An investigation into the cold start performance of automotive diesel engines
The cold start performance of automotive diesel engines is currently poor when compared to similar gasoline units. This thesis describes an experimental and theoretical investigation into the factors limiting diesel cold start behaviour. Studies have been carried out on IDI and DI designs of engine. Start behaviour has been characterised in terms of times taken to complete various stages of startup, engine speed variations and processes which affect these. Combustion and friction behaviour have been investigated in detail. Engine friction losses are dependent on temperature. During start-up these losses are relatively high compared to those when the engine is fully-warm. The work output from combustion is low at low speeds, and prone to a further deterioration at lower temperatures. Consequently, combustion output during cold cranking is initially insufficient to overcome frictional losses. The start times are extended by the need to keep the starter motor engaged until heat generated in the engine causes frictional losses to fall. Eventually, when combustion output is able to overcome friction without the aid of starter motor work, idle speed is reached. Changes to fuel injection and glowplug parameters have been used to achieve a limited improvement in low-temperature starting. Measurements of engine friction have been carried out to determine the influence of temperature and speed, and the relative contributions from each of the main component assemblies. At low temperatures, much of the friction originates in hydrodynamically lubricated components such as journal bearings, due to high oil viscosity at low temperature. Additionally, engine friction as rotation begins has been shown to be far higher than measured by conventional "steady-state" motoring tests (over twice the quasi-steady state friction at -200 e). This initially high friction transient decays towards the quasi-steady values throughout the start. For crankshaft bearings, a friction model has been developed for cold start-up through to fully warm engine conditions. The friction behaviour in the bearings is dependent on thermal conditions around the friction surfaces. Models for the starter system and blowby processes are also presented as part of a broader theoretical investigation to assess the impact of design changes on start quality.