Piston Engines - Carburettors — Page 115, Lesson 141

Right, let's pick up with the accelerator pump. When you snap the throttle open quickly, the airflow through the carburettor responds almost instantly — a bigger volume of air rushes through the Venturi. But the fuel metering system is slower to react to that sudden change. So for a brief moment, you get a temporary weakening of the mixture. That's called a flat spot, and in the worst case it can cause what we call a 'weak cut' — the engine stumbles or cuts out — before the fuel flow catches up with the airflow. To overcome that, the carburettor is fitted with an accelerator pump, which is linked directly to the throttle. Whenever you open the throttle, it forces fuel straight into the Venturi, so the mixture stays correct during that transient moment. That's the pump shown in Figure 8.11. Now, there are two refinements you need to know about. In some pumps, there's a controlled bleed past the pump piston. That bleed allows you to open the throttle slowly without passing fuel to the engine — so the pump only delivers its shot when you actually need it, on a rapid opening. In other pumps, instead of a bleed, there's an additional delayed-action plunger. That plunger supplies an extra quantity of fuel to the engine for a few seconds after the throttle movement has ceased — so it keeps enriching the mixture briefly even after you've stopped moving the throttle. Then we come to priming. Normally, a priming pump supplies fuel to the induction manifold, close to the inlet valve. That's the proper way to get fuel into the engine for a cold start. But in the absence of such a device, it is permissible on some aircraft to prime the engine by pumping the throttle — in other words, exercising the accelerator pump several times to squirt fuel in. Now, this practice must be discouraged in any other circumstance, because it increases the chance of carburettor fires. So it's a last resort, only where the aircraft is approved for it. Let me walk you through the simple light aircraft fuel system shown in Figure 8.12, because it ties all this together. The fuel tanks are rigid tanks fitted in the wings, and they're filled by the overwing method — you open the filler cap on top of the wing and fill from above. The fuel is drawn from the tanks by a mechanical or electrical fuel pump. It passes through a tank selector — that's how you choose which tank you're drawing from — and then through a filter, before being delivered to the carburettor. The carburettor here is the gear or vane type, which refers to the type of fuel pump built into it. Engine priming is achieved by use of a priming pump, which takes fuel from the filter housing and delivers it directly to the inlet manifold. So you can see the difference: the main fuel pump feeds the carburettor, while the priming pump feeds the inlet manifold for starting. The fuel system is monitored for contents and pressure — you have gauges for both. And finally, the fuel drains allow any water to be removed before flight. Water is denser than fuel, so it settles at the bottom of the tanks and in the drains, and you must drain it off before you fly. That's the whole system: tanks, selector, filter, pump, carburettor, priming, and the drains that keep the fuel clean and dry.