Can't start a fire without a spark

by Jim Kerr . May 06 2016
Changing spark plugs at the recommended intervals is cheap insurance to prevent misfiring and costly parts repairs.

Changing spark plugs at the recommended intervals is cheap insurance to prevent misfiring and costly parts repairs.

Since the creation of the internal combustion gasoline engine, a source of spark has been needed to ignite the air fuel mixture.

Spark plugs were, and still are today, the answer.

Subjected to high heat, extreme pressures, and large temperature changes, the spark plug has perhaps the most difficult task in the engine, yet they spark millions of times without failure.

Improvements over the years in ignition systems and spark plug design have extended the useful life of these small but mighty components. In the 1960s, it was recommended to change spark plugs every 16,000 kilometres. Ignition systems of that period typically produced a maximum of 20,000 volts to fire a spark. A spark plug in good condition requires 8,000 to 12,000 volts to create a spark across the electrodes while operating in an engine. A worn plug requires much higher voltage to fire; this could easily have exceeded the voltage available from the ignition systems of that time.

Electronic ignition systems of the 1970s could produce higher maximum voltages. Some systems could produce 30 to 35,000 volts, which was enough to fire even partially worn plugs. Thus, recommended change intervals were extended to between 40 to 50,000 kms.

The challenge of meeting emission laws brought about the development of coil pack ignition systems. These systems use one coil to fire two plugs, as compared to the earlier systems that used one coil to fire all the plugs. Coil pack systems can produce 50,000 volts or higher and are capable of firing even badly worn spark plugs. For best performance, the plug change interval was still around 50,000 kilometres, but many drivers drove the car much further before problems would occur.

Operating an engine with worn plugs causes the coils to produce higher voltages necessary to jump across the worn electrodes. Any areas of weak electrical insulation could allow the spark to jump somewhere else, and the misrouted spark can damage electronic ignition modules, coils and spark plug wires. Changing the spark plugs at the recommended intervals was cheap insurance to prevent misfiring and costly parts repairs.

Some of the latest engine designs use one ignition coil for each spark plug. This is not for higher voltage output, but rather to shorten the path of electricity from the coil to the plug. The shorter the path, the less chance of electrical leakage. The short path also reduced electromagnetic interference in the engine compartment. This is very important as cars continue to increase in the amount of computer wiring and onboard sensors. Electromagnetic interference can cause a computer to malfunction. Another advantage of one coil per plug is the computer can easily diagnose a coil problem and set a trouble code to help in diagnosis.

Several manufacturers are using special high mileage spark plugs that will last for 160,000 kilometres. These plugs cost about three to four times as much as regular versions, and use special alloys for the electrodes that wear very slowly. While the ignition systems have adequate voltage to fire these plugs as they wear, it is often recommended to change them much sooner, simply because on some engines, leaving the plugs in place that long makes them difficult to remove and the repairs cost much more than changing sooner.

Some plugs feature multiple electrodes or electrodes with special shapes. These use premium electrode materials for longer life, and their special design helps to ensure a spark will fire across the electrodes. As electrodes wear, they round off. Spark jumps much easier from a sharp, pointed surface than a round one, so extra electrodes and special shapes with sharp edges help the spark jump across. These plugs are costly to manufacture, so they cost more as well.

Why should spark plugs be changed when the engine seems to be running fine? When a plug begins to fail, it does not stop completely. It may misfire only when you accelerate or when you are driving on the highway. With only one of eight plugs partially misfiring on an eight-cylinder engine, your fuel economy could be reduced by 10 per cent. At today’s price, the wasted fuel would easily pay for new plugs and give better performance as a bonus.

Some engines use two spark plugs per cylinder. This creates two sources of ignition in the cylinder and two flame fronts as the fuel air mixture burn. This faster burn can create more power, but the big advantage is less time for the fuel to pre-ignite by itself, often referred to as “pinging” which can damage engines.