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If
you don’t think that automotive ideas are changing to take into account
increased awareness of global warming and finite fuel resources, take a look at
the latest Mercedes research vehicle – the F700. Sure it might still be a large,
luxury vehicle but it’s what is under the bonnet that shows the changing
priorities of this German manufacturer, perhaps the most respected in the
world.
So
does the car use a V12?
No.
A
V8?
No.
Try
instead a genuinely innovative 1.8 litre four cylinder, one that can accelerate
the 1.7 tonne car to 100 km/h in 7.5 seconds and yet still deliver test cycle
consumption of 5.3 litres/100km.
So
let’s look at that engine technology in more detail.
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DIESOTTO Drive
An automobile’s fuel consumption is determined by
a great many factors. They include weight, air drag and rolling resistance
(tyres) or the management of the many energy consumers in a vehicle. But the
central and most crucial factor of all is the drive system. Enhancing engine
efficiency remains the primary and decisive factor in cutting fuel
consumption.
Mercedes-Benz is convinced that in the coming
years, internal combustion engines will remain the dominant type of drive in
automobiles – albeit appreciably improved in all dimensions. Depending on
operating conditions and use profile, supplementing it with a hybrid module can
be a sensible step.
Until now gasoline and diesel engines have had
clearly distributed roles: the diesel was more economical, but emission control
was easier to manage in the gasoline-powered car.
“Our strategic objective is to make the
gasoline-powered car as economical as the diesel, and the diesel as clean as the
gasoline engine,” says
Dr. Thomas Weber, Board member of DaimlerChrysler AG
responsible for Group Research & Mercedes Car Group Development.
One part of this strategy already has been
implemented: the BLUETEC technology, which reduces nitrogen oxide emissions,
gives the diesel engine the capacity to comply even with the world’s harshest
emission standards – including those to come in future. BLUETEC is already on
the market in the USA and will be available in Europe from the end of 2007.
The DIESOTTO drive in the new research car is a
unique, integrated system for consumption reduction.
It involves:
-
downsizing by reducing cylinder size and smaller
displacement;
-
turbocharging;
-
direct gasoline injection;
-
variable valve control;
-
controlled auto ignition (CAI);
-
hybrid module with integral
starter-generator.
DIESOTTO brings the benefits of diesel technology
to the gasoline engine: in the CAI mode the air-fuel mixture ignites itself the
same as in a diesel engine. And yet DIESOTTO makes no special demands on fuel
quality: the engine runs on normal gasoline.
Controlled auto ignition, moreover, not only is
more economical than the conventional combustion triggered by a sparkplug, it is
also cleaner as regards emissions. In controlled auto ignition, as the
Mercedes-Benz engineers term their new process, hardly any nitrogen oxides (NOx)
are produced. DIESOTTO thus points to the future of the gasoline engine – a
future imagined, researched and developed by Mercedes-Benz.
Two-Stage Charging and Hybrid Module
The first step to less consumption is to downsize
the engine. Instead of a big-volume, naturally aspirated engine, a compact,
turbocharged power plant is used. This leap backwards in cylinder size and
displacement reduces internal friction in the engine. The smaller unit
increasingly runs in a more efficient combustion range.
The drive system in the research car is a
four-cylinder with a displacement of a mere 1.8 litres. At first, this is
somewhat of a surprise for a vehicle of the luxury class, where up to now at
least six cylinders and three litres displacement are the standard.
The DIESOTTO drive gets additional power from a
two-stage turbocharger system. A larger low-pressure turbocharger and a
high-pressure turbocharger of smaller diameter are connected in series and
together optimally cover the complete engine speed range. The complex system
makes for good responsiveness from low revs, provides high torque and pulling
power and delivers high peak output. For driving off, the internal combustion
engine additionally gets assistance from the electric motor of the hybrid
module.
The maximum output of the internal combustion
engine is 175 kW (238 hp); the electric motor develops another 15 kW (20 hp);
the maximum torque of the system is impressive 400 Newton meters.
The hybrid module, with its integral
starter-generator, helps to even further reduce the excellent fuel consumption
mainly in urban stop-and-go operation. The internal combustion engine is shut
off when not needed. On moving off, the combination of internal combustion
engine and torquey electric motor is used. During coasting and braking, the
electric motor recovers energy which is stored in a special high-voltage battery
and used for the next starting operation. This starter-generator is integrated
in the housing of the 7G-TRONIC automatic transmission.
Direct Injection and Variable Valve
Lift
The next, decisive element in the DIESOTTO system
is direct injection. With the CLS 350 CGI, Mercedes-Benz has pioneered
second-generation spray-guided gasoline direct injection. Compared with
conventional manifold injection, this technology provides a consumption
advantage of some ten percent, among other things due to lean-burn operation at
partial load.
In the DIESOTTO engine direct injection is used
differently than in the CGI engine, which in lean-burn operation needs an
ignitable mixture cloud around the sparkplug. DIESOTTO operates with a lean
mixture, but needs direct injection for a homogeneous mixture in the cylinder
and for the free control of the injection cycles.
Another component is variable valve control. The
valve opening times can be adjusted over a large range, and the valve lift can
be altered too. To achieve this, the camshaft is provided with two cams, a
smaller one and a larger one, for each valve. The large valve lift is needed for
full-load operation and high engine speeds, the small valve lift mainly for CAI
at partial load.
Variable Crank Mechanism
In order to meet the different requirements for
partial and full-load operation, the DIESOTTO engine is fitted with a variable
crank mechanism with which the geometric compression can be continuously
adjusted. So despite high specific output an economical drive system can be
realized.
Path-Breaking New Combustion Cycle
The consumption advantage results mainly from the
relatively lean mixture and high exhaust gas content, and from adaption of the
compression ratio depending on operating point. The low emissions of nitrogen
oxides are a result of homogenous combustion at a constant, relatively low
temperature. For further emission control a common three-way catalytic converter
is used.
CAI is suited for the most frequently used driving
modes, at low and intermediate speeds as well as for intermediate load
requirements. At higher loads, DIESOTTO switches to conventional combustion
within a single power stroke. In this case it breathes using the large valve
lift, fires using sparkplugs and runs as a direct-injection unit in Lambda1 (ie
14.7:1 air/fuel ratio) mode.
Specifications
|
F 700 |
Gasoline-powered |
|
Length (mm) |
5180 |
|
Width (mm) |
1960 |
|
Height (mm) |
1438 |
|
Wheelbase (mm) |
3450 |
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Inertia weight class (kg) |
1700 |
|
Tires |
195/50 R21 |
|
Displacement (l) |
1.8l |
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Rated power DIESOTTO
+ electric motor |
175 kW/238 hp
+ 15 kW/20 hp |
|
Max. torque |
400 Nm |
|
Acceleration 0–100 km/h |
7.5 sec |
|
Top speed, governed |
200 km/h |
|
Consumption* |
5.3 l/100 km |
|
CO2 emissions |
127 g/km |
|
Emission rating |
EU6 |
NEDC overall consumption*
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