2007-Mech-130 ICE LAB Assignment # 01

Four-Stroke Internal Combustion Engine

1) Engine

It is defined as the machine which is used to convert energy, especially Heat

energy into Mechanical work. It is the Heart of automobiles.

1.1) Classification of Engines

In broad sense engines are usually classified into two main types;

Internal Combustion Engine (IC Engine)

External Combustion Engine (EC Engine)

Internal Combustion Engine

In internal combustion engine or IC engine the combustion of fuel takes place

within the working fluid which produces heat energy which is being used later to produce

mechanical work output.

External Combustion Engine

In external combustion engine or EC engine the combustion of fuel does not take

place within the working fluid. Combustion of fuel takes place separately and then heat

of flue gases is being used for the heating of working fluid and finally we get mechanical

output by using this heated working fluid under different setups.

1.2) 4-Stroke Engine

It is the type of internal combustion engine. A four stroke engine is that one in

which thermodynamic cycle is completed by the four strokes of the piston in the cylinder

or by the two revolutions of crankshaft. These four strokes are named as follows; also

shown in figure [1.1]

Induction stroke or Suction stroke

Compression stroke

Expansion stroke or Working stroke

Exhaust stroke

Induction stroke or Suction stroke

During the intake stroke, the piston travels downward in the cylinder from Top

Dead Centre (TDC) to Bottom Dead Centre (BDC), the volume of the cylinder chamber

is enlarged. This enlargement of the cylinder chamber causes a decrease in air pressure

which creates a partial vacuum inside the cylinder. Atmospheric pressure forces air or air

and fuel mixture into the cylinder through the open intake valve. The exhaust valve

remains closed during this stroke and the crankshaft turns through 180º or half a

revolution.

2007-Mech-130 ICE LAB Assignment # 01

Compression stroke

The intake valve closes, the piston moves up from BDC to TDC, and the gas in

the cylinder is compressed in the combustion chamber. This stroke is called the

compression stroke and, depending on the type of engine, it usually ranges between 850

kPa and 1000 kPa. Atmospheric pressure is 100 kPa. The fuel is now ready to be ignited.

This is accomplished by an electric spark at the spark plug in case of Spark Ignition (SI)

engine while in case of Compression Ignition (CI) engine fuel is injected in the

compressed air and no spark plug is used. Both valves remain closed during compression

and combustion processes.

Expansion stroke or Working stroke

As combustion process immediately takes place, and the gas, as it burns, heats

and expands instantly. The rapid expansion of the gas greatly increases the pressure in the

cylinder. This pressure increase is approximately five times greater than the compression

pressure, being between 4200 kPa and 4900 kPa and forces the piston down from TDC to

BDC, causing the crankshaft to turn. This is known as the power stroke. Both valves

remain closed during this stroke.

Exhaust stroke

The crankshaft has now rotated 1.5 revolutions, and the cylinder has become

filled with burnt gases that must be removed. The exhaust valve opens, and the piston

moves from BDC to TDC, forcing the burnt gases out of the cylinder. This is known as

the exhaust stroke of 4-stroke engine. During this stroke the intake valve remains closed.

Now the crankshaft has completed two revolutions. The piston is at top dead centre and

the engine is ready to repeat the cycle of operations again.

Figure 1.1: Four strokes of IC Engine

2007-Mech-130 ICE LAB Assignment # 01

1.3) Classification of 4-Stroke Engine

4-stroke engines are classified into different types depending upon the

construction, ignition process and on many other basis. Some of these types are discussed

here.

On the basis of Valve Opening & Closing mechanism

a) Over Head Valve (OHV) engine

In this engine design in which camshaft is installed inside the engine block and

valves are operated through lifters, pushrods and rocker arms (an OHV engine also

known as a "Pushrod" engine). Although an OHV design is a bit outdated, it has been

successfully used for decades.

An OHV engine is very simple, has more compact size and proven to be durable as

shown in fig [1.2]. On the downside, it's difficult to precisely control the valve timing at

high rpm due to higher inertia caused by larger amount of valve train components (lifter-

pushrod-rocker-arm). Also, it's very difficult to install more than 2 valves per cylinder, or

implement some of the latest technologies such as Variable Valve Timing.

