Efficiency of the IC Engine

8/2/2019 Efficiency of the IC Engine

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Efficiency of the IC engine:-

IC engines lose 42% of their energy to exhaust and 28% of their energy to the cooling

system. Therefore the true explanation for the poor performance of the engine would seem to

lie in inefficient use of energy and loss of energy through heat transfer. The loss incurred

through inefficient use of energy is easily understood , compressed fuel and air is ignited andis then used to propel the piston down the cylinder with explosive force for a distance of just

a few inches after which all further energy developed by the fuel is lost and in fact becomes a

liability since the piston has to reverse direction , a process which is inhibited by the pressure

of trapped gases on the piston head. The reason that energy loss to heat transfer has been

tolerated , and even welcomed by engineers , is a little more involved and will be referred to

later on in the article. Notwithstanding the improvements made to the RI engine we have to

ask ourselves , and this is the million dollar question , is this really the limit of performance

of the reciprocating internal combustion engine, does this mark the end of the road for this

more than 200 year old concept , some entrepreneurs seem to think not , they have come up

with the idea of a concept IC engine.

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The pressure pushing down on the piston and forces the crankshaft to rotate, converting the

chemical energy in the fuel to mechanical energy. Unfortunately, the internal combustion

engine is not very efficient and a lot of this energy is lost through heat that is absorbed by the

engine, and lost through the exhaust. Though the heat energy that is lost through the exhaust

can be used to drive aturbochargerso it can't be all that bad, can it?

The burnt air/fuel mixture is expelled from the engine during the exhaust stroke. The

exhaust valve opens slightly before the stroke begins. With the exhaust valve open, the

movement of the piston from BDC to TDC forces the burnt air/fuel mixture through the

exhaust valve and out of the engine. Usually, the exhaust valve opens slightly before the

stroke begins and closes slightly after the stroke ends, allowing the engine to expel as muchburnt air/fuel mixture as possible. Any burnt air/fuel mixture or exhaust gasses that remain in

the combustion chamber after the exhaust stroke will contaminate the fresh air/fuel mixture

that in drawn into the cylinder on the next intake stroke, and will effectively reduce engine

power.

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Basic Engine Power

by "Langer"

There are four ways in which you can increase engine power:

Increase the engine displacement by boring the motor out or stroking the crankshaft. Increase the engine speed. Improve the Volumetric Efficiency of the engine. Increase the air density.

Essentially, all these engine tuning methods seek to improve air flow in and out the engine.

DISPLACEMENT

Engine capacity or displacement is measured by the formula (/4 bore2) stroke cylinders. The bore is the diameter of the cylinder; thus (/4 bore2) gives us the area of the
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cylinder. The stroke is the distance the piston travels from TDC to BDC and gives us the

length of the cylinder. Multiplying these two measurements gives us the volume of one

cylinder. Multiplying the volume of each cylinder by the number of cylinders that engine has

will give us the total displacement of the engine. Thus, by increasing the area, length, or

number of cylinders, we can increase the displacement of the engine.

Unfortunately we can't increase the number of cylinders so we're left with the area and thelength. We can increase the cylinder area by boring the motor. This is the easiest way of

increasing displacement, but is restricted by the thickness of the cylinder walls, and the space

between the cylinders. We can also increaser the length by stroking the crankshaft. This is

more complicated as it requires the offset machining of the big-end journals on the

crankshaft and possibly on the conrods. If the big-end journals of the conrod cannot be

ground, you must either find slightly longer conrods that will fit, or pistons with a shorter

compression height, i.e., the distance between the center of the gudgeon pin and the piston

top. Stroking is restricted by the clearance between the rotational diameter of the crankshaft

and the engine block.

ENGINE SPEED

Increasing engine speed does not increase the power per cycle, but increases the rate at

which power in produced as the number of cycles per time frame increase. In other words,

power is being produced more often as the Otto cycle is being completed much quicker.

Increasing engine speed above the red line of the stock engine generally requires a complete

engine rebuild with forged pistons, stainless steel conrods, stainless steel crankshaft, and a

more robust valve train.

VOLUMETRIC EFFICIENCY

The Volumetric Efficiency (VE) of an engine is the amount of air/fuel mixture that is

ingested by the engine during the intake stroke, relative to the engines displacement. There

are a number of factors that prevent a stock engine from achieving a 100% VE. Chief among

these are restrictions in the airflow path of the intake and exhaust, valve overlap effects, and

reversion.

Restrictions in the airflow on the intake side include the air filter, the throttle body, the

plenum and runners, and the intake port. These restrictions can be overcome to some

degree by fitting a high-flow air filter, and improving air flow through porting and gas

flowing, especially on the cylinder head.

Restrictions on the exhaust system include the exhaust header, the catalyst converter, and

the mufflers. Unfortunately, anti-emission legislation requires that the catalyst converter beretained on street legal cars but restrictions in other areas of the exhaust system can be

overcome by fitting a free flow exhaust header and free flow exhaust mufflers.

Fitting a free flow exhaust system will also reduce reversion, which is the flow of exhaust

gasses back into the combustion chamber. Reversion causes contamination of the air/fuel

mixture and takes up space that the air/fuel mixture should fill, thus reducing volumetric

efficiency. Too much back pressure in the exhaust system will cause reversion. As Bre

suggests in ourexhaust guide, fitting a free flowexhaust headerthat is slightly larger than the

exhaust port on the cylinder head reduce reversion but an anti-reversion (AR) header that is

specially designed to inhibit reversion would be even better.

