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 08-03-2000, 12:09 Post: 18496
Brad Robertson



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 Horse power to horse power

I am upgrading my 1957 35hp two wheel drive gas engine tractor for a diesel fwd compact. How many engine/pto horse power will I need to get in order to have simliar pulling power? If I want hydrostatic trans how much more?Thanks, Brad






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 08-03-2000, 12:49 Post: 18497
Russ DenBleyker



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 Horse power to horse power

Brad,
Assuming the manufacturer is truthful, horsepower is the same whether gas or diesel. I traded a 32hp gas Ford 2000 for a 33hp diesel NH 1925 and they are very similar in horsepower. I figure I lose about 5hp with the hydro transmission, but that is almost entirely offset by 4wd. The 1925 pulls almost as well as the 2000 when pulling irrigation creases. What is not the same is the fuel consumption. The diesel uses about 2/3 the fuel that the gas tractor used. Of course, the diesel engine is only about 2/3 the size of the gas one!






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 08-03-2000, 15:43 Post: 18498
Phil Serre



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 Horse power to horse power

Brad,
It is true that gasoline horsepower and diesel horsepower are the same BUT you have to consider torque at the said horsepower. HP = Torque * RPM and diesels are operation at a much lower RPM to get the equivalent HP of a gas engine. If you look at the output curves of comparable HP gas and diesel units you will see that the gas units have a smaller power band then the diesel. SO WHAT DOES IT MEAN - It means that a diesel with lower RPMs and a broader power band will tug along and not bog down like a gas unit and thus get you through a lot more.






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 08-04-2000, 09:32 Post: 18511
Roger L.



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 Horse power to horse power

I rarely use the HP that I've got with any of my tractors. It is usually more traction that I'm after....not more power. I have noticed that my 1959 gasoline JD outweighs the diesel compact by more than double - even though they have about the same horsepower. So the traction of the JD is much greater. There is really no comparison in ability to pull. It might have to do with the fact that the JD has larger diameter tires and a better weight distribution. The compact tractor has the advantage that it is small and handy.
BTW, the old gasoline JD is a "torquer" with pretty much the same excellent lugging response from idle up to max RPM of 1200. The compact diesel is a high RPM engine which doesn't peak until 2600 RPM - more than double where the gasoline engine peaks. Horsepower is torque times RPM, so the diesel gets a lot of its power from RPM whereas the gasoline-burning JD gets most of its power from torque. The compact diesels are real "screamers" compared to the old farm tractors in terms of torque vs RPM. The old farm tractors had a torque rise when load is encountered that is hard to beat.
What I am saying is that the 30 hp in an old gasoline farm tractor is quite potent at getting work done. If you want to match your 35 hp farm tractor, you had better get at least that much horsepower in a compact, and as much more as you can afford.






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 08-06-2000, 03:43 Post: 18548
droz



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 Horse power to horse power

Actually, I think you have that reversed. A diesel engine produces more torque at the low rpm, which is why people buy diesel pickups when they have to do some heavy towing because of that low end torque. Now there are other factors involved in comparing an old gas engine tractor to a modern diesel. A Harley motorcycle with two big cylinders produces a lot of low end torque compared to a more modern 4-6 cylinder Honda which may have considerably more horsepower. So you have to compare number of cylinders and displacement and also of course, gearing. An old tractor with huge heavy cylinders can have a lot of torque with little horsepower.






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 00-00-0000, 00:00 Post: 18565
Roger L.



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 Horse power to horse power

Naw, I don't have it reversed...although I'll confess to having turned the normal way of looking at such things upsidedown in order to emphasize that these things can be different in the tractor world. In fact, I'm glad that you challenged my thinking.....I was surprised at my conclusions myself! It's always helpful to sit down and grind through the physics of some gadget every once in awhile.
Yep, the ancient low-revving JD gasoline engine really does have twice the torque as the typical compact's multicylinder diesel.....And this was common in tractor/industrial engine design in the 50s - although it is not the way things are done today. However, it is valid in this discussion because Brad is wanting to compare these very engines types. And since horsepower equals torque times RPM.....and if both engines produce the same horsepower....then the one that does it at a lower RPM has proportionately more torque. Hard to argue with that arithmetic!! What the math doesn't tell us is how "driveable" or not that the old engines were..... How well they conserved their momentum....and how smoothly they responded to a need for torque rise (lug-ability). From owning them, I can tell you that they were very good at doing these things, although the accomplished these things at the expense of economy. They were and are terrible fuel hogs. The modern diesel is many times more efficient.
On bikes: I don't know if the Harley has more torque than a similar displacement Honda....although I agree that the HD should have more torque than a Honda of similar horsepower. I'm a bit out of date on this, because I haven't looked at motorcycles design much in the last 20 or 30 years - although motorcycles have always fascinated me. In general, an engine with lots of small cylinders can easily have more torque than an engine with fewer and larger cylinders if the designer wants to orient it that way. There are other reasons (than torque) that explain why multi-cylinder designs are usually run at higher RPMs.






