A well-designed modern engine will last for more than 250 000km if it runs under ideal conditions. Sadly, though, these conditions aren’t easy to achieve, says Jake Venter. But if you want to get the most out of your vehicle, it is still worth knowing the best way to treat it.
An engine is happiest in the middle of its speed range (between 2000 and 4000 r/min if the red sector starts at 6000 r/min) while cruising at a constant speed. The loads inside an engine increase as the square of the speed ratio. What does this mean? Well, when engine speed increases from 3000 to 6000 r/min, for example, the loads are four times higher. [(6000/3000)2 = 22 = 4].
This does not mean that the higher engine speeds should never be used, but it should give you an idea of the effect high engine speeds have on an engine, and also explains why acceleration is usually more stressful than cruising.
Another important issue is temperature. An engine needs clean air, oil and water. Both water and oil should ideally be at temperatures between 90 and 110C. If the water is too cold, the engine will require a slightly rich mixture that will result in unburnt fuel migrating to the sump, where it will contaminate the oil. It will also result in the sump oil being too cold.
If the water is too hot, it will turn to steam and the engine will overheat.
If the oil is too cold, it may not flow freely. And if it’s too hot, it will break down and form sludge.
So what are the best ways of ensuring engine longevity? Quite simply, by cruising at the middle of the rev range and keeping the temperature constant. And this is most easily done by driving at a constant speed on the highway. That’s why so many trucks and buses travel incredible distances without requiring engine overhauls. Most of the time they work under ideal circumstances.
Needless to say, commuters that cover 10km to work and back each day place far more strain on their vehicles, because neither the water nor the oil is at ideal temperature levels for prolonged periods. No wonder, then, that vehicles used for commuting tend to give problems at low mileages.
The best advice I can give motorists who use their vehicles primarily for commuting is to use them for occasional long-distance trips. The higher temperatures achieved during these trips will ensure that any fuel that has mixed with the oil will evaporate.
Opening up your vehicle every now and then on the highway isn’t enough, however. There are filters for all the fluids that go into an engine, and these should be cleaned or replaced regularly.
Moreover, excess wear often takes place even when these filters are serviced at the correct intervals because of severe operating conditions. Off-road motoring is often severe enough to warrant cleaning or changing these filters more often than the servicing schedule suggests. Driving on sand dunes is particularly severe because sand clogs up the air filter, and slow-speed motoring sends both oil and water temperatures to very high levels.
Many modern air filters employ special paper elements that won’t let air through when they get wet. This means that the air filter should be inspected after you drive through water and should even be removed if it is very wet. On a petrol engine, a partially-blocked filter will cause a rich mixture that will lead to heavy fuel consumption. On a diesel, the lack of excess intake air will lead to the engine overheating.
Old oil is another big issue. Engine oil is more than just a lubricant. It also helps to keep the engine cool and transports small particles to the oil filter, where most of them are removed. These particles are the result of wear and will promote an even faster wearing process if they are not removed.
Modern oil contains a number of additives that are blended with the oil to enhance the natural qualities and make the base oil more suitable for use in engines. Most of these desirable qualities are weakened when the oil gets too hot, so that repeated exposure to high engine temperatures has an ageing effect on the oil. Changing the oil at the manufacturer’s recommended periods, and even sooner after driving in severe conditions, is thus an engine-saver.
One final factor to keep in mind when trying to prolong an engine’s life is fuel mixture. In a modern engine, rich fuel mixtures are unavoidable during idling. The airflow in the manifold is so slow that some of the fuel never reaches the engine. This means a rich mixture has to be supplied. A small percentage will not burn in the few milliseconds available for combustion, but will end up in the sump oil where it will reduce the oil quality. Prolonged idling should therefore be avoided.
It has to be added, though, that only so much can be done to promote engine longevity. A lot of it also has to do with the design of the engine. Some manufacturers test their designs more thoroughly than others, so that their engines tend to last longer. If you want a good indication of a particular brand’s durability, check the resale value of its vehicles.