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#81
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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...). 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. Diesel fuel burns slower and will knock when the injection timing is shortened to allow higher speeds. |
#82
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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. |
#83
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On 16 May 2005 11:01:36 -0700, "Sport Pilot"
wrote: Steve wrote: Luke Scharf wrote: Steve wrote: But there are more and more small diesels that run just as fast as gasoline engines. There's no fundamental limit thats any different than a gasoline engine, really, but up until now there's not been a demand for high-RPM diesels. When I was reading about the Volkswagen TDI engine, I vaguely remember coming across someone who said that the redline of that engine was set by the speed which which the burning fuel expanded. Sounds rather fundamental to me - but, then again, I'm a computer guy. -Luke 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. 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. Ever seen a "super modified" pulling tractor??? I think, from the sound of one at full tilt, that 9000 RPM would be on the low side - turboed, and sometimes also supercharged (turbo-compounded) they scream like a wildcat in heat. |
#84
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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 |
#85
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wrote: On 16 May 2005 11:01:36 -0700, "Sport Pilot" wrote: Steve wrote: Luke Scharf wrote: Steve wrote: But there are more and more small diesels that run just as fast as gasoline engines. There's no fundamental limit thats any different than a gasoline engine, really, but up until now there's not been a demand for high-RPM diesels. When I was reading about the Volkswagen TDI engine, I vaguely remember coming across someone who said that the redline of that engine was set by the speed which which the burning fuel expanded. Sounds rather fundamental to me - but, then again, I'm a computer guy. -Luke 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. 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. Ever seen a "super modified" pulling tractor??? I think, from the sound of one at full tilt, that 9000 RPM would be on the low side - turboed, and sometimes also supercharged (turbo-compounded) they scream like a wildcat in heat. Compare that maximum to the gasoline maximum. Formula 1 cars will exceed 20,000 RPM. |
#86
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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. |
#87
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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. |
#88
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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. |
#89
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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 |
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