Stroke Engines

8/3/2019 Stroke Engines

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DEFINATION

A two-stroke engine is an internalcombustion engine that completes the

process cycle in one revolution of the

crankshaft (an up stroke and a down stroke

of the piston, compared to twice that

number for a four-stroke engine.
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Development

The idea to build a two-stroke engine goes back to the

year 1879. But this engine became a qualitatively goodproduct only after many years, when the German DKW

company accelerated its development. Because of its

disadvantages compared with the four-stroke engine, the

two-stroke engine is used practically just in a small range

of capacity, e.g. in small motorcycles. Formerly the engine

was even used to power tiny cars.

BASICS

Like other types of engines, a two-stroke engine has a

crankcase that surrounds and protects all other parts of

the engine. Inside, it has a crankshaft, connecting rod and

single piston. It's also got an intake port, a reed valve, an

exhaust port, and a cylinder---all in addition to thecombustion chamber, where the power is produced that

moves whatever the engine is powering.


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PARTS OF TWO STROKE ENGINE

CRANKSHAFT The crankshaft in a two-stroke engine rotates, moving the

piston by means of the connecting rod. These three partsare the only moving parts in a two-stroke engine. All powerproduced is a direct result of the action of these threemoving parts.

CONNECTING RODThe connecting rod is connected to the crankshaft at oneend, and to the piston at the other. It translates themovement of the crankshaft so that the piston is moved upand down.

PISTON The piston is moved up and down inside the cylinder by

the crankshaft, which is connected to it via the connectingrod. A vacuum is formed as it takes its upward stroke,drawing air and fuel down through the reed valve. Whenthe piston reaches the top, the spark plug then lights theair/fuel mixture, burning it and sending the piston back

down. On the downward stroke, the reed valve gets closedbecause of the increased pressure of the fuel and airmixture within, which is being compressed. New fuel andair travel via the intake port into the cylinder, ready to beburnt. The exhaust is expelled through the exhaust port,


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and an unpleasant side effect is that it usually takes someof the unburned fuel mixture with it.


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WORKING OF TWO STROKE ENGINE

1st stroke: The piston is at the bottom of the cylinder. A

pipe at the left side is opened and lets the fuel mixture,which is already compressed a bit, flow from the lower to

the upper part of the cylinder. The fresh gases expulse

now the exhaust through an ejection pipe, which is not

closed by the piston at this moment.

2nd stroke: After being hurried upward, the piston now

covers the pipe on the left side and the ejection pipe.

Because there is no way out any more, the upper, fresh

gas mixture gets compressed now. At the same time in the

part below fresh gas is taken in by the piston driving

upward through the open suction pipe. At the upper dead-

center, the compressed fuel mixture is ignited by the

sparking plug, the piston is pressed downward while hecompresses at the same time the fresh gas below. The

process begins again as soon as the piston arrives at its

lowest point.


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LUBRICATION

Most small petrol two-stroke engines cannot be lubricatedby oil contained in their crankcase and sump, sincethecrankcaseis already being used to pump fuel-airmixture into the cylinder. Traditionally, the moving parts(both rotating crankshaft and sliding piston) werelubricated by a premixed fuel-oil mixture (at a ratiobetween 16:1 and 50:1). As late as the 1960s, petrol

stations would often have a separate pump to deliver sucha premix fuel to motorcycles. Even then, in many cases,the rider would carry a bottle of their own two-stroke oil.Taking care to close the fuel-tap first, he or she wouldmeter in a little oil (using the cap of the bottle) and thenput in the petrol, this action mixing the two liquids.

Modern two-stroke engines pump lubrication from a

separate tank of oil. This is still a total-loss system with theoil being burnt the same as in the older system, but at alower and more economical rate. It is also cleaner,reducing the problem of oil-fouling of the spark-plugs andcoke formation in the cylinder and the exhaust. Almost theonly motors still using premix are hand-held two-strokedevices, such as chainsaws (which must operate in anyattitude) and the majority of model engines.

All two-stroke engines running on a petrol/oil mix will sufferoil starvation if forced to rotate at speed with the throttleclosed, e.g. motorcycles descending long hills andperhaps when decelerating gradually from high speed bychanging down through the gears. Two-stroke cars (such
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as those that were popular in Eastern Europe in mid-20thcentury) were in particular danger and were usually fittedwithfreewheelmechanisms in thepowertrain, allowing the

engine to idle when the throttle was closed, requiring theuse of the brakes in all slowing situations.

