Question about ADI and MAP

[F4UDOA]

My stupid question for this year,

The R2800B engine (F4U-1/F6F-3/5) used ADI to reach 60" MAP using 100 octane Av-gas. This ADI serves two functions.

1. Cooling
2. More importantantly- Anti-detonation.

However the P-51B/D uses the same fuel (100 octane) and has a mile power rating of 61" MAP and a War Emergency rating og 67" MAP.

The P-51 however uses no ADI. So how does the P-51 V1650-3/7 attain such high MAP with no Anti-detonate?

[MiloMorai]

F4U, what is the CR of the 2800 and 1650?

[hitech]

Is it possible the liquid cool engine had a lower cylender head temp, And could hence run a higher pressure?

And as milo suggest Crompression ratio would have a big impact.

Sorta curious my self about this.

HiTech

[Morpheus]

Higher temperatures in the cylinder will help promote detonation. Increase of cylinder pressure will also prompt detonation. Design of combustion chamber shape and mixture motion, spark plug location and piston design can all make an engine more or less inherently prone to abnormal combustion. Spark timing and fuel quality are at the top of this list as well.

Two different engines,  different animals in the "engine kingdom".

[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.

[Kurfürst]

The asnwer for your question : INTERCOOLER on the P-51D. It provides the same as ADI, cooling of the charge.

[gripen]

Hm... Below is the induction system of the F4U from Zeno's site. What can be seen in the right side of the picture (marked as F)?

Regarding the temp, PW redesigned the R-2800 entirely during the war to improve cooling.

gripen

[Grits]

Morph and I know from owning (and building) high output turbo or supercharged cars that many many factors can change the amount of boost an engine can run without detonation. As he stated, piston shape, combustion chamber shape and finish, spark plug location, turbo/supercharger size, cylinder head material, can all have huge effects on an engines inherent resistance to detonation.

My car for example (Ford 2.3 stock Iron head) is running 20lbs of boost right now. I can turn it up to 23lbs but it makes no more power because the head just wont flow more. Detonation is more likely because the turbo is having to work harder to develope the extra PSI and is heating the carge unnecessarily. The same engine, but with an aftermarket aluminum head can make much more power, but a lower boost (18PSI) because the head can flow more allowing the turbo to heat the carge less.

Basically what I am getting at is unless you compare two identical engines it means nothing, and also more PSI does not always equal more HP.

[Widewing]

I think the bottom line is that the typical radial engine does not discipate heat with a high degree of efficiency. Thus, cylinder head temperatures can rapidly rise to levels where engine oil breaks down and essential lubrication fails. On the other hand, the typical fluid to air heat exchanger is far more efficient and is able to discipate heat much better.

One reason you don't see cylinder head temp gauges in liquid cooled aircraft is that water temperature tells you everything you need to know. Head temps can't rise any higher than the fluid cooling them. Heat is transmitted through the heads and block into the ethylene glycol, which is pumped to the heat exchanger (radiator) and then back to the engine (just like your automobile).

Running high boost in liquid cooled engines has its risks as you can burn valves, melt piston crowns and over-stress connecting rods, wrist pins, melted spark plug electrodes and the like. As long as the the cooling system has the means to remove heat from the block and heads, there is usually no danger to the engine. However, one can increase boost to the extent that the intake charge intercooler and radiator can no longer maintain acceptable temperatures and damage will eventually result. I've seen fluid bath type intercoolers used for turbocharged drag racers. These are more efficient than fluid to air over a short time span, but rapidly become heat saturated. Ok for the short period of operation required for drag racing.

Most radials used in high performance fighters used intake charge intercoolers to lower the temperature enough to improve power (a cooler charge has greater density) and keep cylinder head temperatures reasonable  . However, radials almost always operate at higher temperatures than liquid cooled powerplants. This is normal and accounted for in the engine design. Radials depend more on a cool supply of oil as the oil greatly helps carry heat out of the engine. For example, the R-2800 installed in the P-47 had an oil tank with a 28 gallon capacity, while the P-51 tank held just 12.5 gallons. Since oil is more important to cooling a radial than it is for a liquid cooled V-12, the engine must hold a greater volume of oil and that is reflected in the size of the tank. Radials are also more prone to leaking oil... Another good reason to make sure there's plenty onboard. I can't count how many pushrod tubes I've repacked (replaced seals)....

My regards,

Widewing

[Grits]

What Widewing said. I will also add that the same thing can be seen in motorcycles. The air cooled engines depend on the oil in the system to remove heat from the cylinder head and move it to the sump where it dissipates. When an aircooled motorcycle comes to a stop at a traffic signal cylinder head temps can (and do) skyrocket, if it were not for the oil acting as a heat removal agent the engine would have a very short life, while the water cooled engines run at essentially a constant temperature either stopped or moving at highway speeds. This is the same thing as the 28 gallon oil tank example he gave, but on a smaller scale.