In automotive engineering, an overhead valve internal combustion engine is one in which

the intake and exhaust valves and ports are contained within the cylinder head.

b) Over Head Camshaft (OHC) engine

It is also called as SOHC means Single OverHead Cam. In the SOHC engine the

camshaft is installed in the cylinder head and valves are operated either by the rocker

arms or directly through the lifters as shown in fig [1.3]. The advantage is that valves are

operated almost directly by the camshaft, which makes it easy to achieve the perfect

timing at high rpm. Also it's possible to install three or four valves per cylinder.

Figure 1.2: OHV Engine

2007-Mech-130 ICE LAB Assignment # 01

Compared to OHV pushrod systems with the same number of valves the reciprocating

components of the OHC system are fewer and have a lower total mass. The disadvantage

is that an OHC engine requires a timing belt or chain with related components therefore it

becomes more complex and more expensive design.

c) Twin Cam Engine

It is also called DOHC or Double OverHead Cam - this setup is used in many

today's cars. Since it's possible to install multiple valves per cylinder and place intake

valves on the opposite side from exhaust vales, a DOHC engine can "breathe" better,

meaning that it can produce more horsepower with smaller engine volume as shown in

figure [1.4].

Comparison: The 3.5-liter V6 DOHC engine of 2003 Nissan Pathfinder has 240

hp, similar to 245 hp of the 5.9-liter V8 OHV engine of 2003 Dodge Durango.

Twin Camshaft engines have High efficiency, possible to install multiple valves per

cylinder and adopt variable timing but it is more complex and more expensive. A double

overhead camshaft valve train layout is characterized by two camshafts located within the

cylinder head, one operating the intake valves and one operating the exhaust valves.

Figure 1.3: OHC Engine Figure 1.4: DOHC Engine

2007-Mech-130 ICE LAB Assignment # 01

On the basis of Cylinders Alignment

There are many types of 4-stroke engine on the basis of alignment and number of

cylinders in which piston moves back and forth. These are as follows;

a) Single cylinder b) In-line or straight cylinder c) V-engine

d) opposed cylinder e) opposed piston f) W Engine

g) Radial engine h) opposed piston opposed cylinder engine (OPOC)

a) Single cylinder engine

It is very simple and common type of engine construction and used

very commonly in bikes, lawn movers and other automoblies where

compact size of engine is required. It only consists of one cylinder in

which pistons moves back and forth and in order to complete the

thermodynamic cycle

b) In-line or straight cylinder engine

Inline engines consist of cylinders placed in a single bank one

after another. These engines often offer more torque and a smother

power delivery than a v-type due to the larger block and distance

between cylinders. They are longer in length and height but are

narrower in width than v-types.

c) V-engine

An engine is classified as a V-type if there are two banks

of cylinders attached to a single crankshaft. The cylinders are

usually positioned at either 90 or 60 degree angles from each

other. This block is shorter in length and height than a

corresponding inline block of the same cylinder count and

displacement. This means that the overall hood area of the car can

be smaller or substituted for more passenger or storage area.

Because these engines use a short, strong crankshaft they are

better suited to tolerating higher rotational speeds and higher torsional stresses.

d) Opposed Cylinder engine

Continued improvements in engine design led

to the development of the horizontally-opposed engine

which remains the most popular reciprocating engines

used on smaller aircraft. These engines always have an

even number of cylinders, since a cylinder on one side

of the crankcase “opposes” a cylinder on the other

side. The majority of these engines are air cooled and

2007-Mech-130 ICE LAB Assignment # 01

usually are mounted in a horizontal position when installed on fixed-wing airplanes. Opposed-

type engines have high power-to-weight ratios because they have a comparatively small,

lightweight crankcase. In addition, the compact cylinder arrangement reduces the engine’s

frontal area and allows a streamlined installation that minimizes aerodynamic drag.

e) Opposed Piston engine

In engines that have the opposed-piston

arrangement, two crankshafts (upper and lower) are

required for transmission of power. Both shafts contribute

to the power output of the engine. In opposed-piston

engines that are common to Navy service, the crankshafts

are connected by a vertical gear drive which provides the

power developed by the upper crankshaft. This power is delivered through the vertical drive

shaft to the lower crank-shaft. Large roller bearings and thrust bearings support and guide the

vertical drive shaft.

