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Engineering
>> How Things Work >> How Fuel Injection Systems
Work |
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Introduction |
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In trying
to keep up with emissions and fuel efficiency laws, the fuel
system used in modern cars has changed a lot over the years.
The 1990 Subaru Justy was the last car sold in the United
States to have a carburetor; the following model year, the
Justy had fuel injection. But fuel injection has been around
since the 1950s, and electronic fuel injection was used widely
on European cars starting around 1980. Now, all cars sold
in the United States have fuel injection systems. |
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The
Fall of the Carburetor |
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For most
of the existence of the internal combustion engine, the carburetor
has been the device that supplied fuel to the engine. On many
other machines, such as lawnmowers and chainsaws, it still
is. But as the automobile evolved, the carburetor got more
and more complicated trying to handle all of the operating
requirements. For instance, to handle some of these tasks,
carburetors had five different circuits:
- Main
circuit - Provides just enough fuel for fuel-efficient
cruising
- Idle
circuit - Provides just enough fuel to keep the engine
idling
- Accelerator
pump - Provides an extra burst of fuel when the accelerator
pedal is first depressed, reducing hesitation before the
engine speeds up
- Power
enrichment circuit - Provides extra fuel when the car
is going up a hill or towing a trailer
- Choke
- Provides extra fuel when the engine is cold so that it
will start
In order
to meet stricter emissions requirements, catalytic converters
were introduced. Very careful control of the air-to-fuel ratio
was required for the catalytic converter to be effective.
Oxygen sensors monitor the amount of oxygen in the exhaust,
and the engine control unit (ECU) uses this information to
adjust the air-to-fuel ratio in real-time. This is called
closed loop control -- it was not feasible to achieve this
control with carburetors. There was a brief period of electrically
controlled carburetors before fuel injection systems took
over, but these electrical carbs were even more complicated
than the purely mechanical ones.
At first,
carburetors were replaced with throttle body fuel injection
systems (also known as single point or central fuel injection
systems) that incorporated electrically controlled fuel-injector
valves into the throttle body. These were almost a bolt-in
replacement for the carburetor, so the automakers didn't have
to make any drastic changes to their engine designs.
Gradually,
as new engines were designed, throttle body fuel injection
was replaced by multi-port fuel injection (also known as port,
multi-point or sequential fuel injection). These systems have
a fuel injector for each cylinder, usually located so that
they spray right at the intake valve. These systems provide
more accurate fuel metering and quicker response. |
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When
You Step on the Gas |
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The
gas pedal in your car is connected to the throttle valve --
this is the valve that regulates how much air enters the engine.
So the gas pedal is really the air pedal.
When you
step on the gas pedal, the throttle valve opens up more, letting
in more air. The engine control unit (ECU, the computer that
controls all of the electronic components on your engine)
"sees" the throttle valve open and increases the
fuel rate in anticipation of more air entering the engine.
It is important to increase the fuel rate as soon as the throttle
valve opens; otherwise, when the gas pedal is first pressed,
there may be a hesitation as some air reaches the cylinders
without enough fuel in it.
Sensors
monitor the mass of air entering the engine, as well as the
amount of oxygen in the exhaust. The ECU uses this information
to fine-tune the fuel delivery so that the air-to-fuel ratio
is just right. |
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The
Injector |
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A fuel
injector is nothing but an electronically controlled valve.
It is supplied with pressurized fuel by the fuel pump in your
car, and it is capable of opening and closing many times per
second.
When the
injector is energized, an electromagnet moves a plunger that
opens the valve, allowing the pressurized fuel to squirt out
through a tiny nozzle. The nozzle is designed to atomize the
fuel -- to make as fine a mist as possible so that it can
burn easily.
The amount
of fuel supplied to the engine is determined by the amount
of time the fuel injector stays open. This is called the pulse
width, and it is controlled by the ECU.
The injectors
are mounted in the intake manifold so that they spray fuel
directly at the intake valves. A pipe called the fuel rail
supplies pressurized fuel to all of the injectors.
