Sport Aviation, April 1990

ELECTRONIC  IGNITION
FOR  AIRCRAFT

By Klaus Savier
Light Speed Engineering
P.O. Box 549
Santa Paula, CA 93061

FOR years electronic ignition systems have been standard equipment on engines in cars, boats, motorcycles and lawn mowers.  Some military R.P.V.'s (remotely piloted vehicles) are reported to fly with electronic ignition systems.  We can marvel at magnetos in museums on antique cars and motorcycles.  On airplanes, they still feature the same problems they always had - hard starting, poor reliability at high density altitude and under high temperature conditions.  The fact that they operate independently of an electrical system is appreciated, but is no excuse not to replace one of them.  This situation retains an independent system and opens the door for a more sophisticated system to improve range, ceiling, power, efficiency and weight.  

In designing the optimum ignition system for aircraft, much attention was given to all parameters involved, except for cost.  The primary design goal was to produce a completely solid state system with all the features described later and to win the CAFE Race with it.  Several parameters are very important.  One of them is firing accuracy.  The points of a magneto are driven by several sets of gears which all have lash, but mainly it is the distributor portion which poses the greatest variation in timing.  If moisture is present in the distributor, arcing can occur.  At high altitude, cross firing occurs due to low density air and heat.  Carbon build-up and point wear can vary the timing considerably over time.  

Most high performance engines and some factory designs use "crank trigger" or "crank fire" distributor-less systems.  On a two cylinder opposed 4-cycle engine such a system has a magnetic pick-up coil mounted adjacent to the flywheel or prop extension.  A small bolt is mounted on the extension or flywheel in such a way that it passes by the pick-up coil and induces one electrical pulse for each crankshaft rotation.  This signal is amplified and processed in a specially designed processor before it is passed to the ignition system and from there to a dual ignition coil.  From the coil, one high tension lead goes to each spark plug.  Now both spark plugs fire simultaneously, one at the end of the compression stroke to start the power cycle and the other cylinder when it is at the end of its exhaust stroke.  This second spark is a waste spark, having no effect on function.  

For a four cylinder four cycle, the system described would simply be doubled.  The second pick-up coil has to be mounted 180 degrees apart from the first so that one pulse is generated every 180 degrees.  A four cylinder four cycle fires twice per revolution.  This system generates one pulse for each spark plug so that no spark distribution is necessary.  In a distributor system (electronic trigger or points) one set of points generates two pulses per engine revolution which drives one single coil.  Its high voltage output is distributed by a rotor to four terminals inside a distributor cap leading to four spark plugs.  

_____________________________

"Most aircraft engines
were designed in an era
when fuel savings was
not an issue."
___________________________

While a distributor system reduces parts and cost, it cannot be as accurate as a direct crank trigger system.  It is also more prone to mechanical and electrical wear.  A solid state crank trigger system should be completely maintenance free.  Other benefits such as reduced accessory load and reduced weight are certainly appreciated but are not significant.  

The stronger spark available from a capacitor discharge system, allowing a larger plug gap, greatly improves starting and ignition of leaner mixtures.  Higher spark energy produces a better flame front propagation and thereby burns cleaner.  Having the ignition occur at just the right time for the given condition can improve performance significantly.  The optimum ignition timing varies greatly with operating conditions.  Firing too early can cause engine damage while ignition just before detonation gives maximum power.  The point of ignition for optimum power varies with two parameters.  The main one is "charge pressure".  This pressure is a function of throttle setting, density altitude, pumping losses and compression ratio.  The last two are fixed and the previous two are expressed in manifold pressure.  If the manifold pressure is low, such as during idle or at high altitude in a non-turbocharged engine, charge pressure is low which means that the density of the gases in the combustion chamber is low.  Under these conditions, it takes the flame front longer to advance across the combustion chamber and to develop full pressure on the piston.  When the charge pressure is high, as during a sea level take-off, lots of air and gas are packed into the cylinder and after ignition the flame front spreads much faster. 

