Spark plugs produce the electric spark that triggers the explosive combustion of the air mixture. Although the timing of ignition and high-performance plugs made of special metals attract a lot of attention, have you ever thought about the gap between the center and outer electrodes, or the spark gap? It is often thought that it is just a gap to allow sparks to be generated, but it has a very important role.
Spark plugs are essential to the birth and evolution of internal combustion engines
The combustion condition can be checked by observing the burned condition of the spark plug. When a carbureted engine is smoldering, it is standard practice to determine that the jet setting is too thick, but the burn condition may be affected by variations in the ignition gap. If the gap is small, the flame extinguishing action is stronger, and smoldering due to loss of fire may be mistaken for a thicker carburetor. It is a common mistake to tighten the plug with a narrow gap without noticing that the outer electrode was bent against the cylinder head when the plug was installed.
The internal combustion engine burns fuel inside the cylinder to generate power, whereas the external combustion engine is operated by heat from outside the cylinder, like a steam engine. For the internal combustion engine, it was most important to trigger the combustion of the air mixture at the top dead center of compression (or, more precisely, at the optimum timing before the top dead center of compression), and the spark plug was required for this purpose.
Although the idea and principle of the internal combustion engine had been around for a long time, it was not until the invention of the spark plug in the 1800s that it became concrete as an engine, and while engines have evolved in various ways since then, the mechanism of igniting the air mixture with an electric spark has remained consistent. Since then, engines have evolved in various ways.
The high voltage of 20,000 to 30,000 volts produced by the ignition coil flows from the terminal at the top of the spark plug to the center electrode, and sparks are generated when it discharges to the outer electrode. This can be confirmed by removing the plug and holding it against the cylinder head and pressing the cell button. Although the buzzing blue-white sparks seem to fly continuously as far as the eye can see, in reality, sparks are generated intermittently at each ignition timing.
For example, at an engine speed of 4,000 rpm, the spark plug fires 4,000 times per minute, and in a 2-stroke engine, all 4,000 sparks are used for ignition because the air mixture is burned at each revolution of the crankshaft. On the other hand, a four-stroke engine burns once every two revolutions of the crankshaft, so the mixture is ignited only 2,000 times. However, since the spark plug itself flies once per crankshaft revolution, sparks fly even at the top dead center of the exhaust, and the total number of sparks is 4000. This means that it ignites once every 015 seconds. With such a short and intermittent ignition, it is natural that it looks like a continuous spark.
- Point 1: The invention and manufacture of spark plugs was the key to the practical application of internal combustion engines.
- Point 2: Although the ignition time of the spark plug is extremely short for an engine that rotates at high speed, the mixture can be burned by intermittent ignition.
Spark gap that requires delicate management
The thinner the center electrode is, the less effect it has on the extinguishing action and the easier it is for the spark to grow. In addition, the smaller the surface area of the tip of the electrode, the easier it is for sparks to fly toward the outer electrode, and therefore, ignition can be reduced. However, since the life of the plug is shortened if it is made of the same nickel material that is normally used for plugs, an iridium alloy or platinum that excels in heat resistance and corrosion resistance is used.
The higher the engine speed, the shorter the time available for each ignition, but there are a number of stages before the spark that flies to the spark plug ignites the air mixture, and a certain amount of time is required.
When the ignition time is reached and a spark flies between the center and outer electrodes, it burns into the mixture that exists between the electrodes and creates a spark. The sparks then grow outward from the gaps between the electrodes, eventually spreading to the entire mixture inside the combustion chamber, where they expand and push the piston down hard to generate power.
In the process of the spark trying to grow, it is necessary for the spark to fly out laterally from the gap between electrodes, but the electrodes that seal the top and bottom of the gap have a deflagration effect that removes the energy of the spark. If the engine fails to ignite, the engine will lose the fire. In the previous example, if the engine loses fire once or twice in 4000 times at 4000 rpm, it will not affect much, but if the loss of fire continues, the engine will stop. In the carburetor era of motorcycles, the choke could be used incorrectly and cover the plug when starting cold, but this was caused by the mixture being too thick and not allowing the spark to grow properly.
The gap between the electrodes, or the spark gap, is set in order to allow the spark to grow without being defeated by the extinguishing action of the electrodes. In the case of NGK plugs, the gap for motorcycles without a gap symbol is 0.7 to 0.8 mm, and that for automobile plugs is 0.8 to 0.9 mm. In addition to this, there are also spark plugs with the gap clearly indicated at the end of the part number. For example, the standard plug for the Honda CB400 Super Four is CR8EH-9, and the "-9" at the end of this part number indicates that the spark gap is 0.9mm.
If the spark gap is wide, the spark in the initial stage of ignition becomes large, and it can be expected that the combustion grows without being defeated by the flame extinction effect. However, on the other hand, if the spark gap becomes wide, the load to the side that flies the spark such as ignition coil and igniter becomes large, and if only the gap is widened while the ignition system does not have enough sparking ability, the spark does not fly between the electrodes in the first place, and it may become the end of the world.
