# compression ratio in CI engines

Discussion in 'Mechanical | Automobile | Aeronautics' started by Rohith Reddy, Oct 14, 2011.

1. ### Rohith Reddy Newbie

Engineering Discipline:
Electronics & Communications
Why is the compression ratio in CI engines high??????

#1
2. ### Issue Knight

Engineering Discipline:
Mechanical
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.

#2
3. ### Rohith Reddy Newbie

Engineering Discipline:
Electronics & Communications

#3

Engineering Discipline:
Mechanical
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..

#4
5. ### Issue Knight

Engineering Discipline:
Mechanical
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.

#5