The main advantage of the CGI engine (CGI = Stratified-Charged Gasoline Injection) lies in the stratified operating mode from which it takes its name. During this mode the engine is run with high excess air and thus excellent fuel efficiency. Now, thanks to multiple injection, it is for the first time possible to extend this lean-burn operating mode to higher rpm and load ranges too. During each compression stroke, a series of injections takes place, spaced just fractions of a second apart. This has the effect of significantly improving mixture formation, combustion and fuel consumption. While stratified charge operation was previously only possible in the low part-load range, the new Mercedes direct-injection engine can still operate in this lean-burn stratified mode at speeds in excess of 120 km/h.

When driving on main roads and motorways at largely constant speed and with proper anticipation, the CGI engine outperforms the fuel economy of the six-cylinder engine with conventional injection technology by up to 1.5 litres per 100 km, a saving of up to 15 percent.

Effortless performance twinned with excellent fuel economy

Low fuel consumption and excellent power delivery are not at odds with each other on the second-generation Mercedes-Benz direct petrol injection model. On the contrary, the engine delivers 15 kW/20 hp more power than the conventional-injection V6 and four percent more torque.

Thus the V6 combines its excellent fuel economy with a level of effortless driving enjoyment that is unprecedented in the six-cylinder segment. The CLS 350 CGI accelerates from 0 - 100 km/h in just 6.7 seconds and has an electronically gov-erned top speed of 250 km/h. The key figures for the new CGI engine are:

• Displacement:   3498 cc
  
  
• Compression ratio:   12.2:1
  
  
• Rated output   215 kW/292 hp at 6400 rpm
  
  
• Rated torque   365 Newton metres at 3000-5100 rpm
  

With a fuel consumption of 9.1 - 9.3 litres per 100 kilometres (NEDC combined cycle), the four-door Coupé has a range of approximately 870 kilometres on one tank filling (80 litres).

Pioneering invention: outward-opening piezoelectric injectors

The most important components of this innovative direct petrol injection system are the fast-acting, high-precision piezoelectric injectors. It is on this invention and the way it has been translated into series-production technology that the most important advances associated with spray-guided combustion are based. The piezoelectric valves have injectors which open outwards to create an annular gap just a few microns wide. This gap shapes the fuel jet and produces a uniform, hollow-cone-shaped spray pattern. The microsecond response times of the piezoelectric injectors provide the basis for delivering multiple injections per compression stroke, and thus for lean-burn operation. By allowing flexible and efficient control of the combustion process they play a key part in ensuring the engine's outstanding fuel efficiency.

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 petrol engine.

A high-pressure pump and downstream fuel rail and pressure control valve are responsible for delivering the fuel and regulating the quantity supplied. The peak fuel pressure in this system is up to 200 bar - around 50 times the fuel pressure in a conventional petrol injection system.

Dual effect: low engine-out emissions and high exhaust temperatures

The Mercedes-developed combustion process featuring multiple closely spaced injections on each compression stroke also results in smoother operation and improved emissions performance. Measurements show that engine-out hydrocarbon emissions in the warm-up phase are almost halved. Furthermore, since the injection and combustion processes can be actively controlled, it is also possible to raise temperatures in the exhaust manifold and thus speed catalytic converter warm-up. Just ten seconds after starting from cold, the direct-injection petrol engine reaches an exhaust temperature of over 700 degrees Celsius.

Emissions are controlled by two close-coupled three-way catalytic converters with linear oxygen sensor control, which goes into operation immediately after the engine starts from cold.

To reduce nitrogen oxide emissions, Mercedes-Benz has adopted a two-part strategy. This comprises, firstly, dual electrically controlled and cooled exhaust gas re-circulation which, depending on engine operating conditions, redirects up to 40 percent of the exhaust gases back into the cylinders. Secondly, it also comprises two underfloor NOx storage-type catalytic converters. Under lean operating conditions, these converters adsorb the nitrogen oxides. Periodically, during brief regeneration pulses, the nitrogen oxides are then desorbed, reacting with other exhaust gas constituents to form harmless nitrogen. Sensors upstream and down-stream of the catalytic converters monitor their operation.

In addition, the new CGI engine also incorporates the same unique package of high-tech features as its conventional-injection counterpart. This includes four-valve cylinder heads, variable intake and exhaust camshaft timing, a two-stage in-take manifold, balancer shafts and intelligent thermal management with an electronically controlled thermostat. The crankcase and cylinder heads are of aluminium and the cylinder liners are of low-friction, thermally resistant, lightweight aluminium-silicon alloy.

All fuel-carrying components of the CGI engine are of high-grade steel or brass; the rails in the area of the two cylinder banks and the housing of the high-pressure pump are of forged stainless steel.

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 petrol injection model will replace the current CLS 350.

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