Engine Compression Ratio

August 30, 2012 07:39 by yslim

Compression ratios are affected by the volume of the cylinder head's combustion chamber, the piston top design (domed, dished, flat), the head gasket thickness and the deck height of the block (affects the position of the piston in the bore at TDC).

The higher the compression ratio, the more closely packed the molecules of fuel and air are when the mixture is ignited by the sparkplug, this causes a more powerful explosion by making a more violent reaction  which produces more power.

Increasing compression ratio also increases an engine's volumetric efficiency.  Volumetric efficiency or VE for short is the measurement of how well the engine can move the air fuel mixture into and out of the engine's cylinders.

raising an engine's compression means its octane requirements will increase.  Higher octane is needed if the compression ratio is increased significantly in order to avoid detonation.

A very rough approximate is that milling the head 0.50 mm will increase the compression ratio around 1/2 of a point.  1 mm can increase the compression by around a point.  There are limits to this however.

Compression 11.5:1 is about the threshold of the maximum safe compression ratio that can be run on premium 91 octane pump gas even with modern engine controls

Turbo and supercharged engines usually have lower compression ratios, in the range of 7-9.5:1.  The compression ratio is lower on these engines because the turbo or supercharger packs lots and lots of fuel air mixture into the cylinders.  This raises cylinder pressure considerably and thus the compression ratio must be lower to avoid excess cylinder pressure and detonation.

The perfect balanced air to fuel ratio by mass is 14.7 to 1.

Naturally aspirated engines usually produce the best power at around 13:1 at wide open throttle although 12.5:1 is a bit safer.

leaner air/fuel ratios burn hotter.  If the mixture is too lean and the combustion temps increase, the engine can start to overheat and detonate.

Rich mixtures burn cooler and can help prevent detonation in these high pressure engines. 

Mixture ratios richer than 10:1 can contribute to engine damaging detonation because the excess fuel begins to take up too much room in the combustion chamber and increases cylinder pressure to the point where it can detonate or auto ignite.

Source: http://www.motoiq.com/magazine_articles/id/1631/suck-squish-bang-blow-part-3-compression-ratio-and-air-fuel-ratio.aspx


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