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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
And don't forget, as a general rule diesels are more fuel efficient than gas engines
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Diesel efficiency and Aircraft Engines in WWII
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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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
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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Diesel efficiency and Aircraft Engines in WWII
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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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
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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Diesel efficiency and Aircraft Engines in WWII
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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Diesel efficiency and Aircraft Engines in WWII
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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