Turbo Charging

Hi Jeff,

What I am trying to understand is, why would Rotax go the computer rout, with all its additional cost, when a perfectly satisfactory, simpler, presumably lower cost, mechanical system exists?

I don't doubt the refinement that the ECU may bring but this is like the afterthought, bolt on pollution mitigating solution, so prevalent in the automotive world.

It makes sense to me to have an ECU for a fuel injected turbocharged engine but not for the old tech carburetted engine.😈

Hi All...

The biggest difference with the Rotax engine, 914 in this case, is the fact that you have a turbo boosted engine and not turbo normalized.  In most general aircraft they just run it to sea level pressures to maintain power.  In the case of a 914 it runs the pressure over seal level.  Standard pressure is 29.92 HG (at 59 F) for standard reference.  The non turbo will always be lower pressure manifold that what ever standard is.  So we see a 912 Rotax without turbo, in the carb version, is 80 HP at sea level.  With the 914 we run 39 to 41 HG at WOT take off (and ideally 5800, for 115 HP.  Max continuous power is 33 HG.  The only way to really see maximum performance benefit is with a constant speed propeller.  

To chase these numbers with a mechanical waste gate is possible but not ideal.  The major problems come up in that you must deal with the constant change in engine loading with altitude.  As the air gets less dense so does the prop loading on the propeller, you have to add pitch.  Any slight changes in pitch also create loading issues on the propeller.  The simple control on the 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) 

Yes, all this is completely unnecessary (but I would not fly one and try to manage all this on my own).  If you are conservative and limit your boost along and do not take risks sure you can make a manual cable function.  Rotax will never offer one however.  

Taking a look at the newer turbo offerings, 915 and 916, we get into extremes of management.  The injections systems and the boost pressures that vary much more than the carb engines are one thing.  In the case of the 915 and 916 it also manages the timing, yes they have active timing to limit detonation.  These engines has so much boost that they use forged rather than cast pistons as a note.  

The description in the maintenance manual heavy is actually very good and worth the read for those interested.  I include a sample from that MMH to see what a log file would look like.  It is nice to note that for the type 914 you can now download the latest BUDS reader and you will find a file that allows you to download log files from the 914 without any special dongle.  If you need that you can find it on Rotax Owner website for requirements to use it.  (serial cable and only works on windows PC however) 

Cheers

My friend RW,

You would make an excellent politician, versed in the art of not addressing the core point(s) of a question. Thank you for your dissertation on  the attributes of the 914's ECU while not really answering how it is significantly better than a mechanical wastegate.

In most cases, a wastegate is a relatively simple valve, controlled by a pressure actuator linked to the boost pressure of the turbo. The pressure the actuator is reacting to, is relative to atmospheric pressure ie as the atmospheric pressure drops (with altitude) so the wastegate valve progressively closes, preserving the set boost pressure.

The wastegate is held shut by a spring inside the actuator, but when the boost pressure exceeds the pre-set maximum, it compresses this spring, progressively opening the wastegate. This allows the exhaust gases through, so that they bypass the turbine, regulating its speed.

I can see no reason why a wastegate can not be calibrated to deliver a slightly higher than sea level boost, thus increasing horsepower (if so desired) while maintaining sea level pressure to altitude. The turbo itself (size/design) then becomes the limiting factor.

There is no need for the pilot/driver/operator to control the wastegate.

I accept that this may be a crude, rather than nuanced pressure delivery/response to throttle setting/ altitude, compared with the ECU, however it is more in line with the technology of the fuel metering system - carburettors.

You will note that I have not asked this question regarding the Rotax fuel injected engines, where the ECU can control both fuel delivery and turbo pressures.

Still waiting for a targeted answer😈

OK Sean

How about this...use the TCU to  avoid blowing up your engine.  The risks with manual wastegate control are very high in my opinion.  

Hope that helps

The system you propose, just a spring release one, would eliminate the max power and max continuous speed, you have to pick one.  The 914 at 115 HP max is far more than the 80 HP that the standard 912 develops.  Max conscious is more than double sea level pressure at 33 inch HG.  

Cheers