C0RS4 C v6
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Registered: 5th Jun 07
Location: lincoln
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wat do the first 2 numbers mean on the compression ratio?? 
eg. 9:5:1
cheers
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RCS
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Registered: 26th Jan 05
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It's the ratio of the volume in the cylinder when the piston is at BDC and when the piston is at TDC.
2000cc engine:
Combustion Chamber Volume = Vc = 50cc
Volume at BDC = Vd = 550cc
CR = (Vd+Vc)/Vc = (550+50)/(50) = 12:1
[Edited on 07-06-2007 by RCS]
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Robin
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Registered: 7th Jan 04
Location: Northants Drives: Clio 182 Cup
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Would that not be a 2400cc engine (assuming it's 4 cylinders?)
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RCS
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Registered: 26th Jan 05
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Well spotted, worded it wrong.
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corsaboy_nl
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Registered: 1st Jun 07
Location: Netherlands
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this is the complete formule to calculate the compression ratio if you know the values. I don't know what the englisch translation is of boring x slag. damm
[Edited on 07-06-2007 by corsaboy_nl]
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Rob H
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Registered: 28th Oct 00
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quote:
Originally posted by corsaboy_nl
this is the complete formule to calculate the compression ratio if you know the values. I don't know what the englisch translation is of boring x slag. damm
Basically it's saying calculate the volume of the cylinder (b = bore, s = Stroke), add the Clearance volume (Vc), then divide by the clearance volume.
Using RCS's earlier example of a 4 cyl 2.0 engine, each piston would have a swept volume of 500cc, Then add the clearance volume, 50cc for example (i.e total volume at BDC = 550cc), then divide by the clearance volume again => CR = 11:1.
To go a bit further with detail, ideally you want as high a CR as possible, as it helps with the atomisation of fuel, hence making more power. The downside is more forces on the crank/con-rod due to the higher forces required to compresss the mixture, and also by increasing the pressure it increases the possibility of detonation, more prominent on low grade fuels (like used in the US/other markets where they may use 91 octane fuel or less!).
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RCS
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Registered: 26th Jan 05
Location: Lichfield/Dundee
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Compression ratio is mainly an indicator to engine performance. Higher CR means the engine can physically compress the mixture more. This is why the C16XE head performs better than the X16XE. The pressure in the combustion chamber will be higher in the C16XE when the piston is at TDC after the compression stroke than in the X16XE.
High compression ratios are desireable for racing engines but main issue with high CR is pre-ignition, where the mixture is highly compressed which in turn increases the temperature to the point that specific areas of the mixture ignite before the spark.
The increase in loading due to higher cylinder pressures isn't a worry, as these forces are much smaller than the inertia forces which arise.
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corsaboy_nl
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Registered: 1st Jun 07
Location: Netherlands
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yes but when your engine is turbo charged or supercharged your compression ratio needs to be low.. As low as possible when you want more boost.
An c20ne has the perfect compression ratio to turbo charge up to 0.9-1.0 bar boost.
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RCS
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Registered: 26th Jan 05
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Yes thats true, I was speaking generally for N/A engines. Forced induction cars have lower compression ratios because the air flowing into the cylinder has already been compressed, and to compress it more with a high compression ratio would then lead to pre-ignition.
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