Spark Plugs Information
Figure 1: Spark plug. Source: Pixabay
Inside any gasoline engine, spark plugs are a critical component for powering our world. While high-compression engines like diesel engines combust fuels via high pressure, gasoline engines need a bit of help. Spark plugs provide that small bit of energy to drive combustion in gasoline engines, releasing all of the power stored in the gasoline’s chemical bonds.
Theory of Operation
A spark plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine. This electric current ignites the compressed fuel/air mixture with an electric spark while containing combustion pressure within the engine. For a spark plug to operate properly, the following steps must be achieved in the correct order:
- Voltage creation
- Voltage transfer
- Spark creation
- Ignition of air-fuel mixture
Everything begins with the generation of high voltage in the ignition coil. The ignition system of a car typically generates a voltage between 20,000 volts and 30,000 volts. This voltage is generated by the ignition coil, a kind of high-voltage transformer. It transforms the low voltage from the battery (12 V) into the high voltage needed by the spark plug.
This high voltage is then transferred via ignition cables to the spark plug. Inside the spark plug, there is a central and a ground electrode. The central electrode is connected to the terminal of the spark plug (where the high voltage enters), and the ground electrode is connected to the shell of the plug, which is grounded to the engine block.
Figure 2: Metal spark plug. Source: Pixabay
The gap between these two electrodes is where the magic happens. When the voltage becomes sufficiently high, a spark jumps across the gap from the central to the ground electrode. The spark that's generated is very hot, reaching temperatures of around 30,000 Kelvin, and this heat ignites the compressed fuel-air mixture in the engine's combustion chamber.
The ignition of the air-fuel mixture creates a mini explosion. This forces the piston down, creating the power stroke that eventually turns the car's wheels. Once the cylinder is at its peak in the cycle, the exhaust valve opens to let out the burnt gases. With an empty cylinder, the cycle then repeats.
The exact timing of the spark is controlled by the engine control unit (ECU) to optimize power, fuel economy, and minimize emissions. It's important to note that in modern engines, there can be more than one spark plug per cylinder to ensure complete combustion of the air-fuel mixture.
Over time, spark plugs can wear out or get dirty, which can prevent them from working properly. This can cause a variety of issues, including difficulty starting the engine, reduced fuel economy, and decreased engine performance. Therefore, they need to be replaced periodically as part of regular vehicle maintenance.
Specifications
Spark plugs come with a variety of specifications, each designed for a specific application. Here are some key specifications:
Heat Range
This is the ability of the spark plug to dissipate heat from the combustion chamber. The heat range is determined by several factors, but primarily the length and diameter of the insulator nose. Spark plugs can be 'hotter' or 'colder' depending on whether they absorb heat slowly or quickly. The correct heat range is important to prevent pre-ignition, fouling, and other problems.
Spark Gap
This is the distance between the center and side electrode. The spark gap is important because it determines where the spark will occur. This needs to be set correctly to ensure proper combustion.
Thread Size and Reach
The thread size refers to the diameter of the threaded portion of the plug, while the reach refers to the length of the threaded portion. These need to match the engine's specifications to ensure a proper fit and seal.
Material
Spark plugs can be made from a variety of materials, including copper, platinum, iridium, and others. The material affects the longevity and performance of the spark plug.
Resistor or Non-Resistor
Some spark plugs have a resistor built into them to prevent electrical noise that can interfere with car electronics. Other applications, especially older or racing engines, might use non-resistor spark plugs.
Terminal Type
Spark plugs can have different types of terminals depending on the type of ignition wire used. Some have solid terminals, while others have threaded or snap-on terminals.
Each of these specifications is important and needs to be chosen based on the requirements of the specific engine. Using the wrong spark plug can lead to poor engine performance or even damage. So, it's always important to follow the manufacturer's recommendations when choosing a spark plug.
Types
Spark plugs are typically classified according to the type of material used for the electrode, but they can also differ in terms of design. Here are the most common types:
Copper Spark Plugs
Also known as standard spark plugs, they have a solid copper core and nickel alloy electrode. Copper is a great conductor of electricity and heat, which makes these spark plugs highly efficient. They are often used in older vehicles and those with low-voltage distributor-based ignition systems.
Platinum Spark Plugs
Platinum spark plugs have a platinum center electrode. Platinum is harder and more durable than nickel alloy, so these spark plugs last longer than copper ones. They also have better heat resistance and conduct electricity well. Single platinum plugs have a platinum tip on the center electrode, while double platinum plugs have platinum on both the center and ground electrodes.
Iridium Spark Plugs
Iridium spark plugs have a fine wire center electrode and are typically more expensive than other types. Iridium is extremely hard and has a high melting point, making these plugs very durable. They offer excellent engine performance and longevity.
Double Iridium Spark Plugs
These have an iridium center and ground electrode. They provide superior performance and lifespan compared to other types.
Silver Spark Plugs
Silver has the highest thermal conductivity and electrical conductivity of all metals. These spark plugs are mostly used in high-performance engines.
It's important to note that regardless of the material, spark plugs should be chosen based on the engine manufacturer's recommendations. This ensures compatibility with the engine design and operating conditions.
