Controlled Hot Surface Ignition in Stationary Petrol and Natural Gas Operation

An operation with a lean air-fuel mixture enables smaller cogeneration gas engines to operate at both high efficiency and low NO_x emissions. Conventionally, the combustion process is induced through spark ignition. However, its small reactive mixture volume sets limits on increasing the air-fuel ratio, as a higher dilution reduces mixture inflammability as well as flame propagation speed. In addition, the spark plug durability is limited due to electrode wear, particularly through spark erosion, causing high maintenance costs. The ignition by means of a hot surface has great potential to extend the frequency of servicing intervals as well as to improve the trade-off between engine efficiency and NO_x emissions. Compared to conventional spark ignition, ignition by means of a hot surface is achieved by accelerated combustion. The latter is produced by an increased initial reactive mixture volume.

A hot surface ignition (HSI) system was developed for stationary lean-burn operation, in due consideration of manufacturing costs, lifetime and electrical characteristics that allow for a reliable control of ignition timing. The temperature of the hot surface, and hence the inflammation timing, can be adjusted through its electrical power supply. In order to analyze the impact of the flow conditions on the inflammation process, several designs of inflammation elements were developed.

In a first step, experimental trials were carried out using a single-cylinder test bed engine, running with a lean homogeneous air-petrol mixture at constant load and speed. Compared to prechamber spark ignition, HSI enhances the lean-burn limit, resulting in an improvement of the efficiency-NO_x trade-off. Due to a notable correlation between mean combustion temperature and combustion phasing, a closed-loop algorithm for controlling combustion phasing was developed. Subsequently, the ignition system was applied to a series natural gas lean-burn cogeneration engine. An increase of the lean-burn limit compared to that of the prechamber spark plug was also achieved.