Question about ADI

[drat]

Had this question asked an email group. Wondering if any of you guys have any input on it.

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?

[wmaxt]

Originally Posted by drat

Had this question asked an email group. Wondering if any of you guys have any input on it.

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 P-51 has a liquid cooled engine. Hot spots caused by uneven cooling and associated detonation are the bane of all engines, air cooled engines are particularly susceptible. A richer mixture (more fuel than ideal) is used to keep the engines cool normaly and the adders like ADI for maximum power situations to prevent detnotation.

For instance the 2800 powered P-47N takes 1,165 gallons of fuel to go 2,300mi while the twin engine P-38 can do 2,600 on 1,010 gal and the P-51D does 2,055mi on 440gal, the extra fuel keeps the cylinder head cooler than just air flowing over the fins.

wmaxt

[skychimp]

Hi. My first post here.

Someone asked this exact question on another board. Here was my answer:

The answer is because the Mustang didn’t have an INTERCOOLER, it had an AFTERCOOLER.

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.

Hope that helps.

[wmaxt]

Aftercooler and intercooler are the same thing, just terminology as used by the British ie. the cooler comes after the supercharger, or the American version, the cooler lies between (intermediate to) the supercharger and the carburator.

The intercooler in the Corsair was air to air, and extreamly efficient it is also generaly considered better in some ways than the Mustangs air to water intercooler because the air temp is always lower than that of the coolant system. That becomes debatable at altitude where the air is very thin, limiting its temprature absorbsion while the extreamly efficient cooling system of the Mustang still works well.

An aircooled engine is more suseptable to hot spots and detonation due to less even cooling, compounded by more variations in mixture at the various cylinders in a large radial so the practice of ADI is used more frequently (but not exclusivly) on the aircooled radial engines.

wmaxt

[skychimp]

Aftercooler and intercooler are the same thing, just terminology as used by the British ie. the cooler comes after the supercharger, or the American version, the cooler lies between (intermediate to) the supercharger and the carburator.

This is only correct to the extent that the after and intercooler are both heat exchangers. Otherwise this is wrong. Examine a schematic.

An AFTERcooler lies between both supercharger stages and the carb. The induction air goes from the 1st stage supercharger to the 2nd stage supercharger into the aftercooler then into the carb. The benefit of this system is that the hot, compressed charge is cooled immediately before flowing into the carb.

An INTERcooler lies between the two supercharger stages. The induction air goes from the 1st stage supercharger to the intercooler, then on to the 2nd stage supercharger, then on to the carb. The problem here is that once the charge is cooled, it's compressed and reheated again before flowing into the carb.

Since the most compression and heat came from the second stage of the supercharger, the aftercooler system was better at reducing the induction air temperature because there was no other compression after the cooling phase.

Pratt and Whitney understrood this and tried to develope an aftercooler fror their large radial. However, doing so would have required a major redesign and the effort was abandoned.

It is important to understand there IS a difference between intercoolers and aftercoolers. I understand the terms are used interchangably, but doing so it wrong and leads to wrong conclusions.

[wmaxt]

Originally Posted by skychimp

This is only correct to the extent that the after and intercooler are both heat exchangers. Otherwise this is wrong. Examine a schematic.

An AFTERcooler lies between both supercharger stages and the carb. The induction air goes from the 1st stage supercharger to the 2nd stage supercharger into the aftercooler then into the carb. The benefit of this system is that the hot, compressed charge is cooled immediately before flowing into the carb.

An INTERcooler lies between the two supercharger stages. The induction air goes from the 1st stage supercharger to the intercooler, then on to the 2nd stage supercharger, then on to the carb. The problem here is that once the charge is cooled, it's compressed and reheated again before flowing into the carb.

It is important to understand there IS a difference between intercoolers and aftercoolers. I understand the terms are used interchangably, but doing so it wrong and leads to wrong conclusions.

Technicaly thats correct. I've never seen (in life or in a diagram) a supercharger cooled between the stages, a turbo I've seen aka P-38, I don't know if there is any kind of charge cooler between the supercharger and the carb. If you have a diagram please post it.

