Most engines have some means to ventilate the internal cavities, or spaces, which are related to the lubricating oil system. Systems may be vented directly to the atmosphere or through the engine intake air system. The latter method is preferred in marine installations where the engine is located in an engine room or other compartment.

Venting of heated, fume-laden air directly to the atmosphere in a machinery space will seriously contaminate the air in the space and may create a fire hazard. On the other hand, if the lubricating oil system is not vented in some manner, combustible gases may accumulate in the crankcase and oil pan. Under certain conditions, these gases may explode.

During normal operating conditions, the mixture of oil vapor and air within an engine crankcase is not readily explosive. However, if a working part, such as a bearing or a piston, becomes overheated as a result of inadequate lubrication or clearances, additional oil will vaporize and an explosive mixture will be created. If the temperature of the overheated part is high enough to cause ignition or if a damaged part strikes another part and causes a spark, an explosion may occur.

In addition to the vapor created when

lubricating oil contacts extremely hot surfaces, vapor may accumulate in the crankcase as a result of blow-by past the pistons. Blow-by occurs when the piston is compressing the air and during the power event.

There is little danger of a crankcase explosion or other troubles caused by vapors within the engine if the engine is kept in condition according to the prescribed maintenance program. The ventilation system of an engine greatly reduces the possibility of troubles that might occur because of an accumulation of vapors in the crankcase. Nevertheless, even when an engine is maintained according to prescribed procedures, casualties may occur, or conditions may be created which will lead to an explosion in the crankcase of the engine. When such casualties or conditions occur, they are generally due to abnormal operating circumstances or to the failure of a part.

You should be familiar with the possible causes of crankcase explosions so that you can learn how to prevent their occurrence. The importance of knowing what may cause a crank-case explosion and knowing the precautionary or preventive steps required are apparent when the aftereffects of an explosion are considered. A crankcase explosion may cause serious injury to personnel and extensive damage to the engine. Engine-room fires of a serious nature may occur after a crankcase explosion. Some of the mechanical defects which may lead to a crankcase explosion are crankshaft-bearing failure, damaged or excessively worn liners or piston rings, and cracked or seized pistons.

You may already be familiar with the preventive measures necessary to avoid some of the troubles which lead to the overheating or dilution of engine lubricating oil. Considerably more study and practical experience may be required on your part, however, before you learn how to prevent the conditions that can lead to the overheating or dilution of lubricating oil.