What is Spark Peak Voltage?
Spark Voltage (Informal Description): A measurement of how much ‘static electricity’ it takes to generate a spark.
Spark Peak Voltage (Scientific Definition): The peak magnitude of the electric potential between the electrodes on the spark plug when the spark forms.
This might seem a bit complicated or confusing, but it will make sense when you learn a little more about what’s going on.
How Sparks Form
The engine’s ignition system rapidly increases the energy of the electrons (voltage) on one side of the spark plug. This voltage difference moves electrons in the air from one end of the spark gap to the other, a process called ‘ionization’. This ionized air is much more conductive than regular air, and eventually starts letting electrons cross the gap. This initial ‘spark formation’ takes less than 200 microseconds (<0.02% of a second).
What Sparks Do
What Spark Peak Voltage Means
Why Does Spark Voltage Matter?
Spark voltage tells you two (related) things about the engine:
- The first thing spark peak voltage tells you is how much potential energy the ignition system was able to build up before the spark formed. It is very important that the ignition system builds up and releases this energy at the right time!
- The second thing spark peak voltage tells you is how much potential energy was required to ionize the air between the spark electrodes, and form the plasma. Note that a spark can form anywhere between the ignition coil and electrical ground, not just across the spark plug!
What is a 'Normal' Spark Voltage?
Spark voltage depends on many things, which we’ll get into in the next section, and even one engine in good condition can vary by a lot. That said, most slow engines, and engines with relatively low compression ratios have low ‘nominal’ (optimal) peak voltages, of between 4 kV (kilovolts) and 8 kV. Faster engines, and those with higher compression often have spark voltages above 10 kV, and up to 16 kV. Higher voltages are not necessarily better for engine power or efficiency, and may be a sign of an inefficient ignition system.
How to Measure Spark Voltage
Spark measurement requires specialized equipment, and you will find a run-down of the three most popular options below, along with an explanation of why one familiar tool won’t get the job done. For (even) more information about ignition testers, take a look at our post about all the different kinds of spark testers.
Engine Analyzers and Ignition Analyzers
Engine analyzers and engine ignition analyzers are the most common, versatile, and practical tool for spark voltage measurement. These instruments usually have capacitive pickups, which are attached or placed adjacent to spark plug wires, and sometimes ignition coils (in the case of ignition analyzers). The analyzer category was extremely popular from the 1960s through the 1980s, with brands like SUN Electric and Bear (both now defunct) making large cabinet-type instruments that could be found in most automotive service stations and dealerships. We (GTC) revived the category with our hand-held GTC505 Engine Ignition Analyzer (back in 2015), to provide an instrument for people looking to perform detailed ignition diagnostics.
Spark kV Meters
Spark kilo-volt (kV) meters are a category of tools, which began as a lower-cost alternative to the more popular and capable analyzers. kV-meters were usually connected to one spark plug wire at a time via a capacitive pickup. Our TA100 SmarTach+ is a modern, hand-held incarnation of the traditional kV meter. The TA500 SmarTach+COP is a similar tool, the (added) capability to measure burn times, and work on coil-on-plug systems.
Oscilloscopes
Many professional technicians have adopted oscilloscopes for measuring various ‘dynamic’ signals (including spark patterns, fuel injector waveforms, and oxygen sensor signals). Oscilloscopes are extremely powerful tools, which require a high-voltage probe, or capacitive pickup and attenuator to measure spark voltage, because oscilloscopes are usually limited to something between 20V and 50V at their input (which is 1000 times too low for direct measurement of sparks). Unfortunately, ‘scopes are expensive to purchase, as are their probes and adapters. Oscilloscopes are also difficult to configure for spark measurement, and are incompatible with many coil-on-plug systems. Special care should be taken when using an oscilloscope connected to a computer or computer network to measure anything on a vehicle, because the (potentially) large difference in voltage between (vehicle and computer) voltages can easily damage sensitive computer equipment.
Digital MultiMeters
Many people want to, or try to use a digital multimeter (DMM) to measure spark plug voltage, but this does not work, because multimeters are too ‘slow’. DMMs usually take about 3-4 measurements per second, with each measurement being averaged over 100-400 milliseconds (ms). Catching a spark requires either specialized circuitry (which DMMs do not contain), or a much higher sampling rate (>1000000 measurements per second).
What Affects Spark Voltage?
Engine Configuration and Compression
Engine configuration and design are two of the most important determinants of spark voltage. The engine’s compression ratio and intake system (naturally aspirated or forced induction) determine the air pressure in the cylinder, and air is what conducts the spark across the gap. The higher the cylinder pressure, the lower the voltage (in general); changes in ambient air pressure can have the same effect, but forced-induction counter-acts this.
