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Torque ratings of diesel tractor motors

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greg
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2001-06-09          29098

The brochures always talk about the horse power of the models, what are the torque numbers? More specifically, how many foot pounds of torque does my 20hp Cub Cadet 7205 have at max rpm?

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jb85302
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2001-06-09          29118

I have wondered the same thing. Why don't manufactures post torque ratings for thier engines? I have some friends that when I told them I bought a 19 HP diesel, they thought I bought a small tractor....they said they had a 22hp Sears....uh....yeah...right. Definately 2 different animals!! Dealers are always trying new ways to sell their product, I'm sure if it was benificial they'd do it.
Jeff ....

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TomG
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2001-06-11          29144

I think HP to torque takes a simple conversion formula. Something like: torque equals HP/RPM times some constant. Maybe somebody has the formula in mind. If not, I'll see if I can figure it out. ....

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Rob Munach
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2001-06-12          29173

HP = Torque*2*PI*RPM/33,000 or Torque = HP*33,000/(2*PI*RPM) (PI = 3.14) So knowing the HP at a particular RPM, you can calculate the Torque at that RPM. If the engine has a relatively flat Torque curve, than the Torque you calculate will probably be close to the max torque of the engine. However, if you have a peaky engine, the Torque produced at max HP will be lower than the max Torque at a lower RPM. I knew this Engineering degree was good for something :> ....

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Murf
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2001-06-12          29187

Actually if you're going to go to all the trouble of doing the math, use the long version of Pi, 3.1415926535897932384626433832795, otherwise the rounding off will affect the outcome. Actually I use my Engineering degree all the time, the glass over the certificate makes a perfect mirror to see what is going on in the front office, LOL. ....

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TomG
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2001-06-13          29221

For a calculating formula, I'd use something like (HP*5251.9337)/RPM. I suppose that a text book formula uses PI explicitly to illustrate a principal. Something like RPM goes around in circles. Don't know what principal the 33000 illustrates. Should have been an engineer then I'd know principals, but guess applied math degrees find an occasional use too. However, it's more likely that it just illustrates the ways I spend my time first thing in the morning during these few weeks around here when most stinging insects known to man are out working. ....

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greg
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2001-06-14          29266

So three of you have found the formulas, which one of you wants to do the math? Remember, my question is: 20 hp. at 2500 rpm equals how many foot pounds of torque? thanks, Greg ....

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jb85302
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2001-06-14          29268

Looks like about 42ftlbs@2500rpm ....

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TomG
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2001-06-15          29304

Gee, actually applying formulas usually was my role. Guess I'm not too sorry to be a little late. It gives me opportunity to say that the relation between torque and HP shouldn't be too surprising since HP is defined as 1 hp = 550 ft. lbs. per second. Torque is a static, and HP is a dynamic measure of about the same thing. Both measures have their limitation. In HP, for example, one big bang per second, or a bunch of little bangs equals the same HP, but the torque would be very different, and that can make a lot of difference to how an engine operates. ....

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Norm
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2001-06-15          29306

I'd like to get down to the fundamentals. I know the equations but still don't understand why a 20 hp gas engine has less torque than a 20 hp diesel. I know how they each work, but I don't really understand the diffence in their operational characteristics. Can someone please explain? thanks. ....

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mlmartin
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2001-06-15          29315

The answer to your question is buried in the formula. RPM is the divisor. The lower the RPM required for any given horsepower the greater the torque. Since diesel engines tend to develop maximum horsepower at lower RPM than gasoline engines they tend to develop more torque. 20 HP at 2000 RPM is 52.5 ft lbs of torque while 20 HP at 4000 RPM is 26.25 ft lbs of torque (example exaggerated for clarity). Fuel type is not the determining factor. My wife's car's 5 cylinder turbocharged 2.2 liter gasoline engine develops a maximum of 234 ft lbs of torque at 1900 RPM which is generated by 85 HP. The maximum horsepower (217 HP) is generated at about 6000 RPM which yields 189 ft lbs of torque. A governor on that at 2200 RPM would make it a great tractor engine. Used at 1900 RPM, it would probably last forever (it only has 198,000 miles on it now) and it would be pretty economical, too. --

Matthew



If horsepower goes up linearly with RPM, the torque curve will be flat. ....

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Murf
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2001-06-15          29316

MLM, you raise an interesting point to ponder, I suppose the end result seems to be... applied mathematicians are academics (it only has to work on paper) and engineers are practicing (they have to make it work in real life) therefore we can draw this conclusion, from an analytical point of view the wife's car engine probably WOULD make a dandy stationary power plant, HOWEVER, from a PRACTICAL point of view, by the time you bought her a new car to replace the one you dis-membered, it would NOT be economical.... No disrespect intended Tom, I don't mean to pick on you, I will leave that up to the 'Mozzies.... And finally after all that, the other variable in the picture is the fact that an average gas engine runs at about 8:1 compression while the average diesel runs about 22:1 compression, thus the air/fuel mixture creates a far more violent (read powerful) detonation. Best of luck. ....

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TomG
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2001-06-16          29329

Murf: since you know something about my environment, it's a safe assumption that, after giving the bugs their due, I am incapable of feeling picked on by anything. Well dang, we’ve put up with three days of humid polluted air from the prosperous south at the same time the white pines here are pollinating, and the black fly still isn’t quite finished off. Mosquitoes, sand fly, deer fly and others are alive and well also. About engines: Bits and pieces of this discussion should be in the archives. I believe I gained an understanding that compression ratio is the main difference in engine performance. A diesel should have a smaller displacement than an equivalent HP gas engine. I'm not sure that there is any inherent reason why gas engines develop less torque at low RPM's than diesels. As far as I know high torque/low RPM gas engines are used. It's a matter of how they're tuned--length of intake and exhaust manifolds etc. For most applications, I think it's just efficiency. Since a gas engine will run at high RPM's, why not tune it to develop max torque there and take advantage of the HP gains? I think the situation is that a gas engine can't run at the compression ratios of a diesel, so why design a gas engine to compete with a diesel? I don't know if a diesel is efficient at high RPM's, but I suspect not. I believe that the spill timing on diesel injectors spray fuel over a fair length of time and continue spraying even after combustion starts. I don't know how well that would work at high RPM's. ....

