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Brad Robertson
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2000-08-03          18496


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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Russ DenBleyker
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2000-08-03          18497


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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Phil Serre
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2000-08-03          18498


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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Roger L.
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2000-08-04          18511


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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droz
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2000-08-06          18548


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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Roger L.
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2000-08-06          18565


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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TomG
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2000-08-07          18567


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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Peter Accorti
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2000-08-08          18638


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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Roger L.
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2000-08-08          18658


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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TomG
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2000-08-09          18661


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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Roger L.
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2000-08-09          18665


Oh.,..I see what you are meaning by "instantaneous". Taken that way, I agree with the way that you are using it. I could hardly do otherwise, since we are using it in the same way! :-)...
Yes, It is known that the intake (and exhaust) length does work to change volumetric efficiency. It begins to become an art when the same intake and exhaust must work over a range of RPM - otherwise you end up with something that only runs at one speed - like a granprix engine...or a stationary motor. For multispeed motors you get to think about fun things like the viscous drag of the fluid as it goes around corners in the manifold...and how it changes with RPM. To my way of thinking the answer lies in the variable valve timing that I mentioned. This will make for some really nice engines.
I wonder what other large effects there are that bear on Peter's "volumetric efficiency with RPM" thought? ....


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Peter Accorti
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2000-08-09          18687


TomG, I think you have a valid point on torque being averaged over the revolution of the engine. For the purposes of this argument I think that we can ignore transient effects and just concentrate on average torque. After all we are talking about multi-cyliner engines spinning for at least 100s of rpms, right? So for example, my TC35D has a 3 cylinder engine. Each cylinder has a power stoke once per two revolutions. But, since I have three cylinders I'm getting 3 power stokes per 2 revolutions. A revolution is 360 degrees, so 2 is 720 degrees. 720 degrees divided by three power strokes is 240 degrees. So a power stokes lasts for 240 degrees (in reality an engine can only extract power from 180 degress so we have a small dead spot here). So for my engine even 600 rpm is 15 power stokes per second and my engine dosen't even run that slow! Plus you have flywheels and such smoothing out the power. As far as power from an engine, I honestly didn't know that diesel burns slower than gasoline, but I doubt that is the reason for torque differences. Remember we have to talk averages here. A slow burner and a fast burner if they produce equal torque are totally equivelent! The main reason that diesel is so much torqueier (a new word) is the MUCH higher cylinder pressure. A normal gas engine (running on regular gas) operates with a compression ratio in the high 8s low 9's (meaning that air is squeezed into a 1/8 sized volume). Any more squeezing than that and the engine pre-detonates (the fuel air mixture explodes from the heat of the air as it compresses without even needing a spark plug!) That's why high horsepower engines need higher octane gas (the gas needs to resist this tendency to pre-detonate). Diesels operate at much higher compressions 22 or so to 1. And diesels do pre-detonate (thats why they have no spark plugs). The trick with a diesel is to inject the gas (using very high pressure injectors) at just the right time to bring about this pre-ignition. The very high cylinder pressure is what makes diesels so powerful for a given size (and quantity of fuel). Roger, actually I want PTO speed right at max hp (or near it). Rememeber torque can always be stepped up or down with gears (again no concept of time). A very torquey brush hog spinning at 10 rpms wouldn't be very useful. When you think of an engine, think of it's volumetic efficiency (how well does it breath as rpm's change). That is the torque curve. Horsepower is simply derived from the torque curve. If you know one curve you can always deduce the other. ....


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TomG
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2000-08-10          18693


Yep, max torque defined as average ft. lbs. over a power stroke is probably how it's defined. I also remember back in the days of 'souping up' cars that the milling the head is the single thing that could be done to make the car accelerate faster. Of course, that just raises the compression ratio. I was one of the kids that spent the money on a multi-carb manifold and was disappointed the car didn't go much faster (it did look impressive tho). That was before I knew anything about power curves, and I should have spent less money and just milled the heads. I didn't have the money for a cam and all the other things it takes to actually re-engineer a power curve. By the time I had the money, I'd lost interest and just bought a motorcycle instead. I think my note about intake manifolds illustrates why engines breathe better at certain RPM's and how power curves can be designed. Regarding the duration of burning: If diesel fuel burns much slower than gas, diesels might be expected to be have longer strokes than gas engines. I checked some Ford compact diesel specs against Ford truck gas engines. The diesels were nearly square (bore = stroke) while the gas engines went from square to fairly short stroke engines. There doesn't seem to be a lot of difference, but perhaps most power is transferred to pistons early in the power stroke for both diesels and gas engines. In that case, 'stroke' would be selected for other engineering considerations.

