Thermal Eff. of engine question

[Silverbullet86]

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?

 

[Silverbullet86]

in addition ...

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?

 

[Bribyk]

It probably has more to do with throttling losses than anything else. By raising the intake temp you have to open the throttle more to get the same amount of power which means the engine has less pumping losses (like a diesel only you're controlling the amount of air injected, instead of fuel).

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).

 

[patprimmer]

Two practical limits not apparently considered.

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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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[j2bprometheus]

All-

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.

 

[JSteve2]

Raising intake temp raises exhaust temp almost 1-1 (say 0.8 through a "normal" range of operating conditions). So in your example, (11-6)/11 is really about (11-6.8)/11 or 38%. Raising intake temperature reduces thermal efficiency, so all other things being equal this would reduce fuel economy.

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.

 

[black2003cobra]

SilverBullet - For the ideal Otto cycle, thermal-conversion efficiency, (which is defined as ?t = Wc/Qin), is independent of initial cylinder temp (T1), and works out to 1 - rc^(1-k), where rc is the compression ratio and k = cp/cv. Even with a model that includes heat loss, burn duration, spark timing, etc., I find that ?t is relatively independent of T1.

 

[black2003cobra]

I neglected to add a comment about VE scaling with intake temperatures in my previous post. It follows directly from the expression for steady-state compressible flow through a constriction that the mass-air flow rate will scale as Po/?To, where Po and To are the upstream stagnation pressure and temperature, respectively. Therefore, since volumetric efficiency is proportional to the mass-air flow rate divided by upstream air density, (? = Po/(Rs*To) ), the volumetric efficiency will scale as ?To. Hence, VE actually goes up as the upstream stagnation temperature, To, increases. (Perhaps some were thinking about the correction factor for VE, which would go as ?(Tref/To), where Tref is the reference temperature.)

 

[tomt2]

I have some experience with this. This mod works well with Saturn S Series engines (which I have). I read about a few guys that documented real fuel economy improvements when pulling hot air from there exhaust manifold. The improvement for the well documented case was about 6 mpg. The air temp he was trying to achieve was around 170 deg F. Anymore than that and engine knock was the result which caused spark retard due to the knock sensor. The max intake temp the engine will allow is 247 deg F. After that the check engine light comes on. The key to make this work is finding the ideal intake temp for your engine so that you don't get any engine knock. One Honda owner found that 100 deg F was the ideal temp for his engine. Another thing he tried was adding a resistor (150 ohm) in place of the intake air temp sensor. This fooled the engine into thinking the air temp was hotter than it actually was (about 220 deg F). He got another 2 mpg improvement with the resistor.
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.

 

[Lou Scannon]

Can you distinguish results that were purely due to physics of hotter air & less pumping loss versus moving the engine control to a more efficient calibration based on sensed MAT?

 

[Rob45]

Higher intake air temperatures also typically result in somewhat lower heat loss to the cooling system, a factor many overlook. I think you'd be better off increasing the thermostat temperature if fuel economy is what you're looking for.

 

[patprimmer]

How does higher charge temperature reduce heat losses to cooling system?

I would think higher intake temperature and concequential charge temperature would increase heat transfer due to higher temperature difference.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tomt2]

Hemi: No, I can't distinguish between hotter air and less pumping losses verses ECU adjustments due to hotter air. My guess is they are both pretty small (especially the reduction in pumping losses) but together they add up to a significant savings in fuel. The main reason for this, though, is to get the ECU to lean out the air/fuel ratio so you can see an improvement in fuel economy. The great thing is that it is adding benefit anytime the car is warmed up, not just during acceleration.

 

[black2003cobra]

Less heat loss with higher inlet air temps is clearly not obvious. The heat-transfer coefficient (Woschni) goes down as cylinder temps goes up (and pressure goes down*). So even though the difference in temperature between the gas and cylinder wall goes up with a higher inlet temp, the net result is a lower heat flux. Radiative heat loss would still go up, but that's smaller than the convection component.

*Cylinder pressure goes down because Qin actually goes down with higher inlet air temps...(not up as mentioned above). Recall that Qin = ?c*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.