Aside from the material, there are also variations in the design of spark plugs:
Hot Spark Plugs
These have a longer insulator nose length. They absorb heat from the combustion process slower and keep the spark plug's firing tip hotter, helping to burn off deposits.
Cold Spark Plugs
These have a shorter insulator nose length. They are designed to transfer heat rapidly from the firing tip into the engine head, keeping the firing tip cooler. This type of plug is used in high-performance engines to avoid pre-ignition or knocking issues due to high temperatures.
These variations allow spark plugs to be optimized for a wide range of engine designs and operating conditions.
Figure 3: Illustration of a spark plug. Source: Public domain
Features
Spark plugs come with a number of features that are integral to their design and function. Here are some of the main features:
Seal
The seal prevents combustion gases from leaking out of the engine. It's typically a crushable washer that forms a gas-tight seal when the spark plug is screwed in.
Resistor
Not all spark plugs have this, but many modern ones do. The resistor reduces electromagnetic interference that can disrupt the car's electronic systems.
Heat Range
This is more of a characteristic than a physical feature, but it's an important part of a spark plug's design. The heat range is the spark plug's ability to dissipate heat from the combustion chamber.
Terminal
This is where the spark plug wire or coil boot is attached. The terminal receives the high voltage current from the ignition coil.
Gap
This is the distance between the center and ground electrode where the spark occurs. The gap size can affect the spark plug's performance and must be set correctly for the engine's requirements.
Each of these features plays a crucial role in the spark plug's performance and longevity.
Figure 4: A spark plug. Source: Pixabay
Manufacture
Manufacturing a spark plug involves a number of complex steps to ensure that the final product meets all required specifications for performance and longevity. Here's a general overview of the manufacturing process:
Material Preparation
The first step involves preparing the necessary materials for each part of the spark plug. This includes various metals like steel, nickel alloy, copper, platinum, and iridium for the electrodes and shell, and ceramic for the insulator.
Making the Insulator
The ceramic insulator is typically made by mixing alumina powder with binders and other additives, and then pressing the mixture into the shape of the insulator. The insulator is then fired at a high temperature to sinter it, which gives it its final strength and properties.
Forming the Electrodes
The electrodes are typically formed from metal wire or powder. The center electrode might be made from a copper core with a nickel alloy, platinum, or iridium tip, while the ground electrode is usually made from a nickel alloy.
Assembling the Spark Plug
The center electrode is inserted into the insulator, and a metal shell is added around the insulator. The ground electrode is then welded to the shell. The spark plug is then sealed, either with a glass seal or a mechanical seal.
Gapping
The spark plug's gap — the distance between the center and ground electrode — is set. This gap is crucial because it determines where the spark will occur.
Inspection and Testing
The finished spark plugs are inspected for any defects, and tested to ensure they meet the required specifications. This could include tests for dimensions, performance, and durability.
This is a simplified version of the process — the actual manufacturing of a spark plug involves many more steps and quality control checks. Each manufacturer might have slightly different processes, depending on their specific designs and materials. But in all cases, the goal is the same: to produce a high-quality spark plug that will reliably ignite the fuel-air mixture in an engine.
Applications
Spark plugs are primarily used in internal combustion engines, where they ignite the air-fuel mixture to generate power. Here are some common applications:
Automotive Engines
Spark plugs are widely used in gasoline-powered cars, trucks, and motorcycles. Each cylinder in the engine has at least one spark plug, though some may have two or more for better combustion.
Figure 5: A motorcycle engine. Source: Pixabay
Aircraft Engines
Spark plugs are used in the engines of many types of aircraft, from small general aviation aircraft to larger commercial planes. In fact, aircraft piston engines typically use two spark plugs per cylinder for redundancy and efficiency.
Marine Engines
Spark plugs are used in many types of marine engines, including outboards, inboards, and jet skis.
Small Engines
Spark plugs are used in many small gasoline engines, such as those found in lawnmowers, chainsaws, leaf blowers, and other power equipment.
Figure 6: A used spark plug. Source: Pixabay
Power Generators
Gasoline or natural gas-powered generators also use spark plugs.
Industrial Applications
Various industrial machinery that operates on gasoline or natural gas uses spark plugs.
In all of these applications, the role of the spark plug is the same: to ignite the air-fuel mixture in the engine's cylinders, generating the combustion that powers the engine. The exact specifications of the spark plug, such as its size, material, and heat range, can vary depending on the specific engine and its operating conditions.
Figure 7: An engine block. Source: Pixabay
Standards
Standards are important for spark plugs to ensure consistency and cross-compatibility with different vehicle types. Some of the most important standards include:
- ISO 28741 — Road vehicles — Spark plugs and their cylinder head housings — Basic characteristics and dimensions
- ISO 11565:2006 — Road vehicles — Spark-plugs — Test methods and requirements
- SAE J548/1_201303
- AD 54-04-01 — Airworthiness Directives; AC Spark Plug Model AC LA/87 Spark Plugs
Standards like ISO 28741 are intended to specify the main properties and dimensions of spark plugs. Spark plugs that adhere to these standards are easier for manufacturers to use and find compatible alternatives. Other standards attempt to control the design, performance, or testing of spark plugs to further ensure quality throughout the market.
