The four stroke engine is the backbone of Ford Mustangs, Chevy Camaros, Corvettes, and nearly all race cars, hot rods, roadsters and custom cars. Yet the basic scientific concepts of volumetric efficiency, thermal efficiency and mechanical efficiency and how they relate to engine performance and engine performance parts is not well known.
It is pretty common knowledge among "car guys" that the pistons move up and down in the cylinders and that the valves open and close to create cycles. The four strokes (or cycles) that the engine uses are also common knowledge to car guys. This page provides you with the fundamentals of engine science and will take some of the mystery out of making more horsepower with a four stroke engine and get the most out of your performance engine parts.
An engine is just a machine that converts fuel energy to a rotating motion, which is commonly measured as horsepower. In the engine, gasoline is added to the air as it passes through the carburetor (or fuel injectors) on its way to the cylinder. This mixture is then burned in the cylinder, creating heat which creates pressure. This pressure pushes the piston down in the cylinder to turn the crank shaft.
Now, to the science part. Fuel has a chemical energy. A typical pound of gasoline contains 19,000 to 20,000 Btu. Just like 12 inches equals one foot, it is given that 2,545 Btu per hour = 1 horsepower. So, the power output is directly related to how much fuel the engine can effectively burn. Now, don't run out and put bigger jets in you carb trying to make more power. If the fuel/air mixture is too rich it won't burn properly and you will make less power.
A better way to think about this relationship is that a larger engine would pull more air and fuel into the cylinders; therefore, a larger engine would make more power. Also, super charging or turbo charging packs more fuel and air into the cylinders, so this would also make more power. The trick is relating this science to performance engine parts.
Imagine that you have a 100 cubic inch single cylinder engine. On the intake stroke, the piston moves to the bottom of the cylinder and creates a volume that is 100 cubic inches. The fuel/air mixture that fills this volume is what will be used to create the power.
Now imagine that this engine has some type of restricted intake, such as a small carburetor or restrictor plate like they use in NASCAR. With this configuration, the intake manifold has a fairly good vacuum. In this case, even though the piston pulls a volume of 100 cubic inches into the cylinder, it is not atmospheric air. Here, you have the 100 cubic inches of the vacuum from the manifold. You can think of this as the opposite of super charging because the cylinder ended up with less fuel and air molecules rather than having more fuel/air packed into the same volume.
Volumetric efficiency (VE) is used to describe the amount of fuel/air in the cylinder in relation to regular atmospheric air. If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency. On the other hand, super chargers and turbo chargers increase the pressure entering the cylinder, giving the engine a volumetric efficiency greater than 100%. However, if the cylinder is pulling in a vacuum, then the engine has less than 100% volumetric efficiency. Normally aspirated engines typically run anywhere between 80% and 100% VE. So now, when you read that a certain manifold and cam combination tested out to have a 95% VE, you will know that the higher the number, the more power the engine can produce.
Basically, volumetric efficiency is effected by your carb, intake manifold, headers, and cam specs. All of these items effect how much fuel/air will flow into the cylinder. But remember, the more fuel/air that gets into the cylinder, the more power the engine will produce. This is where software programs such as Engine Analyzer, and Engine Analyzer Pro can be a big advantage. All of these programs calculate volumetric efficiency for different engine configurations that you enter into the software. This allows you to do your own testing without having to buy the parts until you get the right combination.
Getting more fuel/air into the cylinder means more fuel energy is available to make power. Unfortunately, not all of the fuel's energy gets converted to rotating energy. Of the 19,000 to 20,000 Btu per pound of gasoline, typically less than 1/3 ends up as useable horsepower.
Compression ratio, ignition timing, thermal coatings, plug location and chamber design all affect the thermal efficiency (TE). Low compression street engines (smog motors) may have a TE of approximately 0.26. A racing engine may have a TE of approximately 0.34. Because these numbers are relatively small, at first glance there may not seem to be much difference between the street engine and the racing engine. However, when you grind out the math (.34 - .26 / .26), the racing engine produces about 30% more power because of the higher TE.
Finding little improvements to the TE can end up as a significant improvement to the final horsepower that the engine produces. Using the Engine Analyzer Pro software, you can try different compression ratios, coatings, etc. on your computer and see what happens to the thermal efficiency.
Volumetric efficiency identifies how much fuel/air gets into the cylinder and thermal efficiency identifies how much of the fuel gets converted to useable power, but some of this power is robbed by the engine's moving parts. It takes power to overcome the friction between parts and to run engine accessories such as the water pump.
So, depending on how much fuel gets into the cylinder and depending on how much of the fuel gets converted to workable power, some of this power is used by the engine to run itself. The left over power is what you would measure on a dynamometer. The difference between what you would measure on the dyno and the workable power in the cylinder is the mechanical efficiency (ME).
Mechanical efficiency is effected by rocker friction, bearing friction, piston skirt area, and other moving parts, but it is also dependent on the engine's RPM. The greater the RPM, the more power it takes to turn the engine. This means the faster the engine runs the more the ME drops. Often times the ME is expressed in terms of friction horsepower. Or in other words, how much horsepower is needed just to overcome the friction. This is where computer programs like Engine Analyzer and Engine Analyzer Pro are fun to use because you can try different engine combinations in the software and see how much friction HP the combination creates.
However, if you just want to test different engine performance parts to see how much power you can make you might like the Engine Shop or the Mustang Performance Builder
More tech info
Auto-ware main page