How do spark plugs work?
Posted by Maurice | Posted in Diagnostics, Parts, Servicing | Posted on 21-02-2010
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While I’m waiting for my service kit to be delivered – as Royal Mail are taking their time – I thought it would be a good idea not only to know what the ‘gubbins’ under the bonnet looks like, what it is and how to replace it but what it actually does. If I was a surgeon, which I’m not, I think it would be a bit hit and miss to start replacing any old organ or lopping off any old appendage, which I wouldn’t, without knowing why I was replacing it or lopping it off. Surely, part of being a surgeon is to know what makes a person tick in the first place? Then you can replace or lop off with some confidence you’re going to improve them – this is my thinking with Maurice. As he can be a little chuggy at times, particularly when its wet I decided a bit more thought needs to go into it and a bit of diagnostic know-how wouldn’t go amiss. So as I have spark plugs on their way – could they be part of the reason for Maurice’s chugginess??
I have four shiny new NGK BKR6E-11 spark plugs ordered.
The main reason for spark plugs is to produce a spark – no surprise there then! They ignite the air and fuel mixture in the combustion chamber of the engine, which looks like this:
I don’t think Maurice’s is going to look anything like that – maybe a bit of wire wool and some polish? Nope, I don’t think that will work either. Let’s just keep learning and forget about my fanatical need to make everything shiny 
Firstly, the sparkplugs ignite the fuel and air and secondly, they remove heat from the combustion chamber diffusing it into the engine block. Spark plugs can be hot or cold depending on the amount of heat they can remove and are designed to work between 500°C (932°F) and 800°C (1472°F). Below <450°C carbon deposits build up resulting in ‘carbon fouling’ – spark plugs should be self cleaning like a griddle with the carbon burning away when they are hot enough. If not, carbon deposits clog the firing end of the sparkplug and they look blackened as the photo below – this is not good. 
This can make your car misfire – a load noise that sounds like a twelve bore shotgun being fired and has every pedestrian diving for cover as you drive by.
Causes for the charred look are:
- driving like your Grandma – ever so slow driving all the time and only ever going to the corner shop
- spark plugs too cold – so they don’t get hot enough to burn the carbon off
- air-fuel mixture too rich – too much carbon is produced for the spark plugs to burn off
- worn piston rings or cylinder walls reducing compression
- slow ignition timing
- ignition system is on its last legs
If the temperature is too hot you get overheating which is not good either. The ceramic shell of the spark plug, if it gets to hot, can pre-ignite the fuel, over-heating the cylinder and causing a lot of expensive damage to the engine – best not do that then!
Causes of the totally fried look:
- spark plugs too hot – so they can’t pass enough heat fast enough into the engine block
- insufficient torque and/or no gasket – so they aren’t connected to the engine block sufficiently to disperse heat
- too fast ignition timing – too much heat produced in the first place
- fuel octane rating too low
- lean air-fuel mixture
- excessive combustion chamber deposits – because your engine is filthy
- continuous driving with heavy loads – no giving elephants a lift
- insufficient engine cooling or lubrication – you forgot the oil and water
As Maurice is still in factory condition, with no souped up bits or add-ons, I selected the spark plugs recommended by The Book; NGK BKR6E-11 V Power High Performance. As soon as they arrive I’ll whip out the old ones then I should be able to tell if I drive like my Grandma or like Lewis Hamilton! As they haven’t been delivered yet it seems like Royal Mail must drive like my Grandma
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