Nonelectric Hybrid Engines

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Ricardo's engine, called 2/4SIGHT, uses valves like a four-stroke engine, but in two-stroke mode, the engine keeps both the intake and exhaust valves open at the same time so that the fuel and air in the cylinder are replenished each cycle, rather than every other cycle.

There has been a lot of interest in developing a low-emission two-stroke engine. But it's a difficult configuration to perfect because there is little time to get the fuel-air mix in and the exhaust out, says Larmi. "The danger here is that the fresh air intake can go directly out through the exhaust outlet," he says.

Ricardo is using a couple of tricks to get around this problem. First, the design of Ricardo's piston head uses reverse tumbling, a process in which the air intake is directed away from the exhaust valve, to reduce the chances of it flowing straight out of the cylinder. Ricardo has also swapped the cam-controlled valves for electro-hydraulic valves, which, along with the fuel injector, can be controlled by software.

Car manufacturers have showed an interest in building this sort of hybrid engine in the past, says Kee. "But there are a lot of challenges," he says. Indeed, both Toyota and Ricardo looked at this issue in the late 1980s and early '90s.

But in the past, the technology simply wasn't there. According to Ricardo, the only reason the company is able to make a viable system now is because of the software that controls the gas exchange and engine modes. "The engine's control system monitors driver demand," says Jackson. When more torque is required than would be possible in four-stroke mode, it switches, he says. However, the company will not reveal details about when, in the engine cycle, the mode is switched.

Ricardo's prototype, an adapted 2.1-liter V6 engine, has been tested by researchers at the University of Brighton and has been found to be able to produce the kind of performance one would normally expect from a three-to-four-liter engine. Based on the New European Driving Cycle, which is a standard performance test designed to gauge engine efficiency and emissions under typical car usage, the prototype has demonstrated fuel savings of 27 percent, and it reduces emissions by a similar amount. The next phase is to try to incorporate a prototype engine into a working vehicle, says Jackson.