Spark plug gap: definition and use guide

Spark plug gap refers to the gap between the spark plug center electrode and the grounding electrode. The gap spacing of general spark plugs is between 0.6 and 1.3 mm.

The larger the spark plug gap is, the longer the arc will be generated when igniting. It is easier to ignite the gas mixture in the gas cylinder and the kinetic energy will naturally be greater.

If the spark plug clearance is too large or too small, the side electrode should be pressed down or prized to the required range.

Specific spark plug gap is mainly determined by the redundancy of the output voltage of the high-voltage coil.

Spark plug model and its equivalent circuit
Spark plug model and its equivalent circuit

Theoretically, the larger the gap, the longer the arc, the greater the ignition energy.

The gap increases within a certain range, which is conducive to the increase of ignition energy.

However, the larger the gap, the higher the voltage needed to break down the air when igniting, so the spark plug gap matched by different engines is different.

Spark plug gap

Choose the right spark plug gap to make the engine perform better.

Influence of large spark plug gap

  • If the spark plug gap is too large to make the breakdown voltage exceed the redundancy of the high voltage coil output, it will be counterproductive. Not only can the spark plug not increase the energy of igniting the mixture, but it will cause the spark plug not to flash because of the lack of high voltage.
  • At the same time, high voltage lines, especially high voltage coils, will be overloaded for a long time. This leads to the fact that the output voltage can not effectively break the mixture to discharge. It may even cause the ignition coil to be overheated and cause internal short circuit or circuit breakage.
  • Large clearance and strong ignition, the stronger the spark, the full combustion of the fuel mixture in the cylinder, so that the body is easy to start, complete combustion, to a certain extent, more fuel-efficient. In general, the larger the compression ratio and the higher the ignition strength (high voltage) of the engine, the larger the gap required, and the “larger” standard is described in accordance with the manufacturer’s relevant standards.
  • The driver adjusts the spark plug clearance to a larger extent because it improves ignition capability and saves fuel. However, after long use, the spark plug electrode is consumed and the clearance is too large.
  • If the selection criteria (including calorific value, etc.) are incorrect and the clearance exceeds the maximum clearance, it may lead to difficulties in ignition, inability to accelerate, and increased fuel consumption. In addition, it may cause incidental off-fire (misfire), abnormal dynamic conditions, incomplete combustion, increased pollution of exhaust gas and so on.

The spark plug of automotive engine is usually changed from 25,000 km to 30,000 km, and the maximum time is 40,000 km to 50,000 km.

When the spark plug works for a long time, the gap between the two poles becomes larger and larger due to the spark consumption, and the required spark voltage becomes higher and higher.

The experiment showed that the voltage increased by 7 thousand volts for every 1 mm gap.

It’s easy to start the car at ordinary times. When the weather suddenly cooled down, the cold car could not catch fire. It was often caused by the excessive gap between the spark plugs.

The spark plug operates for a long time under the condition of large gap, which requires a higher ignition voltage, resulting in high voltage wire, ignition coil, ignition module work load, shorten the life or damage.

Some vehicles were in good technical condition the day before, but after a day of parking they were difficult to start, or even unable to start.In this way, when the cold car starts, the ignition system is slightly defective, and the spark plug can not jump fire to ignite the mixture.

As for not stepping on the accelerator, the car is easier to start. That is because the intake volume of cylinder is small, the mixture is thinner, and it is easy to jump fire.

Influence of spark plug gap too small

If the spark plug gap is too small, it will not be able to fully discharge high-voltage electricity, resulting in its ignition mixture heat is not enough, resulting in insufficient fuel combustion, resulting in a significant decline in engine power.

If the gap is too small, it may lead to start difficult or even can not start, ignition time is too early, vibration, noise, heavy tail gas and other undesirable.

Reasons for adjusting spark plug gap

As we all know, the working environment of the cylinder is very bad, high temperature and high pressure. Spark plug ignition will generate tens of thousands of volts of high-voltage electricity. For a long time, the spark plug gap between the two electrodes will become larger due to high voltage discharge, so the ignition intensity will be weakened, so the spark plug gap should be adjusted.

Spark plug gap is normal but not ignition

In practice, sometimes the spark plug clearance is within the specified range, but the engine can not start normally. When spark gap exceeds this range, it can start work. The reasons for this phenomenon are:

1. the appropriate working voltage of spark plug is related to gas.When the cold car starts in winter, the gas molecule is not easy to ionize, and a higher jump voltage is needed. At this time, it is easy to jump start if the gap is properly adjusted.

