Mercedes-Benz Introduces New Generation of Four-Cylinder Diesel Engines
10 April 2008
|The new four-cylinder diesel family offers improved performance, torque, emission properties and fuel economy. Click to enlarge.|
Mercedes-Benz has introduced a new generation of four-cylinder diesel engines with improved performance, torque, emission properties and fuel economy. The new engine family will supersede four different powerplants. The new engine will debut in the C-Class this fall, but will eventually be fitted in a number of variants across a wide range of model series, including the Mercedes-Benz Sprinter. Mercedes-Benz says it also earmarking the engine family for use in hybrid vehicles.
In its most powerful variant, the new 2,143 cc four-cylinder unit delivers up to 150 kW (204 hp)—about 20% more power than the engine it replaces. Peak torque has risen from 400 Nm (295 lb-ft) to 500 Nm (369 lb-ft)—an increase of 25%. Despite the 25 kW increase in output, the new four-cylinder diesel burns less fuel than its predecessor, and reduces CO2 emissions by up to 13%. The new engine is Euro-5 compliant.
|Power output graph for the 150 kW variant. Click to enlarge.|
The power-to-displacement and torque-to-displacement ratios of the new engine are 70 kW (95.2 hp) and 233.3 Nm per liter respectively, compared to 58.2 kW (79.2 hp) and 186.2 Nm per liter of displacement for the earlier unit.
When fitted in the C-Class, the new 150 kW unit consumes 5.4 liters per 100 kilometers (NEDC) (43.6 mpg US), 0.5 liters less than previously. When powered by a new 125 kW (170 hp) variant, the C-Class consumes 5.1 L/100 km (46.1 mpg US).
As a consequence, CO2 emissions are reduced by 8% and 13% respectively to 143 g/km and 136 g/km.
Engine-out emissions are reduced to the point where even without an active denoxification process, the new four-cylinder diesel already meets the future Euro 5 emissions standard. Further reductions will be made with BlueTec aftertreatment systems.
Three different variants are initially planned for use in passenger cars.
|New 4-Cylinder Mercedes Diesels|
|Parameter||250 CDI||220 CDI||200 CDI|
@ 4200 rpm
@ 3200 - 4800 rpm
@ 3000 - 4600 rpm
@ 1600 - 1800 rpm
@ 1400 - 2800 rpm
@ 1600 - 2800 rpm
Principal features of the new Mercedes diesel engine include:
Two-stage turbocharging to ensure high power output and optimum torque delivery. The compact two-stage turbocharging unit consists of a small high-pressure (HP) plus a large low-pressure (LP) turbocharger. Both comprise a turbine and a turbine-driven compressor, and are connected with one another in series.
The two-stage turbo system. Click to enlarge.
The HP turbine has a diameter of 38.5 mm and is positioned directly in the exhaust manifold. The flow of exhaust gases flows through this turbine first, causing it to rotate at speeds of up to 248,000 rpm.
A bypass duct is integrated into the HP turbine housing. The duct can be opened or closed by means of a charge-pressure control flap triggered by an actuator. If the duct is closed, the entire exhaust stream flows through the HP turbine—i.e., all of the energy contained in the exhaust gases can be directed towards propelling the HP turbine only. In this way, the optimum charge pressure can be built up at low rev speeds.
As the engine speed increases, the charge-pressure control flap opens to prevent the HP charger from becoming overloaded. A portion of the exhaust stream now flows through the bypass duct to relieve the load on the high-pressure stage.
Downstream from the HP turbine, the two exhaust gas streams join up again, and any remaining exhaust energy drives the 50-millimeter LP turbine at a maximal speed of 185,000 rpm. To protect against overload, the LP turbine also features a bypass duct, which is opened and closed by means of an actuator-controlled flap—the wastegate.
Once the engine reaches medium rev speeds, the HP turbine’s charge-pressure control flap is opened so wide that the HP turbine ceases to perform any appreciable work. This allows the full exhaust energy to be directed with low losses into the LP turbine, which then does all of the turbine work.
The two compressors are likewise connected in series and are in addition connected to a bypass duct. The combustion air from the air cleaner first flows through the LP compressor (diameter 56.1 mm) where it is compressed as a function of the LP turbine’s operating energy input. This pre-compressed air now passes into the HP compressor (diameter 41 mm) that is coupled to the HP turbine, where it undergoes further compression. The result is a genuine two-stage turbocharging process.
Once the engine reaches a medium rev speed, the HP compressor can no longer handle the flow of air, meaning that the combustion air would heat up too much. To avoid this, the bypass duct opens to carry the combustion air past the HP compressor and directly to the intercooler for cooling. In this case, the charge-pressure control flap is completely open too, meaning that the HP turbine is no longer performing any work. This is the equivalent of single-stage turbocharging.
The benefits of this elaborate, needs-driven control of the combustion air feed with the aid of two turbochargers are improved cylinder charging (for high output) with abundant torque even from low rev speeds. Fuel consumption is lowered also.
Optimized intercooler and exhaust gas recirculation. The intercooler has been enlarged compared to the previous series-production version and now lowers the temperature of the air by around 140°C, allowing a greater volume of air to enter the combustion chambers.
