（）are used to digest the sewage to produce innocuous sludge in a biological sewage treatment plant.

By comparing the exhaust gas temperature of each cylinder, the operator can determine if the load is balanced throughout the engine. The device most commonly used is a（）

Diesel engine air start system check valves are opened by （）

The flash point of a residual fuel oil should be used to determine the highest temperature to which oil may be heated（）

One of the reasons that cause oil flowed from sludge outlet of a self-cleaning separator is（）