Sparkplug

Learning Objectives

By the end of this lesson, students will be able to:

Understand the Historical Development and Innovations of Spark Plugs:
Describe the Function and Components of a Distributor in an Ignition System
Explain the Function and Structure of a Spark Plug.

A spark plug is a crucial component in the ignition system of internal combustion engines, particularly in gasoline-powered vehicles. Its primary function is to deliver an electric spark that ignites the air-fuel mixture within the engine's combustion chamber. This ignition creates the explosion needed to push the piston down, generating the power that drives the engine. The spark plug fits into the engine's cylinder head and is connected to the ignition coil, which sends a high-voltage current through the central electrode.

The spark jumps across a small gap between the central electrode and the ground electrode at the bottom of the plug. This tiny but powerful spark must occur at precisely the right moment for optimal engine performance. Spark plugs are designed to withstand extreme temperatures and pressures while maintaining consistent performance. Over time, they can wear out or become fouled with deposits, reducing engine efficiency and requiring replacement. Regular maintenance of spark plugs is essential for smooth engine operation, fuel efficiency, and reduced emissions.

QUICK HISTORY OF THE SPARK PLUG

The actual evolution of the modern day spark plug came about as a solution to the problem with early flame and ignitor ignitions on internal combustion engines. There have been several materials that have evolved as insulation materials. Some materials that have been used are mica, stone and several different kinds of improved porcelain. A lot of early forms of porcelain were not designed to handle the extreme hot and cold changes that engines demanded. Some insulated materials were very porous and would absorb oils and soot easily. This would make for a short life as they would foul out quickly. Gasoline in the early days wasn’t as refined as it is today. It contained more oils and was closer to today’s kerosene. In 1915 a major improvement in porcelain was “775” porcelain, by the Frenchtown Porcelain Company. This new formula produced porcelain that would hold up to heat changes with less absorption of oil. Since that time improvements have continued on porcelain designs. Dr. Joseph Jeffery’s discovery of silimanite in 1933 gave spark plugs a longer life. This was another major improvement in the porcelain design. Over the years, companies have tried to come up with unique designs to be used as selling points for their spark plugs. Some of the unique ideas included:

Quick Detachable – by moving a lever 1/4 of a turn, you could remove the center for quick easy cleaning without tools;
Double Ended Plugs – could be turned over when one end was fouled and still have a plug that worked;
Breathing Plugs – were designed to allow clean, cool air to be sucked in over the hot end of the porcelain, which would assist in further burning away of deposits;
Priming Plugs – allowed you to put a small amount of gas into the cylinder for easy starting;
Coil Plugs – where the coil and the plug were actually combined into a one piece unit;
Intensified Plugs – having a second firing gap, that was advertised as allowing the plug to “fire hotter and longer”.

The Distributor – Function and Components

A distributor is an important part of a traditional ignition system used in gasoline-powered internal combustion engines. Its main function is to direct high-voltage electricity from the ignition coil to the correct spark plug at the right time during the engine’s firing order.

Main Parts of a Distributor:

Distributor Cap – Protects internal parts and connects spark plug wires.
Rotor – Rotates inside the cap and sends high-voltage current to the correct cylinder.
Breaker Points – Open and close to regulate the flow of current to the ignition coil.
Cam – Opens the breaker points as it rotates with the distributor shaft.
Condenser – Prevents arcing and helps the coil release voltage effectively.
Distributor Shaft – Turns with the engine’s camshaft to maintain timing.
Vacuum and Centrifugal Advance Mechanisms – Adjust ignition timing based on load and speed.

PARTS OF SPARKPLUG

INSULATOR

Insulates the terminal, center shaft and center electrode from the housing, preventing escape of high voltage from the electrodes. Since the bottom of the insulator projects into the combustion chamber, high purity alumina with superior heat-proof characteristic, mechanical strength, excellent insulation and thermal conductivity at high temperature, etc. is used.

TERMINAL

The terminal is connected to a high-tension cord through which high-voltage current from the ignition system flows. A terminal nut is installed so this type can support almost any high-tension cord in the world. For vehicles not requiring a terminal nut, the terminal can be removed.

RING, PACKING WASHER

Makes the insulator and the housing fit tightly to each other and maintains the airtightness.

CENTER SHAFT (STEM)

Center shaft connecting the terminal and the center electrode.

This shaft is made of steel and has the role of allowing high-voltage current to flow from the terminal to the center electrode without loss.

HOUSING

The housing forms an outer shell that surrounds the insulator, supports the insulator, and installs the spark plug in the engine.At the bottom the ground electrode is located, so current can flow through the engine itself to the center electrode over the gap.

GLASS SEAL

Mounted between the center shaft and insulator to maintain the airtightness.

DENSO uses the glass seal method.

A special mixture of glass powder and copper powder is charged in the installation section for the insulator and center shaft and center electrode and melted at high temperature. This bonds the center shaft and the center electrode and fuses the insulator and the metal.The sealing for both is good and the thermal ratio of expansion is appropriate, so even under harsh conditions gaps do not occur and good airtightness can be secured.

GASKET

Makes the housing and the engine fit tightly to each other and maintains the airtightness of the combustion chamber.

There is a procedure for tightening and the appropriate tightening margin must be secured.For details, refer to the information in “SPARK PLUG Installation”

ELECTRODE WITH COPPER

Special nickel alloy is used for the center electrode to reduce electrode wear.Copper is sealed into the center section to improve thermal conductivity.

CENTER ELECTRODE

A new iridium alloy tip with a diameter of 0.4 mm is laser welded to the tip of the center electrode to make the center electrode.

This lowers the spark voltage, secure reliable spark, reduces the quenching effect, and improves ignition performance.

Iridium, like platinum, is a precious metal and has extraordinarily superior properties for a spark plug electrode, for example high-temperature withstand, high strength, and low resistance. In order to further improve oxidation resistance at high temperatures, DENSO developed a unique new iridium alloy containing rhodium.

U-GROOVE GROUND ELECTRODE

Nickel chrome material is used for the ground electrode and various measures are taken with the shape to improve ignition performance.

One of these measures is the U-groove.

(1) The surface contacted by the air-fuel mixture is large,

(2) There is much edge section, and sparks occur easily.

(3) The flame core (flame size) widens easily.

There are many other features as well and large ignition energy can be obtained.

DENSO obtained patents for spark plug U-grooves from 1975 to 1992.

TAPERED CUT GROUND ELECTRODE

The ground electrode has a shape in which the electrode tip is cut to a finely tapered shape.This reduces the quenching effect, so it improves ignition performance.

Summary:

Spark plugs are critical to engine performance. Each part—from the terminal to the electrodes—plays a key role in ensuring a stable and powerful spark under extreme conditions. Innovations in design and materials, such as the use of iridium and U-groove electrodes, significantly enhance ignition performance and longevity. Understanding each component helps in diagnostics, maintenance, and improving engine efficiency.

References:

DENSO Corporation. (n.d.). Spark plug technical information. Retrieved from https://www.denso.com

Deutscher, D. (2017). Automotive ignition systems: A comprehensive guide. Motorbooks International.

Stone, R. (2012). Introduction to internal combustion engines (4th ed.). Palgrave Macmillan.

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