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New road for Merc engines

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From next year, Mercedes-Benz plans to introduce a new family of engines across its passenger car line-up – among them four- and six-cylinder units that incorporate 48-volt electrical systems. Wynter Murdoch reports

Mercedes-Benz has launched what spokesmen for the brand call the biggest strategic engine initiative in the company’s history – a return to in-line six-cylinder engines for petrol and diesel units; the introduction of 48-volt electrical systems for four- and six-cylinder plants; and the launch of a small, bi-turbocharged, petrol-fuelled V8 that’s more powerful than its predecessor.

The swing towards the new engine line-up began this year with the introduction of a four-cylinder 2,0-litre diesel unit developed for the E-Class. It will continue in 2017 when S Class derivatives will be equipped with a V8 that displaces just 4,0 litres – 700cc less than the capacity of the current unit.

Other models across the Mercedes-Benz passenger car line-up will be fitted with four- or six-cylinder, petrol- or diesel-powered units that feature 48-volt technology.

The change has been driven by pending legislation which requires vehicle manufacturers to meet ever-tightening global emission laws. According to Prof Thomas Weber, who is responsible for group research at Daimler, the new units have been designed with a view to conforming to current and future legislative requirements.

“More powerful, more economical and cleaner – the new modular family of engines from Mercedes-Benz is right-sized for every vehicle,” he maintains. “The systematic optimisation of the units plays a crucial role in the road map towards sustainable mobility. A key success factor is the extensive electrification of the powertrain.”

Though the V8, which produces around 350kW and 700Nm, eschews a higher voltage electrical system, the engine’s abridged size – coupled with integrated turbochargers between its cylinder banks as well as cylinder deactivation to turn it into a V4 under light loads – is said to have boosted fuel efficiency by about 10% compared with its larger capacity predecessor. What’s more, the unit is about 15kW more powerful than the forerunner.

Says Weber: “Instead of trimming the number of cylinders – thereby foregoing refinement and output – there are much more intelligent solutions. The M 176 engine uses cylinder shut off; at part-load up to 3 600 rpm it is especially efficient.”

That said, the S-Class will also be sold in hybrid form equipped with an in-line, six-cylinder, turbocharged, 3,0-litre, petrol-fuelled unit coupled with a 48-volt electrical system. The engine – designated M 256 – produces 300kW and 500Nm and, in tandem with an electric drivetrain, it is said to offer performance akin to that of the V8.

The unit’s turbo technology relies on an electric auxiliary compressor (eZV) to force air into the intake system. It is supplemented by an Integrated Starter-Alternator (ISG), which is responsible for hybrid functions such as boost for the powertrain or energy recovery.

“The M 256 is the first engine that we have systematically designed for electrification from the outset. The bottom line is that the engine offers the same kind of performance as an eight-cylinder machine while being much more fuel-efficient,” Weber reiterates.

He adds that incorporation of the 48-volt electrical system has dispensed with the need for belt drives for ancillary components at the front of the engine, reducing the plant’s dimensions and resulting in meaningful packaging advantages.

He points out that the unit’s narrow construction, together with physical separation of intake and exhaust systems, has created space for a near-engine exhaust after-treatment resource to further lower emissions.

In terms of design features, he says the 48-volt system serves not only high load items such as the water pump and air-conditioning compressor but, through highly efficient energy recovery, also the ISG, which in turn supplies power to the battery.

“This allows fuel savings that were previously reserved for vehicles equipped with high-voltage hybrid technology,” he says. “In comparison with our superseded V6, CO2 emissions from this engine have been reduced by around 15%.”

On the diesel front, an equivalent six-cylinder plant – the OM 656 – incorporates two-stage turbocharging, variable valve timing, reshaped combustion chambers and steel, stepped-bowl pistons.

According to Mercedes-Benz’s engineers, “Stepped bowl has a positive effect on the combustion process in terms of thermal loading of the pistons and the introduction of soot into the engine oil.

“Efficiency is increased by a higher burn rate in comparison with the previous engine’s omega-shaped combustion bowl. The characteristic feature of the specifically configured combination of bowl shape, air movement and injector is its very efficient utilisation of the fuel mixture, which means that particulate emissions can be reduced to an exceptionally low level.”

The engine’s block is cast from aluminium. What is described as an improved Nanoslide coating has been used for the cylinders, while the unit also features a near-engine exhaust treatment resource.

Power output is said to be 230kW, making it Mercedes-Benz’s most powerful ever diesel engine for a passenger car, while maximum torque is quoted at 650Nm.

Less glamorous than the six-cylinder units and the V8 – but significant for the brand’s higher volume models – is the new M 264, 2,0-litre petrol-fuelled plant. Also known as the Toptype, the four-cylinder unit follows Weber’s right-sizing philosophy by offering output of 100kW – territory previously reserved for higher displacement engines – thanks to turbocharging, variable valve timing, a belt-driven 48-volt starter-alternator (BSA) and a 48-volt water pump.

Like the ISG, the BSA is responsible for hybrid functions that include supplementing the engine’s turbo-boost up to 2 500 rpm and recovering energy to help recharge the battery. In addition, the powertrain system allows the load point in the engine map to continually shift to favourable areas and, when coasting, to turn off the unit.

In the interests of high power output and spontaneous engine response, Mercedes-Benz has opted for twin-scroll turbocharging maintaining that, unlike conventional systems, systematic cylinder flow separation produces high torque in the low revs/min range, together with high specific output.

Further efficiency measures include a friction loss reduction package and, for petrol engines across the line-up, the staggered introduction of particulate filters to improve environmental compatibility.

 

48-Volt battery

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