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Diesel efficiency and Aircraft Engines in WWII
Ok, this has been playing out a long time, interesting discussion. More fuel for thought.
I think efficiency come from the effective use of the power generated by the expanding gas. A diesel engine has been more efficient due to the fact that the long stroke capable with a diesel engine allows the gas to be harness longer, ie think of the muzzel velocity of a revolver and a rifle. The other factor is the higher temperatures reached and the more complete combustion of the material at the higher temperatures. The down side of this is the NOX produced.
Efficiency in a gas engine is dependent on the surface area of the piston as the expansion is less. A few years ago the Ausies developed a new engine called the split cycle. It is a revolutionary design that used large piston that moved only a few mm. It had efficiencies un attainable with current gas technology. The design would work with diesel or gas.
As an aside the diesel engine cycle can use any fuel of choice including finely powdered coal and gasoline.
During WW II, I think, the gas areo engines were super charged not turbo charged. At least I have not seen any. I don't recall whether the diesel areo engines were turbo, super or not.
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Diesel efficiency and Aircraft Engines in WWII
Indeed, WW II Gasoline aircraft engines were supercharged and in some cases used compound superchargers. Some, the F4U comes to mind, also used a water injection system as an emergency power boost to escape pursuing aircraft.
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Diesel efficiency and Aircraft Engines in WWII
The 3 banked P&W wasp I think had only one supercharger (AKA F4U engine). It has been nearly 20 years since I looked at one.
I don't know of an WWII engine with 2 stage. I thought maybe the Napier Sabre, but the 2 stage was never implemented despite 4000 hp out of 2238 cu in. The Sabre is a sight if you have never seen one. It is hard to believe they could develop such an engine in war. It certainly had some teething.
I guess the later Griffons had 2 stage.
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Diesel efficiency and Aircraft Engines in WWII
I think one of the objects of boosts in aircraft engines is allowing them to operate at higher altitudes and not simply power boosts under heavy throttle. I believe that's a better application for superchargers.
As far as I know, Spitfires used vapour phase rather than liquid cooling. The more efficient heat exchange allowed small rads, which improved the aerodynamics. I seem to recall that the engines also were able to operate at higher temperatures than liquid cooled engines. A basic idea from thermodynamics is that the basic efficiency of an engine is determined by the ratio of external to internal temperatures. Thermo is something I know little about, so maybe I'll learn something by throwing in this comment.
I always thought that diesels were long stroke engines too. I figured there may be an advantage to the long stroke since injectors continue spraying following combustion. That would be the 'long push' rifle idea. I was surprised to find that my 1710 engine is square (bore equal to stroke) and so is my gas 1/2 ton. Don’t know, maybe I'll have to think some of this through again.
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Diesel efficiency and Aircraft Engines in WWII
Super chargers require energy a turbo is running on waste energy. I don't believe there was the turbo technology during WWII. The ability to keep a fan running in hot exhaust gasses took a while even for the auto. A seized turbo when flying would be.........!!!!!! Maybe the Germans Diesel had it?
The Rolls Royce Merlin engine used water and 30% glycol mixture at greater than 18 PSI. Yes engine temperatures would dictate that the fluid would be in liquid form at sea level pressure or even more likely at 30,000 ft, but not at that pressure.
The same engine, the Merlin was manufatured by Packard during the war although the parts were not fully interchangable. Something to do with the difference in standard inch.
The Merlin was on the Spitfires, Hurricanes, P-51, Mosquito, Lancaster, Halifax, etc.
The Griffon was larger displacement but was never developed as much as the Merlin so never produced as much HP.
Diesel engine manufactures have gone to short strokes and a precombustion chamber (so have some gas engines). Volume in the smaller combustion area is less than the full surface of the piston therefore allowing similar compression rates with out the longer stroke. This allows the higher RPM engine and greater power to weight. The chamber can be designed to give better mixing and ignition of the fuel, therefore cleaner burn.
Limitation in materials is something that has plagued the internal combustion engine. Better burn could be achieved if hotter temperatures could be reached in the combustion chamber. Much of the energy of the combustion is lost in waste heat. Ceramic materials have shown promise but cost and manufacturing consistancy have kept them from use.
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Diesel efficiency and Aircraft Engines in WWII
Peters: Full of good info as usual. It did clear up my mystery as to why my diesel seemed to be a short stroke engine. I had no idea that the P51, Lancaster and Spitfire engines were basically the same. Despite my interest, I'll try to stick to things that are vaguely tractor related, but maybe with a slight lapse. I worked with a Lancaster pilot in Hamilton, Ontario. He was part of a club that restored a Lancaster and maybe had the only flying one in N.A. I believe it later appeared in some shows. Unfortunately, we never made the connection to go out to the airport to give me a tour during the restoration.
My impression is that boost in superchargers is limited by engine rpm while in turbos it's limited by exhaust gas pressure (throttle). So, I was wondering about operation at high altitude and low rpm? I thought one of the purposes of boosts on aircraft engines was so the engine 'sees' a low enough altitude for the engine to run. I am aware that this may not be an issue for aircraft engines that unlike racing engines come close to running constant rpm's. Even so, I'm sure a turbo could be built for the worst case and excess boost at higher throttle positions could be bypassed. I also doubt that there was a turbo alternative available in the '30's.
Granted, my old high-school notion of putting on dual exhausts and glass pack mufflers to gain power by ‘reducing back pressure’ was entirely naive. Did make a nice sound though--at least to a high schooler. However, the power to run a turbo may not be entirely free.
My impression about operating temperature and efficiency is that power from an engine is related to the expansion of gases. In theory, a hotter engine should produce greater power than a cooler one because the final expansion should be greater. Of course, all sorts of issues such as pre-combustion undoubtedly get in the way of theory. I suspect that an engine running on ether could not produce much power because it couldn't get very hot.
Oh yes, the idea of a difference in standard pitch does make a good oxymoron.
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Diesel efficiency and Aircraft Engines in WWII
Peters, the water injection I spoke of on the F4U was not for cooling but was injected into the cylinders as an emergency power boost. Several times it saved pilot caught on the deck with 'zekes' on their tail. By all accounts the engines were trashed after this was used. I wiil have to look in my corsair pilots handbook, but I remember the engine as a two row radial with 18 cylinders and 2100 horses. The charger I think was a single stage. I remember the two stage units on B-29's and maybe the F8F and certainly it was used on post war airliners like the DC6.
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Diesel efficiency and Aircraft Engines in WWII
Tom;
Both a supercharged engine and a turbo engine will produce more HP per lb. The difference is that the turbo uses less energy than the supercharger and at some combinations or RPM and pressure it is essentually free.
Turbos are used on aircraft engines today, but the piston powered aircraft does not normally fly at the high altitudes and the boost is sufficient from the turbo.
Mark
The F4U had a 3 banked of 6 I believe. The only time I have encountered this engine was on the Martin Mars. I worked mostly with the smaller singles on the Beavers and Otters.
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Diesel efficiency and Aircraft Engines in WWII
My Dad was a Corsair mechanic at the end of the war, and still has all of his manuals. I haven't looked at them for quite sometime, but one thing that always amazed me was the engine layout. They had 3 banks of 9 cylinders. Each cylinder had 2 spark plugs (for a total of 72 plugs!). They also carried 50 gals(!) of oil in the sump. Water injection was used for extra power when needed, but was used sparingly as it really raised heck with the engine. Dad always said the pilots really loved (and were thankful for)the kick in the pants the injection gave them!
Corm
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Diesel efficiency and Aircraft Engines in WWII
From several sources on the internet, gasoline ~125,000 BTU and diesel ~140,000 BTU.
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