In-cylinder air turbulence is another important factor, as fast-moving air disrupts spark formation, and can ‘blow out’ sparks. All engines are designed to create some turbulence in the cylinder, because it helps the flame front spread. In most engines, this turbulence increases with engine speed (RPM), and the amount of turbulence is carefully chosen. Aftermarket modifications including superchargers, turbochargers, intake manifolds, or even exhaust headers can change the amount of turbulence in the cylinder. Most modifications tend to increase peak spark voltage, so if you’re making any of these changes, you should make sure that you account for these effects.
Ignition Coil
The ignition coil dictates what maximum voltage an ignition system can achieve. An ignition coil stores energy in a magnetic field; the number of ‘turns’ in the coil, and the resistance of the wire tell you how strong of a magnetic field the coil can generate. To form a spark, the ignition coil must be able to store enough energy to create a plasma between the spark plug electrode. That said, the more energy-capable the coil, the slower that voltage will rise, which reduces the peak voltage of high-inductance coils. The insulator between the coil’s wires is what limits the voltage that a coil can output; every insulator will ‘break-down’ at a certain voltage (and thickness of insulation), and when the insulator breaks down, the coil’s energy is released as heat (into the insulation).
Ignition System
Direct magneto ignition systems are the simplest, ‘baseline’ type, featuring relatively low spark peak voltages, as the air between spark plug electrodes ionizes slowly. The ionization is relatively slow because one of the spark plug electrodes is directly connected to the ignition coil secondary, so the voltage on the electrode builds up as while the ignition coil is charging. There are ‘transistorized’ magneto systems which increase the rate of ignition coil charging, or trigger a sudden discharge, thus increasing spark peak voltage (and timing consistency), but have only become common in the last few years (after ~2010).
Mechanical distributor-type ignition systems control the ignition coil’s charging and discharge. This control is usually employed to make the spark build up quickly (than it would otherwise), which increases spark peak voltage, but there is almost always a spark in the distributor, which dissipates some energy, and reduces spark peak voltage (as well as spark energy and efficiency). Mechanical distributor systems usually have the largest ignition coils (of all ignition systems), which are generally fully charged, contributing to their high output voltages.
Electronically controlled ignition systems, including electronic distributors, capacitive-discharge, coil-near-plug, and coil-on-plug are theoretically capable of generating the highest spark peak voltages, but usually designed and tuned to produce lower voltages than mechanical-distributor systems.
If you’re looking for more information on the different types of ignition systems, please check our blog-post that describes them in detail.
Spark Plug Gap and Geometry
The spark plug’s geometry, distance between electrodes, and material all impact spark formation and voltage, but there is no ‘best’ type, because these options are all trade-offs.
Sharper electrodes concentrate the ionization and cause the spark to form consistently in the same place, reducing voltage and improving consistency. Increasing sharpness might seem wonderful, but sharper designs are more sensitive to problems as the spark plug wears down, gets fouled, or there are (relatively) large droplets of fuel in the chamber. Increased numbers of electrodes offer the spark multiple paths from cathode to anode, reducing voltages and improving consistency. Unfortunately, these complex designs increase cost, and reduce in-chamber turbulence, which delays flame propagation, that reduces combustion efficiency.
Spark plug gap is probably the single most important factor when it comes to both peak and burn voltage, which is why most spark plug designs are available with a variety of gaps. Increased gaps always cause increased peak and burn voltages, along with less-stable and consistent sparks. It should be noted that modern spark plugs should not be ‘adjusted’, because they are often brittle, and bending them can cause electrode alignment issues.
Spark plug electrode material has almost no impact on spark voltage when a spark plug is brand-new, but can be just as important as design or gap over time. Modern materials (such as iridium or ruthenium) maintain lower peak spark and burn voltages than older materials (such as nickel and copper) because the modern materials are more resistant to corrosion and erosion at high temperatures.
Spark Plug Condition
Wear and erosion of the spark plugs is normal over time, but changes the geometry of the spark plug, and contributes to the gradual reduction in engine performance and efficiency. Both ‘smoothing’ and ’rounding-off’ the edges of the electrodes increase the voltage required to form the spark, which delays it, and retards the ignition.
Fouling of the electrodes, such as oil, or burnt oil residue on either the anode or cathode changes where and how the spark can form. This often causes the spark to form in unusual ways, such as from the ‘sides’ of the central electrode. These unusual sparks are often thinner and less stable than they should be. As a result, the peak voltage and burn voltage are usually higher and more variable than they would otherwise be.
Corrosion (rust) is very bad for spark plugs, because metal oxides are ‘insulators’, which basically ‘block electricity’. A corroded plug will always cause higher peak and burn voltage than a ‘clean’ one, and these voltages will be much less consistent than they should be. If the corrosion is severe, it can eventually cause sparks to form outside the combustion chamber, directly from the spark plug wire, ignition coil, or distributor (if present) to the engine block or chassis.
