Piston Engines - Carburettors — Page 110, Lesson 133

Let’s pick this up with the diffuser, because it’s the part of the carburettor that fixes a real problem we haven’t hit yet. Here’s the problem. As engine speed increases, more air flows through the Venturi, and that airflow gets faster. Now, fuel and air don’t behave the same way when they flow. Air is light and accelerates readily; fuel is heavier and more sluggish. So as the airflow speeds up, the fuel flow doesn’t keep pace proportionally — the fuel lags behind. The net result is that the proportion of fuel to air rises, and the mixture becomes richer than we want. That’s the enrichment effect we’re trying to correct. To overcome this, some carburettors are fitted with a diffuser. Let’s look at how it works. The diffuser is a well fitted in the carburettor float chamber. Inside that well, the fuel level sits at a certain height. As engine speed is progressively increased above idling, the fuel level in the diffuser well drops. As the fuel level drops, it progressively uncovers more air holes in the side of the well. Those holes allow more air into the discharge tube. That incoming air reduces the pressure differential across the fuel, and by reducing that pressure differential, it prevents the enrichment of the air/fuel mixture. So the diffuser is a self-correcting device: the faster you go, the more air bleeds in, and the mixture stays where it should be. There’s a second benefit. The process of drawing both air and fuel through the discharge tube also has the effect of vaporizing the fuel more readily, particularly at low engine speeds. So you get better fuel atomisation, which helps combustion. Now let’s understand the principle behind the air bleed diffuser, because it’s a neat piece of physics. Imagine a tube immersed in a liquid. If you apply suction to the top of that tube, the suction is sufficient to raise a column of liquid to a certain height up the tube. That’s the basic principle — suction lifts the liquid. Now, suppose we make a small hole in the tube, under the same conditions. What happens? Bubbles of air will enter the tube through that hole, and the liquid will be drawn up the tube in smaller drops rather than as a continuous stream. In other words, the liquid is “diffused” — it’s made to intermingle with the air. The air “bleeds” into the tube, and that bleeding air reduces the forces acting on the fuel, retarding the flow of liquid through the tube. So the air bleed both breaks the fuel into droplets and slows its flow. That’s the whole principle: bleed air in, diffuse the fuel, retard the flow. So to tie it together: the diffuser well uses that air-bleed principle to counteract the natural enrichment at higher engine speeds, and as a bonus it improves vaporization, especially at low speeds. That’s the diffuser and the air bleed diffuser principle.