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| FSI engines spray fuel at high pressure, increasing economy |
Similar advances are being made in petrol engines, too. Early direct-injection petrol units such as the Mitsubishi-Volvo GDI engine weren't particularly inspiring, and progress has been slowed by the poor availability of low-sulphur fuel, especially in the UK, but the new wave of advanced petrol engines such as the VW-Audi FSI (fuel stratified-injection) units, the Fiat-Alfa JTS and the Mercedes-Benz CGI are more sophisticated. As with direct-injection diesel engines, these shoot fuel directly into each cylinder's combustion chamber at much higher pressures: the higher the pressure, the cleaner and more efficient the combustion. FSI, however, incorporates a mode known as "stratified charge", which gives the fuel-air mixture in the cylinder an extra swirl to create an electric charge (much as a thunderstorm is created) at the point it reaches the spark plug. This allows for a leaner fuel-air mixture, using less fuel, and the excess air forms an insulating layer in the cylinder to guard against heat energy losses. Exhaust gas recirculation systems - now frequently fitted to non-FSI engines as well - divert some of the waste exhaust gases back up for re-combustion, working with catalytic converters which process the remaining surplus harmful nitrous oxides into inert nitrogen. Besides reducing such emissions, FSI technology can reduce fuel consumption by an estimated 15 percent - which was why it was used in Audi's all-conquering R8 Le Mans racers, cutting down the number of refuelling stops needed. The Mercedes-Benz CGI engine - Mercedes actually developed the first direct-injection petrol unit, for its original 300SL - works in a similar way.
There are numerous other promising developments in petrol engine technology; one with a lot of potential is a British-developed innovation, an engine which can switch between two- and four-stroke operating modes. A collaboration between consultancy Ricardo and Brighton and Brunel Universities, the government-funded 2/4SIGHT project promises emissions improvements of up to 30 percent, the potential for significant engine down-sizing and production costs no greater than those of an equivalent diesel unit. The first test engine built will be a 2.0 V6 giving performance similar to a conventional 3.4 V8, and the next a little 1.0-litre unit which could be used in large family cars, with performance equivalent to a 1.8-2.0-litre. This is a good five years off production reality, however.
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