Air Properties in an Engine Cylinder

It´s all about squeezing more combustion air per unit volume of cylinder intake. Thus more power.

hi, i can't get it.what is the relation between volume of air and power output?

Engine output comes from the expansion of air, forcing a piston down. A greater volume of air creates more force on the piston, and thus more torque. Also, because the turbocharger allows the engine to "breathe" better, the increased torque can be maintained while the engine is running faster, so that power is geometrically increased. For example, a 20% increase in torque at a 20% higher rpm yields a 44% increase in power.

In general terms, the faster one can get air and fuel into the engine, get it burnt and get rid of the combustion products, the more power comes out of the engine.

Turbochargers use the exhaust gases to spin a turbine that forces air into the engine faster.

Superchargers do the same thing except that they use the engine revolutions instead of the exhaust gases.

Hi raj

Two things.

Flow

If you move the air within a tube or pipe that is 1 square foot in area (1.12837 Ft. Dia.) at the velocity of 1200 feet per minute at atmospheric pressure (14.7 psia) the flow would be approximately 1200 cubic feet per minute.

If you increased the pressure (compression ratio 2/1) to 14.7 psig (Gauge pressure) combined with the original (14.7 psia) cubic foot of atmosphere the air flow at the same velocity of 1200 feet per minute would be approximately 2400 cubic feet per minute.

With a fuel injected engine the amount of fuel is controlled by the injector. In older naturally aspirated engines increased flow (air density) would also increase the amount of fuel.

With a finite amount of time it is important to move the volume of air rapidly.

Compression Ratio

When the piston in a cylinder of an engine is forced downward during the intake stroke air enters through the intake valve.

The valve closes as the piston starts upward during the compression stroke.

If the remaining space, volume, in the piston is 1/9 the total volume at the top of the stroke (minimum volume) at a compression ratio of 8/1 x 14.7 = 117.6 psig plus heat.

If the turbo adds 2 psig of "boost" then the compression ratio of 8/1 x (14.7 +2) = 133.6 psig plus greater heat.

The higher internal pressure (like a high compression engine), additional oxygen from increased original volume and heat produce a greater power stroke.

Comment

In a similar fashion an automobile engine has less power when crossing the mountains.

The atmospheric pressure is reduce with altitude. At 5000 feet the ambient air pressure is approximately 12.2 psia. Then 8/1 x 12.2 = 97.6 psig. Approximately 83% of the internal pressure at sea level with the same engine. A turbo with 2.5 psig "boost" could increase the performance at 5,000 foot elevation to that at sea level.

Its all about the fuel/oxygen mixture.

More (denser) air in the cylinder means you have more oxygen so you can inject more fuel and thus get a more powerful explosion for every cycle. Thus you get more horsepower.

That is why all modern cars have a mass air sensor that tells the computer how much air (thus oxygen) is entering the engine (it varies with atmospheric pressure and how clean your air filter is). The computer then injects the correct amount of fuel for the best fuel/oxygen mixture.

By pressurising the intake air in an internal combustion motor you effectively increase the compression ratio; thereby increasing horse power and engine output.

------->INVADER !!

Compressing the air gets more oxygen into the cyclinder (previously stated) to allow for more fuel which leads to more combustion. But compressing the air heats it, making it slightly less dense - in an engine it usually isn't that big of an issue until you get the pressure up very high. On some engines they will cool the air after compression with what is usually called an intercooler - basically a heat-exchanger using air instead of water. That increases the density of the air and can help increase combustion.

Some large power plants do something similar - cool the compressed air before it gets into the gas turbine, usually with large masses of ice - it's a way to increase efficiency and hence output without doing anything to the turbine itself. I remember a power plant in North Carolina that was doing this because their contract with the larger bulk electric supplier that served them required ten years notice before adding capacity at the local plant - they figured they could wring out more efficiency from the turbine by cooling the air with ice and get more output without adding capacity - it didn't require notifying the bulk provider.

To keep it simple:

You're processing more air-fuel mixture for more power per cycle.

You're raising the compression ratio.

You're increasing the load on the cooling system.

By pre-pressurizing the air before it enters the piston chamber, it will increase the mass of air being supplied to the chamber so that when the air-fuel mixture is ignited, a greater mass of air is heated and expanded. This will result in a greater power output than if the engine is not turbocharged.