Patent Publication Number: US-2010127894-A1

Title: Magneto sensor for an aircraft ignition system

Description:
BACKGROUND OF THE INVENTION 
     General aviation piston powered airplanes use magnetos (MAGS) to generate the “spark” for the ignition of the engine. MAGS are basically a permanent magnet that sweeps by a coil and at the correct time an electrical switch called the “points” opens causing a high voltage (HV) pulse to be sent to the spark plug at the correct time for igniting a fuel air mixture in the cylinder. Aircraft use a MAG switch to control the MAGS. The MAG switch turns the MAG off by shorting the “Points” thus stopping the HV pulse to the spark plugs. Thus the MAG switch is a little unusual in that an open switch is MAG=ON and a closed switch is MAG=OFF. Many piston powered airplanes use two MAGS for improved reliability. If an aircraft is flying with two MAGS operating at say 2500 RPM and one MAG fails, the engine RPM drops to 2450 RPM and gets noticeably rougher. Thus it is easy for the pilot to detect something is wrong. 
     Technically advanced airplanes replace one MAG with an electronic ignition. These aircraft use one MAG and one electronic ignition. The MAG doesn&#39;t require electrical power to operate (thus making the aircraft electrical system non-flight critical) and the electrical ignition offers better fuel economy through vacuum advance and higher spark energy so there is good reason to set up the aircraft ignition with both systems. 
     When running the engine at 2500 RPM the electronic ignition is firing at 32 degrees before top dead center (BTDC) and the MAG is firing at a fixed 25 degrees BTDC. If the MAG fails in flight, there is NO sensory or other indication to the pilot that the MAG has failed. There is not an RPM drop or any noticeable engine roughness. 
     SUMMARY OF THE INVENTION 
     The present invention provides systems and methods for indicating status of a piston engine magneto (MAG). An example system includes a MAG sensing circuit that senses a signal from the MAG and an indication device that output an indication of the operational status of the MAG based on the sensed signal. 
     In one aspect of the invention, the MAG sensing circuit samples a signal from a P-lead of the MAG. The indication device determines if the sampled signal is below a predefined threshold value. 
     In another aspect of the invention, the indication device includes a field effect transistor (FET) that receives the sampled signal at its gate. The FET is placed in an “on” state if the sampled signal is above the predefined threshold value, and the FET is placed in an “off” state if the sampled signal is below the predefined threshold value. 
     In yet another aspect of the invention, the indication device includes a light that is activated when the FET is in the “on” state and is deactivated when the FET is in the “off” state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  is a schematic diagram of an example Magneto Status Sensor System formed in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of an example circuit used in the system of  FIG. 1 ; and 
         FIG. 3  illustrates instrument panel components formed in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A magneto (MAG) is controlled by a P-Lead. The P-Lead is a wire that comes from points of the MAG and is grounded by a MAG switch. When the MAG switch is open, the MAG points are not shorted out and the MAG produces a high voltage (HV) pulse to spark plugs when the engine is rotated. The HV pulse is a several hundred volt pulse when the MAG is operating. The present invention includes a circuit that senses the creation of the HV pulse and sends an indication to the pilot through a panel light and/or other system. 
       FIG. 1  illustrates a MAG circuit  20  that senses the status of a MAG. The MAG circuit  20  includes a MAG  22 , a spark plug  28 , a MAG switch  32 , a MAG status circuit  34 , an output light (green light)  38 , and/or other output devices  40 . 
     When a pilot throws a MAG toggle switch in the cockpit into an “on” position, the MAG switch  32  is placed in an open state thereby allowing the MAG  22  to output a high voltage pulse to the spark plug  28 . The MAG  22  includes a rotating magnet  24 , a charging coil  27  located adjacent to the magnet  24  and points  26 . As the magnet  24  rotates past the charging coil  27  at just the right time, the points  26  open and an HV pulse (e.g., 10 kV) is sent by the charging coil  27  to the spark plug  28 . 
     A P-lead  30  from the MAG  22  is connected to a first side of the MAG switch  32  and to the MAG status circuit  34 . A second side of the MAG switch  32  is attached to ground. When the HV pulse is sent to the spark plug  28 , the P-lead  30  outputs a much smaller voltage value (e.g., 200V). Once the MAG status circuit  34  senses the voltage at the P-lead  30 , the light  38  is activated indicating that the MAG  22  is fully operational. In another embodiment, the MAG status circuit  34  sends a signal to one or more different types of output devices  40 . The output devices may be used to store information associated with the signal received by the MAG status circuit  34  for later analysis of the MAG  22 . Or an audio enunciator (not shown) is activated to enunce “MAG Failure Detected”, a comparable message or non-textual alerts (such as beeps). 
     In another embodiment, the other devices  40  includes a flight/engine monitor computer, which would be configured to display a MAG failure message or alert icon on a screen. 
     As shown in  FIG. 2 , the MAG status circuit  34  includes a field effect transistor (FET)  50  and other circuit components for applying a relatively smoothed out voltage value from the P-lead  30  when the MAG  22  is operating properly. When the FET  50  receives a steady voltage value (e.g. 4V reduced from about 200V by the resistor and zener diode) at its gate, the FET  50  is placed in an “ON” state thereby drawing current from a voltage source (e.g., +14 volts), thereby activating the light  38 , such as a light emitting diode (LED). 
       FIG. 3  illustrates lights and switches located on an instrument panel for an aircraft having the MAG circuit  20 . A light  38 - 1  is preferably located proximate to a MAG switch  32 - 1 . The MAG switch  32 - 1  is turned on prior to starting the aircraft. At that time, the light  38 - 1  is not illuminated. Once the engine starts and the MAG is functioning correctly, the light  38 - 1  above the switch  32 - 1  illuminates. During flight if the MAG fails, the green light  38 - 1  will extinguish. 
     When there is no voltage value or a voltage value below a threshold amount at the P-lead  30 , the FET  50  receives a below threshold voltage value at its gate thereby operating in an “OFF” state. In the “OFF” state, the FET switch does not allow current to pass through the light  38 - 1  (i.e., not illuminate). 
     It is possible for the MAG to produce a nominal voltage and still be dysfunctioning. If this were to occur the components of the MAG status circuit  34  are selected to still place the FET  50  in the “OFF” state. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.