Basic Auto Tech Part 3 - by Dr. Tom

[05Roush]

Today we'll begin our stroll through the fuel system. The fuel system is going to be split between three types: carbureted gasoline, propane/natural gas "mixer" and lastly gasoline or diesel fuel injected. First the propane/natural gas system. These fuels are gaseous at ambient temperatures and readily vaporize to mix with air, a device that regulates the fuel's pressure and measures how much fuel and air mixture is fed to the engine, is all that is required here. Gasoline is a little more difficult to blend with air since it is an easily evaporating liquid, but still a liquid. Our predecessors discovered that, when you flow a given volume of gas through a passage, when the passage narrows down, the gas speeds up. When the airflow speeds up the pressure drops! This is how sandblasting tools draw sand from a receptacle full of sand with nothing more than a hose stuck into the sand, and how the garden hose fertilizer/car wash attachments work, sucking fluid out of the attached bottles while the water runs at high flow. This is because of "the Ideal Gas Law" ( PV=nRT) , and to think that the fellow that discovered it never drove! P(pressure) and V(velocity) are inversely related and as one increases the other MUST decrease. An airplane's wing is flatter across the bottom and fatter along its top surface, air is moving faster over the top of the wing and slower along the bottom; (Higher speed=lower pressure). The wing and the plane are sucked up into the sky (or pushed up from under the wing, depending on how you view it), OK Doc, you're thinking, stop showing off and get back to the Car Club topic. All carburetors work this principle in mind, with internal circuits to control fuel and airflow. We'll need a way to supply a tiny bit of fuel and air for it to idle, More to get it to accelerate and a another calibrated circuit to feed it at high speeds. On the outside of the carb are IDLE MIXTURE ADJUSTMENT SCREWS that allow a trickle of fuel into the engine when it's idling and an IDLE SPEED ADJUSTMENT SCREW to control air. If you remove the air cleaner of just about any car, motorcycle, lawnmower or gasoline powered truck, the first thing you'll see is a CHOKE BLADE that forces the engine to use more fuel when it's cold. It is held in an opened or closed position by a calibrated temperature activated spring, depending on engine temperature. When you open this valve you can see a number of large air passages (one, two, three four) through which air is fed to the engine. These are the "Barrels" of the carb. You'll notice that there is a restriction at the top of the barrel that is further constricted by a stem and a ring protruding into the air stream. The restriction is called a VENTURI and the ring and stem are called the VENTURI BOOSTERS. This is why some carbs are called 2-V or 4-V carbs instead of 2 or 4 barrels. Premium four barrel carbs have idle adjustment metering screws on all four barrels. Below the venturis are the THROTTLE BLADES that determine the amount of airflow allowed into the engine. Air and fuel are directed to the individual cylinders by a group of passages called an INTAKE MANIFOLD. That's all we need for now. We want to start our cold engine. The intake manifold is dry inside so we'll have to add fuel by pressing on the gas pedal activating the ACCELERATOR PUMP, squeezing a small amount of fuel through a small spray nozzle into the carb barrels. This pressing of the gas pedal on a cold engine also (1) closes the choke valve, richening the mixture, and (2) sets the linkage so that the engine will run at a higher than normal speed and not stall. Turn the key and the engine inhales and compresses its first gulp of air and fuel and starts. The choke keeps it running at a fast idle and it warms up. When it's warm, either automatically or by driver input via a choke cable for manual chokes, the choke is opened. We give the gas pedal a little hit and the fast idle linkage disengages and the engine speed drops to normal idle. The engine is now running on the idle circut. Air is allowed to enter around the mostly closed throttle blades by the idle SPEED adjustment screw, while the mixture is correct because of the idle MIXTURE adjustment screws. The action of the piston pumping air from the carb but restricted by the closed throttle blades, results in a high vacuum in the intake manifold. All fine unless we want to speed up. When we step on the gas there's a moment when air freely enters the engine, manifold vacuum drops very low, and fuel flow lags behind. This is a lean condition and the engine would likely stall, cough back or just have no power if it continued to run starving for fuel. The accelerator pump squirts extra fuel at this time to cover the momentary lean condition until airflow is high enough inside the venturis to supply fuel through this main circuit. While the vacuum is low and airflow signal at the booster venturi is still too low to supply adequate fuel, another circuit, the power valve, senses the low vacuum and supplies more fuel temporally. You can think of the carb's circuits as layers of a cake, since the circuits are cumulative and add to the previous circuits with the exception of the accelerator pump. Our carbureted fuel delivery system is REACTIVE, we control the airflow and our device reacts by supplying an appropriate amount of fuel. Fuel injection is PROACTIVE which we'll see later. Inside the carb is a well called a fuel bowl that holds gas supplied by the fuel pump. This is normally delivered to the carb at about 4 to 7 pounds per square inch of line pressure. Inside the bowl is a float that is hinged from above and controls a valve that keeps the fuel at a specified level in the bowl. This level is critical to the calibration of fuel delivery. As fuel is used, the pressure insures that more is quickly available so the fuel level is maintained. Too much pressure overpowers the valve and floods the engine with fuel. A restriction called a MAIN METERING JET allows fuel from the float bowl to enter the main circuit. We can change the size of this jet to accurately change the air/fuel mixture ratio of our engine. Inside the main circuit is a system of components that begin to mix the fuel with air. At the bottom of the circuit is the main jet and at the top is MAIN AIR JET. Inside of this internal tube os a perforated smaller tube called an EMULSION tube that helps to introduce air to the fuel and it is this interplay that depends on an exact fuel level to work. Too high a level and we get more fuel than we want, to low and we have too much air. This emulsion is drawn through the stem and into the booster venturi by the vacuum resulting from the air flow in the venturi, and distributed into the airstream. Finally we'll need an additional enrichment device to compensate for work load, our engine will need more fuel to do hard work. When our engine is taxed with a tougher task such as climbing a hill, the air flow through through the carb will be high but the manifold vacuum will be low. We can add more fuel with a vacuum sensing device called a Power Valve or an Economizer circut, depending on the carbs brand. It opens under low vacuum and closes when the vacuum returns to normal. This will supply more fuel when we need it but not waste it when we don't. Control of the carbs available airflow is the driver activating the front two barrels (primary), the rear two (secondary) can be engaged when airflow demands are high and the carb is at Wide Open Throttle by a Vacuum actuator, or an airvalve; or mechanically opened at any speed by linkage. Usually Mechanical Secondary operation is used for racing and vacuum for street driven cars. This more conservative method of operation prevents wasting fuel, bogging the engine down with airflow it can't use that disrupts venturi metering, and washing the oil off of the cylinder walls and diluting the engine's oil with gasoline. We can change jets for more or less fuel, change power valves to adjust when fuel is delivered under load, change carburetor types to control available air delivery and change carb size to fit the demands of our engines size or expected RPM range. Next time Fuel Injection, Diesel, then cams, exhaust and power adders and more. We covered a lot of ground with our trip through the carb today. -Dr.Tom