There are lots of design compromises in diesel engines, and there are
notable differences in DI and IDI engines as currently produced beyond
injection type -- notably, the US made DI engines are almost
exclusively long stroke, limited rpm engines (3000 rpm max) with low
compression and VERY high boost (20 psi) so that the effective
compression ratio under load is similar to a Benz or Volvo short
stroke, high rpm engine (up to 5500 rather than 3000 rpm), high
compression rato (21:1 and up -- Volvo diesels may be as high as 26:1
as built). However, the direct injection engine WILL produce more hp
and torque at the design rpm due to better combustion efficency
(sideways sprays out from the injector rather than a directed flame
from a pre-chamber) -- but ONLY at that rpm, not much higher or lower.
They toss rods when oversped significantly, like any other engine.
The cost is NOISE, and since most US diesels don't have variable
injection timing, smoke and poor efficiency off peak rpm. Used to be
much worse, just watch an old Detroit, Mack, or Cummins over the road
truck at a stoplight -- about the time the black smoke starts to taper
off under load, the driver has to change gears! And I'm not talking
just a little smoke, either!
In modern engines, especially the Cummins sixes, the smoke problem is
mostly gone unless "user modified", but the noise and inflexibility are
still there, hence the lousy milage those engines produce. 17 mpg in a
large empty pickup at 65 mph with a diesel is horrible -- my 72 280 SE
4.5 does that good! That engine should produce 25-28 mpg at that speed
and load. They are nearly bullet proof, produce prodigous horsepower at
peak rpm, and do fine on fuel at that speed.
Prechamber engines, on the other hand, are usually very quiet, very
flexible (wide rpm range with usable hp and torque), but tend to use
more fuel because the thermal effeciency is lower - it's very hard to
get excessive exhaust gas temps because the "fire" will be quenched due
to lack of oxygen in the prechamber when more fuel is added. On a DI
engine, it's fairly easy to keep adding fuel and air and melt the
pistons becuase all the fuel will burn every time. The lower thermal
effeciency is partially overcome by the higher compression ratio, since
higher compression will give you better themal usage reguardless of
other design considerations (and they are easier to start cold, too!)
The fuel efficency overall, however, isn't that simple. Once you're
off the peak rpm in a DI engine, fuel use goes up fast and hp/torque
goes down fast, so unless you have an infinitely variable transmission
or LOTS of gear ratios to choose from, the speed at which you get good
milage is restricted. This was a major problem when speed limits were
lowered in teh 70s as most trucks were running at the wrong rpm -- in
those days, the power band was as small as 1900-2200 rpm! Off that, no
power, stalling on hills, etc, and running a gear down results in even
worse milage....
It's great fun to read engine specs for diesels of all types -- the
difference between US practice and everyone else in the world is
astonishing. Both Detroit and Volvo make large marine engines (3000 hp
or so) -- the Detroit is a huge, triple turbo 18:1 engine that weighs
almost twice as much as the Volvo Penta -- 23:1 compression, 5000 rpm
vs 2200, since tubo, considerably smaller. Both have a history of
dependability, but I'mm willing to bet the Volvo uses somewhat less
fuel in varied use....
The new Jeep diesel is a Detroit, typical low compression engine, and
while it has good torque numbers, the milage STINKS -- barely 20! My
brother's SDL gets 28 on the highway, hauling a heavier body with only
a four speed, not a six speed tranny. I suspect a 60x turbo Benz 2.5L
would give more power, with less fuel consumption (and not much more in
cost, either, since that Detroit adds $4k to the price!)
There are lots of ways around the barn with internal combustion
engines, and diesel engines haven't had the physics worked out as well
as gasoline engines yet (ie they weren't used in aircraft, so real
effeciency wasn't an issue!). Benz has always landed on the side of
extreme longevity and city drivability for the simple reason that the
vast majority of Benz diesels were (and probably are today) used in
taxies. Over-the-road trucks in US form need the same longevity, but
fuel consumption was never a real issue except for a brief period in
the late 70s and today, as the cost is easily passed on to the consumer
with no direct connection. A taxi service will founder quickly if fuel
consumption is high, as most places regulate taxi fares!
One thing I expect to see happen to diesels is the required use of
biofuels, not so much for fuel reasons as for emissions reasons.
Biofuels, since they are oxygenated and don't contain aromatic
hydrocarbons, produce both less soot and "cleaner" soot with less
aromatic content. The carbon dust isn't nearly so much a problem as
the benzene, toluene, and related compounds in it, and nearly everyone
who has tried biofuels remarks on the reduction in visible soot
production.
Peter
- Re: [MBZ] prechambers, was speaking of loop type GP's Peter Frederick
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