Thermal Eff. of engine question
Thermal Eff. of engine question
(OP)
i actively read several car forums and i came across a thread that linked to this thread http://www.gassavers.org/showthread.php?t=244 where this guy raised his intake temp and got better MPG with success, along with others who have tried.
That got me thinking. In i recall from Thermal Apps class that Eta[thermal eff.]=(Qin-Qout)/Qin for an IDEAL otto cycle.
I played around with some numbers: Qin=10, Qout=6. (10-6)/10 = 40%. Now if intake temp was raised, Qin' say 11, came out (11-6)/11 = 45%. 45% > 40%... everything makes sense.
Then i thought, if the engine was sucking in hotter air, wouldn't the temp for ALL of points in the P,V diagram have a higher temp, resulting in the efficiency staying the same?
Or do i have it all confused and can't use the Standard Otto Cycle for real engines and making a class that i took to be completely impractical and useless?
That got me thinking. In i recall from Thermal Apps class that Eta[thermal eff.]=(Qin-Qout)/Qin for an IDEAL otto cycle.
I played around with some numbers: Qin=10, Qout=6. (10-6)/10 = 40%. Now if intake temp was raised, Qin' say 11, came out (11-6)/11 = 45%. 45% > 40%... everything makes sense.
Then i thought, if the engine was sucking in hotter air, wouldn't the temp for ALL of points in the P,V diagram have a higher temp, resulting in the efficiency staying the same?
Or do i have it all confused and can't use the Standard Otto Cycle for real engines and making a class that i took to be completely impractical and useless?





RE: Thermal Eff. of engine question
in my little Thermal Eff equation, the work increased with the higher Qin'; (11-6) VS (10-6); which makes no sense in real life. There wouldn't be more power with hotter intake air.
Someone who has worked with heat engine engineering wanna clear up this mess?
RE: Thermal Eff. of engine question
Also, the higher intake temp does work out thermodynamically but the thermo equations assume the same amount of heat input to the air. This doesn't occur in reality because you can only add an amount of heat proportional to the amount of air (your air-fuel ratio).
RE: Thermal Eff. of engine question
If you raise inlet temp, you get lower VE at WOT, so you use less fuel because you can't make as much power.
If you raise the inlet temp then heat the charge by the same amount as you open throttle more to maintain VE, you reach a pont where peak temperature will damage the engine, so you can't go higher. This reduces the amount of heat you can add when yoi increase inlet temp.
Regards
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RE: Thermal Eff. of engine question
I agree that raising the intake temperature requires the throttle to be further open to get the same power and thus reduces pumping losses. This improves the PMEP (pumping mean effective pressure) and thus improves BSFC (brake specific fuel consumption).
Raising the intake temperature makes knock (autoignition) more likely. Knock can damage the engine.
The electronic controls of the engine might have some logic that retards the spark timing if the intake temperature is hotter and thus minimize the chance of knock. But retarding the spark timing would also make fuel consumption a bit worse.
So the trade-off would probably be this:
Better fuel economy but with knock more likely.
Also, the ECM (engine control module) probably has some logic that takes some protective action if intake temperature gets too high.
A diesel would NOT get better fuel economy if the intake temperature was increased since there is no throttle.
RE: Thermal Eff. of engine question
All other things are not equal. For example, as you get to low enough temperatures secondary effects on the combustion itself can reduce the efficiency. So, at low intake temperatures, increasing intake temperatures helps, but not for reasons related to raw thermal efficiency. At higher intake temperatures increasing intake temperatures reduces thermal efficiency.
RE: Thermal Eff. of engine question
RE: Thermal Eff. of engine question
RE: Thermal Eff. of engine question
I have been experimenting with a similiar setup, but it's too soon to tell what the results are. I need to get some instrumentation to see what is going on.
RE: Thermal Eff. of engine question
RE: Thermal Eff. of engine question
RE: Thermal Eff. of engine question
I would think higher intake temperature and concequential charge temperature would increase heat transfer due to higher temperature difference.
Regards
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RE: Thermal Eff. of engine question
RE: Thermal Eff. of engine question
*Cylinder pressure goes down because Qin actually goes down with higher inlet air temps...(not up as mentioned above). Recall that Qin = ηc*mf*Qhv, and of course ma, and hence mf, goes down as inlet temps go up for a given AFR.
From my model, I find thermal-conversion efficiency actually goes down a hair with higher inlet temps, so my guess would be any mpg improvement might come from the reduced pumping loss and/or leaner AFR, as others mentioned. One does wonder about the impact of better fuel vaporization.