MERCEDES-BENZ recently showed some of its latest technological breakthroughs to the world’s press at an information day. I travelled to Stuttgart to view two new petrol engines and some driverless test cars. Being an old dyno junkie, I also felt at home in M-B’s new engine dynamometer facility.
THE ENGINES
The new engines – a 3 499 cm³ V6 and a 4 663 cm³ V8 – are a step forward in the ongoing endeavour to improve fuel consumption without diminishing the power outputs that buyers of luxury cars expect. They share the following improvements over current production:
• The valve timing adjustment mechanism has been redesigned to give a larger range of adjustment as well as occupy a smaller mount of space than the previous unit.
• The camshafts are now driven from the crankshaft by a short primary chain, an intermediate gear and a secondary chain to each bank, to deliver a quieter and more positive link between the components.
• The water passages in the engine have been redesigned to incorporate an increase in fl ow speeds and heat dissipation at critical points, creating an overall reduction in pressure so that the power needed to drive the water pump has been reduced. The flow is regulated by a threephase management system that allows for a quick warm-up.
• A new vane-type oil pump is chain driven from the crankshaft in such a way that the eccentricity of the vanes in the housing can be varied in two stages to adjust the delivery pressure to the engine’s needs.
• A new stop/start function remembers the position of the piston that was ready for a spark when the engine stopped and fires that cylinder first, after which the starter motor rotates the engine to start the engine.
These two engines differ by NOT sharing the same angle between the cylinders, and there is a story behind this disparity.
Most manufacturers that produce V8 and V6 engines specify 90 degrees for the interbank angle, because it allows them to use the same (very expensive) equipment to machine the cylinder blocks.
This is the ideal angle for a V8 but not for a V6. Even Mercedes used to do this, but 90 degrees increases engine roughness on a V6. The new Mercedes V6 now has a 60-degree inter-bank angle that improves engine smoothness but requires special tooling to produce.
The other difference lies in the combustion process. Mercedes- Benz claims that it was the first company to introduce a seriesproduction spray-guided direct injection system, and the new V6 engine is the first to employ the third-generation version.
The system pressure has been increased to 200 bar and new piezo-electric injectors have been developed that have a response time of 0,1 millisecond. This has made it possible to specify up to five fuel injections per cycle to promote lean-burn (ie, stratified) combustion.
In addition, a multi-spark unit is employed to deliver up to four sparks in rapid succession within one millisecond to create a broad plasma field instead of just a string-shaped spark. This will ensure that the lean mixture associated with lean-burn combustion always ignites.This multiple control over injection and spark enables the V6 to have the following unique combustion map:
• During idling, combustion takes place in a homogeneous mixture (everywhere the same).
• At low partial load, up to 400 kPa MEP (mean effective pressure on the pistons), and up to 3 800 r/min, combustion takes place in a stratified mixture (layers of mixture getting leaner the further away it is from the spark).
• At medium partial load, from 400 to 800 kPa MEP and up to 4 000 r/min, combustion takes place in what Mercedes calls homogeneous stratified combustion (HOS).
This is a combination of the two modes that is achieved as follows: the first injection is sprayed in during the intake stroke to form a homogeneous basic mixture.
A stratified layer of fuel is injected during the compression stroke just before ignition in the form of a single or double squirt, depending on the map characteristic.
• At high load, the combustion is homogeneous or homogeneous split (HSP) throughout entire speed range. The latter is a mode that sees 95 per cent of the fuel being injected in one or two squirts, followed by a very small injection at about the same time as the spark to stabilise combustion. At maximum torque the MEP is 1 329 kPa.
The result of all this technological tomfoolery is greatly reduced fuel consumption. The new V6 has the same displacement as its predecessor, but now develops 370 N.m of torque instead of 350 and 225 kW instead of 200. An S350 fitted with this engine uses fuel at the rate of 7,6 litres/100 km, which is 24 per cent better than the old engine could achieve.
The new V8 engine is far more conventional. It employs the latest spray-guided direct injection to promote homogeneous combustion, but utilises a turbocharger for each bank to generate 320 kW, which is 12 per cent more than its unblown predecessor, in spite of being 15 per cent smaller in swept volume.
The new maximum torque value of 700 N.m is a 32 per cent increase over the previous value, and fuel consumption when installed in a CL500 has decreased dramatically from 12,3 to 9,5 litres/100 km.
The turbochargers are optimised to deliver high torque at low engine speeds. Maximum boost is 90 kPa, with the turbine blades rotating at 150 000 r/min. 600 N.m of torque is available from 1 600 to 4 750 r/min.
TESTING THE ENGINES
The new engines have been tested on the road and in what the company claims to be the most modern engine dynamometer test cells in the world, housed at its Untertürkheim plant. Each cell, and there are 72 of them, is a cage mounted on springs to isolate the rest of the building from the engine vibration.
These cells are in operation day and night to cope with the variety of tests and engines. The latest V-engines have been preproduction tested for 52 000 hours of which 27 000 hours were endurance runs! A further seven million kilometres were covered on roads in all the extreme climates and traffic conditions.
ROBOTIC MOTORING
Radio-controlled cars are not new as any hobbyist will tell you, but on the second information day Mercedes showed two driverless test cars that appeared to be radio-controlled.
However, I soon discovered that they were not radio controlled in the usual sense of having a remote driver. Instead, the throttle, brakes and steering were controlled by industrial robots carrying out a sequence of commands in a software program.
This has the major advantage that every time the car is asked to follow a program, the results will be repeats of the previous run to the extent that the wheel tracks and stopping distances are within 30 mm of each other.
The cars are used for testing and various programs can be beamed to the computer controlling the robots. In an emergency, a panic button in the control tower will bring all the cars to a stop and if any fault develops on the car or the robots they’re programmed to stop immediately.
Repeatability is only one of the advantages to having robotic test drivers. For example, an emergency stop will give repeatable results to an extent that no human driver can achieve.
When a car is jumped over a ramp to test the airbag’s ability to not deploy unless the deceleration exceeds a pre-set value, the landing is unpleasant and may result in injury to a human but a robotic driver does not even work up a sweat.
In addition, there are a number of tests involving present or future safety equipment that involve a high risk factor for any human driver.
For example, I was shown two cars programmed to miss each other by 20 milliseconds (an eye blink is over in 100 milliseconds) at a crossroad. If human drivers were used the slightest timing or judgment error could easily have been fatal. The precise tests that these autopiloted cars can perform are especially relevant at the moment.
Mercedes and other manufacturers have already started to introduce safety technology that turns a car into a thinking partner that can “see and feel” what is going on around it, and even react to some extent when an accident is imminent.
Any one of these systems can only be put in production after a long series of tests, and some of these tests can only now be performed in an accurate, safe and repeatable manner by robots.
