Primer Cooling Trick

The difference is that compustion fuel is metered into the intake airstream prior to the supercharger. It should be throughly vaporized prior reaching the cylinders. Primer fuel, however, is injected directly into the intake manifold shortly before the cylinder. With sufficient quantity some of that fuel will still be in droplet form prior to combustion so it will contribute cooling from vaporization but will not be combusted.

Do you think that the atomised fuel injected into hot streaming air in the manifold will not vapour?

What about automotive engines that sprays metered fuel into the manifold?

What is the difference between fuel injected directly to the cylinder and the fuel injected into the manifold before the cylinder? Where the fuel will have more time to evaporate?

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)

The cooling is effective only before combustion - the mixture has less temperature before it is compressed in the cylinder and thus it begins to burn and not to detonate (high speed burning).

Water and methanol have significantly higher specific evapouration heat - 2200 and 1100 kJ/kg as fuel has only 280. That's the difference... :)

Well, I did say "with sufficient quantity". Is the P51 primer connection a high pressure atomizing injector nozzle? I doubt it (in fact I doubt there was a nozzle at all), so not really a fair comparison and also the difference you asked about.

 

In the absence of other hard data I would have to give the benefit of the doubt to the pilot. He, after all, could observe first hand if his running the primer had an effect on engine temp and performance. Whereas debating it 70 years after the fact with no test bed in front of us is kind of pointless.

There is a nice thing named "physics". It can calculate or estimate many things human's common sense can not explain or predict.

Sometimes pilots have wrong notions - a pilot in Russia (very experienced) sincerely believed that flying MiG-29 it's necessary to extend flaps to have longer glide distance with dead engines.

Sometimes their misbeliefes are harmless like primer injection...

And there is a nice thing called "testing" engineers relay upon because physics and the assumptions made to make it usable can not explain or predict all real world observed phenomena. Physics is great for getting you in the ball park though.

 

I assume you having no physical practical experience testing this particular scenario and therefore no basis for describing the primer squirting (probably a more accurate word then injecting) as a misbelief. It may not conform to your biases based on your knowledge. You do seem to have a good handle on the chemistry and theory of engines. But without testing I wouldn't be so quick to dismiss this experiment.

Maybe Nick Grey knows :)

And there is a nice thing called "testing" engineers relay upon because physics and the assumptions made to make it usable can not explain or predict all real world observed phenomena. Physics is great for getting you in the ball park though.

 

I assume you having no physical practical experience testing this particular scenario and therefore no basis for describing the primer squirting (probably a more accurate word then injecting) as a misbelief. It may not conform to your biases based on your knowledge of theory. You do seem to have a good handle on the chemistry and theory of engines. But without testing I wouldn't be so quick to dismiss this experiment.

Ok, do you need a test if somebody can cool 1 kg of boiling water to 20C using only a little piece of ice?

I am always amased how much money and efforts spend Mythbusters or similar groups sometimes to test things that can be easily calculated and are obvious if you know physics or chemistry, whatever.

Just look at the numbers: for example, a carburettor injected 1.2 kg of fuel. 1 kg reacted in the cylinders giving 43 000 kJ of heat. 200 grams after evaporating can take only 0.2 * 280 = 56 (!) kJ.

As normal mixture strength ratio is about 0.8 you can add not more than 0.8 kg through primer jets, otherwise the total ratio will be too rich to ignite and burn. THus, you can not have cooling capacity more than 0.8*280 + 56 = 280 kJ.

Adding liquid fuel heat capacity (approx 2 kJ/kg/C) and presuming 100C heat we can add approx 200 kJ.

And overall it would be about 1% of heat that fuel combustion gave out. Do you still ask for a test?

For the vapor return line see pg200

http://aviationshoppe.com/manuals/engine_technical_manual/merlin_66_67_70_71_76_77_85.html

Is that for a different engine, and didnt the US built Merlin use a different supercharger, so there could be other differences, but honestly I dont know enough about the differences to say....

EDIT: Just at a quick glance the Merlin the the P-51D was the 69, or US designation V-1650-7, so technically not covered by that manual for whatever that is worth.

Honestly we are probably at the point of over thinking everything, it was used to nurse home a wounded bird, the degree it helped is probably not going to be discovered here ;)

But thanks to Merlin for the interesting read!

Of course, from another of my "inquiring minds want to know threads" lol

Fuel Vapor Return Thread

 

It's in the P-51 manual somewhere.

 

EDIT: Found it...

Good find, although I guess I misunderstood what he was saying, I thought he was saying unused fuel in the cylinder would be returned to the tank....

Interesting. Good analysis. Now let me attack some assumptions.

 

Mixture ratio assumes vaporized fuel, correct? So your assuming all the fuel pump through the priming system is vaporized. A test might be needed to confirm that.

 

THere is no logic in your statement - if fuel is not vaporised it will not absorb heat... no magic, :). If it's vaporised it absorbs heat but shifts mixture strength towards end of flammability.

 

Second assumption that the fuel/air charge is uniform and heat is evenly distributed. The hot gases exiting the exhaust valve aren't really our concern it is the hot gases in contact with the other engine components. Is there a surface effect in play that an excess of fuel might cause to be cooler then expected? Again testing.

As we consider overheating of the engine through the temperature of coolant or oil it has no sense. Actually, having improper timing or mixture (too lean) you can gain local overheat, for example, valves, but overall temperature balance will not suffer.

 

A question that pesters me though is, could he have successfully achieved not excessively overheating the engine with throttle, rpm and mixture control alone. Something that would probably have to be tested out in advanced to find what combination minimized overheat yet produced enough power to sustain flight. Or even did he temporarily prevent combustion with an overly rich mixture thereby cooling the cylinder with the un-ignited charge. In which case would blipping the ignition have the same effect without expending so much fuel. Interesting questions that you would think the USAAF would expend a quite a few Merlins finding the answer to rather then losing planes and pilots that may have made it back if they had an answer.

It's a good idea but the same effect you can get simply reducing MP to get the same average power or simly turn off ignition... Anyway, any games with unburnt fuel is cylinders have a big riskof spark plug fouling...

 

It's a good idea but the same effect you can get simply reducing MP to get the same average power or simly turn off ignition... Anyway, any games with unburnt fuel is cylinders have a big riskof spark plug fouling...

Well thats probably the bottomline on the subject right? I mean if you use the primer for an extended period of time in flight with the engine running, chances are the engine is going to run like a dog, and obviously it was only used to nurse the plane back home, its not like they all of a sudden decided that now in a dog fight they could start priming the engine to get some extra performance. And lets be honest, looking at the situation from the outside, the theory it could have been some what of a placebo effect is very realistic....

All the manual says is that in case of coolant loss expect engine lockup within 10 minutes.

So the answer is no, its not very effective :)