Model Gas Testing Demonstrates Effects on Emissions and Thermal Efficiency: Woody biomass can play a major role in reducing reliance on fossil fuels and contribute to a reduction in carbon dioxide. Low calorific gas produced through gasification of woody biomass can be burned in an engine for such applications as electric power generation and cogeneration systems. However, since the quantity of woody biomass and temperature vary gas composition and the production quantity of gas, a spark-ignited engine fueled with low calorific gas easily changes its performance which is not favorable to electric power generation and cogeneration systems that require stable operation to sustain electric power at constant. Thus, the feasibility of operating an engine on diesel fuel, low calorific fuel, or both to produce constant power was investigated at Yanmar Corporation. Diesel fuel injection timing and gas heat input with respect to the total heat input to the engine were varied to characterize exhaust emissions and thermal efficiency. This chapter reports engine database information about a diesel engine operating on two model gases simulating low calorific gas.