam, tappet and valve must be slowly taken up and valve slowly lifted, at first, ifnoise and wear is to be avoided. For the same reasons the valve cannot be closed abruptly, else it will'bounce' on its seat. (Also the cam contours should be so designed as to produce gradual and smoothchanges in directional acceleration). Thus the valve opening and closing periods are spread over aconsiderable number of crankshaft degrees. As a result, the opening of the valve must commence aheadof the time at which it is fully opened (i.e., before dead centres). The same reasoning applies for theclosing time and the valves must close after the dead centres. Fig.2.3 shows the actual valve timingdiagram of a four-stroke engine in relation to its pressure-volume diagram.b) Dynamic factor;

Besides mechanical factor of opening and closing of valves, the actual valve timing is set takinginto consideration the dynamic effects of gas flow.Intake valve timing.

28

w.UandiStar.org

www.UandiStar.org


29/127

Theory and Design of Automotive EnginesIntake valve timing has a bearing on the actual quantity of air sucked during the suction stroke

i.e. it affects the volumetric efficiency. Fig.2.4 shows the intake valve timing diagram for both lowspeed & high speed SI engines.

Fig:2.4 Valve timing for low and high speed four-stroke SI engine

It is seen that for both low speed and high speed engine the intake valve opens 100 before thearrival of the piston at TDC on the exhaust stroke. This is to insure that the valve will be fully open and

the fresh charge starting to flow into the cylinder as soon as possible after TDC. As the piston moves outin the suction stroke, the fresh charge is drawn in through the intake port and valve. When the pistonreaches the BDC and starts to move in the compression stroke, the inertia of the entering fresh chargetends to cause it to continue to move into the cylinder. To take advantage of this, the intake valve isclosed after BDC so that maximum air is taken in. This is called ram effect. However, if the intake valveis to remain open for too long a time beyond BDC, the up-moving piston on the compression strokewould tend to force some of the charge, already in the cylinder, back into the intake manifold. The timethe intake valve should remain open after BDC is decided by the speed of the engine.

At low engine speed, the charge speed is low and so the air inertia is low, and hence the intakevalve should close relatively early after BDC for a slow speed engine (say about 100 after BDC).

In high speed engines the charge speed is high and consequently the inertia is high and hence to

induct maximum quantity of charge due to ram effect the intake valve should close relatively late afterBDC (up to 600 after BDC).

For a variable speed engine the chosen intake valve setting is a compromise between the bestsetting for low and high speeds.

There is a limit to the high speed for advantage of ram effect. At very high speeds the effect offluid friction may be more than offset the advantage of ram effect and the charge for cylinder per cyclefalls off.

Exhaust valve timing

The exhaust valve is set to open before BDC (say about 250 before BDC in low speed enginesand 550 before BDC in high speed engines). If the exhaust valve did not start to open until BDC, the

pressures in the cylinder would be considerably above atmospheric pressure during the first portion ofthe exhaust stroke, increasing the work required to expel the exhaust gases. But opening the exhaustvalve earlier reduces the pressure near the end of the power stroke and thus causes some loss of useful

29

w.UandiStar.org

www.UandiStar.org


30/127

Theory and Design of Automotive Engineswork on this stroke. However, the overall effect of opening the valve prior to the time the piston reachesBDC results in overall gain in output.

The closing time of exhaust valve effects the volumetric efficiency, By closing the exhaust valvea few degrees after TDC (about 150 in case of low speed engines and 200 in case of high speed engines)the inertia of the exhaust gases tends to scavenge the cylinder by carrying out a greater mass of the gasleft in the clearance volume. This results in increased volumetric efficiency.

Note that there may be a period when both the intake and exhaust valves are open at the same time. Thisis called valve over-lap(say about 150 in low speed engine and 300 in high speed engines). This overlapshould not be excessive otherwise it will allow the burned gases to be sucked into the intake manifold,orthe fresh charge to escape through the exhaust valve.

Table2.1Typical valve timings for four-stroke SI engines

Note.Valve timing is different for different makes of engines.

b-before, a-after TDC-Top dead centre,BDC-Bottom dead centre.

