Experimental investigation of the hydrogen-rich offgas spark ignition engine under the various compression ratios

Abstract

Hydrogen is gaining substantial attention from both public and industries for its promising characteristics as an alternative fuel for internal combustion engines. High flame speed, strong knock resistance, as well as zero carbon dioxide emission put hydrogen ahead of most other alternative fuels. The fast flame and low knock tendency are favorable traits for high compression ratio, which is a key enabler for high efficiency. In this study, engine experiments were conducted to investigate the combustion and emission characteristics of a high-compression ratio, single-cylinder, spark-ignition engine with hydrogen-rich gas mixture. The compression ratio was varied from 10:1 to 17:1 by modifying the piston bowl geometry. The engine load was adjusted through the air fuel ratio by changing the mass flow rate of the fuel gas while the intake throttle valve maintained at wide open position. The consequent excess air to fuel ratio was varied from 3.5 at the lightest to 1.0 at the highest load operation. Spark timing sweep was performed to determine the optimal timings at various load conditions. The highest compression ratio in the present study, 17.0, yielded the highest indicated thermal efficiency, which was 51% at medium load condition. High and low load operations exhibited lower thermal efficiencies, with the estimated excess air to fuel ratio approaching 1.0 and 3.5, respectively. The optimal spark timings of the high load conditions under high compression ratios were retarded to top-dead center or later to avoid backfire and pre-ignition. Results of efficiency loss analysis show that high-temperature combustion is the major contributor to efficiency reduction at high load conditions, whereas the gas exchange process and elongated burn duration were the largest contributors at low load conditions.