P-51 cold start up

[grafspee]

Now we know how p-51 engine behave in extreme weather

Can we have this in DCS ?

[mvsgas]

I thought that the DCS P-51 and its engine performance was based on an actual aircraft. Miss Velma from the Fighter Collection.

[grafspee]

Yes cold start need love.

Magic_zach found out that if pilot move throttle too far prior to start up engine will kick back because ignition will be too early so electric starter will not manage to crank engine. Now you can start up with throttle wide open no problem which is impossible IRL.

Fact that cold start up isn't modeled properly cant get in the way of the engine and aircraft simulation. it is at very good level. both supercharger's and engine's modeling is outstanding imho :)

Edited February 2, 2020 by grafspee

[Yo-Yo]

... as well as the cold start model. It does have kickbacks, but the main reason of avoiding the early ignition is not the starter inability. Overall prop and engine inertia with starter assistance are able to pass top dead center (TDC) even with early ignition timing (before the TDC), but the piston rod will be stressed much more it is stressed at normal conditions.

 

As the slow model used for the engine start-up and shut-down truly simulates the cycle, timings and crankshaft-piston cinematics I can show the difference between normal start-up and abnormal start-up using simulation time histories.

 

The first graph represents the crankshaft total moment from the piston rods along with its angular speed. Electric motor having the same charachteristics as the real starter cranks the shaft (of course with the proper reduction gear). From 2 to 2.5 s the first two cylinders entered compression stroke and later another cylinders entered cold expansion stroke. Then, at t=2.9 the first combustion stroke appeared.

As the ignition moment is quite close to TDC the moment from the pistons is quite smooth, all gas potential is used in right rotational direction.

 

The second case is for a bit early ignition. Take a look at the kickback moment stricking a piston rod as the piston was trying to compress burning mixture. This kickback could not stop or even reverse the shaft but the series of preignited combustions caused slower acceleration.

 

The third case represents maximal preignition value. All effects are much more pronounced but the overall piston work before and after TDC is positive . The piston rods and bearings are hardly abused with strikes.

[grafspee]

Thank you for your post :)

So are you saying that even with throttle at max kick back will not occur ?

[Yo-Yo]

Thank you for your post :)

So are you saying that even with throttle at max kick back will not occur ?

 

It occurs. See lower angular speed after it.

[grafspee]

It occurs. See lower angular speed after it.

 

I can see slower angular speed but i was looking for reversed angular speed :)

check this nice kick back engine merlin 266

3 ignitions in row with reversed angular speed due to kick back.I doubt that throttle was w/o

I know his battery wasn't at 100% sure but war time batteries were't as good as modern one, + manual says that if possible use external power so i woudl agree that with external power starter would overcome kick back but not from battery alone.

And another thing DCS's starters are cranking engines quite faster then real one.

Check later in movie how fast engine come to stop at cut off.

Nice he talks about brakes which start creep at -3 lbs boost heh :)

Edited February 4, 2020 by grafspee

[Yo-Yo]

It can not be preignition. No signes of combustion. And it occurs rare. In DCS the battery is absolutely fresh. 146% of full charge :).

[grafspee]

It can not be preignition. No signes of combustion. And it occurs rare. In DCS the battery is absolutely fresh. 146% of full charge :).

 

What sign would you like to have ?? For me looks like combustion kick back all gases remain in side cylinder because starter keep pushing crank forward if not, only other explenation would be that starter just bouncing from peak compression point which would be even weaker then partial pre egnition.

Maybe you are right probably starter coil cutting out from voltage drop before peak compression

Edited February 4, 2020 by grafspee

[Yo-Yo]

By the way, do you know what voltage is used for P-51 and Spitfire?

 

Normal cranking speed:

Edited February 4, 2020 by Yo-Yo

[grafspee]

By the way, do you know what voltage is used for P-51 and Spitfire?

 

Normal cranking speed:

 

P-51 uses probably 24VDC and Spitfire probably using something taken directly from royal navy i would guess 100-110 voltage ?

Just kidding probably same voltage :)

Edited February 4, 2020 by grafspee

[Yo-Yo]

Nope, 12.

[grafspee]

Nope, 12.

 

meh it does not matter you just need twice thicker electric cables :P

[Yo-Yo]

meh it does not matter you just need twice thicker electric cables :P

 

No, and twice lower accumulator internal resistance... As you use the same accumulator (but only 6 cells), this is, as you understand, impossible to achieve.

So, 24 V gives you more powerful starting moment... even with the same starter nominal power.

[grafspee]

No, and twice lower accumulator internal resistance... As you use the same accumulator (but only 6 cells), this is, as you understand, impossible to achieve.

So, 24 V gives you more powerful starting moment... even with the same starter nominal power.

 

In order to achieve same power level you will have to use exactly same number of battery cells 24V or 12V does not matter for 24V you would use simply 2 X 12V connected in line and for 12V system you would use 2 x 12v connected parallel. If there would be any difrence it would be small.

I would say that 12V starter would have noticeably lower rated power then 24v so it would have greater reduction gear in order to crank engine and lower cranking speed will be provide at same time.

3kv 12v starter would be gigantic comparing to 24v

Edited February 5, 2020 by grafspee