The cylinders of opposed-piston engines do not have valves. Instead, they employ scavenging air

ports located near the top of the cylinder. These ports are opened and closed by the upper piston.

Exhaust ports located near the bottom of the cylinder and are closed and opened by the lower

piston.

f) W-engine

This is a specific type of reciprocating / piston internal

combustion engine configuration. The cylinder banks resemble

the letter W, in the same way a V engine resembles the letter V.

There have been three entirely different implementations of this

concept: one with three banks of cylinders, one with four banks,

and one with two banks of cylinders and two crankshafts.

g) Radial Engine

These were widely used during World

War II and many are still in service today. With

these engines, a row or rows of cylinders are

arranged in a circular pattern around the

crankcase. It may have any number of

cylinders from three to nine. The radial engine

has the same sort of pistons, valves and spark

plugs that any four-stroke engine has. The big

difference is in the crankshaft. Instead of the

long shaft that's used in a multi-cylinder car

engine, there is a single hub. All of the piston's

connecting rods connect to this hub. One rod is

Figure 1.5: Radial Engine

2007-Mech-130 ICE LAB Assignment # 01

fixed, and it is generally known as the master rod. The others are called articulating rods. They

mount on pins that allow them to rotate as the crankshaft and the pistons move. The main

advantage of a radial engine is the favorable power-to-weight ratio.

h) Opposed piston opposed cylinder engine (OPOC)

This engine design is a significant advancement in the art of combustion engines. The

patented engine family consists of extremely compact, lightweight and efficient engines, having

very low noise and vibration and lower costs when compared to conventional engines in similar

power ranges.

This engine is a two-stroke turbocharged two-cylinder with two pistons in each cylinder. It has

four pistons in a two-cylinder engine. The pistons sit opposed to each other, one being pushed,

the other pulled by the combustion of each stroke. Research is still being going on this type of

engine design. Figure [1.6] shows its configuration.

Applications include

engines of all sizes for

military vehicles and

aircraft, automotive,

marine, motorcycle,

scooter, lawn and

garden, power tools

and more.

i) Wankel Engine

German inventor called Felix

Wankel first proposed a design of an ultra

simple and efficient engine which was

called after him the Wankel Engine.

Wankel engine uses rotor instead of a

pistons which allow it to deliver power

without vibration in much higher RPMs.

The design is so simple and brilliant.

Here figure [1.7] shows how is it works?

Figure 1.6: OPOC Engine

Figure 1.7: Wankel Engine

2007-Mech-130 ICE LAB Assignment # 01

On the basis of Fuel Injection System

The fuel injection types used in newer cars include:

a) Single-point or throttle body injection (TBI)

b) Port or multi-point fuel injection (MPFI)

c) Sequential fuel injection (SFI)

d) Direct injection

a) Single-point or throttle body injection (TBI)

This is the earliest and simplest type of fuel injection. Single-point simply replaces the

carburetor with one or two fuel-injector nozzles in the throttle body, which is the throat of the

engine’s air intake manifold. For some automakers, single-point injection was a stepping stone to

the more complex multi-point injection system. Though not as precise as the systems that have

followed now, TBI meters fuel better than a carburetor and it is less expensive and easier to

service as compared to others.

b) Port or multi-point fuel injection (MPFI)

Multi-point fuel injection devotes a separate injector nozzle to each cylinder, right

outside its intake port, which is why the system is sometimes called port injection. Shooting the

fuel vapor this close to the intake port almost ensures that it will be drawn completely into the

cylinder. The main advantage is that MPFI meters fuel more precisely than do TBI designs,

better achieving the desired air/fuel ratio and improving all related aspects. Also, it virtually

eliminates the possibility that fuel will condense or collect in the intake manifold. With TBI and

carburetors, the intake manifold must be

designed to conduct the engine’s heat, a

measure to vaporize the liquid fuel. This is

unnecessary on engines equipped with

MPFI, so the intake manifold can be

formed from lighter-weight material, even

plastic. Incremental fuel economy

improvements result. Also, where

conventional metal intake manifolds must

be located atop the engine to conduct heat,

those used in MPFI can be placed more

creatively, granting engineers design

flexibility.

c) Sequential fuel injection (SFI)

