杏吧原创

WHEN the new Mitsubishi Galant 1.8 saloon hits the streets in Japan this summer and in Europe in 1997, its biggest selling point will lie beneath the bonnet. The car will have a revolutionary new petrol engine that its Japanese manufacturer claims is more fuel-efficient than any of its competitors. Compared with the Galant now on the roads, says the company, the new model will be 25 per cent more fuel-efficient. On a typical city journey it will travel 20 kilometres on a single litre of fuel, or over 55 miles on a gallon. And although the new engine is the same size as the old one, it will offer drivers 10 per cent more power, regardless of the car鈥檚 speed.

The trick, says Mitsubishi, is to inject the fuel directly into the cylinders rather than into the air flowing into them. The company has been so impressed with the performance of the new design that it now plans to introduce direct injection into all its other petrol engines. Mitsubishi has already tested a version of the engine in its futuristic concept car, the HSR-V.

But direct injection engines have an Achilles heel. Their exhaust contains high levels of nitrogen oxides (鈥淣O_x鈥� emissions). These are poisonous gases that are involved in the production of ozone at low altitudes, which in turn has been implicated in the sharp rise in the incidence of asthma in recent years. In Europe and California, new limits on NO_x emissions from new cars will come into force by the end of the decade, and even Mitsubishi admits that its current engine will not meet them. Just how it intends to make the engine acceptable is not yet clear.

In other respects, the new engine relies on the same principles as a traditional four-stroke petrol engine, burning a mixture of fuel and air inside a cylinder containing a piston. Most car engines have four, six or eight cylinders that fire in sequence. Combustion creates hot exhaust gases that rapidly expand, forcing the piston down from the top of the cylinder to the bottom in one stroke. As the engine鈥檚 other cylinders fire in sequence, they drive this piston through three further strokes. First, back to the top of the cylinder, forcing the exhaust gases out of the chamber. Then down again, sucking a mixture of petrol and air into the cylinder. And finally back to the top, compressing this mixture. The cycle starts again when the cylinder鈥檚 spark plug fires, igniting the mixture.

One crucial factor is the way that the mixture burns. In theory, petrol burns most efficiently when mixed as a fine spray with roughly fifteen times its weight of air. In many modern engines, this ratio is carefully maintained by injecting the fuel into the airflow before it enters the cylinder. But the burning process is far from perfect. For example, any fuel that condenses on the cylinder walls is difficult to ignite. And during the burn, the increase in pressure forces roughly 10 per cent of the fuel into the spark plug threads or the crevices between the piston and the cylinder wall, where it cannot ignite at all. Although this fuel eventually burns as the pressure inside the cylinder drops, it contributes little power to the piston鈥檚 downward stroke. In addition, the extra fuel required when the engine accelerates tends to form puddles around the entrance to the combustion chamber. This cannot burn easily either. The fuel injection system must compensate for this lost fuel by adding more petrol to the airflow.

Rush of air

Direct injection overcomes these problems by ensuring that the fuel never gets near the walls of the cylinder. The secret is to control the air flow inside the cylinder, says Akira Kijima, the head of engine design at Mitsubishi. The air intake ports in most engines are at the sides of the cylinder. But Mitsubishi put its air intake ports on the top so that the air rushes down into the chamber 鈥� a design perfected by the British company, Ricardo Consulting Engineers, and used in some racing cars. In addition, the piston heads are cupped in a way that channels the air up and around the walls of the chamber. When the fuel is injected, this airflow ensures that the petrol stays in a small pocket near the spark plug (see Figure). 鈥淥ur in-cylinder air dynamics are quite unique,鈥� says Kijima.

The amount of fuel injected is also carefully controlled so that the ratio of air to petrol is roughly 15:1 inside this pocket. Outside the pocket there is virtually no petrol, so that none ends up on the walls or forced into crevices during the burn. Because of this 鈥渟tratification鈥� 鈥� the uneven distribution of fuel 鈥� the ratio of air to petrol for the cylinder as a whole is roughly 40:1, which is why direct injection engines are so fuel-efficient in most driving conditions.

