The Return of VCR: Variable Compression Ratio (VCR) technology for powertrains is back. Or at least that's what I discovered while attending this year's Geneva Motor Show.
Variable Compression Ratio (VCR) technology is hardly new--Harry Ricardo built and tested the first such engine in the 1920s--but in recent years it has been the focus of some interest. Saab had a bash at it in 2000 with its SVC (Saab Variable Compression) concept, as have other companies including Nissan, PSA Peugeot-Citroen, Renault, and Volvo. For all the benefits that it offers in theory, though, it failed to deliver in practice and soon gave ground to direct injection.
One of the present challenges in automotive engine technology, though, is the improvement of thermal efficiency and hence fuel economy and lower emission levels. And one of the key features affecting thermal efficiency is the compression ratio, which is always a compromise in fixed compression ratio spark ignition engines. This is why the VCR concept remains so compelling to some, including French engineer Vianney Rabhi of MCE-5 Development, the company that I spotted in Geneva. "VCR technology can achieve around 10% fuel saving when running the engine with the optimal compression ratio under all load and speed conditions," says Rabhi, "but this figure can go as high as 18% when downsizing is taken into account while maintaining torque and performance by high boosting. Overall fuel consumption reduction of up to 27% is possible with the new technology without any disadvantages in drivability, pollutant emissions, and with only moderate production cost increase."
He has been working on VCR engines for more than 20 years, registering several patents on the technology in the 1980s. However, the real boost came at the turn of the century with the formation of MCE-5 Development, the name reflecting Rabhi's VCR concept that he had invented in 1997, by a small group of engineers. The major impetus, though, came from the French government through a number of research agencies and the PSA Peugeot Citroen Group which joined the venture at the same time. In the eight years since then, the company has acquired 47 shareholders that include four financial companies and CERTAM, the engine research laboratory based in France. It is also supported by major French institutions for energy saving, innovation and transportation that include the PREDIT program (the French program for energy and transportation), ADEME (the French Agency for Environment and Energy Management), ANVAR (the French Agency for Innovation), Ministry of Research and the Ministry of Industry.
Externally, the MCE-5 engine looks no different than a conventional one and it remains compatible with all combustion chamber shapes and cylinder head designs. All the components are enclosed in the engine block and no moving part is visible from the outside. Another significant factor is that it can be fixed into vehicles and connected to the gearbox, pipes and peripherals as if it were a conventional engine. The difference internally is the piston control and movement. Instead of it being located on a crank, the clever design means that the piston slides absolutely vertically on a geared rail so that piston slap is zero. However, its kinematics remains exactly the same as that of a conventional engine with the same rod/crank ratio. This remains always true whatever the compression ratio which ranges from 7:1 to 18:1.
"The piston is roller-guided and is no longer subjected to rod thrust so there is no piston radial stress or piston slap," says Rabhi. "The forces that generate torque on the crankshaft are entirely assumed by the rollers--this arrangement reduces friction losses and widely extends the cylinder lifespan. This constitutes a strong response to the durability problem of highly downsized high-loaded engines." It is also the first and only intelligent VCR engine with a floating compression ratio as it features integrated hydraulic actuators that provide a wide range, precise and continuous compression ratio control for each cylinder.
The connecting rod and crankshaft have both been designed in collaboration with companies that produce several millions of them a year, says Rabhi. "The forging processes selected for these parts are those already used in the automotive industry. The cost price of each basic part that do not require specific manufacturing processes were identified, all of which has meant that it is possible to mass produce the MCE-5 engine in the near future." This latter point is often the killer for new technologies but Rabhi says that the MCE-5 engine will actually save carmakers money. "Our studies have shown that in the worst case scenario the additional cost to producing it and the turbocharger amounts to around $750, approximately the cost price of a diesel particulate filter. However, when used to replace a large V6 gasoline engine, it generates savings of approximately $1,200 per vehicle while offering the same performance level. At equivalent or lower fuel consumption levels, a vehicle equipped with the MCE-5 engine is less expensive than a diesel and far less expensive than any hybrid, so that the cost price per kilowatt of power or per Newton meter of torque is below current standards."
While the core attribute of a VCR engine is in allowing downsizing and consequent fuel and emissions savings, the downside is that it can affect drivability. However, Rabhi is adamant that this is not the case with the MCE-5. "Drivability is not an issue because it relates to low end torque," he says. "It's necessary when you downsize an engine to have high torque at low revs without any turbo lag--and we have an extremely high torque with that of our 1.5-liter engine being better than that of a conventional 3.0-liter V6. As far as hard numbers are concerned, peak torque is around 3,500 rpm on a normally aspirated engine while we have 265 lb-ft at 1,800 rpm on the MCE-5 with most of it being available from 1,500 rpm. The price that the customer pays per mile must remain low, taking into account the price of the vehicle, its running costs and also the resale price," says Rabhi. "Good performance levels, low fuel consumption and reasonable purchase prices remain indispensable qualities for future cars, conditions that are necessary for cars of the future if cars are to contribute to lowering global [CO.sub.2] emissions and fuel consumption in a significant manner. "However, it is pointless to produce low fuel consumption vehicles that do not sell well or at all because they are too expensive. It's therefore essential to find simple, low cost solutions and this has been our strategy and our solution is the variable compression ratio."
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|Title Annotation:||Euro Auto|
|Publication:||Automotive Design & Production|
|Date:||Apr 1, 2008|
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