Rotax 914 Turbo Charged

[skippydiesel]

In fairly recent times (10-15 years) turbo charged ground dwelling engines, have had their inlet pressure regulated by engine computer management (ECU).

 

Before the ECU turbo pressure was regulated by a simple mechanical system, the wastegate. The wastegate is a fairly simple and trouble free system that, for the most part, operates without driver/operator input - KISS!

 

Dont know how long the Rotax 914 has been around but it has a ECU managed turbo pressure system - why?

 

Before  914 supporters jump down my neck on this,  I should say I support the improvements to power, efficiency and pollution reduction, that the ECU management of fuel injected/turbo charged engines has delivered

BUT

The 914 is a carburetted engine ie the ECU has little ability for direct control the fuel input side of the engines performance, unlike an fuel injected engine.

AND

The aftermarket turbo conversions of Rotax 912 (carburetted versions) seem to use the tried and true mechanical wastegate turbo pressure control system.

 

My question is;

 

• What significant benefit does the ECU bring to the Rotax 914 that a simple (lower cost?) wastegate could not do almost as well???

😈

 

 

[facthunter]

Why worry?  Boosting THROUGH a Carby is a BAD idea. Suitable air/fuel ratio must be available through the range so it must be regulated. The THROTTLE restricts the Mix and therefore the Power when not fully open. A turbo Needs exhaust Heat and pressure to function.  Nev

[BurnieM]

https://www.flyrotax.com/products/914-ul-f

 

I am confused, Rotax says that the 914 has an automatic wastegate and electronic ignition, so no ECU.

 

Edited November 17 by BurnieM

[skippydiesel]

4 hours ago, BurnieM said:

https://www.flyrotax.com/products/914-ul-f

 

I am confused, Rotax says that the 914 has an automatic wastegate and electronic ignition, so no ECU.

 

I stand to be corrected;

 

 "Rotax 914 is watching only 7 channels that are needed to monitor the engine. 

1,RPM, this is obvious.  It also records this and reports overspeed events. 

2,Throttle position, a key element to know for the MAP readings. 

3,Ambient pressure is another, this is needed to know the delta to the pressure within the charge air (airbox pressure)

4,airbox pressure is another and this has limits for reasons such as avoiding detonation. 

5, time stamp, important to track events for failures and diagnostics.  The chip will hold lifetime of peak events (such as overboost events) with a timestamp for example.  It also tracks overspeed such as when you underload the prop and RPM exceeds the limits.  Both overboost and overspeed will trigger alarms to the pilot via a lamp.  Solid red for overboost, flashing red for overspeed, both are serious issues.  Another lamp, orange, is a warning when a sensor shows failure or out of range as in over temp of the airbox. 

6, servo position is also tracked to know if your control is functioning correctly with the MAP you are seeing. 

7, boost time is recorded and events that are over limits are recorded in the event of a failure for diagnostics.  

8, airbox temperature.  This one is super important as high temps lead to detonation.  A type 914 can operate without a intercooler however it is limited to a max of 90C air box temperature.  Should it exceed that the TCU will back off the boost to save the engine from failure.  There is a timer on the amount of boost pressure for the same reason, 5 mins max at WOT and 41 inch HG, over that it will back off.  

In addition this chip holds the lifetime peaks and also the last few hours of full data for all channels.  That is a rolling memory however and it over writes the old data as you fly leaving only the peak values.  (timestamped to the TCU from first use)"

 

From Rotax Owners Forum 

[Thruster88]

So looking at the list of sensors used and their function it should be obvious why Rotax did not use a simple mechanical waste gate. 115hp from only 1211cc at only 5500rpm is a relatively high specific output.

[BurnieM]

https://en.wikipedia.org/wiki/Rotax_914

Wiki says it was introduced in 1996.

 

A little perplexed why they kept the carbs as I would have thought at throttle body with a multi-function ECU would have been the way to go.

This would probably have required a lot more sensors but ECU based systems were not unknown in 1996.

 

[skippydiesel]

2 hours ago, Thruster88 said:

So looking at the list of sensors used and their function it should be obvious why Rotax did not use a simple mechanical waste gate. 115hp from only 1211cc at only 5500rpm is a relatively high specific output.

Please state the "obvious"😈

[skippydiesel]

11 hours ago, BurnieM said:

https://en.wikipedia.org/wiki/Rotax_914

Wiki says it was introduced in 1996.

 

A little perplexed why they kept the carbs as I would have thought at throttle body with a multi-function ECU would have been the way to go.

This would probably have required a lot more sensors but ECU based systems were not unknown in 1996.

