A few weeks ago, a reader wrote in to ask about the use of detonation detectors on motorcycle engines. I wrote then about the microphone and accelerometer types of detector, but there is another, in some respects simpler, type that operates by detecting and analyzing ionization in the combustion chambers. Normally, the matter around us is electrically neutral (we don’t get shocks off everything we touch), made up of atoms consisting of a heavy central nucleus of positive protons and neutral neutrons, surrounded by shells of negative electrons, equal in number to the protons in the nucleus. The electrons are bound to the atom by the attraction of their negative charge to the positive charge(s) on the nucleus. But when something carrying sufficient energy hits an atom, it may dislodge one of its electrons. This atom or molecule, missing one electron, is called an ion. It has a single positive net charge. This being so, we can use an electric field to pull the ion in a desired direction. In any large collection of gas molecules, there are always a few such ions. They result either from thermal collisions (gas molecules are always moving on average at about the speed of sound, but there are always a few moving faster or slower—maybe fast enough to produce an ion or 2. The other source is cosmic rays—the mysterious atomic nuclei that come crashing down through our atmosphere with very high energies, leaving behind them dense trails of ions from all the gas molecules with which they have collided. It is spooky to see this on a suitable detector, and to know that such unseen particles are penetrating us all the time (to say nothing of solar neutrinos, which whiz right through the earth as if it were empty space). The hotter a gas is made, the higher its population of ions. This effect allows automakers to construct simple misfire detectors that use the engine’s spark plugs as their sensors. When the cylinder fires normally, the high temperatures reached produce ions that appear as a small current across the spark plug when it is biased with a small voltage (the bias voltage pushes any positive ions toward the negative plug electrode). When this current is detected, all is well—the cylinder has fired. But when there is a much smaller ion current, we can conclude that the cylinder has misfired, and our ECU’s software adds one to the misfire counter for that cylinder. When the owner or mechanic plugs in an engine error code reader, the dead or weak cylinder is reported. People examining spark plug ion currents in running engines noticed rapid variations; evidently, rapid pressure variations in the hot combustion gas were causing similar variations in ion current. And when engines detonated, there in the ion current record were the high frequency variations caused by the rapid wave reflections of detonation’s violent pressure rise. Here was a way to detect detonation without need of expensive microphones or accelerometers. All you had to do was analyze the waveform in the ion current across the spark plugs. For years, that was easier said than done. It would take fast circuitry, filtering to eliminate signals at the wrong frequency, and it would take knowledge of the frequency range of detonation in a given engine (the smaller the cylinder bore, the more rapidly the pressure variation would bounce back and forth). But today, digital circuitry on a chip can do this at modest cost. An example is Delphi’s Ionization Current Sensing Ignition Subsystem, which provides OBDII misfire detection and detonation sensing.