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Fuel Induction and Ignition
System
Fuel Induction
• In spark ignition engines the air and fuel are usually mixed prior to entry into
the cylinder.
• The ratio of mass flow of air to the mass flow of fuel must be held roughly
constant at about 14.7 for proper combustion.
• Initially a purely mechanical device known as a carburetor was used to mix the
fuel and the air
• Most modern cars use electronic fuel-injection systems
Fuel Induction
•
Nozzles that inject a spray of fuel into the intake air.
•
Controlled electronically and mechanically.
•
Many modern SI engines have multipoint port fuel injectors. Spray fuel directly behind
the intake valve.
•
Multipoint port injector systems are better than carburetors or throttle body injector
system at giving consistent AF delivery.
•
Because of short duration after fuel injection for evaporation and mixing to occur, point
injectors must spray very tiny droplets of fuel. Smaller droplet at high rpm.
•
Engines with pump systems and controls on each cylinder can be more finely adjusted
than those with single-pump systems.
•
The duration of injection is determine by feed back from engine and exhaust sensors
- Oxygen sensor in exhaust manifold.
- RPM, T, air flow rate, throttle position
Fuel Induction
Sensors
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
Fuel Injector
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.
ECU
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.
ECU cont.
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.
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.
Real control systems may have more than 100
parameters, each with its own lookup table.
Performance chips are made by aftermarket
companies, and are used to boost engine
power. The tables in the performance chip
will contain values that result in higher fuel
rates during certain driving conditions.
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.
Ignition System
Ignition Timing
The timing of the spark is critical to success. There is a small delay from the time of the
spark to the time when the fuel/air mixture is all burning and the pressure in the cylinder
reaches its maximum. If the spark occurs right when the piston reaches the top of the
compression stroke, the piston will have already moved down part of the way into its power
stroke before the gases in the cylinder have reached their highest pressures.
To make the best use of the fuel, the spark should occur before the piston reaches the top of
the compression stroke, so by the time the piston starts down into its power stroke the
pressures are high enough to start producing useful work.
The timing of the spark is important, and the timing can either be advanced or retarded
depending on conditions.
Spark advance: The faster the engine speed, the more advance is required
Minimizing emissions, take priority when maximum power is not required. For instance, by
retarding the spark timing (moving the spark closer to the top of the compression stroke),
maximum cylinder pressures and temperatures can be reduced.
Spark Plug
The electricity must be at a very high voltage in order to
travel across the gap and create a good spark. Voltage at
the spark plug can be anywhere from 40,000 to 100,000
volts.
Spark plugs use a ceramic insert to isolate the high voltage
at the electrode, ensuring that the spark happens at the tip
of the electrode and not anywhere else on the plug; this
insert does double-duty by helping to burn off deposits.
Ceramic is a fairly poor heat conductor, so the material
gets quite hot during operation. This heat helps to burn off
deposits from the electrode.
Ignition Coil
The coil is a high-voltage transformer made up of two coils of
wire. One coil of wire is called the primary coil. Wrapped
around it is the secondary coil. The secondary coil normally
has hundreds of times more turns of wire than the primary
coil.
Current flows from the battery through the primary winding of
the coil. The primary coil's current can be suddenly disrupted
by the breaker points.
The key to the coil's operation is what happens when the
circuit is suddenly broken by the points. The magnetic field
of the primary coil collapses rapidly. The secondary coil is
engulfed by a powerful and changing magnetic field. This
field induces a current in the coils -- a very high-voltage
current (up to 100,000 volts) because of the number of coils
in the secondary winding. The secondary coil feeds this
voltage to the distributor via a very well insulated, highvoltage wire.
Distributor
The distributor handles several jobs. Its first job is to
distribute the high voltage from the coil to the correct
cylinder. This is done by the cap and rotor. The coil is
connected to the rotor, which spins inside the cap. The
rotor spins past a series of contacts, one contact per
cylinder. As the tip of the rotor passes each contact, a
high-voltage pulse comes from the coil. The pulse arcs
across the small gap between the rotor and the contact
(they don't actually touch) and then continues down the
spark-plug wire to the spark plug on the appropriate
cylinder.
The points also control the timing of the spark. They may
have a vacuum advance or a centrifugal advance. These
mechanisms advance the timing in proportion to engine
load or engine speed.
Distributorless Ignition
The coil in this type of system works the same way as the larger, centrally-located coils. The
engine control unit controls the transistors that break the ground side of the circuit, which
generates the spark. This gives the ECU total control over spark timing.
Systems like these have some substantial advantages. First, there is no distributor, which is
an item that eventually wears out. Also, there are no high-voltage spark-plug wires, which
also wear out. And finally, they allow for more precise control of the spark timing, which can
improve efficiency, emissions and increase the overall power of a car.
Concept question
1. What is the main reason for a black smoke
coming out from a car exhaust:
• There is too much fuel for the air in the engine cylinder which
results in rich mixture and incomplete combustion;
• There is not enough fuel for the air in the engine cylinder that
result in weak mixture that cause poor combustion and loss of
power.
2. What is the main reason for a white smoke
coming out from a car exhaust:
● Caused by water and or antifreeze entering the cylinder,
and the engine trying to burn it with the fuel;
● Caused by engine oil entering the cylinder area and being
burned along with the fuel air mixture.