 

As I understand it the Rotax 914 is a 30 year old design - the ECU or in the case of the 914 a Turbo Control Unit (TCU) was in its infancy back then. Computer controlled electronic injected fuel delivery was still a long way from the every car/motorcycle/even ride on lawn mowers that we have today.

 

What I don't understand is why a TCU, when mechanical wastegate turbo pressure control, was at the time, a mature technological concept.😈

[Thruster88]

On 18/11/2025 at 9:11 AM, skippydiesel said:

Please state the "obvious"😈

So i see you asked this same question on the rotax forum and dismissed the  answers as is your way. I will not try to convince you that Rotax engineering chose to use electronic control to provide maximum performance while maintaining engine protection from detonation.

 

Some questions for you for you. Do diesel engines detonate?  What happens with the performance of a diaphragm type actuator used on a simple turbocharger waste gate system if such a system climbed to 16,000 feet? Why do many turbo diesel engines not even have a waste gate?

[Blueadventures]

1 hour ago, Thruster88 said:

So i see you asked this same question on the rotax forum and dismissed the  answers as is your way. I will not try to convince you that Rotax engineering chose to use electronic control to provide maximum performance while maintaining engine protection from detonation.

 

Some questions for you for you. Do diesel engines detonate?  What happens with the performance of a diaphragm type actuator used on a simple turbocharger waste gate system if such a system climbed to 16,000 feet? Why do many turbo diesel engines not even have a waste gate?

Saw same this afternoon when popped up on Face book, have to say that I absolutely agree with the content of some of the replies.  Like me at times; over the tone of challenges to informed information shared.

[skippydiesel]

12 hours ago, Thruster88 said:

So i see you asked this same question on the rotax forum and dismissed the  answers as is your way. I will not try to convince you that Rotax engineering chose to use electronic control to provide maximum performance while maintaining engine protection from detonation.

 

Some questions for you for you. Do diesel engines detonate?  What happens with the performance of a diaphragm type actuator used on a simple turbocharger waste gate system if such a system climbed to 16,000 feet? Why do many turbo diesel engines not even have a waste gate?

Not so much "dismissed" as poked holes in Rotax doctrine.

 

Detonation in diesel engines -Good question. Your first to articulate a possible problem with my argument. By definition the diesel compression ignition is a detonation - that is the principal reason why diesel are built to more rugged (heavy) specifications, than petrol spark ignition, lower compression engines.

 

Diaphragm actuators - No expert but would expect that they only operate between certain parameters, in this context atmospheric pressure. 

 

Climb (function) to 16,000' - As above - I don't know.  The naturally aspirated 912 carburettors (diaphragms) are, I have heard, operating at their efficient limit at about 8,000' - obviously they can go much higher however the air:fuel ratio may be increasingly outside optimum. Speculation - Given the ability of the naturally aspirated engine to get into the pilot O2 levels, I would expect the turbo varient with mechanical wastegate to achieve at least 16,000'

 

Turbo charged engines without wastegates - In my experince such engines are operating at quite low boost pressures ie the turbo is small and the inlet/ignition pressures generated are within the structural capacity of the engine . Speculation - engines of this type are most often found in constant rpm service eg hydraulic pumps/ generators, etc where peak rpm/performance is a near constant. 

 

Wastegate 

 

The wastegates overriding function is to to prevent over boost, while maintaining sufficient inlet pressure for optimum (?) performance throughout the rpm range.

Over boost is likly to cause severe fuel detonation, excessive heat and combustion pressures, which will destroy the engine.

In an aircraft the wastegate also acts to  maintain the ability of the turbo to deliver required "boost" as the atmosphere thins at altitude. This boost can be to maintain sea level air density to the combustion process (turbo normalised),  and above sea level pressure (increased power/"Boosted", as in 80 hp delivering 115 hp. 

.

 

[Thruster88]

Think of the simple basic diaphragm type waste gate actuator in this way, on one side we have boost pressure, on the other there is a spring and ATMOSPHERIC pressure. I don't think this will work correctly at altitude. 

 

Recently I removed a turbocharged engine and all components from a Cessna 414. The wastegate is operated by regulated engine oil pressure. Why did they go with this complicated system and not a simple wastegate gate. It is called an absolute controller, designed to work at altitude. 

 

Control at altitude and engine protection is the reason Rotax 914 has the more advanced wastegate control system.

[skippydiesel]

"Think of the simple basic diaphragm type waste gate actuator in this way, on one side we have boost pressure, on the other there is a spring and ATMOSPHERIC pressure. I don't think this will work correctly at altitude." 

 

Simplicity does not necessarily equate with inefficiency or lack of precision. Humans have along history of overcomplicating solutions.

