Tech - February 2017

Mercedes-Benz Straight Six Engines Are Back

In days gone by, Mercedes-Benz were the last word in refinement. Silky smooth straight sixes were the order of the day and while slightly less practical to package than a V6 motor of equivalent capacity, the inherent benefits of a straight six over a V6 made it worth the long bonnet. Because people hate long bonnets.

It would seem, however, that BMW are the only big manufacturer to produce straight six motors these days. Ford Australia did for a while, but they don’t exist anymore and neither do TVR…

This is strange because while a V6 motor makes packaging a breeze thanks to its compact dimensions, it becomes very complicated due to the inherent vibration issues caused by two banks of cylinders with yaw moments on different axis. Balancing shafts can easily cancel out these vibrations but this means that more inertial mass is required to spin the engine – ie: you need more power.

The great news, then, is that Mercedes-Benz are back on the straight-six train as announced towards the end of 2016 and the new M256 promises to be a powerhouse of note. A part of their new range of modular engines, the new six will arrive alongside petrol and diesel straight-fours, straight-sixes and a petrol V8. They all have identical bore spacing and interfaces to vehicle which cuts production costs.

Back to the M256, it features a host of new technology, most notable of which is the Inline Starter Generator or ‘ISG’. The ISG is a 15kW electric motor which drives the crankshaft, starts the internal combustion engine when start/stop is enabled, recovers energy during coasting and braking and acts as a generator for the 12v electrical system. It can also reduce the load on the engine which aids performance and economy.

It is also part of the 48v electrical system which comprises an electric air-conditioning compressor, electric auxiliary compressor and electric water pump which means there is no need for a belt-driven accessory drive. This means that engine length is reduced which, as I have already mentioned, causes packaging issues with the straight-six motor.

Another brilliant up-side to the whole electrification thing is that the 48v compressor is essentially a supercharger which doesn’t have a parasitic effect on the combustion engine. So at low RPM’s, the compressor kicks in and provides boost up until the big exhaust driven turbo kicks in. Expect figures of around 304 kW (407bhp) and 501 N.m and remarkable efficiency, we hope.

Expect to see this exciting new motor in the updated Mercedes-Benz S-Class before trickling down into the rest of the Mercedes-Benz stable.