In order
to provide the right amount of fuel, the engine control unit
is equipped with a whole lot of sensors. Let's take a look
at some of them. |
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Engine
Sensors |
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In order
to provide the correct amount of fuel for every operating
condition, the engine control unit (ECU) has to monitor a
huge number of input sensors. Here are just a few:
- Mass
airflow sensor - Tells the ECU the mass of air entering
the engine
- Oxygen
sensor(s) - Monitors the amount of oxygen in the exhaust
so the ECU can determine how rich or lean the fuel mixture
is and make adjustments accordingly
- Throttle
position sensor - Monitors the throttle valve position
(which determines how much air goes into the engine) so
the ECU can respond quickly to changes, increasing or decreasing
the fuel rate as necessary
- Coolant
temperature sensor - Allows the ECU to determine when
the engine has reached its proper operating temperature
- Voltage
sensor - Monitors the system voltage in the car so the
ECU can raise the idle speed if voltage is dropping (which
would indicate a high electrical load)
- Manifold
absolute pressure sensor - Monitors the pressure of
the air in the intake manifold. The amount of air being
drawn into the engine is a good indication of how much power
it is producing; and the more air that goes into the engine,
the lower the manifold pressure, so this reading is used
to gauge how much power is being produced.
- Engine
speed sensor - Monitors engine speed, which is one of
the factors used to calculate the pulse width
There
are two main types of control for multi-port systems: The
fuel injectors can all open at the same time, or each one
can open just before the intake valve for its cylinder opens
(this is called sequential multi-port fuel injection).
The advantage
of sequential fuel injection is that if the driver makes a
sudden change, the system can respond more quickly because
from the time the change is made, it only has to wait only
until the next intake valve opens, instead of for the next
complete revolution of the engine. |
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Engine
Controls and Performance Chips |
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The algorithms
that control the engine are quite complicated. The software
has to allow the car to satisfy emissions requirements for
100,000 miles, meet EPA fuel economy requirements and protect
engines against abuse. And there are dozens of other requirements
to meet as well.
The engine
control unit uses a formula and a large number of lookup tables
to determine the pulse width for given operating conditions.
The equation will be a series of many factors multiplied by
each other. Many of these factors will come from lookup tables.
We'll go through a simplified calculation of the fuel injector
pulse width. In this example, our equation will only have
three factors, whereas a real control system might have a
hundred or more.
Pulse
width = (Base pulse width) x (Factor A) x (Factor B)
In order
to calculate the pulse width, the ECU first looks up the base
pulse width in a lookup table. Base pulse width is a function
of engine speed (RPM) and load (which can be calculated from
manifold absolute pressure). Let's say the engine speed is
2,000 RPM and load is 4. We find the number at the intersection
of 2,000 and 4, which is 8 milliseconds.
RPM
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Load
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1
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2
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3
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4
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5
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1,000
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1
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2
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3
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4
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5
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2,000
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2
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4
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6
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8
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10
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3,000
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3
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6
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9
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12
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15
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4,000
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4
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8
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12
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16
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20
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In the
next examples, A and B are parameters that come from sensors.
Let's say that A is coolant temperature and B is oxygen level.
If coolant temperature equals 100 and oxygen level equals
3, the lookup tables tell us that Factor A = 0.8 and Factor
B = 1.0.
A
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Factor
A
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B
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Factor
B
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0
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1.2
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0
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1.0
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25
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1.1
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1
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1.0
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50
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1.0
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2
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1.0
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75
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0.9
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3
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1.0
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100
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0.8
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4
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0.75
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So, since
we know that base pulse width is a function of load and RPM,
and that pulse width = (base pulse width) x (factor A) x (factor
B), the overall pulse width in our example equals:
8
x 0.8 x 1.0 = 6.4 milliseconds
From this
example, you can see how the control system makes adjustments.
With parameter B as the level of oxygen in the exhaust, the
lookup table for B is the point at which there is (according
to engine designers) too much oxygen in the exhaust; and accordingly,
the ECU cuts back on the fuel.
Real control
systems may have more than 100 parameters, each with its own
lookup table. Some of the parameters even change over time
in order to compensate for changes in the performance of engine
components like the catalytic converter. And depending on
the engine speed, the ECU may have to do these calculations
over a hundred times per second.
Performance
Chips
This leads
us to our discussion of performance chips. Now that we understand
a little bit about how the control algorithms in the ECU work,
we can understand what performance-chip makers do to get more
power out of the engine.
Performance
chips are made by aftermarket companies, and are used to boost
engine power. There is a chip in the ECU that holds all of
the lookup tables; the performance chip replaces this chip.
The tables in the performance chip will contain values that
result in higher fuel rates during certain driving conditions.
For instance, they may supply more fuel at full throttle at
every engine speed. They may also change the spark timing
(there are lookup tables for that, too). Since the performance-chip
makers are not as concerned with issues like reliability,
mileage and emissions controls as the carmakers are, they
use more aggressive settings in the fuel maps of their performance
chips. |
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