Large two-stroke engines, including diesels, normally usea sump lubrication system similar to four-stroke engines.The cylinder must still be pressurized, but this is not donefrom the crankcase, but by an ancillary supercharger
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Advantages of 2 Stroke Engines:

- Two-stroke engines do not have valves, simplifying theirconstruction.

- Two-stroke engines fire once every revolution (four-stroke

engines fire once every other revolution). This gives two-stroke

engines a significant power boost.

- Two-stroke engines are lighter, and cost less to manufacture.

- Two-stroke engines have the potential for about twice thepower in the same size because there are twice as many power

strokes per revolution.

DISADVATAGES OF 2 STROKE ENGINES

Two-stroke engines don't last nearly as long as four-strokeengines. The lack of a dedicated lubrication system meansthat the parts of a two-stroke engine wear a lot faster.

Two-stroke oil is expensive, and you need about 4 ouncesof it per gallon ofgas. You would burn about a gallon of oilevery 1,000 miles if you used a two-stroke engine in a car.

Two-stroke engines do not use fuel efficiently, so youwould get fewer miles per gallon.
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Two-stroke engines produce a lot of pollution -- so much,in fact, that it is likely that you won't see them around toomuch longer. The pollution comes from two sources. The

first is the combustion of the oil. The oil makes all two-stroke engines smoky to some extent, and a badly worntwo-stroke engine can emit huge clouds of oily smoke.The second reason is less obvious but can be seen in thefollowing figure:

Each time a new charge of air/fuel is loaded into thecombustion chamber, part of it leaks out through the

exhaust port. That's why you see a sheen of oil aroundany two-stroke boat motor. The leaking hydrocarbons fromthe fresh fuel combined with the leaking oil is a real messfor the environment.

These disadvantages mean that two-stroke engines areused only in applications where the motor is not used veryoften and a fantastic power-to-weight ratio is important.

In the meantime, manufacturers have been working toshrink and lighten four-stroke engines, and you can seethat research coming to market in a variety of new marineand lawn-care products


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FOUR STROKE ENGINES

DEFINATION

An internal-combustion engine in which an

explosive mixture is drawn into the cylinder on

the first stroke and is compressed and ignitedon the second stroke; work is done on the third

stroke and the products of combustion are

exhausted on the fourth stroke.


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WORKING

AS their name implies, four-stroke internal combustionengines have four basic steps that repeat with every tworevolutions of the engine:

(1) Intake stroke

(2) Compression stroke

(3) Power stroke and

(4) Exhaust stroke

1. Intake stroke: The first stroke of the internalcombustion engine is also known as the suction strokebecause the piston moves to the maximum volumeposition (downward direction in the cylinder). The inletvalve opens as a result of piston movement, and thevaporized fuel mixture enters the combustion chamber.The inlet valve closes at the end of this stroke.

2. Compression stroke: In this stroke, both valves areclosed and the piston starts its movement to the minimumvolume position (upward direction in the cylinder) andcompresses the fuel mixture. During the compressionprocess, pressure, temperature and the density of the fuelmixture increases.

3. Power stroke: When the piston reaches the minimumvolume position, the spark plug ignites the fuel mixtureand burns. The fuel produces power that is transmitted tothe crank shaft mechanism.


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4. Exhaust stroke: In the end of the power stroke, theexhaust valve opens. During this stroke, the piston startsits movement in the minimum volume position. The open

exhaust valve allows the exhaust gases to escape thecylinder. At the end of this stroke, the exhaust valvecloses, the inlet valve opens, and the sequence repeats inthe next cycle. Four stroke engines require tworevolutions.


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ADVANTAGES OF 4 STROKE ENGINES

The two-stroke vs. four-stroke argument has been goingon ever since Dugals Clarks patented the design in 1881,exactly 20 years after Alphonse Beau de Rochas patentedthe four stroke in 1861. Though 2-strokes are undoubtedlylighted and produce more power per revolution than fourstrokes, they do have a number of disadvantages. Whilemodern technology has narrowed the gap between thesetwo engine designs, the fact is that 4-stroke engines arepreferred for almost every road-going vehicle on the

planet.

Fuel EconomyThe primary reason that 2-strokes tend to get worse fueleconomy than four strokes is that they pull air in throughthe intake port while simultaneously pushing used gases

out through the exhaust port. Along with other factors, thiscrossover often results in fuel being expelled from theexhaust before it has the opportunity to burn. 4-Strokeengines have a dedicated intake, power and exhauststroke, which keeps fuel-to-exhaust crossover to aminimum. All else being equal, a 4-stroke engine with thesame type

of direct injection system used by modern 2-strokes willstill get better fuel economy.


