Driving Impressions and Tech Talk

Hey, where did all the carburetors go?
The evolution of the fuel delivery system
David Cipolloni
January 7, 2003

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Since the early days of gravity fed fuel systems, designers and engineers have worked to provide a better means of delivering fuel to the combustion chamber of an internal combustion engine.  Stoichiometric or Theoretical Combustion is the ideal combustion process during which a fuel is burned completely. Delivering the fuel (in our case we will reference gasoline) in the proper atomized/vaporized air/fuel ratio under all conditions is a daunting task.

If we try to send gasoline to the combustion chamber in a liquid state the engine will not burn it all, we then have a condition known as ďfloodingĒ. We must first atomize the fuel, just like the atomizer nozzle on top of a bottle of Windex glass cleaner. The first real float-type carburetor was invented around 1896 in Germany. Prior to this invention carburetors used very primitive means of delivering fuel, like wicking the fuel into the air stream.

The Maybach carburetor, named after its inventor, was very similar in principle to the carburetors that followed for many years known as updraft carburetors. Even sidedraft and downdraft carburetors use the same basic principles of fuel bowl, float and mixing chamber. Around the turn of the century the venturi was integrated into the carburetor to help atomize the fuel before sending it to the engine. By reducing the diameter of the air inlet (venturi) in the carburetor the speed of the airflow increases, just like putting your thumb over the end of a garden hose. The increase in airspeed siphons the fuel from the fuel bowl and breaks it into small particles (atomization). This basic principle remains today on any gas engine utilizing a carburetor, pretty much limited to lawn tractors and NASCAR.

Old 1-barrel carburetor

Carburetors began as single barrels (one venturi), then evolved into two, three and four barrel carburetors. In their heyday it was common to find 2 four barrel carburetors on everything from dragsters to street machines. Most foreign machines would utilize multiple sidedraft carburetors, while Detroit iron sported multiple downdraft carburetors. Fuel injection first appeared as a means of fuel delivery around the same time as the carburetor, but didnít gain in popularity until WWII when the Germans started using it in the aviation field. Both fuel injection and turbocharging proved their worth during WWII.

Carburetors rely on one essential element, a fast moving stream of airflow at all times. On a mild day down around sea level this is typically not a problem, but at higher altitudes everything changes. When a piston moves down in its cylinder a vacuum is created. By definition a vacuum is an absence of pressure. When this vacuum is created the atmospheric pressure existing outside the engine will quickly rush to fill the cylinder, and by doing so will pass through the carburetor. The fast moving air will siphon the fuel from the bowl, the fuel will be atomized in the venturi where it will then pass through the intake manifold on its way past the intake valve and into the cylinder.

Typical newer-generation carburetor

However, at higher altitudes or on very warm days, atmospheric pressure can be much less. When the piston moves down in the cylinder on the intake stroke there will be a vacuum created in the cylinder. With less air pressure at altitude the air passing through the venturi will be diminished. With very little airflow through the venturi there is little chance of siphoning much fuel from the carburetor. During WWII the American Flying Tigers quickly realized their teeth bearing P-40 Warhawk lacked the ability to fly at the altitudes of their enemy, due to its carbureted non-turbocharged engine.

Lets take a closer look at some of the more specific problems that have led to the demise of the carburetor. Providing a proper air/fuel ratio to any given gasoline engine under all conditions is not easy. There are many factors that will affect the proper delivery of fuel, since this delivery relies on the movement of air through the induction system. Letís face it, siphoning fuel by running some air past an orifice, and creating a low pressure area at that orifice, is only constant when the airspeed remains constant. Carburetors, by nature, typically deliver a little more fuel than is needed. To understand this letís take a look at a few of the systems on a typical carburetor.

Carburetor Exploded View

Idle Circuit: Delivers fuel by allowing the siphon effect to take place beneath the throttle plate. This happens when your foot is off the accelerator and the engine is at idle.

Power Enrichment Circuit: Under acceleration an engine will require more fuel. To supply more fuel a carburetor is typically equipped with metering rods or a power valve. These devices are sensitive to manifold vacuum and will open as manifold pressure drops under acceleration. We would routinely modify these systems during the 70ís in order to correct severe hesitation problems. As manufacturers scrambled to make their vehicles EPA compliant the amount of fuel delivered under acceleration was reduced.

Main Metering Circuit: Delivers fuel in the venturi area when the throttle is open, also by the siphon effect. There is a transition from idle circuit to part throttle and then main metering as you step on the accelerator.

Accelerator Pump Circuit: This is a mechanical device, usually a small plunger or diaphragm. When you step on the accelerator there would be a moment before fuel would begin to siphon from the main metering circuit. The mechanical pump inside the carburetor would spray a stream of fuel into the venturi every time you step on the accelerator. If not for the accelerator pump there would be a drastic hesitation or stalling condition every time you stepped on the accelerator. At this point the carburetor usually provides an excess amount of fuel to cover up this problem. This is one of the main reasons a gasoline engine will emit high levels of unburned hydrocarbons (gasoline) under acceleration.