Engine Speed, Lambda, & Load
The air-fuel mix ratio, known as lambda, has a large impact on spark peak and burn voltage, because the fuel creates resistance between electrodes. A richer mixture, with more fuel means more resistance and increased voltage. A leaner mixture, will result in lower voltages. An uneven mixture also reduces the voltage, because there are ‘gaps’ between the fuel droplets which can be large enough for the spark to pass between. An uneven mix will also cause very inconsistent sparks, because there are (relatively) large droplets which can stick to the plug electrodes, or pass between them.
Engine chamber pressure affects spark peak and burn voltages because the compressed air-fuel mix must conduct the spark, and low-pressure air is a very poor conductor. Increased pressure (because of higher compression ratios, high engine temperature, or valve timing) reduces the resistance of the air-fuel mixture between spark electrodes, thereby reducing the spark peak and burn voltages.
Because of the effects of chamber pressure and air-fuel ratio, increased load tends to increase spark peak and burn voltages.
Magneto-type ignition systems are more impacted by engine speed than others, because the energy and voltage generated by a magneto is proportional to its speed. This increased power output causes longer lasting, more consistent, and higher-voltage sparks.
How To Increase Spark Voltage
- An increase on in-cylinder turbulence, which causes sparks to be ‘blown-out’. This phenomenon usually happens because someone has installed a high-flow exhaust, high-flow intake manifold, or forced induction system.
- The engine has increased peak cylinder pressure (compared to its original design), which has resulted in erratic spark timing (as the spark is forming early in the higher pressure). The changes that cause this are most often the installation of high compression parts (cylinder head or piston), or an aftermarket forced-induction system.
Solid Conductor Cables
The cheapest, common spark plug wire ignition system ‘mod’ (modification) is changing the spark plug wires from (conventional) carbon-filament to low-resistance ‘solid conductor’ (usually copper) wires. This change has no measurable impact on the spark peak voltage inside the combustion chamber, because spark plug wire resistance is insignificant compared to the resistance of the air between the electrodes. There is some evidence that solid conductors do (very) slightly increase spark burn voltage, but this is not clearly ‘better’.
Special Spark Plugs
The studies we’ve seen do support the idea that some novel spark plug geometries can improve engine efficiency, and that different materials and coatings could affect spark voltage. Smaller electrodes which are effectively further apart (due to grooves or other fine features) seem to delay spark formation and increase peak voltage. Electrode material also seems to matter, and some materials seem to slow plasma formation, but we should remember that over the medium to long-term, corrosion-resistance will be very important. Corrosion can increase the size of the gap, and (negatively) affect the surface finish of the electrodes; each of these changes will increase spark voltage.
All of that being said, we (at GTC) believe that these increases in spark voltage actually delay spark formation, reduce spark heating, and worsen combustion efficiency. If you want to change ignition timing, you can do that directly, and observe the results. We don’t recommend doing this, unless you have very good reason for it.
Spark plugs with more than two electrodes actually reduce spark voltage by allowing the electrons to ‘choose’ the shortest path to ground, increasing the heat delivered into the combustion chamber, and effectively advancing spark timing (by a relatively short period of time). There is a performance cost though, as the complex geometry also reduces fluid flow across the spark plug, thus slowing flame propagation, and negatively impacting efficiency. We’re not sure whether this is a good deal, but think it’s probably not worth it.
"High-Voltage" Ignition Systems
There are many different ‘high voltage’ and high-energy’ ignition systems, but the two most common types are variants of the capacitive discharge and electronic distributor designs. Increased voltage is usually achieved by speeding-up the spark build, which allows the electrode voltage to reach higher levels before the air-fuel mixture between electrodes is ionized (and the spark forms).
High-voltage ignition systems usually improve the consistency of ignition timing, but reduce ignition system efficiency, and heating of the air-fuel mixture in the combustion chamber. For these reasons, we generally discourage people from installing high-voltage modifications, especially on forced-induction (supercharged or turbocharged) engines, because they need all the energy they can get. That said, high-voltage ignition systems may improve the performance of very high-speed, timing sensitive engines, such as alcohol-fueled race cars.
Better Indicators of Ignition Performance
As you have probably realized, almost everything affects spark peak and/or burn voltage, which makes it a very sensitive indicator of ignition performance. The problem with this sensitivity is that it can be very hard to tell what the cause of high, low, or variable voltages. Because of how misleading a simple peak voltage reading can be, we recommend that diagnosticians examine spark burn time, and (ideally) either burn voltage or the dwell waveform to better understand what is really going on, both inside and outside of the combustion chamber.
That's All!
If you work on spark-ignition systems, you should take a look at our GTC505 Engine Ignition Analyzer, it was made to help people like you.
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