In an engine system wherein the engine coolant is permitted to boil and the gaseous coolant used as a vehicle for removing heat from the engine, a heating jacket associated with the induction conduit of the engine is supplied with gaseous coolant via a control valve during cold engine starts and during modes of engine operation wherein it is advantageous from the view point of fuel economy to raise the temperature of the engine and/or the incoming fuel charge. The supply is terminated under other modes of operation to avoid heating the incoming charge and reducing charging efficiency.

In an internal combustion engine air intake control system in which a valve is actuated by a vacuum motor in response to engine intake vacuum so as to control an admixture of heated air with ambient air in an engine air intake passage, and in which the action of said intake vacuum on said motor is modified by a thermo-responsive air bleed valve which bleeds air into said motor at a rate which increases with intake air temperature, a thermo-responsive non-return valve is disposed in the bleed valve housing and is constructed and arranged so that, when the engine is cold the non-return valve is closed but permits fluid flow from said vacuum motor to said vacuum source but not in the opposite direction, but, when the engine has warmed up, opens and permits free communication in either direction between said vacuum motor and said vacuum source, this arrangement ensuring that sudden acceleration of the engine before it has warmed up will not result in a sudden surge of cold air being delivered to the engine.

In an internal combustion engine air cleaner, a thermal sensor delivers a vacuum signal to a vacuum motor which actuates a valve unit controlling admission of ambient temperature and heated air. The sensor receives induction passage vacuum and has a bimetal strip which positions an acetal resin valve ball in an air bleed to create a vacuum signal which varies inversely with changes in the induction air temperature, the bimetal strip and valve ball being enclosed within a protective metal housing which permits prompt response to changes in induction air temperature. This construction permits the induction air flow to be maintained at a predetermined temperature.

An improved intake air heating device of internal combustion engine for motorcycles, having a compact and durable construction and adapted to heat the intake air to be fed to the engine by the heat derived from the exhaust gas. The device has an intake system leading to an intake port of the engine, an air cleaner connected to the intake system, an exhaust system connected to an exhaust port of the engine, an air heater disposed to surround a part of the exhaust system, and a heated air conduit providing a communication between the air cleaner and the air heater. The heated air conduit is arranged within the transverse breadth of the engine.