Piston Engines - Carburettors — Page 110, Lesson 131

I want to walk you through the heart of the carburettor system now — how fuel actually gets into the airstream, and then the first important modification we make to a simple carburettor. Let's start with the basic layout. Where fuel pumps are used, we include a fuel pressure gauge in the system. That gauge gives us an indication of pump operation — it tells the pilot that the pump is actually working and delivering fuel at the right pressure. Now, here's the key principle. Air intake pressure — that's the atmospheric air pressure at the mouth of the carburettor — acts on the fuel in the float chamber. The float chamber is connected to a fuel discharge tube, and that tube is located in the throat of the Venturi. Remember, the Venturi is that narrowed section of the intake where airflow speeds up. Here's the driving force behind the whole system: the difference in pressure between the float chamber and the throat of the Venturi. That pressure differential is what provides the force necessary to discharge fuel into the airstream. Think of it this way — the float chamber is at relatively high pressure, the Venturi throat is at low pressure because the air is speeding through it, and that difference literally pushes the fuel out of the discharge tube. And here's the beautiful proportionality in this design. As airflow through the Venturi increases, the pressure drop increases. That means a higher pressure differential acts on the fuel, which increases its flow — and crucially, it increases in proportion to the airflow. So the carburettor naturally meters more fuel when more air is flowing. That's the self-regulating magic of the Venturi. Now, what controls exactly how much fuel flows at any given pressure differential? The size of the main jet in the discharge tube. The main jet is the calibrated restriction. Its size determines the quantity of fuel discharged at any particular pressure differential, and therefore it controls the mixture strength — the ratio of air to fuel. The simple carburettor I've just described contains all the basic components necessary to provide a suitable air/fuel mixture — but only over a limited operating range. That limitation is exactly why we need modifications, and that's what I want to look at next. The first modification is the pressure balance duct. Here's the problem it solves. To maintain the correct rate of discharge of fuel through the main jet, the pressure in the float chamber and the pressure at the air intake must be equal. You might think — why not just drill a hole in the float chamber cover plate and let atmospheric pressure in? Well, that's not satisfactory. Here's why: due to manoeuvres and the speed of the aircraft, the changes in pressure localized around the air intake would not be readily transmitted to the float chamber. In other words, the pressure at the intake changes quickly and locally as the aircraft manoeuvres, and a simple vent hole can't keep the float chamber in step with it. The pressures would drift apart, and the fuel discharge rate would be wrong. So how do we get equalized pressure conditions? We connect the float chamber directly to the air intake by a duct — and that duct is called the pressure balance duct. That's the modification. It ties the float chamber pressure directly to the intake pressure so they stay equal. And here's a bonus — this duct does more than one job. It also supplies air to the diffuser, and in some carburettors it's used to provide altitude mixture control. So the pressure balance duct is a multi-purpose component: it equalizes pressure, feeds the diffuser, and in some designs helps adjust the mixture as altitude changes. Let me show you the layout. shows the general layout and what the carburation system must achieve. is the simple float chamber carburettor with the U-tube principle. And contains all the basic components. Then shows the air bleed diffuser, and Figure 8.3 shows the pressure balance duct mechanism itself. So to sum up where we are: the Venturi creates a pressure differential that drives fuel flow, the main jet sizes that flow to set mixture strength, and the pressure balance duct is our first fix to keep the float chamber pressure equal to the intake pressure so the metering stays accurate through manoeuvres.