Intercoolers Pro and Con

[KyleTownsend]

I have seen a lot of remarks about intercoolers here and there, but I have never found a good explanation of when you need them, and why. In particular, I have never seen a rigorous analysis of their application in the skymaster.

Two "typical" arguments that I see go something like this:

PRO - Intercoolers reduce the temperature of the induction air, sometimes by 100 degrees or more. Your airplane is always happier breathing "cooler" air. Just look how it performs in winter! Intercoolers are a definite plus!

CON - Intercoolers can lead pilots to unintentionally operate at power settings that are really higher than the pilots think. This can be bad for the engines, and may even make them run hotter.

I was bored last night, so I tried to think about the subject a little. This is what I came up with, but I would sure like to hear from those of you who know about this kind of stuff as to whether or not my reasoning is correct.

1. The real reason that one might want intercoolers is that they improve the detonation margin.

2. We would care about this only if the detonation margin was inadequate under conditions that we encounter when in flight.

3. Presumably, our engines are designed to operate at the power settings, altitudes, and temperatures shown in the POH with sufficient detonation margin. So, for example, in my P337, I should be able to cruise along at 20,000 feet and 71% power on an ISA+20 day with sufficient detonation margin. If I can do this, then I don't need intercoolers (since this represents the "worst case" operating point permitted by the POH). However, if I am encountering detonation under these conditions, then I do need intercoolers.

That should be the whole argument, unless I am missing something.

Nonetheless, being curious about numbers, I did a little crunching.

Assuming normal temp and pressure lapse rates (no funky inversions that complicate the analysis), the thinnest detonation margin in a skymaster will occur at the point where bootstrapping first starts to occur, because the combined adiabatic warming of the induction air resulting from (1) compression due to the turbocharger, and (2) compression due to the piston, will be maximal at this point. I figure this point is near 18,000 feet on an ISA+20 day at 72% power in my bird, based on actual experience. This is also about as high as I ever fly, so I used it as my analysis point.

On an ISA+20 day the surface temp is 95 F, and the temp at 18K feet will be 30 F.

The ambient pressure at altitude will be 5.65 PSI and the target upper deck pressure will be 17.14 PSI (equivilant to a desired 33 in hg manifold pressure + 2 in hg for losses across the intake system). This gives a compression ratio across the turbocharger of about 3:1. Adiabatic compression would heat the air to 213 F, but since "real" turbochargers don't have 100% thermal effeciency (actually more like 70%), the air will be heated to around 291F (a rise of 260 F).

In the piston, assuming a 7.5:1 compression ratio, the air will be further heated by the compression stroke. At Top dead center (TDC), the temperature would be increased to 875 F (since this heating is almost purely adiabatic).

Just as a point of contrast, this would compare to 527 F at sea level and 37 in hg (takeoff power setting).

This is where things get a little confusing to me. I believe the autoignition point of 100LL is around 760 degrees. Therefore, anything in excesss of that should cause preignition? According to my calculation, we would be about 100 F over that point.

So, I am thinking that there must be some problem with my analysis.

Can anyone help me out here?

One thing I thought of is since the spark actually fires a few degrees before TDC, the temperature rise would be less at this point, and might still be OK (I haven't calculated this out yet). I don't see that we would care what the temperature and pressure due to compression did after the spark event, since it is going to start rising much more rapidly due to combustion in any case.

Of course, I wouldn't be surprised if I don't just have some kind of math screw up here. Can somebody tell me if these numbers sound right?