Mercedes unveils new four-cylinder diesel engine
What better way to mark the 150th anniversary of Rudolf Diesel’s birth than a brand new generation of four-cylinder diesel engines from Mercedes-Benz which outstrip all previous benchmarks for performance, torque, emission properties and, most notably, fuel economy in their segment? In its most powerful variant, the new four-cylinder unit musters up 204 hp from its 2143 cubic centimetres, meaning that it delivers around 20 per cent more power than the engine it replaces.
At the same time, peak torque has risen from 400 Nm to 500 Nm, equating to an increase of 25 per cent. Despite the 25 kW increase in output, the new four-cylinder diesel burns substantially less fuel than its predecessor, which was itself highly economical. As a consequence, CO2 emissions are reduced by as much as 13 per cent and the new four-cylinder diesel unit already complies with the future EU5 emissions standard.
The new four-cylinder diesel generation from Mercedes-Benz can be briefly summed up as follows: greater power, greater economy, greater cleanliness. The new power unit from the Untertürkheim plant needs to be explained at greater length to be fully appreciated, however. It really does charter territory from which diesel engines - and four-cylinder units particularly so - have previously been excluded. It redefines standards for power output and torque on the one hand and for fuel consumption and exhaust emissions on the other, setting benchmark figures which no other comparable series-production engine is able to match at the current time.
The technical advance which the design engineers at Mercedes-Benz have achieved with this new four-cylinder diesel is not only evident on paper, its effects can also be experienced to an intense degree behind the wheel. As far as the figures are concerned, the most powerful variant of the new diesel engine extracts 150 kW/204 hp from its displacement of 2143 cubic centimetres. This represents an increase of some 20 per cent compared to its predecessor, despite the displacement being almost identical. Meanwhile, the engine’s peak torque has been upped by 25 per cent from 400 Nm to 500 Nm. The power-to-displacement and torque-to-displacement ratios of the new engine from Untertürkheim make just as impressive reading, with figures of 70 kW/95.2 hp and 233.3 Nm per litre respectively (the figures for its predecessor by comparison: 58.2 kW/79.2 hp and 186.2 Nm per litre of displacement).
Lower fuel consumption despite substantial gain in output
The engineers also took care to ensure the new diesel engine is a paragon of fuel efficiency. In spite of the substantial power boost of 25 kW, the engine makes even more frugal use of diesel than its predecessor, which was itself a most modest consumer of fuel. This is immediately apparent from the fuel consumption figures for the C-Class, in which the powerpack will be making its debut in the autumn. When fitted in the C-Class, the new 150-kW unit burns just 5.4 litres of diesel per 100 kilometres (NEDC), 0.5 litres less than previously. And when powered by the 125-kW/170-hp variant that is also newly available, the C‑Class returns even lower fuel figures of 5.1 litres for every 100 kilometres (a drop of 0.8 litres). As a consequence, CO2 emissions are reduced by 8 and 13 per cent respectively to 143 and 136 g/km. The Mercedes-Benz engineers have also succeeded in further reducing the amount of untreated engine emissions. Even without an active denoxification process, the new four-cylinder diesel already meets the future EU5 emissions standard.
"This takes our new four-cylinder unit into a realm which has so far been the preserve of three-litre six-cylinder diesel or large V8 petrol engines - all combined with exemplary fuel economy," commented Dr. Thomas Weber, who is responsible for Group Research and Development at Mercedes-Benz Cars on the Daimler AG Board of Management.
Tangible progress and intense motoring pleasure
Drivers are able to savour the advances that have been made with all of their senses. The new drive unit has a powerful feel to it, its response is agile, it delights with its tremendous pulling power and impresses with admirable levels of smoothness for a four-cylinder engine. It enables sports-car-like performance, propelling the C-Class Saloon from standstill to the 100-km/h mark in a mere 7.7 seconds. The engine’s supreme flexibility permits quick turns of speed for rapid overtaking on country roads, taking just 9.4 seconds to pick up from 60 to 120 km/h. This all adds up to a high degree of fun at the wheel combined with great economy.
Quite apart from its outstanding power output data, the new drive unit also boasts markedly superior torque build-up from low revs compared to the engine it replaces, along with a class-beating torque characteristic curve. This means that the engine can be run extremely economically at low rev speeds in routine driving situations.
The new diesel engine is set to supersede four different powerplants in all, and will be fitted in a number of variants across a wide range of model series, even including the Mercedes-Benz Sprinter. Thanks its high power potential it has been possible to apply the downsizing principle, where smaller engines with fewer cylinders are used in order to lower fuel consumption very effectively. Thanks to the agility, pulling power and optimum running characteristics of the new diesel drive unit, the vehicles it is fitted in will continue to be able to live up to the high standards of comfort and motoring pleasure expected of models from Mercedes-Benz. Three different variants are initially planned for use in passenger cars.
