Go BottomDiesel efficiency and Aircraft Engines in WWII
 | TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley 
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2002-01-30 35087
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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| MarkS
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2002-01-30 35106
And don't forget, as a general rule diesels are more fuel efficient than gas engines ....
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| BCalvin,Texas
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2002-02-01 35147
There is no question that a gasoline engine could and has been a significant contributor to the american and for that matter the world agricultural community. However, both engine as we are all aware operated totally on different priciples.
The advantage of the diesel is the low flashpoint for safety the low rpm/torque characteristics and the fact that they thrive on heat.....running hours on end at very low speeds as we all know generates tremendous heat through the whole machine....heat bring us friction and wear...hence the gasoline engine is not efficient with today demands on farm equipt.
At todays costs the gasoline vs. diesel per gallon cost is no longer a bargain but longevity is the trade off. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-02-01 35150
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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| MarkS
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2002-02-01 35153
Any Mechanical engineers out there who spend alot of time calculating efficiencies of engines? Its been too long since my Thermo or Heat transfer couses to remember the exact equations etc, but I do know that the Compression Ratio of the engine has a ton to do with how efficient it is. Therefore the diesel does better than gasoline (i.e. MOST gas engines are anywhere from 8-10:1, racing engines are much higher, while diesels are somewhere in the neighborhood of 20:1 for compresion ratio) unfortunately double the compression ratio does not=double the efficiency.
TomG, it would be interesting to know what a gallon of diesel can produce in BTU's vs. a Gallon of gasoline. I had never put much thought into that, but if diesel is much higher we all might be a little embarassed and find out gasoline engines are more efficient, but I doubt it. ....
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| BCalvin,Texas
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2002-02-02 35200
Man we have really got one going here now...I think we are all on the same page but the question in TOURQUE....if you examine the hp/torque ban on deisel engine ant a given HP vs. gasoline you will answer the farm tractor question. In gasoline engines as our hot rod days it was torque that we where able to apply to the concrete that made us the fastest but we needed high HP to acheive that mission of course the high HP then gave us the top end to win overall.
Farm equipt. Tractor trailers pull a lot of weight and to move the weight they need a lot of torque but the engines must met thst power ban at a low HP...ie. Cummins 6cyl diesel 245hp@320 lbs.torque...etc.
Gasoline farm tractors used Bull Gear systems in the final drives at the wheels to generate additional torque to move the loads these bull gears where another source of breakage and maintaince. Again deisel has a low flashpoint you know kerosen and is safer in day to day operations...have you seen a gasoline JET??? ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-02-02 35205
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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| BCalvin,Texas
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2002-02-02 35210
This is really getting exciting now....you know I have forgotten totally what the orginal mission was...but I enjoy the discussion ....
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| Jim Nation
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2002-02-02 35220
The biggest source for the diesel's higher efficiency is the lack of throttling losses, particularly at lower loads. In a gasoline engine there is a pressure loss across the throttle plate of up to 18" Hg. This results in less energy use in pumping or sucking the air in. Some of this advantage is lost in the higher exhaust losses of the diesel. This is also part of the reason a turbo is so much more effective on the diesel than the equivalent gas engine. The next most important is the higher compression ratio. ....
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| BCalvin, Texas
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2002-02-03 35257
Jim I agree with your assesment another important factor in comparing the two modes of power as applied to the farm scene is sea level.......As I am sure many of us have forgotten but certainly not the manufaterers of heavy equipt. a deisel engine shows no apprecible power band loss at higher altitudes but gaslone must be tweeked to maximize the thinning of the air it must have to burn cleanly. Again that was the ultimate downfall of the piston driven aircraft high tec combustion engines could not get the aircraft to the new operating ceilings that where demanded of them hence the turbo and then the jet...
This discussion has been very interesting I am not real sure we helped out the guy who had the orginal question though. ....
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 | Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-03-02 35996
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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 | DRankin
Join Date: Jan 2000 Posts: 5116 Northern Nevada
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2002-03-02 36013
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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| Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-03-02 36016
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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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-03-03 36022
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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| Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-03-03 36026
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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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-03-04 36044
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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| DRankin
Join Date: Jan 2000 Posts: 5116 Northern Nevada
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2002-03-04 36052
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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| Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-03-04 36055
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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| Corm
Join Date: Feb 2002 Posts: 7 Northern Vt
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2002-03-04 36064
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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| Norm
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2002-03-04 36082
From several sources on the internet, gasoline ~125,000 BTU and diesel ~140,000 BTU. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-03-05 36095
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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| DRankin
Join Date: Jan 2000 Posts: 5116 Northern Nevada
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2002-03-05 36104
Tom, people need supplemental o2 above 12,000 feet and so do air breathing engines. It is a safe bet that any piston driven aircraft with an operational envelope or a service ceiling over 14 or 15 thousand feet has some sort of manifold boost. Another clue is whether or not the aircraft produces a contrail. In the case of the early WW II bombers (B17,B24,25&26)all had non-pressuized hulls and operated in excess of 25,000 feet. Thats why they had to wrap those boys in sheepskin and why they were always wearing o2 masks. In fact the B-17 had large open windows on the side out of which the waist gunners operated. There was no provision to close these openings. The B-29 was pressurized and I believe it drew its cabin pressure from the supercharger output of a selected engine. If that engine lost power they had to resort to o2 or fly below 12,000 feet. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-03-05 36109
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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