After the intercooler, an electrically controlled flap ensures precise regulation of the fresh air and recirculated exhaust gas. To optimize the quantity of exhaust gas recirculated and thereby achieve high recirculation rates, the exhaust gases are cooled down as required in a heat exchanger with a large cross-sectional area.
This combines with the HFM (hot-film air-mass sensor) modules, which are integrated into the fresh-air supply and provide the engine management unit with exact information on the current fresh air mass, to bring about a substantial reduction in nitrogen oxide emissions.
Intake port shut-off for optimum air supply. The combustion air subsequently flows into the charge-air distributor module, which supplies air to each cylinder in a uniform manner. Built into the distributor module is an electrically controlled intake port shut-off which allows the cross-sectional area of each cylinder’s intake port to be smoothly reduced in size. This alters the swirl of the combustion air in such a way as to guarantee that the charge movement in the cylinders is set for optimum combustion and exhaust emissions over the full spectrum of engine loads and rev speeds.
Fourth-generation common-rail technology with a rail pressure of 2,000 bar—an increase of 400 bar—plus a new piezoelectric injector concept featuring direct injector needle control. The system enables more flexible injection timing, leading to smoother engine running, lower fuel consumption and reduced emissions.
Fuel injection system. Click to enlarge.
New piezoelectric injectors are one of the key components in the fourth-generation CDI technology. The new injectors are fitted with a piezo stack, basically made up of piezoelectric elements connected in series. In contrast to the customary systems used to date, the movement of these elements controls the injector needle directly and enables even greater alterations in volume that are accurate to within a few thousandths of a millimeter.
This enables an increase in the available injection volume as well as particularly fine and fast metering of the injection quantities. In turn, this enables the fuel injection process to be adapted to the momentary engine load and rev speed with greater exactness, with concomitant positive impacts on emissions, consumption and combustion noise.
Injector operation is also completely leak free. This dispenses with the need for a leak oil line to return the negligible quantities of fuel that used to accumulate unavoidably in the system on account of the operating principle. This improves the injection system’s thermal circuit to such an extent that, even at a rail pressure of 2,000 bar, fuel cooling is superfluous. This reduces the high-pressure pump’s operating energy input by around one kilowatt at high engine loads.
Maximum ignition pressure of 200 bar and optimized combustion chamber. The fuel is injected into a combustion chamber with a geometrical form that includes precision-calculated recesses in the piston crowns. Compared to the engine it replaces, the combustion chamber has been made flatter and the diameter somewhat larger. The compression ratio was reduced from 17.5:1 to 16.2: 1.
This optimizes the combustion process by achieving a lasting reduction in engine-out emissions—NOx levels in particular have been cut.
To guarantee spontaneous starting, the engine is fitted with ceramic glow plugs which attain a temperature approximately 200°C higher than metallic glow plugs (1,250°C as opposed to 1,050°C) and are virtually wear-free. Mercedes-Benz put these glow plugs into series production for the first time in the predecessor diesel engine.
Controllable water and oil pumps. Electrically controllable water and oil pumps can be activated in accordance with requirements. Piston cooling is taken care of by an oil pump with a central valve for controlling all four piston-cooling sprayer units with large oil-spray nozzles. The result is identical basic thermal conditions for all cylinders. The larger nozzles provide optimum piston cooling, even when operating under full load, guaranteeing a long service life in the process. The oil pump’s controllable design additionally reduces the oil flow rate and therefore fuel consumption.
Just like the controllable oil sprayer units, the water pump also helps to quickly warm up both the combustion chamber and the friction partners, at the same time lowering fuel consumption and untreated emissions.
Rear-mounted camshaft drive. The rear-mounted camshaft drive allows statutory pedestrian protection requirements to be fulfilled when the engine is installed lengthways. The vibration stimuli originating from the crankshaft are furthermore lower on the rear face of the engine than at the front, which benefits the engine’s smooth running.
The valve timing mechanism is another new development and reduces friction at the 16 intake and exhaust valves, which are controlled by one overhead intake shaft and one overhead exhaust shaft acting via cam followers featuring hydraulic valve clearance compensation. The camshaft, Lanchester balancer as well as the ancillary assemblies are driven by a combination of gearwheels and just a very short chain drive.
The engine block is made from cast iron, the cylinder head from aluminium.
Two water jackets guarantee maximum cooling even at the points of greatest thermal radiation; it is this that enables a ignition pressure of 200 bar and such a high power-to-displacement ratio.
The aluminium pistons slide up and down in cast-iron barrels for minimum frictional resistance.
The connecting rods are made from forged steel, and their weight has been optimized.
In the interests of vibrational comfort, the forged crankshaft with its eight counterweights turns supported by five bearings. The radii of the crankpins are rolled for high strength.
To compensate for the free vibration moments which are inherent to four-cylinder inline engines, there are two Lanchester balancer shafts at the bottom of the engine block running in low-friction roller bearings rather than conventional plain bearings.
A two-mass flywheel, featuring a primary flywheel mass fixed to the crankshaft that is connected to the secondary flywheel mass on the transmission by means of springs (spring-mass system), isolates the crankshaft’s vibration stimuli from the drivetrain, thereby contributing to the engine’s excellent smoothness.
Mercedes says that it is continuing to work on the possibilities offered by ultra-flexible injection timing with a view to reducing engine-out emissions even further.
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