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#81
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"Steve" wrote In practical terms, no one really wants or needs a 9000 RPM diesel, though. Oh, I don't know. NASCAR might be interested in going diesel ! g -- Jim in NC |
#82
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Steve wrote:
In almost ALL real-world engines, the actual limit is set by the point at which some mechanical component would fail. The engine's torque *may* drop off well before the mechanical failure point if it can't ingest enough fuel or air at high speed. In the case of a diesel, you can pretty much increase the burn rate to as high as the mechanical parts can tolerate by increasing turbocharger boost (and injection rate to match). Since detonation isn't possible (no fuel exists in the cylinder until combustion is supposed to begin anyway) the only limits to boost pressure are mechanical in nature. In practical terms, no one really wants or needs a 9000 RPM diesel, though. Actually, detonation can and does occur in Diesels. With today's Diesel fuels and injection methods the fuel does not burn INSTANTLY upon injection. The droplets are too large. The droplets tend to evaporate from their surface area. It IS possible for unburned gases evaporated from the fuel to exist in the chamber and detonate in local regions. The effect is nowhere as severe as the detonation that occurs in spark engines at times. Interesting traces made with high frequency pressure sensors indicate these local "peaks" in combustion pressure when this occurs. It is the cause of the "tinkling" sound that Diesels occasionally make. Detonation is not a full or nothing situation. Local regions can undergo detonation even while the majority of the charge undergoes normal wavefront burning, in either Diesel or SI. |
#83
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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...). I would assume that use of hydrogen as a fuel would allow very high rpm in a Diesel. |
#84
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Don Stauffer 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...). I would assume that use of hydrogen as a fuel would allow very high rpm in a Diesel. But no one has come up with a completely satisfactory way to do direct injection with a gaseous fuel, which is the same problem diesels that run on natural gas face. NG diesels are interesting beasts. They actually mix the gas with the intake air and compress it like a spark-ignition engine would, setting up the possibility of detonation, but since NG is has a very high relative "octane" rating, it doesn't ignite until a *tiny* shot of diesel is injected to initiate combustion. They do have to operate at somewhat lower compression than a straight diesel, but its still in the neighborhood of 14:1 or 15:1 which is much higher than you can achieve with gasoline, at least on any fuel short of leaded aviation racing fuel that is brewed up in ridiculously small (and expensive) quantities for the Reno air racers. |
#85
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The OTHER Kevin in San Diego wrote:
On Mon, 16 May 2005 15:08:37 -0500, Steve wrote: High RPM is an aggravation, not an advantage (no matter what Honda VTEC drivers think...). Shhhh.. Don't tell the guys at Pratt & Whitney that... It depends... gas generator shafts need to spin fast, but the LP spool on a PW4000 doesn't turn any faster than the engine in my car |
#87
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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? |
#88
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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. |
#89
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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. |
#90
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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. |
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