Spark Plugs

Claimed By Christopher Skretkowicz Claimed By Edward Mauger Spring 2017

This page chronicles the inner workings and hidden mechanisms involved with spark plug applications in the real world.

The Main Idea

Spark plugs are essentially what their namesake says they are: plugs that screw into the cylinders of an engine and produce sparks that ignite fuel in combustion engines. Spark plugs take advantage of Faraday's Law and induction to create large spikes of voltage that would otherwise be impossible to achieve with a regular 12 volt car battery.

A Mathematical Model

The primary physics equation used when analyzing spark plugs is Faraday's Law, which states [math]\displaystyle{ \mathcal{E} = -{{d\Phi_B} \over dt} \ }[/math], where [math]\displaystyle{ \mathcal{E} }[/math] is the emf produced from the time-varying magnetic flux Φ_B. The flux is given by [math]\displaystyle{ \int_{\Sigma} \mathbf{B} \cdot d\mathbf{A}. }[/math]

The ignition system essentially consists of a coil pack and the spark plugs. The coil pack, which is grounded, typically of 2 coils, a primary coil and a much larger secondary coil. As current, typically 12V DC, from the car battery runs through the primary coil, it is interrupted by a device timed to the camshaft, which varies the electric current running through the circuit. Because the primary coil is wrapped around the secondary coil, a varying magnetic field induces an electric current to be run through the secondary coil, which in turn produces a much larger EMF due to the number of loops in the coil. This will take the inputted DC voltage (again, typically 12-24V) and output 15,000V-40,000V AC. That current is fed to the spark plugs, which have a center electrode that is isolated from grounding electrode which is grounded through the spark plug casing to the engine, which shares a ground with the coil. The two electrodes have a gap between them that is set typically between .4mm and 1.7mm. A smaller gap requires less voltage from the coil, a larger gap requires more voltage from the coil. In this case, the spark plug's electrodes act as a capacitor with the air in the cylinder being a dielectric insulator. For a spark to occur, the voltage from the coil must be sufficient to cause dielectric breakdown of the air. The spark created ignites the fuel in the cylinder which pushes the piston away which turns the crankshaft to create angular momentum which is then transferred through the drivetrain to the wheels of the vehicle.

The basic equation for dielectric breakdown is given as [math]\displaystyle{ {{V_d}_b}= {{E_d}_sd} }[/math]

V_db represents the dielectric breakdown voltage and E_ds represents the dielectric strength of the insulator and d is the distance. The dielectric strength of air is 3x10^6 V/m, so at a gap of 1mm, the required voltage for dielectric breakdown is 30,000V.

That is a good model for the spark outside of an engine at ambient temperature but there are other variables to consider for voltage required inside of the engine because the dielectric strength will change. If it is assumed the spark plugs have no sort of buildup from carbon or oil, the other variables that effect dielectric breakdown voltage are electrical frequency, temperature, the fact that the dielectric material is not just air but a mixture of gasoline and air at a specific ratio, plus the shape, size, and thickness of the electrodes of the spark plug. The temperature inside of the combustion chamber is typically around 1500 degrees Celcius, increased temperature decreases the dielectric strength. Increased electrical frequency decreases the dielectric strength which is why some manufacturers have researched using higher frequency ignition systems. Gasoline's chemical makeup can vary but Benzene, a component of gasoline, has a dielectric strength of 163x10^6 V/m which is much higher than the dielectric strength of air.

Explanation of System

Ignition System Function Description of Spark Plug

Examples

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Anatomy of a Typical Spark Plug

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Firing Spark Plug

Connectedness

Spark Plugs allow gasoline engines to run. The heat from the spark generated by the ignition system causes the gasoline to combust and drive the piston away to turn the crankshaft. This only works if the spark's heat is above the flashpoint of the fuel. Gasoline has a relatively low flashpoint while diesel has a relatively high flashpoint which is why gasoline engines operate at lower pressures with and with spark plugs (spark ignition) and diesels operate at much higher pressures with no spark (compression ignition). Those concepts would apply more so in a course covering thermodynamics. I am a DIY mechanic who works on both gasoline and diesel engines so understanding the physics behind the ignition system of an engine is useful for my tinkering and modifications of the vehicles I work on. A spark ignition system is mostly related to electrical engineering but as it's fundamental concepts are fairly basic in terms of electrical engineering, they are covered in the mechanical engineering curriculum. The stresses, forces and heat generated by combustion are certainly integral concepts of the mechanical engineering curriculum.

History

Spark plugs have been around since at least 1859. There are reports that a rudimentary version of the spark plug was originally invented by an African-American man by the name of Edmond Berger in 1839 but he did not patent it. Spark ignition was first used in an early internal combustion engine created by Belgian inventor Étienne Lenoir in 1859. British physicist Sir Oliver Lodge provided many improvements to the design in the 1880s. At the turn of the century, many patents for spark plugs were filed by Nikola Tesla, Robert Bosch, Frederick Richard Simms but Bosch is usually credited with the first commercially viable high-voltage spark plugs (1802) and Albert Champion is credited with expanding its use when he built a spark plug factory in Boston in 1805 and supplied American automobile manufacturers. Sir Oliver Lodge's children started Lodge Plug Company in Britain which allowed the British automobile industry to take off. Similarly, Bosch's spark plugs were largely used by automobile manufacturers in mainland Europe.

Throughout the years, improvements have been made such as adding resistors to the plugs to decrease radio interference, different electrode materials, and the ignition system itself has moved from points and distributors to electronic controllers.