Also irigardless of intercooling, all aircooled engines have cooling issues, finns that get no airflow, and spark plugs, valve pockets etc. have the added problem of being heat sinks. A richer mixture is always used to aid cooling and ADI is more prominent in aircooled and esp radials for these reasons. I again refer to my earlier post where the P-47N producing less total hp still uses more fuel than a P-38. That is not to say that incoming air, to the carb, of 200f or more cannot add to the detonation problem.

wmaxt

[skychimp]

Ok. Here's a schematic of the Corsair system. Actually, I said the air flowed from the 2nd stage to the carb, actually if flows from the carb to the 2nd stage. No matter, this schematic shows the intercooler is between the 1st and 2nd stage superchargers:


The Merlin used glycol to keep the supercharger itself cool. But the primary heat exchanger for the induction air was the aftercooler located between the second stage impellor and the carb:


I agree, all engines have heating and cooling issues. And all this stuff is interrelated. But the engine was what it was and how it generated and disipated heat was hard to change without a major resdesign. They way to increase MAP was to either reduce the charge temp or increase fuel octane rating. Those planes with systems that introduce an acceptably cool charge into the engine, like the Mustang, were rated for higher MAPs than those that couldn't introduce an as acceptably cool charge, like the Corsair. Engine manufacturers and the military knew this, and that's why ADI was added to many R-2800 powered planes.

[wmaxt]

Thanks for the diagrams, where did you find them, if you don't mind I'd like to earmark them.

I agree with you princapal.

The P-38s final stage supercharger fed the carb and it still used up to 67" too. It's not all dependant on that last supercharger stage adding heat. Do you have any data on the compression of that last supercharger stage? That last stage may not add any more temp than the Mustangs standard cooling system temp of ~200f. Liquid cooling and better fuel distribution are key eliments to high but efficent and reliable output wothout ADI.

Anything that contributes to detonation is a problem because you can't avoid some of them. The British went to sleeve valves to try eliminating the issues associated with poppet valves but jets took over by the time they had it perfected.

PW did some tests at very high Manifold Pressures with great results but did need ADI, rich, high octane can only go so far and it was on a test bench with ideal airflow.

wmaxt

[skychimp]

The 1st pic is from Zeno's site. It comes from the Pilot's Handbook for the F4U-1.

The second pic comes from Gruenhagen's book "Mustang: Story of the P-51 Mustang Fighter."

With respect to the original question: we know the R-2800 B series engine (the engine in the F4U-1, F6F and P-47D) was capable of operating at 67" hga MAP and even greater on 100/130 grade fuel. We know this because the Commanding General of the Material Air Command set limitations for the P-47D with ADI at 64” hga in a letter dated 26 May 1944 – Subject: Extra Boost in P-47. He wrote that 64” hga was now approved for use in the P-47 with ADI, after acknowledging that "the engine could withstand 75” hga for several hours" (on 100/130 grade fuel).

The R-2800 B series engine in the P-47D could operate at 75" hga on 100/130 grade fuel for several hours. That's well above the 60" hga limitation set for the F4U-1 on 100/130 grade fuel. If the F4U-1's 60" hga limitation was set at the point just before detonation set it, then it wasn't because of an engine limitation, it was because of an induction system limitation. I think that's a safe assumption.

BTW, the late P-38's had max MAP limitations of 60" hga on 100/130 grade fuel.

My understanding from several books I have was that the two-stage Merlin's induction system cooled the induction air to 180 degrees Far, which allowed a max MAP of 67" hga on 100/130 grade fuel.

The same books indicate the V-1710 engines with 6.0:1 compression ration pistons were able to cool the induction air to 230 degrees Far which allowed for a max MAP of 63" hga. The V-1710s used in the P-38 J and L used 6.65:1 compresion ratio pistons and were limited to 60" hga MAP on 100/130 grade fuel.

But I think the engines were close enough to for rough comparisons, with the conclusion being the Merlin cooled the charge better allowing for higher MAP than did the Allisons.

[Lunatic]

The P-51 was able to achive 67" of boost using 150 grade av-gas, not 100 grade. There is sometimes confusion because 100 is in fact the maximum possible "octane" rating, all high grade av-gass is 100 octane. The numbers we are familiar with are not really "octane" ratings, I cannot recall what the technical definition is.

Anyway, the point is mute because using 150 grade av-gas the P-51 could pull 67" of boost, where the F4U-4 could pull 70".

Without ADI, the F4U would pull less boost than the P-51, but I am not sure what the relative initial compression ratios are (I don't have most of my info on this laptop) and this may have something to do with it.

