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#101
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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. Hydrogen, for example, is a very rapid burning fuel. Hydrogen can and has been used in Diesels. I also suspect that natural gas, another common diesel fuel, burns at a high rate. The slow burn rate with many liquid diesel fuels is due to the time it takes the surface area of the droplets to vaporize. |
#102
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#103
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Don Stauffer wrote:
SOME diesels are turboed. Many are not. Some Diesels are normally aspirated. Some are supercharged with geared chargers. Some are turboed. It is by far an overgeneralization to claim that all Diesels are supercharged, even more so to say they are all turbo-supercharged. You forgot supercharged plus turbocharged http://www.wilksch.com/ -- Pub: http://www.slowfood.fr/france Philippe Vessaire Ò¿Ó¬ |
#104
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Thomas Tornblom 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. uh? huh. Read it again. More torque at the same speed *is* more power. That's exactly what I've been saying. You can get more power by increasing torque OR rpm or both. |
#105
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Don Stauffer wrote:
SOME diesels are turboed. Many are not. Some Diesels are normally aspirated. Some are supercharged with geared chargers. Some are turboed. It is by far an overgeneralization to claim that all Diesels are supercharged, even more so to say they are all turbo-supercharged. I believe that if you look at modern diesels currently being produced and sold, you'll find virtually NONE rated at more than 50-60 horsepower that are not turbo-supercharged. And equally few that are strictly mechanically blown (the Detroit Diesel 2-strokes are no longer in production). The VAST majority are, indeed, turbo-supercharged. |
#106
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Sport Pilot wrote:
Steve, As you said power is torque * RPM, so for the same torque more speed is power. Or for the same SPEED, more TORQUE is more power. That's the part of the equation that you've never acknowledged. Your example is a poor one most diesels of equivelant size will deliver more torque at less RPM and have less total horsepower. Not always true (obviously), and when looking at modern diesels its not even GENRALLY true anymore. That's EXACTLY why I picked the example that I did! If you don't like the Dodge comparison, go look at a Ford/Navistar 6-liter diesel and compare it to the closest-sized Ford (5.4L) gasoline engine tuned for a truck application. 10 years ago, the diesel would have won on low RPM torque, but been lacking in peak HP. That is no longer the case most of the time, because diesels are now *always* turbocharged, and most of the time have modern electronic injection control over both injection rate AND duration. I don't know where you found that pitiful Dodge 5.9 liter engine. There's nothing pitiful about it at all, it was an EXCELLENT truck engine used from 1972 until 2003. When built for power rather than torque, it can easily put out well over 400 Hp without turning 7000 RPM, but as delivered in factory trucks, it was tuned for a torque band that's flat as Kansas over about a 3500 RPM span, and that results in a rather modest 230 HP. I have a 4.7 V8 in my Grand Cherokee and it puts out 260+ HP. The 4.7 is the replacement for the 5.2, and while a fine engine in its own right, its a little bit low in the torque department. It doesn't move the fullsize Ram pickup or even the Durango with much authority- the old lower-HP rated 5.9 actually "feels" a lot peppier in around-town driving because it has more torque below 3000 RPM than the 4.7L does, despite a lower peak HP rating. That's a symptom mostly of its small size- when you excessively constrain the displacement of an engine, you start HAVING to spin it faster and faster to get the same power and you sacrifice torque at the lower RPM levels- Which is a key part of the point I've been driving home. Modern gasoline-powered cars and trucks tend to have very high peak horsepower ratings- and yet many of them feel weak compared to older cars, simply because all that high-RPM horsepower comes at the expense of useful low-RPM torque, and they need 5- and 6-speed transmissions just to match the performance of older torquier engines. Gas engines have been pushed smaller and lighter by fuel economy and emissions considerations, resulting in peaky torque curves, poorer low-RPM torque, and higher peak HP to compensate. |
#107
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Steve writes:
Thomas Tornblom wrote: Steve writes: More speed is NOT more power any more than more torque at the same speed is more power. uh? huh. Read it again. Ok. English is not my native language, and I read the sentence such that the "NOT" in the first half also implied that there was an implicit "NOT" in the second half, which made the statement wrong. More torque at the same speed *is* more power. That's exactly what I've been saying. You can get more power by increasing torque OR rpm or both. Then we agree :-) Thomas |
#108
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Thomas Tornblom wrote:
Steve writes: Thomas Tornblom wrote: Ok. English is not my native language, and I read the sentence such that the "NOT" in the first half also implied that there was an implicit "NOT" in the second half, which made the statement wrong. And I'll admit, it was a poorly written sentence. My bad. |
#109
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"Steve" wrote I believe that if you look at modern diesels currently being produced and sold, you'll find virtually NONE rated at more than 50-60 horsepower that are not turbo-supercharged. And equally few that are strictly mechanically blown (the Detroit Diesel 2-strokes are no longer in production). The VAST majority are, indeed, turbo-supercharged. Not true of some of our school busses that were produced in the past 5 or so years. They make up for the lack of super or turbocharging with more cubic inches. -- Jim in NC |
#110
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On Wed, 25 May 2005 09:37:53 -0500, Don Stauffer
wrote: wrote: 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) SOME diesels are turboed. Many are not. Some Diesels are normally aspirated. Some are supercharged with geared chargers. Some are turboed. It is by far an overgeneralization to claim that all Diesels are supercharged, even more so to say they are all turbo-supercharged. The post being replied to was comparing a turbo cummins to a gas engine. Nobody ever said all diesels are turboed. |
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