Primer Cooling Trick

[Merlin-27]

Anyone ever read about P-51 pilots extending the life of an over-heating engine with the use of the fuel primer?

 

I just read a story about one such event where John Godfrey was about to bail over enemy territory but then used the advice of a fellow pilot to cool the engine by frequently priming the engine all the way back to England. Interesting stuff.

 

Of course, the next question is can you do that with the DCS P-51D? I have not tried it...yet. :)

 

I even found the same story in his online biography. (Page 4)

 

John Godfrey Bio

Edited October 23, 2013 by Merlin-27

[WildBillKelsoe]

interesting story. But we don't have the exact distance in DCS, nor could we prime for 2 hours straight while keeping her trimmed. I just wonder why hasn't he climbed so the cold air might cool the engine a bit?

[Merlin-27]

interesting story. But we don't have the exact distance in DCS, nor could we prime for 2 hours straight while keeping her trimmed. I just wonder why hasn't he climbed so the cold air might cool the engine a bit?

 

I'd imagine if it cooled the engine sufficiently for 15 minutes it would be fine for 2 hours (if the priming pump could hold up) and I'm pretty sure they didn't climb because of the added strain on the engine and the low speeds to just keep the A/C airborne with minimal temp cost.

[seikdel]

I have a dumb question: what about priming cools the engine?

[ARM505]

I have a dumb question: what about priming cools the engine?

 

Not dumb, but a simple answer: Throw some fuel on your leg - cool, isn't it? :)

 

It takes energy to heat the fuel, hence it carries some energy away from the engine. In aircraft engines, one of the benefits of a 'rich' mixture (too much fuel vs air) is it's ability to aid in cooling the engine. Priming constantly is an extreme example of this.

[Fifi]

Interesting stuff to know! Thanks for sharing.

Let us know if it works in DCS...

[26-J39]

Yes fuel, oil, air and coolant are all part of the 'cooling system', in any engine.

[xxJohnxx]

It takes energy to heat the fuel, hence it carries some energy away from the engine. In aircraft engines, one of the benefits of a 'rich' mixture (too much fuel vs air) is it's ability to aid in cooling the engine. Priming constantly is an extreme example of this.

 

Good explenation. Take the Boeing 377 (Stratocruiser) as an example, which had some very large piston engines. It had a water injection to cool the engines during takeoff and other high power flight stages.

If you wouldn't turn on the water injection, and firewall the throttles than it would automatically inject much more fuel than necessary. This is a waste of fuel, but it protects the engines from overheat.

[komemiute]

So... tested working in DCS?

[Yo-Yo]

Good explenation. Take the Boeing 377 (Stratocruiser) as an example, which had some very large piston engines. It had a water injection to cool the engines during takeoff and other high power flight stages.

If you wouldn't turn on the water injection, and firewall the throttles than it would automatically inject much more fuel than necessary. This is a waste of fuel, but it protects the engines from overheat.

 

The injection of additional liquids is not a mean to signfcantly change overall heat balance after fuel is combusted: specific heat is 43 MJ per kg and specific vapourisation heat s 0.28 MJ. Anyway, it effectively reduces fresh mixture temperature preventing DETONATION.

Nevertheless, richer mixture slghtly reduces heat amount transferred into engine body.

[howie87]

Why not just fly on 'emergency rich'?

[Yo-Yo]

Why not just fly on 'emergency rich'?

 

It depends on the flght altitude. By the way, anyone can try to fly with closed water radiator shutter at 2-3 km alt usng RUN and then EM RICH at the same MP (30-36 inches) and check water temp. Be sure the engne is not running rough.

[howie87]

Awesome... I love the detail in this thing. Yo-yo, please never leave ED!

[Merlin-27]

The injection of additional liquids is not a mean to signfcantly change overall heat balance after fuel is combusted: specific heat is 43 MJ per kg and specific vapourisation heat s 0.28 MJ. Anyway, it effectively reduces fresh mixture temperature preventing DETONATION.

Nevertheless, richer mixture slghtly reduces heat amount transferred into engine body.

 

That makes sense. Thank you Yo-Yo. I wasn't trying to pull you away from more important work but I am still amazed at what a great simulation it is. SO many things correlate with real world accounts of the actual combat pilots. It all adds a lot to the experience of flying the DCS Mustang.

 

In those tense moments before bailing out, I wonder who was the first to think "maybe I should just sit here and prime the engine more" :) That is one cool character. I'm guessing it was a suggestion from a member of the ground crew. Either way, it prevented at least two great fighter pilots I know of from bailing out of their aircraft.

