Ricardo's special 'two stroke'
No-smoke two-stroke promises power with economy
British engineering research company Ricardo is developing a new petrol engine which combines two-stroke and four-stroke operating cycles. Past two-strokes have compressed the fuel and air mixture in the crankcase, which precludes the use of an oil sump so oil has to be delivered with the fuel, causing polluting oil smoke. But this new engine, dubbed 2/4SIGHT, uses normal overhead valves and a normal oil supply. So how can it be both a two-stroke and a four-stroke? By altering the valve timing and ignition timing radically. The valves are electro-hydraulically actuated so there are no camshafts. This means that at certain speeds and loads, the valves can open during every piston stroke instead of every other stroke, and direct injection means the air swirl and flame front can be controlled to let the downward-moving piston suck in intake air even while the power stroke is finishing. Forced induction, using both a supercharger and a turbocharger, ensures sufficient intake air enters the cylinder. In two-stroke mode the engine delivers remarkable torque: a one-litre verion would manage about 170lb ft at 2500rpm. This means, for example, that a 2.0-litre V6 2/4SIGHT engine would match a conventional 3.5-litre V6 when worked hard while proving much more economical when cruising in four-stroke mode. Ricardo estimates a 27 per cent improvement in economy and CO2 output. The control system, co-developed with Denso, also promises 'seamless' switching between the two modes. Currently the engine exists as a three-cylinder testbed, but the project is backed by a 'consortium of automtove partners'. We've seen revoltionary two-stroke engines before – Ford came close to productionising the Australian Orbital unit – but with today's computing power to control it, and today's need for smaller, lighter, more frugal engines, the Ricardo 2/4SIGHT seems like a promising idea.