 

No offence, you are clearly wrong - a mechanical wastegate will adjust inlet pressures, as atmospheric pressure reduces/increases. 

 

The question really comes down to the precision and range of the mechanical wastegate V TCU.

 

It would seem that at least two of the 912 aftermarket turbo upgrade suppliers, use a mechanical wastegate. I assume that in this, their product is successful/reliable. If it isn't they would  have gone out of business very quickly.

 

"Recently I removed a turbocharged engine and all components from a Cessna 414. The wastegate is operated by regulated engine oil pressure. Why did they go with this complicated system and not a simple wastegate gate. It is called an absolute controller, designed to work at altitude. "

 

Sorry cant comment - don't know what is meant by ".... wastegate is operated by regulated engine oil pressure" in this context.

 

One would certainly hope that in going with "... this complicated system and not a simple wastegate gate. " they achieved some benefit over a mechanical system.

 

It may be that an aircraft turbo system  having to perform over such a large atmospheric pressure range, eg  sea level to 35,000ft,  the mechanical may not be able to equal the TCU

In this speculation, I would still wonder why Rotax didnt offer  a cheaper mechanical option, with say a 20,000ft ceiling.😈

 

Edited November 20 by skippydiesel

[facthunter]

A Hamilton Standard Hydramatic prop is" Moved" using engine oil pressure but it's regulated by a Governor.  It's just a convenient source of Oil. There is a feathering reserve, in case the engine Has Lost Oil Pressure.. . Nev

[BrendAn]

 

1.  No offence, you are clearly wrong - a mechanical wastegate will adjust inlet pressures, as atmospheric pressure reduces/increases.

                                                                                                                                                                                                                                                                                                                             2. it may be that an aircraft turbo system  having to perform over such a large atmospheric pressure range, eg  sea level to 35,000ft,  the mechanical may not be able to equal the TCU

In this speculation, I would still wonder why Rotax didnt offer  a cheaper mechanical option, with say a 20,000ft ceiling.

 

 

which is it skip. you appear to be contradicting yourself in your quest to prove others wrong.😁

[skippydiesel]

2 hours ago, BrendAn said:

 

1.  No offence, you are clearly wrong - a mechanical wastegate will adjust inlet pressures, as atmospheric pressure reduces/increases.

                                                                                                                                                                                                                                                                                                                             2. it may be that an aircraft turbo system  having to perform over such a large atmospheric pressure range, eg  sea level to 35,000ft,  the mechanical may not be able to equal the TCU

In this speculation, I would still wonder why Rotax didnt offer  a cheaper mechanical option, with say a 20,000ft ceiling.

 

 

which is it skip. you appear to be contradicting yourself in your quest to prove others wrong.😁

Not at all.

 

A turbocharges mechanical wastegate performs as I have described. I merely speculated that it may have a ceiling limit (I don't know what that night be).

Knowing that the naturally aspirated 912 ,can (with the right airframe) get into the mid teen altitudes, I further speculate that a turbocharged varient, with a mechanical wastegate, may get to 20,000ft (again I don't know).

I suspect that few small aircraft pilots are intersted in cruising at altitudes requiring O2 supplementation. This may change for plots wishing to cross the high mountain ranges of the World.

 

So logical speculation;

• For pilots who are looking for enhanced (115hp) sea level performance up to at say 12,000ft -  turbo with mechanical wastegate may be quite acceptable and attract a lower acquisition cost.
• For those few pilots who want to top Mt Everest etc the TCU may be an attractive proposition along with pressure suite & O2 -$$$$$$$$

I am not trying to "prove anyone wrong" just asking for an answer to a question - Why does the carburetted Rotax 914 have a sophisticated/complex TCU, when its highly likly a mechanical wastegate would have done the job  - Yet to get an informed response 😈

[skippydiesel]

From the Rotax Owners Forum;

 

"In 1993, the Rotax 912 engine was modified and equipped with a turbocharger for an altitude flight test. The airplane (HK36 Super Dimona model) reached an altitude of 33,000 feet (approx. 11,000 m). The concept was successful. The company started the development of the turbocharged Rotax 914 engine with 115 hp, and serial production began in 1996."

 

It would seem that the desire to achieve high altitude flight, may have been the reason for selecting a TCU, rather than a mechanical wastegate.

 

I am assuming that a mechanical wastegate would not have the "range " to respond to sea level - 33,000ft changes in atmospheric pressure.😈

[Thruster88]

Yes the main purpose of turbocharged aircraft engines is to maintain manifold pressure and power at altitude to clear mountains, weather and or have a much higher True AirSpeed, TAS.

