CI runs cooler because more of the energy is going to work rather than heat, and because there is so much inert material to absorb the heat energy. At load, they can run very high exhaust temperatures.
Expansion ratio and nonhomogeneous fuel air mixtures are not the correct way to view this. Expansion ratio is incorrect because you also have equivalently high compression ratio, more than offsetting this effect. The nonhomogeneous mixture is not correct because it is irrelevant to the final bulk temperature after mixing.
The type of fuel can certainly make a difference. Primary effects there are different fuel heat energies and air-fuel ratios.
BigClive, I'm not sure what you mean by that, but without modification that is a misleading way to think about it. The single greatest effect is that you have a more efficient heat engine from a greater temperature difference enabled by the high compression ratio. Since one side of this is expansion, perhaps that is what you mean. However, since there is generally not an intentional increased expansion ratio, a normal compression ignition engine is not considered an engine having a greater expansion ratio.
Your post is suggestive that an Atkinson-cycle type of approach is causing some efficiency gain in a normal compression ignition engine. However, a higher compression ratio engine will be more efficient than a lower compression ratio engine, all other things being equal, including the expansion ratio being roughly the same as the compression ratio as normally occurs. Further gains might be obtained with an Atkinson or Miller cycle approach, but that is independent of base compression ratio, and can be applied to either low or high compression engines.
The efficiency improvement with Atkinson cycle varies substantially with CR. Heywood has a nice section on what he calls 'over expanded' engines. The tradeoff of course is that WOT torque is reduced.
Cheers
Greg Locock
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Starting wasn't straightforward. The engines needed preheat to working temperature and the vehicle was push started but once on boost it took the world speed record for a diesel powered car. The company is JCB and the vehicle is called DieselMax.
Also, even with low compression from piston displacement, if it has a positive displacement, like a roots blower for instance, that might somewhat increase total compression and therefore charge temperature.
Regards
Pat
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JSteve2 – on the subject of expansion ratios etc.
If I can I like to be able to visualise in a commonsense way the thermodynamic principles behind the various types of engine cycle. As I see it the variation in CR is not the controlling factor in Otto cycle efficiency – it is the ER. As I see it, the temperature after compression alone (that is; without the burning of fuel) is not a “real” temp gain – after the following expansion stroke the temp returns to its initial value. The final temp after compression and combustion is the sum of the temp from compression plus the temp increase from the burning fuel. Increasing the CR may raise the final compression+combustion temp but after expansion the temp rise due to compression cancels itself out. Thus, the way I see it, only the heat gain from combustion is subject to the ER – the increased CR is merely a way of exposing the heat released by combustion to an increased ER. The heat released by combustion would seem to me to be independent of CR etc.
As one of my old thermodynamics textbooks remarks after deriving the formula for Otto cycle efficiency – “It will be noticed that the only variable in this equation is the expansion ratio r”. It seems to make a point of not saying “compression ratio r”.
Does the increasing the CR raise the peak cylinder temp by more than that which results from compression alone? I don’t see how it could.
The above is more of a suggestion than a statement – I am not totally sure about what I have written – perhaps you have a different view on these matters.
BigClive,
I don't think there's anything technically incorrect with what you are saying.
Here is why I wouldn't state things that way - first if I make an engine with high CR relative to low CR, and run the thing without fueling it, the exhaust temperature of the high CR is higher than the low CR. Entropy dictates that less useful work is performed, unless that heat is what you were after.
Second, the efficiency gains are due to the Carnot temperature differences. Remember that, although doubling compression only doubles temperature (even less, actually), it also increases the Carnot efficiency meaning you can get more useful work out of the combustion. There are also some ancillary temperature effects, like reaction rate positive feedback, that drive your peak temperatures higher than might be indicated by the CR alone. So you affect the high side of Carnot with the high CR. You can affect the low side of Carnot with a high ER, but note that for a nominal engine, as stated above, the diesel will result in a higher post ER temperature (all other things being equal such as the same charge temperature and moles). Atkinson tries to go deeper, but that's not what I see as the thrust of this discussion.
OK, so the useful work is available due to Carnot, but you have to recapture it mechanically in some fashion, and that happens during the expansion period, so that could be a way to state this. I just think it's odd to think of this as an expansion ratio effect when the ER relative to CR is the same for a nominal gasoline or diesel. However, as I stated, I'm not sure anything you are saying is technically incorrect.