30

w.UandiStar.org

www.UandiStar.org


31/127

Theory and Design of Automotive Engines FOUR-STROKE CI ENGINES- PRINCIPLE OF OPERATION

The four-stroke CI engine is similar to four-stroke SI engine except that a high compressionratio is used in the former, and during the suction stroke, air alone, instead of a fuel-air mixture, isinducted. Due to high compression ratio, the temperature at the end of compression stroke is sufficientto ignite the fuel which is injected into the combustion chamber.

In the CI engine a high pressure fuel pump and an injector is provided to inject fuel intocombustion chamber.

The carburettor and ignition system, necessary in the SI engine, are not required in the CIengine.

The ideal sequence of operation for the four-stroke CI engine is as follows:

Fig.2.5 Ideal P-V Diagram Fig.2.6 Cycle of Operation

1.Suction stroke

Only air is inducted during the suction stroke. During this stroke intake valve is open andexhaust valve is closed.2.Compression stroke

Both valves remain closed during compression stroke.3. Expansion or power strokeFuel is injected in the beginning of the expansion .stroke. The rate of injection is such that the

combustion maintains the pressure constant. After the injection of fuel is over(i.e. after fuel cut off) theproducts of combustion expand. Both valves remain closed during expansion stroke.4. Exhaust stroke.

The exhaust valve is open and the intake valve remains closed in the exhaust stroke.Due to higher pressures the CI engine is heavier than SI engine but has a higher thermal efficiency

because of greater expansion. CI engines are mainly used for heavy transport vehicles, powergeneration, and industrial and marine applications.

The typical valve timing diagram for a four-stroke CI engine is as follows

IVO about 300 before TDCIVO up to 500 after BDC

EVO about 450 before BDC

EVO up to 300 after TDC

Injection about 150 before TDC

31

w.UandiStar.org

www.UandiStar.org


32/127

Theory and Design of Automotive EnginesTWO-STROKE CYCLE ENGINE-PRINCIPLE OF OPERATION

In two-stroke engines the cycle is completed in two strokes, i.e., one revolution of the crankshaftas against two revolutions of four-stroke cycle. The difference between two-stroke and four-strokeengines is in the method of filling the cylinder with the fresh charge and removing the burned gasesfrom the cylinder. In a four-stroke engine the operations are performed by the engine piston during thesuction and exhaust strokes, respectively. In a two stroke engine suction is accomplished by aircompressed in crankcase or by a blower. The induction of compressed air removes the products ofcombustion, through exhaust ports. Therefore no piston strokes are required for suction and exhaustoperations. Only two piston strokes are required to complete the cycle, one for compressing the freshcharge and the other for expansion or power stroke.

Types of two stroke engines

Based on scavenging methodi) Crankcase & ii) Separately scavenged engine

Based on scavenging process (air flow)i) Cross flow scavenging,

ii) Loop scavenging (MAN, Schnuerle, Curtis type)

iii) Uni-flow scavenging (opposed piston, poppet valve, sleeve valve) Based on overall port-timingi) Symmetrical & ii) Unsymmetrical

Crankcase-scavenged two-stroke engine

Figure 2.7 shows the simplest type of two-stroke engine the crankcase scavenged engine.Fig.2.8 shows its ideal and actual indicator diagrams. Fig.2.9 shows the typical valve timing diagram ofa two-stroke engine. The air or charge is sucked through spring-loaded inlet valve when the pressure inthe crankcase reduces due to upward motion of the piston during compression stroke. After thecompression, ignition and expansion takes place in the usual way: During the expansion stroke the air inthe crankcase is compressed. Near the end of expansion stroke piston uncovers the exhaust port, and the

cylinder pressure drops to atmospheric as the combustion products leave the cylinder. Further motion ofthe piston uncovers transfer ports, permitting the slightly compressed air or mixture in the crankcase toenter the engine cylinder. The top of the piston sometimes has a projection to deflect the fresh air tosweep up to the top of the cylinder before flowing to the exhaust ports. This serves the double purposeof scavenging the upper part of the cylinder of combustion products and preventing the fresh chargefrom .flowing directly to the exhaust ports. The same objective can be achieved without piston deflector

by proper shaping of the transfer port. During the upward motion of the piston from bottom dead centre,the transfer ports and then the exhaust port close and compression of the charge begins and the cycle isrepeated.

