The efficiency of natural-gas-fuelled reciprocating engines increased from an average value of 35% in the year 1990 to over 46% for large-bore engines in 2005. Increasing the load (bmep) of the engines helped to reduce the negative effect of friction losses on efficiency and to reduce the relative heat losses from the combustion process. Doubling the cylinder bore decreased the heat escaping from the cylinder process by about a factor two. The analyses show that further increases in bmep and bore diameter will hardly help in achieving a better efficiency. Reduction of the friction losses appears the best path to follow for an even higher efficiency. The authors do not expect that the efficiency of an Otto-cycle gas engine will substantially exceed 49%.

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