Spark Plugs – searching for the hidden volt

So this is how it works. The ignition system generates a very high voltage and sends it at just the right moment to the spark plug where it jumps the gap between the electrodes in the form of an intense and very hot spark, which sets fire to the petrol. The voltage needed to lump the gap and generate the spark depends mainly on two factors. One is the size of the gap.

To revert to a rule of thumb I was taught in my imperial youth, it takes roughly 1,000 volts to jump a gap of 1/000th of an inch. Modern spark plugs are typically set to have a gap of 1mm which is about 40/1000 of an inch, so the voltage at the electrodes needs to be about 40,000 volts. This would have been well beyond the capabilities of the old points and condenser systems fitted up to the early 1980s, but it’s no problem for today’s electronic systems.

The other factor is the strength of the mixture of fuel and air. A rich mixture – one where there’s quite a lot of fuel – will help the spark jump between the terminals more easily. This becomes important as the spark plug ages.

The constant blasting of tens of thousands of volts between the electrodes makes them burn away slowly – increasing the gap and therefore increasing the voltage needed. If the car does lots of short trips or runs on low detergent petrol, combustion deposits also build up on the plug which can coat the electrodes and again increase the voltage needed to generate a good spark.

It’s the ignition coil that generates the high voltage, and a simplified version of what happens is that the voltage rises higher and higher until it reaches the point at which it can bridge the gap in the spark plug, then off it goes, blasting between the electrodes in an instant. However, it takes time for the voltage to build up – not much time by our standards, but time nonetheless.

If the voltage has to rise higher because of a worn or dirty plug, the spark is a little later than the engine management computer expects it to be – the ignition is retarded, meaning the engine loses power and wastes fuel. Most modern engines will automatically compensate for this by advancing the timing as the plugs age, but only up to a point. Several systems also use feedback from the spark to measure the accuracy of the mixture, efficiency of combustion and various other factors – a modern engine management system is an extremely sophisticated piece of work! The upshot of all this is that the engine is not quite responding the way it should and although the computer can tweak various things to compensate, eventually it runs out of ways to keep it performing at peak efficiency, and there’s only one way that sends the fuel consumption.

There has been a drive to extend service intervals in the hope that we’ll be convinced that our cars have lower running costs. However, even though fuels, ignition systems and spark plugs have improved, they still burn electrodes and get coated with deposits. Some cars have very hard to get at plugs – notably V6 engines in front wheel drive cars – some of them involve lifting or even removing the engine to get to the three at the back. As a result, various different spark plug designs have been introduced that will last up to 60,000 miles, as well as plugs specially designed for high performance, high revving engines. But these are the exception and they are also two or three times the price of “normal” plugs. Most cars are still using plugs of the same design as their 1980s ancestors. To me, if they needed changing after about 12,000 miles then, they must be worth at least checking and adjusting after the same sort of mileage now. That’s what I do anyway!

Oil and damnation

Oil has to do some very important work inside an engine. The most obvious function is to lubricate the moving parts. An engine relies on lots of bits of metal moving up and down or rotating at very high speeds, and for it all to work properly they have to fit very closely together. But if they touch, friction will very quickly heat the surfaces and they will melt. So, oil is pumped around all the moving parts to provide a continuous thin film between them to stop them touching.

The second purpose oil serves is to carry heat away from the moving parts, heat from friction and the heat from the burning fuel. And finally, it has to mop up a lot of nasty stuff produced as a by-product of the burning fuel and store it in suspension until the next oil change – you might view an oil change as the automotive equivalent of a trip to the loo!

In order to do all this the oil has to be thick enough for the thin lubricating film to stay intact at very high pressures to withstand all the buffeting which goes on – bear in mind that a cylinder “firing”, is actually a mixture of fuel and air exploding and you’ll get the idea. And, of course it has to stay thick as it heats up. At the same time it has to be thin enough to circulate freely, especially when the engine’s cold.