 

I doubt your simple mechanical diaphragm actuator wastegate has any range. As you climb reduced atmospheric pressure acting on the spring side of the diaphragm will cause the wastegate to open when we need it to be closing. 

 

All aircraft turbo systems have electronic or an absolute controller or they get the pilot to manually close the wastegate( and hopefully not over boost the engine)

[skippydiesel]

"..... the main purpose of turbocharged aircraft engines is to maintain manifold pressure and power at altitude ..."

 

I think this is well understood by all.

 

The thrust of my curiosity, is not the reasons for using a turbo charger or how it works but why was a simple wastegate was not used for the 914.

 

Without anyone actually stating the reason, it seems likly that the (Rotax designers) desire for what is an extreme altitude (33,000ft) for the vast majority of recreational level aircraft, is that reason. Success in this venture, carried over into the production engine.

 

I think it highly likly (speculation) that a mechanical wastegate system could have a Rotax 9 operating to 20,000 ft.

 

Comparisons with Turbo  LyCons & similar is questionable. Remember the Rotax 9's use an automatic diaphragm carburettor mixture control (no pilot input)- as far as I know, no legacy aircraft engine does this.😈

 

 

[Thruster88]

6 minutes ago, skippydiesel said:

"..... the main purpose of turbocharged aircraft engines is to maintain manifold pressure and power at altitude ..."

 

I think this is well understood by all.

 

The thrust of my curiosity, is not the reasons for using a turbo charger or how it works but why was a simple wastegate was not used for the 914.

 

Without anyone actually stating the reason, it seems likly that the (Rotax designers) desire for what is an extreme altitude (33,000ft) for the vast majority of recreational level aircraft, is that reason. Success in this venture, carried over into the production engine.

 

I think it highly likly (speculation) that a mechanical wastegate system could have a Rotax 9 operating to 20,000 ft.

 

Comparisons with Turbo  LyCons & similar is questionable. Remember the Rotax 9's use an automatic diaphragm carburettor mixture control (no pilot input)- as far as I know, no legacy aircraft engine does this.😈

 

 

I think i did state the reason why a simple wastegate system is not used in my last post. Here is a picture to make it clear.

 

All Rotax, Continental and lycoming engines have pistons valves and throttles and they all fly in the same atmosphere. No magic for any engine or airframe.

Edited November 22 by Thruster88

[BrendAn]

18 minutes ago, skippydiesel said:

"..... the main purpose of turbocharged aircraft engines is to maintain manifold pressure and power at altitude ..."

 

I think this is well understood by all.

 

The thrust of my curiosity, is not the reasons for using a turbo charger or how it works but why was a simple wastegate was not used for the 914.

 

Without anyone actually stating the reason, it seems likly that the (Rotax designers) desire for what is an extreme altitude (33,000ft) for the vast majority of recreational level aircraft, is that reason. Success in this venture, carried over into the production engine.

 

I think it highly likly (speculation) that a mechanical wastegate system could have a Rotax 9 operating to 20,000 ft.

 

Comparisons with Turbo  LyCons & similar is questionable. Remember the Rotax 9's use an automatic diaphragm carburettor mixture control (no pilot input)- as far as I know, no legacy aircraft engine does this.😈

 

 

So what do you hope to achieve. 

Do you want Rotax to make a special mechanical wastegate for you.

Are you intending to buy a turbo Rotax.

 

 

 

[BurnieM]

Yep, a 916 when I win the Lotto next week.

Oh, not talking to me ?

 

[BrendAn]

6 minutes ago, BurnieM said:

Yep, a 916 when I win the Lotto next week.

Oh, not talking to me ?

 

Spare change for you anyway.😁

[facthunter]

Reaching 33,000 has no practical application. It was a Publicity STUNT. Also the CV Bing Carby is NOT specifically designed for Aircraft. It's CHEAP and not properly altitude compensated. Pressurising through it can't help either. You can't beat a properly designed Mixture Control and you can shut down an engine with it much more cleanly than with switches.  Mechanic superchargers Don't have wastegates. They have 2 speeds for the supercharger. A wastegate failure Means a lot of Power loss. Nev

[skippydiesel]

49 minutes ago, BrendAn said:

So what do you hope to achieve.  Satisfy my curiosity - Knowledge is no load to bear

Do you want Rotax to make a special mechanical wastegate for you. No

Are you intending to buy a turbo Rotax. No again

 

I had assumed that the 914 used a mechanical wastegate, to modulate inlet air pressure - some posts on the Rotax Owners Forum referring to "lanes" (computer talk) got me intersted - simple as that, no agenda, ill intent or sinister objectives

😈