Warmup of the Du Pont model V reactor during unchoked engine operation with air injection has been characterized by a nonreactive period, followed by a transition to an ignited condition. The early period is quenched by heat loss. The transition is gradual for hydrocarbons, but more abrupt for carbon monoxide.Model building for the warmup period is directed to the objective of developing a rapid computer simulation to predict light-off times and temperature histories for various reactor designs and operating conditions. Reactor gas temperature and chemical conversions are calculated as solutions for an ideal backmix reactor. Heat balances maintain a record of all reactor metal temperatures for the given configuration. Heat transfer by radiation, convection, and conduction is considered.The presence of a hot spot in the reactor has a strong effect on time to light-off. In addition to lowering the time, such an ignition source shows a great sensitivity to combustible concentration. The relative amounts of CO, H2, and HCs are also important, since there is a sequential transition to high conversion. Initial reactor temperature and exhaust temperature are also important variables.Variables of reactor design, such as insulation thickness, materials of construction, internal configuration, and reactor size, also affect warmup. A brief exploration of these parameters has been made.