Compression Ratio:
The compression ratio significantly affects engine performance. The higher it is, the higher the final compression pressure will be, thus more energy can be extracted from the fuel, resulting in more power. However, there are limitations to this. More on this later.
The compression ratio is a fixed ratio between the volume above the piston at BDC and the volume above the piston at TDC. The compression space is the volume of the combustion chamber where the air is compressed by the piston. The compression space also depends on the thickness of the head gasket, the angle at which the valves are installed in the cylinder head, and the space taken up by the spark plug and injector. This makes it difficult to calculate. Therefore, the compression space is measured by pouring a certain amount of liquid into the closed-valve head and measuring this amount.
Calculating the Compression Ratio:
The displacement is noted as Vs and the compression volume as Vc. Both are in cubic centimeters (cm9). With these two data points, the compression ratio can be calculated. The compression ratio is represented by the Greek letter 35 (Epsilon).
The engine data are as follows:
Vs = 460 cm9
Vc = 50 cm9
Filled in, this gives:
Calculating this formula gives the answer 10. This means that this engine has a compression ratio of 10:1. In the image below, the ratio between the volume above the piston at BDC (10) and the volume above the piston at TDC (1) is clearly visible. The displacement + compression space are 10 times larger than the compression space.
An indirect injection gasoline engine often has a compression ratio between 7:1 and 11:1. A direct injection gasoline engine between 14:1 and 20:1.
The compression ratio of a diesel engine is often between 18:1 and 24:1.
As previously explained, the compression volume (Vc) is measured because calculating it is very difficult. The displacement can be calculated, however. The formula is shown on the right side.
0 (pi) = approximately 3.14
d9 = the diameter of the cylinder squared
s = the stroke in millimeters
In the standard formula of the compression ratio, Vs is replaced by the formula with which Vs is calculated. The brackets on the left and right of the Vs formula indicate that this calculation must be done first. The result of this must be added to Vc and then divided by Vc.
As an example, we take an engine with a bore x stroke of 81.0 x 86.4 mm. The bore is the diameter of the cylinder which must be squared, and the stroke is the distance the piston travels from BDC to TDC. The compression volume of this engine is given: 45 cm9.
Of course, the formula can also be executed in steps. Filling in this data in the formula gives:
The displacement of this engine can now be filled into the formula for the compression ratio:
Limitations:
With a higher compression ratio, more power can be achieved. More energy can be extracted from the fuel (higher efficiency). The compression ratio cannot just be increased; due to the higher final compression pressure, there is a risk of knocking. The fuel ignites due to the higher pressure and temperature earlier than intended. An engine equipped with a turbo has a higher final compression pressure due to boost. With the same compression ratio, this would mean there is a risk of knocking. Therefore, the compression ratio in turbo engines is lower than in naturally aspirated engines.
Car manufacturers also apply techniques to increase the compression ratio without the risk of knocking. Consider a knock sensor to advance the ignition (now present on every engine), water injection to cool the combustion chamber, and different fuels such as methanol and ethanol (used in racing).
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