Gasification of Porous Combustion Chamber Deposits in a Spark Ignition Engine

A computational investigation of the effects of activation energy, porosity, and pore size on the gasification of combustion chamber deposits in spark ignition engines has been performed. The oxygen in the combustion gases reacts with the carbonaceous deposit and causes the deposit to burn away. Experimentally measured deposit parameters such as heating value, surface temperature, surface pressure and porosity were used in the model.

Several models for predicting the gasification of the deposit were investigated. A random pore intersection model developed by Petersen was used to predict the gasification of the deposit. The chemical reactions were modeled with a simple Arrhenius expression. The flow within the deposit was modeled using Darcy's Law. The Kozeny-Carmen equation was used to relate deposit permeability and porosity. The model was incorporated into a finite difference code that predicts the heat transfer and fluid flow through the deposit.

Computed deposit gasification rates were compared to observations in a laboratory engine. The deposits formed in the engine were made up of three distinct layers in which the middle layer of the deposit appeared to gasify at a faster rate than the surface. Uniform activation energy always resulted in the deposit burning in a surface recession. Porosity distribution and specific surface area effects were not strong enough to account for middle layer gasification. A non-uniform activation energy was the only variable in the computational model which resulted in faster gasification rates in the middle layer.