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.
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
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
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.
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)
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
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
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
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
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
Mark Demarco installs the carburetor on Kevin Legape's Winston Cup
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.
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