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More Torque

In general, 4-stroke engines almost always make moretorque at low RPM than 2-strokes. This extra torque has a

lot to do with the efficiency of the fuel burn; a 4-strokeuses almost all of its fuel to impart power to thecrankshaft, whereas fuel crossover in a 2-stroke meansthat it will produce less power per RPM. 2-strokes doenjoy an advantage in high-RPM power output, but simplydon't produce the torque of a 4-stroke.

More Durability

Because 2-strokes must rev to very high RPM to makeany power, most applications using them are gearedtoward maintaining that RPM. Any engine designer will tellyou that the more times an engine goes around, thequicker it will wear out. It's pretty simple math; if an engine

can go through ten million RPMs before it wears out, thenone that revolves at 5,000 revolutions per minute will go2000 minutes between rebuilds. The same engine runningat 10,000 RPM will only last 1,000 minutes.

Cleaner Emissions

Above all else, the primary reason that 2-strokes aren'tmore popular in mass-vehicle applications is that they tendto run very dirty. 2-stroke engines require that oil beinjected with the fuel in order to lubricate the crankcase;that oil gets burned along with the gasoline, whichdrastically increases emissions and soot. 4-Stroke engines


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have a dedicated oiling system that's kept largely separatefrom the combustion chamber, which help to ensure thatthe only thing burning in the engine is gasoline. If you've

ever seen an old car blowing huge plumes of blue smokefrom its tailpipe, then you've witnessed the effect that oilburning can have on emissions.


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Because a 2-stroke engine gets a power stroke twice as often as

a four-stroke engine, it puts out about twice as much power (and

makes twice as much noise) as a four-stroke engine of the same

size.The downside is that, because the 2-stroke engine is sloppier

about how it expels exhaust and takes in fuel, doing them almost

at the same time, it is more polluting. Also, the 2-stroke engine

lets fuel into the crankcase, where the piston can pressurize it

prior to intake. In order to keep the crankcase lubricated, you

have to add expensive lubricants to the fuel, and even so, 2-

stroke engines don't last very long.

CYCLE CATEGORIZATION:This is one of the important points to discuss, which is thethermodynamics of the combustion process. There aretwo main cycles based on which we can categorizeinternal combustion engines, which are: Otto cycle andDiesel cycle.OTTO CYCLE:Otto cycle is the typical cycle for most of the cars internalcombustion engines, that work using gasoline as a fuel.Otto cycle is exactly the same one that was described forthe four-stroke engine. It consists of the same four majorsteps: Intake, compression, ignition and exhaust.


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Figure 6

PV diagram for Otto cycleOn the PV-diagram,1-2: Intake: suction stroke2-3: Isentropic Compression stroke3-4: Heat addition stroke4-5: Exhaust stroke (Isentropic expansion)5-2: Heat rejectionThe distance between points 1-2 is the stroke of theengine. By dividing V2/V1, we get:

where r is called the compression ratio of the engine. Theefficiency is taken to be:


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DIESEL CYCLE:In the Diesel Cycle, named after Rudolf Christian KarlDiesel (1858-1913), only air is admitted in the intake

stroke. The air is then adiabatically compressed, and fuelis injected into to the hot air in the form of many smalldrops (not a vapor). Each drop burns over a small time,giving an approximation of a isobaric explosion. Theexplosion pushes the cylinder outwards. The powerstroke, valve exhaust, and exhaust stroke which follow areidentical to those in the Otto Cycle.

A - 1 to 2: Isentropic compressionB - 2 to 3: Reversible constant pressure heatingC - 3 to 4: Isentropic expansionD - 4 to 1: Reversible constant volume cooling

In other words, the only difference between is the Ottoengine and diesel engine is that the latter does not requirea spark plug to ignite the fuel; the fuel here is ignited underthe effect of increase in pressure and temperature. InDiesel engines, compression ratios are as high as 22.5 to

1, where for Otto engines it normally does not reach evenone fifth that number.


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The four cycles of the diesel engine are:1 - The piston is moved away from the cylinder head by

the crankshaft, drawing only air into the cylinder.2 - The piston moves towards the cylinder head,

compressing the air. At the end of the stroke vaporizedfuel is injected into the cylinder and is ignited by the high

temperature of the air.3 - The piston is forced away from the cylinder head bythe gas, expanding after the ignition of the fuel.

