According to Professor Heinz Junker, the hybrid-electric vehicle (HEV) and the downsized combustion engine have more in common than is sometimes perceived. During a technical briefing at Stuttgart, the Mahle CEO and Chairman of the Management Board said, “Both approaches seek to shift the dominant load area to a more efficient part of the map. After all, the electric motor does little else than to boost a small combustion engine in the lower rev band where the combustion engine alone cannot provide the required driveability.”
Pure electric driving, which is often advertised as an advantage of full hybrid vehicles, is an option that will only go so far, Junker said: The high overall weight of a full HEV dramatically limits its pure electric reach.
This analysis may be backed up by a recent trend of HEV manufacturers, such as Toyota, to install larger combustion engines in new HEV model generations to improve the long-distance driving behavior and fuel consumption. The 2009 Prius HEV, for instance, is equipped with a 1.8-L gasoline engine rated at 98 hp (73 kW) which has replaced the earlier generation’s 1.5-L engine.
The true bottom-line effect of the electric motor and its contribution via recuperation of braking energy is fairly small, according to Junker. “If you take a naturally aspirated gasoline engine as reference, an aggressively downsized gasoline engine alone can improve the fuel efficiency by up to 40%," he explained. "If you consider the additional 5% gain that may be contributed by the electric motor of a full HEV, it is clear that downsizing offers the biggest single potential."
Economically, it makes sense as well, Junker noted, since the total additional cost for a gasoline downsizing efficiency gain in the 30% league "is between 2000 and 3000 euro. In a full HEV, this does not even pay the traction battery.”
Aggressive downsizing of up to 50%
To underpin this ambitious statement, Mahle has developed a radically downsized gasoline engine that squeezes 144 kW (around 200 hp at 6500 rpm) and up to 287 N•m (between 2500 and 3000 rpm at 30 bar brake mean effective pressure (bmep) out of a three-cylinder 1.2 L gasoline powerplant. It is designed to replace a 2.4-L engine in a family size car with a curb weight of up to 1.6 tons. This demonstrator power pack was designed to meet the Euro 5 emissions standard and is equipped with key enabling technologies the supplier has to offer.
To overcome the current limits to downsizing that are in the 30% area, the weight-optimized demonstrator engine is equipped with a low-friction variable valvetrain, featuring Cam-in-Cam technology, split cooling, friction optimized power cell, controlled oil pump, lightweight valves with interior cooling, high-load exhaust gas recirculation (EGR), and two turbochargers.
The specific stationary consumption of the engine is as low as 295 g/kWh at 2000 rpm and 4 bmep. At its optimum point, the three-cylinder engine consumes a mere 234 g/kWh. This compares to the average specific consumption of standard downsized gasoline engines, which is often in the span between 360 and 390 g/kWh.
Maximum EGR strategy
“EGR, in particular, is an enabling technology for downsizing,” says Junker, “as it helps to bring down NOx emissions and fuel consumption. With high EGR rates, there is no need for gasoline enrichment to protect components from too much heat at full load."
Stoichiometric operation at full load, thanks to very high EGR rates, can save up to 10% of fuel in a highly charged gasoline engine,” he noted. “Just as importantly, very high EGR rates can help to avoid the need for a selective catalytic reduction system,” explained Jörg Rückauf, Mahle's Director of Research & Advanced Engineering.
Currently, one threshold for very high EGR rates is a lack of exhaust pressure in parts of the engine map. If the air pressure in the intake duct, for instance, is higher than the exhaust pressure, high EGR rates will simply be impossible to achieve, the Mahle engineer noted. Hitherto solutions, based on throttle valves, increase throttle losses and bring down the gasoline engine’s efficiency. Mahle avoids this side effect by installing a fast rotating charge air valve (called SLV) that only closes the intake duct for the shortest of moments and, thus, briefly decreases the pressure downstream where the exhaust gas inlet is located.
By this momentary effect, it becomes possible to use high EGR rates despite comparably low exhaust gas pressure. The rotating flap motion is electronically synchronized with the cylinder movement to maximize the benefit.
“As there is no permanent throttle effect, the rotating air valve does not affect fuel consumption,” Rückauf explains.
Controlled oil pump
Another new strategy for increasing the efficiency of a combustion engine is to reduce the losses in ancillary components. By controlling the volumetric flow of an oil pump, for instance, the component will not always run along with the engine speed. This makes perfect sense, as the engine’s oil demand levels off from a certain speed.
“Depending on the chosen control strategy, an oil pump can, therefore, contribute up to 2% better fuel efficiency in the New European Driving Cycle,” said Dr.-Ing. Uwe Mohr, Vice President Corporate Research, Advanced Engineering & Services. To exploit this potential, the supplier uses its patented controlled pendulum slider oil pump.
Compared to conventional rotary vane pumps, the pendulum slider pump is claimed to have a 10% better efficiency – and shows dramatically less friction. This is due to the principle of operation: Compared with vane pumps, the individual pump cells of the pendulum slider pump are not sealed via sliding friction but by the rolling motion of pendulums in grooves.
“As a consequence to the low wear, the pump maintains its high efficiency over the complete lifetime," said Mohr.
Currently, the supplier is preparing to use the pendulum slider pump for a map-controlled oil pump application in a passenger car.