Internal combustion engines tolerant to tar-containing producer gas are disclosed. Two concepts are described. The engines are tolerant to producer gas from a biomass gasifier with minimal pretreatment. When biomass is gasified to be burned for power generation or to be used to synthesize chemicals such as biofuels, a large fraction of the installation cost is spent on equipment to clean up the heavy organic components (also referred to as ‘tars’) from the gas stream, hereafter referred to as ‘producer gas’. The invention described herein may be used to enable power generation from gasified biomass with minimal treatment. It may also be used to treat biomass at a very low cost for other uses such as synthesizing chemicals. The producer gas is not necessarily limited to biomass derived. Producer gas derived from coal or other sources has similar issues and the invention described herein would be equally applicable.