After all this, I'm still left with some curiosity. I'll observe that diesel engines, despite their torque, haven't made successful racing engines. So I'll turn the question around and wonder why diesels don't run at 8 - 12,000 rpm?
....


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MJB
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2000-08-10          18699


I apologize in advance for this long post but I could not resist adding to this thread. I think that I read in one of my old tractor magazines that given equal displacement a gas engine will develop more horsepower and greater torque than a diesel engine. The reason for using diesel is simply a matter of fuel efficiency and durability. As one of the earlier posters stated diesel engines are best suited for applications where they are used under a fairly stable load. This makes them ideal for trucks and tractors that run at a reasonably fixed speed and load for many hours. If we assume that an engine receives X amount of wear per revolution it follows that an engine turning at half the RPMs will last twice as long, thus lower operating speed gives a durability advantage. Many old tractor gas engines used large displacement, slow running engines for this reason. These engines also developed great torque at low RPMs due to their long stroke. I have a 1952 Case DC-4 tractor that has a 4 cylinder gas engine with 3-7/8" bore and a 5-1/2" stroke that has a maximum governed speed of only 1200 RPM. This engine will lug down until you can almost hear each individual cylinder fire and it still pulls like a mule. My point is that an engine doesn't have to a diesel to be a good low RPM puller. It is also an advantage to have the normal operating speed set well above the maximum torque RPM of the engine. When the tractor is working and encounters a "tough spot" the engine speed will begin to lug down, which actually increases the torque output of the engine to pull you through. This is known as "Torque Rise" and actually acts as sort of an automatic transmission, which matches the engines torque output to the load placed on it. Thus diesels, and many old gas engines, were designed with a fairly large displacement for their rated horsepower. The problem with this approach is that displacement costs money. Modern gas engines are designed with smaller displacement and thus must turn much higher RPMs to make the same power. An extreme example would be an Indy car engine, which puts out around 900 hp from a very small displacement. These engines have no real lugging ability however as they need to keep the RPMs very high to make their horsepower. Just watch how they rev the engines before they release the clutch to keep from stalling on pit stops. Modern diesel engines haven't gone this route however because the fuel efficiency and durability factors are more important than initial cost for the market segment they target. Once again, please forgive my rambling. MJB ....


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Roger L.
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2000-08-10          18720


OK. I'll take a stab at your question, Tom. "Why don't diesels run at high RPM ? "
First of all, take another look at the basic difference between gasoline and diesel engines: The gasoline engine compresses air and fuel together. The heat of compression will cause autoignition (knocking)if you get above about 12 to 1 compression ratio. Because of this, you stay below the autoignition point which means that you need a spark plug to ignite things. When you ignite the compressed mixture it almost explodes.
Now the diesel doesn't compress the fuel and air together. It only compresses the air. BIG Difference! Because there is no fuel in with the air autoignition is not a problem and you can compress the air very tightly. Like Peter said, a compression ratio of 22 to 1 is common. Air compressed this tightly is very hot...so hot that when the diesel fuel is injected into the heated air mass at the top of the compression stroke the diesel fuel begins to burn immediately. And more diesel is injected as the piston travels down on the power stroke.
So that is my best guess, Tom....I don't know the answer to your question for sure, but I do know that the diesel doesn't explode all at once like the compressed gasoline+air mixture does. The diesel burns slowly and all during the power stroke. My guess is that this diesel burning takes substantially longer than the gasoline+air explosion and that is the reason you can't have a really high RPM diesel. High RPM means that everyting has to happen quickly and that diesel burn is just too slow.
....


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TomG
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2000-08-11          18724


Thanks Roger. That was my guess as well, that diesel combustion takes too long for use at high rpm's. I didn't know that additional fuel was injected during the power stroke. That sure would extend the duration of combustion.

I also recall 12:1 as about the compression ratio limit for gas engines. However, I seem to recall seeing ads in racing parts magazines a long time ago for 14:1 pistons. Perhaps they were intended for drag strip use with different fuels.

