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. 😈