DaimlerChryser has developed a new 3.5 litre V6 with cutting edge technology
that allows it to develop a peak power of 200kW (272hp) at 6000 rpm and a superb
torque curve that gives 350Nm from 2500 – 5000 rpm. Even more impressive, 87 per
cent of this peak torque is available from just 1500 rpm! As expected from this
manufacturer, the engine incorporates some new technology including moveable
tumble valves in the intake ports and an electronically-controlled
thermostat.
The development was undertaken at the Stuttgart-Unterturkheim facilities
where a team of around 500 engineers, technicians and mechanics developed the
new design. Approximately 400 prototype engines were built for testing, both on
the dyno and also mounted in cars and tested in different climates around the
world.
The key development objectives for the engine were:
- Power
output
- Torque
output
- Fuel
consumption
- Comfort
- Exhaust
emissions
In order that these objectives were met, the engineers decided to include in
the new design variable camshaft timing of both the inlet and exhaust cams, a
two-stage intake manifold, tumble flaps in the intake ducts, heat management
strategies and lightweight engine construction.
Camshaft Control
Continuously variable camshaft timing is used on both the two intake and two
exhaust cams – the Mercedes engine is the first V6 with variable intake and
exhaust valve timing... although it is not the first six cylinder engine with this
feature. The cams are controlled as a function of engine load and are able to
varied over a range of 40 crankshaft degrees.
The variable timing is used to allow internal exhaust gas recirculation at
low loads, reducing energy losses and improving fuel consumption. At higher
loads, valve overlap is altered to better provide for the admission of fresh
mixture, improving power and torque outputs.
Electrohydraulic vane-type camshaft adjusters are mounted on the inner
exhaust and intake cams; these cams drive their brothers through directly
meshing gears. The main cam drive is via a duplex chain.
Intake System
A magnesium intake manifold is used, bucking the worldwide trend to composite
materials for this engine component. It incorporates both a two-stage,
variable-length design and also tumble flaps in the ducts.
Under partial load, the tumble flaps are extended, increasing the turbulence
of the gasflow passing into the combustion chambers and causing it to enter at a
higher speed and with a more uniform distribution. This improves mixing and
flame propagation within the combustion chamber, giving part-load fuel
consumption gains of up to 0.2 litres per kilometre and also improving engine
smoothness.
At higher loads, the flaps are recessed into the walls of the intake manifold
and so have no effect on gasflows into the combustion chambers. The tumble flaps
are driven by the ECU electronically controlling a vacuum canister. The action
of the tumble valves is fully mapped.
Extensive computer modelling was undertaken not only of the flows into and
within the combustion chambers, but also ahead of the throttle body. Not
mentioned in DaimlerChrysler literature – but clearly able to be seen here in
this cutaway intake system view – are the turning vanes employed between the
hot-film airflow meter and the throttle body. (For more on turning vanes, see http://www.autospeed.com/cms/A_0491/article.html.)
The intake system uses further rectangular flap valves to vary its effective
runner length. Above 3500 rpm, the flaps are open and air takes the shortest
path to the combustion chambers. This shorter runner length reduces pressure
drop and aids breathing. (Here the red arrow points to the flap valve that
controls intake runner length while the green arrow highlights the tumble
valve.)
However, at lower engine speeds, intake runner length resonant tuning is
maximised by the valves closing and the engine breathing through longer intake
runners.
Electronics
The Bosch ME 9.7 engine management system ECU is mounted on top of the
engine. In addition to controlling fuel injection, ignition timing, the action
of the tumble flaps and the variable length intake manifold, the system also
controls coolant circulation. This is function is carried out by an
electronically-controlled thermostat which allows rapid warm-up and stable oil
and coolant temps. An oil-water heat exchanger is positioned on the engine.
Internals
The cylinder head and block of the engine are made from aluminium, with the
block not using cast-in iron liners but instead an aluminium silicon coating.
The pistons are iron-coated aluminium, the con-rods are made from forged steel
and are 20 per cent lighter than “other V6 engines”, and the crankshaft is also
forged. A balance shaft is positioned between the two cylinder banks.
Other Highlights
Some of the intake ducts are made from woven nylon, which reduces noise
transmission.
The twin cartridge airfilter is mounted on top of the engine under the
cover.
Close-coupled twin catalytic converters (one per bank) are each equipped with
two oxygen sensors. Air-gap insulated exhaust manifolds promote quick cat
converter heating. A secondary air injection system is fitted.
Conclusion
Perhaps the most impressive aspect of the engine is its ability to provide
such a wide and high torque plateau with an excellent specific power figure.
That combination should make for a very driveable design that also retains
plenty of punch when the throttle is floored.
Specifications
| Cylinder arrangement: |
V6 |
| Cylinder angle: |
90 degrees |
| Valves per cylinder: |
4 |
| Displacement: |
3498cc |
| Bore/stroke: |
92.9/86.0mm |
| Distance between cylinders: | 106 mm |
| Compression ratio: |
10.7:1 |
| Output: |
200kW / 272hp at 6000 rpm |
| Output per litre: |
57kW / 78hp |
| Max. torque: |
350 Nm at 2500-5000 rpm |
|
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