MERCEDES-BENZ ENGINE TECHNOLOGY AT FRANKFURT 1999

At the Frankfurt Motor Show (IAA), the Stuttgart-based marque is giving the public an insight into it's current state of engine development. The most important aim of the engine designers is to achieve even further reductions in the exhaust emissions of car engines, in order to meet the strict limits of EU exhaust directives and future US pollutant limits.

Mercedes-Benz reckon that one highly promising process for the spark ignition engine of the future is variable valve timing. 'Variable' here means that the valves are not opened and closed by the conventional continuous control principle of the camshaft but that their opening times are adapted to the particular driving situation. This method avoids the otherwise common throttling of the engine, which detracts from efficiency in the partial load range and is therefore responsible for the higher fuel consumption in urban and short-haul traffic. With an eye to the future, engineers are therefore speaking of 'throttle-free load control'. Other manufacturers have already launched such systems, but none yet are infinitely variable. This is Mercedes' aim.

In the advanced engine development division at Mercedes-Benz, two variable valve timing methods are being put through their paces on test rigs and in practical road tests: one system is mechanical and uses two camshafts, whilst the other is electromagnetic, with actuators on each valve. The decision on whether to implement these technologies in full-scale production will depend on the final outcome of these tests.

Variable valve timing by mechanical system: two camshafts

In the mechanical timing system, two camshafts share the work of opening and closing the valves. Between these two shafts, a sliding carriage with three rollers combines the effect of the cam strokes, so that the valves are opened and closed to the precisely required extent. This mechanism is driven by a continuously variable electric drive motor, which is in turn coupled to the throttle pedal. With variable valve timing by two camshafts, the valve opening strokes and timing can be adjusted so accurately that exactly the right amount of air flows into the cylinder, with the result that a throttle valve should be no longer needed for load adjustment.

For drivers, the tangible effects of this throttle-free engine control system are more agile engine response at low and medium revs and an increase of about 10% in torque in the engine speed range up to 3000rpm. Both of these features provide good foundations for improved elasticity and for even greater driving pleasure.

The advantages of this mechanically variable valve timing system in terms of fuel consumption are just as impressive: with the 3.2-litre V6 engine, Mercedes engineers have recorded a fuel saving of about 8% in the New European Driving Cycle (NEDC), equivalent to about 0.9 litres per 100 kilometres in the Mercedes-Benz S320.

Variable valve timing by electromagnets: free valve play

With the electromagnetic valve timing system, there are no camshafts, bucket tappets, timing chain or any other of the components previously required to drive the inlet and exhaust valves. All these components are replaced by 'actuators', which in this case are powerful, electronically controlled magnets with their armatures directly connected to the valves. The actuators operate the valves with the aid of valve and actuator springs. With this system, each individual valve can be opened or closed at any time on demand, either alone or simultaneously with others.

The system consists of one actuator per valve, so that the cylinder head of a modern 4-cylinder engine will have a total of 16 of these powerful actuators. Microcomputers on each cylinder bank control the actuators by regulating the power supply to the electromagnets. These microcomputers also control the ignition and fuel injection functions.

The almost unlimited variability of valve timing provided by electromagnetic actuators, and the resulting ability to dispense with a throttle valve for load control, leads to a substantial reduction in the fuel losses normally incurred in the charge exchange process, so that the NEDC fuel consumption of a Mercedes-Benz E-Class saloon is reduced by about 10%. This fuel saving effect is even more marked in combination with the automatic cylinder cut-out system that Mercedes-Benz has developed for the V8 engine of the S500: in this combination, the 8-cylinder engine uses about 15% less fuel - a saving of just under two litres per 100 kilometres.

Direct petrol injection: many questions still unanswered

Direct petrol injection remains one of the concepts that Mercedes engineers are researching intensively with the aim of achieving substantial reductions in the fuel consumption of future models - especially in the partial load range. This technology is by no means new to Mercedes-Benz, since back in 1954 the Stuttgart-based auto marque was the first car manufacturer in the world to use direct injection in the 4-stroke petrol engine of the legendary gullwing-door 300 SL sports car. However, given today's strict environmental legislation, the operating conditions for this principle have fundamentally changed.

On the basis of the modern 4-cylinder engine of the A-class, Mercedes engineers have already tested a large number of DI engines on test benches and in test vehicles. This major investment is necessary, because a whole range of basic questions still need to be solved for this injection and combustion system. These questions include emission control, because conventional three-way catalytic converters are not suitable for DI petrol engines: the chemical conversion process of conventional converters only works optimally with a fuel-air mix ratio of 1:14.6 (lambda 1/stoichiometric), whereas DI petrol engines benefit from mainly running on lean-burn mixtures, which may permit considerable fuel savings but are incompatible with the principle of the regulated three-way catalytic converter.

Emission control by accumulator-catalyst: sulphur-free fuel required

If, despite this incompatibility, exhaust emissions are to be kept at the same low level as for an engine with conventional fuel injection, a new type of accumulating catalytic converter is required. This converter adsorbs the nitrogen oxides during lean-burn operation and releases them again in short regeneration phases, so that they react with the other exhaust components to form harmless nitrogen. During these regeneration phases, which last only a few seconds, the engine is operated with a rich mixture. The snag with this system is that in addition to the nitrogen oxides, the catalytic converter also accumulates the sulphur contained in the fuel: this sulphur is converted to sulphate and remains in the converter despite the regeneration process. The result is that, with time, the converter is rendered partly ineffective due to contamination with sulphur, so that the temporary accumulation of nitrogen oxide is reduced and exhaust emissions increase.

Use of the accumulator-converter and compliance with the strict emission control regulations of the EU-4 Directive would therefore only be possible if the oil industry were to supply petrol with a sulphur content of less than 10 ppm (parts per million), i.e. a 'sulphur-free' fuel according to the technical definition. Unfortunately, the currently permissible sulphur content and the EU levels planned for the years 2000/2005 are much too high and are incompatible with future emission control technologies such as the accumulator-converter. In the view of Mercedes-Benz, one solution would be to grant tax concessions for sulphur-free fuel - a measure which has already proved highly successful in the introduction of unleaded fuel.

Fuel economy with the DI principle: between eight and fifteen percent

Various different combustion methods are currently being tested by Mercedes for use in direct injection petrol engines. These tests focus on the decisive factors determining fuel consumption, namely how the fuel is injected in the cylinders, how it is distributed and mixed with oxygen and when it is ignited. Most other car makers are also researching this technology.

However, the considerable fuel savings achieved for the diesel engine by direct injection cannot be attained on the same scale with petrol engines. Although this system, in combination with lean-burn operation, theoretically offers a high potential for fuel savings in the SI engine, in practice, and taking into account the strict EU-4 emission standards, the results are less impressive than those obtainable with the CDI engine. In addition, in the particularly high-economy operating mode in the partial load range, the DI petrol engine can only develop up to about a fifth of its maximum power. Consequently, with a slightly more sporty driving style, there are no measurable fuel savings. In the NEDC driving cycle, Mercedes experts in Stuttgart estimate an overall fuel saving of eight to fifteen percent.

Designed & maintained by caint.com