Engine Knock Sensors, Part 1

Engine knock limits performance and can cause permanent damage. This time, we’ll look at its causes.

Under certain conditions, combustion in a spark ignition engine can degrade into an abnormal preignition process that causes a “knocking” or “pinging” sound. This undesirable combustion process limits the engine’s output and specific efficiency levels. It occurs when the fresh air/fuel mixture preignites in spontaneous combustion before being reached by the expanding flame front.

Under normal combustion chamber conditions, the spark at the spark plug starts the burning process. A wall of flame spreads rapidly in all directions from the spark, moving outward through the compressed mixture in the combustion chamber until all of the charge is burned. The speed with which the flame spreads is called the rate of flame propagation. During combustion, the pressure in the combustion chamber increases to several hundred pounds per square inch (psi), and may exceed 1,000 psi in a modern high compression engine.

Under certain conditions, the last part of the compressed air/fuel mixture, or end gas, will explode before the flame front reaches it. The end gas is subjected to increasing pressure as the flame progresses through the air/fuel mixture. This increases the end gas temperature (due to the heat of compression and also radiated heat from combustion). If this temperature increases beyond the critical point or is maintained for a sufficient length of time, the end gas will detonate before the flame front arrives. Flame velocities in excess of 2000 meters per second (m/s) can occur, compared to speeds of roughly 30 m/s during normal combustion.

When the end gas explodes before the flame front reaches it, there will be a sudden and sharp pressure increase, followed by a very rapid oscillation of pressure in the combustion chamber. Shock waves from this explosion progress rapidly through the burned gases in the combustion chamber and strike the exposed surfaces of the piston, cylinder head and cylinder walls. These shock waves, or pressure pulses, bounce off the metal surfaces and pass back and forth at sonic speeds through the gases, creating a series of pressure pulses in the gases which cause the characteristic engine knocking noise.

The repeated shock wave blows can impose severe stress on engine parts. Shock loads are applied to the piston, connecting rod, crankshaft and bearings. Bearings, in particular, are susceptible to rapid failure under severe knocking conditions, although pistons, rods and crankshafts have also failed from this condition. Chronic preignition also causes increased thermal stresses at the cylinder head gasket and in the vicinity of the valves. All of these factors can lead to permanent mechanical damage.

A number of environmental factors can influence an engine’s tendency to knock. For example, a hot engine will knock more easily than a cold engine. A 20° F rise in air temperature increases an engine’s octane requirement by about three octane numbers. An increase in humidity from 40 to 50 percent at 85° F reduces an engine’s octane requirement by one octane number. This follows the common perception that an engine will run better and more quietly in damp weather. Engine deposits increase octane requirements because they increase the compression ratio. Advancing the spark or leaning the air/fuel ratio increase the engine’s octane requirement. Last of all, higher altitudes reduce an engine’s octane requirement because the air is less dense.

It requires an appreciable time, measured in microseconds (0.000001 second), for the mixture to begin to burn. Increasing the temperature in the combustion chamber reduces this time. So if the temperature in the combustion chamber gets hot enough or is maintained long enough, the end gas will explode prematurely. Several methods may be used to prevent knock. Increasing the rate of flame propagation allows the flame to reach the end gas in time. Subtracting heat from the end gas reduces the likelihood that it will preignite. And using a fuel that is chemically more stable will allow the engine to tolerate higher temperatures without knocking.

Knock can also be controlled by limiting the amount of spark advance. On engines with a fixed spark advance curve, the advance is normally designed with a safety margin to limit total advance at a point before the knock limit is reached. Because the knock limit depends upon fuel quality, as well as engine and environmental conditions, the spark advance is normally retarded more than necessary to maintain an adequate safety margin. The result is increased fuel consumption and reduced performance.

This disadvantage could be avoided if the knock limit were determined continuously during operation. The ignition advance then could be continuously adjusted, in a closed loop operation, just below the point where knock begins. The only problem is, how do we let the control unit know when the engine has begun to knock?

In 1880, Jacques and Pierre Curie made a discovery regarding the characteristics of certain crystalline minerals. The crystals became electrically polarized when subjected to a mechanical force. Tension and compression generated voltages of opposite polarity that were in proportion to the applied force. The converse of this relationship was also confirmed. If a voltagegenerating crystal was exposed to an electric field, it lengthened or shortened according to the polarity of the field, and in proportion to the field strength. These behaviors were labeled the piezoelectric effect and the inverse piezoelectric effect, respectively. Piezo is taken from the Greek word piezein, meaning to press or squeeze.

The magnitude of piezoelectric voltages, movements or forces are small and often require amplification to make them useful. A typical piezoelectric ceramic disc will increase or decrease in thickness by only a fraction of a millimeter, for example. Despite these limitations, piezoelectric materials have been adapted for use in a wide range of applications, including the subject of our discussion: the knock sensor.

The knock sensor consists of a piezoceramic ring, a seismic mass and contact electrodes. The complete unit is attached to the engine block at an appropriate location. The knock sensor is accelerated due to engine vibrations, causing the seismic mass to generate a force to the piezoceramic ring. The piezoceramic ring generates an electrical signal which is proportional to the vibrations in a specific frequency range. If the engine starts to knock due to low octane fuel or other operating condition changes, the knocking signal is detected by the PCM and the ignition timing map is adjusted accordingly.

Two major knock sensor designs are used today: broadband single wire and flat response two wire knock sensors. Both sensor designs use piezoelectric crystals to produce and send signals to the PCM. The amplitude and frequency of this signal varies, depending upon the vibration levels within the engine. Broadband and flat response knock sensor signals are processed differently by the PCM.

We will dig deeper into knock sensor design and operation in the next Counter Point. We also will share some valuable information on engine management system strategies and knock sensor diagnosis.