Fig.2.7-Crankcase-scavenged

two-stroke engine

32

w.UandiStar.org

www.UandiStar.org


33/127


Fig. 2.8 Ideal and actual indicator diagrams for a two-stroke SI engine

Fig.2.9. Typical valve timing diagram of atwo-stroke engine

33

w.UandiStar.org

www.UandiStar.org


34/127

Theory and Design of Automotive EnginesSeparately scavenged engine

Inthe loop-scavenged engine(Fig. 2.10) an external blower is used to supply the charge, undersome pressure, at the inlet manifold. During the downward stroke of the piston exhaust ports areuncovered at about 65before bottom deadcentre. At about 100 later the inlet ports openand the scavenging process takes place.

The inlet ports are shaped so that mostof the air flows to the top of the cylinder for

proper scavenging of the upper part of thecylinder. Piston deflectors are not used as theyare heavy and tend to become overheated athigh output. The scavenging process ismoreefficient in properly designed loop-scavenged engine than in the usual crank-casecompression engine with deflector piston.

Fig.2.10. Loop-scavenged two-stroke engine (separately scavengedengine)

Opposed piston or end to end scavenged engine (uniflow scavenged) two stroke engine.

In this type of engine the exhaust ports orexhaust valves are opened first. The inlet ports giveswirl to incoming air which prevents mixing of freshcharge and combustion products during thescavenging process. Early on the compression strokethe exhaust ports close. In loop scavenged engine the

port timing is symmetrical, so the exhaust port mustclose afterthe inlet port closes. These timings preventthis type of engine from filling its cylinder at full inlet

pressure. In the end-to-end scavenged engines counterflow within the cylinder is eliminated, and there isless opportunity for mixing of fresh charge and burntgases. The scavenging should therefore be moreefficient.

Fig. 2.11. 'End to end' scavenged or uniflow two-strokeengine

34

w.UandiStar.org

www.UandiStar.org


35/127

Theory and Design of Automotive EnginesValvetiming for two-stroke engines

Fig. 2.12(a), (b) and (c) show typical valve timing diagram for a crankcase-scavenged two-stroke engine and supercharged two-stroke engine and a four-stroke engine, respectively.

Fig 2.12

In case of two-stroke engine the exhaust port is opened near the end of the expansion stroke.With piston-controlled exhaust and inlet port arrangement the lower part of the piston stroke is alwayswasted so as far as the useful power output is concerned; about 15% to 40% of the expansion stroke isineffective. The actual percentage varies with different designs. This early opening of the exhaust ports

during the last part of the expansion stroke is necessary to permit blow down of the exhaust gases and,also to reduce the cylinder pressure so that when the inlet port opens at the end of the blow downprocess, fresh charge can enter the cylinder. The fresh charge, which comes from the crankcase forscavenging pump, enters the cylinder at a pressure slightly higher than the atmospheric pressure. Someof the fresh charge is lost due to short-circuiting. For petrol engine this means a loss of fuel and highunburnt hydrocarbons in the exhaust.By comparing the valve timing of two stroke and four-stroke engines, (Fig. 2.12), it is clear that the timeavailable for scavenging and charging of the cylinder of a two stroke engine is almost one-third thatavailable for the .four-stroke engine. For a crankcase-scavenged engine the inlet port closes before theexhaust port whilst for a supercharged engine the inlet port closes after the exhaust port [Fig. 2.12 (b)].Such timing allows more time for filling the cylinder.

35

w.UandiStar.org

www.UandiStar.org


36/127

Theory and Design of Automotive EnginesScavenging process

At the end of the expansion stroke, the combustion chambers of a two-stroke engine is left fullof products of combustion. This is because, unlike four-stroke engines, there is no exhaust strokeavailable to clear the cylinder of burnt gases. The process of clearing the cylinder of burned gases andfilling it with fresh mixture (or air}-the combined intake and exhaust process is called scavenging

process. This must be completed in a very short duration available between the end of the expansionstroke and start of the charging process.

The efficiency of a two-stroke engine depends to a great degree on the effectiveness of thescavenging process, since bad scavenging gives a low mean indicated pressure and hence, results in ahigh weight and high cost per bhp for the engine. With insufficient scavenging the amount of oxygenavailable is low so that the consequent incomplete combustion results in higher specific fuelconsumption. Not only that, the lubricating oil becomes more contaminated, so that its lubricatingqualities are reduced and results in increased wear of piston and cylinder liners. Poor scavenging alsoleads to higher mean temperatures and greater heat stresses on the cylinder walls.

