Question:Why did the car makers change from carburetors
to fuel injection?
Answer:The standard reply to this question is because
fuel injection provided a better way to meet government fuel economy
and emission standards, which is true. But equally important
is the fact that fuel injection is an all-round better fuel delivery
system.
Fuel injection has no choke, but sprays atomized fuel directly
into the engine. This eliminates most of the cold start problems
associated with carburetors. Electronic fuel injection also integrates
more easily with computerized engine control systems because the
injectors are more easily controlled than a mechanical carburetor
with electronic add-ons. Multiport fuel injection (where each
cylinder has its own injector) delivers a more evenly distributed
mixture of air and fuel to each of the engine's cylinders, which
improves power and performance. Sequential fuel injection (where
the firing of each individual injector is controlled separately
by the computer and timed to the engine's firing sequence) improves
power and reduces emissions. So there are some valid engineering
reasons as well for using fuel injection.
TYPES OF FUEL INJECTION
The earliest fuel injection systems were mechanical and were
more complex than carburetors. Consequently, they were expensive
and their use was limited. Chevrolet introduced a Rochester mechanical
fuel injection system back in 1957, and it became the "hot"
setup on Corvettes up through 1967.
The Europeans, however, were the real leaders in fuel injection
technology. Bosch offered an early electronic system on Volkswagen
Squarebacks in the late 1960s and early 1970s. By the early 1980s,
almost all of the European auto makers were using some type of
Bosch multiport fuel injection system.
In the mid-1980s, the domestic auto makers first turned to
"throttle body" injection as a stop-gap system as they
made the transition from electromechanical carburetors to fuel
injection.
THROTTLE BODY INJECTION (TBI)
Throttle body injection is much like a carburetor except that
there's no fuel bowl, float, needle valve, venturi, fuel jets,
accelerator pump or choke. That's because throttle body injection
does not depend on engine vacuum or venturi vacuum for fuel metering.
Fuel is sprayed directly into the intake manifold instead of being
siphoned in by intake vacuum.
A TBI fuel delivery system consists of a throttle body with
one or two injectors and a pressure regulator. Fuel pressure
is provided by an electric pump. It's a relatively simple setup
and causes few problems -- but doesn't provide all of the advantages
of a multiport or sequential fuel injection system.
MULTIPORT INJECTION
The next step up from TBI was multiport injection. Engines
with multiport injection have a separate fuel injector for each
cylinder, mounted in the intake manifold or head just above the
intake port. Thus, a four cylinder engine would have four injectors,
a V6 would have six injectors and a V8 would have eight injectors.
Multiport injection systems are more expensive because of
the added number of injectors. But having a separate injector
for each cylinder makes a big difference in performance. The
same engine with multiport injection will typically produce 10
to 40 more horsepower than one with TBI because of better cylinder-to-cylinder
fuel distribution. Injecting fuel directly into the intake ports
also eliminates the need to preheat the intake manifold since
only air flows through the manifold. This, in turn, provides
more freedom for tuning the intake plumbing to produce maximum
torque. It also eliminates the need to preheat the incoming air
by forcing it to pass through a stove around the exhaust manifold.
There are other differences between multiport injection systems.
One is the way in which the injectors are pulsed. On some systems,
all the injectors are wired together and pulse simultaneously
(once every revolution of the crankshaft). On others, the injectors
are wired separately and are pulsed sequentially (one after the
other in their respective firing order). The latter approach
is more complicated and requires more expensive electronic controls,
but provides better performance and throttle response by allowing
more rapid changes in the fuel mixture.
