Spitfire and mixture, a few questions

[Uriah_]

I have noticed that there seems to be an evolution on mixture control on the Spits from the Mk.1 to the 9.  Somewhere along the line there seemed to be two positions on mixture - Auto Lean and Auto Rich to the Spit 9 where it is mixture cut off and just plain auto mixture. 

Is my perception right that there is indeed a difference between Auto lean/rich and cut off/auto?  Or is it all just terms that mean the same thing?

 

Why on cold start does the pilot (from all the videos of Spit 9 that I have watch) that the pilot only moves mixture to AUTO after pushing starter and boost coil buttons?

[Terry Dactil]

My understanding is that the venturi type carburetor cannot draw enough fuel from the jets with the very low intake airflow at the low rpm provided by the starter.

You therefore use the primer pump to inject an air temperature dependent measured amount of fuel into the intake manifold, and this vaporizes and will enable the engine to start and run briefly. This increases the rpm sufficiently for the carburetor to work properly when you select the mixture to the run position.

[Uriah_]

So there is no point in putting mixture to 'auto mixture' because it just is not going to pull in the fuel?  Is there any harm in doing so?

 

Moving the lever forward is 'auto'?

 

[grafspee]

Spitfire's engine is equipped with with injection type carb and mixture control is 100% automatic, so there is only cut off and RUN/AUTO position, AUTO will adjust mixture according to engine power settings so when cruising carb will lean out mixture so engine will burn less fuel and it will rich mixture if power setting increase this is step above, from cut off / lean / rich mixture control.

In that type of injection carb if you have fuel pressure in the system and you move mixture lever from cut off to AUTO, carb will start injecting fuel regardless if engine is cranking or not. If engine is not cranking fuel will simply pour out on the ground making high fire risk. If you switch fuel pump on and move mixture lever forward it will blow fuel on the ground because engine  is not inducting.

So you have 2 ways of starting up spitfire. You can start with mixture set to cut off or AUTO. If you want to start with mixture lever in AUTO fuel pump must be off, if you start engine with mixture lever in cut off you can switch fuel pump on but as you hit starter you need to move mixture forward.

At cold start engine won't start with out priming not because carb can't provide fuel, this carb will provide fuel with no problem, but low temp makes fuel evaporation very slow and only evaporated fuel burns addition to this carb is placed very far from engine intake manifolds. So you need additional fuel so engine can catch up faster.

Edited January 20, 2023 by grafspee

[Skewgear]

2 hours ago, Uriah_ said:

So there is no point in putting mixture to 'auto mixture' because it just is not going to pull in the fuel?  Is there any harm in doing so?

 

Moving the lever forward is 'auto'?

 

Think of the big red lever next to the Spitfire IX's throttle as another on/off switch. There is no "rich" or "lean" setting as such. Forward = engine will run, aft = engine will not run. Technically it is called the "carburettor cut off".

As grafspee says, you prime the engine before starting because it needs a fuel-air mixture to burn. When the engine is stopped, the parts that produce this mixture won't do so in the correct proportions. Hence we have a manual primer pump.

When the carb cutoff lever is forward, fuel can flow from the carburettor into the cylinders. If the lever is forward when you press the starter button, this results in the cylinder being flooded with fuel from the carb. In DCS you see yellow flames coming from the exhuasts as unburnt fuel is pumped out and ignites: you have flooded the engine with too much fuel for it to start and run.

The Ki-gass primer pump squirts a fuel-air mixture into the cylinders via the induction system (basically, the fuel delivery lines downstream of the carb). Befire starting the engine you also need to prime the carb with fuel by pressurising the fuel system upstream of it with either the electric pump or the wobble pump.

This is why you start the Spitfire and P-51 with the carb cut-off closed until the engine coughs and splutters. That initial coughing is the engine burning the fuel you squirted in from the primer pump: turning on (pushing forward) the carb cutoff at that point gives the engine a steady supply of fuel and air. (In reality you're meant to keep pumping the primer after opening the carb cutoff, until the engine catches and runs steadily. This is not modelled in DCS)

The Spitfire LF.IX's engine, the Merlin 66, (and the P-51's almost identical V-1650-7) is fitted with a carb that automatically selects the right fuel-air mixture for whatever throttle (boost) setting the pilot has picked. On earlier Merlins the carb did not do this, so the pilot had to select rich or lean as necessary. The lever used to do this in early Spitfires was the same one we use in the Spitfire IX to turn on or off the carb fuel supply. In those earlier Spitfires there was a separate cutoff control, a ring pull on the right hand side of the instrument panel.

The Merlin 66 runs lean up to +7lbs boost and rich over that figure - in game you'll see this as the boost setting being very sensitive to throttle movements until increased past +7lbs.

[Uriah_]

Great answers guys.  Very much appreciate it.  Seems odd the videos all seem to skip or gloss over this important step.