The spark plug is what produces the electric spark that triggers the explosive combustion of the air mixture. Ignition timing and high performance plugs using special metals are the focus of attention, but have you ever considered the distance between the center electrode and the outer electrode = spark gap? It is often thought of as just a gap for sparks to ignited, but it has a very important role.
Spark plugs essential to the birth and evolution of internal combustion engines
By observing the burned condition of the spark plug, the combustion condition can be checked.
In the case of smoldering on a carbureted motorcycle, it is a standard practice to determine that the jet setting is too thick, but the burnout may be affected by variations in the ignition gap. If the gap is small, the deflagration effect will be stronger, and smoldering due to loss of fire may be mistaken for carburetor density. It is a common mistake to tighten the plug with the gap narrowed without noticing that the outer electrode was bent against the cylinder head when the plug was installed.
For internal combustion engines, it is extremely important to trigger combustion in the air mixture at the top dead center of compression (or more accurately, at the optimum timing before the top dead center of compression), and spark plug was needed 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 took on a concrete form 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 been consistently inherited. The mechanism of igniting the air mixture by electric spark has been consistently inherited even though engines have evolved in various ways since then.
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 onto the outer electrode. This can be checked by removing the plug and holding it against the cylinder head, etc., and pressing the cell button. The buzzing blue-white sparks seem to ignite continuously as far as the eye can see, but in reality, sparks are generated intermittently at each ignition period.
For example, the spark plug fires 4,000 times per minute at an engine speed of 4,000 rpm. In a 2-stroke engine, the air mixture burns with each revolution of the crankshaft, so all 4,000 times are sparks for ignition. On the other hand, a 4-stroke engine burns once every 2 revolutions of the crankshaft, so the mixture is ignited only 2,000 times. However, since the spark plug itself fire once per crankshaft revolution, it also fire at the top dead center of the engine displacement, for a total of 4,000 sparks. When a spark fires 4,000 times in 60 seconds (1 minute), it ignites once every 0.015 seconds. With such a short and intermittent ignition, it is no wonder 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 a spark plug is extremely short for an engine that rotates at high speed, it is possible to burn the air mixture by igniting it intermittently.
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. Also, the smaller the surface area of the electrode tip, the easier it is for sparks to fires toward the outer electrode, thus reducing ignition. However, if the nickel material used for the plug is made thinner, the life of the plug will be shorter, so iridium alloy or platinum, which have excellent heat and corrosion resistance, are used.
The higher the engine speed, the shorter the time available for each ignition, but there are a number of steps before the spark sent to the spark plug ignites the air mixture, and a certain amount of time is required.
When the ignition timing is reached and sparks are ignited between the center and outer electrodes, the mixture that exists between the electrodes catches fire and creates a spark. The sparks then grow outward from the gaps between the electrodes, eventually spreading throughout the entire air mixture in the combustion chamber, and the greatly expanded pressure pushes the piston down hard to generate power.
In the process of a spark trying to grow, it is necessary for the spark to fire out horizontally through the gap between the electrodes, but the electrodes blocking the top and bottom of the gap have a deflagration effect that takes away the energy of the spark, and if ignition fails, the spark will be lost. In the previous example, if the engine loses its ignition only once or twice every 4,000 revolutions at 4,000 rpm, it will not have much effect, but if the loss of ignition continues, the engine will stop. In the carbureted era of motorcycles, it was possible to cover the plug by using the choke incorrectly during cold starting, but this was caused by an overly dense mixture that did not allow the spark to grow properly.
The gap between the electrodes, or spark gap, is set to allow the spark to grow without being defeated by the extinguishing action of the electrodes. Spark plug manufacturers have established standard gap values through various studies and experiments. In the case of NGK plugs, the plugs for motorcycles without a gap symbol are 0.7 to 0.8 mm, and the plugs for automobiles are 0.8 to 0.9 mm. In addition to this, there are also spark plugs that have the gap clearly marked 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.
The wider the spark gap, the larger the spark in the initial stage of ignition, and the more combustion can be expected to grow without losing its ability to extinguish the flame. However, on the other hand, if the spark gap becomes wider, the load on the side of the ignition coil, igniter, etc., will also increase, and if the gap is widened only when the ignition system does not have sufficient sparking ability, it may end up falling apart because sparks will not be produced between the electrodes in the first place.
