removed with certainty by evaporation or combustion. On the other hand, the compression of the mixture being generally higher, premature ignition is very likely to occur.
Back Pressure to the Exhaust.—How the pipes and chests for the exhaust should be arranged in order not to exert a harmful influence on the motor has already been explained. Even if the directions given have been followed, however, the exhaust may not operate properly from accidental causes. Among these causes may be mentioned obstructions in the form of foreign bodies, such as particles of rust, which drop from the interior of the pipes after the engine has been running for some time and which, accumulating at any place in the pipe, are likely to clog the passage. Furthermore, the products of combustion may contain atomized cylinder oil which finds its way into the exhaust-pipe. This oil condenses on the walls of the elbows and bends of the pipe in a deposit which, as it carbonizes, is converted into a hard cake and which reduces the cross-section of the passage, thereby constituting a true obstacle to the free exhaust of the gases.
These various defects are manifested in a loss in engine power as well as in an abnormal elevation of the temperature of the parts surrounding the exhaust opening.
Sudden Stops.—Sudden stops are occasioned by faulty operation of the engine, and by imperfect fuel supply. Among the first class the chief causes to be mentioned are the following:
1. Overheating, which has already been discussed and which may block a moving part.
2. Defective ignition.
3. Binding of the admission-valve or of the exhaust-valve, preventing respectively suction or compression.
4. The breaking or derangement of a member of the distributing mechanism.
5. A weakening of the exhaust-valve spring, so that the valve is opened by the suction of fresh quantities of mixture.