Question about ADI and MAP

[justin_g]

Never seen the term "aftercooler" in any period documents(NACA, P&W etc). In fact this is the first time i have ever seen it used...

[MiloMorai]

Originally posted by Kurfürst
I often seen the Merlins "aftercooler" referenced as an "intercooler". And while you are correct with your points, and certainly understand the subject far better than me, let me note that, as I heard, the British and the Americans use different terms to describe the same thing, the former call it an IC, the latter know it as an AC.

 An aftercooler is after the supercharger. An intercooler is between the the 2 stages of the supercharger.

justin, the P&W H-3730, the Chrysler IV--2220 and the Allison V-1710-119 had aftercoolers. Pics show the cooler between the supercharger and the induction manifold.

description of the V-1650

The supercharger is a two-stage two speed gear driven unit with intercooling and aftercooling.  Two stages are used to obtain relatively high pressure ratio's efficiently, two-speed operation allows improved performance at high altitude without the loss of power at sea-level which is inherent at high blower speeds, and intercooling and aftercooling significantly reduce charge temperature, allowing higher boost without detonation.

For low speed operation, the supercharger turns at 6.391 times crankshaft speed, for high speed the blower spins at 8.095 times crankshaft speed.  The supercharger is driven through a gear train coupled to the spring drive through the supercharger driving gear in the wheelcase.  Three independent planetary gear trains are arranged at 120 degree intervals around the driving gear, the driving gear thus driving three planetary pinions.  The ring gears for the three planetary drives have internal and external teeth, the external teeth are coupled to the supercharger pinion which directly drives the supercharger, the internal teeth are coupled to planetary gears coupled to planetary pinion gears.  For low speed, the planetary pinions are directly locked to their respective ring gears through planetary gears and clutches, so the ring gear is turning at the speed on the input to the planetary geartrain.  For high speed, the planetary gears are allowed to rotate about the sun gear, increasing the speed of the ring gear with respect to the input shaft.  A hydraulic clutch (three really, one for each planetary gear train) combined with over-running "sprag" clutches is used to effect speed changes.  The hydraulic clutch uses moderate engine oil pressure controlled with 24V electrically actuated solenoid -- the solenoid is actuated to apply oil pressure for high speed, and pressure is released for low speed operation.

The supercharger itself consists of two impellers on the same shaft, both turning the same speed.  The first stage uses a 12.0" diameter impeller, while the second stage uses a 10.1" impeller.  The intercooler is an integral part of the intermediate volute case, located between the first and second stage impellers.   The compressed air from the first stage passes through the cooled volute and passage to the second stage.  The aftercooler is located between the exit of the second supercharger and the intake plenum, and is a conventional air/water heat exchanger (liquid/air radiator).  A separate cooling system with its own pump was provided (permitting cooler water than is possible by using the engine coolant) was provided for aircraft use -- a 40% reduction in intake temperature was reported by Rolls-Royce at maximum speed and power with a coolant flow of about 30 gallons per minute.  For boat racing, the aftercooler is usually replaced with simple plenum tube (called a tube or ADI tube) due to disruptions in airflow and mixture which occur in the aftercooler matrix at very high power levels -- at least some of which is though to result from air-fuel-ADI separation.

An automatic boost regulator is standard equipment for aviation use.   The boost regulator automatically retards the throttle as full boost is reached, eliminating manual control of this critical function.  War emergency boost is often provided whereby the pilot can over-ride the regulator when necessary, an indication such as a broken seal is usually provided to alert maintenance that the engine has been overstressed.  For planes with ADI, there is usually an interlock provided that restricts maximum boost to a lower setting when ADI is not functional.

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[justin_g]

NACA Technical Note No. 794, Feb. 1941

An intercooler is a heat exchanger, and the name "intercooler" was chosen because this heat exchanger was interposed between two stages of compression. The name now has come into widespread aeronautical use as meaning any heat exchanger used for cooling the engine air, regardless of its position in the induction system.

NACA Technical Note No. 795 uses the term "intercooler" in reference to single stage superchargers...

"Intercooler" was the term in use at the time, which was the point I was making.

[ShortyDoowap]

Originally posted by justin_g
NACA Technical Note No. 794, Feb. 1941



NACA Technical Note No. 795 uses the term "intercooler" in reference to single stage superchargers...

"Intercooler" was the term in use at the time, which was the point I was making.

The only US plane to use the aftercooler in any number was the P-51 with the Merlin engine.  That's a post 1941 aircraft.  

