In the process of lubrication, some of the oil goes past the piston rings in the cylinders and enters the combustion chamber. This oil causes carbon deposits. As carbon accumulates it makes itself known by loss of power and knocking of the engine, especially on a hard pull, as in climbing a hill. The carbon on the inside of the chamber, on top of the piston and sometimes on the under side of the piston, keeps the heat in the combustion space, causing the knocking. Sometimes the carbon itself becomes so hot that it burns or glows, exploding the charge of fuel before the proper time. This produces the pre-ignition knock which involves a great loss of power. Sometimes pieces of car bon get between a valve and the valve seat, holding the valve open. When the exhaust valve is thus held open, the flame of the explosion can pass completely around it, burning or warping the valve.
The best way to remove carbon is to remove the cylinder head from the engine and scrape out the deposit. The valves should be ground while the head is off and the gaps on the spark plugs reset. Before replacing the head every particle of loose carbon or grinding compound should be removed to avoid scratching the cylinder walls, and a new gasket put in between the head and cylinder block. Carbon troubles may be minimized in the following ways:—(i ) by using an oil of low carbon-depositing tendency ; (2) by operating the engine on a lean air-fuel mixture; (3) by running the engine at a sufficiently high oper ating temperature and giving it a little real work once in a while ; (4) by use of an efficient air cleaner ; (5) by use of anti-knock fuels when necessary ; (6) by keeping the engine in such mechanical condition that oil consumption is not excessive.
When engines are operated only for short trips in cold weather, or used in service where much idling is encountered, water from the combustion of the fuel may accumulate in the crankcase. At the
same time excess fuel or unburned lubricating oil may wash carbon deposits down past the piston rings into the lubricating oil. When this happens the oil and water may be emulsified, the carbon particles acting to stabilize the emulsion in much the same way as do egg particles in the making of mayonnaise dressing. The resulting emul sion or winter sludge may cause stoppage of the lubricating system. Troubles from this source may be avoided by increased operating temperatures (through use of cooling water thermostats and water fronts) and frequent changing of the lubricating oil. Low temperature service is destructive to engines through increased wear and will often lead to clogged oil rings and resulting high oil consumption if adequate protection is neglected.
When engines are operated under high temperature conditions such as encountered with high speeds and loads in hot weather, the lub ricating oil may oxidize, forming materials in the oil which may cause trouble if the oil is not changed before their concentration becomes serious. These materials may be acids corrosive to certain engine parts, oxidation sludge which may clog oil lines and screens, or varnish-like materials which will cause sticking of pistons, rings, valves, and other moving parts. In less serious cases and where the lubricating oil has not been changed at proper intervals gradual clogging of oil control rings may be expected. Since this will result in increased oil consumption any savings through prolonging the crankcase change period would be nullified. Consequently it is advisable to follow the recommendations for oil change made by the manufacturer of the lubricating oil.