of intake
On 11/25/2012 07:06 PM, graeme bird wrote:
> Thats very neat, thanks. I am wondering though if its really a
> problem to be running at 80deg.
There are a few reasons to keep the engine temperature higher:
1) Wear of the engine. Different parts of the engine expand at different
rates when the engine gets warmer. Ideally, you have the engine running
always at the same temperature, so all the parts wear in for exactly
that specific temperature, and fit nicely together around that specific
temperature, without too much friction of leakage. If the temperature
changes, friction in some parts increases, and other parts become too
sloppy. So, aim to keep the engine always wearing at the same
temperature. For that reason, I always keep my engine at 115C and avoid
wear at other temperatures. 80C could also if you manage to keep the
engine running at that temperature during the summer, but then there are
still a couple of other reasons to aim for a higher temperature:
2) Efficiency. The engine gets its energy from burning fuel, more
exactly from producing heat, the temperature difference between the
pressure of the air before and after the combustion. Ideally, there
would be no losses. Unfortunately, we have to cool the engine, and by
cooling we take precious energy away. The more we cool, the more we harm
the efficiency. So to get the most bang for the buck, you will want to
run the engine as hot as possible... without melting some parts. For the
Rotax, this sweet point is close to 115C, which is a safe distance from
135C where the cylinder heads become soft and start to deform.
3) Condensation. Byproduct of combustion is water. Part of the
combustion gasses including the water is escaping between the pistons
and cylinder walls, anything below 100C will condensate this water. So.
the condensation in the engine is not just something like the
condensation in the fuel tank, but is actually produced at a high rate
during your flight. (Don't underestimate the amount of these so called
blow-by gasses; these gasses are actually responsible for transporting
the oil back from the crankcase into the oil tank). If you run the
engine at 80C you will collect a lot of water in the engine, and due to
other substances in the fuel, the water will have a low pH and eat away
the engine. If your engine only gets hot during take off, it is not
sufficient to boil away all the water, and the water collected during
the cruise will remain in the engine until the next take off, which
might be a long time away, especially in the winter. So, it is best to
keep the oil temp during the cruise above 100C to avoid collecting too
much water.
(You won't find the water at the bottom of the oil tank except in
extreme cases, as the oil is formulated to keep a lot of water in
suspension, but still this suspended water is corrosive and also worsens
lubrication.)
Frans
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