primer lock & rpm's

[peachmonkey]

hi, all,

I've been enjoying the P47 more and more every day, such an awesome module.

Recently I've stumbled upon something weird, not sure if it's something to do with my system, hotas, etc.. or it's an expected behavior.

If I start the engine (from cold start) and forget to lock the primer (don't twist it to the right) the RPM's will continuously fluctuate with 100-150RPM range, i.e. you could hear the engine speed up/slow down, the RPM needle will dance, etc..

Is this normal to be expected? If it is, could anyone possibly enlighten me on what's actually taking place in the engine that causes such flux ?

cheers.

 

p.s. can't wait for when ED introduces the extended fuse delay... P47 will be an unstoppable ground pounder.

Edited November 22, 2021 by peachmonkey

[saburo_cz]

When you do not lock primer, fuel will run into fuel system and will cause too rich mixture.

It resulting in heavy smoke and sometimes fire, IRL. In DCS it does what you wrote..

Edited November 22, 2021 by saburo_cz

[peachmonkey]

1 hour ago, saburo_cz said:

When you do not lock primer, fuel will run into fuel system and will cause too rich mixture.

It resulting in heavy smoke and sometimes fire, IRL. In DCS it does what you wrote..

 

logically it makes sense, but it's pretty high level. I guess maybe a fuel delivery diagram would help solve it, however I can't find any high resolution ones on google.. Here's the best one I could find..

• Looking at #4 and tracing the solid line to the left, it ends under number 9, i.e. the fuel is delivered directly in to the cylinder.
• But does the fuel line (starting from #7) actually have enough pressure in it once the engine is running?  
• If the pressure is created by #13 main booster pump why is it 'pulsating' and causing the RPM fluctuation? Wouldn't the pressure stay more or less static?
• Or is the fluctuation due to the ignition (timing?) getting messed up because of the extra fuel in the cylinders?

 

Edited November 22, 2021 by peachmonkey

[grafspee]

Leaving primer unlocked makes whole situation unstable, this is higher resolution.

As you can see primer supply is just before main fuel pump, when engine running it will suck fuel from primer line ending up eventually with air baubles ingested in to carburetor main supply, this will fluctuate fuel pressure in to carburetor, which results in rpm fluctuation as well. 

 

Edited November 23, 2021 by grafspee

[peachmonkey]

6 minutes ago, grafspee said:

Leaving primer unlocked makes whole situation unstable, this is higher resolution.

 

thank you for the diagram, indeed a lot more clear.. and TOP Secret

unstable -> agreed.

looking for the answer to "why pulsating" now..

[saburo_cz]

13 minutes ago, grafspee said:

you can see primer supply is just before main fuel pump, when engine running it will suck fuel from primer line ending up eventually with air baubles ingested in to carburetor main supply, this will fluctuate fuel pressure in to carburetor, which results in rpm fluctuation as well. 

Acctualy no.

Fuel from the primer line goes to manifold after the carburetor. That is reason why the fuel from the primer line increase richness of mixture.

[grafspee]

6 minutes ago, saburo_cz said:

Acctualy no.

Fuel from the primer line goes to manifold after the carburetor. That is reason why the fuel from the primer line increase richness of mixture.

richness of mixture  does not explain rpm fluctuation, it would end up with lower rpm or engine refuse to run if mixture was too rich. 

And primer line if fed from main line as main pump so when pump is running it will draw fuel from main tank and from primer line at the same time.

And fluctuation should increase when higher pressure will appeared in manifold, and when you open up throttle above elevation pressure it will force air/fuel mixture exactly in to main fuel pump supply .

Edited November 22, 2021 by grafspee

[peachmonkey]

6 minutes ago, grafspee said:

richness of mixture  does not explain rpm fluctuation, it would end up with lower rpm or engine refuse to run if mixture was too rich. 

And primer line if fed from main line as main pump so when pump is running it will draw fuel from main tank and from primer line at the same time.

 

 

do any of the auxiliary pumps run in normal operation? I.e. the fuel is 'pushed' vs. sucked..

the air bubbles make sense, but it'd mean they come straight from the manifold.. lowering the A/F ratio, until the same bubbles end up in the carb, and then back in to the manifold...  full circle..

[grafspee]

I thin auxiliary pumps are engaged when fule pump rheostat is turned on.

[peachmonkey]

Just now, grafspee said:

I thin auxiliary pumps are engaged when fule pump rheostat is turned on.

in the manual the Rheostat is called "Fuel Booster Pump Rheostat".

Also, the manual states that the Booster Pump is on when the main fuel tank is selected (on the floor switch)..

So.. is #19 (aux pump) = booster pump, however it's just idling maybe until the rheostat is rotated beyond its minimum setting?

