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In order to obtain fundamental data to employ direct injection in gas-fueled engines, an experimental study was carried out using a constant volume vessel. Heat release rates and shadowgraph photos were acquired for natural-gas and hydrogen jets simulating the changes in engine-combustion-control factors. The results show that although a higher temperature is needed for ignition, the temperature dependencies of ignition delay and heat release rate in natural-gas jets are similar to those of diesel sprays. The ignition delay and heat release rate are sensitive to injection and ambient conditions. Hydrogen jets have shorter ignition delays compared with natural gas jets. At sufficiently high ambient temperatures, the heat release pattern shows an entire diffusion combustion. Under such conditions, the ignition delay is not greatly influenced by injection conditions and the heat release rate can be controlled by the injection rate.