Effects of diluents on knock rating of gaseous fuels

Abstract

Concerns on energy security, emissions, and the recent hike in the price of fossil fuels have prompted the rapidly growing interest in the use of various alternative and renewable fuels, including low heating value fuels such as land-filled gases, biogases, coal bed methane gases, and others. Generally, the low heating value (btu) fuels contain substantial amounts of diluents such as carbon dioxide, nitrogen, water vapour, and other trace gases in the fuel composition. The present contribution describes the results of the investigation of knock in a single-cylinder variable compression ratio (CR) spark-ignition engine fuelled with gaseous fuels, such as natural gas, methane, and hydrogen, in the presence of different amounts of diluents, specifically carbon dioxide and nitrogen, in the fuel mixture in order to represent closely the general composition of land filled and biogases in practice. The knock characteristics of the fuels were quantitatively evaluated in terms of the methane number using various methods. Generally, the addition of either diluent carbon dioxide or nitrogen in the fuel mixtures augmented the knock resistance characteristics extending the engine operational limits. With every 10 per cent increase of carbon dioxide in the fuel mixture, the CR was increased by one point, whereas for a 25 per cent of nitrogen content in the fuel mixture, the CR was augmented by a half point in the operating conditions considered.