Importance of Coolant To Long Engine Life—Part 2
During the last 20-30 years the heat load on the engine cooling system has more than doubled, though the physical size of the radiator which provides the principal means of heat transfer to the atmosphere has only increased about 10 percent.
As stated in this column in the November issue, the power output per cubic inch of displacement of engines has nearly doubled during this period, resulting in nearly double the amount of heat (Btu) that must be transferred to the atmosphere by the cooling system. This heat load, to which is added the heat load from increased engine displacement for higher power output to handle the heavier apparatus, has required a marked increase in cooling system efficiency.
Added heat, which the cooling system must handle, is contributed by: (a) nearly doubling the output capacity of the electric generator (alternator) from 50 amperes to 100-125 amperes; (b) increased fan diameter and blade pitch to move more air; (c) larger engine coolant pump to increase volume and flow velocity of coolant; (d) power steering pump and (e) air compressor for air brakes. Each adds to the power output required from the engine, thus increasing the heat rejected to the cooling system. No small contribution to the cooling system load is the heat load added by the engine lubricating oil cooler, with the most recent addition being the heat load from cooling the automatic transmission hydraulic oil.
Engine designers strive to combine technical advance in foundry practice with design for minimum engine weight consistent with operating requirements. A greater uniformity in foundry production has permitted a gradual reduction in wall thickness of cylinder blocks and cylinder heads. This permits a higher and more uniform rate of heat transfer to the coolant.
During the time fuel is burning in the cylinder, flame temperature is approximately 3,000-3,500°F. The cylinder head, valves, piston and cylinder wall are exposed to this heat, though for only a short time period, but the average temperature reached by these components exposed to the heat of combustion is relatively high. Exhaust valves will reach 1,200°F, piston crown temperature 500°F, and cylinder wall temperature (upper half) will reach 250°F, on the water side. The coolant flow in these areas must be in relatively large volume and at high velocity to swiftly carry away the heat. Any condition adversely affecting the rate of heat transfer to the coolant or to slow or stop coolant circulation, can be critical.
Metals unstable: Most of the metals are not stable in the environment in which they are used. Of the several metals used in the average fire apparatus cooling system, iron, copper, brass (alloy of copper, tin, lead and zinc) and aluminum, all are subject to oxidation in varying degree, with iron having the highest rate. Iron oxidizes slowly in air, but in the presence of moisture the oxidation rate increases—the highest rate when it is immersed in water carrying dissolved oxygen. Heat is an important factor in the rate of corrosion, the rate increasing with temperature rise. The relative acidity of the coolant is also a factor in the corrosion rate.
It is in the areas of highest temperature in the cylinder head and in the cylinder block that the maximum rates of corrosion and scale formation are found.
The problem of corrosion control would be relatively easy to solve if water without impurities was used in the cooling system as only the oxidation of iron, producing a light uniform coating of rust, would be the control problem. However, to be realistic, the water used in cooling systems includes, in varying degree, calcium and magnesium salts in solution. These may be present in the form of bicarbonates, sulfates, chlorides or nitrates. In the presence of heat these salts precipitate to form scale deposits which add to the electro-chemical corrosion problem. Unless a control is introduced into the coolant, hot spots develop to cause burned valves, warped valve seats, cracked or warped cylinder heads, cylinders out of round, piston ring scuffing, overheating of both the coolant and engine lubricating oil with a loss of engine power.
The lubricating oil cooler function is also adversely affected by scale formation. Heat transfer from the oil to the coolant is reduced with a corresponding increase in lubricating oil temperature. Varnish and sludge form in the engine under these conditions.
Continued next month