In a radial piston engine, before the engine is shut down the rpm is brought to a considerable high power setting of approximately 1950 rpm. This is to burn the unburnt carbon in the spark plugs. Are there other reasons for this specific power setting to be selected before the engine shuts down?
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3$\begingroup$ What engine are you talking about? Your statement indicates this is a general procedure with radials, but it's news to me. I've operated an R-985, and you kill it from idle or just above idle, like any other engine. 1950 rpm is around cruise rpm for a 985. $\endgroup$– John KOct 23, 2020 at 18:25
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No Sir, radials do not get run at 1950 RPM before shutdown. That is a higher power setting than a good runup. But it is true that the idle RPM is brought up to around 1000 RPM for a while and the purpose is not to burn the plugs clean -although it couldn't hurt- but to scavenge oil that has pooled in the engine case during idle operation and return it to the oil tank. These engines have a dry sump system where the oil is stored in a reservoir and distributed to the case for lubrication of engine parts, then sucked up by scavenge lines and returned to the oil reservoir. You don't want excess oil sitting in the engine case after shutdown since it can seep into the lower cylinders and cause a hydraulic lock on startup. Source: I operated R-985, R-1340, R-1820, R-1830, R-2600 and R2800's.
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It could also be to remove lead deposits from the spark plugs. Some avgas has an agent added to remove lead deposits, this only works above a certain temperature which is achieved above a certain RPM. We had to do a similar procedure in the Cessna 152.
As you will know, Avgas 100LL contains a compound known as Tetra Ethyl Lead (TEL) which acts as an octane booster for the fuel. This results in a fuel which is commonly known as a 100 Octane lean mixture and 130 rich mixture Performance Number fuel.
In practice it is even better than this, with ratings more like 106 lean mixture & 130 rich mixture which are far in excess of the comparable 85 - 87 octane of road fuels. To achieve this a lot of TEL is used - around 5 times the quantity that was used in the old Leaded automotive fuels.
This increase in octane allows aviation engines to produce more power through increased compression ratios or alternatively by increasing the inlet pressure by using a turbo or a supercharger. The problem with using Leaded fuels is that they will always burn with more deposits than unleaded fuels.
The Tetra Ethyl Lead used for octane boost in the fuel naturally degrades to form Lead Oxide when it is burned. In reality it is this oxide which gives the octane boost. The problem is that Lead Oxide is a solid up to about 900 deg C which is well within the wall temperatures inside a piston engine.
In order to prevent these deposits from forming, a Lead scavenging compound is added to Avgas 100LL - this compound is Ethylene Dibromide. This scavenger is designed to react with the Lead oxide to form Lead Bromide which is more volatile - becoming a gas at around 200 - 250 oC. This is a low enough temperature to ensure that the Lead is removed from the engine as a gas end it subsequently goes back to the solid phase as the exhaust gas cools in the atmosphere.
As a point of interest the pale brown / ash coloured staining that is often seen leading from the exhausts of high powered engines, such as those found on the warbirds, is in fact Lead Bromide.
To enable this reaction between the Lead Oxide and the scavenger to work, there needs to be a relatively high combustion temperature.....
Read more at this link.
