Recently, for the protection of the environment, the regulation of automobile fuel consumption and exhaust gas emission has been strengthened. To improve fuel economy, it is demanded that each engine part contributes to reducing the workload of the engine, even the engine lubrication oil pump. In response to this, a new variable discharge oil pump was developed. It is the world's first internal gear type oil pump that has electronically controlled continuously variable discharge. The work performed by the pump chiefly takes two forms: sliding friction of the rotor and pumping work which moves the oil. First, in developing a variable discharge oil pump, a new tooth profile of the rotor was developed to reduce its sliding friction. As a result, the sliding friction of the rotor was reduced by 34% while maintaining the same theoretical oil discharge rate. Next, a variable discharge mechanism using an internal gear was developed. Since it is an internal gear pump, the flow rate can be controlled by changing the eccentric direction of the rotor relative to the ports of the housing while maintaining the eccentricity distance. In order to realize this, guide grooves in the ring holding the housing were given a unique design. To reduce pumping work, the variable discharge mechanism was designed to limit hydraulic pressure to 100 kPa or less. In addition, design efforts focused on quick response to signals for increased pumping. Specifically, to achieve low hydraulic pressure in the engine, the oil pump was made highly responsive, supplying high hydraulic pressure quickly when the engine needs it. In this way, the pumping work of the continuously variable discharge oil pump could be reduced to 63% less than that of a conventional pump, which improved fuel economy by 1.2% in actual testing.