One of the ways to solve this dilemma is to use platinum or iridium electrodes as opposed to the nickel electrodes of standard plugs, which have a high ignition performance. The electrodes made of platinum or iridium are overwhelmingly thinner than those made of nickel. The slimmer electrode has less of a quenching effect that interferes with the growth of the spark, and the flame spreads faster.As a result, this plug has the merit of improving combustion efficiency. If you look at the cross-sectional area of the center electrode, the thin electrode has an effect similar to that of a lightning rod, and it is also characterized by the fact that it generates sparks more easily than a standard plug with a larger area. This leads to the advantage that if the capacity of the ignition coil is the same, it is difficult to lose the fire. In addition, the life of the plug is also longer because the electrodes are less worn than those of nickel.
High ignition performance plugs are expensive in relation to standard plugs, but it is good to know their advantages in terms of the combustion mechanism.
- Point 1: The gap between the center electrode and the outer electrode becomes the ignition gap, and the spark generated in this gap spreads to the mixture and triggers explosive combustion.
- Point 2: Because the central and outer electrodes have a quenching effect that hinders the growth of sparks, plugs with high ignition specifications that have thinner electrodes made of platinum or iridium were developed.
Make the gap wider when the ignition is strong, and narrower when it is weak.
A coin-type gap tool is like a stepless, slope-shaped thickness gauge that is attached along the periphery. Insert the gauge into the ignition gap, and the current gap is the place where a thicker gauge cannot fit. In the case of the photo, the gap is 0.8 mm.
SPII full power kit made by AS Utani is popular as ignition system tuning parts that can feel the change of engine feeling. In order to improve the combustion power of the mixed air, the three elements of the voltage, current, and discharge time to the spark plug are greatly increased. After installing the full power kit, the ignition gap is reduced to 1.1 to 1.It is a sign of confidence in the strength of the ignition power that it is instructed to spread to 3mm.
Normally, when widening the ignition gap of a plug, it is often necessary to insert a flat head screwdriver into the gap between the center electrode and the outer electrode and pry, but if a tool with a special gap owner is used, only the outer electrode can be opened without applying force to the center electrode. However, if you use a tool with a special gap owner, you can open only the outer electrode without applying any force to the central electrode. When widening the gap of an iridium plug with an extremely thin central electrode, it is strictly forbidden to pry the central electrode as a fulcrum.
If sufficient power is not supplied from the ignition coil, spark will not fly to the spark plug. If the primary voltage flowing from the igniter to the ignition coil is low, it can be assumed that the igniter is defective, and if the secondary voltage flowing from the ignition coil to the spark plug does not increase sufficiently even though there is no problem with the primary voltage, it can be assumed that the coil is the problem. In either case, by narrowing the ignition gap from the standard value, the required voltage may be lowered and the spark may start to fly. When the ignition starts to lose fire at the destination, it is good to narrow the gap as an emergency measure.
If the ignition system is stock, the plug gap should be adjusted to the standard value, but engines with two or more cylinders should be careful. As mentioned earlier, the gap of plugs for motorcycles that are not specifically specified is 0.7 to 0.8 mm, and NGK plugs are available with other gaps in 0.1 mm increments.
This is because the ignition gap is so severe that it is set every 0.1mm, and the flame extinction effect that affects the ignition performance is affected by the gap. Of course, there are variations in the state of the air mixture and the compression of each cylinder, so it is not absolutely necessary to control the spark gap alone. However, it is easier to align the spark gap than to check for variations in the air mixture and compression.
There are several types of gauges for measuring the spark gap, but here we will introduce the measurement using a coin-shaped gauge. However, it is easy to use. Just slide the gauge from the thin part to the thick part at the outer circumference and read the scale when it stops. If the gap is uneven, set a standard and adjust it.
We can also focus on the ability of the ignition system and the ignition gap in terms of customization, tuning and maintenance. If the ignition gap is large, the first spark is large, and the combustion pressure can be expected to become larger than burning lazily by spreading the fire to the whole combustion chamber at once. However, if the ignition gap is widened, there is a risk that the spark will not be able to fly.
The SPII Full Power Kit from AS Uotani, which is the most reliable ignition system tuning parts, is a combination of SPII Control Unit and SPII High Power Ignition Coil. The fact that the plug gap when the SPII Full Power Kit is installed is designated as 1.1-1.3mm means that we are confident in the size of the ignition energy. And if you can spark in a wide ignition gap, it will be easier for the spark to grow bigger without being affected by the extinguishing action, which means that the engine will feel better.
Conversely, if the spark plug tends to lose fire due to age-related deterioration or trouble with the ignition system, narrowing the spark gap can reduce the load on the ignition coil and igniter, making it easier for sparks to fly. In this case, the combustion itself tends to be small and weak because the proximity of the center and outer electrodes increases the flame extinguishing effect. However, if narrowing the gap reduces ignition loss and stabilizes the engine, it is an effective first-aid measure for problems encountered on the road, and can also help to identify the cause of ignition system problems.
Although it is often taken for granted that spark plugs emit sparks every time the ignition time comes, we would like to know that the ignition gap is important in various situations and keep proper maintenance.
- Point 1・If the spark can fly even if the ignition gap is widened by the ignition system tuning parts, it comes to be able to obtain a big combustion power from a big spark.
- Point 2・When the spark plug of an old motorcycle or a discontinued mdel tends to lose fire, it may be possible to temporarily improve the symptoms by narrowing the ignition gap and lowering the required voltage.