The next chapter in the Mercedes-Benz diesel success story
This new diesel powerplant is Mercedes-Benz’s resounding answer to questions over the future of motoring, and marks yet another milestone in the evolution of diesel technology. At the same time, the Stuttgart-based automotive manufacturer is perpetuating a long-standing tradition. It was as long ago as 1936 that the diesel engine received its world premiere in a passenger car from Mercedes-Benz - the now legendary 260 D. Ever since, Mercedes-Benz has been hard at work advancing and honing the technology it pioneered. There have been many momentous occasions over the years, including the first ever turbodiesel passenger car engine in the Mercedes-Benz 300 SD (1977), the world premiere of four-valve technology (1995), the first diesel-powered saloons with particulate filter system in the US state of California (1985), common-rail direct injection technology (1997), the maintenance-free diesel particulate filter (2003) which has in the meantime become available for all Mercedes-Benz diesel models, as well as the introduction of BlueTec technology (2006) for the cleanest-running diesel engines in the world.
100,000 hours on the test rig and ten million test kilometres
Mercedes-Benz is now adding the latest chapter to this long-running success story with the arrival of its all-new, groundbreaking four-cylinder diesel engine. After a 48-month development period, during which time the design engineers employed the very latest computer technology, the first prototype of the new engine was put into operation in August 2005. Some 100,000 hours on the test rig were required to elicit optimum performance characteristics from the new engine under all conditions and regardless of the intended purpose. The power units were subject to tough endurance testing, including acutely demanding cycles on the test rigs that were designed to truly put them through their paces. Over a distance of ten million test kilometres in a variety of vehicles, the engine had to prove its mettle in the baking heat of the desert and the icy cold of the polar regions, withstanding dust, mud, water and the very harshest treatment in the process.
The new engine celebrates its premiere in autumn 2008, when the first power rating variant will be launched in just the C-Class initially. The power unit is due to be deployed in various model series from Mercedes-Benz, returning outstanding fuel consumption figures in all cases. It can be installed both lengthways and crossways and is envisaged for all-wheel-drive vehicles too. Naturally, the new engine can be supplemented by the cutting-edge BlueTec emissions control system developed by Mercedes-Benz, and it is also earmarked for use as a fuel-efficient internal combustion engine in hybrid vehicles.
Innovative technologies without parallel
The exemplary figures achieved by the new engine for output and torque characteristics, economy, exhaust emissions and smoothness are the result of a whole raft of innovative technologies. These include a number of new developments, the likes of which cannot currently be found in any other standard-production passenger car diesel engine. The principal features of the new Mercedes diesel engine:
- Two-stage turbocharging ensures high power output and optimum torque delivery.
- Fourth-generation common-rail technology with a rail pressure that has been increased by 400 bar to 2000 bar, plus a new piezoelectric injector concept featuring direct injector needle control creates the ideal basis for more flexible injection timing, leading to smoother engine running, lower fuel consumption and reduced emissions.
- The maximum ignition pressure is 200 bar which also contributes to the high output.
- Both the oil-spray nozzles and the water pump are activated in accordance with requirements to save energy.
- The camshaft drive is positioned at the rear in order to enhance running refinement and satisfy the exacting pedestrian protection requirements.
- 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 optimised by the Mercedes engineers.
- 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 (technical term: spring-mass system), isolates the crankshaft’s vibration stimuli from the drivetrain, thereby contributing to the engine’s excellent smoothness.
Injection with the fourth generation of the common-rail principle
The new diesel unit from Mercedes-Benz ushers in the fourth generation of the tried-and-tested common-rail direct injection technology. The distinguishing characteristic of the latest generation is the increase of 400 bar in the maximum rail pressure, which now equals 2000 bar. This rise in pressure potential was of crucial importance for boosting the engine’s output to 150 kW/204 hp and its torque to 500 Nm, whilst at the same time bringing about a marked improvement in the engine’s untreated emissions.
Piezoelectric injectors which are a completely new development form one of the key components in the fourth-generation CDI technology. They harness the ability of piezoelectric ceramic to alter its crystalline structure with microsecond speed when an electrical voltage is applied. The actual spatial movements produced are tiny however, For this reason, the new injectors are fitted with a piezo stack, which is 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 millimetre. The benefits of this are in increase in the available injection volume as well as particularly fine and fast metering of the injection quantities. This enables the fuel injection process to be adapted to the momentary engine load and rev speed with yet greater exactness - by means of high-precision multiple injections of fuel for example - which has a positive impact on emissions, consumption and combustion noise. Plus, the engine runs even more quietly when idling than its predecessor.
As a result of the innovative actuation concept, injector operation is 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 2000 bar, fuel cooling is super-fluous to requirements. Not only does this save energy, it reduces the high-pressure pump’s operating energy input by around one kilowatt at high engine loads.
In order to continue to deliver optimum injection quantities over the engine’s entire service life, an adaptive learning function is able to compensate for any tolerance deviations that may occur as a result of minimal component wear.
200 bar ignition pressure and optimised combustion chamber
The fuel is injected into a combustion chamber with a meticulously devised geometrical form that includes the 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 optimises the combustion process by achieving a lasting reduction in untreated emissions - NOx levels in particular have been cut drastically.
One of the determining factors for maximum power output and for fuel consumption at full throttle, from an emissions point of view, is the maximum ignition pressure. With a pressure of 200 bar, the new four-cylinder diesel from Mercedes-Benz is one of the top-ranking passenger car diesel engines in this regard. To guarantee spontaneous starting, the engine is fitted with ceramic glow plugs which attain a temperature approximately 200 degrees Celsius higher than metallic glow plugs (1250°C as opposed to 1050°C) and are virtually wear-free. Mercedes-Benz put these glow plugs into series production for the first time in the predecessor diesel engine.
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.