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Rob said:
All sorts of engines run without intercoolers, without damage. Detonation is a function of temperature AND pressure in the cylinder at, or near, full compression, not the temperature in the intake. Besides, if you are flying up high, where an add on system makes sence, the air starts out colder so it isn't all that much warmer than it would have been at ground level.[emphasis mine]
The principle reason for adding an intercooler is the same as for adding a supercharger, to make the intake air denser and increase the mass flow through the engine.
In particular, I take issue with your suggestion that intercoolers are not needed at altitude. Let’s take a look at a supercharger (mechanical or exhaust driven) installation on a 7.5:1 compression ratio engine providing 30” MAP at 20’000 pressure altitude and 20C above ISA. While a perfect supercharger (100% efficient) might only raise the adiabatic (no heat added or lost in the process) motored (no spark or fuel, just air) compression temperature 9%, a real world supercharger (76% efficient) would raise the motored compression temperature over 31% (using the Rankine temperature scale). I think that is a significant number.
Mike,
I think you caught me on that one. It's good to know somebody is paying attention out there. I forgot to consider what happens to the energy of the inefficiency and you are correct that in a centrifugal pump/compressor nearly all of it goes to heating the fluid. Actually, when I ran the numbers in the calculation, I thought the discharge temperature looked a little low. The math checked out, but I should have looked a bit further.
By way of example, this Christmas, I was flying in Texas at 17,000’ with a pressure altitude of nearly 20,000’ (13.8”) and an OAT +22C over ISA (my Density Altitude was 23,500’). My Compressor Discharge Temperature (“CDT”) was 212F at 33” MAP with a pressure ratio of 2.4 for a compressor efficiency of 76% (TSIO-520R). This is an 180F temperature increase over ambient! I think that is a significant number.
An intercooler with only 60% efficiency would have reduced my inlet and turbine temperatures by over 100F ! That would have moved my TIT from 1622 to 1522, well below my max TIT. Similarly, it would decrease the heat rejection requirements of the engine and improve my CHTs a bit. Furthermore, it would result in decreased peak combustion pressures and a slight delay in the pressure peak, resulting in less stress on the engine. All of these factors significantly increase the detonation margin between safe operation and engine damage. More importantly, the lower temperatures substantially reduce the probability of preignition, and this is the real killer.
If you throttled back to maintain the same mass flow rate, then I would completely agree with you. But if you take advantage of the additional mass flow the intercooler permits, then I think you will find that the some of these values go the other direction. The intercooler isn't a polytropic process, like the compressor. If you cool the air 10%, then you increase its density by 10%. Yes the intake air is cooler, but the heat output of 10% additional fuel will result in only 6% reduction in temperature and a 14% INCREASE in pressure in the cylinder (assuming we avoid detonation/preignition in both cases). Then there are changes in the heat transfer effectiveness at the combustion chamber surface, which further reduce the 6% reduction in temperature.
Also, keep in mind that simply cooling the fuel air mixture in the cylinder by 60 degrees does not buy a full 60 degrees of detonation margin. As density increases, both the autoignition temperature and the minimum ignition energy decrease.
In the end, an intercooler still has some favorable benefit on detonation/preignition margin, but this benefit is quite small compared to its effect on power output.
Rob
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