2007 Mercedes CLS in Depth

Press Release

Page 1: Short Version
Page 2: Model Range/Equipment
Page 3: Engine (I)
Page 4: Engine (II)
Page 5: Active & Passive Safety
Page 6: Chassis
Page 7: Specifications

Engines: New direct petrol injection system and of a more powerful eight-cylinder engine

• New CLS 350 CGI consumes ten percent less petrol

• Output increased by 60 kW/82 hp in new V8 model CLS 500

Three new developments are being added to the CLS-Class engine line-up. Besides the tried-and-tested V6 diesel of the CLS 320 CDI, Mercedes-Benz is now offering the four-door Coupé with an all-new direct-injection petrol engine and the new, more powerful V8 originally developed for the S-Class. At the upper end of the engine range is the new 6.3-litre V8 engine from Mercedes-AMG, which is used in the CLS 63 AMG (see page 28).

Mercedes engineers first implemented the idea of spraying fuel directly into the combustion chambers, and only then mixing it with air, in an aircraft engine around 70 years ago. On land, Mercedes direct-injection engines caused a sensation during the 1950s. After numerous racing victories in the 300 SLR, the model M 198 six-cylinder in-line unit entered series production in the legendary 300 SL “Gullwing� in 1954. This engine generated 158 kW/215 hp and allowed a maximum speed of up to 260 km/h.

As can be seen, direct petrol injection has a long tradition at Mercedes-Benz. Nonetheless, the researchers and engineers in Stuttgart were entering uncharted technical territory when they began the development of a spray-guided combustion process more than ten years ago. In the view of specialists, this has the greatest potential for overcoming two of the most important automotive engineering challenges of the future, namely an even lower fuel consumption and reduced exhaust emissions.

The greatest advantage of this new technology compared to direct injection with wall-guided combustion is its significantly better thermodynamic efficiency: the fuel is sprayed into the cylinders with great precision - according to requirements and the driving situation - where it is almost entirely burnt with a very high amount of excess air and is therefore put to the best use.

The potential of a spray-guided combustion system had been recognised as a result of research work carried out in the early 1990s; however, the injection technology necessary to put this idea into practice in series production was not yet available at the time. Specifically, the injection valves must form a uniform spray of fuel which is stable under all operating conditions in the immediate area of the spark plugs. This makes a spray-guided combustion system much more technically demanding than the previous, wall-guided process in which mixture formation mainly depends on the (not always uniform) charge movement in the cylinders.

Injectors provide stable jet control on the basis of piezoelectric technology

The aim of creating a spray of fuel which was always uniform and precise required the development of a completely new injector. In 1994 the laboratories at the DaimlerChrysler Research Centre began a series of conceptual studies in which the scientists opted for the latest piezoelectric technology from the very start. This is based on special ceramics and metal alloys which change their shape within milliseconds when subjected to an electrical pulse.

Although these material characteristics were discovered by the brothers Pierre and Jacques Curie back in 1880, this invention has only been put to industrial use in recent decades. In the automotive world the term “piezo� has only been in general use since 2004, when the first diesel engines with third-generation common-rail injection entered the market.

The developers of the direct-injection petrol engine make even better use of the positive attributes of piezo-ceramics, namely power and speed. In contrast to the diesel injector, where the actuator only operates a valve, the piezo module in the petrol engine directly controls the injector needle. The piezo movement is therefore directly translated into needle movement, determining the flow through the valve. This direct actuation enables partial strokes to be set, and also ensures a constant flow over the entire cycle time by means of charge adjustment at the piezo actuator. By virtue of its very uniform stroke, piezo technology also ensures a highly reproducible spray pattern, thereby creating an important basis for the effective control of the combustion process.

The developers of the new direct-injection petrol engine were also very particular where the shape of the injection spray was concerned. These requirements were met with a new type of injector which opens outwards to create an annular gap just a few microns wide. The shape of the gap and the nozzle forms the spray pattern. Under all injection and operating conditions the result is a uniform, hollow cone-shaped spray pattern which even retains its shape if the electronic engine management system changes the angle of the intake camshafts or the length of the intake ducts when a high output is required. The high fuel pressure of 200 bar also makes a major contribution to the consistent stability of the fuel jet.

