Turbo Charging

• Formulas:
  $kW=(Torqu{e}_{Nm}\times RPM)/9549.3$
•  
  HP = (Torque ft.lbs x RPM) / 5,252

Rotax peak power is at 5800, only for 5 mins.  Above this you risk detonation and it is only for take off power normally. Max continuous is at 5500 RPM and in theory you can operate at this RPM all the time however fuel burn would be extreme and wear levels/temperatures would be near max. 

The 914 can achieve 115 HP at 5800 RPM but not at 5500, there it is only 100 HP.  A turbo with a auto-dump to control boost cant control those variables, you would have to select what would be the pressure you want to design it for.  The turbo size issue is far more complex.  There are a lot of places you can research papers on this subject.  The big problem for aircraft is the changing ambient pressures and finding an inlet that will work from sea level to normal flight altitudes.  Small turbos work great with sea level and drop off badly at relatively low altitude changes.  Large inlets will work great at high altitudes and perform very badly at low ones.  The basic design of the 914 was starting with a Garrett T25 turbo and then modifications to come up with a balanced inlet to outlet match.  The off the shelf turbos generally a good thing with autos, mostly fail long before TBO.  (wastegate failures due to extreme resonate vibrations, high shaft speeds at altitude leading to seal failures) 

Having worked on calibration issues from the early 914 I can assure you it is not as simple as a cable.  The Rotax philosophy of only building to a certified standard and then making a version available for experimental also plays into this.  Given that concept they would not consider a mechanical only system.  

The answer to your question is in the lines, sorry I can't just give you a one sentence answer.  Let us remember that the only option in the early 1990s was carburetors.  I however believe that within the next 5 to 10 years they will disappear from the products, just like they did on cars and motorcycles and most powersports products. 

Naturally you can disagree,  but to question a 30 year old decision from Rotax kind of seems a bit odd to me is all.  

Cheers

RW,

"Naturally you can disagree,  but to question a 30 year old decision from Rotax kind of seems a bit odd to me is all"

I am not disagreeing or agreeing, I am merely trying to understand why a TCU/ECU (computer) is used to control the wastegate of an engine whose design is 30 years old ( ECU/computer management systems around but in their infancy) and carburetted to boot. At the time most turbo engines (majority diesel) would have had mechanical wastegates (mature/proven technology).

 "Small turbos work great with sea level and drop off badly at relatively low altitude changes.  Large inlets will work great at high altitudes and perform very badly at low ones. 

My ground equipment understanding of turbo selection -

 # Small  diameter facilitates acceleration, initial power delivery at lower rpm, but looses performance at high rpm. (Small turbo unit may not need a wastegate)

 # Large diameter delivers slower pressure build but gets to higher pressure at high rpm. Larger diameter turbines will most always require wastegate control.

 # The terms "small/large diameter" is relative to engine size (cc) ie a small capacity engine will normally have a small turbo.

 # The matching of turbo performance (combination of turbine diameter & design) to desired engine performance (volumetric efficiency) is      critical and will fall between the above extremes.

 # I do not agree with your statement "Large inlets will work great at high altitudes and perform very badly at low ones" The pressure the turbo generates is controlled by the wastegate, releasing exhaust gas in response to inlet duct pressure /relative to atmospheric pressure. Of course selecting an over small/large turbo will present engine performance deficiencies. Select the right turbo and control its delivery of pressurises air to the engine inlet, will result in expected engine performance from sea level to altitude (?)

I would expect an aircraft turbocharger to be relatively large (compared with a vehicle engine of similar CC), to accomodate climbs to altitude ("thin" air). This means that the wastegate will be be somewhere close to fully open at sea level, when full power is demanded for TO/Climb and fully closed, near the limits of its effective altitude. This will be the same no matter what is controlling turbine inlet pressure.

The aircraft turbo being near fully open at sea level TO/Climb does not necessarily mean that this is a "turbo normalised" arrangement ie non turbo power maintained to altitude. It may be that the engine is benefiting from greater inlet air pressure (higher hp) that is maintained to altitude.

"The basic design of the 914 was starting with a Garrett T25 turbo and then modifications to come up with a balanced inlet to outlet match."

I assume the "balanced inlet to outlet match" you are referring to is pressure ie the ability of the exhaust driven turbine, to drive the inlet turbine, to deliver required inlet pressures??

