In 2005 Mercedes-Benz introduced newly developed V6 engines that opened up a brand-new chapter in its long and successful history as a maker of diesel engines. These six-cylinder powerplants combine all of the current and future-oriented technologies developed for diesel engines - from the mechanical systems and thermodynamics to the flow dynamics, electronic engine management and exhaust gas purification. That guarantees top values in performance and torque characteristics, economy, exhaust emissions and smooth operation. In the future, the V6 diesel engine in the R-Class will be available in two output ratings: 140 kW/190 hp in the new R 280 CDI 4MATIC and 165 kW/224 hp in the R 320 CDI 4MATIC.
The most striking characteristics of the new Mercedes diesel engines can be summed up as follows:
- Six cylinders in a V-shaped configuration
- Aluminium crankcase with grey cast iron cylinder liners
- Third-generation common-rail technology with piezo injectors
- Four-valve technology with two camshafts per cylinder bank
- Exhaust-gas turbocharger with electrically adjustable turbine vanes
- Peak combustion pressure up to 180 bar
- Exhaust gas recirculation with electrically regulated valve
- Electrically regulated throttling of the intake air
- Swirl control through electrically controlled intake port shut-off
- Quick-start glow-plug
First aluminium V6 diesel engine
More output, higher torque and lower exhaust emissions — these were the main goals of the teams that developed the V6 diesel engine, and they used a number of different strategies to achieve these goals. One example is the engine’s lightweight engineering.
This is the world’s first diesel engine to feature an aluminium cylinder crankcase and grey cast-iron cylinder liners. It weighs only 41 kilograms, which makes it one of the best examples of the concept of lightweight engineering. Other elements that are made of aluminium include the cylinder heads, cylinder head covers, pistons, coolant pump, sump and boost pressure distributor. The use of plastics also helps to reduce the engine’s total weight. The engine components for ducting fresh and charge air, the damper filters and the engine shielding are made of plastic.
A newly developed valve timing system lowers friction and reduces the moving masses: the 24 intake and exhaust valves per cylinder bank are controlled by one overhead camshaft per cylinder bank, via roller-type cam followers with hydraulic valve clearance adjustment. The camshafts are driven by a tried-and-tested double-bush chain; the balancer shaft and the high-pressure fuel injection pump are also integrated into the chain drive.
Compact “one box” design concept
Thanks to a newly developed “one-box” concept, the V6 engine is one of the most compact diesel engines in its displacement class. "One box" design means that the engine, together with its components and ancillary parts, forms a compact unit. The complete air filtration system is also directly attached to the engine and thus requires no additional space.
The CDI six-cylinder engine wouldn’t be a true Mercedes engine if it didn’t meet the brand’s strict standards of rigidity, vibration characteristics and longevity, in addition to its ultramodern lightweight engineering, compact dimensions and low-friction valve operation. Precise calculations and computer simulations provided the engineers in Stuttgart with valuable information and helped them achieve their ambitious goals. A look at the interior of the V6 engine reveals:
- The forged crankshaft rotates in four bearings that have been enlarged by five millimetres each by comparison to the in-line six-cylinder engine — an alteration that was made in the interests of increasing the vibration smoothness. The radii of the crank pins are rolled and are therefore highly rigid. The crankshaft’s stiffness under flexure and torsion is more than double the values of the predecessor engines.
- The connecting rods are also made of cast steel. The Mercedes engineers have further optimised their weight by using a new alloy and making geometric improvements.
- The painstaking design of the combustion chamber geometry, which includes the precise calculation of the recesses in the piston crowns, optimises the combustion process and thus helps to sustainably reduce the engine’s untreated emissions.
- The free vibration moments which are inherent to a V6 engine are compensated for by a balancer shaft between the cylinder banks. The balancer shaft rotates at the same speed as the crankshaft, but in the opposite direction.
