It was no less than around 70 years ago that Mercedes engineers implemented the idea of spraying fuel directly into the combustion chambers of an engine, and only then mixing it with air. At that time, the first Mercedes aircraft engines (DB 601) with direct petrol injection were taking to the skies. In April 1939 a Messerschmidt M 209 equipped with a 2035 kW/2768 hp engine from this series achieved a speed of 469.1 mph, establishing a world record which was only beaten 30 years later.
On land, Mercedes direct-injection engines caused a sensation during the 1950s. After numerous racing victories in the 300 SLR, the model M 198 in-line six-cylinder unit entered series production in the legendary 300 SL "Gullwing" in 1954. This engine developed an output of 158 kW/215 hp and allowed a maximum speed of up to 161 mph.
This brief review shows that direct petrol injection has a long tradition at Mercedes-Benz. Nonetheless the researchers and engineers in Stuttgart were entering uncharted technical territory in 1994, when they began the development of a spray-guided combustion process. 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 burns almost completely 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. 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 charging movement in the cylinders.
Injectors: stable jet control based on 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 milli-seconds when subjected to an electrical impulse.
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. Piezo movement is therefore directly translated into needle movement, determining the flow through the valve. This direct operation allows finely graduated strokes, and also a constant flow over the entire cycle time thanks to charge adjustment at the piezo actuator. By virtue of its very uniform stroke, piezo technology also ensures a highly reproducible spray pattern as the basis for effective control of the combustion process.
The developers of the new direct-injection petrol engine were also very demanding 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, hol-low-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 turbulences 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 high excess 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 combus-tion 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 74.5 mph, only later switching to homogenous opera-tion where the fuel/air ratio is 1:14.6 (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 working stroke and thereby considerably improve both mixture formation and ignitability. Combustion is more rapid, uniform and complete than with single injection, while the thermodynamic efficiency of the engine improves significantly and engine-out emissions (hydro-carbons) 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 therefore also plays its part in ensuring near-total combustion, low fuel consumption and low emissions in the direct-injection 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 new CLS 350 CGI is designed to operate on sulphur-free unleaded premium fuel and its state-of-the-art technology gives it the potential to adapt to emissions standards of the future. In Western Europe, the CLS direct-injection petrol model will replace the current CLS 350. The 7G-TRONIC seven-speed automatic transmission is standard equipment.
Emission control: in-engine measures plus four catalytic converters
As in the V6 with conventional fuel injection, the emission control concept of the new CGI engine has two stages: it is based both on in-engine measures which ensure low engine-out emissions and on effective exhaust gas aftertreatment by a total of four catalytic converters.
The in-engine measures specifically include the Mercedes-developed combustion process featuring multiple closely spaced injections on each compression stroke. This improves the exhaust quality of the V6 engine in the warm-up phase, as actively controlled injection and combustion using low quantities of fuel ensures higher temperatures in the exhaust manifold and accelerates catalytic converter warm-up. Just ten seconds after starting from cold, the direct petrol injection engine reaches an exhaust temperature of over 700 degrees Celsius. Dual electrically controlled exhaust gas recirculation is also employed, with which up to 40 percent of the exhaust gases can be returned to the combustion chambers. This achieves a considerable reduction in nitrogen oxide emissions.
Emission control begins with two close-coupled three-way catalytic converters, each of them monitored by two oxygen sensors - a control sensor and a diagnostic sensor. This linear oxygen sensor control goes into operation immediately after the engine starts from cold, providing information about the exhaust gas constituents which the electronic control unit of the V6 uses for a controlled warm-up.
As conventional catalytic converters require a "stoichiometric" fuel-air mixture (lambda = 1), but stratified charge operation uses high excess air (lambda >1), the CLS 350 CGI is equipped with two NOx storage-type catalytic converters. Under lean operating conditions these converters adsorb the nitrogen oxides, then de-sorb them during brief regeneration pulses so that they react with other exhaust gas constituents to form harmless nitrogen.
The NOx storage-type catalytic converters are also monitored by sensors - a temperature and a nitrogen oxide sensor.
High-tech package: new Mercedes six-cylinder unit as an innovative basis
The new direct-injection petrol engine is based on the port-injected V6 powerplant first presented by Mercedes-Benz in 2004. In addition to its pioneering injection process, this engine excels with a number of other technical innovations:
- Variable camshaft timing on the intake and exhaust sides – no other V6 engine has this feature – improves the available output. The camshaft angles are adjustable by anything up to 40 degrees to ensure that the valves are able to open and close at the most favourable time in any driving situation.
- A variable intake module varies the air supply as required. The length of the intake ducts leading to the cylinders is adjusted by means of flaps: at lower engine speeds the flaps are closed to increase the length of the intake duct. This creates pressure waves which support the intake process and make a lasting improvement to the torque yield in the lower engine speed range. As a result 317 Newton metres – around 87 of the maximum torque – is already available from 1500 rpm.
- Fuel economy is improved by an intelligent thermal management system. Coolant circulation is stopped during the warm-up phase, so that the engine reaches its normal operating temperature more rapidly. The result is an improved oil flow and considerably less in-engine friction, as well as lower exhaust emissions. When the warm engine is operating under full load, the thermal management system always keeps the engine oil and coolant at the best possible temperatures. This ensured by an electronically controlled ther-mostat which is active in all driving situations.
- The cylinder head and crankcase are of aluminium. The pistons, connecting rods and cylinder liners also follow the latest design principles, not only helping to save weight but also making a positive contribution to responsiveness and smooth running.
- The cylinder liners are surfaced with a low-friction aluminium-silicon alloy which has proved its worth in other Mercedes-Benz car engines. Other advantages include high dimensional stability, exemplary thermal characteristics and low weight. The weight-saving compared with conventional grey cast-iron liners is around 500 grams per cylinder.
- The forged crankshaft is equipped with four counterweights. Four wide crankshaft bearings with transverse reinforcing struts attached to the crank-case also help to reduce vibrations.
- A balancer shaft between the two cylinder banks compensates the free vibration moments which are inherent to a V6 engine, ensuring exemplary smooth running. It counter-rotates at the same speed as the crankshaft.