Rotax iS vs ULS

[Warmi]

I keep reading about 912 iS being a smoother running engine .... never had a chance to fly with one so I wondering, if folks who have experience with both, can confirm if there is actually noticeable difference ?

Thanks

[Mark Gregor]

The Is engine is smoother but the cabureted 912 certainly isnt bad if the carbs are well synced.
The nice thing about the Is is no need to sync the carbs.

Mark

[MrMorden]

The iS engine is very nice, but it's $3000 more than the ULS (current CPS pricing) and weights ~22lb more. The extra cash is probably not a huge deal when you are already spending $17k on an engine or $100k+ on a new airplane, but on an LSA 22lb can be significant. It's 3.5 gallons (45min) of fuel, or an extra overnight bag.

The 912ULS is a great engine and pretty easy to keep running, but the iS is probably the future direction Rotax is going, and it would not surprise me to see the carb'ed engines stop being made in 5-10 years. I just wish it were a bit lighter. For now, either engine will do great on an LSA or other small airplane.

[MrMorden]

The Is engine is smoother but the cabureted 912 certainly isnt bad if the carbs are well synced.
The nice thing about the Is is no need to sync the carbs.

True, but the iS has its own quirks. It needs dual electrical systems, because if if loses electrical power, the engine stops. And diagnosis of engine issues is more complicated, can require a special (expensive) Rotax ECU scan tool for some problems.

Each has pros and cons.

[drseti]

I concur with Andy. Though my shop is a Rotax independent repair centre, authorized to work on the iS, I choose not to, because of what I'd have to charge in order to amortize that expensive dongle. Besides, I already invested in the equipment to sync carbs.

[jetcat3]

Forgive my ignorance, but if a 912 ULS or 914 UL engine loses electrical power, what happens to the functionality of these carbureted engines? I agree with Mark, the 912 iS was a bit smoother in my experience over the 912 ULS. Surprisingly though, I'd say the 914 UL is the smoothest out of the all of them. Must be due to the turbocharger and lower compression ratio.

[FastEddieB]

Forgive my ignorance, but if a 912 ULS or 914 UL engine loses electrical power, what happens to the functionality of these carbureted engines?

Those engines runs independent of electrical power, so the engine will continue to run with full functionality.

From the ROTAX 912 Owners Manual:

The ROTAX 912 engine is equipped with a dual ignition unit of a breakerless, capacitor discharge design, with an integrated generator.

The ignition unit needs no external power supply.

Two independent charging coils located on the generator stator supply one ignition circuit each. The energy is stored in capacitors of the electronic modules. At the moment of ignition 2 each of the 4 external trigger coils actuate the discharge of the capacitors via the primary circuit of the dual ignition coils.

The 912is is different in that it requires "external" power to run its ignition, hence the extra weight and complexity of redundant systems.

[jetcat3]

Awesome, thanks so much!

[drseti]

The 912is is different in that it requires "external" power to run its ignition,

Which is one of the reasons that I tend to prefer the ULS.

[3Dreaming]

The 912is is different in that it requires "external" power to run its ignition,

Which is one of the reasons that I tend to prefer the ULS.

I think you guys are incorrect. While there is no doubt the 912iS is more complicated than the 912ULS, it doesn't use external power for its ignition. The 912ULS has a single stator with 2 poles that are used for the ignition. The 912iS has 2 stators one dedicated to engine operation and one to provide electrical power for the airplane. If the one for the engine fails it automatically switches to the other stator. Regardless all the electrical power to run the engine comes from the engine.

[FastEddieB]

I think you guys are incorrect.

Well, for me it certainly would not be the first time.

Do you have a source? I was really led to believe that the ignition n the 912is did require external power to run - as was Prof. Schuch, apparently.

edited to add: you are apparently correct and I stand corrected.

From the installation manual from the 912is:

"SYSTEM DESCRIPTION
The ignition unit operates with four double ignition coils with double ignition. If the LANE selector switch is in the "AUTO" position, the active LANE controls all four double ignition coils which are connected to the eight spark plugs. In normal Dual LANE Mode (Auto Mode), both spark plugs ignite each cylinder at the same time. In LANE A or LANE B Mode, only one spark plug ignites per cylinder.
The ignition unit does not require an external power supply."

[MrMorden]

The 912is is different in that it requires "external" power to run its ignition,

Which is one of the reasons that I tend to prefer the ULS.

I think you guys are incorrect. While there is no doubt the 912iS is more complicated than the 912ULS, it doesn't use external power for its ignition. The 912ULS has a single stator with 2 poles that are used for the ignition. The 912iS has 2 stators one dedicated to engine operation and one to provide electrical power for the airplane. If the one for the engine fails it automatically switches to the other stator. Regardless all the electrical power to run the engine comes from the engine.

My understanding is that it's not the ignition that requires external power, it's the fuel injection system. If no power flows to the injectors, no fuel = stopped engine.

[FastEddieB]

My understanding is that it's not the ignition that requires external power, it's the fuel injection system. If no power flows to the injectors, no fuel = stopped engine.

Eureka! I think you may be onto something.

Back to the manual when I have a chance, unless someone beats me to it!

[drseti]

I've yet to dig through the manual, guys, but I think Andy nailed it. Electronic fuel injection requires electrons. No power means no fuel (regardless of how much spark you have), resulting in...no power! (Tom, do you concur?)

[FastEddieB]

There was a crash of a Diamond Twinstar. I believe it had been started but had a very weak battery.

When the pilot raised the gear, that extra load on the electrical system caused a large enough voltage drop that the engines quit. Not sure whether ignition or fuel injection was involved, but since a running diesel engine requires no ignition source I suspect the latter.

As an aside, none of this discussion is meant to "diss" the 912is or electronic fuel injection in general. I think both represent the future and are obvious improvements over carburetors - especially the somewhat finicky BINGs on the regular 912.

But at this early stage for the 912is, it's naive to think there are no downsides. For me, beyond the weight, is the difficulty for owner maintenance of the system, and the current difficulty in finding a properly-equipped shop. Pulling a float bowl is pretty straightforward - having the right, and expensive, dongle at hand and the training to use it when there's a problem is somewhat less so.