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?

[skymaster]

hot day in tx you will appreciate intercoolers. inside

[Walter Atkinson]

Intercoolers are a VERY good thing. As you correctly pointed out, they increase the detonation margin. All other stated reasons for having an intercooler are questionable at best.

You made some faulty assumptions, though. The addition of the intercooler does not increase the HP output. That is logical, but incorrect since the increased exhaust backpressure lowers the volumetric efficiency enough to make the HP output just about the same with and without an intercooler. Most people neglect to include that effect in their calcualtions.

The most detonation prone condition is with high IAT (without an intercooler) and with high CHTs and high internal cylinder pressures. It's not at cruise at 17,000 feet as you postulated.... it's on takeoff on a hot day with high CHTs. It is almost impossible to detonate these engines unless the pilot does something inappropriate with the mixture.

The MOST detonation-prone condition will occur at high powers settings at about 40-50dF ROP where internal cylinder pressures are the highest with high IAT. An intercooler significantly mitigates that.

[KyleTownsend]

>> Intercoolers are a VERY good thing. As you correctly pointed out, they increase the detonation margin. All other stated reasons for having an intercooler are questionable at best.

Makes sense to me. The only caveat I would add is that intercoolers are a good thing on engines that need them. If the detonation margin is already sufficient for a given engine, then intercoolers are superflous.

>>You made some faulty assumptions, though. The addition of the intercooler does not increase the HP output.

I guess you are referring to the "Con" argument I citied. This was just a comment that an RTC instructor made that provided food for thought. I believe the core of his argument hinged on the fact that pilots may be tempted to use the POH power settings that apply before an intercooler is installed, to the same engine after an intercooler is installed. His theory was that if it is an "ISA+20" day, but the intercooler is providing 40 C of cooling, the pilot should apply the "ISA-20" power settings rather than the "ISA+20" power settings, and that he might not know this (although I believe that some (or all ?) of the intercooler manufacturers provide supplements that adjust the power settings appropriately).

>>That is logical, but incorrect since the increased exhaust backpressure lowers the volumetric efficiency enough to make the HP output just about the same with and without an intercooler. Most people neglect to include that effect in their calcualtions.

Yup. I hadn't thought of that.

>>The most detonation prone condition is with high IAT (without an intercooler) and with high CHTs and high internal cylinder pressures. It's not at cruise at 17,000 feet as you postulated.... it's on takeoff on a hot day with high CHTs. It is almost impossible to detonate these engines unless the pilot does something inappropriate with the mixture.

Now you are getting into an area where my ignorance truly shows. I have read more about detonation since my original post, and it is an amazingly complex and intricate subject. There are, in fact, a number of engines that operate at internal temperatures well in excess of the spontaneous combustion point of 100LL. Apparently, this has to do with the time-delta of the pressure rise vs temp rise. I was just thinking about where the maximum temperature rise across the turbocharger would occur. This will be much higher at altitude than at sea level (by a couple of hundred degrees, at least). At this point, I suppose I would ammend my thesis to specify the "critical point" as the altitude at which the turbocharger starts bootstrapping at 100% power (rather than 75%). I believe that this would yield much higher CHT's than 100% power at sea level, because the heating due to the turbocharger would be much more than the somewhat lower ambient temperatures existing at altitude.

[Walter Atkinson]

Kyle:

The thoughts about adjusting MP for an intercooler are misplaced for the reason I cited... the exhaust backpressure is negating the increase in power from the lower temperature. Recommendations to adjust MP from the manufacturers are in error since they have neglected to take the exhaust backpressure into account in their calculations.

Your concerns about high temp across the turbo charger are not warranted. That is before the intercooler. The rise is based on the compression, not the altitude. If you do not have an intercooler, your concern is warranted.

There is as much confusion over intercoolers as any topic. None of the oft-cited reasons for having an intercooler hold any weight as compared to the increase in the detonation margin. The omission of the effect of exhaust backpressure on HP output is a serious omission and has led to many erroneous conclusions concerning the effects of an intercooler.

