Spark Plug Tech for Internal Combustion Engines

This section of the Tech Zone will hopefully educate you in the how the Spark Plug works, and also introduce you to the techniques of proper plug reading.

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INTRODUCTION:

The spark plug's location exposes it to extreme temperature variations, chemicals, fuels and oils. It is attacked by cylinder pressures produced by the piston and cam timing, then it is assaulted by high-output ignition units. Through all of that the biggest concern the spark plug has is that of the air/fuel mixture. If the fuel mixture is not accurate the spark plug will never be able to accurately do its job. As a result of all of this, one can effectively learn what the engine is doing by reading (viewing close up) the firing end of the spark plugs.

By careful examination of the plug's color, gap, and any deposits that reside on it, you will be shown the efficiencies as well as deficiencies of what is going on in the engine. Spark plugs should be checked at least yearly, and replaced as often as necessary. In most cases you can follow the manufacturers recommendations, but in a race car, our replacement intervals are quite frequent.

 HOW A SPARK PLUG WORKS:

The basic understanding of a spark plug is that it must perform two primary functions.

1. To Ignite the Air/Fuel Mixture
2. To REMOVE the heat out of the combustion chamber

Spark plugs transmit electrical energy that turns fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to cause the spark to jump across the spark plug gap, thus creating what is called Electrical Performance.

Additionally, the temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called Thermal Performance, and is determined by the heat range of the spark plug.

It is important to understand that spark plugs CANNOT create heat, only remove it. The spark plug works as a heat exchanger, pulling unwanted thermal energy away from the combustion chamber and transferring the heat into the engine's cooling system. The Heat Range is described as the spark plug's ability to dissipate heat. The rate of heat exchange is determined by:

• The insulator nose length
• Gas volume around the insulator nose
• Materials and/or construction of the center electrode and porcelain insulator

Now to the actual function. As the ignition is triggered and spark energy leaves the coil (typical distributor-type ignition) and is looking for the closest path to ground. Hopefully this is the gap to ground at the spark plug. The spark travels from the coil into the distributor, across the rotor, jumping to the corresponding terminal on the cap for that cylinder, and through the plug wire lead to the spark plug. Once it reaches the plug it generates a spark kernel as it jumps the gap. This kernel is what ignites the compressed air/fuel mixture in the combustion chamber. Proper timing of the spark is very important and controlled by the timing settings (either mechanically by the distributor, or by computer or ignition control unit). Inaccurate firing intervals will cause lost performance, reduced fuel efficiency, spark plug damage, and internal engine damage. For all this to work together you still need to have the correct heat range (described later) and correct spark plug gap settings.

OPENING THE SPARK PLUG GAP:

For as long as engine builders and racers have been tuning engines there has been talk of enlarged or modified spark plug gaps. The theory is that by opening the gap that you increase the spark kernel, thus improving flame propagation (complete burn across the entire area of the piston), and increasing performance output of the engine. Well, I can say that it does work, but I can also say that it does not work. The explanation for this is that on some applications you will see gains, on others you won't. In some cases you may actually lose performance by opening the plug gap.

On weaker, or stock ignitions, opening the gap puts added strain on the other ignition components in the system. On race ignition systems you are already taking advantage of the output of the ignition box, extended spark duration, and more efficient energy transfer. The only honest way to discuss modifying the plug gap is to experiment on your individual application. You'll either see a benefit, or you won't. Spark plugs gaps are a trial and error type of science when you deviate from the manufacturer recommended settings. If you increase the gap at 0.001" - 0.002" steps and you get to a point where you lose power you will need to close the gap up 0.002"-0.003" to be at the optimum gap. This is what you need to think about:

• The ignition coil may not have enough stored energy to fire, or in the least case, not enough energy by the time it gets to the plug to be able to jump the gap.
• Plug wire leads will break down at a faster rate due to the added resistance as the spark tries to reach ground.
• Rotor, distributor cap, points/condenser (if using an antiquated breaker point distributor), and the center carbon in the distributor cap will show early failures.
• The chance of spark scatter in the distributor cap increases.

All of this occurs because the greater the gap, the higher the voltage requirement necessary to get through the components to jump the gap. The harder these components have to work, the shorter their life span. As a side note, on aftermarket, and high performance rotors the tip is a bit longer to control spark scatter on higher output ignitions.

SPARK PLUG HEAT RANGES:

A spark plug's heat range has no relationship to the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range measurement or specification is determined by several factors:

• The length of the ceramic insulator nose
• The insulator nose's ability to absorb and transfer combustion heat
• The Material composition of the insulator
• The material composition of the center electrode

Let's first look at the length of the insulator. The longer the insulator nose, the larger the surface area that is exposed to the combustion gasses and heat. With the long insulator nose heat is dissipated slowly. This also means that the firing end of the plug will heat up more quickly. Make sure you understand that we are talking about the actual exposed insulator length, not the extended tip length.

The insulator nose length is the distance from the firing tip of the insulator to the point where the insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. No matter what engine the plugs are installed in, be it a lawnmower, a boat, your daily driver or your race car, the spark plug tip temperature must remain between 450°C to 850°C. If the tip temperature is lower than 450°C, the insulator area surrounding the center electrode will not be hot enough to deter fouling and carbon deposit build-ups, thus causing misfires. If the tip temperature exceeds 850°C, the spark plug will overheat which can cause the ceramic around the the center electrode to blister as well as the electrodes will begin to melt. This may lead to pre-ignition/detonation and expensive engine damage. (see the plug pictures that are part of this article)

In identical spark plugs, the differences from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. Note, that a projected style spark plug firing temperature is increased by 10°C to 20°C due to more area exposed to the combustion temperatures.

The firing end appearance also depends on the spark plug tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled, and overheated. The borderline between the fouling and optimum operating regions (450°C) is called the spark plug self-cleaning temperature. This is the temperature point where the accumulated carbon and combustion deposits are burned off automatically.

Next Page - Influences on Spark Plug Temperature & Performance

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