The feasibility of argon-circulated hydrogen engines for use in vehicles was recently investigated. The substitution of the noble gas argon (Ar) as the working gas in a hydrogen engine led to the realization of a zero-emission, high-efficiency engine that allows low-ignitability hydrogen to ignite instantaneously after injection. The objective of this study was to investigate the combustion processes of hydrogen jets under argon-circulated hydrogen-engine conditions. Experiments were conducted in a constant-volume combustion vessel under varying conditions with respect to atmosphere, oxygen concentration, injection pressure, ambient temperature, and nozzle-hole diameter. Furthermore, the ignition characteristics and the combustion processes of the hydrogen jets were observed using high-speed shadowgraph images. Under short ignition delay conditions, the experimental results for the Ar-O-2 atmosphere indicated a lower heat-release rate, which continued to the end of the injection and resulted in a longer combustion period, whereas the heat-release rate under the air (N-2-O-2) atmosphere terminated simultaneously with the end of the injection. The ignition delay for the hydrogen jet under an Ar-O-2 atmosphere increased with decreasing ambient temperature, similar to the behaviour observed under the air atmosphere. Other fundamental characteristics that may help control the operation of argon-circulated hydrogen engines were obtained and discussed. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.