CARE OF TRACTORS

Wairarapa Times-Age, 24 August 1940, Page 9

CARE OF TRACTORS

CAUSES OF CARBON DEPOSIT BEST LUBRICANT NEEDED The rapid formation of carbon of a hard nature in a tractor engine is due to oil in excessive quantities passing the piston rings and entering the combustion chamber. An over-rich fuel mixture is the cause of a soft, sooty deposit. At the same time, however, should oil be passing the piston rings in fairly large quantities and the temperature of the engine be sufficiently high, the oil will be combined with the dry deposit, and thus form a hard crust

Some engines are very prone to carbon formation, even when the mixture is correct, and the amount of oil reaching the bores of the cylinders is the minimum necessary to provide adequate lubrication. On the other hand, some engines, even when the pistons and rings are noticeably sloppy, show very little tendency to become coated with carbon. As a general rule, the hotter the surfaces on to which the oil finds its way. the harder the carbon deposit will be. In the case of the exhaust valves, however, which work at very high temperatures, most of the deposit is burnt oil.

The manner in which the heat of the combustion chamber affects the formation of carbon, is strikingly illustrated in the case, of engines in which the cooling tends to be irregular. Particularly with side-valve engines, parts of the combustion space may show only a very light deposit, while in other places the layer of carbon may be thick and soft, indicating that the particular portion of the head was far cooler than the rest.

Unequal cooling of the combustion chamber is often confirmed, when an engine shows signs of detonating while the water temperature is still below 90 degrees C. This trouble is often put down to the fact that the critical detonating temperature for the particular engine has been reached. The actual cause, however, is nearly always the formation of local steam pockets in the water space in the cylinder head. These steam pockets cause the adjacent metal to be isolated from the cooling system and thus reach a very high temperature. It is often recommended that, when an engine is being carbonised, the piston crowns and cylinder heads should be carefully polished. This condition greatly reduces the formation of carbon on these parts, and increases the ease with which the deposit is removed when the engine is next dismantled. The tractor owner can do much to reduce the formation of carbon. by choosing the right oil. It is false economy to use cheap oils, since laboratory tests have proved that the deposit of carbon and gummy residues is very much higher in the case of an inferior lubricant, as compared with a first-class specially treated oil. Should excessive carbon deposit be experienced with a good quality oil in use. the oil itself should not necessarily be blamed. It is possible that too much lubricant is reaching the combuustion chambers through the use of an unsuitable grade. Oil will be allowed to be pumped into the cylinders if the cylinder bores or piston ring grooves are worn. Similarly, slack big-end bearings will often result in an excessive amount of oil reaching the cylinder bores. This is brought about by the fact that, since the oil is being fed under pressure to the bearings, it will find its way out at the sides oil' the bearings and be Hung over the interior of the cylinders.