A plasma combustion system was developed to improve fuel economy and efficiency without modifying the engine configuration. Non-thermal plasma generation technology with microwave was applied. Plasma was generated by spark discharge and expanded using microwaves that accelerated the plasma electrons, generating non-thermal plasma. Even at high pressures, spark discharge occurred, allowing plasma generation under high pressures. The durability and practicality of previous plasma combustion systems was improved. The system consisted of a spark plug without a resistor, a mixer circuit, and a control system. The mixer unit used a standard spark plug for plasma combustion and functioned as a high-voltage and high-frequency isolator. A commercially available magnetron produced microwaves of 2.45 GHz. The spark and microwave control system used a trigger signal set to the given crank angle, from the engine control unit. Spark and microwave control signals were sent to a pulsed power supply according to the trigger delay and duration. The spark and microwave signals were determined individually and used to control the plasma characteristics. The system was tested in a practical multi-cylinder engine to investigate the effects of plasma on the lean limit and fuel efficiency. Under low loads and lean mixtures, plasma ignition improved the initial combustion period and fuel consumption and reduced the indicated mean effective pressure (IMEP) cyclic variation. Moreover, microwave plasma improved the brake-specific fuel consumption (BSFC) under high-emission gas recirculation conditions due to its stable ignition and initial flame propagation.