Lean burn combustion uses clocked compression stroke injection; the last injection just before ignition serves to stabilize the mixing and turbulence conditions. This produces stable and virtually stoichiometric mixtures in the area of the spark. In conjunction with multi spark ignition (more on this below), optimum ignition conditions are created under a range of load and speed conditions.

The principle of stratified charge operation is to deliver a mixture that is sufficiently rich for combustion in the immediate vicinity of the spark plug, while the remainder of the cylinder receives a very lean mixture that could not be used in a traditional engine. Stratified charge operation enables sharply reduced fuel consumption when running at lower loads.

In stratified charge mode, the high-compression engine operates with a high level of excess air. To this end, the fuel is not injected into the air until the compression phase, after the air has been compressed by the pistons. A precision-controlled electronic multiple injection process ensures that a fuel-air cloud localized in the area of the spark plug forms only at the time of ignition.

This fuel-air cloud is ignited by the spark plug and in turn ignites the very lean mixture which is distributed throughout the rest of the combustion chamber. This enables extremely efficient combustion with a high level of excess air.

Because the last injection takes place immediately before ignition, with the piston located almost at TDC, the requirements for depth of spray penetration and vaporization behavior are much higher than for intake stroke injection in order to prevent the moistening of pistons and walls. I.e., free spray length is reduced significantly. Thus, one of the key enablers of this BlueDIRECT combustion system is a piezo injector with an outward-opening nozzle, which can distribute very short injection impulses extremely precisely and stably.

Outward-opening piezo nozzle (left) compared with the standard multi-hole valves (right) showing a completely dethrottled operating point with 200 bar injection pressure for the piezo injector versus 130 bar for the multi-hole valve. The high injection pressure in conjunction with the outward-opening nozzle with a very small valve gap produces a tapering hollow spray, for which the depth of penetration is only about 50% of that of a conventional multi-hole valve. Vaporization occurs four times faster than with multi-hole valves of the type currently used. This reliably prevents wall moistening, which is the basis for extremely low particle emissions and thus compliance with the EU6 Stage 2 exhaust gas standard. It also creates increased robustness with respect to carbonization and residue build-up. Source: Vent et al. Click to enlarge.

Lean burn stratified mode encompasses characteristic map ranges from idle mode up to 3,500 rpm and mean effective pressures of up to 5 bar, Daimler says; this corresponds to approximately half the full load of a naturally aspirated engine.

In the load range above 5 bar, the HOS mode (homogeneous stratified lean combustion), which was specially enhanced for turbocharged 4-cylinder engines, is used. Here, a combination of intake stroke injection and late compression stroke injection in conjunction with turbocharging are used in order to extend the lean-mixture operating range as far as the full intake load. The load range above this is, in turn, operated stoichiometrically and since the engine is already being operated almost dethrottled due to the high loads, it results in a thermodynamic optimum even without lean-mixture operation.

Operation strategies on the characteristics map. Source: Vent et al. Click to enlarge.

The four-cylinder BlueDIRECT engines feature third-generation direct gasoline injection with spray-guided combustion. The system pressure is now up to 200 bar and is adjusted to the optimum level based on the engine’s performance map. The newly developed piezo injectors allow up to five injections per power stroke for the best possible mixture formation—and thus optimal flammability.

The third-generation direct injection system also features “rapid multi-spark ignition” (MSI). Following the first spark discharge and a brief combustion period, the coil is quickly recharged and a further spark is discharged. The MSI system enables up to four sparks to be discharged in succession within one millisecond, creating a plasma with a larger spatial expansion than conventional ignition.

The rapid multi-spark ignition can be actuated to vary both the timing of the sparks and the combustion period to suit the relevant operating point. This provides scope for the best possible center of combustion and improved residual gas compatibility. This in turn reduces fuel consumption.

In conjunction with continuous adjustment of the intake and exhaust camshaft, the direct injection system also provides the essential basis for scavenging. As a result of a partial overlap of the opening times of the intake and exhaust valve, part of the drawn-in cold fresh air conveys the hot exhaust gas contained in the cylinder into the exhaust manifold, thereby improving cylinder charging substantially in comparison with the conventional mode of operation. Due to the increased mass flow in the exhaust line, the turbocharger additionally responds much more quickly at low revs in particular, avoiding turbo lag.

A new thermal management system has also been developed: in a cold state, a switchable water pump with flow-optimized ball valve ensures that no coolant flows through the engine, providing for swift heating-up of the combustion chambers after starting up the engine. The thermostat is electronically controlled and the coolant temperatures are adjusted according to driving style and ambient conditions. The thermostat itself is also a flow-optimized ball valve. In the interest of high efficiency, the volumetric flow of the oil pump is also controlled as in the V engines.

The 155 kW (211 hp) CLA four-door coupé generates maximum torque of 350 N·m (258 lb-ft), which is available throughout the wide range of 1200 to 1400 rpm. It accelerates from 0 to 100 km/h in 6.7 seconds.

Diesels. The 200 CDI will have a displacement of 2.2 liters, as opposed to the previous 1.8 liters. It retains a rated output of 100 kW (136 hp) and maximum torque of 300 N·m (221 lb-ft), and emits 102 g of CO₂ per kilometer with the 7-speed DCT: 7 g less than its predecessor. This corresponds to a fuel consumption figure of 3.9 liters per 100 kilometers (60.3 mpg US). In addition, the A 200 CDI already complies with the Euro 6 emissions standard.

The top diesel models—A 220 CDI, B 220 CDI and CLA 220 CDI—have also been revamped. In-engine optimization measures have resulted in a further reduction in fuel consumption with the same performance data.

All-wheel drive diesels. The new A 200 CDI 4MATIC and A 220 CDI 4MATIC models offer efficiency combined with traction, as the first all-wheel-drive diesel models in the new generation of compact cars from Mercedes-Benz.

The most innovative components of the new 4MATIC include the power take-off to the rear axle, which is integrated into the 7G-DCT automated dual clutch transmission, and the rear-axle gear unit with integrated, hydraulically actuated multi-disc clutch. This set-up allows fully variable distribution of the drive torque between the front and rear axles. Additional benefits of this design are a lower system weight compared with the competition and also high efficiency.

Resources

• Guido Vent, Christian Enderle, Dr. Norbert Merdes, Fritz Kreitmann, Dr. Ralph Weller (2012) “The new 2.0l turbo engine from the Mercedes-Benz 4-cylinder engine family,” 2^nd Aachen Colloquium China