An improved liquid propellant airbag inflator including a tubular sidewall internally defining a combustion chamber and a propellant reservoir separated by a piston. A propellant injection port connecting the combustion chamber to the propellant reservoir is defined by a propellant guide surface of the tubular sidewall receiving the piston. According to one embodiment of the invention, the propellant injection port is provided in the form of a plurality of longitudinal grooves. According to another embodiment of the invention, an inner diameter of the propellant guide surface is larger than an outside diameter of the piston so that a gap, acting as the propellant injection port, is created between the propellant guide surface and the piston. The present invention also provides an improved liquid propellant airbag inflator including a tubular sidewall internally defining a combustion chamber and a propellant reservoir separated by a piston, wherein the sidewall defines a series of gas exhaust ports communicating with the combustion chamber. A cylindrical exhaust sleeve extends from the piston into the combustion chamber and seals the gas exhaust ports until the inflator is initiated and the piston is forced into the propellant reservoir. The present invention additionally provides an improved liquid propellant airbag inflator including a tubular sidewall internally defining a coolant reservoir, a combustion chamber and a propellant reservoir, with a propellant piston separating the propellant reservoir from the combustion chamber, and a coolant piston, independent from the propellant piston, separating the coolant reservoir from the combustion chamber.