CrazyEngineers Archive
Old, but evergreen and popular discussions on CrazyEngineers, presented to you in read-only mode.
@balamurali.a • 03 Sep, 2009
What is the max thermal efficiency of a CI & SI engine, Why CI engine is having more thermal efficiency ?
@nany_mech • 03 Sep, 2009 Thermal efficiency of any IC engine is the ratio of " what you get to what you pay for "

that is its the ratio of Work output to Heat input.

Thermal efficiency states how well we are utilizing the energy stored in fuel.

When we compare SI & CI engines , due to large compression ratio or more chance for better expansion CI engines will have higher Thermal efficiencies when compared with SI engines.
@balamurali.a • 03 Sep, 2009 thx nany,

what is the max thermal effy for a Diesel engine (Idle conditions)

what is the max thermal effy for a Petrol engine (Idle conditions)
@nany_mech • 03 Sep, 2009 The maximum thermal efficiency of a diesel cycle is dependent on the compression ratio and the cut-off ratio.

for example , for a CI engine working at compression ratio 25 :1 and for diesel No.4 flame temperature being 527K and air inlet temperature being 329 K , efficiency is 44.12 % and for compression ratio 14:1 the efficiency is 55.7395 %

The additional complexity for the diesel formula comes around since the heat addition is at constant pressure and the heat rejection is at constant volume.

The Petrol engine(Otto) cycle by comparison has both the heat addition and rejection at constant volume. The efficiency of petrol engine is dependent on compression ratio alone.

For example for a petrol engine having compression ratio 9 and compression index 1.4 (air)
it is approx 58.47% and for compression ratio of 12 (max) it is 62.98%
@balamurali.a • 03 Sep, 2009 one confusion, you mentioned for CI 25:1 Effcy is 44.12 and for CI 14:1 Effcy is 55.7, why for a high CI, effcy is less ?
@nany_mech • 04 Sep, 2009 Sorry for that wring math !..I was wrong then , but now I got the actual values !

for example , for a CI engine working at
compression ratio 25 :1 and for diesel No.4 flame temperature being 527K and air inlet temperature being 329 K , efficiency is 75.73% and

for compression ratio 14:1 the efficiency is 68.62% and

its true that with increase in compression ratio in CI engines, air standard thermal efficiency (Idle conds.) will increase.
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