Intercooling versus exhaust gas temperature

[thirdworld]

hello fellas,
I am reading a book by Hugh Macinnes called "turbochargers". In this book he emphasis dropping intake air temp by any degree should also drop the EGTs by that amount. can someone substantiate this with formula, explanation, experience? I am baffled!?!? I am currently working on a small 4 cylinder and will like to understand this.
thanks.

 

[kakarot]

hmm, I will try.
when you cool the intake air, you in other words increase the amount of air (density). this is one advantage of intercooler, more air=more fuel=more power.
but another advantage is that when you lower your intake air you also prevent ditanation.
if you have ditanation, it heats up very high (bad). thously increasing the exaust temp. but if there is non, then the exaust temp would be lower.
in other words, you lower the intake temp, you lower the exaust temp. having too low exasut temp will reduce the output of the turbo.

instal a temperature gauge so that you could monitor it easily.

best thing is to have maximum temp the turbo alows.

PS what is the ISDN number of these book?

 

[thirdworld]

thanks I am still miffed. if you decrease IATs shouldn't hat mean more potential for power, which means more heat, which also means high EGTs?

verbatum...."As a rule of thumb, one degree decrease in intake manifold temp will give a one degree drop in exhaust temp"

the ISBN is 0-89586-135-6

 

[crysta1c1ear]

I would guess that his logic is along the lines of it's the fuel in the engine that is providing the power, and crushing air on the compression stroke is just acting like a spring and giving the compression back on the power stroke, plus what you get from the petrol; if the air is initially hotter, it is harder to compress but gives more power back on the power stroke, the two roughly cancelling out.

Temperature is a measure of the kinetic energy per molecule. What he seems to be saying is put a bit more heat in and you get the same bit more out, the fuel providing a heat difference ideally between compression and power strokes, and that difference being what really runs the engine.

It seems to be a very simple view of things, as a warmer engine should cause more heat to be conducted away through the cylinder walls. However, with warmer (less dense) air, you couldn't burn as much fuel and so there would be a smaller quantity of the hotter gases actually in contact with the cylinder walls.

Balancing conflicting factors like this can be tricky and call for a rule of thumb, eg what you gain on the swings, you loose on the rounabouts. I'd guess that his rule of thumb is what he has observed in practice and grossly rounded.

thanks I am still miffed. if you decrease IATs shouldn't that mean more potential for power, which means more heat, which also means high EGTs?

more heat, which also means high EGTs ....

No, let's say heat is total energy and say temperature is (average kinetic) energy per molecule. Just because you have more heat because you have more molecules doesn't mean that any particular molecules are moving faster.

 

[mattsooty]

Detonation per se does not cause high exhasut temps, in actual fact it will cause the reverse and temps will reduce.

This can be explained by more of the work released from the charge being expended onto the piston crown - very similar infact to advancing the spark timing. This is simply because the centroid of combustion is advancing as the combustion event takes less time.

The result is overpressure in the combustion chamber and mechanical overload on the piston/cranktrain.

MS

 

[Rob45]

One possibility is that increasing the intake charge density will allow you to get more work out of the turbocharger.
The turbo is driven by exhaust enthalpy, and an increase in turbo work, that is, an increase in the energy removed from the exhaust by the turbo, reduces the exhaust gas temperature.

What do you think?

 

[Fabrico]

It makes some sense for a given power output.

Up to a point, more air means greater efficiency. As long as you can put extra air in without heating it up and spending too much energy pumping it, you are, in effect, doing the exact same thing as making the engine bigger. You get more power without increasing anything else.

Normally it would be thought of as increasing power. But in this case it would be letting up on the throttle.

 

[dgallup]

I can tell you from experience with highly turbocharged DI truck diesel engines that at the same power level, there is slightly more than a degree decrease in exhaust temperature for each degree drop in inlet manifold temperature (because of reduced BSFC and heat release).

I mapped out the first production release of a 16 liter V8 400 HP engine. We varied inlet restriction, exhaust restriction, intercooler discharge temp, injection timing & RPM at constant power to produce multi dimensional engine maps. Took me about 2 straight months of dyno time.

A gas engine will respond differently because you will make more power with the cooler (denser) inlet temperature. But I have never run that test.