T.O. No. 1F51-D-1 "Pilot Training Manual For The F-51D Mustang," 20 January 1954, properly refers to the system as an aftercooler.

So did AAF Manual 51-127-5 "Pilot Training Manual For The P-51 Mustang," 15 Aug 1945.

[gripen]

The differences in the charge cooling can't really explain the differences in the reachable MAP between the Merlin and R-2800. The E-series V-1710 (same compression ratio as in the B-series R-2800) with auxilary stage as used in the P-63 was rated at for 75" with ADI (no intercooler) while the highest rating for the B-series R-2800 with ADI and intercooling was around 64" (and R-2800 probably needed richer mixture). With ADI the Merlin would have reached even higher MAP, IIRC Merlin was type tested for +36lbs with water injection (probably with grade 150 fuel).

The reasons for lower MAP are much more likely in head cooling.

gripen

[ShortyDoowap]

Gripen, it all goes together.  Remember, hot spots could ignite a hot charge easier than a cooler charge.  That’s the reason the charge needed to kept as cool as possible.  The Intercooler/ADI system of the Corsair/Hellcat was unable to reduce the charge temperature enough that high manifold pressures could be used.   The Mustang's aftercooler system could reduce the charge temperature enough that higher MAPs could be used.  

The simple fact is that if the Corsair or Hellcat was to use a higher MAP on the same fuel grade, it had to find a way to reduce the charge temperature further.  Pratt and Whitney tried to develop an aftercooler, but abandoned the attempt after realizing that it would have required a major redesign of the engine and supercharger system.  Therefore, the only other way was to increase ADI flow.  That was hard.  As it were, the Corsair carried only about 10 gallons of ADI, and the Hellcat 16 gallons, both set for a flow for 15 minutes use, IIRC.  

Also, comparing the Merlin, R-2800 and Allison is difficult.  They all had different types of superchargers, and engine cooling methods.  As you know, the R-2800 was air/oil cooled, the Merlin was mostly liquid cooled, and the Allison as a combination liquid/oil cooled engine.  

The V-1701 in the P-63 had no intercooler or aftercooler, true.  But it also has a single speed 2nd stage supercharger, compared to the two speed 2nd stage supercharger in the Mustang’s Merlin.   The Allison’s 2nd stage supercharger compressed much less, and imparted much less heat to a charge than did the Merlin’s 2nd stage supercharger.  Couple that with the fact that the P-63 carried a huge amount of ADI (25 gallons for 15 minutes use) and you can see why the Allison could obtain 75” hga.  

The simple fact is that if higher MAPs are to be used on the same fuel, the charge temperature has to be reduced.

[joeblogs]

Based on my rading HiTech is right on this. Water cooling reduces hot spots in the cylinder so you can typically get more MAP out of a water cooled engine without detonation.

The cylinder head tolerances you see in most engine specs have to do with the temperature where the cylinder actually loses it's strength and may experience structural failure.

It's hard to argue this is about an intercooler when we are comparing engine models with two stage superchargers that both take advantage of large intercoolers.

-Blogs

Originally posted by F4UDOA
HT, I have my handy dandy POH's ready for action.

Well Cylinder head tempatures of the F4U-1 R2800-8W B-block are max 500 degrees Farenheit in both Mil and WEP.

On the F4U-4 R2800-18W and 42W are both 473 degrees F. So from the B to C block the head temps are marginally lower however my engine charts only show 60"MAP on the chart for WEP so higher temps if alloable are not shown.

My P-51D POH only shows coolant temp not cylinder head temps. The coolant temp is much lower at 135 Degrees C (275 F). Indeed there is no mention of Cylinder heads throughout the manual however they do warn against pre-ignition and detonation due to increased engine temps caused by too lean a mixture etc.

Based on this I would have to say that your explanation is correct although I would like to know what the head temps of the P-51 really are. Apparently there is no gauge to inform the pilot of this so he is just looking at his coolant temps and listening for cylinder knocking.

[joeblogs]

The Merlin made up for its smaller displacement with a higher max RPM. But this only places more stress on the ability to cool the engine.

-Blogs  

Originally posted by Kurfürst
BTW, one thing I can think about is the rather vast volume difference between the rather small Merlin (27 liter) and the R2800 (well over 40 litres). I'd believe that to provide the same pressure for a larger volume, much larger amount of air need to be compressed, and it may be that leads to higher temperatures in the charge eventually in the larger engine. just a guess.

