In the early days of radio communications there was a need for portable frequency measuring standards for transmitters. Resonating cylinders with movable end plungers were employed to perform this function. The end plunger was connected to a lead screw having a very fine thread similar to that used in a micrometer barrel. The micrometer barrel was calibrated directly in frequency. Some barrels were carefully designed to show variation in resonance frequency of one part in 10,000.
The method of making a measurement of frequency was to introduce some of the transmitter energy into the variable cylinder system. The screw would be turned to move the end plate until a resonance in the cylinder could be detected at a detector also coupled to the cylinder. The detector was usually a point contact whisker into a silicon crystal and its detected current was used to deflect the needle of a microammeter. The microammeter current would dip when a resonance was excited. A pointer on the barrel would show the frequency of the source.
The cylinder of an internal combustion engine with a moving piston is an ordinary case of a cylindrical microwave resonanter having a variable cylinder height. This height is related to the crank angle of the crankshaft of the engine through a simple trigometric function of this angle.
The concept of employing microwave energy in order to study the internal workings of a cylinder of an internal combustion engine is disclosed in the Merlo U.S. Pat. Nos. 3,589,177 and 3,703,825,(each of which is hereby specifically incorporated by reference) and the paper of Merlo entitled "Combustion Chamber Investigations By Means of Microwave Resonances", in IEEE Transactions on Industrial Electronics and Control Instrumentation, April 1970, Volume 1ECI-17, No. 2. The above-mentioned United States patents and the paper disclose the use of microwave energy in the combustion chamber of an engine as a diagnostic tool. A spark plug was modified to serve the function of a microwave probe. Radio energy was transmitted into and received from the cylinder while the engine was operating. Strong absorption of the input energy occurred at various times during the cycle. Strong symmetry was present in the microwave signal during the various strokes of the engine.
In the paper of W. G. Rado entitled "Characteristics of a Plasma Generated by Combustion in a Spark Ignition Engine", in the Journal of Applied Physics, Vol. 46, No. 6, dated June, 1975, the author performed bulk absorption measurements at microwave frequencies on a plasma generated by the combustion in a spark ignition engine. From these measurements the author estimated the charge concentration and characteristics of the space-charge region formed around the plasma probe.
In the paper of J. H. Lienesch and M. K. Kragge entitled "Using Microwaves to Phase Cylinder Pressure to Crankshaft Position", SAE Paper 790103, dated Mar. 2, 1979, the authors describe a phasing technique which involves the transmission of microwave energy through a sparkplug hole and into the cylinder of a motored engine. The authors observed the microwave resonances which were symmetric about a minimum volume of piston position. Resonances were recorded on an oscilloscope screen along with one degree pulses generated by the crankshaft.
The Wilson U.S. Pat. No. 4,331,029 discloses a method and apparatus for measuring the ignition timing of an internal combustion engine wherein angular position of the engine crankshaft is monitored while microwave radiation is injected into a selected engine cylinder. The angular position of the engine crankshaft at an apparent top dead center position of the piston in a selected cylinder is identified as a function of microwave resonances within the cylinder. The angle of piston top dead center position is then compared with an event corresponding with ignition of the selected cylinder to determine the ignition angle relative to the piston top dead center position.
Many of the ideas and concepts disclosed and claimed herein are described in the papers of Merlo entitled "Microwave Signal Analysis For Engine Diagnostics" Midcon/81 Professional Program Session Record #4 Vehicle Diagnostics; and "Detection of Missing Bearing Clips Using Microwave Resonances" SAE Paper 820484, dated Feb. 22, 1982, both papers of which are hereby incorporated by reference.