Date registered: Mar 2007
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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.