Thus it goes without saying that every improvement in the scavenging leads to improvement inengine and its efficiency in several directions and hence, a detailed study of scavenging process anddifferent scavenging systems is worthwhile.

The scavenging process is the replacement of the products of combustion in the cylinder from the

previous power stroke with fresh-air charge to be burned in the next cycle. In the absence of an exhauststroke in every revolution of the crankshaft, this gas exchange process for a two-stroke engine must take

place in its entirety at the lower portion of the piston travel. Obviously, it cannot occur instantaneouslyat bottom dead centre. Therefore, a portion of both the expansion stroke and the compression stroke isutilized for cylinder blow-down and recharging.

The scavenging process can be divided into four distinct periodsFig. 2.13 show the pressure recordings inside the cylinder for a Flat 782 S engine. When theinlet port opens the gases expanding in the main cylinder tend to escape from it and to pre-dischargeinto the scavenge air manifold. This process, called pre-blowdown, ends when the exhaust port opens.As soon as the exhaust ports are open, the gases existing in the cylinder at the end of expansion strokedischarge spontaneously into the exhaust manifold and the pressure of the main cylinder drops to avalue lower than that existing in the scavenge air manifold. This process, called blowdown, terminatesat the moment the gas pressure inside the cylinder attains a value slightly lower than the air-pressureinside the scavenge manifold. During the third phase, called scavenging, which starts at the moment thespontaneous exhaust gases from the cylinder terminates and ends at the moment the exhaust ports areclosed; the scavenge air sweeps out all residual gases remaining in the main cylinder at the end of the

spontaneous exhaust and replaces them as completely as possible with fresh charge. After scavenging iscomplete the fresh charge continues to flow till the scavenge ports are open and the pressure in thecylinder rises. This results in better filling of the cylinder. This last part of the scavenging process iscalled additional-charging.

36

w.UandiStar.org

www.UandiStar.org


37/127


Fig. 2.13 Fiat 782 S engine standard scavenging & typical valve timing diagram of a two-stroke engine

37

w.UandiStar.org

www.UandiStar.org


38/127


Fig.2.14shows, a typical pressure-volume diagramfor a two-stroke engine. In this diagram the total pistonstroke has been divided into power strokeand scavengingstroke (This division is arbitrary). The area of the p-vdiagram for the power stroke depends very much on thescavenging efficiency. With proper scavenging efficiencythe pressure rise due to combustion is lower and hencethis area is smaller and lower thermal efficiency isobtained.

Fig. 2.14 Typical pressure-volume for a two-stroke engine.

Theoretical scavenging processes

Fig. 2.15 Three theoretical scavenging processes.

Fig.2.15 illustrates three theoreticalscavenging processes. They are Perfect scavenging, Perfect mixing and Complete shortcircuiting.

{ The delivery ratiomassreference

cyclepermixture)(orairdeliveredofmass=delR , compares the actual

scavenging air mass (or mixture mass) to that required in an ideal charging process.(If scavenging is done with fuel-air mixture, as in spark-ignition engines, then mixture mass is usedinstead of air mass.)

The reference mass is defined as displaced volume ambient air (or mixture) density.Ambient air (or mixture) density is determined at atmospheric conditions or at intake conditions.

This definition is useful for experimental purposes. For analytical work, it is often convenient to use the

trapped cylinder mass mtras the reference mass. OR in other words the delivery ratio is a measure tothe air (mixture) supplied to the cylinder relative to the cylinder content.

If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to thecylinder volume (or displacement volume whichever is taken as reference).

Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where itis less than unity.

The scavenging efficiencychargecylindertrappedofmass

retainedmixture)(orairdeliveredofmass=sc ,

indicates to what extent the residual gases in the cylinder have been replaced with fresh air.If 1=sc , it means that all gases existing in the cylinder at the beginning of scavenging have

been swept out completely.}

38

w.UandiStar.org

www.UandiStar.org


39/127

Theory and Design of Automotive Engines(I)Perfect scavenging.

