Four-cylinder diesel engine (OM 651), Part 1

Leading the way in terms of power output, consumption and emissionsMercedes-Benz C 250 CDI BlueEFFICIENCY Prime Edition, (W 204) 2008

  • Two-stage turbocharging for high power output and spontaneous response
  • Fourth-generation common-rail technology
  • Technical innovations reduce consumption and emissions

Stuttgart, October 13, 2008. The basic character of the new four-cylinder diesel generation from Mercedes-Benz can be described in just a few superlatives: 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.

C 250 CDI BlueEFFICIENCY Prime Edition, 2009

C 250 CDI BlueEFFICIENCY Prime Edition, 2009

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 (295 lb-ft to 368 lb-ft). 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 (172 lb-ft) per litre respectively (the figures for its predecessor by comparison: 58.2 kW/79.2 hp and 186.2 Nm (137 lb-ft) per litre of displacement).

Lower fuel consumption despite substantial gain in output

C 250 CDI BlueEFFICIENCY Prime Edition: Efficiency and driving pleasure on a new plane

C 250 CDI BlueEFFICIENCY Prime Edition: Efficiency and driving pleasure on a new plane

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 new C 250 CDI BlueEFFICIENCY Prime Edition, in which the powerpack will be making its world debut. When fitted in the C-Class, the new 150-kW unit burns just 5.2 l/100 km (45.23 mpg/US, 19.23 km/l) of diesel (NEDC), 0.7 litres less than previously. 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.

C 250 CDI BlueEFFICIENCY Prime Edition, 2009

C 250 CDI BlueEFFICIENCY Prime Edition, 2009

“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 pleasureMercedes-Benz C 250 CDI BlueEFFICIENCY Prime Edition, (W 204) 2008

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 (62 mph) mark in a mere 7.0 seconds. The engine’s supreme flexibility permits quick turns of speed for rapid overtaking on country roads, taking just 9.2 seconds to pick up from 60 to 120 km/h/37 to 75 mph (5th gear). The top speed is 250 km/h (155 mph). These figures all add up to a high degree of fun at the wheel combined with great economy.

C 250 CDI BlueEFFICIENCY Prime Edition, 2009

C 250 CDI BlueEFFICIENCY Prime Edition, 2009

Quite apart from its outstanding power output figures, the new drive unit also boasts markedly superior torque build-up from low engine speeds compared to its predecessor model, along with a class-beating torque characteristic curve. This means that the engine can be run extremely economically at low engine speeds in routine driving situations.

Innovative technologies without parallel

Mercedes-Benz OM 651, new generation of four-cylinder diesel engines

Fourth-generation common-rail technology with a rail pressure that has been increased by 400 bar (5,802 psi) to 2000 bar (29,007 psi)

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 (5,802 psi) to 2000 bar (29,007 psi), 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 (2,900 psi) 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 and therefore fuel. The oil pump’s controllable design additionally reduces the oil flow rate – and therefore fuel consumption.
  • 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.
  • A two-part water jacket in the cylinder head ensures maximum cooling in the area of the combustion chamber plate; it is this that enables an ignition pressure of 200 bar (2,900 psi) and such a high power-to-displacement ratio.
  • The cast-iron cylinder barrels have been more finely honed than in the predecessor model and as such help to improve consumption.
  • To compensate for the second-order forces which are inherent to four-cylinder in-line 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.
  • The two-mass flywheel has been specifically designed for high engine torque at low engine speeds in order to isolate the crankshaft’s vibration stimuli, thereby contributing to the engine’s excellent smoothness.

Two-stage turbocharging for high torque at all engine speeds

Mercedes-Benz OM 651, new generation of four-cylinder diesel engines

To compensate for the second-order forces which are inherent to four-cylinder in-line engines there are two Lanchester balancer shafts at the bottom of the engine block.

The new diesel engine marks 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). 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.

Mercedes-Benz OM 651, new generation of four-cylinder diesel engines 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.

C 250 CDI BlueEFFICIENCY Prime Edition: Efficiency and driving pleasure on a new plane

C 250 CDI BlueEFFICIENCY Prime Edition: Efficiency and driving pleasure on a new plane

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.Mercedes-Benz OM 651, new generation of four-cylinder diesel engines

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.

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