Tecogen Division, Thermo Power Corporation, has vast experience using automotive derivative natural gas and propane engines in both stationary and vehicular applications. These lower cost engines have been modified internally to take advantage of these higher octane fuels and to demonstrate equivalent durability and reliability when compared to industrial grade engines which cost significantly more. This paper will show some of Tecogen's experience in using these engines, including their use in hybrid vehicles as a range extender. We will also discuss how the waste heat off the engines can be used efficiently for heating and air conditioning of hybrid vehicles. Emphasis will be on the engine as part of an effective hybrid vehicle system.