This will give you some
understanding of Electronic Fuel Injection
BASIC EFI THEORY
This article is intended for those people who have had little experience with
electronic fuel injection and would like to have a handle on the basics.
History
EFI as applied to automobiles in mass production use was first introduced in the
late 1960s. The Bosch D Jetronic analog system was widely used by Volvo and VW.
This system was well engineered and quite reliable given the technology
available at the time.
The '70s saw the introduction of the excellent L Jetronic system and licensed
spinoffs built in other countries. Emission regulations and the energy crisis in
the mid '70s caused most car manufacturers to consider the switch to EFI.
Nissan, Toyota and BMW notably equipped almost all of their higher end models
with the Bosch system by 1982.
Many German and American car companies were slow to embrace EFI for reasons
unknown. By 1985, the first digital systems were in widespread use worldwide by
most manufacturers to some degree and the days of the carburetor were numbered.
Today, over 95% of all cars produced are EFI equipped. EFI is certainly not new,
as its roots were firmly established over 30 years ago.
Basic Theory
EFI uses solenoid valves called
injectors to meter fuel delivery. Most vehicles today use 1 injector per
cylinder. When the solenoid is energized, fuel sprays out into the valve port.
Fuel is delivered to the injector by a high pressure electric pump at around 40
psi. Fuel delivery is controlled by the injectors which are cycled by the
computer. The computer produces a signal to open the injectors for a certain
length of time depending on engine conditions relayed by sensors. The longer
that the injector is open, the more fuel is injected. As engine load and rpm are
increased, the injector open times are increased to match increasing airflow.
This computer output signal is called the injector pulse width. The longer the
pulse width, the more fuel is injected.
Injector
Engine Requirements
Standard spark ignited, 4 stroke engines require that the correct proportion of
fuel be mixed with the incoming air for efficient operation. This proportion is
in the range of 13 parts air to 1 part fuel for best power, 15 to 1 for best
emissions and 17 to 1 for best economy. Most modern engines aim for a ratio of
around 14.7 to 1 for the majority of cruising and medium power conditions. This
is the chemically correct ratio which results in the lowest average emissions
and reasonable power. A rich condition is characterized by an excess of fuel and
a lean condition is characterized by an excess of air or lack of fuel.
As rpm is increased, up to a point, airflow also increases and fuel flow must
increase to match it. As the throttle is opened at a given rpm, airflow
increases to a certain point and again, fuel flow must follow airflow.
Fuel System
EFI fuel systems consist of a tank, pump, fuel rail, regulator, injectors and
return line. Fuel is drawn from the tank by the pump which steps up pressure to
around 40 psi. Fuel pressure is controlled by the fuel pressure regulator
located on one end of the fuel rail by bleeding fuel back to the tank through
the return line. The pump always puts out an excess of fuel so large quantities
are returned back to the tank during idle and low speed conditions and less as
engine demand increases. The fuel rail is essentially a tubular fuel manifold
designed to carry fuel to the injectors as well as hold them in place on the
intake manifold. The injectors are usually sealed with O-rings on each end. One
end has the fuel entering from the rail and the other end spigots into the
manifold. The injector has an 2 pin electrical plug to carry switching current
to the solenoid windings. When energized, the solenoid core is pulled back which
pulls back a sealing pintle, disc or ball, allowing fuel to spray out in a fine,
conical pattern.
External type pump
In-tank type pump
Fuel rail
Fuel pressure regulator
Air Metering and Measurement
The amount of air entering the engine is controlled by a conventional butterfly
valve on most engines located in a throttle body assembly.
Throttle body with TPS
Airflow measurement is by one of two basic methods; Mass Airflow and Speed
Density. The mass airflow method uses either a spring loaded flap attached to a
potentiometer or a heated wire mounted in front of the throttle body to sense
actual airflow. The position of the flap or amount of current required to keep
the wire heated to a certain temperature is relayed to the computer as a voltage
signal. A certain voltage equals a certain airflow rate.
Nippondenso/Bosch vane type airflow meter
Flap detail. Note air temp sensor.
