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#91
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Stuart & Kathryn Fields wrote: We own a 2003 Jetta TDI and it poops out before it gets to 4500. However, it goes uphill at 80mph gaining speed at around 2500RPM. BTW. Hp= Torque X RPM. If your 100hp engine peaks at 2500 rpm and your 200horse engine peaks at 7500 rpm, the 100hp engine has to develop 1.5 times more torque than your 200hp engine at the peak hp rpm. -- Stuart Fields Experimental Helo magazine P. O. Box 1585 Inyokern, CA 93527 (760) 377-4478 (760) 408-9747 general and layout cell (760) 608-1299 technical and advertising cell www.vkss.com www.experimentalhelo.com "Steve" wrote in message ... Sport Pilot wrote: Steve wrote: Sport Pilot wrote: Diesel fuel is not conducive to high speed running. Nor is a long injection period through much of the expansion phase. Yes you can boost the turbocharger and the other things, but an Otto cycle engine will respond with even higher speeds. Parts failure from speed is not a problem with diesel engines, the rotating parts have to be bigger than an otto engine because of the higher compression, yet the otto engine will turn higer RPM's with smaller parts. All of the above is true in the common practice of diesel design, but none of it is necessarily true. There is nothing FUNDAMENTAL that limits a diesel to low-RPM designs only. One can build a screaming high-RPM diesel with light-weight rotating parts, but one would have to ask "why?" Gasoline engines are made high-revving in order to increase power output from a small package, but diesels can develop a lot more low-RPM torque through high boost because they don't detonate when "lugged," so there's no NEED to make them scream. If you need more power, don't spin them faster, just boost them harder. High RPM is an aggravation, not an advantage (no matter what Honda VTEC drivers think...). Actually diesels don't really deliver that much torque at similar speeds. Ok they do but only because the valving is timed for the slower speeds. Many gasoline tractor engines will diliver similar torque, but with a higher fuel consumption due to the lower compression ratio. It depends on what you're comparing to. A lot of tractors use normally-aspirated indirect-injection diesels, which are no comparison to a turbocharged diesel. Or to a good normally-asipirated gasoline engine, except in terms of longevity. Gasoline engines are hard to beat for the *width* of their torque curve, but modern turbo-diesels do a good job against them, and do so with better efficiency. Diesel fuel burns slower and will knock when the injection timing is shortened to allow higher speeds. Higher speed requires more than just rapid injection, it also demands higher boost pressure, which causes the fuel to burn faster without any knock-like phenomenon. All serious diesels are turbocharged for that reason (as well as others), and normally-aspirated diesels are limited to very small power outputs and relatively low efficiency. You can only get so much combustion chamber pressure through the compression ratio of the engine- the rest has to come from forced induction. As someone pointed out earlier, the VW TDI develops power up to 4500 RPM, which is comparable to many gasoline engines, but it is able to do so only because it is a turbo-diesel. As I said before, there's no reason that building a 7000-RPM diesel isn't possible, but there's no REASON to build one. I think most of us know that torque is only a force and you need speed (RPM) to develop power. More speed is more power. Or was there some other point you are trying to make? |
#92
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Sport Pilot wrote:
I think most of us know that torque is only a force and you need speed (RPM) to develop power. More speed is more power. Or was there some other point you are trying to make? II think the point is that there are TWO ways that do an equally good job of increasing power. More TORQUE is more power too. Since power=K*Torque*RPM, the result is that if you hold the speed constant and double the torque, you get double the power. There are diesel engines in this world that develop 20,000 horespower at only 150 RPM. Naturally, the torque value is astronomical. No, they don't fly. ;-) They swim. |
#93