[ShortyDoowap]

The original question was why could the Merlin use utilize higher MAPs than the R-2800 on the same fuel.

The answer is that the induction system on the Merlin was able to introduce a charge with a temperature sufficiently cool enough that detonation wouldn't be experience at its rated MAPs.  The induction sytems in the Corsair and Hellcat could not introduce a charge that was cool enough to sustain the same MAPs as the Merlin.   The Mustang's induction system could cool a charge enough that it could tolerate 67" hga.  The Corsair's couldn't.

The engine was what it was.  The way an engine created and disipated heat was hard to change without a major redesign of the engine.   The way to increase MAP, then, was the reduced the charge temperature, or switch to a higher octane fuel.  Temperature was reduced by way of an intercooler, aftercooler, ADI, or a combination of any or all of them.  For the most part, when manufacturers and the Military wanted to increase ratings, they didn't redesign cooling systems, they added water injection.

And that you can get higher MAP out of a watercooled engine is not always correct.   Maximum ratings for the P-38 were only about 60" hg - with the utilization of an intercooler.  And the Allison was considered to have better cooling properties than the Merlin, and also had a less compressive 2nd stage supercharger than did the Merlin.    

Whatever the cylinder head temperatures were in the engines discussed is incidental.  If you introduce a 220 degree charge into a Merlin, it will detonate quicker than than a 200 degree charge, even if the cylinderhead temp is the same.    The induction systems had to deal with the engine temperatures.   The Merlin's induction system was better able to cool a charge, and thus deal with the engine heat better than the R2800's induction system could.

[ShortyDoowap]

And BTW, when you say

It's hard to argue this is about an intercooler when we are comparing engine models with two stage superchargers that both take advantage of large intercoolers.

remember the Mustang and the Corsirs had significantly different induction systems.  

The biggest heat inducer to a air/fuel charge is the 2nd stage supercharger.  

The Mustang's system sent the charge from the second stage supercharger to the aftercooler.  The benefit was that the charge didn't have to go thru another heat-inducing compression.

The Corsair's system sent the charge from the 1st stage supercharger to the intercooler, and from there it was compressed again (and heated again) by the 2nd stage supercharger.

Again, P&W realized and aftercooler was better, but could not develope one for its engines.  

Want higher MAPs, then the charge has to cooled more.  Again, when the military wanted higher MAPs, they didn't improve engine cooling, they strove to reduce charge temp.

Engine heat had everything to do with it.  But MAP ratings were determined by how cool a charge could be made.  As charge cooling was improved, MAP ratings increased.

[Grits]

Originally posted by ShortyDoowap
Whatever the cylinder head temperatures were in the engines discussed is incidental.  If you introduce a 220 degree charge into a Merlin, it will detonate quicker than than a 200 degree charge, even if the cylinderhead temp is the same.    The induction systems had to deal with the engine temperatures.   The Merlin's induction system was better able to cool a charge, and thus deal with the engine heat better than the R2800's induction system could.

Cylinder head temps are a factor just as charge temp, combustion chamber shape, flow capability of the ports in the head, and all the other things said. Yes, charge temp is the most critical factor, but it is not the only one. A cylinder head (either water or air cooled) can have localized hot spots and cause detonation no matter cool the carge temp.

Its not an either/or situation, there are many interrelated factors that all contribute to the amount of boost an engine can handle which is why unless they are identical, not just similar, its meaningless to compare them.

[ShortyDoowap]

I didn't say it was the only factor, in fact, I specifically said it wasn't.  I said all these thigs are interrelated.

Having an engine with hotspots sufficient to detonate a charge "no matter how cool it is" would not have been usual and would been an indicator of a serious cooling problem.  

Nevertheless, the answer to the question remains that the Mustang's system could deliver a charge cool enough that it could deal with the heat and all the other stuff that factors into detonation such that it could tolerate 67" hga.  The Corsair's couldn't.

When Vought and P&W wanted to run the Corsair at higher MAPs, they added water injection, they didn't modify the cowling so that the engine ran cooler.

[ShortyDoowap]

Here's what Graham White says in "R-2800:  Pratt and Whitney's Dependable Masterpiece"

[Grits]

LOL....I'm very aware of how charge cooling works and its ramifications, I have 3 cars with turbo's, one of them making 300+ HP out of 2.3ltr's.

[ShortyDoowap]

Calm down, Grits, that wasn't intended for the experts.