AIR DENSITY
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Denser air produces more power because it has more air molecules per volume. There are

two ways in which air density can be increasedby lowering the air temperature, or

increasing air pressure. Unfortunately, we can't really lower the air temperature but we can

increase air pressure. The easiest way to increase air pressure would be to drive at lower

altitude but this isn't really practical. The other way is to use forced induction. The three

forms of forced induction are:..........................................................................................................................

During the exhaust stroke, a good way for an engine to lose power is through back

pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston

pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the

piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust

valves rather than one improves the flow by making the hole that the exhaust gases travel

through larger.

In a normal engine, once the exhaust gases exit the cylinder they end up in the exhaust

manifold. In a four-cylinder or eight-cylinder engine, there are four cylinders using the same

manifold. From the manifold, the exhaust gases flow into one pipe toward thecatalytic

converterand themuffler. It turns out that the manifold can be an important source of back

pressure because exhaust gases from one cylinder build up pressure in the manifold that

affects the next cylinder that uses the manifold.

The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a

common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe.

These pipes come together in a larger pipe called the collector. The individual pipes are cut

and bent so that each one is the same length as the others. By making them the same length, it

guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so

there is no back pressure generated by the cylinders .

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First, thecylindersucks a mixture of oxygen and fuel (gasoline or diesel) into its chamber,

where it is then ignited by a spark plug or glow stick. These three ingredients create a

controlled explosion, orcombustion, which generates both energy and waste gases. That

energy transforms into wheel-turningtorqueto propel you down the road, but the exhaust is a

cloud of useless particles that would clog up your engine if left to linger in the piston

chamber.

That's where the exhaust system comes into action. The spent gases are exhaled, traveling outof the engine through your exhaust manifold, and they eventually exit out the back of your

vehicle. The faster the exhaust gets out of the way, the easier it is for your engine to breathe,

which means more horsepower,torqueand fun behind the wheel.

The problem with stock exhaust systems is that they are not optimized for the best possible

flow. Auto manufacturers spend much more time thinking about the shape and placement of

your air vents than they do planning your exhaust pipes. What's the result of their neglect?

Your motor has to work harder duringcombustion, which robs you of valuable horsepower,

torqueand fuel economy. A performance exhaust system will harness the potential power that

your engine is wasting because of its inefficient stock exhaust. AutoAnything's quick-

installing, bolt-on performance exhaust systems improve your power by:

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INCREASING THEINCREASING THE AMOUNT AND TRAVELING SPEED OF EXHAUST FLOW is the first step toincrease engine performance and efficiency. The factory exhaust system represents arestriction in the stock configuration. By designing a system that increases the amount and

traveling speed of exhaust flow, it allows for a reduction in backpressure and thermal stress,thus unleashing the engines potential to improve power, torque and responsiveness.power, torque and responsiveness.

Producing More Engine Power

HorsepowerFor a complete explanation of what horsepower is and what horsepower means, check outHowHorsepower Works.

Using all of this information, you can begin to see that there are lots of different ways to make anengine perform better. Car manufacturers are constantly playing with all of the following variables tomake an engine more powerful and/or more fuel efficient.

Increase displacement - More displacement means more power because you can burn more gasduring each revolution of the engine. You can increase displacement by making the cylinders biggeror by adding more cylinders. Twelve cylinders seems to be the practical limit.Increase the compression ratio - Higher compression ratios produce more power, up to a point.The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burstinto flame (before the spark plug ignites it).Higher-octanegasolines prevent this sort of earlycombustion. That is why high-performance cars generally need high-octane gasoline -- their enginesare using higher compression ratios to get more power.Stuff more into each cylinder - If you can cram more air (and therefore fuel) into a cylinder of agiven size, you can get more power from the cylinder (in the same way that you would by increasingthe size of the cylinder). Turbochargers and superchargers pressurize the incoming air to effectivelycram more air into a cylinder. SeeHow Turbochargers Workfor details.Cool the incoming air - Compressing air raises its temperature. However, you would like to havethe coolest air possible in the cylinder because the hotter the air is, the less it will expand whencombustion takes place. Therefore, many turbocharged and supercharged cars have an intercooler.

An intercooler is a special radiator through which the compressed air passes to cool it off before itenters the cylinder. SeeHow Car Cooling Systems Workfor details.Let air come in more easily - As a piston moves down in the intake stroke, air resistance can robpower from the engine. Air resistance can be lessened dramatically by putting two intake valves ineach cylinder. Some newer cars are also using polished intake manifolds to eliminate air resistancethere. Bigger air filters can also improve air flow.Let exhaust exit more easily - If air resistance makes it hard forexhaustto exit a cylinder, it robsthe engine of power. Air resistance can be lessened by adding a second exhaust valve to eachcylinder (a car with two intake and two exhaust valves has four valves per cylinder, which improvesperformance -- when you hear a car ad tell you the car has four cylinders and 16 valves, what the ad

is saying is that the engine has four valves per cylinder). If the exhaust pipe is too small or themuffler has a lot of air resistance, this can cause back-pressure, which has the same effect. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system. When you hear that a car has "dual exhaust," the goal is to improvethe flow of exhaust by having two exhaust pipes instead of one.Make everything lighter - Lightweight parts help the engine perform better. Each time a pistonchanges direction, it uses up energy to stop the travel in one direction and start it in another. Thelighter the piston, the less energy it takes.Inject the fuel - Fuel injection allows very precise metering of fuel to each cylinder. This improvesperformance and fuel economy. SeeHow Fuel Injection Systems Workfor details.
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Efficiency of the IC Engine