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Steve wrote: Sport Pilot wrote: I think most of us know that torque is only a force and you need speed (RPM) to develop power. More speed is more power. Or was there some other point you are trying to make? II think the point is that there are TWO ways that do an equally good job of increasing power. More TORQUE is more power too. Since power=K*Torque*RPM, the result is that if you hold the speed constant and double the torque, you get double the power. There are diesel engines in this world that develop 20,000 horespower at only 150 RPM. Naturally, the torque value is astronomical. No, they don't fly. ;-) They swim. Why the obvious apple and orange comparision? I can show you a model diesel engine of less than 1/2 cubic engine that turns about 10,000 RPM? So what. A large engine is going to turn less RPM because of its rotation mass. You need to compare engines of equal size. The model diesel engines will not turn as fast as the gas or methanol/nitro engines because the fuel (ether and kerosene) will not burn as fast. But they will put out more torque than the other two types, though only marginally more than gasoline. This is because the fuel burns slower, not because of the cycle, because all, the spark ignition gas engine, the glow plug methanol/nitro engine, and the compression ignition engine, are actually otto cycle engines. |
#94
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Sport Pilot wrote:
Why the obvious apple and orange comparision? To refute the statement that "more speed is more power." You need to compare engines of equal size. OK, Lets do it! Dodge 5.9 Liter v8 gasoline engine: 230 horsepower at ~5000 RPM Compared to: Dodge/Cummins 5.9 Liter turbo-diesel engine: 325 horsepower at 2900 RPM More speed is NOT more power any more than more torque at the same speed is more power. |
#95
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Sport Pilot wrote:
The model diesel engines will not turn as fast as the gas or methanol/nitro engines because the fuel (ether and kerosene) will not burn as fast. I say again: there is no magic rule that says "diesel burns slower" than gasoline. Increasing the boost pressure can increase the burn rate AS MUCH AS YOU WANT. Its just not done most of the time, because no one WANTS a 7000 RPM engine when a 4000 RPM engine is available to do the same job. People only build 7000 RPM engines when there's no other way to get the power. This is because the fuel burns slower, not because of the cycle, because all, the spark ignition gas engine, the glow plug methanol/nitro engine, and the compression ignition engine, are actually otto cycle engines. No, that's already been covered. Diesel (true diesel with either direct or pre-chamber injection) is a subtly different cycle than the Otto cycle. Its constant-pressure combustion instead of constant-volume combustion. |
#96
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Steve writes:
Sport Pilot wrote: Why the obvious apple and orange comparision? To refute the statement that "more speed is more power." You need to compare engines of equal size. OK, Lets do it! Dodge 5.9 Liter v8 gasoline engine: 230 horsepower at ~5000 RPM Compared to: Dodge/Cummins 5.9 Liter turbo-diesel engine: 325 horsepower at 2900 RPM More speed is NOT more power any more than more torque at the same speed is more power. uh? More torque at the same speed *is* more power. power = torque * speed Had the gas engine produced the same torque at 5000 rpms as the diesel do at 2900, then it would have had about 560 hp at 5000. Thomas |
#97
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"Thomas Tornblom" -to-reply wrote in message ... Steve writes: Sport Pilot wrote: More speed is NOT more power any more than more torque at the same speed is more power. uh? Yeah, that was my reaction! (?) ;-) More torque at the same speed *is* more power. power = torque * speed Had the gas engine produced the same torque at 5000 rpms as the diesel do at 2900, then it would have had about 560 hp at 5000. Thomas Actually, the equation to calculate power (or horsepower (HP) in this case) is HP = Torque X rpm / 5252. Torque being a measurable force that turns the crankshaft and ultimately the wheels, main rotor, or propeller (I just noticed that this is going to two aviation and one automotive newsgroups). Horsepower is defined as a "measurement" of work performed. By the equation, at least as it applies to internal combustion reciprocating engines, you can't have HP without torque! So, if you can increase the torque value at a given rpm, you'll increase the power output at that rpm. Likewise, if you can maintain a given torque value at a higher rpm, you'll produce more power in that case too. It doesn't matter what kind of engine you're talking about or the fuel burned. Gasoline, diesel, methanol, it makes no difference. The equation still applies. FWIW! Fly/Drive Safe, Steve R. |
#98