One of the solutions to this dilemma is a high ignition performance plug that uses platinum or iridium for the electrodes of the standard nickel plug. Electrodes made of platinum or iridium are much thinner than those made of nickel. The slimmer electrode has less of a quenching effect that interferes with the growth of the spark, allowing the flame to spread faster, resulting in the advantage of improved combustion efficiency. If you look at the cross-sectional area of the center electrode, the thin electrode has an effect like that of a lightning rod, and it also has the characteristic of generating 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 harder to lose fire. In addition, the electrode wears less than nickel, resulting in a longer life.
High ignition performance plugs are more expensive than standard plugs, but it is good to know their advantages in terms of 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 flame arresting effect that prevents the growth of sparks, plugs with high ignition specifications were developed with thinner electrodes made of platinum or iridium.
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 sicnes gauge along the periphery. Insert the gauge into the ignition gap, and the current gap is the place where a thicker gauge will not fit any further. In the case of the photo, it is 0.8 mm.
AS UOTANI's SPII Full Power Kit is a popular ignition system tuning part that allows you to feel the change in engine feeling. In order to improve the combustion power of the air mixture, the 3 factors of voltage, current, and discharge time to the spark plug have been greatly increased. The fact that the ignition gap is instructed to be widened to 1.1-1.3mm after the full power kit is installed is a sign of confidence in the strength of the ignition power.
Normally, when widening the ignition gap of a plug, it is often necessary to insert a flathead screwdriver into the gap between the central and outer electrodes and pry it open. However, if a tool with a special gap owner is used, only the outer electrode can be opened without applying force to the central electrode. When widening the gap of an iridium plug with a very thin center electrode, it is strictly forbidden to pry the center electrode as a fulcrum.
If sufficient power is not supplied from the ignition coil, the spark will not ignite 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 rise sufficiently even though there is no problem with the primary voltage, it can be assumed that the coil is the problem. In either case, narrowing the ignition gap from the standard value may lower the required voltage and allow sparks to ignite. If you are on the road and a fire starts to break out, you can try narrowing the gap as a first aid measure.
If the ignition system is OEM, the plug gap should be adjusted to the standard value, but engines with more than 2 cylinders need to be careful. As mentioned earlier, the gap for motorcycle plugs 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.
The reason why the gap is set at every 0.1mm is because the ignition gap is so severe, and the extinguishing action that affects the ignition performance is affected by the gap. If the plug gap of each 4-cylinder engine is different, the combustion state of each cylinder may also be different. Of course, there are variations in the state of the air mixture and compression in each cylinder, so it is not absolutely necessary to manage only the spark gap closely. 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 I will introduce the measurement using a coin-shaped gauge. It's easy to use, though: just slide it from the thin part of the perimeter to the thicker one, and read the scale when it stops. If the gap is uneven, set a standard and match it.
In terms of customization, tuning, and maintenance, attention can also be paid to the capabilities of the ignition system and the ignition gap. If the ignition gap is large, the initial spark will be large and spread throughout the combustion chamber at once, resulting in greater combustion pressure than if it burns lazily. However, widening the ignition gap also carries the risk that the spark will not be able to be ignited.
The SPII Full Power Kit from AS UOTANI is a highly reliable ignition system tuning part, especially for users of discontinued models. The combination of the SPII Control Unit and the SPII High Power Ignition Coil doubles the voltage applied to the spark plugs compared to the normal spark plugs to approximately 40,000 volts. Discharge current is increased by 1.5 to 2 times, and discharge time is increased by 2 to 3 times. The fact that the plug gap when the SPII Full Power Kit is installed is designated as 1.1-1.3mm means that it has that much confidence in the size of the ignition energy. Moreover, if you can spark a wide ignition gap, the engine will feel better because the spark can easily grow bigger without being affected by the deflagration effect.
On the other hand, if the spark plug tends to lose fire due to aging or trouble with the ignition system, narrowing the spark gap can reduce the load on the ignition coil and igniter, making it easier for the spark to ignite. In this case, the proximity of the center and outer electrodes increases the flame extinguishing effect, and the combustion itself tends to be smaller and weaker. However, if narrowing the gap reduces ignition loss and stabilizes the engine, it is an effective first aid for problems encountered on the road and can help identify the cause of ignition system failure.
Although it is often taken for granted that spark plugs emit sparks every time the ignition time comes, it is important to know that the ignition gap is essential in various situations and to keep proper maintenance in mind.
- Point 1: If the spark can be generated even if the ignition gap is widened by the ignition system tuning parts, it will be possible to obtain a large combustion power from a large spark.
- Point 2: When the spark plug of an old or discontinued vehicle tends to lose fire, it may be possible to temporarily improve the symptoms by narrowing the ignition gap and lowering the required voltage.
