Hi folks, Long time lurker but usually keep quiet as I don’t often have anything productive to say. I’ve googled and trawled all over for the answers to the questions I’m about to ask but don’t seem to be able to find them. Any answers / pointers in the right direction appreciated.
I’ve been looking at cockpit images of N3200 etc and noticed a few things:
1)They seem to have starting magneto? As far as I’m aware this doesn’t appear on mk IIs onward or at least as a separate control. So simply why or what changed on later Merlins to not need this?
2)Mk Is seem to have a rotary valve next to the primer marked all off, carb and cylinders. I am assuming this directs the prime to either, but again why is this omitted on later marks? and if so is fuel only fed to cylinders?
3)Mk Is don’t have slow running cut-offs. What are the benefits to having one compared to simply turning the mags off? Is there a technical difference between slow running and idle cut offs and why did UK designs have it as a separate control and US designs have it as part of the leaning control?
Sorry for so many questions, but thanks in advance for any responses.
Ben
By: Seafire - 29th September 2015 at 10:42
The Merlin XII of the Mk.II used a Coffman cartridge starter, rather than electric start. Some magnetos use an “exciter” (I think that’s the term, in US, anyway) for starting, which boosts the amperage, causing a “hotter” spark. This requires an electrical source besides that generated by the magneto itself, which may mean it wouldn’t work with the Mk.II’s starting system. I can’t speak for other Merlin varieties offhand. (I’m also going on old memory, so some of the details here may be wrong!)
bob
By: MerlinPete - 29th September 2015 at 09:39
Hi Ben
I don`t know what the priming control does, I assume the early versions had electric fuel pumps which serve to prime the carburettor?
As Tony said, stopping the engine with fuel in the cylinders has all sorts of implications, including hot starting difficulty, and they have a tendency to run-on (or backwards!) when hot, see the end of this video of a 1938 Merlin II:
https://www.youtube.com/watch?v=Qv7qi_ZGHfo
Pete
By: Moggy C - 29th September 2015 at 06:02
On Lycomings in my experience stopping the engine by switching the mags off will often result in a fairly hefty backfire or two. I’m not sure if that applies also to a Merlin.
Moggy
By: TonyT - 29th September 2015 at 00:26
Welcome Ben,
If you simply turn of the mags the engine will continue pulling fuel in as it runs down, this can result in an unburnt charge in the cylinders, not a good scenario on a warm engine and also dangerous if it’s turned over etc, chopping the fuel means it stops but no fuel should remain in the engine as it gets burnt.
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September 28, 2015 at 5:12 pm