[ShortyDoowap]

Question about ADI and MAP
My stupid question for this year,

The R2800B engine (F4U-1/F6F-3/5) used ADI to reach 60" MAP using 100 octane Av-gas. This ADI serves two functions.

1. Cooling
2. More importantantly- Anti-detonation.

However the P-51B/D uses the same fuel (100 octane) and has a mile power rating of 61" MAP and a War Emergency rating og 67" MAP.

The P-51 however uses no ADI. So how does the P-51 V1650-3/7 attain such high MAP with no Anti-detonate?

The answer is because the Mustang didn’t have an INTERCOOLER, it had an AFTERCOOLER.  And while Kurfurst used the wrong name for the system, I believe he was correct in pointing it out.  

An intercooler is a heat exchanger between superchargers.  An aftercooler is a heat exchanger between the supercharger(s) and the engine.   An aftercooler reduces induction air temperature better than an intercooler.  Higher manifold pressures can be tolerated the lower the induction air temperature is kept.  Induction air temperature is key.

An aftercooler is more efficient than an intercooler in lowering induction air temperature.   Using an aftercooler system, induction air temperature in the Mustang was low enough that a higher manifold pressure could be tolerated.  The Corsair’s intercooler and ADI system was apparently unable to reduce the induction air temperature enough that higher manifold pressures could be utilized.  

Had the Mustang used an intercooler instead of an aftercooler, it probably would have needed ADI to achieve its boost.   Conversely, had the Corsair used an aftercooler (and P&W tried to develop one), it probably would not have had to use ADI to achieve the same manifold pressure.

The obvious manner to achieve higher manifold pressures (on the same fuel grade) on an engine with an intercooler (ie. Corsair) is increase the amount of ADI injected into the engine.   The quantities of ADI required increase in a linear manner with the desired power level.  The more ADI dumped into the system, the more cooling of the induction air temperature, and the greater manifold pressure that could be tolerated.  

100/130 grade fuel could be boosted to very high levels (I’ve seen as high as nearly 90” hga) as long as the induction air temperature was kept low enough.  The Mustang's aftercooler did it better than the Corsair's intercooler and ADI.

[flakbait]

Yup, P-51 had an aftercooler because both blowers were rigged in series. Blower #2 compressed the air blower #1 fed it, and ran the intake air up to temps that were too high for the engine to digest. So they plummed in an aftercooler to reduce the air temp between blower #2 and the carb. See p104 (fig 143) and p363 of America's Hundred Thousand for diagrams on the inline blowers and aftercooler layout.

As a side note: the max CHT for any aluminum head is 450F. Any higher than that and the head starts to lose structural strength. The 500F limit noted by DOA above is for short periods (takeoff, max-speed flight, emergencies) according to P&W and not a sustained limit. Absolute max operating CHT is 450F, but P&W suggests holding 50F lower than that to make sure the cylinder head material maintains high operating strength. Lycoming and Continental both recommend CHT remain lower than 420F for their air-cooled (non radial) engines built today.

http://www.avweb.com/news/columns/182084-1.html

About 3/4 the way down on that page is an excellent chart from P&W regarding CHT, along with a quote from a manual they published. Deak still has copies of P&W's "The Aircraft Engine" for sale. He's asking $100 per copy over at http://www.advancedpilot.com/store.html There's also a few other books available for any aircraft nut.



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Flakbait [Delta6]

[HoHun]

Hi F4UDOA,

>So how does the P-51 V1650-3/7 attain such high MAP with no Anti-detonate?

While there is plenty of good stuff in all of the above posts, let me point out that boost pressure in itself means nothing - you'd have to compare the pressures in the combustion chamber right before ignition.

It comes down to absolute boost pressure multiplied with compression ratio as a key parameter for comparisons.

Due to the complexity of real life, it's only an approximation, but I'd expect it's a rather good approximation. (Hooker et al. go into considerable detail there, but I have only skimmed the book so far and don't quote it as a source here :-)

With regard to cylinder head temperatures, I believe the indicated values might not be directly comparable between different engine types since the temperature is not evenly distributed, and the sensor is probably placed in a convenient location for each engine type, reading a bit higher or lower depending on the exact installation.

I don't think that liquid-cooled engines actually have a great advantage with regard to peak temperatures as it's my impression that the detonation usually is caused by small hot spots in places that are as difficult to cool by liquid as they are by air.

(As pointed out above, liquid cooling has several advantages, but I think they mostly concern handling and not necessarily performance.)

Just some impressions from a few books and one or two NACA reports - I might be wrong, but thought I'd add my thoughts to this thread anyway :-)

Regards,

Henning (HoHun)

[Kurfürst]

Originally posted by ShortyDoowap
The answer is because the Mustang didn’t have an INTERCOOLER, it had an AFTERCOOLER.  And while Kurfurst used the wrong name for the system, I believe he was correct in pointing it out.

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.

[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.