Rotax 914 power setting question

bones

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cant remember, didnt have log books before, but since over 3000
More manifold pressure makes more power which wears the engine out faster.

It is instructive to note that Rotax decided to limit 35inches to five minutes. This suggests to me that during some tests they found what they considered excess wear with too much boots for too long.
Vance correct me if I'm wrong but I was sure the 914 was max boost of 39" for 5 mins?

Edit; I went looking and found the answer myself and I was right, they are 39" some may get close to 40" for the full 5 mins, then the max 100% boost is actually 36"
 
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Vance

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I stand corrected.

I stand corrected.

I looked it up in the Cavalon pilot's operating handbook and the yellow arc is from 27 inches to 39 inches.

The maximum continuous manifold pressure is shown as 0 to 27 inches.

The message of my post doesn't change. Rotax felt what they considered high boost (39 inches) for more than five minutes was harmful for the 914 and limited it for that reason.

With a pilot controlled waste gate it is important to recognize that more boost is harder on the engine.

I should have looked it up before I posted, I am sorry for my sloppy post.
 

Rick E

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The maximum continuous manifold pressure is shown as 0 to 27 inches.


I don't believe this is correct for the 914, maybe the 912. Otherwise my recent trip around the country sitting on 30" of MAP will have caused some problems?????
 

Vance

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The maximum continuous manifold pressure is shown as 0 to 27 inches.


I don't believe this is correct for the 914, maybe the 912. Otherwise my recent trip around the country sitting on 30" of MAP will have caused some problems?????
I stand corrected again.

Maximum continuous power (green arc) for a Cavalon with a 914 is listed as 0 to 31 inches in the POH.

I went by the colors so I didn't pay too much attention to the numbers.

Sorry.
 

Rick E

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I stand corrected again.

Maximum continuous power (green arc) for a Cavalon with a 914 is listed as 0 to 31 inches in the POH.


G,day Vance,

Not sure if we have a different POH over here but MCP in my handbook is 35"

Regards Rick
 

ckurz7000

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Chris, with an automatically controlled waste gate and a constance speed prop, short of pulling the throttle back to idle, does your MP ever change? Doesn't your prop control change engine RPM or MP? I believe you need an in flight over ride on the auto waste gate so you can better manage your power. The way I understand it, Rotax doesn't want you to run your 914 below 75% power. I run my Piper Arrow (non turbo) at WOT and control the mixture for peek EGT at 10,000'. This yealds around 65% (or less). At this setting my IO360 is burning 8 gal per hr.
Stan, the automatic waste gate control does not prevent you from setting any manifold prssure and rpm combination you like. How else would I have gotten the data shown in the graphs attached to my original post?

Prop control adjusts rpm and throttle adjusts MAP.

Rotax specifies certain power settings in the engine manual. Those range from 115%, 100%, 75%, 65% down to 55%.

Greetings, -- Chris.
 

ckurz7000

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I stand corrected again.

Maximum continuous power (green arc) for a Cavalon with a 914 is listed as 0 to 31 inches in the POH.


G,day Vance,

Not sure if we have a different POH over here but MCP in my handbook is 35"

Regards Rick
Maximum continous power for the 914 is given as 35"/5500 in the engine manual.

-- Chris.
 

ckurz7000

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Rob, I read with interest that you have a manual waste gate control on your engine. It seems to me that you now have three independent inputs (waste gate position, throttle and prop pitch) to control only two outputs (rpm and MAP). You probably need to consider to include another variable, like airbox temperature. Otherwise you have too many controls for too few variables. That would seem to make it hard to choose proper settings.

On the Rotax there is an electronic Turbo Control Unit which looks at throttle position, boost pressure, ambient temperature and rpm to set the waste gate.

-- Chris.
 

Vance

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I stand corrected again.

Maximum continuous power (green arc) for a Cavalon with a 914 is listed as 0 to 31 inches in the POH.


G,day Vance,

Not sure if we have a different POH over here but MCP in my handbook is 35"

Regards Rick
From the Cavalon Pilot Operating Handbook last revised 1/7/2015:

Manifold Pressure ROTAX 914 UL Marking Maximum manifold pressure Red radial 39 In Hg Yellow arc 31 – 39 In Hg Maximum continuous MAP Green arc 0 - 31 In Hg

According to the Cavalon POH Puff’s manifold pressure gage appears to be marked correctly with the red an inch low.

I don't know why it is different than the Rotax engine manual or your Cavalon POH.
 

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Stan V

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The Rotax 914 has an automatically controlled waste gate. If I reduce rpm, MAP stays the same.

-- Chris.
I got the impression from this post that you had no cockpit control if the automatic waste-gate.
 

Stan V

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Stan, the automatic waste gate control does not prevent you from setting any manifold prssure and rpm combination you like. How else would I have gotten the data shown in the graphs attached to my original post?

