Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: hazed- on October 07, 2002, 11:30:45 AM
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Just seen a program called coltrains planes and Automobiles :
and i'll quote an old guy (spit pilot) who said this about the Spitfires gravity fed carberetta's when you nose down and the engine cuts out:
First of all we admired the principle of the fuel injection because we had carburettas on the merlin, and one of the weaknesses of that particular system was when the aircraft dived the engine cut out, due to negative G the fuel was thrown up to the top of the chamber and the engine cut out.This was a bit of a panic at the time and we had to overcome that problem, but theres another factor which you may not know about, The evaporation of the fuel drops the temprature by 25 degrees, that raises the pressure ratio of the compressor, or the blower to your advantage.So you get that gain with the carburator system that you do not have with the injection system'
Now what on earth does he mean here?
anyone out there understand what he means and have some clue as to how/or in what way this might help?
Does this mean that with gravity fed carbs you could touch on negative G cut for a split second and gain an extra boost of power due to the effects described?? hehe probably not but its the first thing i thought of :D
VERY interesting :D
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The evaporation has nothing to do with neg-g cutout, the function of the carburetor is to evaporate the fuel into a fuel-air mixture that is combustible. The effect what he's talking about must be the cooling effect of the fuel mixture in the compressed air vs normal airfeed of the direct injection engines. This difference is very easy to remedy with an intercooler though.
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That's a great program; I have it on tape! :)
What he means is this:
When fuel is changed from a liquid to a vapour state, it takes energy (heat) to do so. This energy it obtains from the ambient air. In effect, liquid high octane fuel finely divided into droplets sucks the heat out it's surroundings to vapourise, resulting in a colder, denser atmosphere.
The colder (and there fore denser) your intake charge is; the more fuel/air you're squeezing in each cycle; and the bigger the resulting bang.
It has nothing to do with gravity feed nor negative G cut out. The guy (not, I beleive a Spit pilot; but a former Rolls Royce engineer) is just acknowledging a design drawback, and counter balancing it with some design advantages.
I found it fascinating the way the engineers and fans of both engines found differing ways to wring every horse power they could out of their designs, and the differeing approaches they made.
Really good television.
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This was a bit of a panic at the time and we had to overcome that problem, but theres another factor which you may not know about, The evaporation of the fuel drops the temprature by 25 degrees, that raises the pressure ratio of the compressor, or the blower to your advantage.So you get that gain with the carburator system that you do not have with the injection system'
Now what on earth does he mean here?
With a carburetted fuel system, the air from the air intake passes through a carburetor -- a device that sprays a fine mist of fuel into the incoming air, and the resulting fuel/air mixture enters the cylinder through the intake valve. In a fuel-injected engine, the air is ducted into the cylinder through the intake valve, and after the intake valve is closed, an injector nozzle sprays a fine mist of fuel directly into the cylinder. For purposes of fuel delivery, it's kind of like having a carburetor in each cylinder.
Fuel injection allows the amount of fuel in each cylinder to be more accurately controlled, resulting in better fuel economy (the fuel in each cylinder is burned more completely) and (for automobilies) a slight increase in power over a carburetted engine.
However, in aircraft, the amount of air entering the engine is much greater. With an aircraft engine, you control the amount of air entering the engine by controlling the manifold pressure -- the pressure of the air (or air/fuel mixture, for carburetted engines) being fed into the cylinders. And this is where caruburetion gets its advantage.
Take a spray bottle full of water and spray some water into the air in front of you, then walk into the mist. The water is at room temperature, the air in the room is at room temperature, but the misted air feels cooler to you. When you spray a liquid into the air, some of the liquid evaporates, sucking heat out of the air, making it cooler. And cooler air is denser, meaning that there are more air molecules in a given volume. So when you have a carburetted engine, each cylinder's worth of fuel/air mixture will have more fuel and air in it than a fuel-injected cylinder would at the same manifold pressure.
For example, let's assume that the air coming through the manifold is at a temperature of 80° C after it passes through the compressor of both a fuel-injected and carburetted engine. If the fuel injection drops the temperature of the intake air 25° C, then the intake temperature of the carburetted engine will be 55° C. Boyle's Law shows that the intake air for the carburetted engine will have 8% more air mass than the injected engine -- the equivalent of going from, say, 30" of manifold pressure to 32", without stressing the engine. The carburetted engine develops more power at the same manifold pressure than a fuel-injected engine of the same displacement.
