Spark Plugs

Claimed By Christopher Skretkowicz Claimed By Edward Mauger Fall 2016

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 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 spark to occur, 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. 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.

Explanation of System

Ignition System Function Description of Spark Plug

Examples

[#[File:Spark Plug(1).jpg]]

Anatomy of a Typical Spark Plug

[#[File:firing spark plug.jpg]]

Firing Spark Plug

Connectedness

Spark Plugs are literally the spark behind all combustion engines that power motorized vehicles such as cars, trucks and motorcycles. I have always had an interest in cars when I was growing up, but never really knew (and still don't completely know) how they work. Spark plugs may be a small part of a cars engine, but they are vital to creating the power that drives engines. It is very intriguing to see how exactly these spark plugs are able to created large voltage spikes to produce sparks within engines by using Faraday's law, a concept covered in an intro Physics class. Spark plugs do not necessarily link directly to my Civil Engineering major, but I have gained an appreciation for the electromagnetic ins and outs that drive mechanical systems in the real world.

History

The first known spark plug was invented by Étienne Lenoir as a part of the first internal combustion engine in 1860. Lenior and his French buddies dominated the spark plug market in the early 1900s, supplying gasoline engine manufacturers with only a limited line of plug configurations. They were also known to be quite costly and had substandard quality. It wasn't until in 1903 when Oliver Lodge created his spark plug and manufactured it throughout the United States.