Choke Circuit: This is the small butterfly valve you usually see when you remove the air cleaner assembly from the carburetor. In order to get the engine started upon cranking a generous supply of fuel is needed in each of the cylinders. Since some of the fuel will cling to the intake runners on its way to the cylinder a rich fuel mixture will be needed to get the engine started. Itís been a few years but does anyone remember the procedure for starting an engine with a carburetor? You would usually step on the accelerator a couple of times.

This would work the accelerator pump inside the carburetor and send copious amounts of fuel into the intake system. The butterfly valve would close causing a low pressure in the venturi so a full siphoning affect would occur in the idle and main metering circuits. Thatís a whole lot of fuel folks, get it wrong and the first whiffs of exhaust would be black with soot and other nasty exhaust emissions. The choke butterfly (valve) would open progressively as the engine temperature rose. This usually occurred with the use of a bimetal spring attached to the choke valve and heated by electricity or exhaust gases.

The carburetor, essentially a mechanical device, was a fabulous invention, used for over 100 years and going strong on lawnmowers, weed whackers and other lawn equipment. Itís not uncommon for a lawnmower to emit much higher levels of exhaust emissions than a modern automobile. In order for automobile manufacturers to meet EPA guidelines for exhaust emissions and fuel economy a move from carburetors to fuel injection was inevitable.

Typical fuel injector sprays the exact amount a fine mist of fuel right into combustion chamber
Animated image courtesy of RC Fuel Injection

Since throttle body fuel injection is considered an inexpensive replacement for a carburetor, we will make reference only to multi-port fuel injection, a system that utilizes one fuel injector for each cylinder. A multi-port fuel injection system is capable of delivering a pressurized and atomized charge of fuel to each cylinder right at the intake valve. This charge of fuel can be regulated by the fuel injection system through a network of sensors and a computer processor. We will not reference the older mechanical fuel injection units in this article. Each fuel injector, a small electro-mechanical device, is capable of regulating the delivery of fuel by opening and closing (pulsing) many times per second. The amount of time the injector remains open is known as the pulse width.

To create a precise delivery of pressurized fuel to each cylinder a management system is required. The management system is controlled electrically and consists of several sensors and a computer processing unit. To understand this lets take a look at a few of the sensors in a typical electronic fuel injection system.

Throttle Position Sensor: This small sensor is connected to the throttle valve shaft, it sends throttle position data to the computer processor. This is how the computer knows how far you have depressed the accelerator pedal.

Mass Air Flow Sensor: This sensor is located in the air intake and tells the computer the mass of air that is entering the engine. This is how the computer knows how much fuel will be needed to mix with the air.

Coolant Temperature Sensor: This sensor is located in the engines cooling system, the sensor tells the computer if the engine is at operating temperature or not. A cold engine will typically require more fuel for initial start and warm-up.

Manifold Absolute Pressure: This sensor can determine the pressure in the intake manifold. As the throttle valve is opened the pressure in the intake manifold drops. This occurs because the pressure inside the cylinders starts to equalize with atmospheric pressure as the throttle is opened. In this way the computer will know approximately how much power the engine is making.

Engine Speed Sensor: This sensor provides engine RPM data to the computer. This is the only way the computer knows the actual engine speed in order to regulate the pulse width of the injectors.

Oxygen Sensor: This sensor is located in the exhaust system close to the engine. The amount of oxygen in the exhaust is an indication of how rich or lean the fuel mixture is. This data allows the computer to alter the air/fuel ratio.

The computer processing unit will take data from all the sensors and create complex algorithms in order to control fuel delivery. Data tables and parameters are embedded in the computers memory, these tables/parameters allow the computer to extrapolate a fuel delivery plan hundreds of times per second. Many racers will reprogram their computers with new data/parameters in order to provide more fuel needed in high performance applications.

Mark Demarco installs the carburetor on Kevin Legape's Winston Cup race engine.
Photo: Ford

Even though NASCAR continues to use antiquated carburetors on their cars, the rest of the world has progressed to modern day technology. You wonít see technicians changing carburetor jets at a CART or F1 race, but you will see them reprogramming the computer processing units with their laptop computers.

There have been rumors that NASCAR may adopt the 3.5 Liter IRL engine as their next engine standard.  And why not, at least it's fuel njected.

But, I still have a fondness for nostalgia soÖ.. until the EPA comes looking for me Iíll always keep a few screwdrivers handy so I can fiddle with the carburetors on my lawn equipment. And who wonít miss those backfires from the exhaust pipes on the stockers, every time they back off the gas, and all that fuel builds up in the exhaust system? Now thatís the kind of stuff the fans love. 

Comments can be sent to the author at feedback@autoracing1.com.




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