Prop control adjusts rpm and throttle adjusts MAP.

Greetings, -- Chris.
Then here you explain that your prop and MP works like other machines that DON'T have an automatic waste gate. What am I missing?
 

ckurz7000

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Then here you explain that your prop and MP works like other machines that DON'T have an automatic waste gate. What am I missing?
The facts are thus:

1) I can set any rpm and MAP by adjusting prop pitch and throttle, respectively.
2) Waste gate position is controlled automatically by the TCU.

I guess the waste gate is set so that there is no overboost. Maybe someone else can explain it better?

-- Chris.
 

ckurz7000

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From the Cavalon Pilot Operating Handbook last revised 1/7/2015:

Manifold Pressure ROTAX 914 UL Marking Maximum manifold pressure Red radial 39 In Hg Yellow arc 31 – 39 In Hg Maximum continuous MAP Green arc 0 - 31 In Hg

According to the Cavalon POH Puff’s manifold pressure gage appears to be marked correctly with the red an inch low.

I don't know why it is different than the Rotax engine manual or your Cavalon POH.
Vance, the POH for the Cavalon says, on page 9-1-2:

Power setting Engine RPM MAP Fuel flow [ltr/h]
Max. TOP 5800 39 33
Max. MCP 5500 35 26
75% MCP 5000 31 20
65% MCP 4800 29 17.5
55% MCP 4300 28 12.5

MAP limits do not apply at engine speeds above 5100 RPM, marked by a yellow triangle at the RPM gauge / engine speed indicator.
-- Chris.
 

bones

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cant remember, didnt have log books before, but since over 3000
The facts are thus:

1) I can set any rpm and MAP by adjusting prop pitch and throttle, respectively.
2) Waste gate position is controlled automatically by the TCU.

I guess the waste gate is set so that there is no overboost. Maybe someone else can explain it better?

-- Chris.
The waste gate controller is governed by the TPS on one of the carbs, it is also governed by boost pressure, if boost is too high the waste gate will open, the 914 will develop up to 180 hP if you know how to "trick" the TCU :D
 

ckurz7000

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The waste gate controller is governed by the TPS on one of the carbs, it is also governed by boost pressure, if boost is too high the waste gate will open, the 914 will develop up to 180 hP if you know how to "trick" the TCU :D
Do tell...
 

ckurz7000

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Thanks, Mike, this looks very interesting. What I don't quite get is that the literature says it only takes effect at the 105% throttle setting, i.e., for take-off and climb out. But the graph shown indicates an effect covering a much wider area. Do you know more? And I couldn't find a price an the webpage. I guess I have to get in touch with the guy directly...

-- Chris.
 

Mike G

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Chris
Sorry I don't have any experience with this. It seems to for sale at 500euros plus VAT.
Here is a discussion about it on the French forum (sorry in French).
http://www.forum-autogire.com/t2410-Bo-tier-lectronique-914-additionnel-gyro-perfo.htm
I presume it simply prevents the wastegate from opening when it should and over boosts the engine. I don't quite see the need to change the signal to change the float chamber pressure. I'll have to go back and read up on when the 914 TCU sends that signal.
I assume the text saying it only works at 105% is an over simplification, as you say the performance curve shows it coming into effect at 4500 rpm.
The guy on the French Forum who says it's great also sells them;)
Mike G
 

Mike G

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Chris
The 105% could be correct. The 914 throttle position goes from 0 to 115% according to the manual. At 108% throttle the TCU increases the Airbox pressure and it rises from 36"Hg at 108% to 40.5"Hg at 110%, it remains constant at 40.5" up to 115% throttle. At the same time, as the Airbox pressure rises through 37.5"Hg the carb float bowl reference pressure switches from static Airbox pressure to the higher dynamic Airbox pressure and enriches the mixture for detonation prevention.

Perhaps this device increases Airbox pressure earlier (at 105%) and therefore also switches to dynamic reference pressure for the float chamber earlier.
I can see this increasing power output a bit between 105% and 108% due to richer and cooler mixture but don't see how it increase 115% power unless it somehow delays the signal to open the wastegate when the Airbox pressure reaches 40.5" and increases the Airbox pressure and MAP.

The description suggests that they only interfere with the output from the TCU (that's wastegate actuator motor and float chamber reference pressure change over valve), I'm sure they must also be monitoring Airbox pressure and throttle position (and I hope temperature) to ensure that they don't overboost the engine.

In theory, all you need to do is put a small resistance between the Airbox pressure sensor output connection N°3 and the TCU to fool the TCU into thinking the Airbox pressure was lower than it is. That would increase the Airbox pressure and hence MAP.

I no longer have a 914 or I'd try it and see.

Mike G
 
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