Ideally, the fresh fuel-air mixture should remain separated from the residual combustion products withrespect to both mass and heat transfer during the scavenging process. Fresh air pumped into thecylinder by the blower through the inlet ports at the lower end of the cylinder pushes the products ofcombustion ahead of itself and of the cylinder through the exhaust valve at the other end. There is nomixing of air and products. As long as any products remain in the cylinder the flow through the exhaustvalves consists of products only. However, as soon as sufficient fresh .air has entered to fill the entirecylinder volume (displacement plus clearance volume) the flow abruptly changes from one of productsto one of air. This ideal process would represent perfect scavenging with no short -circuiting loss.

(ii) Perfect mixing.

The second theoretical scavenging process is perfect mixing, in which the incoming fresh charge mixescompletely and instantaneously with the cylinder contents, and a portion of this mixture passes out ofthe exhaust ports at a rate equal to that entering the cylinder. This homogeneous mixture consistsinitially of products of combustion only and then gradually changes to pure air. This mixture flowingthrough the exhaust ports is identical with that momentarily existing in the cylinder and changes with it.For the case of perfect mixing the scavenging efficiency can be represented by the following equation:

delR

sc e

=1 , where sc and Rdelare scavenging efficiency and delivery ratio respectively.This is plotted in Fig. 2.15. The result of this theoretical process closely approximates the results ofmany actual scavenging processes, and is thus often used as a basis of comparison.

(iii)Short-circuiting.

The third type of scavenging process is that of short-circuiting in which the fresh charge coming fromthe scavenge manifold directly goes out of the exhaust ports without removing any residual gas. This isa dead loss and its occurrence must be avoided.The actual scavenging process is neither one of perfect scavenging nor perfect mixing. It probablyconsists partially of perfect scavenging, mixing and short-circuiting.Fig. 2.16shows the delivery ratio and trapping efficiency variation with crankangle for three different

scavenging modes., i.e.,perfect scavenging (displacement), perfect mixing and intermediatescavenging.Fig. 2.17shows the scavenging parameters for the intermediate scavenging. This represents the actualscavenging process. It can be seen from this Fig. that a certain amount of combustion products isinitially pushed out of the cylinder without being diluted by fresh air. Gradually, mixing

and short circuiting causes the out flowing products to be diluted by more and more fresh air untilultimately the situation is the same as for perfect mixing, i.e., the first phase of the scavenging processis a perfect scavenging process which then gradually changes into a complete mixing process.

Fig,2.16 Delivery ratio and efficiency variation with ` Fig. 2.17 Scavenging parameters forcrankcase for three different scavenging modes. intermediate scavenging

39

w.UandiStar.org

www.UandiStar.org


40/127


Scavenging parameters ..

The delivery ratio - The delivery ratio represents the ratio of the air volume, under the ambientconditions of the scavenge manifold, introduced per cycle and a reference volume. This referencevolume has been variously chosen to be displacement volume, effective displacement volume, totalcylinder volume or total effective cylinder volume. Since it is only the quantity or charge in theremaining total cylinder volume at exhaust port closure that enters into the combustion, the total

effective cylinder volume should be preferred. The delivery ratio is mass of fresh air delivered to thecylinder divided by a reference mass,

i.e.,massreference

cyclepermixture)(orairdeliveredofmass=delR ,

The delivery ratio compares the actual scavenging air mass (or mixture mass) to that required inan ideal charging process. OR The delivery ratio is a measure to the air (mixture) supplied to thecylinder relative to the cylinder content.

If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to thecylinder volume (or displacement volume whichever is taken as reference).

Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where it

is less than unity.(If scavenging is done with fuel-air mixture, as in spark-ignition engines, then mixture mass isused instead of air mass.) The reference mass is defined as displaced volume ambient air (or mixture)density.

Ambient air (or mixture) density is determined at atmospheric conditions or at intake conditions.This definition is useful for experimental purposes. For analytical work, it is often convenient to use thetrapped cylinder mass mtras the reference mass.

