Quote:
Originally Posted by Agnew
Alan, I would advise against re-purposing the “anti-dieseling” solenoid. On the Volvos and the side-draught carburetors, these open a channel from the air side of the throttle valve to the engine side, which has an effect very similar to opening the throttle valve by the same amount (by pressing the accelerator pedal for example). On down-draught carburetors, the solenoids block off the idle jet when the ignition is switched off. In practice, their main purpose is not to prevent dieseling, as this shouldn’t really be an issue in a properly set up spark ignition engine: The compression is not high enough for compression-ignition of the fuel/air mix, the ignition is switched off so there is no spark, and the cylinder head/valves/etc should really not get that hot to ignite the mix. But what is a real problem is that the passive “flywheeling” of the engine to a halt still creates vacuum on the intake side, which sucks in a small amount of fuel/air. This does not get ignited (normally), but remains in the cylinders, washing down the cylinder walls and eventually ending up in the sump, contaminating the engine oil and greatly increasing wear next time the engine is started. The solenoids reduce the amount of fuel ending up in the cylinders after the ignition is switched off. On down-drought carbs, the solenoids have nothing to do with idle speed (at least until they break).
But in our Volvo, the channel they control is adjustable. This is best left as is, but your excellent idea can be better implemented through the use of an A/C bypass solenoid (a second one, in my case).
The idle bypass channel can be set for 900 rpm at idle in N and the “transmission compensation” solenoid can be adjusted for 900 rpm with the transmission in gear, via the starter inhibitor switch with an inverted state (via a logic gate or just a simple relay). Then the idle speed would remain unconditionally stable.
In fact, this is what Volvo should have done in the first place! Now, where do we get a couple of extra A/C bypass solenoids of the adjustable type? It would take a certain amount of time until the solenoid opens the channel and more fuel/air reaches the cylinders, but it would also take a certain amount of time for the transmission to actually engage the epicyclic gearing, loading up the engine, so with a bit of luck, this could be very smooth!
This is something I’d gladly add to my 244! These are mounted remotely and are hooked up to the intake manifold and carb via fuel hoses (for vacuum hoses, but I prefer if they are rated to withstand fuel since it will be the fuel/air mix that they will be passing).
I actually thought that this was how it was meant to work when I first bought the car, and assumed that the solenoid may have given up, but after looking everywhere, I was unable to find such an arrangement for the transmission (confirmed by the manuals). It is only there for the idling and A/C.
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Hi,
I have no prior knowledge of these types of carburettors (most of my experience is with motorcycles) - so I'm just going by what I read in the Volvo green book. As you may see from this screen shot, it describes the purpose as being to stop engine run-on (I always thought that meant dieseling) and says that in some markets the solenoids is re-purposed to increase engine speed for cars equipped with air conditioning (rare here in the UK):
... so, that is why i thought the purpose was anti-dieseling, and that is what gave me the idea of re-purposing it to compensate for transmission drag at tick-over. You are probably right though; I have no more experience than reading the book.
My Pierburg 175CD carburettor does not seem to have any channel built in to it for air conditioning compensation (that I can see, but again I'm just going from the book):
... it seems to have just the metering (throttle), idling and solenoid bypass circuits, so where would an air conditioning bypass solenoid plug in? I'm wondering whether such a solenoid would replace the adjuster screw for the anti dieseling circuit (that seems to be what is shown in drawing - but then the current fitted solenoid would have no effect (the channel being closed when the car is in P or N anyway).
If my carburettor has a suitable air conditioning circuit and if I can find a solenoid to fit, then I'd be really keen to see if I can get this to work as you have described - I'd even worked out how to use an ordinary 5 pin relay to provide a NOT gate control from the starter inhibitor switch. It does seem to me that if I just fitted a second solenoid to the place where the anti dieseling channel adjuster is now, then the two would be operating in series on the same channel, so the anti dieseling one would be superfluous (perhaps I have misunderstood something?).
I'd be really pleased to hear whether you think this might work - not because there is a problem that really needs fixing, but because it would be a really clever enhancement to the RB.
Alan