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 08-07-2000, 07:13 Post: 18567
TomG

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 Horse power to horse power

I'll add a couple of bits to an engine concept discussion. I'll probably learn something, and I've always got plenty I should learn.

I think the idea that HP = torque x rpm is a rough approximation. The idea is that HP is a measurement over time. In that concept, the more times the same force (torque) is applied over a particular duration, the higher the HP. A small force applied more times than a large a large one can equal more HP. Therefore, more bangs per second (higher RPM) equals higher HP.

However, the simple idea of HP = torque x RPM treats the force of ignition as instantaneously applied and of zero duration, which is a simplification of the physics. Burning a fuel charge takes time. The pressure builds in a cylinder due to engine compression and fuel burning. Force is being applied more or less continuously on a power stroke until the exhaust valve opens. Of course, all this happens while the piston is moving. Ignition in gas engines usually occurs before the piston reaches top dead centre. I don't know about diesels.

What all this means is that how diesel fuel burns, compared to gasoline, makes a difference in both the torque and HP of diesel and gasoline engines. Here's where I've got to speculate, since I'm not an engineer. I suspect that diesel engines apply more force over a greater part of the power stroke than gas engines. The much higher compression ratio of diesel engines means that the atmospheric charge 'unwinds' over a greater distance. In addition, the heaver hydrocarbons in diesel fuel may burn slower, which would mean that cylinder pressures take longer to build up but are maintained for a longer time over the power stroke.

If all this is true, diesels should have an inherent advantage in producing power at low RPM's. However, it is true as Roger notes that gasoline can be designed (tuned) to for power at low RPM's, but they may not be very efficient. Low RPM gas engines use long intake and exhaust manifolds, mild cam durations, small ports, heavy flywheels etc.






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 08-08-2000, 19:00 Post: 18638
Peter Accorti



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 Horse power to horse power

This is a great discussion! I'll add a couple of thoughts as well. Maximum torque occurs at maximum volumetric efficiency (the ability of the engine to breath the most air) per revolution. Of course per revolution has no concept of time. If the engine speeds up and is less volumetrically efficient (can't breath as much air) but is turning faster then you get an increase in horsepower. This is why maximum horsepower always occurs at a higher RPM than maximum torque. If the torque curve is flat across RPM there will be a big difference between max torque and max horsepower. If torque drops sharply then max horsepower will be very close to max torque. Looking at a Kubota brochure in front of me, I see that max horsepower (on a L4310) occurs at 2600RPM, max torque occurs at 1500 RPM. This Kubota has a pretty flat torque curve (varies from 150 at low idle, 153 max to 120 lb*ft at max hp RPM). The "lugability" of the Kubota might be perceived as small because such a large RPM change has to occur to get to max torque from PTO speed for example. On the other hand it might be considered very lugable because it's torque is so constant. Roger's old tractor might seem more "lugable" because a smaller RPM change brought about MAX torque more quickly. With all that said, I'm having a hard time thinking that old engines are truly more lugable than newer engines. And what do we mean by lugable? Is it just a quick transient change for a moment (plowing through a momentary hard spot where rotating engine and flywheel mass can temporarily aid the engine in getting through)? In that case I could see an advantage for the older engine. Low RPM engines typically have large flywheels and heavy rotating mass. Or is lugability the ability of the engine to resist temporary (but longer than say 1 second) changes in RPM due to load?






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 00-00-0000, 00:00 Post: 18658
Roger L.