Always more questions. Could get to be a pain I suppose. However, I'd sure appreciate knowing the difference between direct and indirect injection. I thought I had my mind around that one, but I guess I never heard an actual explanation.
....


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Randy Eckard
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2000-08-11          18725


I must first say I have really enjoyed this thread. A wealth of information.
Here is what I understand about direct and indirect injection. In the Direct injection engines, the injectors spray fuel directly into the cylinder and onto the piston. Indrirect injection engines spray the fuel into a small chamber next to the cylinder. I dont fully understand the physics behind this but I do know the results. Direct injection engines are easier to start in cold weather and are more powerful per cubic inch. Indirect injection engines are much quieter. There have been several posts on here stating that Kubota engines are quieter than the Yanmars in the JD tractors. One big reason is that the Kubota is Indirect(at least the last time I looked) and the yanmar is direct. I think the Yanmar is easier to start in cold weather. It's also interesting to note that if you are around them much you will also notice that indirect diesel exhaust smells much different than direct. If anyone has any other thoughts, I'd love to hear them.
Randy ....


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TomG
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2000-08-11          18726


No problem with length MJB, it's a good read. I recall hearing some sort of engine used in California oil fields. Seemed to be a single cylinder and probably had an extremely long stroke. You'd hear a bang, a definite pause and then another bang. I'm not sure what the engines were used for, but you could mistake them for the acetylene generators used to scare birds out of the fields, except not as loud, and the pauses weren't nearly as long. Guess really long stroke engines work if there is a heavy enough flywheel.

I don't know, but I guess it's possible to build a gas engine that out-torques a diesel at low rpm's. As you mentioned about racing engines, tuning can do amazing things to performance.

I believe that a racing tune uses independent manifolds so each cylinder can have the same tuning at a given rpm. Then manifold lengths and other engine characteristics are selected to resonate around the same rpm. The result is a lot of power per displacement but over a narrow rpm range. I recall hearing that in the days of manual racing transmissions, small formula car drivers shifted about every 15 seconds to keep the rpm within its power range. Conventional engines use combined manifolds. Each cylinder sees different manifold lengths, and each produces different outputs at a given rpm. On a tractor, I suppose one cylinder may tend to wear faster if the tractor always runs at PTO rpm.


....


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Bird Senter
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2000-08-11          18729


TomG, your mention of those single cylinder diesels with the bang, pause, then bang again reminds me of the first time I drove to Alaska in 1972. After hearing that at several of the service station/lodges, etc. in the wilderness of northern Canada and Alaska, I had to inquire as to what that noise was out in the woods and learned it was their generators (located pretty far from the buildings to reduce the noise as much as possible). I was told they run continuously for many years between overhauls. I believe something similar is (or was) used in a lot of oil fields. ....


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Steve in Buffalo NY
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2000-08-11          18732


On the subject of big, 1 cyl engines. When I was a kid, my neighbor was sort of a mad scientist type. His hobby was restoring these old 1 cyl engines and running them in shows. His biggest project had flywheels about 6 feet in diameter, and a clyinder abound 10-12" and a reeeeeeealy long stroke. As previously described, it would run about 50 RPM at full tilt. I remember him saying it was a pretty powerful setup at 7 hp. It has a govenor that would cut the ignition when it was up to speed so at no load the thing would go 30-40 seconds between fires. He said that where they were used these engines supplied power over wide areas, usually by rotating shafts suspended on pilot bearings. He said it was not unusual for 1 engine to drive several wells over a good sized field. ....


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Peter Accorti
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2000-08-11          18737


