An improved plasma reactor has been designed, built and evaluated. It is characterized by a reduced power per area ratio, relative to previous designs, and includes several improvements to run the whole system safely in a car. The new reactor design includes a concentric inner high voltage electrode, a grounded outer electrode, a shielded high-voltage and high temperature resistant electrical connection. A generator controller has been developed for better control of operating conditions as required during the engine cold start phase.The new generator/reactor system was installed in the exhaust pipe of a gasoline direct injection engine. HC emissions could be reduced up to 30 % in the first 40 seconds of a cold start test.In addition to HC treatment the dielectric barrier discharge has also been investigated as a method for regenerating a diesel particulate trap.