Heat exchangers for oil cooling, heating and exhaust gas recirculation
The Mercedes engineers use a separate roller chain to drive the oil pump. The efficient and quiet external-gear-type pump transports the oil through a large-volume full-flow oil filter to an oil-water heat exchanger located between the cylinder banks. The heat exchanger’s high output of 15 kW ensures that the oil temperature never exceeds 130 degrees Celsius, even during periods of peak engine performance. The tunnel of the balancer shaft also serves as the main oil gallery. The lubricant flows from here to the main bearings, into the cylinder heads and to the piston sprayers, which open automatically and cool the pistons with oil when a certain oil pressure is reached.
The key component of the water cooling system is a belt-driven pump located on the crankcase. It operates as a double spiral pump and presses the coolant from the front into the cylinder banks of the crankcase, where it flows through special bores in the cylinder head seals, mainly toward the outlet side. The cooling system is controlled by a thermostat according to the crossflow concept.
The coolant flow for the oil-water heat exchanger comes from the right crankcase; the cooler of the exhaust gas recirculation system and the heat exchanger of the heating system are supplied with coolant from the left cylinder head. This means that the cooling circuit is designed to provide adequate cooling under all torque and engine performance conditions. High flow velocities are reached in particular at the valve crosspieces, around the cylinder head injector shafts, in the oil-water heat exchanger and in the exhaust gas recirculation cooler. This makes excellent heat transfer possible.
Turbochargers with variable nozzle turbines
The V6 diesel engine is aspirated by means of a VNT (variable nozzle turbine) turbocharger. This technology is a major prerequisite for generating powerful torque and high output even at low engine speeds. VNT turbochargers can adjust the angle of their vanes quickly and precisely, thanks to an electric management system, thus using the largest possible exhaust gas volume to compress the intake air and generate boost pressure. This variable, adaptive turbocharger control results in improved cylinder charging, which leads to higher torque.
What’s more, the electric VNT technology permits precise interaction with other systems that are responsible for lowering engine-out emissions and conducting exhaust gas aftertreatment.
The turbocharger is connected with a downstream intercooler, which reduces the temperature of the compressed and heated air to around 95 °Celsius, with the result that a larger volume of air can be fed into the combustion chambers. Installed downstream from the intercooler is also an electrically adjusted valve that is used to throttle the V6 engine when the exhaust gas recirculation is active. With the help of this electrically controlled valve, it is possible to regulate with great precision the volumes and mixtures of the exhaust gases that are recirculated into the combustion air. In order to optimise the recirculated exhaust gas flow, the exhaust gas is substantially cooled in a high-performance heat exchanger. The HFM (hot film air mass measurement) modules, which provide the engine controlling system with precise information about the current flow of fresh air, are integrated into the fresh air flow management system. The cooling of the exhaust gas, together with the HFM modules, significantly decreases nitrogen oxide emissions.
The combustion air subsequently flows into the charge-air distribution module, which supplies each cylinder equally. An integral component of the distribution module is an electrically controlled system in the intake port that progressively reduces the cross-section of the intake port for each cylinder. This alters the swirl of the combustion air, thereby ensuring that the optimum tumble — for both combustion and exhaust emissions — is achieved in each cylinder, at every engine load and speed.
Piezoceramics for precisely metered injection in microseconds
These V6 diesel engines represent the third generation of series-produced common-rail direct injection engines at Mercedes-Benz. This means that the injectors, the high-pressure pump and the electronic valve timing all operate even more efficiently, which in turn further reduces fuel consumption, exhaust emissions and combustion noise.
In place of the solenoid valves formerly used, the injectors are now fitted with piezoceramics whose crystalline structure changes within milliseconds under an electric voltage. This effect, which was discovered in 1880 by the brothers Pierre and Jacques Curie, was used by the engine developers to lift the valve needles at the tip of the injectors with a precision of just a few thousands of a millimetre, thereby ensuring an extremely fine distribution of the fuel injected into the combustion chamber.
