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Is the Higher Engine Compression Ratio, the Better of Efficiency

As we all know, when we talking about the technical parameters of an engine, there are two points: one is the power and another factor is the torsion or static thrust. While there are many factors which may affect the power and torsion, among these factors, one of the important is engine compression ratio.

Engine compression ratio is an essential thing for the engine. It respects the extent of the engine gas mixture compressed. The ratio of the maximum piston volume to the minimum compressed volume is called the "compression ratio."

The higher compression ratio, the better power

Generally speaking, the higher compression ratio, the better of the power, and with sufficient combustion, of course the emission will be less. High compression ratio engine always requires high quality fuel oil.

In theory, large compression ratio, the higher pressure and temperature at the end of compression, mixture combustion rate will get faster, and the combustion efficiency of engine will get better, but the engine's compression ratio cannot be too high, because for gasoline engine, too high compression ratio will produce deflagration; while for diesel engine, the higher compression ratio, the bigger of the mechanical load, so we should not turn a too high compression ratio. For gasoline engine, the compression ratio between 9 and 11, while diesel engine's compression ratio between 14 and 22 are the best.

The higher compression ratio, the higher pressure of the engine cylinder will produce. Then what's the relationship of compression ratio and overall pressure ratio? (Read more at http://en.wikipedia.org/wiki/Compression_ratio)

Compression ratio 2:1 3:1 5:1 3:1 15:1 20:1 25:1 35:1
Pressure ratio 2.64:1 4.66:1 9.52:1 25.12:1 44.31:1 66.29:1 90.60:1 145.11:1

Compression ratio is defined via the volume reduction CR=V1/V2 while pressure ratio is defined as the pressure increase PR=P2/P1.

In the case of natural air intake and under the same displacement, the higher will produce more power and torque. General gasoline engine compression ratio is range within 8 to 12. But the compression ratio is too high; it is easier to make the engine to knock. At the same time, different compression ratio needs to use a different grade of gasoline. The higher compression ratio, the higher gasoline label, which is due to the higher grade of gasoline, the higher of the octane number, that you can avoid detonation.

Take the AGM-30cc airplane engines for example. You are required to use fuel of 2C regular gas mixed with 30:1 to run the engine work better. And the compression ratio of 7.6:1, even though the ratio is a little lower than the standard level 8, operating this engine is enough with this ratio. Since we should make sure that our engine works stable and smooth, if we make the compression ratio to a higher level, then the engine may be worked to overpower, and you cannot control it at that time.

So the question of whether the higher compression ratio, the better of efficiency is a relative conclusion not absolute. Recommend reading How to Choose the Best Gasoline Engine?

What do you think about it? Share your opinions to AGM Engine team at jimmy@agm-engine.com.