2. engine magnetic steel demagnetization. Coil insulation performance decline and other reasons; magneto voltage is relatively low, at this time the spark plug gap can only be adjusted smaller, to break through the air, and produce sparks.

3. the piston, piston ring and cylinder liner of the engine are seriously worn out.The reduction of cylinder compression force makes it difficult to start, but the performance of the magneto is very good. At this time, the gap of the spark plug can be enlarged appropriately, and the spark produced will be stronger than before. As the gap increases, the arc is longer, so it is easier to ignite the mixture in the gas cylinder to improve the starting performance and working performance.

Adjustment method of spark plug gap

The problem of replacing spark plugs: the gap between old and new spark plugs is different.

The length and calorific value of the new spark plug are the same as the old one, but the gap between the electrodes of the spark plug is larger.The electrode gap of the old spark plug is 1.0 mm, while the spark gap of the new spark plug is only 0.75 mm. This 0.25 mm gap is enough to make the engine power and economic characteristics different.

The first task before installing the spark plug is to adjust the electrode gap of the spark plug. Here we will only discuss the gap adjustment of the uni-polar spark plug.

Generally speaking, a large screwdriver and thick gauge are needed to adjust the gap between the spark plugs. But this rule is not universal.

We can substitute all kinds of bank cards.

Here’s how it works: Put all kinds of cards between the old spark plug electrodes, and the cards that just fit in (with a little resistance) are the standard cards we want.The thickness of the standard card is the appropriate spark plug gap. We plug the standard card into the new spark plug electrode.

If you can’t plug it in, you need to pry open the side electrode with a screwdriver and measure it again; if you can plug it in easily, you need to knock back the side electrode a little bit.

The above method is only applicable to spark plugs with short mileage.If the spark plug is used for a long distance, the spark plug gap will increase, so it can not be our standard reference.

At this point, the spark plug clearance must be adjusted by thickness gauge and screwdriver according to the spark plug clearance parameters of vehicle instructions.

New spark plug gap

Whether new or used spark plugs, their clearance should be adjusted to the clearance specified by the automobile manufacturer:

  • This gap has a direct effect on the combustion process of the engine. The gap spacing between general spark plugs is 0.7~1.1 mm.
  • When the spark plug fires, the electrode will slowly be ablated, resulting in an increase in gap. The gap between the spark plugs used is higher than that at the initial installation time, which is also normal for 0.5 mm.
  • For the new spark plug, the two surfaces of the spark plug electrode are smooth and flat. As the use time increases, the electrode will become smooth and spark plug gap will also increase.4, so when adjusting the gap, we should consider the gap becoming longer with the increase of the use time.

Spark plug ignition process

First, explain the ignition process. From spark plug discharge to flame combustion, it is not immediately fired. A successful ignition must go through these three periods.

1. Ignition Period: The arc breaks down the mixture and ignites it, forming a tiny flame that can be formed anywhere from the positive to the negative through the arc. This period is called the igniting period.

2. diffusion period: after the fire point is formed, it will gradually become a bigger fire spot. The arc has disappeared and the fire point has turned into a fire core. This period is called diffusion period.

3. combustion: expansion of fire core ignites more mixed gas to form flame. This period is also called the spreading period.

The ignition period is mainly related to the mixture density, discharge intensity and electrode shape.

The higher the compression ratio is, the more dense the mixture is, the less likely it is to ignite; the higher the discharge voltage is, the easier it is to ignite.

But too high voltage will accelerate the consumption of spark plug; the finer the electrode, the more easily ignited, the thicker the rounder the less easily ignited.

Ignition gap has a certain impact but not the main, in the normal range of theory can break down the mixture every time, because the discharge curve of the electrode is not the naked eye to see the hit on the end, the ignition process has a cumulative area, discharge area, continuation area, generally in the continuation area there will be 1-2 small discharge process.

The effect of discharge gap on engine working condition is mainly the ignition loss rate.

Diffusion period is the most important period of ignition, during which the size of the ignition nucleus determines the performance of spark plugs with different gaps and different electrode designs at different rotational speeds.

The size of the fire core determines the number of mixtures that are eventually ignited at the same time, and determines the combustion rate of the mixture (only within a relatively small range, do not misinterpret it as a spark plug that can change the absolute combustion rate of the mixture).

Generally speaking, the larger the diffusion of the ignition core, the faster the combustion speed of the mixture is, the better the power is, but the diffusion of the ignition core is affected by the heat release speed of the spark plug and the ignition gap.