Now I actually wonder if I can alleviate these fluctuations by increasing the fuel pressure via the rheostat  (will test tonight)..

 

[grafspee]

3 minutes ago, peachmonkey said:

 

do any of the auxiliary pumps run in normal operation? I.e. the fuel is 'pushed' vs. sucked..

the air bubbles make sense, but it'd mean they come straight from the manifold.. lowering the A/F ratio, until the same bubbles end up in the carb, and then back in to the manifold...  full circle..

Air baubles would induce pressure fluctuation on main carburetor fuel supply and this would directly impact engine rpm, if other way was true than enriching fule would end up with fire from exhaust or engine grind to the stop, but i cant think of any reason why engine would fluctuate rpm.

[peachmonkey]

Just now, grafspee said:

Air baubles would induce pressure fluctuation on main carburetor fuel supply and this would directly impact engine rpm, if other way was true than enriching fule would end up with fire from exhaust or engine grind to the stop, but i cant think of any reason why engine would fluctuate rpm.

agreed, the Venturi effect will be .. affected.. resulting in some unbalanced a/f mixture ratio..

[grafspee]

4 minutes ago, peachmonkey said:

in the manual the Rheostat is called "Fuel Booster Pump Rheostat".

Also, the manual states that the Booster Pump is on when the main fuel tank is selected (on the floor switch)..

So.. is #19 (aux pump) = booster pump, however it's just idling maybe until the rheostat is rotated beyond its minimum setting?

Now I actually wonder if I can alleviate these fluctuations by increasing the fuel pressure via the rheostat  (will test tonight)..

 

Main fule pump is on when you drain fule from main tank, this behavior should be gone when you select auxiliary fule tank 

According to this diagram selecting aux tank should pressurize primer line and carb main supply so rpm fluctuation should be gone, but expect some heavy spark plug fouling. 

Edited November 22, 2021 by grafspee

[peachmonkey]

2 minutes ago, grafspee said:

Main fule pump is on when you drain fule from main tank, this behavior should be gon when you select auxiliary fule tank 

I think i'm getting lost in the terminology.

is #22 Main Fuel Pump = Main booster Pump = controllable by the Rheostat ?

[grafspee]

8 minutes ago, peachmonkey said:

I think i'm getting lost in the terminology.

is #22 Main Fuel Pump = Main booster Pump = controllable by the Rheostat ?

According to manual, those electric pumps are engaged when fuel selector is in main or aux position, Rheostat enable only to operate those pumps at higher  speeds.

So when Rheostat is at START AND ALTIDUDE position those booster pumps are operating at normal speed. 

And booster pumps are assisting mechanically driven pump all the time, unless drop tanks are selected.

Edited November 22, 2021 by grafspee

[peachmonkey]

4 minutes ago, grafspee said:

According to manual, those electric pumps are engaged when fuel selector is in main or aux position, Rheostat enable only to operate those pumps at higher  speeds.

So when Rheostat is at START AND ALTIDUDE position those booster pumps are operating at normal speed. 

And booster pumps are assisting mechanically driven pump all the time, unless drop tanks are selected.

 

cool, thank you for the clarification..

I think I'll try out switching to Aux pump, then boosting it via the rheostat to see what effect it produces on the RPM with the primer unlocked.. If the diagram is showing what I think it's showing the system shifts from suck to blow..     .. to test out the bubbles theory ..

This has been brought to you by "fun times in DCS P47".

[grafspee]

9 hours ago, peachmonkey said:

cool, thank you for the clarification..

I think I'll try out switching to Aux pump, then boosting it via the rheostat to see what effect it produces on the RPM with the primer unlocked.. If the diagram is showing what I think it's showing the system shifts from suck to blow..     .. to test out the bubbles theory ..

This has been brought to you by "fun times in DCS P47".

Question is how deep fuel system is simulated in the sim, it could be simple effect of rpm fluctuation triggered by unlocked primer, in this case switching to aux tank or increasing booster pump rpm won't change a thing.

The best way is to test this in real plane hehe

Edited November 23, 2021 by grafspee

[peachmonkey]

4 hours ago, grafspee said:

Question is how deep fuel system is simulated in the sim, it could be simple effect of rpm fluctuation triggered by unlocked primer, in this case switching to aux tank or increasing booster pump rpm won't change a thing.

The best way is to test this in real plane hehe

 

yup, agreed.

Tested the 'bubbles' theory last night :   switching to AUX tank/pump or increasing the fuel pressure with either pump doesn't eliminate or reduce the fluctuations one bit.

oh well.. lack of a result is a.. result !

cheers

Edited November 23, 2021 by peachmonkey