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Originally Posted by joselu43
I respectfully disagree. I believe Diesel cycle engines are a better bet than Otto as far as efficiency goes, and, if gas prices keep moving up, we are going to see a lot more of them. Today's Diesels are a new breed and very close in performance to gas engines. In terms of efficiency they easily beat gas engines. Despite the fact that the Otto cycle is more efficient than the Diesel one, because Diesels operate at much higher pressure ratios (from 16 to 24:1) their efficiency is higher. The maximum possible efficiency for an Otto engine at a 10:1 compression ratio is about 45%. Typical efficiencies for cars are 18 to 20% city and 26 to 28% highway, where is the other 17 to 27% going? Well, friction (valves, piston rings etc), combustion loss (small usually) and more importantly the losses associated with letting the air into the cylinders. These are particularly high at light engine loads. We have reduced them by going to 3,4 and 5 valves per cylinder, using variable inlet geometry and variable valve timing and a number of other things. We can go to lighter pistons, fewer rings, leaner burning and so on at increased complexity and cost but I do not think gas engines could even then match diesel efficiency.
That's what I think anyway.
JL
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I pretty much agree with everything you've said. It wasn't clear who you are disagreeing with. Assuming it was me, I didn't mean to imply that I thought gas engines will become more efficient than diesels. I assume that the same types of advances will be applied to both diesels and gas engines. If I was to bet, I would say that diesels will remain more efficient for the foreseeable future. I'm a big fan of diesels, I have two.
Also, gas engines only approach their theoretical efficiency under full throttle conditions. Under low load they are controlled by throttling the inlet air, which is not included in the Otto Cycle model. In other words, diesels operate closer to their maximum efficiency than gas engines in the real world.
One clarification to what you said. Modern diesels do not actually operate on a classic "Diesel Cycle," they are actual operating closer to a "Air-Standard Duel Cycle" (had to look that up) which is more efficient and similar to a gas engine at full throttle. The difference has to do with the fuel injection timing. I think this helps support your point.