Why diesels get good economy - application to petrol engines

[Ted Hart]

Originally Posted by brick

Diesels don't have "ignition timing" because there is no ignition system. You get it started using glow plugs and compression takes care of business from there.

Is not the injection point of the diesel fuel the "timing point" of the diesel engine? The air is squeezed (and HOT!); when the fuel is injected..."Bang!"

[omgwtfbyobbq]

Yup, that's how it usually works.

[Sludgy]

Diesels get good economy for three main reasons. These factors can be mimiced in gas engines:

Diesel combustion is always lean, meaning more complete fuel combustion. Gas engines can lean also out the mixture, but the resulting NOx emissions are excessive, and USEPA would never allow it.

The compression (actually expansion) ratio is higher. Compression ratio in gas engines is limited by knock. A high compression ratio with late-closing intake valve timing that reduce the compression that prevents knock, while still providing a high expansion ratio. This is the "Atkinson" cycle.

There are no part-throttle pumping losses. Variable valve timing can minimize the pressure drop across the throttle.

[Mike T]

EGR will abate NOx to acceptable levels. It sure does the trick in diesels.

[Ted Hart]

Originally Posted by diamondlarry

... it would jump and buck so bad it would be pretty much un-driveable. Now, if you could figure out how to totally vaporize gasoline...

I have a problem with your question: what does the % vaporization have to do with jumping & bucking? My reason for asking...I can vaporize 100% of the gasoline...but bucking & jumping seem outside the picture of results. Please elaborate....

[rh77]

Originally Posted by Ted Hart

I have a problem with your question: what does the % vaporization have to do with jumping & bucking? My reason for asking...I can vaporize 100% of the gasoline...but bucking & jumping seem outside the picture of results. Please elaborate....

I think it's impossible to vaporize 100% of the fuel, with the current technology.

RH77

[GasSavers_Brock]

I could be way off base but also isn't the fact that a diesel optimum rpm for fuel usage also very similar to where the engine typically cruises (2000rpm). I was under the impression gasoline engines tended to turning a lower RPM then what gave you the most mpg from the fuel? I am probably not saying that correctly.

[landspeed]

Brock : diesels have peak torque lower down in the RPM range, meaning that you get the most efficient power output while the engine is turning more slowly, so less drag from the engine mechanicals. I think what you are saying is also true, that peak torque occurs at cruise speed, which means you get most efficiency at the speed you tend to drive at.

[GasSavers_Brock]

Yes, that’s what I meant to say. Thanks for saying it correctly

[omgwtfbyobbq]

I think you two are making a big assumption. Iirc, peak torque isn't always peak efficiency for gasoline or diesel engines, it usually isn't imle. Diesels just don't have pumping losses at low load, while gassers do. Both have optimal efficiency around 1,500-2,000rpm based on the few BSFC maps I've seen.