Starting a engine on electronic ignition.
 

I have heard that starting on a electronic ignition is not a good idea. On my RV-4 I've been told to always start on the mag with the electronic ignition turned off and then switch it on after engine is running. (I'm running the ElectroAir system). But in the Emag/Pmag setup they recommend starting with the electronic ignition on. Why the difference? And also it sounds like some problems with the emag/pmag may have been during startups, so why not just start the engine on a normal mag and then switch over.

My last thread on questions on electronic ignitions got stopped short. Just looking to get some questions answered.

Chris.

It depends....
 

A good thing about electronic ignition is the good strong spark for starting. But timing is critical. An electronic ignition that does not retard the spark to TDC for starting is going to cause a kickback and break the starter.

So, conventional wisdom said its better to start on an impulse-coupled mag.

But some of the newer, better EI have a built-in spark retard for starting. The P-mags and the LiteSpeed ones do. Now you get the best of both worlds, a spark that is good and strong, and retarded to TDC for starting.

It is possible under very low voltage available to have the spark retard feature in the EI get disabled. This would be bad. You would have to have a fairly weak battery that the voltage would drop a bunch during cranking to cause that.

Which is almost certain to happen at some point in the life of the airplane -hence the advice to start on an impulse-coupled mag and switch the EI on after start. Batteries get old and die slowly; master switches get left on; winter days make for hard cranking. Look at the facts and play the odds.

The Emag/Pmag have a (programmable) "Run Mode Start Delay." This disables the coils until the engine has turned through a chosen number of blades, typically 4.

My understanding for the reason behind this is that the Emags, like most EI setups, are "wasted spark" which means they fire every time the timing magnet comes around, not just on the compression stroke. This means they will fire sometimes with the intake valves open, and may ignite any fuel vapors in the intake manifold/sump.

Turning the engine over a few times will clear any vapors out so you don't blow up your airbox.

It's a mixed bag. If you have dual EI's then obviously you have no choice. Given a choice, I'd setup to start on an impulsed mag every time, because I would always have one mag, and it makes sense that should the EI fail, you have the option of starting the engine. Non-impulsed mag, no option. Also I don't think i'd trust a EI in any kind of hand proping situation... which is something to think about.

The issue here is how do EIs know when to spark? Most count from a point before the furthest advance that will ever be required, based on engine rotational speed (rpm). Say the most spark advance that the EI will ever select is 40 deg, it might start counting at 50 deg, or even 60 deg.

When starting it is essential that the spark occurs at TDC or later to avoid kickback. So an EI that counts will have to count for 50 or 60 degrees of rotation to ensure the starting spark occurs at TDC. If the battery is weak, and compression is strong, and its not your day, it is possible for the engine to slow down enough as the compression rises for the counting process to get to TDC before the engine does. So the spark occurs too early and you risk a kick-back, and potentially a broken starter, etc. So the advice has been to start with the EI off. I'm sure that people like Electro air and Litespeed have improved their algorithms, but I understand that's how they work.

P/E-mags work on a different principle in that they measure the rotation position of the engine many times per revolution, so they don't have to count. They guarantee that no sparks will occur before TDC (and do a few other things to enhance starting). So they are recommended to be on for starting.

There's probably more to it than that, but it gives the general idea.

Pete

Dual P Mags Start Great
 

I can tell you that my dual p-mag IO-360 has been an easy starter since day one.

Hans

Some people add a very small isolated battery just for the EI (uses a diode so the small battery will charge when ship's power is available, but will not discharge otherwise). This ought to give you the best of both worlds.

I have one mag and a Lightspeed EI. I always start with both ignition sources. No problems to date. In fact, one thing i really like about the EI is the ability to put out a good hot spark, which helps starting, especially in cold weather or on a hot engine re-start, such as during a bunch of young eagle flights.

Steve Ciha

I have dual Lightspeeds and it starts just fine with the starter and it also starts fine hand propping it. No kick back at all.