Press Release
Two-stage turbocharging for high torque at all engine speeds
The new diesel unit draws the air it needs to breathe from not one but two turbochargers, marking the first ever instance of two-stage turbocharging in a series-manufactured passenger car diesel engine from Mercedes-Benz. The aim of this concept is to eliminate the inherent drawbacks of a single-stage turbocharger. These include, for instance, the moment of inertia of a large turbocharger, which drivers may perceive as sluggish start-off characteristics (turbo lag). What’s more, it is virtually impossible to reconcile good start-off abilities and maximum power along with low fuel consumption even at full throttle when deploying just a single-stage turbocharger.
The compact-sized module for the new two-stage turbocharging concept 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 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 revolutions per minute.
- Integrated into the HP turbine housing is a bypass duct, which 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, the whole exhaust stream flows through the HP turbine, meaning that 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-millimetre LP turbine at a maximal speed of 185,000 revolutions per minute.
- To protect it against overload, the LP turbine also features a bypass duct, which is opened and closed by means of an actuator-controlled flap known as 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), meaning abundant torque even from low rev speeds. Besides this, fuel consumption is lowered too. The upshot of this as far as the driver is concerned is harmonious driving characteristics with zero turbo lag, good torque delivery over the entire rev band, tangibly superior performance, plus better communication between engine and accelerator.
Intercooler and exhaust gas recirculation have been optimised
The new turbocharger system is perfectly complemented by an intercooler that has been enlarged compared to the previous series-production version and now lowers the temperature of the air - that has been first compressed and therefore heated up - by around 140 degrees Celsius, 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. So as to optimise the quantity of exhaust gas recirculated and thereby achieve high recirculation rates, the exhaust gases are cooled down as required in a powerful 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. The results are highly impressive: efficient engine warming, reduced emissions when engine is still cold due to warmer combustion temperatures, reduced emissions when engine is warm thanks to good EGR compatibility and good EGR cooling, no tendency for deposits to build up, as well as a long service life. The engine can be started at temperatures as low as approximately zero degrees Celsius with no preglow waiting period, while the effective turbocharging technology ensures that the engine runs stably without misfiring even when cold.
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.
Rear-mounted camshaft drive
The list of the new four-cylinder diesel engine's principal innovations also includes the rear-mounted camshaft drive. This allows statutory pedestrian protection requirements to be fulfilled when the engine is installed lengthways with the bonnet rising towards the rear. 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 exceptionally 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. It was possible to reduce the increased noise levels usually associated with a gearwheel drive by carrying out painstakingly detailed refinement.
Controllable water and oil pumps save fuel
The electrically controllable water and oil pump which can be activated in accordance with requirements are also unique features for a standard-production diesel engine. Piston cooling is taken care of by an oil pump with a central valve for controlling all four piston-cooling sprayer units with their large oil-spray nozzles. The result is identical basic thermal conditions for all cylinders. The generously sized nozzles promise 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.
The controllable water pump is yet another innovative new feature. 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.
Development potential has not yet been exploited to the full
In spite of its unrivalled power output, model running characteristics and outstanding fuel consumption figures, the new four-cylinder diesel engine from Mercedes-Benz holds yet further, untapped potential. Development work is continuing on the possibilities offered by ultra-flexible injection timing with a view to exerting an even more positive effect on engine emissions.
In addition to this, combing the new star of the diesel sector with other consumption-optimisation technologies - such as those found in hybrid vehicles - will achieve further significant reductions in fuel consumption.
