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Richard Keller is the director of product ignition engineering for Federal-Mogul, the company who produce Champion spark plugs. Richard has been working for Champion since 1980 and so knows a thing or two about spark plugs… “A spark plug has a very simple function, it takes the energy provided by the ignition coil and delivers it to the combustion chamber in the form of the spark.” “The spark plug itself is constructed of a shell, which is the part you screw into an engine, and trapped within this shell is the ceramic. The ceramic is the insulator that keeps the high voltage that is passing through the centre of the spark plug from earthing. In the centre of the plug you have a terminal, which is the bit you connect to the coil via the plug cap, then a resistor seal with maybe 5,000 Ohms resistance that is for EMI suppression. This seal is typically made of glass and is mechanically and hermetically sealed and then below this is the central electrode, which is the part you see passing through the ceramic and out of the bottom of the spark plug and is constructed with nickel alloy. Often this has a little bit of precious metal attached to it to help wear resistance. The final component is the ground electrode, which earths the spark plug through the plug’s body and is made of nickel alloy.” “When a spark plug sparks there is electrical wear, but also at the temperature a spark plug operates you get a certain amount of erosion and corrosion and the precious metal helps prevent this and retain the gap. Without a precious metal in place an engine could wear out a plug in 30,000 miles, with the metal it doubles or triples the figure.” “A spark plug’s life depends on the current discharged and the temperature of the electrodes, the number of reps it goes through isn’t a very good predictor. The higher the temperature within the engine, the higher the chance of oxidation and therefore the higher the wear rates. Around 400-degrees Celsius is the temperature that carbon will start to burn off and you won’t get a build up of deposits, so just above this is ideal. However, when you get to above 850-degrees you start to see high temperature oxidation and that’s when the precious metals start to come to their own. This is a fairy common operating temperature for a high output engine and we design plugs to deal with this, however it wasn’t always so easy. Modern fuel injection systems are very good at maintaining this temperature as they control the fuel into the system very accurately, carbs are far worse which is why carbed bikes oten foul plugs. Carbed bikes start to leak and introduce more fuel, richening the mixture and leaving unburnt carbon deposits on the plug. Under-fuelling is far worse than over-fuelling.” “With over-fuelling you will see deposits on the plug and poor running caused by the plug earthing through the deposits and not the ground electrode, meaning there is no spark. Get the motor running hot and this will generally burn off, so it is a minor irritation. Under-fuelling means the air/fuel ratio is quite lean and the temperature inside the cylinder increases until miss-fires happen. If pre-ignition occurs the tip temperature of the spark plug can increase by 1,000-degrees in a few seconds and it will melt and molten metal inside the combustion chamber is bad…” “The spark itself is typically only a few milliamps, which isn’t that much due to the system being an inductive discharge. The coil transforms the bike’s low 12 volt electrical system into a higher 40,000 volt output, which is why it hurts when someone kicks a bike over and you are holding the plug! Luckily, it is current that kills, not volts, so you will just get woken up a bit!” “The spark itself only exists for a few milliseconds but there are lots of things we can do to make the most out of the spark. Plugs have different end designs as they have different properties. Modern engines run very lean, and in these conditions a thin electrode gives good mixture access to the spark and therefore spark exposure. The electrode can interfere with the flow of fuel/air inside the cylinder, so the design does make a big difference. However, on older vehicles that run cooler there is no real benefit.” “You may have noticed the ceramic part of a spark plug is ridges, which is a hag-up from the good old days. The plug caps used to be very poor at keeping the electricity within them, and ‘flash-over’ would happen, which is when the charge goes down the outside of the plug to earth rather than inside. These ribs helped stop it happening, although nowadays they are nonsensical. Earlier I mentioned that plugs have EMI suppression within them. EMI, or electro magnetic interference, is radio noise. Prior to around 1980 most engine didn’t have most engines didn’t have many electrical components so the ringing effect from the ignition system in the spark plug caysed by the arc creating an electrical disturbance wasn’t an issue. Nowadays there are so many electrical components that without these suppression the plugs could affect the car’s electronic systems. Without the suppression you plugs could make the lights flash on and off or the windows open!” “Will there be spark plus in the future? We are working on some technology called Corona that has excellent spark exposure and therefore improved combustion, but it is in its infancy. I kind of doubt spark plugs will be gone before I retire, but other technology will start to appear within the next ten or so years.” Find our more motorcycle parts information on our website!
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