Piston Engines - Carburettors — Page 110, Lesson 128

Let’s start with the big picture. We’re looking at the simple float chamber carburettor, and I want you to hold two physical principles in your head, because the whole device is built from them. The first is the ‘U’ tube principle, and the second is the Venturi principle. Let’s take the ‘U’ tube first. Imagine a tube bent into the shape of a ‘U’, filled with liquid. If the pressure acting on both legs is the same, the liquid level in either leg will be the same. Now, if you create a pressure difference across the two legs, that difference will cause the liquid to flow. That’s the whole idea — a pressure difference drives flow. In practice, we don’t use a literal ‘U’ tube. One leg of the ‘U’ is opened out to form a small tank — that’s the float chamber. A constant level is maintained in that chamber by a float and valve mechanism, which regulates the flow of fuel coming from a fuel pump, or pumps, delivering a supply from the main aircraft tanks. So the float and valve act as the level keeper, and the fuel pump is the source. Now the second principle, the Venturi. This rests on Bernoulli’s Theorem, which states that the total energy per unit mass along any one streamline in a moving fluid is constant. The fluid possesses energy because of its pressure, its temperature, and its velocity. If one of those changes, one or both of the others must also change, so that the overall energy stays the same. Here’s how that plays out in the carburettor. As air passes through the restriction of the Venturi, its velocity increases. Because total energy is constant, that increase in velocity causes a drop in pressure and a drop in temperature. The pressure drop at the throat of the Venturi is proportional to the mass airflow. And that pressure drop is what we use to make fuel flow from the float chamber — we place one leg of the ‘U’ tube in the Venturi. So the Venturi creates the low pressure, and the ‘U’ tube principle uses that pressure difference to draw fuel out. Now let’s put the whole system together. In a float chamber carburettor, airflow to the engine is controlled by a throttle valve, and fuel flow is controlled by metering jets. Engine suction provides a flow of air from the air intake, through the Venturi in the carburettor, to the induction manifold. The air speeds up as it passes through the Venturi, and a drop in pressure occurs at that point. But here’s an important contrast: within the induction manifold, pressure rises as the throttle is opened. So the Venturi throat is the low-pressure point, and the manifold pressure behaves differently — it rises with throttle opening. Fuel is contained in the float chamber. It can be supplied by gravity, or by an electrical booster pump, or by an engine-driven fuel pump. And a constant level is maintained in the chamber by the float and needle-valve. That needle-valve is the same float-and-valve mechanism we mentioned with the ‘U’ tube principle — it keeps the fuel level steady so the pressure difference across the system stays predictable. So the whole story is this: the Venturi creates a pressure drop proportional to mass airflow, the float chamber maintains a constant fuel level, and the pressure difference between the chamber and the Venturi throat draws fuel through the metering jets into the airstream. The throttle valve controls how much air flows, and the metering jets control how much fuel flows. Take a look at Figure 8.2 — that’s the simple float chamber carburettor showing the ‘U’ tube principle in action. And Figure 8.1 shows the general layout of the carburation system.