Re: 7.3 swap

[bearbvd@domain.elided (Greg Hermann)]

>Tom:
>
>In general diesels are more efficient for a couple of reasons having to do
>with their inherent characteristics:
>
>1) They operate at 100% (or close to that) volumetric efficiency at all
>speeds due to their varying amount of fuel injected but not having throttle
>plates to vary the density of the air entering the cylinders.

No, they do not. diesels typically have a far LOWER VE than spark motors at
WOT--because they need to trade off turbulence in the air charge for VE in
order to help the fuel "find" the air in the chamber.
>
>i.e. They are "open to the atmosphere" at all speeds. To get this same
>effect on a gasoline engine, you'd have to floor the throttle.

Diesels DO have lower pumping losses than spark motors at part throttle due
to the lack of a throttle--but this advantage is non-existent at WOT!!

The Otto (spark engine) cycle is actually significantly more thermally
efficient than the Diesel cycle!!!
>
>Add a turbocharger and you can get better than 100% volumetric efficiency in
>a diesel!

Same applies to all--If I were building that 605 to go in a boat, using
water cooled intercoolers, a (slightly) bigger cam, more like 18 or 20 psi
of boost, and turning it more like 5200 rpm max (YES, it would do it OK),
we would be talking about 1400 or 1500 HP (1400 or 1500 ft lbs, also) so
fast it would be funny!! That 925 ft. lbs. is really a pretty mellow state
of tune for the thing!
>
>2) They make "working torque" at lower RPMS. And, lower RPMS increases
>efficiency because of less internal friction and rotational forces to be
>overcome.

Diesels torque curves have a lower peak than spark motors--mostly because
the fuel cannot find the air in the chamber quickly enough to burn soon
enough to do any good at high engine speeds. A diesel must be bigger (or
boosted to a higher pressure) to make anything like power comparable to a
given spark motor--thus the high low speed torque--but its torque falls off
much sooner than a spark motor's does.

And--let's don't forget that diesels are utterly incapable of burning all
of the air that they breathe in--they foul on soot LONG before they get to
a stoichiometric air/fuel ratio! So--they HAVE to have a larger
displacement to make the same power/torque at the same level of manifold
pressure as a spark engine!!

Diesels also have higher internal friction because of their higher
compression ratio. The point where the increase in thermal efficiency due
to higher compression is more than cancelled out by the increase in
friction due to a higher compression ratio typically occurs at about 11 or
12 to 1 compression--and this c r  range is where best fuel economy will
occur, either for a spark or a diesel engine.
>
>3) Diesel fuel has 40% (I think that was the figure I saw - I have no
>reference to check) more potential energy per gallon than gasoline. Hence,
>more work can be done per gallon of diesel burned than gasoline.

I do have a reference--(Internal Combustion Engines, by Edward F. Obert)
comparing isooctane to heptadecane (cetane) , the cetane (standard diesel
fuel) actually has 99.6% of the heat of combustion per unit volume that the
isooctane (standard spark motor fuel) does! Of course, these are pure
hydrocarbons, not the typical blends, but each is pretty representative of
what goes into a blend of either type. So--the real difference is not
nearly what your impression was!!
>
>O.K. you say, if diesels are so great, why isn't everyone driving them?
>
>Well, a lot of reasons, mostly education and habit. Mostly, I think it's our
>American love of speed and horsepressure. The diesels current and past, are
>somewhat lacking here.

Diesels are improving, but there are basic limitations to how well they can
do, and there are definite, well proven, non-mechanical limitations to
their maximum engine speed. When total weight is considered as an important
design factor, diesels fall by the wayside very quickly. Ever seen a diesel
cycle engine in a piston engine powered aircraft??
>
>But, give it about ten more years, you'll see a lot more diesels on the
>road. Take a look at Europe where 70% of all light trucks sport diesel
>power, compared to the U.S. where it is currently only 20%.

The situation in Europe is driven by politics and taxation, not by
technological and scientific considerations, let alone by free economics!
I, for one, hope that our country stays freer than that!!

Understand please--none of this is intended as a rant or a flame--just
trying to present facts for due, objective  consideration! I HATE wive's
tales!

Regards, Greg
>
>Tom H.
>
>2)