4 - The exhaust valve is opened and the piston movestowards the cylinder head, driving the exhaust gases fromthe cylinder.


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CONCEPT ENGINE:

5 STROKE ENGINE

The patented 5-stroke concept, which was

invented by Gerhard Schmitz, utilizes two fired

cylinders operating on a conventional 4-stroke

cycle, which alternately exhaust into a central

expansion cylinder, where the hot exhaust gasesact on a third cylinder. The third cylinder is an

additional low-pressure expansion cylinder

decoupled from the expansion and compression

processes of the 4-stoke pair, and enables the

optimum expansion ratio to be selected

independently of the compression ratio.


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From that one has learned the expansion ratio, thevalue between the intake air into the 4-stoke

cylinder and its exhaust gases are of considerable

interest. The engine runs an overall expansionratio in the region of 14.5:1, which approaches theratio of a diesel engine. The advantage to

following the expansion ratio in the design is thecompression ratio can be reduced to delay knock(pre ignition, where the conditions inside the

combustion chamber ignite the air fuel mixture)

onset without a reduction in performance. Theseinsights in the use of the thermodynamics has the

running of the lab engine producing impressive fuelconsumption readings over a very wide operatingrange.The building of the new engine is a cast cylinder

head, a machined solid cylinder block and separate

electrically powered oil and water pumps. Twooverhead camshafts operate the conventional coil

spring valve gear with the camshaft for the 4-stokecylinders running at one half crankshaft speed andthe fifth stoke cylinders camshaft running at

crankshaft speed. The engine is also turbochargedto increase the engine rating. The entire

construction uses current manufacturingtechnology as well.The payoff is in the early results. From 700ccpeak power is 130 bulk horsepower @ 7000 rpm


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with torque up to 166 newton meters @ 5000 rpmusing a paltry 226 grams of fuel per kilowatt hour.

Ilmor says the prototype is up and ready for road

testing and could be used as a conventional engineor as part of a hybrid drive train. The companysays it is keena British term expressing a strong

motivation, to find a partner to help develop theengine and it is talking to automakers and topsuppliers about the technology.

Principle of operation

The 5-stroke concept engine utilises two fired cylinders

(High Pressure - HP) operating on a conventional 4-stroke

cycle which alternately exhaust into a central expansion

cylinder (Low Pressure - LP), whereupon the burnt gases

perform further work. The LP cylinder decouples theexpansion and compression processes and enables the

optimum expansion ratio to be selected independently of

the compression ratio.

Running of the concept engine has produced impressive

fuel consumption readings over a very wide operating

range. This is because at the onset of knock a greater

percentage of work can be extracted in the LP cylinder,

giving a degree of self compensation.


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5-stroke performance figures Peak power 130 bhp @ 7000 rpm Engine capacity 700cc (turbocharged) Peak torque 166 Nm @ 5000 rpm Fuel consumption of only 226 g/kWh

ADVANTAGES OF 5 STROKE ENGINE CONCEPT

A secondary cylinder provides an additionalexpansion process enabling extra work to beextracted, hence increasing thermodynamicefficiency.

The engine runs an overall expansion ratioapproaching that of a diesel engine in the region of14.5:1

Minimised pumping work due to the downsizing effectfrom highly rated firing cylinders.

The compression ratio can be reduced to delay knockonset without a reduction in performance.

Because the firing cylinders can be very highly rated,the engine is relatively compact.

The fuel consumption does not rise as rapidly withincreasing BMEP, as retarding rejects more energyinto the expansion cylinder.


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The engine uses 100% conventional technology andso requires no new manufacturing techniques.


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An engine is defined as the machine that converts the

chemical energy liberated through combustion of a certain

fuel, into a mechanical energy that is used to derive a

certain vehicle. The definition highlights two importantfacts about the engines. First, an engine is a machine,

hence a mechanism exists. This mechanism can vary, and

thus we can have more than one mechanism of operation.

The two most famous mechanism of actions are the two-

stroke and four-stroke engines. As clear from its name, the

only difference exists in the so-called stroke. This leads todifferent design considerations, and accordingly leads to

distinguishable efficiency for each kind. There is a third

design for engines that is called the Rotary Engine. MENG

491students are working on designing and manufacturing

a rotary engine this semester.


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Stroke Engines