So in order to get the same power out of a fuel-injected engine, it has to be larger and heavier than a carburetted engine. Not a lot heavier and larger, but with a larger engine, the airframe has to be larger and sturdier, which gives a plane with a carburetted engine an edge.
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Or of course the same boost could be had for a fuel injected engine with a bit of water injection added. Still its more complication and weight and power required to do it so you still have a penalty for playing catch up.
Has nobody ever noticed that under the correct damp conditions your car suddenly seems to have a power boost ?
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The advantage of the carburated supercharger over the injected one depends on where the carburator is placed. The same applies to the injector. The placement depends on wheter it is before or after the air is compressed by the supercharger or turbocharger. The advantage mentioned in that persons comment is this.
The turbo or super charger compresses air. This adds heat to it. Hot air has less oxygen by volume as it expands the molecules and spreads them and it also adds to the cooling problem for the engine. Adding an intercooler does help but it also adds ducting and resistance to the aircharge on it's way to the engine. More resistance means less efficiency.
Placement of the injector or carburator comes into play here. If you place the carb before the compressor the entire charge and fuel are heated and the engine gets hot air/fuel. If you place the carb AFTER the compresor the fuel helps to cool the incomiong air by evaporation of the fuel. The gasoline absorbs some of the heat and the engine runs cooler requiring less fuel to help keep the engine cool. Yes, a rich mixture is one way to cool an aircraft engine.
Some engines had a single injector, instead of one for every cylinder like most injected cars today. You still got the advantage of a system without a carburator as the fuel was sprayed (injected) into the manifold for vaporization before the air charge got to the cylinders. There would be no cut out as there would be no carb floats to affect by a negative G maneuver. Fuel is delivered under presure from the tank to the injector nozzle. You can see here again where the placement of the injector in relation to the compressor could aid in cooling the intake air.
The power advantage of the compressor has to overcome the power needed to run the compressor before it can boost the power output of the engine. At some point a reciprocating engine reaches a point of diminishing returns in that it cannot compress enough air due to the lack of air density at altitude and it runs out of boosting ability for the manifold. Depending on the design, (turbo, turbo supercharger, mechanical supercharger, single or multiple stage) you get a differant amount of power advantage to differing altitudes. All of them would be superior to a normally asperated engine alone irregardless of whether they were injected or carburated.
Does this answer your question? Sorry for the lecture but my class in A&P school just went through this part of power plants. :)
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Hi everyone,
I just looked up the world's first series production aircraft engine with fuel injection, the Jumo 210G. For our discussion, it's particularly interesting since it was derived from a carburetted engine.
Well, the Jumo 210G (which appeared 1937) has 730 HP take-off power compared to the Jumo 210Da/Ea's 720 HP (1936). Engine (dry) mass is up to 425 kg from 422 kg.
The greatest difference is neither power nor weight, but optimum specific fuel consumption - the fuel-injected Jumo 210G is down to 216 g/PSh from 235 g/PSh for the carburetted engine.
(Figures from von Gersdorff et al.)
Regards,
Henning (HoHun)
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Fantastic explanations! every one as i read them made it all clearer and clearer!
I cant believe it but AH may well have started me on yet another subject for intense reading! lol This game is amazing.I used to read about aircraft in WW2 but until I played AH and started to understand the real principles behind air combat maneuvers I really didnt 'read it' if you know what I meanEvery book I had read suddenly had something new to tell me.
This programme has piqued my interest and your thorough and I have to say eloquent explanations have made me even more interested in the dynamics behind what really goes on inside these engines.
Its obvious what im going to ask for now isnt it hehe
come on guys whats the best book to start out with? preferably one that has constant referals to WW2 engines?
:D thnx again
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Best guy to ask about Allisons (P38, P39, early Mustang) is Hilts.
He knows his stuff, and he's not (too) biased.
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Originally posted by hazed-
come on guys whats the best book to start out with? preferably one that has constant referals to WW2 engines?
:D thnx again
You can try
"Allied Aircraft Piston Engines of WW2"
by Graham White for starters.
R-R, Napier, Bristol, GE, Allison, P&W, Wright, hypers
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White's book is great. He's a degreed engineer and he has rebuilt and restored and operated most of the engines in the book.