Inductively sense aircraft engine valve position

-April 25, 2014


Presenting one of the runners-up in the TI LDC1000 inductive sensor design contest.

This design uses an inductor to sense the position of a valve on an aircraft engine as a way to get camshaft position information. Knowing the camshaft position along with the crankshaft position allows fuel injection while the intake valve is open and the use of individual coils for each cylinder.

Benefits of Design
All the available designs I have found for aircraft ignition systems use a single coil for two spark plugs, including a system I designed more than 10 years ago. This design only requires crankshaft position, as it fires one cylinder on compression at the same time as the other cylinder is on the exhaust stroke. There are a couple disadvantages to the coil for two plugs. The waste spark on the exhaust cylinder requires extra voltage. The polarity of the two spark plugs is opposite by design. A spark plug should have a negative polarity applied to the center electrode which runs hot due to the thermal isolation of the insulator and the plug design. The hot center electrode prefers to emit electrons. The plug will spark at a lower voltage with a negative polarity on the center electrode. The last disadvantage of the dual plug coil is that a single coil failure affects two cylinders.

For fuel injection systems, the best designs only spray fuel in the intake when that cylinder is on the intake stroke. Gasoline has many different chemicals with different vapor pressures and if the fuel and air mixture is present in the intake manifold for even one revolution some of the fuel will condense on the manifold walls, requiring richer mixture than needed for combustion. Most modern cars use timed injection, at least during lower RPM ranges. Aircraft engines are designed to run at low RPM – most redline around 3,000 RPM – and would benefit from timed injection

Sensor Design
The sensor is designed to protrude through the valve cover. Most aircraft engines have individual heads and valve covers. My first design was to use a ferrite core shaped like a "C", placed such that the rocker arm would complete the "C", increasing the inductance. This design had three drawbacks. There is little clearance inside the valve cover, so the ferrite had to pass through the sheet metal cover. The losses from the currents generated in the cover used much of the available sensor “proximity” range (the rocker arm is made from cast iron, which has poor permeability). And, the shape of the rocker arm is a triangular cross section, exposing little area to the ferrite core.

The prototype that came next senses the position of the top of the valve spring assembly. The top of the valve spring is further from the cover, decreasing interference from it. The assembly can be done with a tube which makes it much easier to seal where the sensor passes through the cover. The sensor can be placed closer to the valve spring assembly because the height of the assembly is well determined by the valve stem height and seat where the rocker arm has lifter clearance that can allow extra movement.

Function of the Sensor
Mounting the completed sensor on an engine shows a substantial change in the raw proximity data, from just over 15,000, to about 19,000 with the valve up. There is also a significant change in inductance. If valve position is determined by the combination of inductance and proximity data, a very robust sensor will result.

Using the digital sensor with a controller allows the unit to adjust the threshold as the engine wears, or even as it warms up, providing a very robust sensing system.

Further work
The winding of the sensor was not optimal. It was wound around the plastic tubing, and extended about a quarter inch up the tube. With a small winding holder, the entire winding could be located at the end of the sensor, which should improve the sensitivity.

My prototype was sealed with silicone sealant; using a captured O-ring would be preferred.

Integration into systems
Some time ago I designed a crankshaft triggered ignition system for small aircraft. At the time, I experimented with single coils for each spark plug but never found a satisfactory way to determine which of the two coils to fire. I actually prototyped a single coil system, disabling the undesired output with an AND gate. Such a change is easily accomplished with a small extra board outside the existing system.

While I have not designed a fuel injection system at this time, the same idea of disabling an output would work, requiring extra output drivers.

Potential Issues
The main concern has to do with how the system could cause problems for the engine.

Oil leaking at the sensor location is possible; using a tubular sensor can minimize this possibility because you can make a captured O-ring seal. Using an O-ring seal also allows the valve to easily push the sensor to a higher position if the clearance should decrease for any reason. The crankcase is low pressure, which decreases the chance of oil leaks.

The sensor breaking off would cause problems, but locating it in the valve cover area reduces the chance of serious damage. The pieces would have to fall through a pushrod tube to get to the crankcase, and at that point they would be away from the gears at the engine end, most likely falling into the sump. Constructing the sensor out of plastic makes it less likely to damage other engine parts.

If the sensor fails, the drive circuitry could default to a safe mode, with extra sparks and/or injection pulses to keep the engine running until a safe landing could be performed. Aircraft engines almost all have two ignition systems as well, decreasing the chance of an ignition failure causing the engine to stop.

Sensing Design Challenge 2013 winners:

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