RE: What is ion-sensing knock detection? PH Explains

RE: What is ion-sensing knock detection? PH Explains

Tuesday 9th October

What is ion-sensing knock detection? PH Explains

How readings from the spark plugs can be used to avoid harmful knock in an engine



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.

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Author
Discussion

stavers

Original Poster:

172 posts

82 months

Tuesday 9th October
quotequote all
"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"

This might have been true for very early knock detection systems but all the ones I have worked with, at least at OEMs, are cleverer than this. Whilst they may only have 1 or 2 knock sensor for every 3/4 cylinders, the ECU has algorithms to calculate which cylinder had the knock issue and how severe the knock intensity is thus allowing each cylinder to be (fairly) accurately controlled.

This is usually done in the early calibration phase by deliberately driving each cylinder in turn in to knock and then mapping the transfer path of the noise through the block with relation to spark event so the ECU only retards the spark of the cylinder(s) with knock and can adjust how much spark is removed based upon the intensity of the knock.

The ion-sensing system is still a much more accurate system though but does bring a lot of cost. And there would probably be traditional knock sensors as a back-up in case someone fits cheaper spark plugs!

aeropilot

16,565 posts

163 months

Tuesday 9th October
quotequote all
Nice to see some credit being made to Saab for it's Trionic engine management system, which was years ahead of all other makers at the time of its introduction.


3795mpower

269 posts

66 months

Tuesday 9th October
quotequote all
stavers said:
"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"

This might have been true for very early knock detection systems but all the ones I have worked with, at least at OEMs, are cleverer than this. Whilst they may only have 1 or 2 knock sensor for every 3/4 cylinders, the ECU has algorithms to calculate which cylinder had the knock issue and how severe the knock intensity is thus allowing each cylinder to be (fairly) accurately controlled.
As I pressed the reply button...this sums it up nicely.
Selective knock Control was the norm on “advanced” engines I was trained on during
The early ‘90’s (those with combined fuel injection & ignition system, even some
With just programmed ignition)

A single or dual knock sensor, an ecu that knew crank position (and tdc) was
Plenty sufficient control knock and automatically adjust for fuel quality/octane rating.

CedricN

364 posts

81 months

Tuesday 9th October
quotequote all
aeropilot said:
Nice to see some credit being made to Saab for it's Trionic engine management system, which was years ahead of all other makers at the time of its introduction.
Absolutely, they really were brave going through with the ion sensing, lots of people said it would never work. The cars might have lacked in other departments, but the control system was really ahead of its time as you point out

mikeg15

216 posts

136 months

Tuesday 9th October
quotequote all
Is ion sensing or knock sensing available for dabblers like myself in after market ECU applications. Some years ago I inquired with an ECU supplier at the Autosport show but was told that they couldn't get Delphi to talk to them about it.
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Fastdruid

5,602 posts

88 months

Tuesday 9th October
quotequote all
mikeg15 said:
Is ion sensing or knock sensing available for dabblers like myself in after market ECU applications. Some years ago I inquired with an ECU supplier at the Autosport show but was told that they couldn't get Delphi to talk to them about it.
It would be nice if so. On a related note I was reading this build diary the other day and really interesting looking at someone putting a scope on and looking at the spikes generated by the cylinder events https://www.rodsnsods.co.uk/forum/garage/medusa-20...

Niffty951

1,684 posts

164 months

Thursday 11th October
quotequote all
Interesting topic. I knew nothing of this systems' existance. Thank you for sharing. Nice concise introduction, I may well dig to find out more.

Max_Torque

12,946 posts

153 months

Thursday 11th October
quotequote all
mikeg15 said:
Is ion sensing or knock sensing available for dabblers like myself in after market ECU applications. Some years ago I inquired with an ECU supplier at the Autosport show but was told that they couldn't get Delphi to talk to them about it.
Until recently, ION sensing was tied up in several Patents, meaning you had to pay to use the tech. As processing power in engine controllers rapidly increased, conventional acoustic knock sensing became cheap and as of similar high performance. Today, only a few manufacturers use Ion sensing, and aftermarket wise it would be a nightmare as without a reference knocking pressure calibration, how do you rate the knock intensity. Most aftermarket systems that offer knock control are typically very badly calibrated, as it's an expensive and time consuming thing to do properly, and really requires significant engine dyno time to get a stable base map and appropriate gain of response.

Today, some engines are moving to real time measurement of in-cylinder pressure in order to optimise combustion across all operating points, and from that data it's trivial to calculate the mean knocking pressure (and the mass fraction burn angular position) and hence optimise ignition angle in real time