Activities for high-efficiency small gas engines

Abstract

Interest in the potential of natural gas is growing, owing to its low CO2 emissions per unit of heat produced, and to the development of techniques for the exploitation of shale gas. Research and development relating to natural gas is also being pursued in view of its potential to increase Japan’s energy security, because deposits are found around the world.Osaka Gas is pursuing the medium-to-long term development of gas engines. Increasing compression ratio, improving combustion under lean combustion conditions, and increasing specific output will be key to improving thermal efficiency. This paper summarizes the results of tests Osaka Gas to date with a view to improving the efficiency of small gas engines.First, tests with downsizing through forced induction were conducted. A naturally aspirated gas engine with a displacement of 3318 cm3 was fitted with a high-efficiency turbocharger, and performance tests were conducted at brake mean effective pressure (BMEP). It was found that thermal efficiency under lean conditions reached 40%, but that NOx emissions exceeded 1500ppm. When exhaust gas recirculation (EGR) was applied with a view to reducing NOx emissions, at the EGR limit (EGR ratio 18%), thermal efficiency was around 39% (NOx 500ppm). With stoichiometric combustion, at the EGR limit, thermal efficiency reached a maximum of 39%.Next, the effectiveness of multi-point ignition, pre-chamber spark plugs and homogeneous charge compression ignition (HCCI) in improving lean/diluted combustion were studied, and each was found to improve stability, extend the lean limit and increase thermal efficiency under lean combustion conditions.Studies were also conducted in relation to the dedicated EGR system proposed by Southwest Research Institute (SwRI) as a means of improving EGR. It was determined that around 9% H2, which has a combustion-promoting effect, was produced under conditions where the equivalence ratio was 1.5.