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A new twist has been given to the
dialogue about whether the future of the hybrid leans toward the
gasoline or diesel solution, with the announcement by Mercedes of
its highly complex DiesOtto (Diesel Otto-cycle) unit—a mix of the
two based on a gasoline engine block. Claimed to combine all the
advantages of both engine types, it was revealed to AEI shortly
before the Frankfurt Motor Show where it was installed in a concept
vehicle.
Volkswagen unveiled its work on a similar combined combustion system
(CCS) engine late last year based on a diesel block that burns
biofuel; however, the Mercedes engine uses a gasoline block and is
fueled by gasoline. Professor Herbert Kohler, Vice President, Group
Research and Advanced Engineering, Vehicle and Powertrain, and Chief
Environmental Officer, Daimler, said that in a sense, diesel and
gasoline engine technology—which had gone their separate ways for
decades—“will be celebrating a marriage beneath the hood.” He also
said that if the specific strengths of each were combined, the
result would be an almost optimal concept, which he described as “an
engine that achieves top ratings for output, torque, emissions, and
fuel consumption, and is also very appealing in terms of cost.”
Kohler admits,
however, that turning this “marriage beneath the hood” into reality
is not quite so easy, although “we are well ahead with our
development work. And the DiesOtto engine becomes even more
economical if combined with a hybrid module—for which it was
designed.” Kohler would not be drawn on the likely timescale for a
production DiesOtto other than to say that it was feasible “in the
mid-term.”
The DiesOtto has
direct gasoline injection, turbocharging, and a variable compression
ratio. It also has “controlled auto ignition, a combustion process
similar to that of a diesel,” as well as variable valve control. |
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The engine uses spark ignition
when starting and also under full load, with the controlled auto
ignition sequence taking over under partial loads and low to medium
engine speeds. The technology is said by Mercedes to provide “very
low” NOx emissions typical of homogeneous combustion at reduced
reaction temperatures. The engine has a regular three-way catalytic
converter.
“While the direct injection of
our 350CGI engine is intended to stratify an ignitable mixture
around the spark plug in lean-burn operation, its main purpose in
the DiesOtto is to get an excellent homogeneous mixture and to
control the self-ignition by the injection process.” He added: “The
art lies in controlling the combustion point using the fuel
quantities and injection timing. As the mixture ignites
simultaneously at numerous points, the resulting combustion is very
uniform—i.e., homogeneous.” |
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A “variable crank train” is fitted,
but as AEI closed for press, Mercedes had not released full details
of how its variable combustion system operated.
The research four-cylinder
DiesOtto unit produces 175 kW (235 hp) and for the hybrid
application is allied to a 15-kW (20- hp) electric motor. Maximum
torque for the total system is 400 N·m (295 lb·ft).
“A comparison with our current
gasoline 2.5-L V6 naturally aspirated engine and the 3.0-L V6
turbodiesel confirms that the DiesOtto engine shows performance and
torque values for 1.8-L displacement which are presentable,” said
Kohler. “The specific values that correspond to particular
displacement demonstrate peaks that compare with conventional
powertrains. This proves that we really have combined the best of
two worlds,” added Kohler. “The rated fuel consumption achievable
together with hybridization is below 6.0 L/100 km and we are not
talking about a compact car but a research car with the approximate
dimensions of the present S-Class.”
Mercedes is to place ever-greater
emphasis on hybrids, not only in conjunction with a gasoline IC
engine or, further ahead, the DiesOtto. The company is also
developing diesel hybrid solutions and expects to overcome the
challenge of high costs; typically a complete diesel engine may be
up to 50% more expensive to build than a gasoline unit.
“In future, Mercedes-Benz will
only develop vehicles and engines which can be enhanced with hybrid
technology,” said Kohler.
Stuart Birch |