The trapping efficiency - The amount of fresh charge retained in the cylinder is not same asthat supplied to the cylinder because some fresh charge is always lost due to short-circuiting. Therefore,an additional term, trapping efficiency, is used to indicate the ability of the cylinder to retain the freshcharge. It is defined as the ratio of the amount of charge retained in the cylinder to the total charge

delivered to the engine, i.e.,(mixture)airdeliveredofmass

retainedmixture)(orairdeliveredofmass=tr

Trapping efficiency indicates what fraction of the air (or mixture) supplied to the cylinder isretained in the cylinder. This is mainly controlled by the geometry of the ports and the overlap time.

The scavenging efficiency Scavenging efficiency is the ratio of the mass of scavengeair which remains in the cylinder at the end of the scavenging to the mass of the cylinder itself at themoment when the scavenge and exhaust ports of valves are fully closed. It is given by

chargecylindertrappedofmass

retainedmixture)(orairdeliveredofmass=sc ,

indicates to what extent the residual gases in the cylinder have been replaced with fresh air.

If 1=sc , it means that all gases existing in the cylinder at the beginning of scavenging havebeen swept out completely.

The purity of the charge:chargecylindertrappedofmass

chargecylinderin trappedairofmasspurity = , indicates

the degree of dilution, with burned gases, of the unburned mixture in the cylinder.

The charging efficiencydensityambientxvolumedisplaced

retainedmixture)(orairdeliveredofmass=ch , indicates

how effectively the cylinder volume has been filled with fresh air (or mixture)Relative cylinder charge.- The air or mixture retained, together with the residual gas, remaining

in the cylinder after flushing out the products of combustion constitutes the cylinder charge. Relative

cylinder charge is a measure of the success of filling cylinder irrespective of the composition of charge.The relative cylinder charge may be either more or less than unity depending upon the scavenging

pressure and port heights.

40

w.UandiStar.org

www.UandiStar.org


41/127

Theory and Design of Automotive EnginesExcess air factor, - The value (Rdel-1) is called the excess air factor.If the delivery ratio is 1.4,

the excess air factor is 0.4.

Classification based on scavenging process

The simplest method of introducing the charge into the cylinder is to employ crankcasecompression as shown in Fig.2.7. This type of engine is classified as the crankcase scavenged engine.In another type, a separate blower or a pump (Fig.2.8) may be used to introduce the charge through theinlet port. They are classified as the separately scavenged engines.

Fig.2.16 Methods of Scavenging (a)Cross Scavenging (b) Loop Scavenging, M.A.N. Type(c)Loop Scavenging Schrle Type, (d) Loop Scavenging, Curtis Type

Another classification of two-stroke cycle engines is based on the air flow.Based on a transversal air stream, the most common arrangement is cross scavenging, illustrated

in Fig.2.16 (a). Most small engines are cross-scavenged. The cross scavenging system employs inlet andexhaust ports placed in opposite sides of the cylinder wall. The incoming air is directed upward, tocombustion chamber on one side of the cylinder and then down on the other side to force out theexhaust gases through the oppositely located exhaust ports. This requires that the air should be guided

by use of either a suitably shaped deflector formed on piston top or by use of inclined ports. With thisarrangement the engine is structurally simpler than that with the uniflow scavenging, due to absence ofvalves, distributors, and relative drive devices. The inlet and exhaust of gases is exclusively controlled

by the .opening and closure of ports by piston motion. The main disadvantage of this system is that thescavenging air is not able to get rid of the layer of exhaust gas near the wall resulting in poorscavenging. Some of the fresh charge also goes directly into the exhaust port. The result of these factorsis poor bmep of cross-scavenged engines.

Based on a transversal air stream, with loop or reverse scavenging, the fresh air first sweepsacross the piston top, moves up and then down and finally out through the exhaust. Loop or reversescavenging avoids the short -circuiting of the cross-scavenged engine and thus improves upon itsscavenging efficiency. The inlet and exhaust ports are placed on the same side of the cylinder wall.In the M.A.N. type of loop scavenge, Fig.2.16(b), the exhaust and inlet ports are on the same side, theexhaust above the inlet.

