You might not know it, but we owe this man, Daniel Bernoulli, a considerable debt. Bernoulli was a Swiss mathematician born into a famous family of mathematicians in 1700 (famous to mathematicians and engineers, that is).
What he described, in 1738, was derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a flowline is the same at all points on that flowline. This requires that the sum of kinetic energy (the energy of motion) and potential energy remain constant. Thus an increase in the speed of the fluid occurs proportionately with an increase in both its dynamic pressure and kinetic energy, and a decrease in its static pressure and potential energy.
What this means, put more simply, is that if an airflow is speeded up (by sucking it through a pipe for example), then more of the energy contained in the airflow is present as motion and since you cannot invent energy from nowhere then it’s pressure energy must decrease.
This means that when an aircraft wing moves through space, the pressure on the longer, curved upper surface is lower than that on the shorter, straight lower surface because the air on the upper surface moves faster just to keep up.
Talking of keeping up, are you getting this?
Back to the pipe with the air being sucked through it. Because the air in our pipe moves faster than the air outside, the pressure outside is higher than the pressure inside. This means that if you introduce a small tube of fluid into the side of the pipe, the fluid will get sucked up the tube due to the lower pressure.
In our world, the pipe and its little tube are called a carburetter!
So there we have a concept that allows us to draw air and fuel into the engine. We now need some refinements: we need to be able to set the ratio of air to fuel for different engine speeds, and it would be helpful for the rider to be able to vary the speed of the engine. The other small problem is that the engine needs different amounts of fuel & air according to its load, its running speed and the air temperature. And the amount of oxygen in the air…and… and…
So, that little tube poking up into the air stream – we can control the size of that by changing the size of the hole through it. That will allow us to control the amount of fuel passing into the air stream for the whole range of the carburettor’s operation. It’s shown in the diagram as item 21, and is properly called the main jet. Of course, we also need a way of opening and closing the carburetter, and for that we have a slide that will block off the airflow, or will allow it to fully open. It’s shown as item 15, and is opened by pulling on the cable K.
Now, remember we said that the air/fuel ratio needed to be controlled across the whole range of operation of the engine. You’ll soon realise that if the fixed main jet provides the correct amount of fuel when the slide is fully open, then it will provide way too much when the slide is not fully open… so we need some more widgets to deal with that, ideally something which is variable. Enter the tapered needle, item 19, to fanfare & drum roll! The tapered needle slides up and down with slide 15, in a tube 18 called the needle jet. The fact that it is tapered offers a variable area between the needle and the needle jet, which gives us a variable fuel flow, and now, we can control the mixture at any slide opening!
More or less. Bantam carburetters of the era we are dealing with are not sophisticated devices, and when you shut the slide, the engine stops (in fact, that is how you stop the engine since there is no ignition switch either!). Most motorcycle carburetters feature yet another jet arranged to bleed some fuel into an air passage around the slide, just a small amount, to let the engine tick over while you put your helmet on and say goodbye to the cat.
But there is more. The most casual observer cannot fail to notice that a motorcycle fuel tank is typically placed above the engine, and, especially if you are inclined to walk the fells & dales of Yorkshire, or you have ever spilt a cup of coffee, that liquids are usually inclined to flow downhill. Thus, we need a device to stop the fuel pouring out of the tank and flooding the carburetter and washing over your feet. We’ll use a float, item 2, coupled to a needle valve, item A to stop the fuel coming in through the fuel pipe, item 7. This is doubly important because the level of the fuel in the needle jet (item 18) affects the ability of our air flow to suck it out – too high, and it will come out too easily and there will be too much fuel in the air flow. Therefore, the float (item 2) and the float needle (item A) are coupled together to set the fuel level at just the right height, just like the float valve in the cistern in your toilet. There are two passages, C and D, to allow the fuel to get to the needle jet.
You will remember also that we said we would need to arrange for the engine to start when it was cold. Engines need more fuel in the air/fuel mixture when they are cold, and to provide this we have spring loaded plunger (item 6) which can be used by the rider to hold the float down and the float needle off its seat – thus allowing a lot of fuel into the carburetter, temporarily.
