Mercedes-Benz's new BlueDirect engine improves fuel economy by 7%

    Mercedes-Benz has announced seven new versions of its A-Class, B-Class and CLA series that will be significantly more fuel efficient than older models; the A-Class 200 CDI 4MATIC and 220 CDI 4MATIC are the first two models in the company's new generation of compact cars to use four-wheel drive technology and diesel engines.

    The 2.0-liter four-cylinder engine in the new Mercedes-Benz CLA 250 is a product in the BlueDIRECT gasoline engine series. The engine first appeared in the E 250 coupe launched last year, and also in the C-Class car that was eye-catching at the North American International Auto Show. It cleverly integrates automobile exhaust turbocharging and high-pressure exhaust gas external circulation. Depending on the specific driving conditions of the car, this engine can independently select stratified or uniform lean combustion mode, helping the CLA 250 to achieve a fuel consumption of 5.4 liters per 100 kilometers under comprehensive conditions (in other words, one gallon of fuel supports the vehicle to travel 43.6 miles), saving 7% fuel compared to the previous generation.



BlueDIRECT 2.0-liter four-cylinder direct injection engine, with the option of stratified or uniform lean combustion mode

    Lean burn utilizes compression stroke fuel timing injection technology, and the last injection close to ignition time helps stabilize the mixing and turbulence environment, thus ensuring the accuracy and stability of the fuel-gas mixture ratio around the spark plug. Combined with multi-spark ignition technology, the best ignition conditions and timing under different load and speed conditions can be obtained.

    The principle of stratified combustion operation is to spray a high proportion of fuel gas to the area around the spark plug to achieve instantaneous and efficient combustion. The fuel content in the gas in the rest of the combustion chamber is very scarce, and it is difficult for traditional engines to effectively utilize this part of the lean gas. When the vehicle is running under low load conditions, stratified combustion technology can significantly reduce fuel consumption. The engine uses stratified combustion mode, and the high-pressure compressor will introduce excess air. The fuel is not directly sprayed into the intake air, but the air is directly sprayed into the combustion chamber after the piston completes the compression stroke in the cylinder; the multiple injection process precisely controlled by electronic components ensures that the fuel mixture is near the spark plug at the moment of ignition. The high-concentration fuel mixture cloud is ignited by the spark plug, and then they further ignite the lean gas mixture distributed in the rest of the combustion chamber. The excess air makes the combustion efficiency reach the maximum value.

Left: Outward-opening piezoelectric fuel injector, fuel injection shape at 200 bar pressure; Right: Standard multi-hole fuel injection valve, fuel injection shape at 130 bar pressure. The outward-opening piezoelectric fuel injector with extremely small valve clearance has a fuel mist length of only half that of a multi-hole fuel injection valve under high pressure; the time it takes for a piezoelectric fuel injector to generate fuel mist is 1/4 of that of a traditional multi-hole valve, effectively preventing residual oil droplets on the cylinder wall.

    Because the last injection in each cycle occurs immediately before ignition, when the piston is basically at top dead center, the requirements for the gas injection distance and vaporization characteristics are much higher than for intake stroke fuel injection, in order to prevent the formation of oil droplets on the piston head and cylinder wall. The free injection distance needs to be significantly reduced, so a core component of the BlueDIRECT combustion system is a piezo injector with an outward-opening nozzle, which can spray very short fuel pulses with extremely high accuracy and stability of the fuel injection amount. Stratified lean combustion mode is suitable for low load conditions, with engine speeds below 3500 rpm and mean effective pressures below 5 bar, which is close to half of the maximum load of a naturally aspirated engine.

    When the load exceeds the mean effective pressure of 5 bar, the turbocharged four-cylinder engine's specially enhanced homogeneous stratified lean burn (HOS) mode will be used. The combination of intake stroke fuel injection and compression stroke fuel injection, combined with turbocharging, is used to extend the operating range of the lean fuel mixture to all intake loads. By precisely controlling the fuel mixer injection amount and inhaling excess air at high loads, the engine has good thermodynamic efficiency even without lean burn technology.

    The four-cylinder BlueDIRECT engine is equipped with a third-generation gasoline direct injection device and uses spray-guided lean combustion technology. The system's maximum pressure can reach 200 bar, and it can be adjusted to the optimal amount according to the engine's performance map and working conditions; the newly developed piezoelectric injector allows each power stroke cycle to complete up to five injection operations, which helps to form the best fuel mixture ratio and cloud shape, thereby optimizing the combustion process and improving fuel efficiency.

    The third generation of direct fuel injection system uses fast multiple discharge ignition technology (MSI): the first spark discharge is followed by a rapid combustion process, the ignition coil is immediately charged and the second spark discharge is completed, and so on. The multiple discharge ignition system can complete four spark discharges in a millisecond, generating a plasma zone with a larger expansion space than ordinary ignition. The fast multiple discharge ignition system can be used to adjust the spark ignition time and the entire combustion process to meet the corresponding power output needs; it helps to define the range of the optimal combustion area and improves the utilization of the residual lean fuel mixture, which in turn reduces fuel consumption.