Thermodynamic equation for combustion in a spark ignition engine

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SUMMARY

The discussion focuses on the complexities of determining combustion pressure in spark ignition engines. Key variables include compression ratio, stroke, spark timing, burn delay, and fuel composition, among others. A rough estimation method suggests that peak combustion pressure can be approximated as 75% of maximum pressure occurring approximately 13 degrees before top dead center (aTDC). However, this method is highly imprecise and should only be used as a last resort.

PREREQUISITES
  • Understanding of spark ignition engine mechanics
  • Familiarity with combustion thermodynamics
  • Knowledge of engine performance variables such as compression ratio and air-fuel ratio
  • Basic principles of thermodynamic cycles
NEXT STEPS
  • Research the impact of compression ratio on combustion pressure in spark ignition engines
  • Study the effects of spark timing and burn delay on engine performance
  • Explore advanced combustion modeling techniques using software like GT-Power
  • Learn about historical methods for engine design, particularly those used in the 1930s
USEFUL FOR

Engineers, automotive designers, and students studying internal combustion engines who seek to understand combustion dynamics and pressure calculations in spark ignition engines.

chhitiz
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if i have calorific value of mixture, volume at time of compression, bore diameter, can i know the pressure being exerted on the pistons due to the combustion?
 
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No, there are lots more variables here, sorry. Think about compression ratio, stroke, spark timing, burn delay, end gas temperature, fuel composition, fuel temperature, fuel mixing ratio, engine speed, torque, piston bowl design, cylinder head design, thermal conductivity of the block, piston and head, air fuel ratio, expansion ratio...
 
Plug and chug baby. This is precisely the reason for practical tests.
Short of making lots of assumptions, there is no way to have an equation to show the pressure.

A VERY rough and ready method is to constuct an air standard cycle, and simply assume that the peak combustion pressure is 75% of maximum, that occurs approx 13 deg aTDC.

Disclaimer: I cannot stress just how 'pulled out of the backside' this method is. But it's good if you really are stuck for a better option. This is similar to a method they used in the 1930s to design piston engines aircraft (I can't remember the name of the book that the method was in)
 

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