Thermodynamic analysis using the otto cycle.
Discussion
Hi
I hope some ph members could help me with a question for a assignment I need to do?
We have to do a complete thermodynamic analysis of a 4 stroke single cylinder Tecumseh engine. I'm struggling to find out how to calculate the pressure within the combustion chamber ( is it the same as the mean effective pressure?).
Also struggling with combustion chamber temperature aswell.
I hope some ph members could help me with a question for a assignment I need to do?
We have to do a complete thermodynamic analysis of a 4 stroke single cylinder Tecumseh engine. I'm struggling to find out how to calculate the pressure within the combustion chamber ( is it the same as the mean effective pressure?).
Also struggling with combustion chamber temperature aswell.
is there a way to change the title?Hi there.
I assume you are talking about peak temperature and pressure?
Firstly you need to make some assumptions.
1. The working fluid is air that is at ambient tempertature.
2. The compression exhaust strokes are isentropic.
3. The heat addition is at constant voulume. (ideal otto cycle)
4. Air is an ideal gas
Peak temp and pressuere occure at point 3. The end of the heat addition process.
First step find temp and pressure after compression (2). This is done using the isentropic relations, and the compression ratio v1/v2
http://en.wikipedia.org/wiki/Isentropic_process
Then you find the the temp at 3 using Qdot=mdotcv(T3-T2) (you know T2 and you can work out heat if you know fuel flow rate (AFR=14.6 for petrol) and assume 100 percent efficient combustion and no heat transfer.
(or you can calculate a mass flow of air trating the engine as a pump. mdot=(rpm * volumetric efficiency * density * swept volume)/120. Mass of fuel is then mdot/14.6. You can then multiply mdot by the LCV of petrol and this is your Qdot value in kJ/s or kW
You can thern find out pressure by (P2V2/T2)=(P3V3/T3).
Obviously this will provide an approximation. I assume you are early on in your studies. There are more rigorous methods, but thi is a good starting point. Due to it not allowing for heat transfer etc you will get fairly high temps and pressures. You can get a reasonable answer with some fudge factors. I recomend you try a thermodynamics text book for basics and Heywoods for engine tech.
Hope that helps.
I assume you are talking about peak temperature and pressure?
Firstly you need to make some assumptions.
1. The working fluid is air that is at ambient tempertature.
2. The compression exhaust strokes are isentropic.
3. The heat addition is at constant voulume. (ideal otto cycle)
4. Air is an ideal gas
Peak temp and pressuere occure at point 3. The end of the heat addition process.
First step find temp and pressure after compression (2). This is done using the isentropic relations, and the compression ratio v1/v2
http://en.wikipedia.org/wiki/Isentropic_process
Then you find the the temp at 3 using Qdot=mdotcv(T3-T2) (you know T2 and you can work out heat if you know fuel flow rate (AFR=14.6 for petrol) and assume 100 percent efficient combustion and no heat transfer.
(or you can calculate a mass flow of air trating the engine as a pump. mdot=(rpm * volumetric efficiency * density * swept volume)/120. Mass of fuel is then mdot/14.6. You can then multiply mdot by the LCV of petrol and this is your Qdot value in kJ/s or kW
You can thern find out pressure by (P2V2/T2)=(P3V3/T3).
Obviously this will provide an approximation. I assume you are early on in your studies. There are more rigorous methods, but thi is a good starting point. Due to it not allowing for heat transfer etc you will get fairly high temps and pressures. You can get a reasonable answer with some fudge factors. I recomend you try a thermodynamics text book for basics and Heywoods for engine tech.
Hope that helps.
I suppose I needed some assurance that i was heading down the right path. Heywoods book is one I will have to get a copy of . Pulkrabeks Engineering fundamentals of IC engines is another I have had recommended to me.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff