Or, OBD, Ob-la-la, life goes on . . .
The EPA and the California Air Resources Board keep tightening the emissions noose on new cars; the next step, to be phased in starting with the 1994 model year, is a requirement known in the industry as OBD II -- On Board Diagnostics II. The law standardizes some aspects of the interface between engine control units (ECUs) and diagnostic equipement, requires that ECUs be smart enough to diagnose themselves for many defects, and -- perhaps most importantly -- requires that a car ECU shut fuel off to a misfiring cylinder, preventing super-high hydrocarbon emissions and preventing the catylytic convertor from frying itself permanently burning all the excess fuel and air being pumped to it. The question for the industry to solve was this: How can an ECU inexpensively and reliably detect a misfiring cylinder?
The brute force approach is neither cheap nor reliable: a pressure transducer screwed into the combustion chamber, housed in what might appear to be a second spark plug. Instead, car companies are looking at making the most of already installed sensors. It's possible, for example, to use the crankshaft position sensor already installed to handle fuel injection and ignition timing to look at how quickly the crank accelerates in the 30 or 40 degrees after an ignition event; measure the crankshaft speed over that distance precisely enough, and you can tell whether it's being kicked along by a firing cylinder, or is limping because of a misfire. If the misfire persists, it's a simple matter to shut the injection off for that cylinder, and light an idiot light on the dashboard. One benefit in this beyond what the EPA sought is increased engine life; the raw gas washing oil from the cylinder wall in a misfiring cylinder increases bore and piston wear.
The second approach is yet more elegant; in it, the ECU looks at the ignition voltage across the plug while the plug fires, just as mechanics sometimes hook oscilloscopes up to ignitions to diagnose problems. The ionized gases of combustion have much lower resistance than an unburned charge, so a sufficiently powerful ECU can use the spark trace to determine burn/no burn. To make this work reliably, it's necessary to mount the coil directly over the spark plug, eliminating the variable resistance effects of long coil wires, just as SAAB has done on its four-cylinder engines since 1990. In a further refinement of this techinque, a second spark is fired post-combustion; according to some researchers, it's possible to determine the residual oxygen in the spent gases remaining in the cylinder by the voltage trace, and thus open up the possibility of removing the oxygen sensor from the exhaust system.