Lotus unveils Omnivore flex-fuel concept at Geneva show

Lotus hasn't been short of news lately, what with the announcement of the four-seat Evora (pictured) and all. But the UK sporting specialist's most exciting revelation at the Geneva motor show was arguably not a car but an engine -- and a decidedly unsporting one at that.

With only one cylinder, the two-stroke Omnivore concept is a long way from climbing beneath the bonnet of anything in a Lotus showroom. But marketability isn't the point here.

As the name suggests, the Omnivore is designed to take the flex-fuel concept into new places, overcoming a number of problems conventional engines experience in being asked to move seamlessly from one type of fuel to the next.

Many of those problems arise from variations in combustion behaviour between bio- and fossil fuels. Ethanol and petrol have distinctly different ignition temperatures and combustion envelopes, making it difficult for a statically configured piston engine to run at optimal efficiency from fuel type to fuel type.

Turbocharging goes part of the way because it's relatively easy to vary boost levels to help make the most of ethanol's higher octane count; Lotus has taken this a step further by taking an apparent backward step in two-stroke combustion cycles.

The advantage of this, says the company's powertrain engineering boss, Geraint Castleton-White, is that it allows them to dispense with poppet valves.

"The absence of poppet valves in two-stroke engines makes the incorporation of a variable compression ratio system relatively straightforward," he told the press in Geneva.

"Our research on four-stroke engines showed us that while it's thermodynamically desirable, practically speaking it's very difficult, particularly taking into consideration production feasibility. This two-stroke engine could solve these practical difficulties and simultaneously permit a much larger range of compression ratio adjustment, with the potential to perform at a much higher efficiency when running on renewable fuels."

They achieve this with some Aussie help, incidentally, in the form of an air assisted direct injection system provided by Orbital Corporation of Australia. 

The key to the Omnivore's interfuel nimbleness is the replacement of conventional valves with what Lotus calls a moveable 'puck', allowing for real-time variations in compression ratio. With the help of sensors in the fuel intake, the engine management system can vary compression according to fuel type and driving conditions. It harnesses the motion of the piston to open the intake and exhaust ports, using a valve to trap exhaust gases, allowing asymmetric timing and real-time adjustment of exhaust flow to accommodate changes in driving conditions.

Dispensing with poppet valves allows the Omnivore to find further efficiencies in the use of a monoblock -- a cylinder block and head hewn from a single piece of metal. This helps keep the weight down and, by eliminating the need for a head gasket, dispenses with one of the internal combustion engine's major reliability bugbears.

One point of excitement here is that it makes possible -- eventually -- a shift to homogeneous charge compression ignition. HCCI is a crucial part of what makes diesels so much more energy efficient than petrol and ethanol engines -- once the engine is turning over, ignition is triggered by compression, dispensing with the need for complex and energy draining extrinsic spark-plug systems. (If you want to see just how energy draining the spark system is in a petrol engine, disconnect your alternator. Have a mobile handy -- you'll be needing roadside assistance or a new battery within an hour or two, even in daytime when you're not running head and taillights.)