Unburned hydrocarbons from SI engine exhaust are a primary component of smog. At present they are attributed to several sources, one of which is the gap between piston and cylinder wall. Unburned charge is pushed into this crevice during the compression stroke. The crevice is narrow enough to quench the flame front, leaving unburnt crevice gases, so that during the power stroke, as the piston descends and the exhaust valve opens, these unburnt gases re-emerge.We have used reactive and non-reactive CFD simulations to study piston crevice outflow of unburnt fuel for a propane-fueled engine. We investigate the time and spatial dependence of the crevice outflows and the degree to which they either ignite or remain unburnt when they emerge into a time-varying, engine-like environment. We calculate rate of fuel consumption and depict fluid flows and plume shapes of the emerging gases.