Hitachi Non-Vane meter
The speed density system uses a solid state pressure transducer to measure the
pressure in the intake manifold combined with rpm and air temperature to
indirectly determine airflow. Again, a certain pressure relates to a certain
voltage which is relayed to the computer.
From left to right: 1 Bar, 2 Bar, 3 Bar MAP sensors
Sensor Inputs
Most EFI systems measure the same basic 6 inputs;
RPM
Most systems measure rpm off of the ignition coil tachometer pulse or crank
triggered magnetic/Hall effect sensors. Rpm is considered a primary input signal
on all EFI systems. Most systems generate an injection pulse for every tach
pulse so as rpm is increased, the frequency of injection pulses is also
increased.
Airflow
On mass airflow type systems, this input is also considered a primary input
signal. X amount of air requires Y amount of fuel. As rpm and throttle opening
is increased, airflow increases to a point.
Manifold Pressure
On speed density type systems, this input is essential when combined with the
rpm signal to calculate airflow. As the throttle is opened, the manifold
pressure increases which will require more fuel.
Throttle Position
This input is a secondary input on most systems. It is required mainly for
acceleration enrichment when the throttle is rapidly opened. By looking at the
rate of change of throttle blade angle, the computer can determine how quickly
the throttle is being opened and can supply the extra fuel required momentarily
to alleviate the lean condition. Throttle position is measured by a
potentiometer attached to the throttle shaft. Think of the TPS input as acting
like the accelerator pump on a carburetor.
Throttle position sensor
Some systems can use blade angle instead of manifold pressure for load
information. This method is frequently employed on racing engines with hot
camshafts and 1 throttle plate per cylinder where the use of a MAP sensor is
difficult.
Water Temperature
Water temperature is a secondary input required mainly to ensure proper starting
and warm-up of the engine. When the engine is cold, the air to fuel ratio must
be very rich to enable enough fuel to vaporize for proper starting. The computer
increases the injector pulse width to supply extra fuel when cold and tapers
this fuel off as the water temperature increases. Once the water warms past 120
degrees or so, the computer does not need to add any extra fuel.
Water temperature sensor
Where a carburetor chokes off air to richen the mixture when cold, EFI squirts
in extra fuel to achieve the same effect.
Air Temperature
This is a secondary input required especially on speed density systems. The
sensor is usually mounted in the intake manifold or air filter area. As the air
temperature drops, its density increases. Denser air requires more fuel. As the
temperature of the inducted air increases, the computer reduces the pulse width
to compensate for lower density. Mass airflow systems are not critically
affected by operation without an air temperature sensor because the airflow
meter is already measuring the air mass entering the engine.
Air temperature sensor
Oxygen Sensor
This sensor is employed in closed loop systems to modify the basic pulse width
after the fact. It is mounted into the exhaust manifold area. By looking at the
oxygen content of the exhaust gasses after combustion, the computer can
determine if the air/fuel ratio is too rich or too lean for optimum combustion
and adjust the next few injections accordingly. This sensor is primarily
employed for emission control and to a lesser degree, fuel economy. For the
lowest average emissions, the air/fuel ratio must be kept around 14.7 to 1.
Oxygen sensor
Under full throttle conditions, this sensor input is ignored by the computer so
that the engine can produce more power by runner a richer mixture. This is
called open loop mode and the computer is supplying the injector pulse width
from tables based on all of the other sensor inputs. Once throttle opening and
rpm are reduced to cruising conditions, most systems will jump back into the
closed loop mode where they will stay for a large portion of the time on most
street driven applications.
Basic Operation
As explained in the Basic Theory section, the computer processes all of the
voltage signals from the various sensors to determine the engine operating
conditions at the moment and delivers the appropriate pulse width to the
injectors. If engine airflow increases by 10%, the pulse width is also increased
by about 10% to keep the air/fuel ratio constant. If the rpm is doubled from
2000 to 4000 rpm, the number of injections are also doubled to double the fuel
flow.
The computer looks at the changes in sensor inputs every few milliseconds in
order to be ready to modify the pulse width if any parameter changes.
Information provided by
Racetec,INC