In the Schnuerle type, Fig.2.16(c), the ports are side by side. the inlet ports are placed on bothsides of the exhaust ports so that the incoming air enters in two streams uniting on the cylinder wallopposite the exhaust ports, flows upwards, turns under the cylinder head, then flows downwards theother side to the exhaust ports. Such a system of air deflection reduces the possibilities of short-circuiting to minimum. With this system flat-top pistons without deflectors are used. The speed of loop

or reversed scavenged engine is not restricted by mechanical limitations because valves are not used,the charging process being controlled by the piston only. The speed can thus, exceed that of valvecontrolled two-stroke engines. Owing to the absence of cams, valves and valve gear, engines are simpleand sturdy. They have a high resistance to thermal stresses and are, thus, well suited to higher

41

w.UandiStar.org

www.UandiStar.org


42/127

Theory and Design of Automotive Enginessupercharge. The major mechanical problem with a loop scavenged two-stroke engine is that ofobtaining an adequate oil supply to the cylinder wall consistent with reasonable lubricating oilconsumption and cylinder wear. This difficulty arises because when the piston is at top dead centre thereis only a very narrow sealing belt available to prevent leakage of oil from crankcase into the exhaust

ports. Since for loop scavenging greater cylinder distance is necessary to accommodate scavenge-airpassage between the cylinder, a strong connecting rod and crankshaft need for supercharged engine canbe used.

The Curtis type of scavenging, Fig.2.16(d), is similar to the Schnuerle type, except thatupwardly directed inlet ports are placed also opposite the exhaust ports.

The most perfect method of scavenging is the uniflow method, based on a unidirectional airstream. The fresh air charge is admitted at one end of the cylinder and the exhaust escapes at the otherend flowing through according to parallel flow lines normally having a slight rotation to stabilize thevertical motion. Air acts like an ideal piston and pushed on the residual gas in the cylinder after the

blowdown period and replaces it at least in principle, throughout the cylinder. The air flow is from endto end, and little short-circuiting between the intake and exhaust openings is possible. Due to absence, atleast in theory, of any eddies or turbulence it is easier in a uniflow scavenging system to push the

products of combustion out of the cylinder without mixing with it and short circuiting. Thus, the

uniflow system has highest scavenging efficiency. Construction simplicity is, however, sacrificedbecause this system requires either opposed pistons, poppet valves or sleeve valve all of which increasesthe complication.

The three available arrangements for uniflow scavenging are shown in Fig.2.17 A poppet valveis used in (a)to admit the inlet air or for the exhaust, as the Case may be. In (b) the inlet and exhaust

ports are both controlled by separate pistons that move in opposite directions. In (c) the inlet andexhaust ports are controlled by the combined motion of piston and sleeve. In an alternative arrangementone set of ports is controlled by the piston and the other set by a sleeve or slide valve. All uniflowsystems permit unsymmetrical scavenging and supercharging.

Fig.2.17 Uniflow Scavenging(a) Poppet Valve(b) Opposed Piston(c) Sleeve Valve

42

w.UandiStar.org

www.UandiStar.org


43/127


Reverse flow scavenging is shown in Fig.2.17 In this type the inclinedports are used and the scavenging air is forced on to the opposite wall of thecylinder where it is reversed to the outlet ports. One obvious disadvantage of

this type is the limitation on the port area. For long stroke engines operating atlow piston speeds, this arrangement has proved satisfactory.

Fig2.17 Reverse Flow Scavenging

An interesting comparison of themerits of two cycle engine air scavengingmethods is illustrated in Fig.2.18. In fact,specific output of the engine is largelydetermined by the efficiency of thescavenging system-and is directly related tothe brake mean effective pressure. Asshown in Fig.2.18 scavenging efficiencyvaries with the delivery ratio and the type ofscavenging. In this respect cross scavengingis least efficient and gives the lowest brakemean effective pressure. The main reasonfor this is that the scavenging air flowsthrough the cylinder but does not expel theexhaust residual gases effectively. Loopscavenging method is better than the cross

scavenging method. Even with a deliveryratio of 1.0 in all cases the scavengingefficiencies are about 53, 67 and 80 per centfor cross scavenging, loop scavenging anduniflow scavenging systems withcorresponding values ofbmep as 3.5,4.5 and5.8 bar.

Fig.2.18 Scavenging Efficiency

Comparison of different scavengingsystems

Fig.2.19 compares the scavenging efficiencies of three different types of scavenging system.The cross-scavenging system employs inlet and exhaust ports placed in opposite sides of the cylinderwall. In the loop scavenging system, inlet and exhaust ports are in the same side of the cylinder wall andin uniflow scavenging system, the inlet and exhaust port are at opposite ends of the cylinder.It can be seen that uniflow scavenging gives by far the best scavenging, that loop scavenging is good,and that in .general, cross-scavenging is the worst.

