At low outside temperatures, the combustible mixture burns better (emission of harmful substances is lower), if the air required for combustion is heated.
The preheated air prevents the central injection from icing up, which can occur at temperatures from +3°C to +8°C and high air humidity. When the engine is running, some of the fuel evaporates; The resulting evaporative cold, with the appropriate air humidity, can lead to the formation of a layer of ice and prevent the preparation of a perfect air-fuel mixture.
At higher outside temperatures, cooler intake air is better. Air expands when heated. Because the engine can only take in a certain amount of air, it gets less of it when it's hot. This leads to an enrichment of the air-fuel mixture and increased fuel consumption.
Automatic control of intake air preheating
The preheating of the air is carried out depending on the load and temperature: cold air enters the air filter housing from the front through the intake pipe. Through the second channel, heated air from the exhaust manifold can flow. In the air filter housing there is a regulating damper, which regulates the flow of heated air. The so-called thermal valve determines from which channel air is sucked in. When the throttle is closed or only slightly open, there is a high underpressure in the intake manifold under it. This reduced pressure acts on the diaphragm of the regulating damper, as a result of which it opens the inflow of warm air and at the same time reduces the inflow of cold air. When gas is supplied, the underpressure in the intake manifold is reduced, and now the air damper closes the warm air inlet.
As soon as the intake air reaches a certain temperature, the thermal valve in the vacuum pipe closes the passage, the damper closes the warm air supply, now only cold air can be sucked in.
Left: Thermal valve (1) and damper (2), regulating the air flow, are structural elements of the preheating system.
On right: The intake manifold in a 1.6 liter 8-valve engine is heated by coolant. The arrow shows the coolant hose to the intake manifold.
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