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BMW Valvetronic (1)


François Dovat

For the first time, a spark ignition engine (petrol engine) does without an intake manifold throttle.

Diesels engines have none: they suck a quantity of air independent of the load, i.e. their power is regulated only by the volume of fuel injected. It is one of the reasons of their better efficiency. A partially closed intake throttle blade increases the pumping losses by strangling the engine to control its power. Thus a spark ignited engine has its best efficiency (inversely proportional to specific consumption – sfc or bsfc) at almost wide open throttle (WOT), in other words close to full load, and this efficiency drops more with the reduction in load than in the case of a diesel. Many OEMs try to solve this problem by means of charge stratification, a technology which consists to use extremely lean air/fuel mixture in the peripheral zones of the combustion chamber when the engine is under low load, so allowing the throttle to remain wide open. The direct injection of gasoline in the combustion chamber at the end of compression stroke makes it possible to concentrate a mixture in flammable proportions close to the spark plug. But it is a difficult way because the production of nitrogen oxides is increased due to the combustion in excess air, and also in consequence of an increased thermal load – in particular on the top ring and land. Moreover, the engines using the stratified charge process require sulfur free gasoline, or else their advantage in fuel economy becomes illusory.

Too many copiers are running on this part, ± 30% of the text is shown, the full text is available on the HIstomobile's CD ROM.

(© François Dovat)
#118

 /   BMW Valvetronic (2) 

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