IntroductionThe ignition system has proved to be one of the more challenging aspects of the Humphrey Pump project, with the requirement to ignite a varying gas mixture in a combustion chamber which is being continually splashed with water. From the very outset a dual ignition system was used.
Ignition SourceA 'trembler coil' ignition source is provided by a pair of transistor switched high-tension automotive coils. Timing for the transistor switches comes from a pair of 555 timer chips. The switching frequency can be adjusted to give the optimum sparking power, too high a frequency and the sparking power decreases as the 'dwell' time for the low tension current recovery becomes too short.
The ignition system is housed in a wooden cabinet attached to the side of the main water tank and is shown in the photo below. The 555 timer chips are on the circuit board to the lower right. The relays on the lower left drive the valve interlock.
Sparking PlugsThe first attempt at producing waterproof sparking plugs was to reproduce the method used in the original pumps. This method uses a standard sparking plug, with the gap between the insulator and the outer body filled in with lead or solder and has the end ground flat. This was soon abandoned in favour of a sparking plug with a single projecting electrode and an earth electrode fixed within the combustion space. This methond brings the spark into the centre of the combustion space.
The photo below shows two of the several variations made over the years. The one nearest to the camera is being used in the most recent pump.
Ignition TimingWith no rotating parts, the ignition timing is derived from sensing pressure. One of the early designs used a diaphram sensor and an adustable microswitch. The drawback with this arrangement is that the peak pressure can vary considerably. Humphrey himself was aware of this problem and his original paper details mechanism to sense when the peak pressure has occured. The method currently used had a PIC micro controller sampling the voltage accross a potentiometer which is connected to a piston working against a spring in a cylinder. A peak is detected when 10 successive samples show a voltage reduction. an output from the micro controller then switches on the 555 timer chips for each of the ignition coils for a period of 1 second. Once a peak has been detected, the software has to detect a corresponding trough before detecting the next peak. It is often observed during pump operation that the mixture would ignite on the second peak.
The photo below shows the box housing the PIC electronics.