Sequential fuel injection, also called sequential port fuel injection (SPFI) or timed

injection, is a type of multi-port injection. Though basic MPFI employs multiple injectors, they

all spray their fuel at the same time or in groups. As a result, the fuel may “hang around” a port

for as long as 150 milliseconds when the engine is idling. This may not seem like much, but it’s

enough of a shortcoming that engineers addressed it: Sequential fuel injection triggers each

Figure 1.8: Schematic of MPFI system

2007-Mech-130 ICE LAB Assignment # 01

injector nozzle independently. Timed like spark plugs, they spray the fuel immediately before or

as their intake valve opens. It seems a minor step, but efficiency and emissions improvements

come in very small doses.

d) Direct injection

Direct injection takes the fuel injection concept about as far as it can go, injecting fuel

directly into the combustion chambers, past the valves. More common in diesel engines, direct

injection is starting to pop up in gasoline engine designs, sometimes called DIG for direct

injection gasoline. Again, fuel metering is even more precise than in the other injection schemes,

and the direct injection gives engineers yet another variable to influence precisely how

combustion occurs in the cylinders. The

science of engine design scrutinizes how the

fuel/air mixture swirls around in the

cylinders and how the explosion travels

from the ignition point. Things such as the

shape of cylinders and pistons; port and

spark plug locations; timing, duration and

intensity of the spark; and number of spark

plugs per cylinder (more than one is

possible) all affect how evenly and

completely fuel combusts in a gasoline

engine. Direct injection is another tool in

that discipline, one that can be used in low-

emissions lean-burn engines.

On the basis of Engine Cooling System

a) Water cooled engines

In this type of engine is being cooled by

circulation of water through the which are made in

the block of the engine during its design. Usually

large engine are cooled by water cooling system.

If no cooling system is used in engine then engine

can seize at higher speeds and its internal parts

like piston , connectiong rod and cylinder can be

damaged which ultimatly destroy the engine.

b) Air cooled engines

These types are usually cooled by air. This is done with

the help of fins made around the cylinder head and also

arounde the block of the engine. This technique to cool down

the engine is mostly used in small engines which are usually

Figure 1.9: Direct Injection system

Figure 1.10: Water cooled system

Figure 1.11: Air cooled system

2007-Mech-130 ICE LAB Assignment # 01

used in motor bikes, lawn mover etc.

On the basis of Speed of the engine

Low speed engine, medium speed engine and high speed engine.

On the basis of Type fuel used

Petrol engine, Diesel engine, Gas engines, Kerosene engines LPG, CNG etc.

On the basis of Method of igniting fuel

Spark ignition (SI) engine and Compression ignition (CI) engines.

1.4) P-V Diagram of 4-Stroke Engine

Four-stroke engine usually operate on a thermodynamic cycle which is otto cycle i.e otto

cycle is the ideal cycle for studying the operation of 4-stroke spark ignition engine. First I will

discuss the different processes invovled in the otto cycle and then how we can get maximum

power from engine.

Ideal Otto cycle

The ideal P-V diagram of the Otto cycle is shown in figure [1.12]. Cycle begins at the

lower left at Stage 1 with the beginning of the intake stroke of the engine. Due to difference in

pressure across the cylinder inlet valve charge will move inwards through inlet valve. Between

Stage 1 and Stage 2 the piston is pulled out of the cylinder (towards BDC) with the intake valve

open. The pressure remains constant, and volume increases as fuel/air mixture is drawn into the

cylinder through the intake valve. As Stage 2 begins, the compression stroke of the engine starts

with the closing of the intake valve. Between Stage 2 and Stage 3, the piston moves back into the

cylinder (towards TDC), the charge volume decreases, and the pressure increases because work

is done on the charge by the piston. Stage 3 is the beginning of the combustion of the fuel/air

mixture. The combustion occurs very quickly and the volume remains constant. Heat is released

during combustion which increases both the temperature and the pressure, according to the

equation of state.

Stage 4 begins the power stroke or working stroke of the engine. Between Stage 4 and

Stage 5, the piston is driven towards TDC or the crankshaft, the volume in increased, and the

pressure falls as work is done

by the product gases on the

piston. At Stage 5 the exhaust

valve is opened and the

product gases are exchanged

with the surrounding. The

volume remains constant and

the pressure adjusts back to

atmospheric conditions. Stage

6 begins the exhaust stroke of

the engine during which the

piston moves back into the

cylinder (toward TDC), the

volume decreases and the

pressure remains constant. At

the end of the exhaust stroke, Figure 1.12: Ideal P-V Diagram

2007-Mech-130 ICE LAB Assignment # 01

conditions have returned to Stage 1 and the process repeats itself.

