MCE-5’s Variable Compression Gas Engine Promises Power and Efficiency

Could a radical new engine from Europe make American pickups more fuel-efficient? MCE-5 Development, based in France, has created technology that it says promises power figures similar to current V-8 engines with fuel economy that’s nearly 50 percent better. The secret: MCE-5 is able to adjust an engine’s compression ratio on the fly, cylinder-by-cylinder.

“We are able to vary the compression ratio for each cylinder independently,” said Vianney Rabhi, MCE-5’s head of strategy and development and the engine’s lead designer. “Each cylinder can do the best with its own physical properties and operating conditions to ignite at just the right time, depending on driving conditions.”

MCE-5 says a 2.2-liter engine using its Variable Compression Ratio intelligent technology could generate 330 horsepower at 4,000 rpm and 395 pounds-feet of torque at only 2,000 rpm while returning 21/25 mpg city/highway. In comparison, the 5.4-liter V-8 available in today’s Ford F-150 is rated at 310 hp at 5,000 rpm and 365 pounds-feet at 3,500 rpm, and it gets 14/20 mpg city/highway.

“The compression ratio can range from 7:1 to 20:1,” Rabhi said.

An engine’s compression ratio refers to the amount of volume a piston will compress the fuel/air mixture into in a cylinder at the top of a piston’s stroke compared to the total volume of the cylinder when the piston is at the bottom of its stroke. Generally -- depending on fuel type -- the greater the compression ratio, the more efficient an engine is and the more force it can generate for towing and hauling.

By varying the compression ratio over such a wide range -- rather than maintaining a fixed compression ratio like a conventional spark-ignited gas engine -- combustion can be precisely controlled and matched to driving conditions for optimal efficiency without resulting in knock that can damage the engine. Knock occurs when the fuel/air charge combusts in an uncontrolled manner.

Rabhi rethought the gas engine’s moving parts to create the VCRi engine. Each cylinder head houses a primary combustion piston that drives the engine, but it’s connected via a set of gears and controls to a new secondary piston, called a control jack. The control jack’s height is changed hydraulically depending on driving demands, which in turn changes the pivot point and stroke length of the primary piston, altering the volume in the combustion chamber and changing the combustion ratio. MCE-5 says changes can happen in as little as 100 milliseconds.

“In the case of MCE-5, we’ve added three new parts, but the design is still compact,” Rabhi said. “It has a piston, [control] jack, [gear] racks, a [gear] wheel, a rod and a crankshaft. [Today’s engines] only have a piston, rod and crankshaft. This is how we control the compression cylinder by cylinder.”

According to Rabhi, the design will get more efficient over time as MCE-5 redesigns the combustion chambers, adds gasoline direct injection to the engine and, eventually, combines it with another new combustion technology: homogeneous charge compression ignition. HCCI enables a normally spark-ignited gasoline engine to operate similar to a compression-ignited diesel engine. Those engines rely on very high compression ratios (like the MCE-5 can generate) to squeeze the air in the combustion chamber so much that friction alone causes the fuel and air mixture to burn. Up until now, though, HCCI engine combustion cycles have been notoriously difficult to control, depending on engine load.

“HCCI is unstable, at this moment,” Rabhi said. “You need to be very precise with the valve timing, fuel intake temperatures, blow-by per cylinder, effective valve timing and exhaust gas recirculation -- all parameters which have a high impact on the self-ignition of the charge. If you can control the compression ratio within different parameters, you can ignite [the fuel] when you want. MCE-5 can compensate for this. The engine will self-adapt to operating conditions.”

According to Rabhi, HCCI, combined with variable compression, could yield another 10 to 15 percent boost in fuel economy over variable compression alone.

MCE-5 is testing the VCRi engine jointly with a European car manufacturer. The company hopes to leave the engine test bench and dyno behind this month and operate a small car with a VCRi engine. But it’s not small cars Rabhi wants to stop with; he’s very interested in full-size pickups, like the F-150.

“The bigger the vehicle, the higher the fuel consumption,” Rabhi said. “Our technology is much more interesting for big vehicles like American trucks that need fuel efficiency and high power.”

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