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1. Description of an Internal Combustion Engine

There are many different types of Internal Combustion Engines.  Essentially all of them work the same way.  A mixture of air and fuel is sucked into the engine, where it is compressed.  The mixture of air and fuel is then ignited.  The burning gasses expand, and then they are expelled from the engine.

Click here for an explanation of the four cycles.

2. Ratings of Power

The amount of work an engine exerts is measured in foot * pounds of torque.  The amount of power that an engine can do is measured in horsepower or watts.

1 horsepower = (550 FT * LB) / Sec = 746 Watts = 2,545 BTU / Hour

To convert torque into horsepower: (Torque * RPM) / 5,252

*NOTE* Horsepower will always equal torque at 5,252 rpm, torque will always be greater than horsepower under 5,252 rpm, and horsepower will always be greater than torque over 5,252 rpm.

Click here for an explanation of the difference of torque and horsepower.

3. Ratings of Efficiency

There are many different ways to find the efficiency of an engine, and many different parts of an engine that you can rate the efficiency.

Thermal efficiency is the percentage of energy taken from the combustion which is actually converted to mechanical work.  In a typical low compression engine, the thermal efficiency is only about 26%.  In a highly modified engine, such as a race engine, the thermal efficiency is about 34%. 

Mechanical efficiency is the percentage of energy that the engine puts out after subtracting mechanical losses such as friction, compared to what the engine would put out with no power loss.  Most engines are about 94% mechanically efficient.

This means that for a stock engine, only 20% of the power in fuel combustion is effective.

Click here for a diagram of power losses.

Volumetric efficiency is the relationship between how much air and fuel has been taken into the cylinder, compared to how much air and fuel is required to fill the cylinder under normal atmospheric pressure.  Volumetric efficiency is expressed in a percentage.

Click here for more information on volumetric efficiency and compression.

Another way to rate an engines performance is its horsepower to cubic inch ratio.

An engine has 351 cubic inches and 330 horsepower.

330 / 351 = .94 so its horsepower to cubic inch ratio is .94:1

4. Hopes for a more efficient engine.

Engines aren't very efficient.  Just think only 20% of the power produced by the burning fuel is useful, and even less than that ever makes it to the wheels of your car.  Luckily, people are always working on ways to improve the performance of engines.  The last few years have really opened the door for more efficient engines with the improvement of computers.  Computers now control almost every aspect of how new car engines run.  The one thing that they do not control on your engine, that would have the most improving effect, is the valve timing.  The cam is the brains of an engine.  It tells the engine when to fire its spark (not all cases), when and how long to breathe in and out.  Since a cam can't change when and how long to do these things, an engine is setup to run most efficiently at one speed.  Every other speed the engine has no chance to run as efficiently.  There are companies that are now trying to make engines with variable valve timing.  There are a couple ways this will work, but the best is with solenoids that are computer controlled.  The computer analyzes information and decides how to control the valve events.  This gives an engine almost equal chances to run as efficiently as possible at all speeds!  Some companies already sell engines with variable valve timing, but the ones on the market have limits to how far they can change the valve timing events.


Fun Facts

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