[NineLine]

That makes sense. Thank you Yo-Yo. I wasn't trying to pull you away from more important work but I am still amazed at what a great simulation it is. SO many things correlate with real world accounts of the actual combat pilots. It all adds a lot to the experience of flying the DCS Mustang.

 

In those tense moments before bailing out, I wonder who was the first to think "maybe I should just sit here and prime the engine more" :) That is one cool character. I'm guessing it was a suggestion from a member of the ground crew. Either way, it prevented at least two great fighter pilots I know of from bailing out of their aircraft.

 

 

Isnt the saying... "Fear of Death is the mother of invention" :P

[doright]

The injection of additional liquids is not a mean to signfcantly change overall heat balance after fuel is combusted: specific heat is 43 MJ per kg and specific vapourisation heat s 0.28 MJ. Anyway, it effectively reduces fresh mixture temperature preventing DETONATION.

Nevertheless, richer mixture slghtly reduces heat amount transferred into engine body.

 

But all that fuel is not combusted (insufficient O2). Even a "lean" mixture on these engines had more fuel then stoichiometrically required for the oxygen content of the air. That is why you frequently see flames at the exhaust stacks.

[blkspade]

But all that fuel is not combusted (insufficient O2). Even a "lean" mixture on these engines had more fuel then stoichiometrically required for the oxygen content of the air. That is why you frequently see flames at the exhaust stacks.

 

That makes even more sense given the mention of non-combusted fuel flowing back to the LH main tank in the P-51.

[Yo-Yo]

But all that fuel is not combusted (insufficient O2). Even a "lean" mixture on these engines had more fuel then stoichiometrically required for the oxygen content of the air. That is why you frequently see flames at the exhaust stacks.

 

What does it change? Maybe I gave not very detailed explanation due to lack of time. The total amount of heat given off for the cycle almost does not depend on mixture strength. The overall effect barely can be seen within several percents of the total heat amount if you move through 0.95 (auto-lean) to 0.6 mixture.

[Yo-Yo]

That makes even more sense given the mention of non-combusted fuel flowing back to the LH main tank in the P-51.

 

From the cylinder??? :)

[NineLine]

What does it change? Maybe I gave not very detailed explanation due to lack of time. The total amount of heat given off for the cycle almost does not depend on mixture strength. The overall effect barely can be seen within several percents of the total heat amount if you move through 0.95 (auto-lean) to 0.6 mixture.

 

 

I did test to see if there was noticeable cooling effects from dooing this, but I didnt notice any, maybe I needed to be at a more critical temp, so maybe I will play with it a little more...

 

That makes even more sense given the mention of non-combusted fuel flowing back to the LH main tank in the P-51.

 

Where was that referenced, seems strange that un-combusted fuel would flow back to the tanks, in fact that seems impossible :)

[Yo-Yo]

I did test to see if there was noticeable cooling effects from dooing this, but I didnt notice any, maybe I needed to be at a more critical temp, so maybe I will play with it a little more...

 

 

 

Where was that referenced, seems strange that un-combusted fuel would flow back to the tanks, in fact that seems impossible :)

 

I think he meant excessive fuel from the carburettor :)

 

Though we use real average curves for heat distributed to oil, water, etc, the effect is very slight, as I wrote before. Maybe more detailed-scale gauge is required and very accurate test. :)

 

But the longer I am looking at the curves and formulas the more I am in doubt that this described effect was not more than a simple placebo... :)

[NoJoe]

Where was that referenced, seems strange that un-combusted fuel would flow back to the tanks, in fact that seems impossible :)

 

I think it's the extra fuel from the fuel pump. The pump supplies more than the engine needs, so extra fuel is routed back to the left main tank. (I think).

 

There are a lot of real-world planes that do this, too. It's the reason why the P-51 manual recommends flying on the left tank first: so there's room for the return fuel. Otherwise you'd overfill the left tank and end up venting fuel overboard. The Cessna 340 has a similar design "feature". ;)

 

--NoJoe

 

[EDIT] Damn, sniped by the man himself! :thumbup:

[NineLine]

I think he meant excessive fuel from the carburettor :)

 

Well either way, if using the primer to cool the engine slightly, unused fuel from that action wouldnt end up back in the tanks, more likely down the side of your plane :)

 

Fuel that never made it to the cylinder... sure I could see that back in the tanks...

[Yo-Yo]

By the way - extra fuel, injected before or into the blower could act as MW-50 for the planes where metered fuel injected directly to the cylinders.

[NineLine]

But this probably wouldn't work as well as water or Methanol (or a mixture of both), correct? (I am assuming because those two liquids wouldnt combust like the fuel, making them better for cooling)

 

By the way - extra fuel, injected before or into the blower could act as MW-50 for the planes where metered fuel injected directly to the cylinders.