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compression ratio in CI engines

Question asked by Rohith Reddy in #Coffee Room on Oct 14, 2011
Rohith Reddy
Rohith Reddy · Oct 14, 2011
Rank E1 - BEGINNER
Why is the compression ratio in CI engines high?????? Posted in: #Coffee Room
ISHAN TOPRE
ISHAN TOPRE · Oct 14, 2011
Rank A2 - PRO
Compression ratio is the ratio of the volumes which the engines can afford to compress.
It depends on various factors. It depends on metallurgical considerations while most important parameter in this context is the engine knocking.
For CI engines, it is called Detonation. A CI engine operates on a diesel cycle. While an SI engine works on Otto cycle.
In a diesel cycle (referring to air standard cycle) heat addition occurs at constant volume. While in an Otto cycle, the heat addition occurs at constant pressure (again imagine an air standard cycle).

Now for the same compression ratio, the work produced in Diesel cycle is less as compared to that obtained from SI engine.

Hence, the compression ratio of a CI engine is usually kept high, around 22 or so, while it is around 9 for SI engines.
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Rohith Reddy
Rohith Reddy · Oct 14, 2011
Rank E1 - BEGINNER
thanx for your answer
Siddhant Mukherjee
Siddhant Mukherjee · Oct 15, 2011
Rank D1 - MASTER
QUOTE::"In a diesel cycle (referring to air standard cycle) heat addition occurs at constant volume. While in an Otto cycle, the heat addition occurs at constant pressure (again imagine an air standard cycle)"::

Reply:: This is wrong and the vice versa is true

The reason behind keeping compression ratio high for any engine is to get higher mean effective pressure thus larger power output.This is not possible in case of petrol engines due to the problem of knocking at higher compression ratios so comp. ratio is decreased and spark plug is used for fuel combustion. Whereas diesel engines do not suffer such problems and for getting higher power output compression ratio is raised and fuel is self ignited leading to removal of spark plug..
ISHAN TOPRE
ISHAN TOPRE · Oct 15, 2011
Rank A2 - PRO
S.mukherjee
QUOTE::"In a diesel cycle (referring to air standard cycle) heat addition occurs at constant volume. While in an Otto cycle, the heat addition occurs at constant pressure (again imagine an air standard cycle)"::
Friend, please refer to the diesel cycle and Otto cycle diagram for once. In the diagram above I have tried to plot both cycles in a same graph.
I have also mentioned about engine knocking and detonation.

Issue
Compression ratio is the ratio of the volumes which the engines can afford to compress.
It depends on various factors. It depends on metallurgical considerations while most important parameter in this context is the engine knocking.
For CI engines, it is called Detonation.

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