One reason why direct injection has taken so long to develop is that fuel injectors designed to spray a small volume of atomised fuel into the cylinder cannot easily handle the larger amounts needed at high speed and when the car is accelerating.鈥� This was the major area of innovation,鈥� says Kijima.

Mitsubishi鈥檚 direct injection system produces a stream of atomised petrol that fans out into the cylinder in a cone-shape. The size of this cone, and therefore the amount of petrol it contains, depends on the pressure inside the cylinder. During normal driving, when the car is not accelerating and is travelling at under 120 kilometres per hour, fuel is injected as the piston reaches the top of its compression stroke, when the pressure inside the cylinder is high. This creates a small cone of atomised petrol. But when the engine needs more fuel, injection occurs earlier, during the air intake stroke when the pressure is low 鈥� leading to a large cone containing more petrol. Under these conditions, the air-fuel ratio can be as high as 13:1, making the engine no more efficient than conventional ones.

Before its time

The on-board computer which controls when injection occurs is another crucial factor in the engine鈥檚 success. Lack of electronic controls had stifled early attempts to make this type of engine work. The American oil giant Texaco was one of the first to experiment with direct injection engines in the 1940s. The objective was to build military vehicles that could run on any fuel, says Ray Paggi, an engineer in the company鈥檚 fuel research group in Beacon, New York. The company thought that injecting the fuel directly into the cylinder might be the way to make this work. At one time, Texaco engineers were road-testing its prototype engines in some sixty delivery vans. They tried burning a huge range of fuels, including jet fuel and soyabean oil. The problem was that some combusted spontaneously when they were mixed with air and put under enough pressure, whereas others required a spark. Work on direct injection only ended in the early 1980s when Texaco finally decided that there was no way to build an engine that could run on any fuel. 鈥淚t was a thought before its time,鈥� says Paggi.

A completely different set of advantages of direct injection engines has now emerged. Their extra power over conventional ones of the same size is a good example. Much of any engine鈥檚 power depends on the extent to which its pistons compress the air-fuel mix before ignition. In theory, a mixture at high pressure can release more energy when it burns. But if the combustion chamber is already hot, attempting to achieve too high a pressure can trigger combustion prematurely causing the pistons to work against each other. This phenomenon is known as 鈥渒nocking鈥� and drastically reduces the engine鈥檚 power.

In a direct injection engine, however, the small droplets of petrol begin to evaporate as they are injected into the cylinder. This absorbs heat and keeps the mixture of air and fuel cooler than in a conventional engine. As a result, it can be safely compressed to a higher pressure to produce more power, without causing knocking.

But without further advances, direct injection engines will not last long on the roads of Europe and the US. Ironically for an engine which offers such good fuel economy, its high NO_x exhaust emissions pose a serious threat to its viability. 鈥淏y 2000 we will need a low-NO_x catalyst,鈥� says Kijima. Research is under way around the world, since similar catalysts will be needed for diesel engines.

For the moment, the engine meets European and American emission criteria because it produces low levels of other pollutants such as carbon monoxide and hydrocarbons. But even these emissions can only be kept within reasonable limits if the engine runs perfectly.

In practice, achieving this will not be easy, argues David Cole, director of the Transportation Research Institute at the University of Michigan, in Ann Arbor, and a professor of engine design for more than twenty years. For example, hydrocarbon emissions soar if any fuel escapes from the small pocket around the spark plug. 鈥淭he trick is to manage the interface between that area and the surrounding air,鈥� he says.

Even if Mitsubishi can do this, it must still find a way to reduce NO_x emissions. The company is banking on somebody coming up with a catalyst that reduces NO_x levels in car exhaust. 鈥淚t鈥檚 a tremendous challenge,鈥� says Cole. 鈥淭he Mitsubishi people have done some good work, but it鈥檚 a tough, tough problem.鈥� Despite five years鈥� research, a working low-NO_x catalyst seems as far away as ever 鈥� and without one, Mitsubishi鈥檚 revolutionary engine could be short-lived.