Lycoming’s advice goes further into detail if you read a copy of their old publication ‘Flyer reprints’ nothing to do with the UK GA magazine but a collection of info collected over the years.
Lycoming recommend the 1800rpm prior to shut down as there are scavenging agents in 100LL that are designed to keep the plugs clean (in particular lead fouling). The scavenging agent works at higher combustion chamber temperatures hence the requirement to increase the rpm prior to (and immediately before) shut down.
The advice works, especially on O-235s in Cessna 152s which are a lead generating machine. Been there and proven that after maintaining a fleet for some 20,000 hours with rarely a mag drop between 50 hour checks due to lead fouling. When I fly Lycomings (or ground run them) I do this and check the mags whilst the rpm is up which tends to highlight any ignition problems.
It does of course go with the proviso to check behind you prior to increasing the rpm….
Taking the engine up to 1800 rpm enables the chemical in the fuel that evaporates the lead to do so, this chemical requires the heat that is produced at 1800 rpm to do its job.
Which chemical is this, that needs 1800rpm to work?
When it is burned in an engine, tetraethyl lead – now called avtel by makers Innospec – can
leave a deposit which has a corrosive effect, particularly on valve heads and stems. This is called
lead fouling. The lead oxides must be scavenged as far as possible, and for this purpose ethylene
dibromide is added to avgas.
Ethylene dibromide reacts with the lead oxides to form lead bromide, a gas (at high
temperatures) which is exhausted from the cylinders. It doesn’t work very well at low
temperatures, which is why lead fouling is at its worst during operations at low power settings
such as taxying and hanging around waiting for take-off clearance.
Because of its effects on the environment, ethylene dibromide was banned by international
treaty 20 years ago.
It’s worth running the engine up before shut-down, especially if you’ve had a long taxy at
relatively low power, to get rid of lead oxides. Increase power from the ground idle range to
around 1800 rpm for 15 or 20 seconds. This should raise the temperature enough to let the
ethylene dibromide do its work.
When you’ve gone back to ground idle, check the magnetos. Lead fouling also affects plugs
– temperatures are lower around the plug, before the flame really gets going. Some pilots also
lean the mixture off as much as possible when faced with a long taxy, to reduce the amount of
fuel and increase the temperature in the pot.
from here: IAOPA
Will fuel without lead give the same fouling? I have never heard of anyone doing this procedure. It does create a lot more noise around the hangar which is not really nice.
Many thanks Chess.
That raises more questions:
Increase power from the ground idle range to around 1800 rpm for 15 or 20 seconds. This should raise the temperature enough to let the ethylene dibromide do its work.
However, running the engine full-rich at 1800rpm is likely to produce the same EGT as running it fully leaned at say 1200rpm. I can get some data next time I fly. And everybody should lean maximally anytime they are on the ground. So this “1800rpm” is unlikely to be the whole story, and may originate from the bad old days when “everybody” taxied full-rich – just as most people are taught in their PPL today (never touch the red lever).
The other thing is that even though I had never previously heard of this 1800rpm thing, and neither I suspect have most people, most people don’t get leaded-up spark plugs. And I haven’t had them even when erroneously not leaning on the ground.
That article has some funny stuff it in e.g.
Rotax engines flying
on 100LL tend to break down every couple of
hundred hours because of lead interfering with
the valve system
Is that real?
The article is clearly (also) a marketing piece for Hjelmco 91/96 – which BTW I would use, if available here.
I know of one airfield with UL94 in the region (Mengen) – UL94 cheaper than 100LL, but still way above alternatives (not to mention AT, FR…)
I like the outlook: 100LL gone in 20 years, not sure about 10 years. MMhhh 11 years since publication and 100LL is going pretty strong.
Absent a regulatory ban of lead, I would not make a prediction even now.
@Peter – the article does not suggest you do the 1800rpm full rich, or does it :-) ? That said, I have not been aware of it, just the ground leaning and the “leaned cruise rpms” for spark cleaning after a failed magneto check as per Lycomings recommendations
The article is clearly (also) a marketing piece for Hjelmco 91/96
I thought the same; Hjelmco may have placed an advert in the associated magazine so this was a bit of an “advertorial”.
the article does not suggest you do the 1800rpm full rich, or does it
Indeed, and what I am getting at is that the 1800rpm bit may be entirely spurious. Well, it is meaningless without specifying whether full-rich or peak-EGT.
Above we have seen a lot of opinions about the effectiveness of the 1800 RPM shutdown in cleaning the spark plugs some of this has a basis in scientific fact and others no so, however now for some hard fact.
My Cessna 152 fleet had spark plug fowling problems being a regular occurrence when the engine hours got towards 50 ( oil change and other Lycoming SB’s actioned at this point ). The introduction of of the Lycoming recommended shut down procedure has eliminated this problem ( and the three hours Labour getting a licence engineer to drive to the aircraft & fix the problem. ) so all in all from an operational point of view complying with this Lycoming SB saves me a lot of money.
Peter wrote:
Rotax engines flying
on 100LL tend to break down every couple of
hundred hours because of lead interfering with
the valve system
Our club’s Rotax-powered Aquila is run on Mogas because avgas is too ‘dry’ so the oil has to be changed every 50 rather than 100 hours. I believe it also attacks or chokes some soft engine parts but don’t know which. Just avoid the unleaded with 10% ethanol because that will apparently make the engine go pop (I accidentally bought it and had to re-sell 200 litres of SP95-E10 – with a diesel car 200l was too much for the lawnmower 
)
