Vapor generators for powering external combustion engine systems. Their combustor generates heat in full response to engine system power demands. Emissions have low noxious pollutant content over the wide power demands of a vehicle on the road. Internal arrangements maximize combustion of the fuel that is atomized in the primary combustion zone. Hot spots above 2,100* F in the combustor are minimized to inhibit the formation of nitrous oxide. Further, quenching spots below 1,500* F are controlled to minimize the formation of carbon monoxide and unburned hydrocarbon particles. An important feature in the combustor section is that the direction of flow of input air is opposite to that of the flow of the hot combusted gases. This moderates the rate of gas flow, and turbulates to intermix the gases towards their complete combustion. A wide range of available fuels may be used, and operation over at least a 20:1 range of air to fuel input is effected. The combustor-vapor generator hereof meets the Emission Standards for 1976 set by the Environmental Protection Agency. It is suitable for mass production at reasonable unit cost with relatively inexpensive materials.