Piston Engines - Carburettors — Page 115, Lesson 139

I want to walk you through power enrichment in a carburettor. This is the system that keeps your engine alive at high power settings. Here's the core problem we're solving. At power settings above the cruising range, a richer mixture is required to prevent detonation. Detonation is the uncontrolled, explosive burning of the fuel-air charge, and it can destroy an engine. So we deliberately add extra fuel at high power to cool the combustion and keep it controlled. There are two ways to get that rich mixture. The first is an additional fuel supply — a separate jet that adds fuel. The second is to set the carburettor to provide a rich mixture for high power, and then bleed off float chamber pressure to reduce fuel flow for cruising. That second method is the back-suction economizer, which we'll get to. Let's start with the first method — the Power Enrichment or Economizer Jet. Look at Figure 8.9. Here we have a carburettor with an additional needle valve. This valve may be known as a power enrichment jet, or an economizer jet. The needle valve is connected to the throttle control. It is fully closed at all throttle settings below that required to give maximum cruising power at sea level. But as the throttle is opened above this setting, the needle valve opens progressively, until at full throttle it is fully open. So the valve only starts to open when you push past maximum cruise power, and it opens more and more as you approach full throttle. Now, on some engines, the power jet is operated independently of the throttle. Instead of being connected to the throttle, it's operated by means of a sealed bellows which is actuated by manifold pressure. Manifold pressure is the absolute pressure in the intake manifold — it's a direct measure of engine power. In this way, high-power enrichment is related to engine power rather than to throttle position. That's a subtle but important difference: throttle position tells you where the butterfly is, but manifold pressure tells you what the engine is actually doing. Now for the second method — the Back Suction Economizer. This is an air-operated economizer, and it works on the opposite principle. Instead of adding fuel at high power, it reduces fuel at cruise. Here's how it works. When the throttle valve is at a high power setting, the pressure of air flowing past the valve is only slightly below atmospheric pressure. So it has little effect on air pressure in the float chamber. The float chamber stays at near-atmospheric pressure, fuel flows freely through the main jet, and you get a rich mixture. That's what you want at high power. But as the throttle is closed to the cruising position, air flowing past the throttle valve creates a suction. That suction is applied to the float chamber through the economizer channel and air jet. The reduced float chamber pressure reduces fuel flow through the main jet, providing the economical mixture required for cruising. So you see the contrast. With the power enrichment jet, you add fuel at high power. With the back-suction economizer, you reduce fuel at cruise by pulling a vacuum on the float chamber. Both achieve the same goal — rich at high power, lean at cruise — but through opposite mechanisms. Let me make sure you've got the key terms. The power enrichment jet, also called the economizer jet, is the additional needle valve connected to the throttle. The back-suction economizer uses the economizer channel and air jet to apply suction to the float chamber. And the float chamber is the reservoir that holds fuel at a constant level, feeding the main jet. One thing to note — the excerpt mentions Figure 8.10 for the back-suction economizer, but that figure isn't in our available set. The principle is what matters: throttle position controls whether suction is applied to the float chamber, and that suction controls fuel flow.