Thermodynamic equation for combustion in a spark ignition engine

In summary, the conversation discusses the possibility of determining the pressure on the pistons due to combustion based on the calorific value of a mixture, volume at the time of compression, and bore diameter. However, there are numerous other variables that need to be taken into account such as compression ratio, stroke, spark timing, and fuel composition. Without making assumptions or conducting practical tests, there is no equation to accurately determine the pressure. A rough method is to construct an air standard cycle and assume that the peak combustion pressure is 75% of maximum, occurring at 13 degrees aTDC. However, this method is not reliable and was used in the past for designing piston engines in aircraft.
  • #1
chhitiz
221
0
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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  • #2
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...
 
  • #3
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)
 

What is the thermodynamic equation for combustion in a spark ignition engine?

The thermodynamic equation for combustion in a spark ignition engine is the ideal gas law, which states that the pressure (P), volume (V), and temperature (T) of a gas are related by the equation PV = nRT, where n is the number of moles of gas and R is the gas constant.

How is the thermodynamic equation used in spark ignition engines?

The thermodynamic equation is used in spark ignition engines to calculate the amount of work done by the expanding gases during the power stroke. This work is then converted into mechanical energy to power the engine.

What factors can affect the thermodynamic equation in a spark ignition engine?

The thermodynamic equation in a spark ignition engine can be affected by factors such as the air-fuel ratio, engine speed, and spark timing. These factors can change the pressure, volume, and temperature of the gas, thus altering the equation.

Why is the thermodynamic equation important in spark ignition engines?

The thermodynamic equation is important in spark ignition engines because it helps engineers optimize the engine's performance and efficiency. By understanding the relationship between pressure, volume, and temperature, they can make adjustments to improve the engine's operation.

What are some limitations of the thermodynamic equation in spark ignition engines?

Some limitations of the thermodynamic equation in spark ignition engines include the assumption of ideal gas behavior, neglecting the effects of heat transfer and friction, and not accounting for the chemical reactions that occur during combustion. These limitations can affect the accuracy of the equation in real-world engine conditions.

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