Engine coolant is considered as any solution which is circulated through the engine to provide the means for
heat transfer from the different engine components. In general, water containing various materials in solution is
used for this purpose.
The function of the coolant is basic to the design and to the successful operation of the engine. Therefore,
coolant must be carefully selected and properly maintained.
A suitable coolant solution must meet the following basic requirements:
1. Provide for adequate heat transfer.
2. Provide a corrosion-resistant environment within the cooling system.
3. Prevent formation of scale or sludge deposits in the cooling system.
4. Be compatible with the cooling system hose and seal materials.
5. Provide adequate freeze protection during cold weather operation.
The first four requirements are satisfied by combining a suitable water with reliable inhibitors. When operating
conditions dictate the need for freeze protection, a solution of suitable water and a permanent-type antifreeze
containing adequate inhibitors will provide a satisfactory coolant.
Any water, whether of drinking quality or not, will produce a corrosive environment in the cooling system. Also,
scale deposits may form on the internal surfaces of the cooling system due to the mineral content of the water.
Therefore, water selected as a coolant must be properly treated with inhibitors to control corrosion and scale
To determine if a particular water is suitable for use as a coolant when properly inhibited, the following
characteristics must be considered: the concentration of chlorides, sulfates, total hardness, and dissolved
solids. Chlorides and/or sulfates tend to accelerate corrosion, while hardness (percentage of magnesium and
calcium present) causes deposits of scale. Total dissolved solids may cause scale deposits, sludge deposits,
corrosion, or a combination of these. Chlorides, sulfates, magnesium, and calcium are among but not
necessarily all the materials which makeup dissolved solids. Water, within the limits specified in Tables 1 and
2 of Fig. 1, is satisfactory as an engine coolant when proper inhibitors are added.
A corrosion inhibitor is a water soluble chemical compound which protects the metallic surfaces of the cooling
system against corrosive attack. Some of the more commonly used corrosion inhibitors are chromates,
borates, nitrates, nitrites, and soluble oil.
Depletion of all types of inhibitors occur through normal operation. Therefore, strength levels must be
maintained by the addition of inhibitors at prescribed intervals. Always follow the supplier's recommendations
on inhibitor usage and handling.
Sodium chromate and potassium dichromate are two of the best and most commonly used water system
corrosion inhibitors. However, the restrictive use of these materials, due to ecology