The major components that require refurbishing in a worn-out engine, namely the pistons and rings, the cylinder bores, as well as the crankshaft and its bearings, are all affected by three causes of wear – abrasion, attrition and corrosion
Abrasive wear is caused by small particles, introduced through air, fuel or oil. This kind of wear can be reduced to a minimum by ensuring that the various filters are kept in good condition. Small particles of carbon, formed during combustion, can also serve as a lapping agent.
This refers to wear by metal-on-metal contact. This type of wear depends entirely on the thickness of the oil film. The rings are, of course, lubricated, but the nature of the oil film depends on the motion of the piston.
When the piston is travelling down the bore at an incredible rate of acceleration, the oil film is thick enough to eliminate direct metal-on-metal contact, but at the top and bottom dead-centres, where the piston comes to a complete stop, the oil film is very thin, causing wear by attrition.
If we examine a worn cylinder, we find that the deepest wear marks are at the top of the cylinder where the top ring comes to rest. Then there is a section in the middle showing virtually no wear. Finally, there are much smaller markings at the bottom of the cylinder, where the top ring again comes to rest.
Why does the top ring do most of the damage? Because the ring’s slight tension against the cylinder wall is increased hugely by the combustion pressure. This penetrates behind the ring, causing it to do its job of sealing-in the combustion gases and transferring heat to the cooling water. This pressure obviously depends mainly on throttle opening rather than engine revs. Planting your right foot down at low engine speeds (lugging) is harmful because low revs equate to a thinner oil film.
The ring behaviour also leads to corrosive wear. The very thin layer of oil left behind by the ring at the top and bottom of its travels is not sufficient to protect the cylinder wall from acids formed by the combustion process. In a hot engine these acids leave the combustion chambers as gases, but in a cold engine they condense onto the walls as droplets.
The chief culprits are formic and other organic acids and, if a fuel contains some sulphur, then a vicious acid called sulphur trioxide is also present. Tests have shown that the cylinder walls should be above 120C at the point where the top ring comes to rest, otherwise these acids will condense onto the wall. This roughly translates into a radiator temperature of at least 80C.
Corrosive wear is by far the most problematic, as shown by research done in the US using the radio-tracer method. If a part wears, a proportional amount of the metal will appear in the crankcase oil. If the source of the wearing metal can be traced, measuring wear becomes a simple matter of determining the rate at which the wearing metal accumulates in the oil. This is done by making only the part to be investigated radio-active and then measuring the radioactivity in the oil. The amount of radioactivity in the oil is then directly proportional to the wear of the part concerned. Furthermore, this radioactivity can be measured continuously while a vehicle is travelling and a record can be obtained of the wear as it happens.
When this technique is applied to the wear rate at the cylinder bore, it is found to be temperature dependent, and it is very instructive to look at the wear taking place on a typical journey.
If you start the car in the morning and let it idle for a minute or two with the choke in operation to warm the engine and improve drivability, you experience maximum wear for three reasons:
- The low temperature causes corrosive wear.
- The choke causes an over-rich mixture which allows droplets of fuel to wash some of the oil away from the rings.
- The cold oil does not reach all the parts it has to lubricate.
Now you set off on a journey, travelling through a town. The water temperature gauge rises steadily and the wear rate decreases dramatically to about 50% of the initial rate. It must be remembered that the water temperature is an average value and the stop/start driving typical of city conditions causes fluctuations in cylinder wall temperatures, especially at the top, hence the corrosive wear.
Out in the country, the average speed rises with just an occasional slower section, and the wear rate drops to about 25% of the initial value.
Finally, we reach a freeway and are now able to cruise for long periods at a constant speed. The temperature gauge should remain steady at the normal temperature determined by the thermostat (if it’s working correctly) and under these conditions the wear rate drops to almost nothing. That’s right, if you travel 1000km in a day, most of the wear takes place during the first 15 minutes! For this reason, some hard-working diesel engines have been known to cover more than one million kilometres without needing an overhaul.