Technical Report 187, c4e-Preprint Series, Cambridge
An adsorption-precipitation model for the formation of injector external deposits in internal combustion engines
ref: Technical Report 187, c4e-Preprint Series, Cambridge
- Injector leakage causes accumulation of fuel near the nozzle post-injection.
- The leaked fuel degrades under the action of NOx and O2 in the quench layer.
- Radical chain oxidation mechanism involving branching reaction with NO is proposed.
- The pressure drop during the power stroke causes the leaked droplet to boil.
- The boiling triggers receding of the droplet toward the nozzle and precipitation.
The occurrence of deposits on fuel injectors used in gasoline direct injection engines can lead to fuel preparation and combustion events which lie outside of intended engine design envelope. The fundamental mechanism for deposit formation is not well understood. The present work describes the development of a computational model and its application to a direct injection gasoline engine to describe the formation of injector deposits and their effect on engine operation. The formation of fuel-derived deposits at the injector tip and inside the nozzle channel is investigated. After the end of an injection event, a fuel drop may leak out of the nozzle and wet the injector tip. The model postulates that the combination of high temperature and NOx produced by the combustion leads to the initiation of a reaction between the leaked fuel and the oxygen dissolved in it. Subsequently, the oxidation products attach at the injector wall as a polar proto-deposit phase. The rate of deposit formation is predicted for two limiting mechanisms, adsorption and precipitation. The effects of the thermal conditions within the engine and of the fuel composition are investigated. Branched alkanes show worse deposit formation tendency than n-alkanes. The model was also used to predict the impact of injector nozzle deposit thickness on the rate of fuel delivery and on the surface temperature of the injector wall.
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