 "The off the shelf turbos generally a good thing with autos, mostly fail long before TBO.  (wastegate failures due to extreme resonate vibrations, high shaft speeds at altitude leading to seal failures)"

I am not surprised that an "off the shelf turbo" designed for ground applications would be found wanting, they are designed for very diffrent engine applications. The turbo must, in the end be configured/designed for the application - ground diesel grunt, high performance sports car, race car or aircraft - they are not interchangeable HOWEVER the principal of function remains the same.

I am not yet convinced, that the combination of an appropriate turbo size/design, including calibrated mechanical wastegate, would not have delivered similar performance & durability, as the current Rotax 914 TCU delivers, but at significantly lower cost/complexity. 😈

The rational for the Rotax 914 using a TCU, instead of a simple tried & true mechanical wastegate, would seem to be "lost to the sands of time" - I guess I will never know the truth of the matter. Curiosity not satisfied. 😈

This is an interesting discussion! (I'm a long time forum lurker here).

What was the motivation for this unique product development??? ... There were probably a lot of factors but the decision at the time will likely remain a mystery until the day some retired ROTAX engineer writes their autobiography!

Have a look at this little article about blends of company management styles that I stumbled on the other day: https://itamargilad.com/product-discovery-models/

Where is ROTAX on  that spectrum, I wonder?

All: the excerpt is taken from the www.rotax.com web pages, history is there.  The reason for the 914 and its development was mostly due to desire from the glider aircraft (motor gliders are huge in Europe)  At that time it was the only way to get more than 80 hp from this size of engine.  The 100 hp, higher compression with bigger bore, was not in production until 1999.  

Rotax has always built to market demands for the most part.

Cheers

1989: A 4-stroke engine concept that prolongs the success of Rotax aircraft engines
Despite these developments, market demand for a modern, reliable high-performance engine increased in the 1980s. In 1985, the company began developing the Rotax 912; a project dedicated to the aircraft market. For the very first time, all conditions needed for an aircraft engine were considered, like high security, high-quality standards for airplanes, etc. In consideration of the experimental airplanes and gliders, the power-to-weight ratio was one of the main targets.

A major advantage in the development of the Rotax 912 engine was the opportunity to influence the whole design engineering. The Research & Development effort was impressive but the big advantage was the ability to supervise the project from concept to production.

"With the development of a flat four-cylinder engine, we wanted to reach the next level – the 80-horsepower engine category. The engineers of the 912 engine concept were pilots. They fully understood the market requirements. To put it in a nutshell, the engine was developed by pilots for pilots,” said Uhr.

Finally, in 1989, serial production began of the first Rotax 912 UL engine. The target was the JAR-22 certification of the responsible aviation authority (Austrian Type Certification), which was received in 1990. The Rotax 912 A was born; The ‘Type Certificate’ of the Rotax 912 F engine in the United States followed in 1994.

In parallel, BRP-Rotax worked on the national approval as a production and maintenance organisation. The approval was received from the national aviation authority in 1990. The Rotax 912 engine, with 80 hp and weighing only 56 kilograms, was quickly recognized in the gliding sector and substituted other heavy engines. The TBO was 600 hours when the series was launched, but is now just 2,000 hours.

Gradually, different aircraft manufacturers developed airplanes and installed the new Rotax 912 engine.

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.

The certification of the Rotax 914 F engine under the FAR 33 and JAR-E programs was obtained in the same year.

In parallel with the gliding sector, the ultralight (UL) – as well as the advanced UL – and the experimental airplane category further expanded. Due to the heavier take-off weight, more power was required.

At the same time, the General Aviation Revitalization Act was signed in the United States. It was intended to counteract the effects of prolonged product liability on general aviation aircraft manufacturers by limiting the duration of their liability for the aircraft they produce.

The development of the 100-hp Rotax 912 ULS engine began following market demand for more horsepower. The first engines for the ultralight market were delivered in 1999, followed by the certified version – the Rotax 912 S – also in 1999. More horsepower was one of the characteristics and the engine was intended for airplanes with a higher load capacity.

"Without any doubt, the Rotax 912 / 914 engines have substantially benefitted the light and ultralight aviation business. There is quite a large number of aircraft that were designed specifically for the Rotax aircraft engine series,” mentioned Uhr.

More than 50,000 engines of the Rotax 912 / 914 series were sold since 1989, resulting in more than 45 million flight hours of the fleet.

Important milestones for the Rotax aircraft engines business were the receipt of the European Aviation Safety Agency (EASA) Design Organisation Approval (DOA) in 2003 and the Production Organisation Approval (POA) in 2005. "We are very proud since a small number of organizations hold both approvals,” said Uhr.