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Steve wrote: Sport Pilot wrote: Why the obvious apple and orange comparision? To refute the statement that "more speed is more power." You need to compare engines of equal size. OK, Lets do it! Dodge 5.9 Liter v8 gasoline engine: 230 horsepower at ~5000 RPM Compared to: Dodge/Cummins 5.9 Liter turbo-diesel engine: 325 horsepower at 2900 RPM More speed is NOT more power any more than more torque at the same speed is more power. Steve, As you said power is torque * RPM, so for the same torque more speed is power. In fact with 0 RPM you have no power only a force. No I have not argued that desiels cannot deliver power by increasing torque. Only that their inherent design and fuel limits their maximum speed. Your example is a poor one most diesels of equivelant size will deliver more torque at less RPM and have less total horsepower. I don't know where you found that pitiful Dodge 5.9 liter engine. I have a 4.7 V8 in my Grand Cherokee and it puts out 260+ HP. I know that the 5.7 Liter hemi V8 puts out about 320 or so HP and it is not turbocharged. So that is a more equal comparison. Unlike the apple orange examples you put up. |
#99
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Steve wrote: Sport Pilot wrote: The model diesel engines will not turn as fast as the gas or methanol/nitro engines because the fuel (ether and kerosene) will not burn as fast. I say again: there is no magic rule that says "diesel burns slower" than gasoline. Increasing the boost pressure can increase the burn rate AS MUCH AS YOU WANT. Its just not done most of the time, because no one WANTS a 7000 RPM engine when a 4000 RPM engine is available to do the same job. People only build 7000 RPM engines when there's no other way to get the power. This is because the fuel burns slower, not because of the cycle, because all, the spark ignition gas engine, the glow plug methanol/nitro engine, and the compression ignition engine, are actually otto cycle engines. No, that's already been covered. Diesel (true diesel with either direct or pre-chamber injection) is a subtly different cycle than the Otto cycle. Its constant-pressure combustion instead of constant-volume combustion. I was not quite correct if I said desiel fuel burns slower, it is however made to ignite better with desiel engines so that the fuel ignites spontanously when injected. The fuel burns slower because it is injected over a period of time. However, I may have been correct about the model fuel. There are diesel conversion heads for model glow engines. This replaces the head and glow plug with a head with a varible compression. You adjust the timing by adjusting the compression. The fuel is a mix of kerosene, ether, amyl nitrate, and oil. This fuel will ignite at a much lower temperature than true desiel fuel. The ether and amyl nitrate are the componets which ignite at a low temp. Though ether is extremely volatile it is not a very high energy fuel and I think it burns slowly. These converted engines will turn large propellers at slower speeds than the glow engines they were converted from. Another example of a high energy fuel that limits speed is nitromethane. Because it is a monopropellent the racers can pretty much inject as much as their ignition can ignite, except for hydrolocking. That is if more fuel is injected than the combustion chamber volume the engine will blow up. But nitro burns slowly so RPM is more or less limited to just over 10,000 RPM loaded. The slow burning is why you see huge flames coming out the exhaust. |
#100
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On Tue, 24 May 2005 21:54:29 GMT, Thomas Tornblom
-to-reply wrote: Steve writes: Sport Pilot wrote: Why the obvious apple and orange comparision? To refute the statement that "more speed is more power." You need to compare engines of equal size. OK, Lets do it! Dodge 5.9 Liter v8 gasoline engine: 230 horsepower at ~5000 RPM Compared to: Dodge/Cummins 5.9 Liter turbo-diesel engine: 325 horsepower at 2900 RPM More speed is NOT more power any more than more torque at the same speed is more power. One BIG factor is being forgotten here. The diesel is turboed. This makes it roughly equivalent to an 8 liter engine at about 6psi boost. Any combustion engine produces power in proportion to the amount of air consumed. On a diesel it does not necessarily "consume" all the air that goes through it - but the maximum power output is definitely limitted by how much air can be put through it. A turbo can eisily double the amount of air an engine pumps through it at a given speed. Running an engine at double the speed also increases the amount of air going through the engine - not quite double due to reduced volumetric efficiency at speed. Double the CFM gives double the horsepower, before factoring in frictional losses and / or pumping losses. A naturally aspirated diesel engine generally produces less HP per unit of displacement, but more torque at low RPMs due in part to less pumping loss (no air throttle) uh? More torque at the same speed *is* more power. power = torque * speed Had the gas engine produced the same torque at 5000 rpms as the diesel do at 2900, then it would have had about 560 hp at 5000. Thomas |
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