Ideal Otto cycle consists of following processes as described below;

2-3 Adiabatic Compression process

3-4 Heat addition at constant volume

4-5 Adiabatic expansion process

5-6 Heat rejection at constant volume

Actual Otto Cycle

In actual practice, cycle varies from the ideal cycle due to some irreversibilities in the

process and also due to entropy generation. Figure [1.13] below shows the actual cycle during

the operation of 4-stroke engine.

It is clear from the figure that in actual case peak pressure occur as the piston starts it motion

towards the BDC (Bottom Dead Cente) i.e peak pressure occur before TDC (Top Dead Centre)

whereas in ideal case peak pressure occure at TDC. Also the spark plug ignites before the piston

reaches TDC, it is due to the reason to allow the flame to penetrate properly through the entrie

mixture which is generated from the spark.

It can also see that exhaust leaving the engine cylinder has higher pressure than atmospheric

pressure which allows it to exhaust automatically. There are two loops formed in the actual

cycle, smaller loop is called Pumping loop and bigger loop is called Power loop.

Figure 1.13: Actual P-V Diagram of Four Stroke Engine

2007-Mech-130 ICE LAB Assignment # 01

1.5) 4-Stroke Engine Optimization

In the optimization of any engine P-V diagram plays a very important role. Suppose if

someone wants to increase the power of engine then he can do that by doing some changes in the

engine if and only if these changes also change the area of PV diagram. Because, if the area of

power loop increases or area of pumping loop decreases then power of engine increases and vice

versa; otherwise it is wastage of time and money.

The power of four stroke engine can be increased by changing some of the following parameters;

By increasing pressure Ratio

By increasing displacement

By increasing rate of combustion of fuel

By increasing the calorific value of fuel

By using Turbo-charging

a) It increases the peak pressure point in the P-V diagram which ultimately increases the

power of engine.

b) It also enters the compressed air into engine through the inlet valve of engine, so area of

pumping loop decreases.

By using ultra charging

a) By sucking the exhaust

By sucking the exhaust

By reducing the friction losses

a) It can be done by reducing the piston skirt

b) By increasing surface finish

c) By increase lubrication

d) By using high quality material such as PTFE (Poly-tetra-flouro-ethene)

By increasing the Volumetric Efficiency of engine

2007-Mech-130 ICE LAB Assignment # 01

References

http://www.grc.nasa.gov/WWW/K-12/airplane/engopt.html

http://www.cdxetextbook.com/engines/motivePower/4gasEng/basicprincpetrol.html

http://classicmotorcycles.about.com/od/historicaldevelopment/ss/4strokeengines_2.ht

m

http://auto.howstuffworks.com/engine.htm

http://www.mechlook.com/2010/04/working-of-ic-engines/

http://www.tpub.com/content/construction/14264/css/14264_46.htm

http://www.deepscience.com/articles/engines.html

http://www.samarins.com/glossary/dohc.html

http://www.arthursclipart.org/carmechanic/mechanic/page_01.htm (pics)

http://en.wikipedia.org/wiki/Overhead_camshaft

http://howautowork.com/list_of_contents/part_1/ch_1/Engine_Types_with_respect_t

o_cylinders_arrangement_3.html

http://www.engineeringtv.com/video/Opposed-Piston-Opposed-Cylinder

http://www.ecomotors.com/

http://www.propulsiontech.com/aboutapt.html

http://science.howstuffworks.com/transport/engines-equipment/radial-engine2.htm

http://ask.cars.com/2008/08/types-of-fuel-i.html

http://www.kewengineering.co.uk/upgrades4mgs/Electronics/fuel_injection.htm

http://www.kfz-tech.de/Engl/pVDiagramm.htm PV diagram

http://www.mechadyne-int.com/vva-reference/papers/the-impact-of-variable-valve-

actuation-on-engine-performance-and-emissions.pdf

http://www.tuning.wanadoo.co.uk/tuning-basics.htm (important)

http://www.custom-car.us/basics/default.aspx (important)