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01/06/2011
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It's a challenge that has engaged the minds of many down the years: changing the compression ratio in an engine on the fly. In these fuel efficiency-conscious times,
a breakthrough was never more needed. William Diem reports
Changing the compression ratio in an engine on the fly is an idea that has been around for decades without successful industrialisation, but its potential for improving fuel consumption means that it continues to attract investment.
Variable compression ration (VCR) "offers the largest potential improvement in part-throttle fuel efficiency and CO2 emissions, when compared to other competing technologies, if applied to highly pressure-charged downsized engines," concluded British engineering firm Prodrive in a 2002 SAE paper. And today, with all automakers in the world looking for ways to reduce consumption, without harming customer satisfaction, is the right time for invention.
Small engineering companies in France and The Netherlands are aiming at just that market. In Lyon, France, MCE-5 Development S.A. is refining a technique to vary the compression in a petrol engine from 6:1 to 15:1, depending on the driver's demand for torque. Combined with direct injection and supercharging, VCR could improve fuel efficiency by 15-20%, says Vianney Rabhi, the inventor and founder.
Meanwhile, in Naarden, The Netherlands, Gomecsys BV inventor and technical director
Bert de Gooijer is demonstrating a supercharged two-cylinder, 800cc engine, producing 95kW and 150 Nm of torque, in which compression ranges from 7:1 to 18:1, reducing fuel consumption by 18%.
The fuel efficiency potential within VCR comes from the fact that all fixed compression ratios are a compromise between what the engine needs at full throttle and what it needs at part throttle. The threshold of unwanted detonation at wide open throttle limits the maximum useable compression ration to 11:1 or 12:1. But at part throttle – the situation at cruising speeds – compression could be higher, because the temperatures and pressures in the cylinder are much lower.
"VCR is a way of enabling aggressive downsizing, without downgrading performance," says Kean Harrison, team leader for advanced engines and hybrids at Prodrive. "Most VCR systems have been quite cumbersome: interesting academically, but not that feasible."
Automakers and inventors have patented many ideas over the years. None has reached series production, but Saab Automobile demonstrated a variable compression engine to journalists in 2000. When the hinged cylinder head was tilted hydraulically, the volume in the cylinder changed, while the stroke remained the same. However, Saab was at the time run by General Motors, who killed the project.
Prodrive's 2002 paper reviewed patents filed by Ford Motor Co., Volvo Car, Daimler AG, Nissan Motor Co, PSA Peugeot-Citroen, Mayflower and others. The ideas under review included a moving cylinder head, variation of combustion chamber volume and of the piston deck height, modification of the connecting rod geometry, moving the crankpin within the crankshaft to vary the stroke and moving the crankshaft axis.
Rahbi's system in France involves a modification of the connecting rod geometry, and the theory has moved off the test bench and into two Peugeot 407s.
MCE-5 has built nine engines by hand so far. The 1.5-litre, four-cylinder test engine produces 162kW and 420 Nm of torque at 1800 rpm, giving the cars the feel of a diesel. Fuel consumption in the demonstration vehicles is about 150 g/km of CO2. In the 2010 Peugeot 407, with a 2.6L 157kW V6 and 290 Nm of torque, fuel consumption was 233 g/km.
Rather than the crankshaft, the connecting rod is attached to a hinge or teeter-totter, known as the gear wheel, which is connected to the crankshaft. A second rod, or control jack, is connected to the other side of the hinge and controlled hydraulically by the engine controller. When the control jack is forced downwards hydraulically, the gear wheel tilts and pushes the piston rod and piston higher in the cylinder, resulting in a smaller volume and higher compression. Internal losses are small, because the system uses forces available in the engine to control the hydraulics, rather than a separate pump.
Swept volume remains the same – 84mm in the test engine – while the dead volume at the top of the cylinder can be made larger or smaller. "Each cylinder can be independently controlled," notes Rabhi. "It takes three combustion cycles, about 0.3 seconds, to change the compression ratio. … Most of the time, the VCR waits for the turbine to react."
The advantages of VCR are minor, in the case of a naturally aspirated engine, perhaps 3-5%, says Rabhi, but "with forced intake, we gain up to 35%, in comparison to a naturally aspirated engine, and, in comparison with a GDI turbo, which already has made improvements, we will gain between 15% and 20%."
The Dutch engineering company is displaying its third generation engine in a Mitsubishi Colt and, "we are getting good results for fuel reduction", says Bert de Gooijer.
The system involves a new crankshaft, in which eccentric bearings, controlled by a fixed ring gear at the front of the engine, can lift or let fall the bottom of the connecting rod. A series of gearings means the bearings change position at half the rotation speed of the crankshaft, which actually reduces internal friction losses in the engine. Compression can be reversed from highest to lowest in half an engine cycle under hard acceleration and the change from low to high compression take two complete cycles. Implementing the system in an existing engine will require the new gears, a small electric motor to adjust the controlling ring gear and a new press-fit crankshaft, rather than a solid machined one common today.
"Harley Davidson uses a press-fit crankshaft, so the process is known," adds de Gooijer. "But, for OEMs, it is a big step to change that."
Philippe Coblence, the Renault powertrain engineer who used to run Renault's F1 engine programme and was the architect of the automaker's newest 1.6L Energy dCi 130 going into the Renault Scenic and Nissan Qashqai this summer, says that "variable compression is a good idea, but it is difficult to realise".
The problem for an OEM is to assure durability and reliability in series production, and Prodrive's Kean Harrison points out that, in the future, US rules on emissions will require manufacturers to warranty their powertrains for 15 years or 240,000 km (150,000 miles).
Prodrive worked with Gomecsys on its first generation engine and Harrison liked the simplicity of that approach, while new linkages, as in the MCE-6 engine, might be more liable to failure, long term, exposing a manufacturer to financial risk. Both MCE-5 and Gomecsys are now talking with automakers about licensing their inventions. MCE-5 had a development deal for several years with PSA Peugeot-Citroen a decade ago, but has since been on its own.
The two companies have different ideas about what the first production VCR engine might look like. De Gooijer believes it will be a high volume, in-line 2- or 3-cylinder engine, because that would bring the fastest fleet reduction of CO2 emissions. An engineer at MCE-5 thinks it will be a small-volume, high-power engine for a niche vehicle, because that would limit a manufacturer's investment risk.
In any case, Harrison adds, "now is the time" for these developments to get a hearing in OEM powertrain departments. "There is enormous focus on new technologies."
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Author
William Diem
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