Homogeneous charge compression ignition with nondilute stoichiometric methane–air at extreme compression ratios

Abstract

This article reports continuing research into operating a free-piston internal combustion engine at extremely high compression ratios. In particular, the use of a homogeneous charge compression ignition combustion strategy with a premixed, stoichiometric methane–air mixture is investigated. A stoichiometric mixture was chosen to correspond with potential use of a three-way catalyst for meeting emissions regulations. To achieve correct autoignition phasing at very high compression ratios, methods for cooling rather than heating the intake charge are discussed, including intercooling and water injection. Results of a set of experiments designed to represent an intercooling approach are reported for effective compression ratios up to 80:1. Ignition phasing slightly after the minimum volume point was achieved for all cases. Indicated efficiency approached 53%. CO, NOx, and hydrocarbon emissions are also reported for equivalence ratios in the range of 0.96–1.04. The measured emissions profiles were suitable for use with a three-way catalyst. Results are reported from a second set of experiments in which the injection of water during compression is used to evaporatively cool the gas. The desired ignition phasing was achieved for compression ratios up to 60:1; however, the amount of water required was far greater than predicted. A dramatic reduction in pressure ringing resulting from combustion was noted with the water injection as compared to the experiments without water injection.