I would not want to operate a turbocharged engine without an intercooler since I have seen the test stand effects of not having one! The detonation margin is minimal in most non-intercooled turbocharged engines and all it takes is a slightly reduced FF at takeoff power to wipe away that margin away. An intercooled installation has a much wider safety margin.

The increased CHT at altitude is from poor cooling in the thinner air, not increased heat inside the cylinder. The same heat is produced at any given power at any altitude. It's how efficiently that heat is removed that alters the CHTs.

[KyleTownsend]

OK. I think I understand what you are saying now about the intercooler causing increased exhaust backpressure which offsets the HP gained by lower intake air temp.

On the next subject: Actually, I was talking about engines (the stock TSIO360) that do not have intercoolers. At sea level, the ambient pressure is about 30 in hg, and the required boost from the turbocharger is about 9 in hg, to generate full power. The turbocharger will heat the air about 62 F at this boost level.

In comparison, at full power and 15,000 feet (for example), the ambient pressure is about 14.6 in hg, and the required turbocharger boost is about 24.4 in hg. At this boost level, the temp. rise across the turbocharger will be around 232 F. This is partially offset by the 54 F cooler ambient air at that altitude (assuming normal lapse rate). However, this still means that the air admitted to the cylinders will be 115 F hotter than it would be at sea level and the same power setting. This will contribute to higher CHT's won't it?

[Walter Atkinson]

Kyle:

OK, now I see what you are saying. Yes, but not significantly. CHT is basically a function of internal cylinder pressure. Other factors are pretty minimal by comparison. What the higher IAT does is lessen the detonation margin such that detonation will occur sooner at the same CHT. Higher CHTs and higher IATs are a bad combination. In that light, you are absolutely correct.

That's why an intercooler has such a significant advantage over non-intercooled installations. It really is a big advantage.

Walter

[kevin]

I don't have the engineering background that either of you two do, but Walter's assertion that the HP is the same does not match my practical experience. With American Aviation intercoolers on my 73 P337, if I used the stock MP/RPM settings, I would get higher fuel flows and faster airspeeds, indicating to me that the engine was putting out more horsepower. My mechanic gave me the same advice as Kyle's RTC instructor: reduce the MP until you get the correct fuel flow for the percentage of power that you are trying for. In my case, this was 2" lower. When I did that, I got book airspeeds (matched to book fuel flows). If it was only one engine that did this, I would think it could be guage error, but both engines were perfectly matched in this respect.

Kevin

[Walter Atkinson]

** With American Aviation intercoolers on my 73 P337, if I used the stock MP/RPM settings, I would get higher fuel flows and faster airspeeds, indicating to me that the engine was putting out more horsepower.**

Could that have been a fixed wastegate installation? If so, your observation would be correct. That will not be the case with a variable wastegate which alters exhaust backpressure.

[kevin]

No, it is a variable wastegate. It is a stock P337 turbocharging system (variable wastegate) with only the addition of intercoolers.

Kevin

[Walter Atkinson]

That is most curious. That is not at all what George's research has shown to be the case.

Walter

[KyleTownsend]

I would think that the amount of back pressure caused by intercoolers would vary from one installation to another. Perhaps, on average, it is true that the amount of backpressure attributable to the intercooler would offset horsepower "gains" due to cooler (more dense) indicution air.

However, it is certainly conceivable that some intercoolers could be more "free-flowing" and would not fully offset the power increase. This would primarily be a function of intercooler size and/or effeciency.

Back on the subject of CHT's, detonation, etc. If I am understanding correctly, CHT's are primarily a function of internal cylinder pressure (which, in turn, is influenced by the timing of the combustion event), but detonation margins are more heavily influenced by IAT?

[Walter Atkinson]

Kyle:

**Back on the subject of CHT's, detonation, etc. If I am understanding correctly, CHT's are primarily a function of internal cylinder pressure (which, in turn, is influenced by the timing of the combustion event), but detonation margins are more heavily influenced by IAT?**

Sorta.

CHT's are function of heat IN and heat OUT. Heat IN is a function of ICP. ICP is most affected by timing and mixture (mixture afects the EFFECTIVE timing, or the thetaPP). Heat OUT is a function of baffling and OAT.

Detonation margins are affected most by the combinations of fuel octane, CHT, and IAT.