Piston Engines - Carburettors — Page 110, Lesson 136

We're starting a new topic now: mixture control on a carburettor. Let me set the stage first, because this builds directly on the venturi principle we've already covered. Here's the core problem we're solving. As altitude increases, the weight of air drawn into the cylinder decreases, because air density decreases. For a given intake velocity, the pressure drop in the venturi will decrease as ambient density decreases. But here's the catch: the fuel flow due to that pressure drop will not decrease by the same amount. So the mixture becomes richer. This progressive richness with increased altitude is unacceptable for economic operation. That's the whole reason we need mixture control — to correct that automatic enrichment as we climb. Now, there are two main ways to do this. Let's look at the first: the needle type mixture control. With a needle type, a cockpit lever is connected to a needle valve in the float chamber. Movement of that cockpit lever raises or lowers the needle, and that varies fuel flow through an orifice to the main jet. So the position of the needle controls the mixture strength. And in the fully-down position, it will block fuel flow to the main jet entirely — which gives you a means of stopping the engine. That's your idle cut-off on this type. Now, a practical point before we move on. The smallest orifice in the whole fuel system is the fuel jet. Because it's so small, it's vulnerable. To prevent any blockage of the jets by dirt or debris, a fuel strainer can be fitted just before them in the fuel line. That's your protection against contamination. Now the second method: the air bleed mixture control. This one works differently — it operates by controlling the air pressure in the float chamber, thus varying the pressure differential acting on the fuel. Here's how it works. A small air bleed between the float chamber and the venturi tends to reduce air pressure in the float chamber. A valve connected to a cockpit lever controls the flow of air into the float chamber. When that valve is fully open, the air pressure is greatest, and the mixture is fully rich. As the valve is closed, the air pressure decreases, thus reducing the flow of fuel and weakening the mixture. And there's a clever extra feature on the carburettor illustrated. The valve also includes a pipe connection to the engine side of the throttle valve. When this pipe is connected to the float chamber by moving the cockpit control to the 'idle cut-off' position, float chamber air pressure is reduced and fuel ceases to flow — thus stopping the engine. So to summarise the contrast: the needle type physically blocks fuel flow with a needle in the fully-down position, while the air bleed type stops the engine by reducing float chamber pressure through that idle cut-off pipe connection. Both give you a way to shut the engine down, but through completely different mechanisms.