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Physics and the art of starting a Bullet: Part 2- The Choke
Last time we saw there was a bit more to the simple act of starting the Bullet than we suspected. And then the "choke" is something we could never really swallow. (Ooops, pardon the pun)
I mean, some
people proudly saying that their Bullet never requires the choke even in the bitter-est winter, while every carburettor manufacturer (that we have seen) provides it, and most of us having to rely on it not only on
winter mornings, but just about every morning....sure can do with some explaination!
As usual, we start with a rewind. To run without burning it's valves or sooting it's plug, indeed, to run at all, an engine
must have it's fuel and air in a certain ratio. It may be 1:12 or 1:14 or 1:16, we aren't going to split hair here. (But what does that have to do with temperature?!)
Patience...That was just the Chemistry,
not the whole story. Kinetics makes it clearer: The fuel and air must be mixed well, and be able to burn in a short time- a VERY short time!!!
Imagine an engine running at 3000 RPM. That is 50 rotations per
second. 25 cycles per second. Each cycle of 4 strokes. Only one of which burns the fuel. So the fuel has 1/100 th of a second to burn in, and what doesn't, goes out of the exaust, wasted. And the Bullet engine may
rev almost upto 6000 RPM.
OK, so now we can easily imagine that huge drops are not going to burn completely or properly. The surface may "catch fire", but they will be out of the exaust before the
combustion is complete.
But even that is not the complete story. (sorry!). Remember that the "fire" begins at the spark-plug, and the spark is not present for the entire duration of the
power-stroke! So the fuel nearest to the spark plug is ignited, and the flame spreads from drop to drop until the whole volume is burnt. Even if the amount of fuel is correct, but the drops are large and few, the
combustion may not propogate thruout the volume. In effect, the engine may not run.
Whether it is cold or hot, the carburettor works almost the same. But on the way to the cylinder, the fuel vapourises into
smaller droplets due to heat of the inlet port and is readier to burn than in the case of a cold engine on a winter morning. To enable starting in this condition, either the drops must be made small enough without
requiring heat, or more (big) drops must be supplied until the engine is hot enough to vapourise the fuel by it's heat.
Ah, now the story is clear! Now, let's go on to "chokes"
A classic
choke reduces the apperture thru which the air enters the carburettor, increasing the velocity, and also increasing the suction on the fuel in the carburettor. Which gives the smaller and also more drops required
for the cold engine. However, the "choke" in our Bullets does not work like that. (It's technical name is infact "starting carburettor", not choke.)
What it does is open a valve letting a
lot more fuel get drawn into the engine. NOW we are in a position to analyse the myths about chokes! If the engine can start cold without the choke, it means it is getting enough fuel for that thru the slow-speed
jet. So it's quite likely the jet is bigger than necessary, wasting fuel when the engine is hot! Or, it may be that the "air-screw" is adjusted too rich, with similar effect. Or, the fuel level in the
float chamber is set too high. Or, the engine has been flooded already and hence starts. In short, it is "two negatives making a positive". Nothing to be happy about!
Then, an engine that starts without choke will give poorer mileage ?
Actually, the Bullet carburettor has 4 ranges, determined by: the slow-jet, the slide cutaway, the needle position, and the main-jet, which
are effective at different speeds. The mileage is determined by whichever (combination) is dominant at the riding speed. We will see this in detail when we discuss the working and tuning of the carburettor.
In the above discussion, we have taken a quick overview of the physics of "carburetting" and seen the choke operation in it's light. Our aim is to ultimately understand observations, diagonise problems,
and tune correctly. We shall not be re-designing carburettors, or aiming at PhD. So we have made some simplifications regarding vaporisation/atomisation of fuel. Trust me, it will not hinder us in our modest,
practical goals.
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