A Breakthrough in Performance and Economy

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.

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
Inertia weight class (kg)	1700
Tires	195/50 R21
Displacement (l)	1.8l
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*

Share this Article: 

Digg this story

Seed Newsvine

Post to Reddit

Save on del.icio.us

Email