Diesels the best way to get high MPG?

[Diesel_Dave]

Originally Posted by Frank Lee

I'm not so sure diesels have less friction. In spite of turning lower rpms, they typically have longer strokes; interestingly enough there are many gassers that run 1000-1200 ft/mn piston speed while the two stroke diesels anyway are running 1700! The ring pack is the largest single contributer to friction so higher piston speeds must lead to higher friction.

Plus they have heavier rotating/reciprocating parts which add friction.

Now that I think about it some more, it might not be fair to make a blanket statement that diesels have lower friction. It probably depends a lot more on the specific engine design. Plus, diesels have higher cylinder pressures (which would increase friction).

[George Tyler]

i don't think desiel ngine economy is helped by the constant pressure combusion, it would be better if it was at constant volume. Deisel engine consumption is hurt by not having a stoiciometric mixture, let me explian like this: the efficiency of an engine, all else being equal, is related to the ratio of the maximum temp to the minimum. The minimum depends greatly on the expansion ratio, and here the diesel does well, but the highest temperature at the top of the stroke is accieved with a chemically correct mixture, and also in the shortest time.
With a diesel with constant pressure the fuel is injected over a period of time, lowering temperatures and efrficiency over what it could be, and also the fuel that burns further down the power stroke is expanded with a lower ratio.
neverthe less, I have 3 Priuses, and have used these and also a deisel Pajero for my 4km trip to work in traffic, and the diesel actually is not much heavier than the hybrids like this, and it's also lighter than my Daihatsu charade petrol! My favorite engine is the Isuzu 2.5 non turbo direct injection deisel in a ute I had, it got 14km/L in all conditions. Deisel's gains in efficiency more than compensate for any higher losses.....

[oldbeaver]

Volkswagen developed a turbo diesel engine which is not common rail, and seems to have better combustion control. It is the "pumpe düse" (PD) engine:

http://www.myarchive.us/richc/VW_TDI_with_PumpeDuse.pdf

I have a Skoda Octavia 1.9 TDI PD and seems to have a better fuel use than other diesel engines, probably because of the extremely fine control of fuel injection to produce maximum performance.

OldBeaver

[Diesel_Dave]

Originally Posted by George Tyler

i don't think desiel ngine economy is helped by the constant pressure combusion, it would be better if it was at constant volume. Deisel engine consumption is hurt by not having a stoiciometric mixture, let me explian like this: the efficiency of an engine, all else being equal, is related to the ratio of the maximum temp to the minimum. The minimum depends greatly on the expansion ratio, and here the diesel does well, but the highest temperature at the top of the stroke is accieved with a chemically correct mixture, and also in the shortest time.
With a diesel with constant pressure the fuel is injected over a period of time, lowering temperatures and efrficiency over what it could be, and also the fuel that burns further down the power stroke is expanded with a lower ratio.

If both engines ran the same compression ratio, you would be correct that a diesel is less efficient. This page explains the details better than I can here:
http://www.classle.net/sites/default...ual_cycles.pdf

[Arragonis]

Yes.

[Piwoslaw]

Diesels may be more efficient (thermally), but EM's Garage shows that more important than what you drive is how you drive it. The question remains, can the best gasser hypermilers get the same/better numbers with a diesel (all else being equal), and vice versa?

[Arragonis]

Originally Posted by Piwoslaw

Diesels may be more efficient (thermally), but EM's Garage shows that more important than what you drive is how you drive it. The question remains, can the best gasser hypermilers get the same/better numbers with a diesel (all else being equal), and vice versa?

And yes too.

[darcane]

My latest issue of AEI magazine had an interesting engine from Honda. It had a linkage on the crank that allowed for a larger expansion ratio than it's compression ratio. IIRC, it was a gas engine with 12:1 compression and about 17:1 expansion. Between that and a heat exchanger built into the catalytic converter, they were trying to hit 50% thermal efficiency. I don't remember if they achieved that goal or not. But, it looks like we still have room to improve on gas engines.

I could dig it up tonight if people are interested.

[euromodder]

When the higher energy content of diesel is taken into account, the numbers aren't that good anymore.
Then there's the matter of the worse emissions: NOx, particulates.

The sheer complexity of today's diesels is another problem area.
I'm not the only one with particulate filter issues.
Current research indicates that having a DPF may well be worse than having none - the particules are still there, but smaller and more harmful as a result.

[oil pan 4]

I don't know of any one with a big gas powered truck getting MPG numbers in the high 20s to low 30s.