Observing a BMW 740 in a tail-down surge from 70 to 130 mph in a patch
of clear traffic is hardly a rare sight on the German autobahns. In a few
months, however, you might see one of these BMW luxury sedans wearing a
740d label, signifying a high-performance breakthrough for diesel engines.
The oil burner under the hood of the 740d is a 3.9-liter V-8 that
develops 235 hp at 4000 rpm and 413 pound-feet of torque at as little as
1750 rpm. Coupled to an automatic transmission, that's enough oomph to
push the 740d to 150 mph while consuming less fuel than a 528i manual.
Similarly energetic diesel V-8s are on the way from Audi and Mercedes.
To find out how a 3.9-liter diesel engine produces more twist than a
5.7-liter Corvette V-8, I asked Dr. Fritz Indra, the engine expert who
once created turbocharged horsepower for Audi and BMW tuner Alpina and who
is now the engineering director of GM's Advanced Powertrain Group. Indra
explains that several major developments, with gasoline-engine parallels,
have recently transformed the diesel engine.
For one thing, these new V-8s, as well as modern oil burners, employ
four valves per cylinder. The advantage of this layout is not so much
improved breathing as it is the centralized positioning of the fuel
injector. By locating the injector in the exact center of the bore, the
pattern of the fuel spray can be perfectly symmetrical to achieve the best
possible dispersion in the combustion chamber. This arrangement also
allows the injection of fuel to be slower and later in the combustion
cycle for reduced emissions. In a four-valve gasoline engine, of course,
the centralized spark promotes faster combustion, which allows a higher
compression ratio.
The injector itself is no longer a mechanical nozzle supplied by a
precision-machined mechanical pump as it was as recently as 10 years ago.
The latest crop of engines uses electronic injectors fed by a common-rail
fuel system. This is much like the electronic fuel injection on every
gasoline engine, except that the diesel fuel is pressurized by an
engine-driven pump to about 20,000 psi, rather than the 20-to-40 psi
generated by the electric pump in most gasoline tanks.
This pressure feeds injectors that spray a mist of fine droplets
through five or six laser-drilled nozzles directly into the combustion
chamber. This is a marked change from older diesels that injected fuel
into pre-chambers (essentially little anterooms communicating with the
main combustion chamber) that reduced the traditional knock of diesel
combustion.
This sharp, almost metallic knock occurs because in a diesel the fuel
basically explodes shortly after it's injected into the cylinder, unlike
in a gasoline engine where the flame front progresses gradually through
the combustion chamber. By reducing the sharp pressure increase produced
by the explosion of diesel fuel, the pre-chamber reduces the diesel's
knocking.
With the electrically controlled common-rail injectors, however, the
fuel can be sprayed directly into the chamber in two or more stages to
reduce the knocking. By eliminating the pre-chamber and the heat lost
through its walls, fuel economy improves about 15 percent.
Further gains are achieved by generating swirl within the combustion
chambers. In the BMW V-8, one of the two intake ports for each cylinder is
angled to generate swirl, but Indra explains that on some of Opel's latest
diesels, secondary throttles can selectively block one of the intake ports
to achieve the same effect.
Still, diesel power remains limited because its combustion process
precludes revving much above 5000 rpm. Moreover, because the fuel sprayed
into a diesel combustion chamber has no time to mix with the air, diesels
generate black smoke if you try to burn more than 85 percent of the
available air (local regions of overly rich mixture start producing the
black smoke we see in many diesels at full power).
So if you can burn only about 85 percent of the available air, why not
add more air with some manner of forced induction? The BMW diesel V-8
employs twin turbochargers, along with a hefty intercooler, to produce a
peak boost of about 15 psi and roughly double the air flowing through the
engine. Knock is not a problem with the diesel because its combustion is
essentially controlled knock to begin with.
The twin turbos on the BMW V-8 are unusually responsive owing to the
variable geometry nozzles on their exhaust turbines. When these
electronically controlled nozzles close down, they essentially make the
turbos act like tiny blowers to produce meaningful boost at low rpm. At
higher speeds, they open up to maintain efficiency and reduce back
pressure. This approach is an excellent way to reduce turbo lag, but for
now, the variable nozzle mechanism survives longer in a diesel's
1500-degree exhaust stream than in a gasoline engine's 1750-degree blast.
These modern diesels employ catalysts to reduce hydrocarbon and
carbon-monoxide emissions to gasoline-engine levels, but oxides of
nitrogen remain a problem in that conventional catalysts require a richer
mixture than the diesel provides to neutralize NOx. What's needed is a
lean-burn NOx catalyst. Several are under development, but they require
low-sulfur fuel to operate properly.
Smoke (particulates) remains a problem. Ultra-strict California
standards (one gram of particulate matter per 100 miles) proposed for the
next decade will, if implemented, essentially outlaw diesels. Such
standards are unattainable even by the various experimental traps that
capture these particles of soot before they can escape from the tailpipe.
Besides, these traps create back pressure in the exhaust system and
require periodic cleansing, which is accomplished by applying a flame to
burn out the captured soot.
Still, Indra remains enthusiastic about these diesels. "You can
speculate that future high-performance engines will all be diesels."
I'm not convinced, but if fuel gets scarce again and the regulators have
some mercy, these powerful new diesels will be the most promising power
plants for the large, heavy cars and trucks of which we're becoming more
and more fond.
