THE formation of varnish or lacquer deposits on pistons and other engine parts is closely related to the formation of sludge deposits. The whole problem of sludge and varnish may be divided into three classes:
The principal cause of the low-temperature sludges is water, and varnish is usually not a serious problem at the low temperatures. The principal cause of the intermediate-temperature sludges appears to be the fuel and in some cases, the oil, and varnish may be serious at the intermediate temperatures. The principal cause of the high-temperature sludges is the oil, and in this case also the varnish may be serious.
Under commercial conditions, all three of these classifications tend to overlap; and in all three classifications maintenance, operating conditions, and engine design are also important factors.