To avoid detonation in a "fixed timing" engine, the point of ignition has to be set for a worst case scenario such as full power take-off on a cold day from Death Valley with a hot engine.  On most engines, this is about 25 degrees B.T.D.C.  Flying the same engine at 10,000 feet and partial throttle would call for the timing to be at 35-40 degrees B.T.D.C. for best power.

Most aircraft engines were designed in an era when fuel savings was not an issue.  And they were installed in airplanes with such marginal performance that they would rarely be flown above 10,000 feet.  If they made it above that altitude, they most likely needed full throttle.

By contrast, we have some incredibly high performance homebuilts today which are often flown well above 10,000 feet where they need very little power for respectable speeds.  Many homebuilts with fixed pitch props cruise at 13-18" MP.  Under those conditions, optimum ignition timing is from 40-47 degrees B.T.D.C., respectively.  At such low MP, detonation would only occur at timing advances beyond the capability of the system and it is questionable if the excess pressures generated from detonation exceed those during normal full power operations.

_____________________________

"... we have some
incredibly high 
performance
homebuilts today
which are often flown
well above 
10,000 feet..."
_____________________________

One other variable changes peak effective pressure in the cylinder and requires a variation in timing for optimum performance: "mixture".  At stoichiometry (which is the optimum mixture of approximately 14.7 pounds of air to 1 pound of gasoline) flame front propagation is at an optimum.  If there is more or less fuel, the flame front will spread at a slower rate and subsequently more timing advance is necessary for optimum power.  

If the mixture is adjusted lean of stoichiometric, a power reduction occurs for two reasons: first, there is less fuel to generate power, and second, the pressures generated in the cylinder are delayed due to the slower flame front propagation.  The losses due to slower burning can be recovered by advancing the ignition timing.  Having the ability to adjust ignition from the cockpit allows manual fine tuning for any MP and mixture combination.  At altitudes above 10,000 feet, it is possible to lean to roughness and advance the timing until the engine smoothes out.

Flight test data from two airplanes indicate that a specific fuel consumption of around 3 lbs/hr/hp is achievable. 

At low MP operation, when mixture is lean of peak and timing is advances, all engine temperatures drop because very little fuel is being burned and the entire piston stroke is used for gas expansion which has a cooling effect.  EGT's drop since all fuel has been burned when the exhaust valve opens.  This condition can be demonstrated on the ground when advancing the timing reduces the exhaust noise so much that a propeller turning at only 500 rpm makes more noise than the pressure pulses emerging from the open exhaust pipes.  At altitude, when mixture is set on the lean side of peak and timing is advanced accordingly, cylinder head temperatures can reduce by 80 degrees F, oil temperature by 25 degrees F, and EGT by 70 degrees F.

It is also possible to tune for octane variations.  Octane number has a significant effect on flame front propagation with lower octane gasoline burning faster.  When the engine is operated near stoichiometric ratio and ignition timing is advanced significantly, turning the one remaining magneto on or off will not produce an rpm variation.  This indicates that the mixture is already burning when the magneto fires some 15-20 degrees later than those plugs operated by the electronic ignition.  Before take-off, when both systems operate at manufacturer designed timing, the mag drop is even.

The latest version of the new digital microprocessor controlled capacitor discharge system optimized timing electronically.  A bar graph in the cockpit indicates full advance by displaying a red bar.  Twenty inches of MP would display one half bar, and full throttle at sea level shows no light.  The bars can be labeled with inches of MP and then substitute a conventional MP gauge.  A manual switch is used to turn the automatic advance off.

For further information, send a self addressed stamped envelope and $5 to the address shown at the beginning of this article.

References

Power Secrets- Smokey Yunik, 1983

Kent's Mechanical Engineers Handbook, Eleventh Edition, "Power", 1938

A Practical Guide to Use of EGT and CHT Systems for Aircraft - G. F. Simpkinson