One reason the P-51 did not have ADI (until the H model) because ADI reduces exhaust gas temperatures which is counter productive to the P-51's exhaust thrust system.

Also, water injection is hard on valve guides and rings (I know this from real life experiance with car engines). Usually about 1% top oil is included in the mixture, but even this does not prevent a dramatic increase in wear.

=S=

Lunatic

[skychimp]

The P-51 was able to achive 67" of boost using 150 grade av-gas, not 100 grade.

67” was the Mustang’s rating on 100/130 PN fuel, not 100/150 PN fuel. The Americans used 72” on 100/150 PN fuel, and the British used as high as 80”.

Anyway, the point is mute because using 150 grade av-gas the P-51 could pull 67" of boost, where the F4U-4 could pull 70".

Again, 67” is the Mustang’s rating on 100/130 PN fuel.

70” is not the F4U-4’s rating on 150 grade fuel, 70” is its rating on 115/145 PN fuel.

The American Corsair never used 100/150 grade fuel. 100/150 grade fuel was a British fuel. The Americans never produced any 100/150 grade fuel. 100% of 100/150 grade fuel was produced in England. Americans produced 100/130 grade and later 115/145 grade fuel. Those are the fuels on which American manufacturers rated their engines. 8th Air Force fighter groups used British 100/150 grade fuel, but their ratings were set by the Material Air Command.

[wmaxt]

There was a field order in '44 concerning P-38L models with 17 series engines that allowed 64" boost (not 67", sorry) giving 1,725hp each. Test bench runs showed the Allisons were capable of 100+" and 2,300hp. With turbo compounding as much as 2,900hp was recorded in a version earmarked for the P-63. ADI was used in these tests

The main point is that engine configuration and cooling method, more than charge cooler placement is the reason for the required ADI in the 2800 over the Merlin or the Allison.

Lunatic is correct in that ADI is harmful in the long run which is one reason it was used sparingly.

wmaxt

[KraziKanuK]

Originally Posted by Lunatic

The P-51 was able to achive 67" of boost using 150 grade av-gas, not 100 grade. There is sometimes confusion because 100 is in fact the maximum possible "octane" rating, all high grade av-gass is 100 octane. The numbers we are familiar with are not really "octane" ratings, I cannot recall what the technical definition is.
Lunatic

The word you are trying to recall is Performance Number. One was for the rich mixture and the other was the lean mixture.

[DaveB.inVa]

Engine configuration and head design all play on making power without detonation too. For instance a smaller bore is more resistant to detonation and you can tolerate higher compression if you have a longer stroke. In this instance its not only how much you compress the charge but also the rate at which you compress the charge.

Duration and timing of the cam and also spark timing especially have a lot to do with detonation prevention.
As said before fuel mixtures can be adjusted to allow a higher MAP as well. When running a rich mixture with a naturally aspirated engine power will be lost but on a forced induction engine power loss is minimal and the remainder of the fuel is used basically for cooling. Its inefficient and wasteful but it works. In this case the gains made by running a higher MAP far outweigh the loss incurred by running a rich mixture.

Most bombers used this method in War Emergency power and added a lot of fuel to cool the charge. This makes sense as bombers would use WE power much less than fighters.

[skychimp]

Originally Posted by wmaxt

There was a field order in '44 concerning P-38L models with 17 series engines that allowed 64" boost (not 67", sorry) giving 1,725hp each. Test bench runs showed the Allisons were capable of 100+" and 2,300hp. With turbo compounding as much as 2,900hp was recorded in a version earmarked for the P-63. ADI was used in these tests

The main point is that engine configuration and cooling method, more than charge cooler placement is the reason for the required ADI in the 2800 over the Merlin or the Allison.

Lunatic is correct in that ADI is harmful in the long run which is one reason it was used sparingly.

wmaxt

I believe that directive was in response to the availability of 100/150 PN fuel. 60" remained the limit for planes using 100/130, but the Material Air Command allowed the plane to be boosted to 64" when 100/150 PN fuel became available. The only USAAF units to use 100/150 PN fuel were fighter groups in the 8th AF. By the time 100/150 PN fuel was available, scarcely any P-38s were left in the 8th. I have a copy of that directive, I'll see if I can find it.

AFAIK, the L model didn't develope 1,725 hp on 64" hga. 1,725 hp was developed at 70" hga, and that was never approved for use.