The mixture formation itself is also of decisive importance. This is optimised by turbulence at the edges and inside the cone-shaped spray; these suck air particles into the fuel spray, forming an optimally ignitable mixture.

Spark plugs precisely positioned at the edge of the fuel/air mix

Correct positioning of the spark plugs was a further challenge requiring sophisticated flow calculations and tests. To ensure that the ignition spark is able to jump rapidly and reliably, the spark plug must reach the cloud of fuel/air mixture but must not be in direct contact with the liquid fuel, otherwise it will gradually carbonise.

In order to meet both requirements, the piezo-injector of the CGI engine extends into the centre of the combustion chamber. It has therefore been moved roughly to the position where the spark plug is located in a conventional port-injection engine; the spark plug has been repositioned closer to the exhaust valves, where it can reach the ignitable mixture at the turbulent edges of the cone-shaped spray. A cross-flow cooling system in the cylinder head ensures that the spark plugs and injectors always operate in the most favourable temperature range.

Stratified charging: even at higher loads and engine speeds thanks to multiple injection

The great fuel economy of the direct-injection petrol engine is mainly based on the stratified charge principle. This means that the engine operates with a high compression ratio and a high excess of air. The fuel is injected into the air compressed by the pistons at a relatively late stage. Such lean-burn operation was previously only possible in the lower load ranges. Thanks to the new, spray-guided combustion system, Mercedes engineers have now been able to extend this lean-burn operating mode to higher rpm and load ranges, achieving further reductions in fuel consumption. The V6 engine in the CLS 350 CGI still operates with stratified charging at speeds of over 120 km/h, only later switching to homogenous operation where the fuel/air ratio is 1:14.6 (stoichiometric ratio lambda = 1).

The conditions for extended stratified-charge operation are created by the extremely fast piezoelectric injectors, as they inject several successive jets of fuel into the combustion chambers during each power stroke and thereby considerably improve both mixture formation and ignitability. Combustion is more rapid, uniform and complete than with a single injection, while the thermodynamic efficiency of the engine improves significantly and untreated emissions (hydrocarbons) are reduced by more than half.

With the aid of simulations for the fuel mixture and the combustion process, the pistons have been designed with special piston bowl geometry which concentrates the lean mixture in the area around the spark plug and prevents it from spreading out towards the cylinder wall. The piston shape can therefore also play its part in ensuring near-total combustion, low fuel consumption and low emissions in the direct-injection petrol engine from Mercedes-Benz.

Fuel delivery: pressure of up to 200 bar in the rails

The injection system of the new Mercedes V6 engine is similar to that of a modern diesel engine with common-rail technology. The centrepiece is a newly developed high-pressure pump which distributes the fuel to the two stainless steel rails on the cylinder banks as required. The piezoelectric injectors are connected to these.

With a pressure of up to 200 bar, the new system develops around 50 times the fuel pressure in a conventional port-injection system. The pump delivers fuel to the rails during every second injection, building up maximum pressure. As fuel is only delivered on every second injection, the pressure is slightly reduced during the cycle. However, the mean pressure for all injectors remains at 200 bar during injection.

A regulating valve ensures that only the fuel quantity required for the engine’s operating point is delivered, thereby reducing the power requirement of the high-pressure pump.

Fuel that is not needed flows back via a water heat exchanger and is mixed with the incoming fuel from the tank of the CLS 350 CGI. The low-temperature coolant circuit of the injection system also cools the electronic control unit of the direct-injection engine, which manages all the working processes of this six-cylinder power unit.

The engine and performance data of the new CLS 350 CGI at a glance:

Thanks to its exemplary torque, the CLS 350 CGI can sprint from 60 to 120 km/h in third gear in just 6.2 seconds.