Start and Ignition Assist Systems
Early vehicles with diesel engines could only be started at low temperatures with intense smoke formation accompanied by loud knocking noises. Glow and engine starting times of several minutes were common. Therefore, startability was and is an important g
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Start and Ignition Assist Systems Wolfgang Dressler and Stephan Ernst
Early vehicles with diesel engines could only be started at low temperatures with intense smoke formation accompanied by loud knocking noises. Glow and engine starting times of several minutes were common. Therefore, startability was and is an important goal of engine manufacturers, and suppliers’ development activities. The refinement of suitable start assist systems and their adaptation to engine characteristics has led to crucial advances in diesel engine starting and cold running performance. Sheathed-element glow plugs (GLP) are used for cars (typically with displacements of less than 1 l/cylinder) to assist engine starting and running at low outside temperatures. Intake air glow systems such as glow plugs, intake air heaters or flame starting systems are used for commercial vehicles with larger displacements. Ongoing development of car diesel engines to increase driving satisfaction by enhancing specific power output and improve environmental compatibility by further reducing exhaust emissions (soot/NOX) is leading to engine concepts with lowered compression ratios. Present day direct injection (DI) car engines have compression ratios of 16–18:1. The currently lowest compression ratio (e) in a production car diesel engine is 15.8:1 [12-1]. Early (prechamber) diesel engines typically had compression ratios of approximately 21:1. Without additional measures, lowering compression ratios from 18:1 to 16:1 impairs cold starting and cold idle performance. This, in conjunction with the desire for cold start and cold idle performance similar to gasoline engines, increases demands on future glow systems (glow control unit, GCU and GLP) for cars: – fastest heating rates (1,0008C in less than 2 s) for cold starts down to –288C similar to gasoline engines, – flexible adaptation of the glow temperature to engine demands, – maximum glow temperatures of up to 1,3008C and continuous glow temperatures of up to 1,1508C to reduce exhaust emissions for low compression engines (17:1 and lower),
S. Ernst (*) Robert Bosch GmbH, Diesel Systems, Stuttgart, Germany e-mail: [email protected]
– extended afterglow capability in the minute range for low emission cold idle with increased running smoothness even in the warm-up phase of low compression engines, – intermediate glow capability, e.g. to facilitate particulate filter regeneration, – high glow system service life of up to 200,000 km, – constant glow characteristics (temperature and heating rate) throughout the service life of sheathed-element glow plugs, – OBDII and EOBD capability and – support of advanced communication interfaces, e.g. CAN or LIN. Typical compression ratios for commercial vehicle engines are presently between 17:1 and 20.5:1. Commercial vehicle engines with two-stage supercharging will be introduced in the future. This will increase peak pressure without additional measures in the cylinder. Lowering the compression ratio to 16–16.5:1 will allow maintaining peak pressure at the pre
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