I thought you guys might enjoy this last part. It is a common misconception that long stoke engines have more torque for a given displacement. Trust me they don't. Let's look at a simple example. We have two engines both with 100 cid. Long engine has 10 square inch piston and 10 inch stroke (10 * 10 = 100). Short engine has 100 square inch piston and 1 inch stroke (100 * 1 = 100). Let's say, in both cases, that the cylinder pressure of the burning fuel is 10 psi. For the sake of discussion, let's measure the torque when the piston is half way down the stroke (it dosen't really matter though). Long engine will have 10 psi * 10 square inch (area of piston) = 100 lbs pushing on 10 inch stroke = 1000 in*lbs of torque. Short engine will have 10 psi * 100 square inch (area of piston) = 1000 lbs pushing on 1 in stroke = 1000 in*lbs of torque. They are obviously the same! The ease by which the stroke can be turned (because it is longer) is exactly offset by pushing that less hard on the piston (because it is proportionally smaller for a given displacement). The rate of fuel burn does not have an effect on long stroke being desirable or not (the pressure in the cylinder dosen't know if its pushing on cylinder walls or piston). There are good reasons for making engines with a larger bore than stroke. An engine with a large bore has more room for large valves in the cylinder head. Obviously this makes it easier for the engine to breath. Also (I think, not sure) large bore engines have less piston to cylinder wall friction compared to long stroke engines. I should not have been so quick to say the diesels have more torque than gas engines (for a given displacement). I don't know. It all depends on the burn characteristics of diesel. That high compression ratio certainly is beneficial though. ....


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george pacheco
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2000-08-12          18770


Hi guys, a long stroke engine has a longer distance from the center of the crank to the center of the connection rod journal. This would give more torque to the crank for a given amount of pressure on piston crown. It kinda like using a cheater pipe on a breaker bar. Another point is that the pressure (psi) above the piston is not linear. This pressure, at full burn, is the highest when the volume above the piston is the smallest. As the piston descends and the burn area increases the psi per sq inch decreases as burn force is applied to a larger surface area. Gasoline has an expansion factor of about 4, that is 15psi compressed to 10 to 1= 150 psi at end of compression stroke. Light this off with the spark plug and will expand to a max of about 600psi and then begins to drop off as piston goes on down. The louder the exhaust the higher the more residual pressure left in the cyl is when the exhaust valve opens. This is why racing engines have a louder exhaust then basic work engines that are designed to get as much pressure per pound of fuel as possible applied to the piston instead of out the door. The downside of this, of course is reduce volumemetric efficiency. Peter, your description of the connection between volumemetric efficiency and torque was terrfic. Hope the above makes some sense.
regards,
george, keoke ....


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MJB
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2000-08-14          18823


This post is in regards to the question of direct vs. indirect diesel injection. Being a Case tractor collector I have read several articles regarding the early development of diesel engines used on Case tractors in the early 1950s. Case developed an indirect injection diesel, which used what was called a "Lanova Power Cell". When the piston approached TDC the injector would spray a jet of fuel horizontally across the top of the piston into the "Power Cell", which was a pre-ignition chamber on the opposite side of the cylinder. The fuel would ignite and begin to burn in this chamber and the resulting expansion would blow the burning air-fuel mixture back into the cylinder thus giving a more thorough mixing of the fuel and air. This design was used because although it was more expensive to produce, it was more fuel-efficient than the early direct injection designs. Case continued to use this design until it became necessary to turbo-charge their engines to compete with other manufacturers published horsepower numbers. The Lanova indirect injection diesels do not take to turbos very well. It seems that as the pressure in the cylinder is increased by the turbo, the fuel penetration into the "power cell" drops off. This tended to concentrate the fuel in a single spot on top of the piston creating a hot spot that could burn a hole in the piston. To overcome this limitation, Case switched to direct injection diesels at this time for both turbo and naturally aspirated engines. This also gave them the benefits of simplicity, lower cost, and commonality of parts. I believe that with improvements in injector and pump designs the modern direct injection diesels are as efficient as the indirect injection engines. I don't know what the reasoning is for Kubota and some other manufacturers to continue to use the indirect injection design. MJB ....


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Roger L.
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2000-08-14          18826


MJB, thanks for the discussion on direct and indirect injection. You might have cleared up opne mystery for me... My wife has a Ford F-250 Direct Injection Turbo diesel, and I had though that they were indirect injection before the turbo version - although I am not sure about this.
One reason for the indirect injection might be the noise reduction. I have two of the US imported compact Yanmars. One is direct injection and the motor has a distinct "diesel knock" at all speeds. This is normal for the engine...and in fact is almost a "clang" at some RPMs. The other Yanmar is indirect injection and is much quieter. Instead of a noise like a hammer on an anvil, it fires with a sort of rolling chuckle and purr. Not quiet, but not like the other one at all. The direct injection is much more fuel efficient in spite of the noise. ....


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