Originally Posted by
Kitdy
So according to you, diesels engines can't rev high because of their strength, a high compression ratio interferes with valve timing...
...but according to jediali, it is because diesel gas takes longer to burn. Is it a bit of both, or is one of you wrong?
If diesel takes longer to burn, would it help the situation if there was a spark plug?
First I mentioned that there are several reasons I didn't mention them all. Yes the relatively low flame front speed of diesel fuel keeps the redline speed down but if you do the math then it should only limit a 2L Inline 4 diesel engine to about 6000-6500RPM yet the fastest stop at 5000RPM or less. Rather then the low flame speed of diesel I would think that Jediali's explaination of how Diesel cycle combustion differs from Otto cycle (ie constant pressure versus constant volume) is vital in understanding the difference. Also I was implying that because the internal componants of a diesel engine have to be made strong they are also heavy and the high compression ratio normally means a longer stroke aswell as difficulties with high RPM valvetrain optimization.
Originally Posted by
Kitdy
The fundamental difference being that the Otto cycle includes spark plugs, yes?
Well that isn't the only fundamental difference there is also the different combustion (like Jediali said) and the fact that they are wholly optimized for high compression (even at the expense of high RPM operation) as Mr. Diesel saw compression ratio as something that couldn't be optimized enough when using a Otto cycle engine.
BTW:
Jediali: I am happy you realised that you switched the meanings of homogeneous and heterogeneous mixtures. Also I would like to point out that Kitdy was right about energy content of diesel fuel. Diesel is more dense (850g/L vs 720g/L for petrol) and has a higher energy content per volume (40.9MJ/L vs 34.8MJ/L for petrol). However you are right about the caloric value (45.3MJ/kg vs 45.8MJ/kg for petrol).
Culver: Thank you for your clear explaination on homo- hetero-geneous mixtures (I was writing up the same thing before I realised you beat me to it!)
I am looking forward to what you Prof. has to say about HCCI technology. Personally I have been following progress on this subject for awhile now and it certainly seems interesting. Apparently instead of stopping detonation this type of engine is designed to create a smaller combustion that is like detonation (sudden almost instantaneous and complete combustion of the injected fuel). It is able to combust a lean homogeneous mixture. It seems the biggest problem is how to throttle (control) the engine properly and get it to work under high load conditions.
Last edited by hightower99; 12-17-2007 at 12:10 PM.
Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
Engine torque is an illusion.