Compression Ratio???

[R2D2]

My favourite subject M271 engines are all the same but the "1.8" has a compression ratio of 10.2:1, the "2.0" has a compression ration of 9.5:1 and the "2.3" has a ratio of 8.7:1.

If they all have a capacity of 1796cc and all have a bore of 82mm and a stroke of 85mm then how is the compression ratio different?

Go slowly with me!

 

[grober]

CR different

Different combustion chamber volume or dish/recess in piston crown. Capacity usually means theoretical swept volume ( BY THE PISTON) bore X stroke. I think??

 

[R2D2]

Thanks Grober. Just thinking why is a lower compression ratio equal to a bigger output then?

 

[Dieselman]

Each engine is 'blown' by the supercharger, but typically if you want to blow harder for greater output you need to lower the compression ratio to stop pre ignition.

 

[R2D2]

Ah. So changing the CR isn't the answer in itself.

 

[Stratman]

To elaborate a bit, the compression ratio is the swept volume divided by the unswept volume.

If you take two engines, identical apart from the conrod length, they will have the same capacity (swept volume) but the engine with the shorter conrods will have a lower compression ratio as it will leave a larger unswept volume at TDC.