What’s more, piezo injectors are considerably lighter and operate at twice the speed of conventional solenoid valves. Thanks to a response time of just 0.1 milliseconds, the fuel injection can be adapted more precisely to suit changing loads and engine speeds, which in turn helps to cut emissions, fuel consumption and combustion noise. The number of fuel injections per power stroke has also been increased from three to five thanks to this piezo technology.
The engineers at Mercedes have also refined the other components of common-rail technology and the fuel injection process:
- The hydraulically optimised injector nozzles have eight injection orifices (compared to seven in previous models). This results in even finer dispersion of the fuel in the combustion chambers, as well as improved mixture formation.
- The demand-regulated high-pressure pump operates with a maximum injection pressure of 1600 bar.
- The pilot injection developed by Mercedes-Benz, which insures a softer combustion process and thus perceptibly lowers the engine’s noise level, takes place twice in a row in the new V6 engine. In less than a millisecond, small pilot amounts of fuel flow into the combustion chambers, generating even better preheating.
- In order to burn off the particulates in the standard-fitted particulate filter, a double post-injection process can be initiated as required.
Exhaust gas purification with two catalytic converters and a particulate filter
Two oxidising catalytic converters clean the exhaust gases emitted by the new Mercedes diesel engine. One of them functions as the starting converter, and thanks to its position near the engine is ready for operation a short time after a cold start. A larger main catalytic converter is connected downstream of this starting converter. The function of the oxidising catalytic converters is to transform carbon monoxide and unburned hydrocarbons by bonding them chemically with oxygen (oxidation).
This efficient exhaust gas aftertreatment, combined with the complex internal engine measures, already enables the V6 diesel engine of the R 280 CDI 4MATIC to meet the stringent EU4 exhaust limits.
In order to further reduce exhaust emissions, in Germany Mercedes-Benz combines the new six-cylinder engine with a maintenance-free particulate filter system as standard equipment. The system is responsible for a further significant reduction of particulate emissions. The particulate filter regenerates itself without additives, which means that it remains effective over a very high mileage.
All-wheel drive, 7G-TRONIC and air suspension at the rear axle are standard equipment
The new diesel model of the R-Class is available in the body version with a 2980 millimetre wheelbase. It is characterised by the same comprehensive array of standard equipment as the R 320 CDI 4MATIC. These include:
- 4ETS traction system
- Adaptive airbags for driver and front passenger
- Air suspension at rear axle
- Aluminium trim
- Ambient lighting in the exterior mirrors
- Anti-lock braking system (ABS)
- Armrest between the front seats with a two-part stowage box underneath
- ASR acceleration skid control
- Audio 20 CD car radio with eight loudspeakers and a CD player
- Brake Assist
- Central display in the instrument cluster
- Central locking with remote control
- Chrome exhaust tailpipe
- Cruise control with SPEEDTRONIC
- DIRECT SELECT gearshift system
- Seat belt tensioners at all seats
- ESP® with tyre pressure loss warning system
- Executive seats with armrests in the first and second rows of seats and backrest angle of the front seats
- Exit lights in the doors
- Headlamp assist
- Headlamps with projection technology
- Heated and electrically adjustable exterior mirror
- Heat-insulating glass
- ISOFIX child seat attachment points
- Leather-covered multifunction steering wheel
- Light-alloy wheels
- Outside temperature display
- Permanent all-wheel drive
- Power windows (4-way)
- Rain sensor for the windscreen wipers with two sensitivity levels
- Reach and height-adjustable steering column
- Rear window wiper
- Rev counter
- Seat belt limiters at all seats
- Seat belt tensioners at all seats
- Seven-speed 7G-TRONIC automatic transmission
- Sidebags in front
- Speed-sensitive power steering
- THERMATIC automatic air conditioning
- TIREFIT tyre repair set
- Wide 235/65 R 17 tyres as standard