Nonelectric Hybrid Engines

A new kind of hybrid vehicle could offer reduced fuel consumption to consumers concerned about gas prices. Mechanical engineers in the United Kingdom have developed a novel kind of combustion engine that is able to switch between being a two-stroke and a four-stroke engine. The system, they say, can reduce fuel consumption by 27 percent.

The improved fuel consumption essentially comes from downsizing the engine, says Neville Jackson, technology director of Ricardo UK, an engineering firm in Shoreham-on-Sea that developed the new engine. “A smaller engine has less internal friction and delivers better fuel consumption,” he says.

But small car engines, which are usually based on a four-stroke design, don’t offer a lot of power. They can be particularly problematic when operated at low speeds with a high load, such as when accelerating uphill. Such conditions can even make a small engine stall if the driver doesn’t downshift.

“Four strokes are most efficient at full throttle; with two strokes, it’s the opposite,” says Robert Kee, a mechanical engineer who specializes in combustion engines at Queen’s University, in Belfast, Northern Ireland.

The difference between two- and four-stroke engines is that the latter carry out the four stages of air intake, compression, combustion, and exhaust in four strokes of a piston. A two-stroke engine, in contrast, does this in just two piston strokes.

Two-stroke engines are intrinsically simpler by design and have higher power-to-weight ratios at high loads and low speeds because they get twice as many power strokes per revolution. But traditional two-stroke engines require oil to be mixed in with the fuel, and therefore produce higher emissions. Because of this, they aren’t typically used in cars. Instead, they’re used for lightweight applications such as chainsaws, lawnmowers, and some motorbikes.

But now, researchers at Ricardo have developed a piston head that operates in both two- and four-stroke mode, and it can switch automatically between the two modes, depending on the needs of the engine. This allows a smaller engine to handle the low-speed, high-load conditions without stalling.

“This is an interesting concept,” says Martti Larmi, head of the Internal Combustion Engine Laboratory at Helsinki University of Technology, in Finland.

The main challenge in building such an engine is perfecting the scavenging process, he says, when the residual gases from the previous combustion cycle are replaced with fresh air and fuel.

“You need some kind of pressure on the intake side to push out the gases that have already burned,” says Larmi.

In a traditional two-stroke engine, the force of the fuel and air intake drives out the exhaust. Unfortunately, this process causes some unburned fuel to be lost as exhaust, resulting in higher emissions. Four-stroke engines force the spent fumes out of the cylinder through a cam-controlled valve using an upward stroke of the piston. During the following downstroke, fresh air and fuel are injected into the cylinder while the exhaust valve is closed.

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.