The Story of the Spark Plug | Part II

By excluding lead from fuel, the temperature required to burn-off deposits had increased. However, this produced a problem at lower engine loads where combustion chamber temperatures are more moderate. Utilizing a hotter plug can contend with this but it also presents the obstacle of overheating at higher engine loads. Furthermore, lead-free gasoline suffers from an increased likelihood of pre-ignition, which is a condition in which the air-fuel mixture ignites before the spark plug fires off. This could potentially lead to severe engine damage and using a hotter plug can further heighten this risk.

Because the central electrode is purposed to release electrons during spark events, it's also meant to be the hottest surface on the spark plug. This also makes it easier for electrons to be released, developing a localized spark formation. The electrical discharge brought on by the ionization of gases within the spark gap is known as a corona discharge and it usually forms where the electric field strength is largest within the electrode. This tends to be where the radius of a curved surface is the smallest, such as an edge or at a sharp point. Though a sharp point would erode far too quickly for practical use, the end of copper core electrodes are designed with a sharp edge that erodes in a consistent manner. As the plug wears, the edge will slowly begin to round over reducing the strength of the spark generated. The service life of a copper core spark plug can be extended by periodically cleaning and refilling the end back to a sharp edge. Copper-cored spark plugs could easily achieve a service life of around 32,000 Km or about 20,000 miles.

Another notable design is known as a surface-discharging spark plug. These plugs are meant for engines that cannot physically accommodate the tip protrusion found on traditional spark plugs. A well-known example of this is the facing sweeping motion of the Wankel engine. To remedy this issue, surface discharging spark plugs have a recessed stubby center electrode that forms a spark gap to the plug’s body, conferring a predominantly flat ignition face.

Also around this time period, the addition of noble metal high-temperature electrode spark plugs was becoming commercially viable. These plugs are composed of center electrodes made of a thin wire of metals such as iridium, tungsten or platinum. Additionally, these class of spark plugs offered fine, extremely sharp-edged electrode ends that have high melting points which were far more durable than copper core spark plugs. The inherent durability of noble metal spark plugs made them an ideal match for the newer higher-voltage ignition system. This negated the requirement for regular maintenance and offering a remarkable service life of over 160,000Km or about 100,000 miles. It was now possible to operate a vehicle for years without any service to the ignition system.

On modern fuel injected cars, higher compression ratios, as well as increased combustion timing tolerances, are used to extract the most energy possible. Which in turn increases power and efficiency. As an engine’s rotating speed increments, triggering an ignition event slightly before the point of maximum compression within a cylinder, or advancing timing is done to give the combustion process more time to occur. Changes to the air-fuel mixture are also made to adjust for load and speed changes.

Under certain conditions, uncontrolled combustion can occur as smalls pockets of air-fuel mixture explode outside the envelope of the normal spark triggered combustion front. This is known as detonation and occurs when the timing is advanced too aggressively or the air-fuel trim is mismatched for the conditions within the cylinder. Though this can be mitigated by using a higher octane fuel, which lowers gasoline’s volatility. Generally, the engine control unit is tasked with finding an equilibrium between preventing detonation and achieving optimal combustion timing.

Because burning air-fuel mixture becomes ionized and conductive shortly after a spark event. It becomes possible to repurpose the electrodes of a spark plug to measure the conductivity of the gases during combustion. This was done by sending a low voltage current through the spark plug’s electrodes, where the chamber ionization can be sampled. Between spark events the ionization curve can be measured and compared to a known “clean burn” set of data, allowing perfect direct detonation detection. Misfires can also be easily detected by sensing the lack of ionization.

As ionic-sensing systems grew in sophistication throughout the mid-2000s, this method now allowed the spark plug to provide individual cylinder information to the engine control unit on fuel trim and peak cylinder pressure position. This level of information now allowed engine control units to quickly adjust each cylinder, improving the overall engine efficiency as well as offering a new diagnostic tool for engine service.