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 Horse power to horse power

Yep. The only way I get some of these things clear in my mind is to talk them over with people. Tom, you are correct in that a diesel burns fuel for a longer portion of each cycle and is more efficient because of it. This is the classic thermodynamic argument in favor of the diesel engine. Congratulations on that thinking.....I'm not so comfortable with the way you said that the simple HP= torque*RPM equation requires that you regard the torque as an instantaneous event. I don't think that this simple equation "cares". Although I certainly agree that the engine output cares a lot!! Of course what really happens is that the torque is positive over some small portion of the revolution, and that the motor actually loses power during the non-burning part of the cycle. But who is to know if we just average it out? The equation doesn't know. Personally, whenever I get a chance to simplify things I take advantage of it....in this case I think you can view torque as being equally spread over the revolution if you want. Good stuff. I couldn't agree more with your conclusion. Those old gasoline motors were and still are excellent for getting work done if you can afford the cost of the energy.
Peter's thinking on volumetric efficiency changing at different RPMs as being the reason why that torque and HP curves are different shaped is cleverly explained. I'd bet that a lot of people have wondered about that very thing. I know that I sure did.....I mean, if horsepower is just torque times RPM, then why aren't the graphs just a couple of straight lines differing only by the RPM? Peter's explanation explains why this isn't so. Cheers to Kubota for publishing a Torque and HP chart in the first place. Like Peter, I'm a little disgruntled that the rated PTO speed is not at the torque peak. Maybe the HP is still going up with RPM faster than it is falling off because the torque is going down due to lowered volumetric efficiency at higher RPM. Yep, I think that must be it. HEY! What happens if you turbo charge this engine? Now the cylinder is filled by pressure, so I'd think that the volumetric efficiency would stay pretty flat across the RPM band. Er, OOPS, that is unless the turbo itself is only efficient at higher RPM....Hmmmm....Maybe what we need here is a variably gear-driven supercharger so that we can fill those cylinders at all RPMS. Shades of James Bond.
This all might be out of date by next year. I notice that Navistar (Ford and International) has just filed a patent on a nifty way to solve the variable cam timing problem. They expect to eliminate the camshafts from their entire diesel motor lineup by 2005. Valves will be electo/hydraulically actuated (like their fuel injectors are now) so that valve duration, acceleration ramping, and opening height will all be under control of the master computer - which also has the same control over the injectors. A Major Improvement..and one which could also be applied to gasoline engines.







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 08-09-2000, 07:22 Post: 18661
TomG

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Roger: I think we have the same idea about torque x RPM. The simple equation doesn't consider the duration and form of the torque across a revolution. In my terms, that is a force instantaneously applied. In practice, manufacturer torque specs probably are nominal values across the power stroke duration. If so, solutions from the simple equation would be similar to those from an elaborate calculus type of equation.

I'm for simplicity, but in my case, I couldn't start to figure out the obvious differences in performance between diesel and gas engines until I started thinking about the likely differences in how diesel and gas charges burn. There are undoubtedly other reasons as well.

Here's a flavour of my understanding of Peter's volumetric idea applied. Normally aspirated engines are ram charged to an extent. The amount of ram charge induction is RPM dependent for a particular engine tuning. For example, air moving in an intake manifold has inertia. There is more inertia when the air is moving fast at high engine RPM. For that reason, greater intake induction can be gained by leaving an intake valve open past bottom dead centre on the intake stroke. The piston may be coming up, but inertia of air in the manifold rams a greater charge in the cylinder than closing the valve at BDC. How long the valve can be left open depends on inertia of air in the manifold, which is affected by both RPM and manifold length. However, that's not the whole story. When an intake valve closes, pressure in that part of the manifold is low, but starts increasing. The best induction is obtained when the valve opens for the next intake stroke at the point of max manifold pressure. That's a reason why long manifolds are not efficient for high RPM engines. By the same token, short manifolds don't provide enough inertia at low RPM's, to support intake cam durations much past BDC.

The main idea here is that the effect of intake manifold and cam duration design on induction is maximum at a particular RPM. Similar mechanisms work in the exhaust and scavenging portions of the cycle. Almost everything in an engine has an RPM related effect. An engineer can pretty well tailor almost any power/torque curve through these RPM dependent effects, and most of them can be interpreted in terms of volumetric efficiency. I think the same sort of standing wave also would exist in the intake manifold of a turbo-charged engine, and the charger would affect the manifold length. There's a chance that charger efficiency does decline at high RPM, but I'm just guessing.

Now this is really getting long. My trouble is that I'm on the net early in the morning when it's interesting to think about these things. During the day, it's more interesting to be on the tractor than think about how it works. I may be more balanced it I netted during the day.






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Discussion Boards > Active Subjects > Messages as Posted > Size Tractor Needed Forum

Thread 18496 Filter by Poster:
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