Strange indication of thermal efficiency
 

Hello everyone,
I am currently doing combustion simulation of a natural gas four-stroke engine. I found that when the ignition time is advanced, the crankshaft angle corresponding to the peak cylinder pressure will be advanced, and the cumulative heat release before TDC will be more and more. This means that most of the energy is consumed before TDC for work, and it should be mostly negative work. . According to theoretical knowledge, the indicated thermal efficiency of this working condition should be very low. But when I did the P-V diagram integration, I found that the integrated area was not small, resulting in a very high indicated thermal efficiency, which could reach 36%. May I ask why this is? Is there something wrong with my settings?

Hi!
The work done will be more since the expansion stroke will have more pressure from TDC. Also combustion will most likely be faster when the ignition timing is advanced, as you build up more pressure (and thus temperature) early on.

The ignition time at this time is 40°CA BTDC. Is it a bit abnormal if the efficiency is so high?