The scavenging curve for the uniflow scavenging is very near to that of perfect scavenging thatfor loop scavenging is near the perfect mixing. With good loop scavenging the scavenging curve is

generally above the perfect mixing curve and that of cross-scavenging engines it is, generally, below theperfect mixing curve.Table 2.2 compares the port areas available for different scavenging systems. Largest flow areas

are available with uniflow system. In such a case the whole circumference of cylinder wall is available

43

w.UandiStar.org

www.UandiStar.org


44/127

Theory and Design of Automotive Enginesand the inlet port area can be as high as 35 per cent of the piston area. Due to the use of exhaust valvethe exhaust flow area is small - about 18 per cent. In cross-scavenging the size of the inlet and exhaust

ports is limited to about 25 and 18 per cent of piston area respectively because the ports are located onthe opposite sides of cylinder wall. Schurnle type of loop scavenging requires that both the ports must

be located within about three-quarters of the cylinder circumference. This limits the size of inlet andexhaust ports to about 18 and 14 per cent of piston area only. The data for a typical four-stroke engineare also given for comparison. However, while comparing with the four-stroke engine it must be kept inmind that though the flow area is small, the time available for flow is almost three times more than thatavailable for the two-stroke engine.

Fig. 2.19 scavenging efficiency, versus delivery ratio of different scavenging system.

Table 2.2 Typical values for areas for different scavenging systems

Loop or cross-scavenged engines with their inlet ports limited half of the cylinder circumferencefall in low speed category. Uniflow scavenged engines with adequate air inlet port are and limitedexhaust port areas fall in medium speed category, whilst the opposed piston engine takes on to highspeeds because of its high rate of exhaust port opening, freedom from valve gear speed limits, good

scavenging and perfect balancing. Un-supercharged uniflow engine has a considerable higher meaneffective pressure than the loop-scavenged engine. There is more freedom in design of combustionchamber for loop scavenging. This results in low fuel consumption and the engine is simple to make andeasy to produce. Table 2.3 compares the typical bmep values obtainable with different types of

44

w.UandiStar.org

www.UandiStar.org


45/127

Theory and Design of Automotive Enginesscavenging systems. The output of both uniflow and loop scavenged engines is limited 'by the thermalstresses imposed. But the loop scavenged engine due to its simple cylinder head can better withstand thethermal stresses.

45

w.UandiStar.org

www.UandiStar.org


46/127


Table 2.3 Typicalvaluesof bmep for the C.I. two-stroke oil engines

Table 2.4compares the representative port timings for different types of two-stroke engines.

Table 2.4. Port timings for different two-stroke engines

Port design

The Design of the inlet and exhaust ports for two stroke engines depends on various parameters.Some of the important basic parameters are;

a) Scavenging method

b) Shape, inclination & width of portsc) Amount of air/charge deliveredd) Scavenging pressuree) Mean inlet velocity fn. Of pr. Ratio, temp. of scavenging & scavenging factorf) Duration(crank angle) of port opening & average port height uncovered by piston

Blowdown time area (for exhaust)[which is a fn. of temperature of exhaust Gas, expansion endvolume(fn. of displacement volume), exhaust Gas pr., scavenging pr., & indicated meaneffective pressure]

g) Inlet duration, exhaust lead* & hence exhaust durationh) Number of ports & height of ports

*during exhaust Lead, only exhaust port is kept open, & during super charging only inlet port is kept

open.

46

w.UandiStar.org

www.UandiStar.org


47/127


THE DIFFERENT SCAVENGING METHODS ARE AS FOLLOWS

BASED ONSCAVENGING PROCESS( AIR FLOW )

I. CROSS FLOW -for low power o/p engines eg. Two wheelers,

Simple, but more short circuiting, hence more charge loss, super chargingis not possible. It is found that port position is limited with in 50% ofcircumference.

II. LOOP FLOW -for medium o/p engines.Air takes loop, less short circuiting, hence less charge loss

A. MAN type -intake & exh. p

Documents

Theory and Design of Automotive Engine [UandiStar.org]