TECH CORNER: Gentlemen, Spark Your Engines!

What Is A Spark Plug And How Does It Affect Performance?

A spark plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded. The central electrode protrudes through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell and designated the side, earth, or ground electrode(s). [1]

Operation

The plug is connected to the high voltage generated by an ignition coil or magneto. As the electrons flow from the coil, a voltage develops between the central and side electrodes. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor and allows electrons to flow across the gap. Spark plugs usually require voltage of 12,000–25,000 volts or more to "fire" properly, although it can go up to 45,000 volts. They supply higher current during the discharge process, resulting in a hotter and longer-duration spark. [1]

As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion. This is the "click" heard when observing a spark, similar to lightning and thunder. [1]

The heat and pressure force the gases to react with each other, and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this fireball, or kernel, depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded, and a large one as though the timing was advanced. [1]

Cars typically have four-stroke gasoline engines, which means there are four strokes, or movements, to the moving parts inside the engine per rotation. Inside each cylinder is a piston, which moves up and down within the cylinder to compress the gas for combustion, and pushes the exhaust gases out after combustion. The piston is operated by an arm that attaches to the crankshaft, a shaft that extends through the bottom of the engine. The piston goes up and down twice during each rotation, hence the four strokes -- up, down, up, down.

The spark plug is positioned at the top of the cylinder, where the air-fuel ratio is compressed. The tip of the plug sits inside the engine, recessed into the side of the cylinder wall. The other end remains outside the engine and is attached to a wire that is also attached to the distributor.

As the piston travels up the first time, compressing the air-fuel mixture inside the combustion chamber, the distributor sends an energy surge along the spark plug wire. When the surge reaches the plug, it fires, igniting the air-fuel mixture. The explosion inside the combustion chamber forces the piston down again, turning the engine further as the same process happens in all of the other cylinders, one at a time. As the piston comes up the second time, the exhaust valve opens, and the burned gases are released into the exhaust pipes.

As you can see, the spark plug is a very important part of the engine. Therefore, in order to maintain the highest performance a car can offer, care should be taken to perform the proper scheduled maintenance in a timely fashion. Most older cars have copper spark plugs, which require yearly maintenance, usually performed at each tune up.

The best course of action is to replace each spark plug during a tune up. However, copper plugs can also be cleaned with emery paper, which is a very fine sandpaper that polishes soil from the plug's tip. Whether replacing or cleaning the plugs, each spark plug should be gapped to the correct specifications, usually found in the owner's manual or shop manual; it will not operate correctly if there is too much or too little space between the tip of the plug and the metal finger that curves over the top of it.

On the other hand, a platinum spark plug requires much less maintenance. Some platinum plugs can last 100,000 miles (160,934 km); if there is a discrepancy between the recommendations in the owner's manual and the manufacturer, adhere to the more conservative mileage rating. Like copper spark plugs, platinum versions need to be gapped to the car manufacturer's specifications. However, emery paper should never be used on a platinum spark plug; if the plug becomes soiled, it will simply need to be replaced. For this reason, regardless of the maintenance timetable, each plug should be removed and inspected at every tune up.

Spark Plug Heat Dispersal

The degree to which a spark plug disperses the heat it receives is called its "heat range". Spark plugs with a high degree of heat dispersal are called high heat range (cold type) and those with a low degree of heat dispersal are called low heat range (hot type). This is largely determined by the temperature of the gas inside the combustion chamber and the spark plug design.

Low Heat Range and High Heat Range

Low heat range plugs have long insulator leg sections and the surface area affected by the flame and the gas pocket capacity are large. Also, since the heat release path from the insulator leg section to the housing is long, heat dispersal is low and the temperature of the center electrode rises easily. On the other hand, high heat range plugs have short insulator legs and the surface area affected by the flame and the gas pocket capacity are small. Also since the heat release path from the insulator leg section to the housing is short, heat dispersal is high and the temperature of the center electrode does not rise easily.

Spark Plug Temperature and Vehicle Speed

There are restrictions on the temperatures at which spark plugs can be used: the lower limit is the self-cleaning temperature and the upper limit is the pre-ignition temperature. A spark plug only functions completely when its center electrode temperature is between these temperatures of about 500°C and 950°C. When the center electrode temperature is 500°C or lower, free carbon generated when the fuel does not combust completely is deposited on the surface of the insulator. Therefore, the insulation between the insulator and the housing falls, electricity leaks occurs, the spark across the gap is incomplete, causes ignition failures. This temperature of 500°C is called the self-cleaning temperature because above this temperature the carbon is naturally burnt away completely by combustion. When the center electrode reaches 950°C or higher, pre-ignition (early ignition) occurs, meaning that the electrode serves as a heat source and ignition occurs without a spark. Therefore, output falls and this can reach the level of electrode wear and insulator damage. Low heat range spark plugs have center electrode temperatures that rise easily and even at low-speed, they easily reach the self-cleaning temperature, so carbon is not deposited easily on the insulator section. On the other hand, high heat range spark plugs have center electrode temperatures that do not rise easily, so they are unlikely to reach the pre-ignition temperature even at high speed. Therefore, this type of spark plug is generally used for high speed, high output engines. That is why it is necessary to select spark plugs with the appropriate heat range for the engine characteristics, running conditions, etc.

Spark Plug Heat Dispersal

The degree to which a spark plug disperses the heat it receives is called its "heat range". Spark plugs with a high degree of heat dispersal are called high heat range (cold type) and those with a low degree of heat dispersal are called low heat range (hot type). This is largely determined by the temperature of the gas inside the combustion chamber and the spark plug design.

Low Heat Range and High Heat Range

Low heat range plugs have long insulator leg sections and the surface area affected by the flame and the gas pocket capacity are large. Also, since the heat release path from the insulator leg section to the housing is long, heat dispersal is low and the temperature of the center electrode rises easily. On the other hand, high heat range plugs have short insulator legs and the surface area affected by the flame and the gas pocket capacity are small. Also since the heat release path from the insulator leg section to the housing is short, heat dispersal is high and the temperature of the center electrode does not rise easily.

Sources:

[1] Spark plug. (2015, December 28). In Wikipedia, The Free Encyclopedia. Retrieved 19:00, February 1, 2016, from https://en.wikipedia.org/w/index.php?title=Spark_plug&oldid=697167995;

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