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MarkS
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2001-06-18          29376

I haven't read the entire thread here, but I do know the higher the compression the more efficient the engine. (Spend a few months in a Thermodynamics class) This is why Diesels are usually more fuel efficient. For performance gasoline racing engines one of the key factors is always increasing the compression. Nascar Stock Cars at one time were running as high as 17 to 1 compression for this reason. But at these high compression levels incresed octance fuel is required to prevent untimely detonation or spark knock (its not uncommon to see 100+ octane in racing fuel). I believe I read an earlier post that stated most diesels are 22 to 1, while cars are running at around 8 to 1. In recent years car manufactures have been slowly working on ways to increase this compression without casuing detonation. You'll notice that several new cars are getting close to 10 to 1 now. This is due to advancement in aluminum cyl heads and fuel injection. Im getting way off base here so I'll quit typing ....

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Rob Munach
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2001-06-18          29382

Another reason Diesels are more efficient than gas motors is that they are not throttled. The pressure drop of air going past a throttle plate is energy lost. Think of how much energy is required for the motor to generate vacuum to "suck" the air past a partially closed throttle plate. That is one reason why gas motors are usually more efficient at wide open throttle. ....

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John Miller, III
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2001-06-18          29385

Torque=(HP X 5252) / RPM. This gives you torque in Foot-lbs. Using 33,000 instead of 5252 is Torque in Inch-lbs... One of the reasons diesels have more torque and efficiency is because the energy developed from diesel fuel is much higher than gasoline. Using 1 gallon of diesel generates about 138,000 Btu. versus gasoline which is about 124,000 Btu. ....

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TomG
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2001-06-19          29413

Another way of looking at the fuel issue is that many compact diesels burn about one gallon per hour. That works out to about four more BTU's per second, which would raise 4-pounds of water 1-degree F each second. Well, maybe that's not something to sneeze at after all. Regarding the formula: the simple formula doesn't seem to consider the number of cylinders. I guess that the formula assumes that an actual measurement would be taken off the flywheel, which would produce an averaging effect. Torque is a static measure. The torque in any ideal engine between ideal combustions would be zero. In addition, at the point of combustion, one big cylinder would produce more torque than several smaller cylinders firing at different times. Of course, real combustion isn't ideal--it takes time. I believe that combustion in a diesel occurs over a longer period than gas engines, since the injectors continue spraying after combustion starts. I don't know if this is an issue in efficiency or performance. ....

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Art White
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2001-06-19          29415

You guys are having fun disscusing torque. Let me throw a curve ball in, how about torque rise? For the contours of NY we really like that here. How can you put your math to figure that one out. ....

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Rob Munach
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2001-06-19          29416

If I am not mistaken, the combustion in a diesel occurs very rapidly - which gives it the trademark knocking sound. That is one reason they have to be built so tough to withstand the "hammer blow" on top of the piston (in addition to the high compression ratio) ....

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Murf
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2001-06-19          29421

Naw, Art, it's just a simple Hyperbolic curve, in an inverse proportion to the torque / horsepower relationship...... or words to that effect.... LOL ....

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MarkS
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2001-06-19          29426

Anyone wanna convert that to Newton-Meters for me? LMAO ....

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TomG
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2001-06-20          29447

Torque rise? I think that's 'The higher the fewer.' Guess I finally found a use for the punch line to a Zen riddle I heard in high school. Still doesn't make any more sense to me though. Well, maybe there is some sense if the thinking is about diesels. Rob's comment about the speed of diesel combustion may be correct. I was thinking about the duration of force applied during power strokes rather than the speed of combustion. Even then I might be wrong. If diesel combustion was longer in duration than in gas engines, then I'd expect that all diesels would be long stroke engines. However, my 1710 engine is nearly square (equal bore & stroke). Many gas engines also are square. I was surprised to hear that diesel injectors continue spraying even after combustion starts. I surmised that the combustion duration would be longer than in gas engines where all fuel is contained in the gas/air mixture charge. ....

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jb85302
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2001-06-20          29455

Ouch my brain is beginning to ache from all this thinkin'. The thing I do know about horsepower and torque(be it gas or diesel) it sure is fun to see in action. It's tractor pullin' season!! Time to watch those cranked up diesels poke holes in the ozone layer(no offense to any treehuggers).
Hmmmm wonder if I could s/c the ole Yanmar??
19hp+$$$$$+heavy foot= Lots of parts on the ground = Very unhappy wife!!
Guess Not,
Jeff ....

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Murf
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2001-06-20          29461

Actually, ROb, Tom, you're both right about the 'burn time' duration of a diesel, which is exactly why a diesel MUST be governed at the fuel pump. If a diesel rev's too fast the engine will 'run away', in other words, the incoming fuel will be spontaneously ignited by the outgoing (still burning) mixture causing the engine to speed up even more. The engine would race out of control until it flew apart. NOT a good thing. As for Newton / Metres, that like any other Metric (or more properly SI, Systeme Internationale) equation is all based on 10's, so the math is ridiculously easy, but it still makes your brain hurt. Best of luck ....

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