Return of the Inline-6 Engine

Mercedes-Benz M256 

Article Richard Simonds

Image MBUSA Media

Three major forces seem to be influencing current engine development: emissions and fuel-economy regulations; performance needed from smaller displacement engines; and production costs due to having too many different types of engines. Based on recent announcements in the automotive press, Daimler AG seems to be developing a new family of engines to meet all three of these factors. According to reports, the company currently has inline-4 and 6-cylinder engines in development to be powered by gasoline or diesel fuels. As someone who has always appreciated the inherent balance and smoothness of inline-6s, I’ve been intrigued with this technical focus.

What is known is that Mercedes-Benz has developed a new 6-cylinder inline gasoline engine (M256) that has for the first time been systematically designed to work with ancillary electrical and electronic components driven by electric motors rather from common external belts. This engine is a modular design with every function – including the air-conditioning compressor, new intelligent turbocharging with an electric auxiliary compressor, and an Integrated Starter-Alternator (ISG) – connected electrically, guaranteeing excellent drivability with no turbo lag and an ISG function with built-in electrical boost for improved performance, lower emissions and improved fuel economy.

To accomplish this, the engine has a 48-volt system that will meet the needs of all electrically powered components (a/c, turbocharging, power steering, fan, electronic-control modules, lights, entertainment systems, and so forth), while providing a performance and fuel-economy boost directly between the engine and the drivetrain. There will be a 48-volt lithium-ion battery and a 12-volt conventional battery with a down-converter from the ISG to maintain the 12-volt battery.

Current indications are that the engine will consist of cylinders displacing 500cc each, with the I-6 displacing three liters. The engine is rated at 408 horsepower (DIN) with 500-plus Newton-meters of torque. With initial production destined for non-U.S. markets, equivalent SAE figures are not yet available.

Unlike V-6 engines, the inline-6 configuration does not require a counter-balance shaft to smooth out harmonic vibrations, thus reducing the friction and complexity of the engine. Each side of the new engine is designed to serve separate functions: The left side is the cool side for the fuel-induction and electrical-compressor systems to provide low-rpm boost; the right side is the hot side for the exhaust and twin-scroll turbocharging system to provide performance in the mid- and high-rpm range. Finally, the new M256 engine provides the power of a V-8 with excellent performance, better fuel economy and lower emissions than the V-8 engines that have been used since the mid-1960s for Mercedes-Benz luxury and performance cars.

The current V-6 engines in use since 1998 were developed in part to reduce production costs by allowing the company to produce V-6 and V-8 engines on the same production line; the new inline design offers the advantage of being able to produce I-4 and I-6 engines on the same production line and fabrication equipment using the same size of cylinder bore and pistons simply by changing the number of cylinders, crankshaft, camshaft and related components.

Though announcements made to date carefully make no commitments to when such an engine might find its way into cars destined for the U.S. market, we can certainly surmise that this development will have broad-reaching significance to the global market in the near future.

A cut-away rear-view of the M256 engine with the integrated charged air cooler (honey-combed component) visible on the left-hand side just below the valve cover, electric supercharger behind and and below it, NVH-optimized engine mount in the middle bottom, and integrated starter-alternator on the right, at the rear of the engine.