What he described, in 1738, was derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a flowline is the same at all points on that flowline. This requires that the sum of kinetic energy (the energy of motion) and potential energy remain constant. Thus an increase in the speed of the fluid occurs proportionately with an increase in both its dynamic pressure and kinetic energy, and a decrease in its static pressure and potential energy.What this means, put more simply, is that if an airflow is speeded up (by sucking it through a pipe for example), then more of the energy contained in the airflow is present as motion and since you cannot invent energy from nowhere then it’s pressure energy must decrease.
This means that when an aircraft wing moves through space, the pressure on the longer, curved upper surface is lower than that on the shorter, straight lower surface because the air on the upper surface moves faster just to keep up.
Talking of keeping up, are you getting this?
Back to the pipe with the air being sucked through it. Because the air in our pipe moves faster than the air outside, the pressure outside is higher than the pressure inside. This means that if you introduce a small tube of fluid into the side of the pipe, the fluid will get sucked up the tube due to the lower pressure.
In our world, the pipe and its little tube are called a carburetter!
So there we have a concept that allows us to draw air and fuel into the engine. We now need some refinements: we need to be able to set the ratio of air to fuel for different engine speeds, and it would be helpful for the rider to be able to vary the speed of the engine. The other small problem is that the engine needs different amounts of fuel & air according to its load, its running speed and the air temperature. And the amount of oxygen in the air…and… and…
So, that little tube poking up into the air stream – we can control the size of that by changing the size of the hole through it. That will allow us to control the amount of fuel passing into the air stream for the whole range of the carburettor’s operation. It’s shown in the diagram as item 21, and is properly called the main jet. Of course, we also need a way of opening and closing the carburetter, and for that we have a slide that will block off the airflow, or will allow it to fully open. It’s shown as item 15, and is opened by pulling on the cable K.
Now, remember we said that the air/fuel ratio needed to be controlled across the whole range of operation of the engine. You’ll soon realise that if the fixed main jet provides the correct amount of fuel when the slide is fully open, then it will provide way too much when the slide is not fully open… so we need some more widgets to deal with that, ideally something which is variable. Enter the tapered needle, item 19, to fanfare & drum roll! The tapered needle slides up and down with slide 15, in a tube 18 called the needle jet. The fact that it is tapered offers a variable area between the needle and the needle jet, which gives us a variable fuel flow, and now, we can control the mixture at any slide opening!
More or less. Bantam carburetters of the era we are dealing with are not sophisticated devices, and when you shut the slide, the engine stops (in fact, that is how you stop the engine since there is no ignition switch either!). Most motorcycle carburetters feature yet another jet arranged to bleed some fuel into an air passage around the slide, just a small amount, to let the engine tick over while you put your helmet on and say goodbye to the cat.
But there is more. The most casual observer cannot fail to notice that a motorcycle fuel tank is typically placed above the engine, and, especially if you are inclined to walk the fells & dales of Yorkshire, or you have ever spilt a cup of coffee, that liquids are usually inclined to flow downhill. Thus, we need a device to stop the fuel pouring out of the tank and flooding the carburetter and washing over your feet. We’ll use a float, item 2, coupled to a needle valve, item A to stop the fuel coming in through the fuel pipe, item 7. This is doubly important because the level of the fuel in the needle jet (item 18) affects the ability of our air flow to suck it out – too high, and it will come out too easily and there will be too much fuel in the air flow. Therefore, the float (item 2) and the float needle (item A) are coupled together to set the fuel level at just the right height, just like the float valve in the cistern in your toilet. There are two passages, C and D, to allow the fuel to get to the needle jet.
You will remember also that we said we would need to arrange for the engine to start when it was cold. Engines need more fuel in the air/fuel mixture when they are cold, and to provide this we have spring loaded plunger (item 6) which can be used by the rider to hold the float down and the float needle off its seat – thus allowing a lot of fuel into the carburetter, temporarily.
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