priming warm engine

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Zacke
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Joined: 27 Sep 2018, 08:18
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priming warm engine

Post by Zacke »

Hey guys,

5° OAT, shut down engine for cooling for 15 minutes, oil 60°C, coolant 70°C, trying to restart but she doesn't catch (throttle cracked, all magnetos on). Do I have to prime full again (10 - 12 strokes)?

Thanks in advance,
Zacke

alan CXA651
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Re: priming warm engine

Post by alan CXA651 »

Hi Zacke.
I have never done 10-12 strokes of primer , on either hot or cold engine , anywhere between 3 and 6 is all i have done , but mostly 3 to 4 i have found works for me , also i find moving throttle to have the horn cutout plate on throttle , just above the horn cutout casting .
I think you might be flooding the engine with 10-12 strokes.
regards alan. 8)
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Zacke
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Joined: 27 Sep 2018, 08:18
Location: EDNY

Re: priming warm engine

Post by Zacke »

Hi Alan,

thanks for your response.

The manual and the shift-2-instructions talking about 12 strokes @ 0°C.

But again my question: When the engine is still warm you don't need that much prime as if it is cold. Am I right?

Zacke

alan CXA651
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Joined: 15 Mar 2016, 08:23

Re: priming warm engine

Post by alan CXA651 »

Hi Zacke.
It should take less primming on a warm engine , as the engine is easier to turn , due to thinning of oil as it warms up , so you are correct , it should take less primer.
I find getting the throttle in the correct position , so as to allow the engine to fire and tickover , without to much power , as this is critical , , due to aircraft could nose over if revs to high , or not fire up if set to too low a setting.
regards alan. 8)
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Zacke
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Posts: 381
Joined: 27 Sep 2018, 08:18
Location: EDNY

Re: priming warm engine

Post by Zacke »

Yes, same observation here with the correct throttle position.

By the way: Could you explain how the starter magneto is working. I have only a rough idea what it's good for. Furthermore: What will happen if I forget to turn it of after engine start?

Thanks,
Zacke

alan CXA651
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Re: priming warm engine

Post by alan CXA651 »

Hi Zacke.
On the A2A spit , this is only fitted to the MK1a , the MK2A and B are cartridge start .
I dont know exactly how it works , but at a guess once the trolly ack is fitted , which is a big external battery , you would turn this starter mag on , this i would guess is a dc motor , which would convert dc power to high ac power for the plugs , once the engine fires up , your engine powered mags if turned on , would be powered from the engine to keep the engine running , then you would turn off the starter mag switch, and give ground crew the signal to disconnect the trolly ack, if you forgot to switch it off before trolly ack disconnected , it might cause a big spark at the electric disconnect plug , which could startle the ground crew , who are very close to the prop , this would not end well for them , and you would eventually burn out the plug disconnect pins .
I tend to fly either the MK2A or B , that undercarriage hand pump is hard work on the MK1A.
regards alan. 8)
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Killratio
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Re: priming warm engine

Post by Killratio »

Starter Mag is basically a boost coil arrangement by slightly different means. it won't damage the engine if not turned off but will annoy the ground crew.

Priming suggestions are based on ambient temperature on the assumption that the engine is cold. if it is warm, it should start with no primer if you have not shut it down correctly and just used the SRCO. If you have drained the lines first, it will require 2 or 3 primes and possibly another as the engine catches. At any temp above 0 C the Merlin should start fairly easily if you have the throttle as Alan described. At down to -1 or -3C it should start with an effort. The best I have managed is -15C but I suspect it was the foul language that made it start, not the primer.... :wink:

Fantastic to see someone want to learn the nuts and bolts still Zacke!

(Note to self.. Change signature to "Real men use hand pumped undercarriage" ) :P :P :P
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Zacke
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Re: priming warm engine

Post by Zacke »

Thanks for the explanations! It's much clearer to me now.

(Why does it annoy the ground buddies? :))

Zacke
Last edited by Zacke on 04 Jun 2021, 15:33, edited 1 time in total.

