An exhaust gas treatment system and method for treating the exhaust gas of an internal combustion engine includes a catalytic converter and a catalytic preheater or igniter upstream of the catalytic converter. A bypass passage extends around the igniter such that during the ignition period about two-thirds of the exhaust gas is bypassed around the igniter and one-third passes through the igniter. The igniter raises the temperature of the exhaust gases passing therethrough to an adiabatic temperature of about 900.degree. K. Accordingly, the flow through the igniter combined with the flow through the bypass passage raises the average temperature of the combined exhaust gases to approximately 700.degree. K., which is sufficient to ignite the catalytic converter at substantially the leading edge thereof. Accordingly, normal convective flow of the exhaust gases through the catalytic converter assists in propagating ignition of the converter. A temperature sensor in the catalytic converter causes a controller to operate a valve to divert substantially all of the exhaust gas flow through the bypass passage after the catalytic converter has ignited.