Detonation, also known as 'knock', can be catastrophic for an engine. It occurs when temperatures and pressures rise too quickly once combustion has started, which can cause pockets of the remaining air-fuel mixture in the cylinder to self-ignite.
The uncontrolled combustion causes erratic spikes in pressure - resulting in a distinctive rattling noise, often most prominent when the engine is under load. At best, this random combustion is inefficient and harms the engine's power output. At worst, persistent knocking can damage the pistons, heads, cylinder walls and plugs.
To help counter this, manufacturers use several detection methods to identify knock. The information obtained can then be used to alter the engine management system's behaviour, usually by modifying the ignition timing, in order to stop the knock.
The detection of knock is often achieved using an acoustic or piezoelectric knock sensor. Attached to the engine's block, these pick up on the distinctive noise or vibration caused by knocking. In many cases, though, only one cylinder or the engine as a whole is monitored - and precisely monitoring in-cylinder conditions is otherwise impossible. This means that, if one cylinder starts knocking due to some anomaly, every cylinder will have to be dialled back - reducing the engine's output and efficiency - just to be safe.
One way of avoiding this is to use an ionisation current-sensing ignition system. This effectively turns each spark plug into an in-cylinder sensor that can be used to monitor knock to a far more accurate degree. The ECU can then adjust the management system's behaviour to tailor the combustion cycle in each and every cylinder, resulting in improved efficiency and reliability.
How do these ion-sensing ignition systems work?
During the four-stroke cycle in a petrol engine, a varying quantity of ions and electrons is released as atoms are ionised by the processes taking place in the cylinder. These can be produced by the spark plug firing, or by the thermal and chemical events that occur during combustion. When the spark has been triggered, the measuring system applies a voltage to the spark plug. The electrical field generated then causes the ions and electrons in the cylinder to move between the centre and ground electrode of the plug - generating a current.
The conductivity of the gas mixture, and thus the current measured across the plug electrodes, depends on the density of electrons and ions around the spark plug. The resulting ionisation-related current measured is subsequently dubbed the 'ionic current' and can be used to ascertain the events taking place in the combustion chamber.
In a conventional firing stroke, the amount of ionic current present in the combustion chamber goes through several distinct stages and is directly related to the combustion pressure and temperature. During the firing cycle, for example, the ionic current gradually climbs and then falls with the rise and drop in pressure and temperature.
Knocking, however, creates uneven spikes in pressure - which causes additional atoms to be ionised. This causes the ionic current to fluctuate wildly, letting the ECU know that knocking is occurring. Steps to negate the issue, such as retarding the timing, can then be taken. Alternatively, if a misfire occurs, there will be no rise and fall in the ionic current; in this case, a warning light could be triggered.
Such systems can also be used to pre-emptively avoid knock by monitoring the state of the cylinder prior to firing the spark. Additionally, other data can be obtained from more advanced setups. For example, the ion sensing system could be used to estimate the quality of the combustion process, or if any pre-ignition is occurring.
A brief history of 'ionic current' knock detection
The concept of measuring ionisation in an engine's cylinder to detect knock has been around since at least the 1940s; myriad patents from that era document systems designed for logging data from aero engines.
More advanced automotive research systems using ion-sensing equipment were then devised and used by companies such as Bosch and Alfa Romeo in the late 1970s. As electronic fuel injection and ignition controls became far more widespread, manufacturers also began investigating the concept of using ionic current sensors to monitor and adjust the operation of each cylinder to peak performance without inducing knock.
Saab, for example, patented such a detection system in 1984 - followed by a setup that could alter the timing in 1986; it subsequently put its 'Trionic' engine management system into production in 1993. This would use ion current measurement to detect knock and misfires, data which could then be used to alter the ignition timing to avoid issues and optimise the combustion cycle. This set-up also allowed for more precise cylinder synchronisation.
BMW drew further attention to the system when it used an 'ionic-current control system' in its S85B50 V10, which arrived in 2005 and powered the E60 M5, E61 M5 Touring and E63 & E64 M6 Coupe. Other manufacturers, including Ferrari, Honda and Mercedes-Benz, also use a similar system.