3. The engine is so regulated, particularly in the case of English-built engines, as to effect what is technically called "scavenging" the products of combustion. In order to obtain this result, the mixture-valve is opened before the end of the exhaust stroke of the piston and the closing of the exhaust-valve. Owing to the inertia and the speed acquired by the products of combustion shot into the exhaust-pipe after explosion, a lowering of the pressure is produced in the cylinder toward the end of the stroke, causing the entrance of
air by the open admission-valve and consequently effecting the scavenging of the burnt gases, part of which would otherwise remain in the cylinder. It is evident that if a charge of the mixture has not been normally exploded, either because its constituents have not been mingled in the proper proportion, or because the ignition apparatus has missed fire, this charge at the moment of exhausting will pass out of the cylinder without any acquired speed, and will flow back in part at the end of the exhaust stroke past the prematurely opened admission-valve, thereby lodging in the air suction apparatus. Despite the suction which takes place immediately following the re-entrance of the gas into the cylinder, a certain quantity of the mixture is still confined in the suction-pipe and its branches, where it will catch fire at the end of the exhaust stroke after the opening of the mixture-valve.
In order to avoid these detonations it is necessary simply to see to it that the mixture is regularly ignited. This is accomplished by mixing the gas and air in proper proportions or by correcting the ignition time.
Retarded Explosions.—Retarded explosions considerably reduce the power which an engine should normally yield, and sensibly increase the consumption. They are due to three chief causes: (1), faulty ignition; (2), the poor quality of the mixture; (3), compression losses. The existence of the defect cannot be ascertained with any certainty without the use of an indicator or of some registering device which gives graphic records. Nevertheless, it is possible in some
degree to detect retarded explosions, simply by observing whether there is a diminution in the power or an excessive consumption, despite the perfect operation and good condition of all the engine parts.
In order to remedy the defect it should be ascertained if the compression is good, if the supply of gas is normal, and if the conditions under which the mixture of air and gas is produced have not been changed. Lastly, the ignition apparatus is gradually adjusted to accelerate its operation until a point is reached when, after explosion, shocks are produced which indicate an excessive advance. The ignition apparatus is then adjusted to a point slightly ahead of the corresponding position. Recalling the descriptions already given of the various systems of ignition, the manner of regulating the moment of ignition in each case may be summarized as follows:
1. For the valveless incandescent tube, provided with a burner the position of which can be varied, ignition can be accelerated by bringing the burner nearer to the base. Retardation is effected by moving the burner away from the base.
2. In the case of the incandescent tube of the fixed burner type, the moment of ignition will depend upon the length of the tube. The retardation will be greater as the tube is shorter, and vice versa.
3. If the tube be provided with an ignition-valve, the time of ignition having been regulated by the maker, regulation need not be undertaken except if the valve-stem be worn or the controlling-cam be distorted.
If these defects should be noted, the imperfect parts should be repaired or renewed.