The last 70 years have seen a dramatic improvement in engine life. In 1950 Volkswagen Beetles achieved fame by often lasting for more than 100 000 km, but at the time many other small engines gave up the ghost at lower mileages. These days one expects a good design to last for at least 250 000 km, and we frequently hear of engines that have lasted that long.
Engine temperature, oil condition, driving style and lack of servicing are some of the factors that determine the wear rate. Driving style plays a relatively minor role unless you habitually lug the engine or keep the revs close to the red sector on the rev counter. Lugging refers to employing a large throttle opening at a low engine speed in a high gear. This article is mainly concerned with the major components that require refurbishing in a worn-out engine, namely the pistons and rings, cylinder bores and crankshaft bearings. The smaller components often have problems and failure modes of their own.
The cylinder bore/piston interface is a good place to start and the important component here is the top ring. It has by far the most pressure behind it. This can easily be seen when we examine a worn cylinder, for the wear pattern is slightly hour-glass shaped. The deepest wear markings are at the top of the cylinder where the top ring comes to rest, then a section in the middle showing virtually no wear and very much smaller markings at the bottom of the cylinder where the top ring again comes to rest. This explains the time-honoured ritual of feeling the ridge whenever an open cylinder block is shown to a mechanically-minded person.
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.
Wear by metal-to-metal contact is called attrition. This type of wear depends entirely on the thickness of the oil film. The rings are 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 metal to metal contact, but at the top and bottom dead centres, where the piston comes to a complete stop, the oil film is very thin.
Abrasive wear is caused by small particles, introduced with the air, the fuels or the oil. This shows the importance of good filtration, which can only occur if the engine air, fuel and oil filters are replaced regularly. Small particles of carbon, formed during combustion, can also serve as a lapping agent.
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 wear rate of 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 driveability, 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 per cent 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 per cent of the initial value. Finally, we reach a freeway and are 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 1 000 km in a day most of the wear takes place during the first fifteen minutes! Some hard-working diesels have been known to cover over one million kilometres without needing an engine overhaul.
Fast warm up
This means that an engine should be warmed-up as fast as possible, without causing damage by revving too high or pulling a heavy load. The best way to achieve this is to drive off immediately after starting and to keep the speed low for the first few kilometres. This method of warming-up also has the advantage of warming-up the gearbox and rear axle at the same time.
Another interesting result of wear research is that if two engines, made of the same materials, are running under the same conditions, the rate of cylinder bore wear in terms of fractions of a millimetre per 100 hours will be the same for both, even if the bores differ greatly in diameter.
If we express the acceptable rates of bore wear as a percentage of the bore diameter we get the following criteria:
If the wear is not more than 0.2 per cent of the bore diameter no appreciable deterioration is observed.
When the wear reaches 0.25 per cent of the bore diameter oil consumption will begin to rise, and in the case of diesel engines cold-starting may become troublesome.
When the wear reaches 0.3 per cent of the bore diameter oil consumption will be getting out of control. Spark plugs may oil up frequently, leakage and blow-by will become noticeable and piston temperatures may rise, leading to the piston rings sticking in their grooves. The extra heat is due to the bad conduction of heat across the wider path to the cooling water from the rings.
When the wear reaches about 0.35 to 0.4 per cent of the bore diameter the deterioration in performance will be very noticeable. The oil consumption will be out of control and blow-by will be excessive, accompanied by the risk of broken rings.
Wear rate vs bore diameter
What does this mean in practice? Compare two engines with bore diameters of 100 mm and 200 mm. Let’s assume they’ve run for the same length of time under the same conditions and that they are constructed to the same design with the same material specifications. They will then have worn the same amount, say 0.3 mm, according to the above research. If we now express the wear as a percentage of the bore diameter we find that the wear for the 100 mm diameter bore amounts to 0.3 per cent, which is over the limit. The wear for the 200 mm bore is only 0.15 per cent and this will have no noticeable effect!
The rule then is: the bigger the bore, the more wear it can tolerate. A four cylinder of the same capacity as a six should then last longer if all the other conditions are the same. A twin should last even longer and a single should live forever. The snag, of course, is the increased piston mass and unacceptable vibration associated with a smaller number of cylinders. These lead to greater bearing loads.
When does an engine need an overhaul
Living with a car is very much like living with a wife or husband. Any change in behaviour is a cause for concern and should be investigated.
The most important criteria are:
Performance. A drop in performance, with no other symptoms, is usually due to incorrect fuel mixture and ignition timing settings. It’s seldom related to engine wear. The latter usually reduces piston ring to cylinder bore friction so that many engines go faster when they drink oil.
Noise levels. Any newly-discovered noise usually means trouble. Worn valve lifters start to make a tapping noise while worn main and conrod bearings make a knocking sound.
Oil usage. Modern engines vary a lot in what is acceptable oil usage. Let your car’s owners’ manual be your guide. Any change in oil consumption is often due to a leak but on older engines, it’s usually due to worn valve guides, valve oil seals, pistons, rings and cylinder bores.
Smoking. This is most likely caused by the worn parts mentioned under oil usage, but a malfunctioning crankcase ventilation system may also cause smoking. If the smoking starts after an oil change one should suspect an overfull sump.
What your exhaust smoke is trying to tell you