As the oil ages the combustion products build up. They are corrosive, so oils contain additives to neutralise them and stop them attacking the structure of the oil itself. The additives combine with the contaminants and new compounds form, some of which are not soluble in the oil, so precipitate out and stay in suspension. Most are filtered out by the oil filter – which is why it’s there – but some settle out as sludge, which can build up in the narrow passageways and restrict or even stop the oil getting through – very much like arteries furring up from bad cholesterol. Like people who don’t exercise, the problem’s worse if the car doesn’t get the occasional decent run. A 100 mile trip along a motorway will clear a lot of sludge, get rid of any water that’s condensed in the oil – and help the exhaust last longer, clean deposits from the combustion chambers, charge the battery properly, clear a particulate filter and do lots more good things. Cars like a good workout too!

Eventually, however, all the additive gets used up and the waste products do begin to degrade the structure of the oil. Even worse, if the filter gets full of sludge and blocked, it’s bypassed, so all the dirty oil circulates round the engine – furring up the narrow oilways even faster and making the oil seem very thick. The harmful effect of this is not just the increase in viscosity and the restricted flow, but the fact that the oil is much less able to sustain the pressures between the bearing surfaces. Poor oil flow and degraded oil makes momentary metal to metal contact increasingly frequent and rapid engine wear results.

Turbo chargers are particularly prone to going bang in a very expensive way if the oil gets even slightly degraded – the high temperatures and huge speeds demand a constant supply of good, clean oil, so walk away from any second hand turbo charged car without a full and exemplary service history – just one late oil change can mean trouble looming. Even cars with a full service history aren’t immune from problems, especially if the manufacturer has put the service interval up to 20,000 miles. Extended service intervals have been blamed for such faults as snapping timing chains. They are usually lubricated by a spray of oil from a small nozzle. After a high mileage, the oil sludges, blocks the small hole in the nozzle and the chain, starved of lubricant, wears very rapidly then snaps.

So, infrequent oil changes run the risk of something breaking – but you might get away with it, so maybe the money saved on oil is worth the gamble. Sadly not, because dirty oil gives another problem. Because the oil is saturated with the unwanted combustion products, it can no longer carry them away and neutralise them, so they hang around in the crankcase. A small amount of gaseous fumes circulating around the empty bit inside an engine is quite normal. These used to be vented to the atmosphere, but for many years now cars have had valves and pipes that feed these noxious fumes back into the engine to be burnt. That way they are treated by the emission reduction equipment of the exhaust system and don’t end up as atmospheric pollution. Normally this works very well – although the fumes are mixed with the air and fuel, the amount is very small and has little or no effect on the efficiency of the combustion. However, if the oil can no longer do its job of absorbing most of the combustion products, there is far more to be fed into the engine, so much that combustion is affected and the engine performs well below peak efficiency. Once again, there’s only one way that sends the fuel consumption. In addition, the exhaust gases are likely to be much dirtier than the emissions equipment is designed to deal with and the result is more pollution, and at MOT time a failed emissions test.

The cure for all this is simple. If you do a normal mileage – 10,000 miles a year or so – have the oil and filter changed every at least 12 months. If you do a very low mileage consisting mainly of very short journeys, you might want to change it twice a year – Spring and Autumn. And if you’re a high mileage driver, change the oil and filter every 10,000 to 12,000 miles, whatever the manufacturer says. Flushing oil used at an oil change will also get rid of all the settled sludge. It’s not vital if the oil has been changed regularly, but it does help preserve the engine and, at around £10, it’s certainly worth considering if you’ve just bought a car that spent it’s previous life poodling down to the shops once a week. It might be low mileage, but it’s the equivalent of a couch potato.

So, it pays to keep the oil clean and change it regularly. Less fuel, fewer emissions and good performance are the rewards – and even if oil prices are at record levels, an oil change is a lot less than a new engine.