Zacke
Staff Sergeant
Posts: 381
Joined: 27 Sep 2018, 08:18
Location: EDNY

Re: priming warm engine

Post by Zacke »

Where can one get the signature banners from? Is there one for the spitfire crew members?

alan CXA651
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Re: priming warm engine

Post by alan CXA651 »

Hi Zacke.
try here :-viewtopic.php?f=23&t=29849
be warned , dont use any that point to photobucket , else you will be disapointed due to potobuckets policy.
regards alan. 8)
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Zacke
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Location: EDNY

Re: priming warm engine

Post by Zacke »

Hey guys,

I've a hard time starting her up some minutes after shut down. Ambient temperature 8-10°C, engine pretty warm (oil and coolant), ideling and closing the fuel cocks, waiting about 60 to 90 sec until she begins running rough, then pulling the SRCO, closing throttle, switching magnetos off. Starting her up after some minutes seems to be much harder than starting her up from cold & dark. (Throttle cracked just a little bit to forward of gear warning casting).

I've now played around an hour or so testing some different settings (poor little starter - sorry!). When the engine is warm you actually do not need to prime (at most one shot), even if you had drained the lines . I think I had a tendency to overprime her.

For me some interrelations are still not clear yet:
a. What does that primer exactly do technically? That's a question I wanted to ask for years although using primers since years on the GA planes, Mustang, Connie. :? (the Spit teaches you lessons!)
b. What's the exact procedure if an overprime situation is suspected with the MkI? (Playing around I realized shutting the fuel cocks, opening the throttle quite far and letting the starter run after a while she begins to catch again. For sure that's not a good way to clear her - danger of nose over).
c. The fuel pressure comes up to a quite "normal" level when the engine is only turned by the starter. If the throttle is cracked (to much) yet does that eventually flood the engine if it needs to long to catch?
d. Eventually my problems with starting her up again have to do with fouled plugs.

I really want to understand all that technical and practical backgrounds. I don't want to destroy my starter and a trolley accumulator isn't always within reach. So good to know how to fire her up in a solid way.

Thanks for your help,
Zacke

alan CXA651
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Posts: 2438
Joined: 15 Mar 2016, 08:23

Re: priming warm engine

Post by alan CXA651 »

Hi Zacke.
The primer is putting fuel in the cylinders , to help the engine fire up , then it can pump its own fuel in once its running, over primeing floods the cylinders and stops the sparking in the plugs , if you over prime , turn off mags and fuel cocks and turn engine over to clear cylinders of unburnt fuel , be aware of the limitations on this to prevent damage to engine and excesseries , once engine clear of excess fuel , go through the startup proceedure again .
regards alan. 8)
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Killratio
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Re: priming warm engine

Post by Killratio »

a. The primer injects fuel into the cylinders while the rotation of the engine is not sufficient to draw/pump enough fuel for combustion through the system. The Spitfire I originally offered the option of priming either cylinders or carby. That turned out to be a less than ideal arrangement and after several fires, the control was wired off to allow Cylinders only. In the A2A Spitfire it is removed altogether and the priming pump is connected directly to the cylinders.

b. As Alan as said..only thing I would add is that the throttle goes fully open while you crank to clear an over-prime.

c. No, provided the throttle is not forward of the start position, it will not flood the engine on normal cranking.

d. Too much fuel, low temp cylinder, dirty and incomplete burn, oil flowing normally = fouled plugs, yep!
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Zacke
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Re: priming warm engine

Post by Zacke »

Thanks again.

b. Isn't that harmfull to the engine? Say there's still enough fuel in the system she catches then with full throttle. In addition even if you slam back the throttle when she catches she jumps away or noses over pretty easy. Maybe in that case one should do that without brakes or trolley acc connected.

c. But if you have the throttle forward of the start position trying to get her going it will probably worsen the flooding because it delivers even more (unburned) fuel to the cylinders? So if you're unsure if the engine isn't still flooded you better close the fuel cocks, crank her and see how she's doing? Or better: Never go over throttle start position except you wan't to clear an over-prime.

The manual states not to use the starter for more than 30 secs (common sense also with the other planes). But is that really that short? Sometimes it really needs more than a half minute to get her going.

Regards,
Zacke

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Killratio
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Re: priming warm engine

Post by Killratio »

b . Remember that your MAGS are OFF whwn doing this, so no danger of engine catching!

c. Yes, correct, better to never go past start position unless clearing.

Starter....absolutely vital not to overheat it. Consider yourself lucky that the A2A bird uses the modified starter cog of the Merlin III that does actually give better torque. Originally the Spitfire suffered from a weak starter that was even harder to use. But if you want long term health of the system, stick to the manual. You may get away with overcranking or not....Accusim doesn't work on a "hard" coded limit, it works as a living breathing mechanical system, albeit digital.
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