What are the advantages of diesel? - Page 4 - Mercedes-Benz Forum

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post #31 of 33 (permalink) Old 03-18-2007, 09:19 PM
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Hi DZL,
You are correct about the Otto and Diesel cycles, but these are really just idealized cycles. Actual engines run somewhere between the two. Aside from the compression ratio, actual gasoline and diesel engine thermodynamic cycles are much more similar than their ideal cycles would imply. In fact, I would say that gasoline engines are closer to the diesel cycle, and diesel engines are closer to the Otto cycle. This is because gasoline is formulated to burn slowly to avoid detonation. Thus the gasoline engine operates with much more constant pressure combustion than the Otto cycle implies (Otto cycle has zero constant pressure combustion). Diesel fuel is formulated to burn quickly, and diesel engine's have a very rapid combustion pressure rise -- like constant pressure combustion. The rapid pressure rise is what makes the characteristic diesel sound.
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post #32 of 33 (permalink) Old 03-19-2007, 12:09 AM
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Originally Posted by Jon L
Hi DZL,
You are correct about the Otto and Diesel cycles, but these are really just idealized cycles. Actual engines run somewhere between the two. Aside from the compression ratio, actual gasoline and diesel engine thermodynamic cycles are much more similar than their ideal cycles would imply. In fact, I would say that gasoline engines are closer to the diesel cycle, and diesel engines are closer to the Otto cycle. This is because gasoline is formulated to burn slowly to avoid detonation. Thus the gasoline engine operates with much more constant pressure combustion than the Otto cycle implies (Otto cycle has zero constant pressure combustion). Diesel fuel is formulated to burn quickly, and diesel engine's have a very rapid combustion pressure rise -- like constant pressure combustion. The rapid pressure rise is what makes the characteristic diesel sound.
Interesting comments from experts. Can you clarify if the above took into account the fact that current diesels with piezo-electric nozzles can inject diesel 5 if not more separate times per engine cycle, this should make the diesel sound quite much attenuated if not making it disappear.

What about the constant pressure, at low loads this multiple injection techniques should help getting more constant-like pressure? Also at higher loads the injection time is relatively long to the whole burning cycle, isn't it? If the fuel burns quickly, it would still not mean an instant pressure peak if you keep on injecting fuel for a relatively long time.
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post #33 of 33 (permalink) Old 03-19-2007, 01:24 PM
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I think you are correct when you say that the modern common rail diesel systems change the whole combustion event in ways that were not possible before. They are much quieter, and that would be an indication that the rate of pressure rise is more controlled and "gentle."

Interestingly, constant volume combustion would be theoretically more efficient than constant pressure combustion. The problem is that with diesel compression ratios, trying to have constant volume combustion (meaning near instantaneous combustion right at top dead center) would result in peak cylinder pressures that would blow the cylinder head off the engine! (In a gasoline engine, it would result in catastrophic detonation). Also NOx emissions would be very high. It might actually be better to get closer to constant volume combustion at low loads (where the pressures are lower, and move more towards constant pressure combustion as the load goes up. The task of the engine development engineer is to use all the capabilities of the injection system to tune the combustion event to provide the best COMPROMISE between economy, emissions, durability, driveability, noise, etc. This has to be done across the whole range of engine speeds and loads (and ambient temperatures, and transient temperatures during warm-up, and transient throttle response, etc, etc!) The best compromise for full power will be different than for idle, different for steady highway cruise, etc. It is a time consuming task. When I was at Cummins, doing this kind of work for over-the-road truck engines, we had some automated test facilities that would run an engine overnight on a dynamometer, keep changing all the many parameters, and take all the measurements. The development engineer would review plots of the measurements and decide how the engine should be set-up at different parts of the speed - load map. This was called "building a calibration." After being satisfied with how it ran on the dyno, a bunch of trucks would be run with the new calibration in some long field tests to make sure the engines behaved well under real-world conditions.
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