For those of you who dug the jetting thing I posted from Kevin Cameron,
here's a follow-on from Dr. Rob Tuluie from the Department of
Astrophysics at Penn State. He's also editor of Motorcycle Online and a
very successful roadracer.
Andy "I don't claim to actually know anything myself but can always find
the answer from somewhere else" Ramm
'67 Midget Mk III -- The Smidgin
'69 MGB -- Emma
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Here are my comments on jetting depending on air conditions:
The mass flow rate of air, dm/dt into the engine is
dm/dt (actual) = dm/dt (STP) / CF
where STP is the Standard Test procedure SAE J1349:
T_s = 298 K = 25 C (C stands for degrees Celsius, K stands for degrees
Kelvin,
and K = C + 273)
P_s = 99.0kPa
P_i = inlet air pressure
T_i = inlet air temperature
and where the correction factor is defined as CF =
(P_s/P_i)*sqrt(T_i/T_s)
for dry air (note that water vapor can play an important role,
but I won't get into it here; it basically affects the pressure term
only).
Note that the mass flow rate of air into the engine, which is the
amount of air mass taken in at every intake stroke, depends on the
*square root* of temperature. This is contrary to KC's scaling of
CF = (P_s/P_i)*(T_i/T_s). Any ideas why we differ, KC? I do know
that that the way I wrote the CF above, CF = (P_s/P_i)*sqrt(T_i/T_s), is
standard SAE practice, though.
So, if say the jetting is spot-on at 25 C (298 K) and a pressure of
99.0kPa, then at 5 C (278 K) and a pressure of say 101 kPa
it should be changed as follows:
CF = (P_s/P_i)*sqrt(T_i/T_s)
= (99/101)*sqrt(278/298) = 0.95
therefore the engine at 5 C and a pressure of 101 kPa will ingest a mass
of air larger by a factor of 1/CF = 1.05 (see formula on top).
Hence the jetting should be increased by a factor
of 1.05 or 5%. This does not neccesarily mean that the jet number should
be
increased by that much (it may be that your jets are not rated by flow
but by the diameter of the ortifice). If indeed the jet is rated via
flow,
then the jet number should be increased by 5%.
There are additional factors, such as the fact that colder temperatures
may decrease the vaporization of the fuel, hence leading to a still
leaner
mixture. Engine operating temperatures are lower, decreasing vaporization
even further, etc. I suspect that at very cold temperatures these are
major effects. But then, SAE J1349 advises that this determination of CF
should not be used at very low or very high temps.
Cheers,
Rob
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Dr. Robin Tuluie |Business: CYCLE SCIENCE
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