Spark plugs and LPG

The plug is the most loaded part of the ignition system due to direct contact of electrodes with hot combustion gases, especially in an engine powered by LPG, wherein the conditions are even more unfavorable. It’s because the nature of combustion process in an LPG engine differs from the same process in petrol-powered engines.

The history of spark plugs

The history of spark plug dates back many years before the first combustion engine was built. How is that even possible? At the end of the seventeenth century, Dutch physicist and mathematician, Christiaan Huygens, invented a fuse ignition machine that ran on gunpowder, and thanks to this machine, he was able to obtain mechanical energy. It could therefore be claimed that the spark plug comes in a straight line from… a fuse.

© BoschOne of the first spark plugs, manufactured by Bosch in 1902

With the development of natural science and the discovery of electricity, research and experimentation took on momentum. 100 years later, an Italian physics teacher was experimenting with detonating gases (explosive gases that are created from electrolysis of water!). He caused an explosion by an electrical spark. These experiments clearly mark the beginnings of the electric ignition of gas-air mixture. The breakthrough came in 1860, when an inventor called Étienne Lenoir, Belgian native living in France, built a single cylinder, two-stroke internal combustion engine fueled by  town gas (coke). The motor powered a simple vehicle that managed to go for 11 kilometers between Paris and Joinville-le-Pont. It’s also worth mentioning that one of the first vehicles with a combustion engine was powered by gas! For the purposes of this experiment, a remote ancestor of modern spark plug was created. Since its lying single-cylinder did not use compression and reached only 80 rpm, heat load, mechanical load and electrical load of spark plugs was small. Therefore, Lenoir’s invention was not really complicated.

A quarter of a century later, Karl Benz manufactured his own spark plugs used in his buzzer-ignition engines. Its work was based on continuous and uncontrolled spark-over between electrodes, causing ignition of air-fuel mixture. A few years later, Gottlieb Daimler managed to construct a high-speed engine with hot-bulb ignition, which was commonly used for some time. This time, the ignition did not arise from electrical discharge. It was not until the early twentieth century that a German industrialist and inventor, Robert Bosch, helped the spark plug obtain current shape and operating methods. All this thanks to a high-voltage battery ignition, which turned out to be the most reliable solution.

In 1902, Bosch started production of spark plugs. An interesting and little-known fact is that Louis Renault – French automotive pioneer – had his share in the whole process, as he came up with the idea to equip the plugs with a thread and screw them into the engine chamber. Another important moment in the history of spark plugs was the introduction of spark plug heat factor by Bosch in 1925. This enabled standardization and common taxonomy of spark plugs, and mass production of plugs with different properties.

The insulator of the plug as we know it today was first made of French chalk, then of mica and porcelain. The only important structural change before 1910 was flanging the rim of the insulator and screwing central electrode into the threaded hole of the plug’s slide. Another material used to create insulators was ceramic material called soapstone. Its era, however, was short. It was quickly (1931) made obsolete by another ceramic mixture – piranit. Even today, the ceramic insulator works best and it’s able to withstand high operating temperatures without being deformed and still perform its tasks.

The spark plug works when spark-over occurs between two electrodes: the inner one, usually made of copper, and the outer one. The number of outer electrodes is substantially the same – between one and four. In the 1920s, American company Champion introduced plugs with two outer electrodes in industrial engines. Ten years later, the company increased their number to four! Over time, and especially with the development of motorsports, plugs underwent further modifications that increased their durability, the ability to self-clean the electrodes, and guaranteed the strongest spark. Most treatments were about using unusual materials. In sports cars, plugs with silver and platinum electrodes were introduced. Thanks to this, durability of plugs rose to 100 thousand kilometers. So today, plugs with electrodes coated with a thin layer of platinum are used more and more. In the 70s, the copper core was put forward to the combustion chamber. It ensured a more reliable ignition and more effective self-cleaning of the plug at higher temperatures.

Heat factor is crucial

The main parameter that characterizes spark plugs is the heat factor matched to a specific engine (its work load). It specifies – by means of a numeral index – the ability of the spark plug to discharge and dissipate heat that is transferred from combustion chambers. Each engine manufacturer specifies this parameter by marking which plugs are suitable for each kind of engine. Higher heat factor translates into greater ability to remove heat, which means that this kind of plug may be used in a more loaded engine without the risk of causing spontaneous combustion. Such plugs (commonly referred to as cold-running spark plugs) are harder to heat up, but their ability of self-cleaning is not that great (more carbon deposits). Lower heat factor means that the plug has smaller ability to remove heat (heats up faster), but has greater self-cleaning ability. Plugs of this type (commonly referred to as hot-running spark plugs) are used in engines with smaller compression ratios and lower power, which have lower combustion temperatures.

Appropriately selected spark plug heat factor allows the plug to operate in temperatures between 500 and 850⁰ C. The first value ensures that the plug will self-clean, that means it will burn all impurities (hydrocarbons and soot) that precipitate on the surface of the insulator. When temperature goes down, impurities will deposit in the insulator, and thereby the surface resistance falls and a so-called conductive bridge is formed. But when the temperature of the electrodes extended into combustion chambers goes above 850⁰ C, it may lead to spontaneous combustion and uncontrolled burning of the blend. Both occurrences in the context of EOBD systems used in modern cars are unacceptable. That’s why selecting spark plugs suitable for a specific type of a car and replacing them periodically according to the manufacturer’s recommendation is essential for correct engine work, especially if the engine is adapted to gas supply.

LPG combustion process

© BoschThe change of ignition voltage depending on the type of fuel (LPG, petrol)

Gas-air mixture, in contrast to fuel-air mixture, burns in an engine at lower speed. It’s one of the reasons why LPG-powered engines have slightly less power (lower pressure rise in the long term).

Later generations of gas supply systems make this loss such small that it’s imperceptible to the user. Such way of combusting gas-air mixture causes the engine to be much more flexible than when it’s fueled with petrol. But a longer combustion process also means that chamber walls are exposed to a longer period of contact with hot exhaust gases. It’s one of the problems that must be faced when a spark plug is screwed into the head. Another one is impeded ignition, resulting from air-gas mixture’s resistance that is about 30% higher.

© BoschBosch offers single- and multi-electrode spark plugs

Single- or multiple-electrode?

Using spark plugs causes them to wear due to spark erosion. This leads to gradual removal of material from electrodes, thus enlarging a gap between them. It always happens, but we can still try and prevent it and extend spark plugs’ durability. One of the things we may do is use multi-ground spark plugs in which discharge is distributed between different lateral electrodes. But these plugs are not recommended for gas-powered engines due to lower ignition energy which is spread across a larger number of electrodes. Gas engines should be equipped with single-ground spark plugs, preferably those with a slightly smaller gap, which ensures greater discharge energy. To make them more durable, other (better) materials are used.