<P> Engines equipped with fuel injection do not require carb heat as they are not as prone to icing - the gasoline is injected as a steady stream just upstream of the intake valve, so evaporation occurs as the fuel / air mixture is being drawn into the cylinder, where metal temperatures are higher . The exception is monopoint or TBI injection systems which spray fuel onto the throttle plate . </P> <P> Some multipoint injection engines route engine coolant through the throttle body to prevent ice buildup during prolonged idling . This prevents ice from forming around the throttle plate but does not draw large amounts of hot air into the engine as carburetor heat does . </P> <P> In light aircraft, the carburetor heat is usually manually controlled by the pilot . The diversion of warm air into the intake reduces the available power from the engine for three reasons: thermodynamic efficiency is slightly reduced, since it is a function of the difference in temperature between the incoming and exhaust gases; the quantity of air available for combustion inside the cylinders is reduced due to the lower density of the warm air; and the previously - correct ratio of fuel to air is upset by the lower - density air, so some of the fuel will not be burned and will exit as unburned hydrocarbons . Thus the application of carb heat is manifested as a reduction in engine power . If ice has built up, there will then be a gradual increase in power as the air passage is freed up by the melting ice . The amount of power regained is an indication of the severity of ice buildup . </P> <P> It must be kept in mind that the ingestion of small amounts of water into the engine following melting in the carburetor may cause an initial period of rough running for as much as one or two minutes before the power increase is noted . Again, the pilot will note this as evidence that icing conditions are present . However, more than one pilot, when confronted with a rough running engine has mistakenly turned the carburetor heat back off, thereby exacerbating the situation . </P>

Where would a carburetor air heater be located in a fuel injection system