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In the automotive industry, extensive research has been devoted
to improving fuel atomization and vaporization by carburetor and
induction tract modifications that use nozzles, heated manifolds,
vibrating plates, exhaust gas recirculation, swirl and other methods.
Results show that fully vaporized fuel may not ignite quite as
readily as ideally atomized tiny fuel droplets. Droplets of less
than 15 micron (.00059") diameter have been found to best follow
the bending airflow path of induction manifolds.The design of
the Smart Carby main jet allows for altering the droplet size
to suit individual applications or experiments. We have found
that controlling the droplet size influences the engines ability
to make more power at high revs. If an engine has a slightly inefficient
cylinder head flow or lower than optimum compression ratio for
the choice of fuel, the result is not enough effective compression
at the top end of the air flow range.
When this occurs the heat built in the chamber during the compression
stroke is not enough to achieve sufficient vaporization of the
fuel droplets. Droplets are vaporized by heat transfer from the
surrounding atmosphere through the surface area of the droplet.
The same volume of fuel spread into many small drops has a greater
surface area than one large drop hence fine atomization improves
the burn ability at high revs. Research has shown that lean ignition
is more sensitive to atomization quality than to ignition energy.
So a lean cylinder will show relative mixtures by observing spark
firing patterns on an oscilloscope. The emitter orifices of the
Smart Carby main jet deliver a finely atomized fuel fog to the
induction system.
This fog distributes more evenly and ignites more readily than
larger fuel droplets. Mixture distribution is also evident in
exhaust gas temperature measurements. The Blackjack car has so
far shown cylinder balance exhaust temperature variations of 50
degrees Celsius during the burnout even while on the limiter and
as low as 20 degrees Celsius during the run. An interesting observation
is the exhaust temperatures with the Smart Carburetors are much
more even throughout the duration of the run than with the previous
carburetors. Exhaust Gas Temperature And Cylinder Burn Exhaust
Gas Temperature (EGT) reflects the leanness or richness of the
fuel mixture. EGT is also an important diagnostic parameter. It
rises significantly during magneto failure or during misfiring
due to a fouled spark plug or other causes, and it falls during
knock or detonation. The EGT probe measures the average gas temperature
just outside a cylinder's exhaust port and this reflects the relative
temperature of the gases that bathe the head of the exhaust valve
when the valve is open.
At a fixed RPM and manifold pressure, the EGT spread observed
across a range of mixture settings reflects the evenness of the
mixture distribution among all cylinders and can be used as a
guide to modify the induction system to improve that distribution.
Such modifications should only use EGT data obtained at wide open
throttle settings due to the highly variable induction flows at
part throttle. There is a general rule that a leaner cylinder
will show a higher exhaust temperature but this is not always
the case. This is a useful rule to follow if you are dealing with
a standard type low power output production engine but don't always
follow it for high output race engines. FACT- 'The temperature
in the exhaust pipe is the left over energy from the combustion.'
Basically the exception to the general rule is when the temperature
is high because the fuel is still burning in the pipe. Slow burning
of the fuel in the chamber leaves more heat in the exhaust pipe.
Lean or rich mixtures both cause slow burning, basically any time
the cylinder output efficiency is less than optimum it means the
energy input to the piston is less, therefore more energy is converted
to heat when the exhaust valve opens.
The temperature in the exhaust can go up as the cylinder is run
leaner and it can also go up as the cylinder is run richer. It
is important to assess the other factors like sparkplugs and timing
and exhaust analysis before making a decision on tune. Droplet
size also influences exhaust temperature, especially when the
mixture is not fully vaporized and/or correctly distributed throughout
the chamber. A large drop stuck out by the cold walls of the chamber
and jammed in the squish may have trouble evaporating the heavy
components of the fuel leading to a gas environment of light hydrocarbons
that may become overheated and start exploding. This is one of
the largely overlooked causes of detonation. Detonation that occurs
firstly in the gap between the piston and cylinder wall is usually
because of this problem.
Remember that gasses burn, liquids don't. This means that you
have to convert liquid fuel into a gas before it will burn. Petrol
and methanol are quite different in their vaporization. Methanol
is a single molecule and all the molecules require the same energy
input to convert them into a gas. Petrol is a blend of various
chemicals and they all boil into a gas at different temperatures,
i.e. energy input. As the pressure of the atmosphere around the
droplet increases the temperature necessary to convert the liquid
chemicals into a gas increases too. This makes it difficult to
convert large drops to gas in the short time available at high
rpm, so the general solution is to lean the mixture slightly so
that there is less liquid to be converted by the same amount of
heat ( a high speed lean out). The cylinder then may make more
power but it has less fuel than the most powerful correctly vaporized
mixture. It is important to not band-aid a good feature to prop
up inadequacy. Testing so far has shown that by using the droplet
size of the Quick Change mart Carby correctly, high rpm power
may be increased. At the present time Blackjack is the only car
using a 6 barrel quick change carburetor. I have available 4 barrel
quick change carburetors for sale and I will be developing large
4 barrel Dominator replacements in the future.
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