Abstract

In this paper, a thermodynamic model of a spark ignition internal combustion engine fueled with natural gas is developed in order to estimate the air-fuel-unburned gas temperature at before top dead center (BTDC). This temperature is used as controlled variable in a control loop in order to avoid the autoignition phenomena when the engine operates with a fuel with different methane number from the methane number requirement of the engine. The model formulation is based on a polytropic compression proccess whose coefficient was determined experimentally in a turbocharged internal combustion engine fueled with natural gas. To make feasible the use of differents gaseous fuels from natural gas, it was necessary to design two control strategies to avoid the knocking phenomenon and choose the best one. The ambient temperature is the disturbance considered, whose changes are significant in different places in the world. The first control strategy that was implemented is called “Robust”, which employs a conventional feedback control loop with a robust controller which is designed. The response of this control loop is compared to the response of the second control strategy called “Feedforward control”. The results obtained reveals that Feedforward control strategy has better performance than robust control strategy for this application. The control strategy and the model proposed will allow increase the range of application of gaseous fuels with low methane number (MN) leading to guarantee a safe running in internal combustion engines that currently are fueled with natural gas.