Rolls Royce nimbus starting problem

[cb1000]

I have a nimbus 103 that has always started with no problems, but now for some reason it just won't fire. there is fuel at the torch igniter and a spark and spins up to 10% on starter ok and fuel in the tank was used less than 3 months ago when it last ran. Does anyone happen to have any idea of why this engine may fail to start ( common probs etc ? )

[gidge348]

Hi there, is the torch igniter lighting? Should see a slight increase in EGT when it lights before the main light?

10% is a bit low for starting do you have the generator field winding hooked to the main start as well? Mine tends to light off about 25-30% www.youtube.com/watch?v=8KYToVX6Fpg If it does light at 10% you may end up with a hung start?

Tried pulling the fuel filter off and see if it's blocked?

One last thing if all that is all ok under the cover in the centre of the intake is the fuel line into the spray system. Might want to see if you have main fuel there?

Good luck, post some pics

Cheers
Ian...

[cb1000]

Thanks for the suggestions , the problem seems to e that the torch igniter is not lighting. I'm sure the starter has always run to about 10% then the torch igniter lights and picks up to 20%+ then the main fuel tap is opened and it runs up to idle.
There is fuel getting in and can be seen spraying out of the igniter when removed ( I even changed the torch igniter just in case it was playing up ), and it is also sparking ok.
The field terminal is at 24v when the starter is operated ( surely if this was missing it would revert to generator mode anyway ? )
Should the starter run at more than 10% before the torch igniter lights ?

[gidge348]

If you have fuel spraying into the combustor and the plug is sparking you should get a torch igniter light?

Is the torch igniter solenoid definitely staying open? A loose wire, sticky solenoid or dirt in the line may cause it close and put out the fire??

Does the EGT stay the same while you are trying to start, I normally see a small (10-50 degree) increase when the torch igniter lights, then goes up to running temp when the main fuel lights.

A video of the start procedure and problems may help?

The field winding (B) needs to go to + terminal during starting or else you only get a small amount power from the starter, I have mine through a switch. Seems to work like a soft start, main winding runs up to say 10-15% then when I switch on the field winding it runs up to 30%.

I have had hung starts before when I disconnect the start before say 40%

Are you using jet fuel or diesel or ?

Cheers
Ian...

[turbochris]

f the batteries are weak the torch solenoid will not open when the starter pulls the batteries down.

[cb1000]

The first thing I suspected was the batteries , but I have checked these and even borrowed two bigger HD commercial batteries to try but the engine still only spins up at 10-15% on starter. I checked the field is connected as you suggested. the fuel solenoid is definitely open and after spinning it up for 20-30 secs a couple of times there is fuel coming out of the drain so it is getting in.
I'll have to start taking it apart to check everything I suppose, but thanks for your input anyway. Its probably something really simple as it was working perfectly..

[jetric]

   A friend of mine had the exact same problem with starting his nimbus engine it is mounted in a classic mini and has been converted to thrust we had done quite a few shows at santa pod with it and it had allways started perfectly until one event at santa pod when he had to do some filming with it for a discovery channel programme the engine flatly refused to light off! there was fuel getting to the torch igniter and the cracker box was working perfectly but it would not light off and there was raw fuel coming out of the combustion chamber drain, After the event he checked a few things on the fuel system all was fine but he noticed that there was a pipe that led from the fuel feed to the torch igniter onto the outside of the combustion chamber and that there was some sort of one way valve in this pipe, inside the valve was just a steel ball that sealed against a face using gravity and noticed that there was some dirt on the sealing face that was stopping the ball from sealing properly so he cleaned this and reassembled the valve and hey presto the engine lit off and ran perfectly.
  The idea of this one way valve is that when the solenoid valve is turned on that feeds the torch igniter the fuel pressure that is fed to the torch igniter holds this valve shut and none of the fuel goes down the pipe that is connected to the ouside of the combustion chamber once the engine has lit off and started the solenoid valve that feeds the torch igniter is turned off and the compressed air from the compressor is fed along the pipe that is connected to the outside of the combustion chamber and it opens the one way valve by lifting the ball off its seat and blows any fuel out of the spray nozzle of the torch igniter in order to stop the nozzle from getting clogged up with fuel that would turn to carbon if it was left in the nozzle due to the heat in the flame tube,
  Its worth just checking that this valve is clean and working properly for a nice easy fix,

   Rich. 
 

Nov 24, 2015 4:18:00 GMT -5 cb1000 said:

The first thing I suspected was the batteries , but I have checked these and even borrowed two bigger HD commercial batteries to try but the engine still only spins up at 10-15% on starter. I checked the field is connected as you suggested. the fuel solenoid is definitely open and after spinning it up for 20-30 secs a couple of times there is fuel coming out of the drain so it is getting in.
I'll have to start taking it apart to check everything I suppose, but thanks for your input anyway. Its probably something really simple as it was working perfectly..

[cb1000]

thanks Chris & Rich for the help , but I got it running now. It turned out to be a bad connection on the starter fuel solenoid valve. The valve worked when testing and there was fuel getting in when cranking , but during cranking it kept shutting off / on and obviously not enough fuel there to light up. re-making connection and it fired up straight away.