Influence of spark plug gap on engine condition

The larger the gap, the faster the burning rate of the mixture. That’s a good thing. Why don’t spark plugs usually use a large gap? There are several reasons for this:

1. Cost: The most common spark plug is nickel-copper alloy. This material is not very good in melting point, hardness, strength, conductivity, corrosion resistance and other aspects, but compared with precious metals such as silver, gold, platinum, iridium, such as performance gap is not big and the price is quite cheap. Due to the lack of its own characteristics, the electrode can not be done very fine, unless the service life is not considered, so the gap can not be very large, otherwise the fire rate will be very high.

2. Effect of airflow: It is well known that ignition does not start from the piston to the top dead point, because it takes time for the flame to spread from start to full combustion. The theory is that the maximum work is done when the flame is completely burned, and the maximum efficiency is achieved if the piston is at the top dead point.So spark plug ignition is actually in the rising stage of piston. The turbulence of the mixture squeezed by the piston in the cylinder is so intense that it blows out the newly formed core of the flame, causing ignition failure.In this case, the large gap spark plug is not as reliable as the small gap.

3. The effect of engine speed: the lower the engine speed, the longer the stroke time and the weaker the relative turbulence. Conversely, the higher the speed, the shorter the time and the stronger the turbulence.

The higher the engine speed is, the bigger the ignition angle is, so the misfire rate has a great influence on the engine power output at high speed.

Usually 2000 rpm ignition angle of more than 20 degrees, 6000 rpm to more than 40 degrees or even close to fifty degrees.

Selection of spark plug gap

The large gap spark plug used in internal combustion engine with low rotating speed is favorable for raising low torsion.

The high speed internal combustion engine should use a small clearance to reduce the ignition rate as much as possible and obtain a good power.

Besides, the ignition position of the spark plug also affects the power output.

Generally, spark plug electrodes used in low power or low compression ratio engines extend more, which is conducive to ignition from the center of the combustion chamber, easy to burn.

The spark plug length of high power and high compression ratio engine is short. The flame is closer to the cylinder head for heat dissipation. Because the temperature of the combustion chamber increases, the engine will have more detonation tendency. The computer will automatically delay the ignition angle, and the reduction of the ignition angle will eventually reduce the power output.

We can see the basic performance of a car through the spark plug, and now most of the new cars use direct injection engines, we can also see that the spark plug electrode is a little shorter.

Expression of spark plug gap

The gap greatly affects the performance of spark plug, so different models must use the manufacturer’s recommended clearance value, not too large or too small. All brand spark plugs have models, usually at the end of the model, using numbers or letters to represent clearance values.

Bosch – at the end of the model using S, T, U, V, W, X, Y, Z in turn for 0.7 mm, 0.8 mm, 1.0 mm, 1.3 mm, 0.9 mm, 1.1 mm, 1.5 mm, 2.0 mm. The general T (0.8mm) is omitted directly, and many models do not indicate the gap value.

NGK – At the end of the model or where the second number appears, use 6, 7, 8, 9, 10, 11, 13, 15 to indicate 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, 1.1 mm, 1.3 mm, 1.5 mm, generally 8 (0.8 mm) is omitted directly.

DENSO – at the end of the model or where the second number appears, the use of 6, 7, 8, 9, 10, 11, 13, 15 in turn to indicate 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, 1.1 mm, 1.3 mm, 1.5 mm, generally 8 (0.8 mm) is omitted directly. Iridium models do not specify clearance values.

Torch – At the end of the model or where the second number appears, the use of 6, 7, 8, 9, 10, 11, 13, 15 in turn to indicate 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, 1.1 mm, 1.3 mm, 1.5 mm, generally 8 (0.8 mm) is omitted directly.

Factors affecting spark plug gap

  • Intake: The spark plug clearance of a naturally aspirated engine is usually greater than or equal to 0.8 mm, and that of a turbocharged engine is usually less than or equal to 0.8 mm.
  • Compression ratio: In theory, the larger the compression ratio, the smaller the clearance, which is determined by the overall engine parameters and calibration.
  • Ignition coil: theoretically, the higher the energy the ignition coil can provide, the higher the breakdown voltage, the clearance can be properly enlarged, except for turbocharged engine, which is determined by the overall engine parameters and calibration.
  • Electrode material: iridium platinum material, because the electrode is thin, easy to discharge, theoretically can be appropriately increased. It is determined by the overall engine parameters and calibration.

Latest Comments

  1. Roger Wilkinson October 15, 2019

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