Crankcase dilution is a phenomenon of internal combustion engines in which unburned diesel or gasoline accumulates in the crankcase. Excessively rich fuel mixture or incomplete combustion allows a certain amount of fuel to pass down between the pistons and cylinder walls and dilute the engine oil. It is more common in situations where fuel is injected at a very high pressure, such as in a direct-injected diesel engine. 所以低溫環境是助長燃燒不全的因素之一,如果在低溫又加上短程行駛更加助燃燒不全,另一個則是噴射時機較晚導致未燃汽油被刮入曲軸箱。整個看來機油被稀釋的現象存在於高壓缸內直噴引擎,但每家 製造商克服的手法不同。