“The combustion engine will have to make one of the important individual contributions to bringing down CO2 emissions,” Heinz K. Junker, Chairman of the Mahle Management Board, said during an interview with AEI at the Frankfurt Motor Show. To demonstrate the influence combustion engine components and systems can have on overall engine efficiency, the supplier has applied its entire range of state-of-the-art technology to a technology demonstrator engine that was developed and constructed at Mahle Powertrain in the U.K. This 1.2-L threecylinder gasoline engine pushes out up to 120 kW (160 hp) per liter. According to Mahle, this would be sufficient for a 1.6- tonne vehicle—a vehicle class that would typically be powered by a 2.5-L engine. Reducing the capacity by half would result in more than 20% higher fuel efficiency, according to the supplier. “The future lies in downsizing,” Junker said.

    As turbocharging technology is key for this kind of downsizing, Mahle will start to manufacture turbochargers by 2010. Current development includes gasolineengine wastegate turbochargers for engines with up to 200 kW (268 hp) output, and variable-flow turbochargers for diesel engines with up to 150 kW (201 hp) output. The first prototypes were shown at Frankfurt.

    The demonstrator engine’s high efficiency owes to a long list of innovative technologies that includes two alternative turbocharging concepts, forged pistons, DOHC, and direct injection with injectortip cooling. Additional technology applications include an air-water intercooler, cooled high-load exhaust gas recirculation, dual camshaft adjustment, and lightweight valves.

    Mahle’s core development targets were to cut total mass by reducing component weight and minimizing internal friction in the engine. One-third of the engine’s friction losses can be attributed to the tribological system of rings and pistons, according to the supplier. To help reduce this effect, the demonstrator engine’s piston pins were coated with a diamond-like carbon coating, cylinder running surfaces were coated with Nikasil, and the piston rings were PVDcoated. The supplier also uses PVD (physical vapor deposition) technology for its Flex Fuel engine piston rings, as they require a much higher wear and fretting resistance than piston rings for conventional-fuel engines.

    When it comes to reducing the friction of bearings in the engine, Mahle is also considering the replacement of plain bearings with roller bearings, according to Junker.

    Among the component innovations shown at Frankfurt were lightweight intake and exhaust valves with internal cooling. These are made of formed sheetmetal parts. The valve has a hollow body, and the cone and disc are produced in a multistage metal-forming process. The stem is made of a steel pipe, and individual components are joined by means of high-precision laser welding. As a result, mechanical finishing is reduced by 25% compared to forged-steel valves. The hollow design allows the valves to be filled with sodium, enabling considerably higher combustion-chamber temperatures and an optimized engine operation with respect to fuel consumption.

    “These lighweight valves can take temperatures that are between 50 and 100°C higher than is possible with other valves. Originally, we developed this valve type mainly to reduce the moving masses which have to be accelerated and decelerated at high frequency during operation. By doing this we increased the component strength in high-speed ranges while reducing wear and noise. Today, it becomes obvious that lightweight valves offer additional benefits for turbocharged lean-burn gasoline engines. We have embarked on three engine programs that are likely to result in [production of] lightweight valves.”

    Mahle’s new gasoline-engine Evotec pistons also feature lower oscillating masses. The box walls of the piston thrust side are narrower than those on the antithrust side. This saves weight in areas subject to less stress. Compared to a standard piston version without recesses behind the ring zone above the bosses, the Evotec piston is approximately 20% lighter, according to Mahle. Improved engine responsiveness, lower fuel consumption, and lower exhaust-gas emissions are the consequence, the supplier claims. To produce this asymmetrical shape, Mahle designed a new casting process. To increase the piston’s service life and wear resistance, the Evotec piston is available with a ring carrier for the first ring groove.

    For passenger-car diesel-engine aluminum pistons, the load limit may be near 200-bar (2900-psi) peak pressure and temperatures in excess of 400°C (750°F), Junker said. “When you take commercial engines as the role model, this was basically the load population when the change to steel pistons began. By 2012 or 2015 we might see the first dieselpassenger- car forged-steel piston applications. In particular, the identical heat rate of piston and cylinder will help to minimize blow-by and oil consumption.”

    High-strength and reduced-mass connecting rods made of forged steel are another element of lightweighting. The forging process results in a favorable fiber orientation. The new 36MnVS4 material provides a steel grade with a unique chemical composition. It has a finegrained microstructure and its fatigue resistance is up to 30% greater than existing materials for fracture-split connecting rods.

    Finally, Junker drew attention to another steel component: the piston pin. “By using shorter piston pins, we can take out another bit of weight of the total piston/conrod system.”
 

                                                                                                                                           Jörg Christoffel