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Diesel efficiency and Aircraft Engines in WWII
The comment finally motivated me to find the fabled Nebraska Tractor Tests. The address is: http://www.ianr.unl.edu/pubs/FarmPower/g579.htm
The page describes some of the testing procedures, and on-line Acrobat publications are available. The intent is to provide Ag tractor owners a source of unbiased performance measures. Too bad the testing doesn't include compact tractors.
There was a recent thread here where weight was discussed. One way of thinking is that many tractors are manufactured heavier than necessary for maximum performance. No conclusions, but it does take HP to move ballast around. But, there is no question no question that a tractor has to be ballasted properly to do the work.
Several less recent treads were into diesel engine theory. Again, no conclusions, but it is possible to build gas engines with torque characteristics similar to diesels. However, gas engines have advantages over diesels at higher RPM's. I guess an attitude of an engineer might be: Why try to build a gas engine to compete with what diesels do better? Besides diesel fuel has been cheaper than gas--at least until recently.
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Diesel efficiency and Aircraft Engines in WWII
I'm making what I think are some add-on comments rather than differing with any point. Maybe some interest in diesel engine theory will be ignited--sparked or compressed as the case may be. I claim to be learning rather than to have any particular expertise.
First is that diesel efficiency in terms of fuel/gallon per hour belongs to diesels--no contest. However, I believe that #2 fuel contains more energy (BTU’s) than gasoline, so I don't know about comparisons in terms of energy efficiency.
Heat, along with reciprocating motion etc. are sources of parasitic energy losses and serve to decrease efficiency. Both engines have high losses, and I don't know if a diesel’s losses are appreciable less. I do know that a diesel looses enough to make co-generators efficient in fairly large applications. A co-generator uses an engine (usually diesel) to run a generator and also recovers heat through an exchanger.
I also know there are large low RPM stationary gasoline engines that run almost constantly for years. They are tuned to produce high torque at low RPM's but probably aren't as efficient as diesels.
I think the main difference contributing to diesel efficiency is the much higher compression ratio. In my teens, a 'poor-man's' hot rod was simply milling the heads to increase the compression ration. It takes more energy to achieve higher compression, but you more than get it back when a fuel/air charge is ignited in the smaller volume. Gas engines simply don't work at all at anywhere near diesel compression ratios.
A bunch more could be said, but I better save more talking about what I don't completely know for another day. For example, ignition in a gas engine is a one-time ‘spark ‘n bang’ while diesel injectors continue spraying after ignition.
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Diesel efficiency and Aircraft Engines in WWII
I think I've seen gas diesel BTU comparisons, maybe in the archives here. If nothing pops up I'll put some time into a search, although I too doubt that a gas engine is going to turn out more efficient.
It's a little difficult to apply these torque and HP comparisons. I try to keep in mind that one can be calculated from the other. Since 1 HP = 550 ft lbs. per second, torque equals HP divided by rpm times a constant that allows force to be expressed in ft. lbs. and time in RPM's. Basically, torque is a static force concept and HP is a force over time concept. Hope I'm right. I'm more than a little out of practice with concepts and formulae myself, but the basic idea is that one big bang every so often or a lot of little bangs produces the same HP.
Running a small multi-cylinder engine at high RPM is a good way to get high HP without having high max torque. However, the engine has to be tuned so that torque doesn’t fall off at high RPM’s, or HP may decrease rather than increase. A racing gas engine usually is designed to produce relatively high torque at higher RPM and within a narrow range. Such designs also have the effect of producing very poor torque at low RPM. You get a very high HP engine for it's displacement, but it barely runs below 3000 rpm or so. The trouble with using such an engine is that you have to get the engine from idle to high rpm before it works. Slipping the clutch or sloppy torque converters are OK for race cars, but are maybe not so good for tractors.
I that what I used to call a 'torquey' engine as one that is tuned to produce its max torque at lower RPM's. It gets you off the ground good but doesn't get you going faster when higher RPM's are needed. In high gear, the only way to go faster is with higher engine RPM's, and torque starts dropping off as RPM's increase in a torquey engine. So, what do I think? I think diesels probably have an advantage over gas engines at producing torque at low RPM and maybe we'll get into some of the reasons.
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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
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
Thanks Peters. Makes sense to me. As is often the case, I'm testing my impressions here against definite experience and knowledge. I recall hearing that the B29, along with the Norton Bomb Sight enabled strategic bombing. To do that, the plane had to fly above ground fire and it was the supercharged engines that enabled planes to fly that high. Of course, this is just a story I heard lord knows when.
My father worked as a mechanic at Lockheed building B17's under 'Lend Lease.' I never heard mention if they were supercharged, but I don't think they had the altitude capability. I also seem to recall that a somewhat modified B29 engine was used on DC7's. The 7's performance was a huge leap over the DC6. I don't know if 6's were supercharged. Too bad that neither my father's manuals nor most of his tools survived family moves after he started flying instead of fixing.
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Diesel efficiency and Aircraft Engines in WWII
It was so cold here this morning that transports on the highway seemed to be leaving con trails, but that’s just my way of griping about the weather. Yes, I think the altitude notes are true. I didn't know that B17's flew that high, but I do know something about altitude. What was billed as the highest mountain highway in the world is in Colorado. I forget which peak it went to, but I think it was higher than 12,000. I know that my '53 Chevy drove there and I ran from the information building to the summit. However, neither of us were working all that well even though both the Chevy and I had high altitude jets.
I went through a military altitude school. I think 12,000' was a mil spec for when pilots start receiving OX. By similar reasoning, performance engines probably need it as well.
Most people need the OX at 12,000' to fly a plane reliably. We survived OK at 14,000' although I could have used some manifold boost myself. At 25,000' I had about 20 seconds of useful consciousness. We filled out a short quiz. The last question, which I missed, was signing the quiz. At 36,000 the partial pressure of OX isn't sufficient to get enough OX across the lung membranes, and masks start feeding pressure. Pressure breathing is strange because the mask blows you up when you relax. You have to learn to talk with full lungs. Pressure breathing is sort of a manifold boost. I guess it works within limits for both people and planes.
Pressure breathing works to 46,000' after which pressure suits are required. At 45,000' a Kleenex can be thrown across the chamber like a baseball and everybody passes gas but nobody can hear it. Fighter pilots eat bland diets when on duty. I think B29's depressurized when entering hostile areas. I've been through explosive decompression and it would really mess up flying a plane. Huge bang and the entire chamber fills with a dense fog. Air comes out your mouth so fast your lips flap. You've got about 15 seconds to get a mask on and you can’t really see it. I don't imagine many airline passengers would get those safety masks on under these conditions.
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