Volumetric efficiency and its relationship to compression.
The amount of compression an engine has is expressed as a ratio. The ratio is found by how many times smaller the cylinder is when the piston is at the top of its stroke (Top Dead Center, TDC) compared to when its at the bottom of its stroke (Bottom Dead Center, BDC).
A cylinder displaces 100 cubic inches at BDC, and 10 cubic inches at TDC. The compresion ratio is 10:1.
100 / 10 = 10 so the compression ratio is 10:1
There are two types of compression static and dynamic.
Static compression is also known as mechanical compression. It is measured by calculations from a fixed volume, just as we've done.
Dynamic compression is measured by the volume of air and fuel that make it into the cylinder compared to the final volume of the cylinder at TDC.
Here is an example of dynamic compression: A cylinder displaces 100 cubic inches at BDC, has a volumetric efficiency of 50%, and the cylinder displaces 10 cubic inches at TDC.
(100 * 50%) / 10 = 5 so the compression ratio is 5:1
As you can see, volumetric efficiency plays a direct role in dynamic compression. But what role does compression play in an engines performance?
Gasoline in a liquid state is not flammable, but once it vaporizes and mixes with gas it can easily be ignited. By compressing a mixture of gasoline vapors and oxygen, this mixture becomes very explosive. The more this mixture is compressed, the more explosive it becomes. Below is a chart on how increasing an engines compression also increases the torque an engine puts out. On the left side is the original compression, and at the top is the new compression ratio. The numbers are in average percentage increases.
| 14:1 | 13:1 | 12:1 | 11:1 | 10:1 | 9:1 | 8:1 | |
| 14:1 | 0 | ||||||
| 13:1 | 1.0 | 0 | |||||
| 12:1 | 2.2 | 1.2 | 0 | ||||
| 11:1 | 3.5 | 2.4 | 1.3 | 0 | |||
| 10:1 | 5.0 | 3.9 | 2.8 | 1.5 | 0 | ||
| 9:1 | 6.7 | 5.6 | 4.5 | 3.2 | 1.7 | 0 | |
| 8:1 | 8.7 | 7.6 | 6.5 | 5.2 | 3.7 | 2 | 0 |
There are drawbacks too high compression. When things are getting compressed they become hot. If the compression is to high the air and fuel mixture can self-ignite, which can cause performnace losses, and even ruin an engine. Also by raising the compression more NOx are released which can be bad for the environment.
Volumetric efficiency has a direct link to dynamic compression. When volumetric efficiency is increased, not only does the engine get more fuel, but it also has a higher dynamic compression ratio which also increases power.
An engine's volumetric efficiency has is always changing depending on the rpm and the way the engine is designed. Most naturally aspirated stock engines have a volumetric efficiency of 80%. Some naturally aspirated race engines have volumetric efficiencys of over 100%!