Operation EFI: Voodoo, Witchcraft, and Carburetors

Carburetors: You will never find a more complex hive of ports and orifices (Image from Wikimedia Commons)	In Operation EFI: The Point, I insinuated that carburetors utilized wizardry to combine air and fuel. While this isn't entirely true, it isn't entirely false. The modern carburetor - which sounds like an oxymoron - is a highly developed device that operates quite ingeniously, with nothing but ports, valves, and orifices, and tricky physics doing the work.

In its purest form, a carburetor is a device that uses the established properties of fluid flow to mix precise amounts of fuel and air. However, achieving this requires satisfying many varying conditions, including engine speed and load, air temperature, pressure, sudden changes in throttle position and others. The carburetor must match four conditions: Idle - The engine needs a relatively constant amount of fuel and air to maintain idle speed. Part Throttle - When the throttle is partially open, but not significantly. Usually right off idle as you accelerate from a light. Cruise - The throttle is open significantly, but not at WOT, such as highway cruising and moderate acceleration. Wide-Open Throttle (WOT) - When the engine is under full load and demanding maximum fuel. A carburetor uses the laws of fluid dynamics to meter fuel. With venturis, or devices that speed the flow of air, useful pressure differentials are created according to Bernoulli's Equation that push fuel and air to-and-fro. In essence, Bernoulli's Equation  relates the velocity and pressure of any fluid stream, ultimately expressing that an increase in fluid flow velocity decreases the pressure within that fluid stream. By using a venturi to speed up flow, fuel can be pushed from areas of atmospheric pressure to this low-pressure area. In order to satisfy all of these requirements in a car, there are five "circuits" in a modern carburetor. The word circuit is used because it defines the path of flow; much in the way wires are a path for electricity, passages and orifices are a path for fluid flow.  Before I get into their functions I will describe how a carburetor regulates fuel flow to the circuits. Via a fuel line, fuel is fed into a float bowl, or a small volume in which a certain amount of fuel is kept. This amount of fuel is set at a certain level, called a float level, because a float-operated valve regulates the level of fuel in the bowl. When fuel is consumed and the level drops, the float lowers, opening a needle valve and allowing fuel in. Once filled to the set level, the float rises and closes the valve.  This level is proportional to the amount of fuel fed to each circuit - increase the level and the amount of fuel delivered to each circuit increases, due to the increasing hydrostatic pressure in the float bowl. The float bowl is always kept at atmospheric pressure by the use of vents. The first of the circuits is the idle circuit. The idle circuit consists of an orifice that leads from the bottom of the float bowl and branches off to a port above the venturi (the idle air bleed) and a port below the throttle blade. The size of this orifice and the air bleed is critical and directly controls the air fuel ratio at idle. Fuel flow can further be adjusted by means of a mixture screw. When the throttle is completely closed, the revolving engine creates a vacuum within the intake plenum and air is pushed through the idle circuit tube, mixed with fuel (or emulsified) and introduced into the intake plenum.  The second circuit is the transition or progression circuit. When the throttle is cracked open, the pressure in the plenum rises. Since the pressure in the plenum and atmospheric are now closer, less fuel/air is pushed through the idle circuit into the plenum. To match this increased airflow, a small orifice is uncovered as the throttle is cracked. The sudden increase in air velocity past the throttle plate again reduces the pressure at the orifice exit, and atmospheric pressure in the bowl and idle air bleed can now push more emulsified fuel into the plenum. The greater the speed of the incoming air, the greater the pressure drop and the more fuel is delivered.  The third circuit is the main or power circuit, so called because it is responsible for delivering the majority of the fuel to the engine when under load. Once the throttle blades are open enough and the transfer slot cannot supply enough fuel, the speed of the air is great enough to allow atmospheric air to push  through the main air bleed, emulsify fuel in the main circuit, (fed by the main jet, the orifice controlling the amount of fuel delivered) and into the plenum. By sizing the air bleeds and main jets properly, AFR can be controlled somewhat at varying engine speed and load. A note concerning jets, bleeds, and orifices: since these all have fixed dimensions, changes in atmospheric pressure or temperature will also change the resultant fuel and air delivery (and ultimately air-fuel ratio), with no on-the-fly modification possible. Thus, to account for summer or winter temperatures, or large altitude changes, the main jets must be exchanged for those of a different size, and the idle bleeds and screws adjusted to maintain proper AFR.

The fourth circuit is the power circuit, which many hot-rodders know from Holley's infamous 'power valve.' The power circuit runs parallel to the main circuit, and is similar except that it is activated by a vacuum-controlled valve. When the pressure in the plenum falls below a certain amount (indicating high load on the engine) the valve opens, allowing additional fuel to be inducted into the plenum. This circuit is only activated under high loads and near WOT.

The fifth circuit accounts for sudden changes in throttle position. When the throttle is opened suddenly, there is a drop in flow speed and an increase in pressure. Because of this increase, fuel is not pushed into the intake stream, despite more air entering. To balance this additional air, the throttle is connected to a lever and a plunger - when the throttle moves, it displaces the plunger and forces additional fuel through a nozzle and into the venturi, where it atomizes. This is known as the accelerator pump. By changing the displacement of the pump (usually only a few ccs,) the size of the nozzle, and the geometry of the activation lever, the duration, amount, and timing of this squirt of additional fuel can be tuned. However, since it is a mechanical linkage, it can also not be adjusted on-the fly, and changes in conditions will change the AFR.

A word about chokes: A choke is an additional butterfly valve that sits above the venturis. Activated in cold  weather, the choke blade closes and the throttle blade opens slightly. When the car is started, the size of the area the air can enter through is reduced, both speeding up flow and reducing total air input. By doing so, the mixture is forced rich, and more fuel is added than is stoichiometrically correct. This condition helps overcome the tendency of fuel to puddle on cold surfaces - more fuel is required to reach the cylinders without falling out of suspension. By cracking open the throttle blade slightly, the engine operates at a higher speed, which aids in fuel atomization. As the engine warms up, the choke opens either by the driver or by means of an electromechanical coil.

The carburetor accomplishes a lot while not being mechanically complicated. Using pressure gradients generated by manipulating air flow velocity, fuel can be metered out relatively precisely. The main drawbacks of the carburetor include lack of on-the-fly tuning or compensation for environmental factors, poorer fuel atomization at low speeds and loads and less uniform fuel distribution. That is not to say that carburetors are not an effective solution - engines have achieved very high power and efficiency levels using carburetors.

Despite this impressive technical achievement, when it comes to efficiency and power in any situation, an EFI system will shine. EFI systems can actually measure the AFR directly, adjust for temperature and pressure changes and a host of other variables. Tuning a carburetor requires experience, knowledge, and patience, and in this technological world I have little practical knowledge regarding them. I do, however, know how to use a computer, and have a basic grasp of EFI tuning. That knowledge and the aforementioned advantages of EFI are the basis for my switch.

For more info see:

    http://www.chevyhiperformance.com/techarticles/83118_carburetor_basics/index.html

    http://www.thisoldtractor.com/gtbender/mg_manuals/dellorto_manual.pdf