Backfiring

Under certain conditions of mixture ratio there will be backfiring in the:

• intake manifold,
• exhaust manifold.

Backfiring in the intake manifold or carburetor: Most frequently occurs during starting of an engine under cold-weather conditions. The priming and choking operation varies the mixture from too lean to too rich. A very lean mixture will burn very slowly and the charge may still be burning when the exhaust valve is closing and the intake valve is about to open. The fresh charge in the intake manifold is not so diluted as when induced in the cylinder and mixed with the clearance gases and consequently will burn more rapidly than the charge in the cylinder. If the fresh charge, upon being induced, is ignited by the residual flame of the previous charge, the flame will travel back through the intake manifold, burning the charge therein. 

Backfiring caused by the slow flame propagation of a lean mixture is not confined to starting, but may occur under any condition of engine operation if the mixture becomes lean enough; it can be made to occur by excessive leaning of the mixture with the mixture control. Backfiring in the carburetor during starting conditions only occurs when the mixture is too lean. Rich mixtures burn faster than lean ones, and under starting conditions the extra fuel which must be supplied to form a rich mixture is probably partially evaporated by the heat of combustion and extinguishes the flame before the next charge is induced.

Backfiring in the exhaust system: Backfiring occurs in the exhaust system under two conditions of operation. The most common occurrence is the somewhat irregular backfiring that occurs when the engine is being motored (driven by the propeller) with the throttle closed. Sometimes you can hear this backfire on airplanes on short final to land. Under this condition the idling system is supplying the mixture. The manifold pressure will not vary much with speed so that the quantity and quality of the mixture in the manifold are practically the same as under normal idling speed. Also, the exhaust product in the clearance space remain the same with speed., consequently as the engine speed is increased due to motoring the amount of the charge per stroke becomes smaller and the dilution greater until firing ceases. The succeeding unburned charges are pushed out into the exhaust system, the dilution in the clearance space is decreased and after a few cycles with no firing, a charge will be fired. It will be a lean and slow burning charge and the opening of the exhaust valve and result in an explosion. This type of backfiring can be eliminated by increasing the richness of the idling mixture.

The other condition that results in backfiring in the exhaust system is usually that of a faulty fuel control. Under part throttle operation, a faulty carburetor may cause an enrichment of the mixture which would cause misfiring. Opening the throttle would reduce the richness, and the firing of the charge would be resumed. In the meantime, the unburned mixtures which have collected in the exhaust system have become combustible probably due to the condensing of some of the heavier ends of gasoline, and these are ignited from the flame of a cylinder which fires resulting in a rather violent explosion.