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Transcript of Sunil Seminar
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INTRODUCTION
MPFI stands for multi-point or multi-port fuel injection system.The term MPFI is used
to specify a technology used in Gasoline/petrol Engines. This system is a replacement
of carburettors and used in almost all the vehicles with slight modifications. Fuel
injection is a system for admitting fuel into an internal combustion engine. The primary
difference between carburettors and fuel injection is that fuel injection atomizes the fuel
by forcibly pumping it through a small nozzle under high pressure, while a carburettor
relies on suction created by intake air rushing through a venturi to draw the fuel into the
airstream. MPFI system injects fuel into individual cylinders, based on commands
from the on board Engine management system computer. These techniques result not
only in better power balance amongst the cylinders but also in higher output from
each one of them. In the MPFI system, there is separate supply of fuel to each
individual cylinder regulated by electronic control module or electronic control unit
depending on the input from various sensors. Carburettors were the predominant
method used to meter fuel on gasoline engines before the widespread use of fuel
injection. A variety of injection systems have existed since the earliest usage of the
internal combustion engine.
One of the first commercial gasoline injection systems was a mechanical system
developed by Bosch and introduced in 1952 on the Goliath GP700 and Gutbrod
Superior 600.
The first commercial electronic fuel injection (EFI) system was Electrojector,developed by the Bendix Corporation and was to be offered by American
Motors Corporation (AMC) in 1957. The system incorporated electronic control
module or on board computer for the purpose of metering & supplying of fuel
depending upon the input from various sensors.
At present the modifications are going in the basic construction and reliability
of engine control unit which is the heart of the modern electronically controlled
multipoint fuel injection systems. Automotive leader BOSCH has contributed a
lot in the development of fuel injection systems and majority of patents filed by
it are blessings to the current automotive sector.
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HISTORY OF SUPERSESSION OF CARBURETTORS
CARBURETTOR
THROTTLE BODY INJECTION OR SINGLE POINT INJECTION
MULTIPOINT FUEL INJECTION SYSTEM
At first, carburettors 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 carburettor, 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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FACTORS CONTRIBUTED TOWARDS REPLACEMENT
Failure to meet the condition of stochiometry was one among the prime reasons
that paved the way for gasoline injection systems.When efficient combustion
does not take place, extra fuel nor extra oxygen molecules remain; each fuel
molecule is matched with the appropriate number of oxygen molecules. This
balanced condition is called stochiometry. In the case of carburettors, the air-
fuel mixture was supplied to the intake manifold of the multi-cylinder engine.
The fuel due to the inertia continued to flow in the straight direction resulting
into pudding of fuel near the corner, thus the charge entering the end cylinders
will be rich and the fuel entering the cylinders close to the supply will be lean in
nature. The problem is known as misdistribution of fuel.
The second reason was the inability to meet the increasingly strict exhaust
emissions, imposed by the federal government in the year 1970 & 1980 in U.S.
So in order to more easily comply with government emissions controlregulations, automobile manufacturers, beginning in the late 1970s, furnished
more of their gasoline-fuelled engines with fuel injection systems, and fewer
with complex carburettor systems. There are three primary types of toxic
emissions from an internal combustion engine: Carbon Monoxide (CO), unburnt
hydrocarbons (HC), and oxides of nitrogen (NOx). CO and HC result from
incomplete combustion of fuel due to insufficient oxygen in the combustion
chamber. NOx, in contrast, results from excessive oxygen in the combustion
chamber. The opposite causes of these pollutants makes it difficult to control all
three simultaneously. Once the permissible emission levels dropped below a
certain point, catalytic treatment of these three main pollutants became
necessary. This required a particularly large increase in fuel metering accuracy
and precision, for simultaneous catalysis of all three pollutants requires that the
fuel/air mixture be held within a very narrow range of stoichiometry.
As a result the fuel injection was phased in through the latter '70s and '80s at an
accelerating rate, with the US, French and German markets leading and the UK
and Commonwealth markets lagging somewhat, and since the early 1990s,
almost all gasoline passenger cars sold in first world markets like the United
States, Canada, Europe, Japan, and Australia have come equipped with
electronic fuel injection (EFI).
Fuel injection systems have evolved significantly since the mid-1980s. Currentsystems provide an accurate, reliable and cost-effective method of metering fuel
and providing maximum engine efficiency with clean exhaust emissions, which
is why EFI systems have replaced carburettors in the marketplace. EFI is
becoming more reliable and less expensive through widespread usage. At the
same time, carburettors are becoming less available, and more expensive. Even
marine applications are adopting EFI as reliability improves. Virtually all
internal combustion engines, including motorcycles, off-road vehicles, and
outdoor power equipment, may eventually use some form of fuel injection.
FUNCTIONAL OBJECTIVES
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The functional objectives of a fuel injection system can vary. There are several
competing objectives such as:
Power output Fuel efficiency
Emissions performance
Ability to accommodate alternate fuels
Reliability
Driveability and smooth operation
Initial cost
Maintenance cost
In a MPFI system, there is separate supply of fuel to each individual cylinder, thus the
more accurate controlling of the fuel supply as well as metering is realized. Fuel
injection generally increases engine fuel efficiency. With the improved cylinder-to-
cylinder fuel distribution, less fuel is needed for the same power output.
A fuel-injected engine often produces more power than an equivalent carburetted
engine. Fuel injection alone does not necessarily increase an engine's maximum
potential output. Increased airflow is needed to burn more fuel, which in turn releases
more energy and produces more power. The combustion process converts the fuel's
chemical energy into heat energy, whether the fuel is supplied by fuel injectors or acarburettor. However, airflow is often improved with fuel injection, the components of
which allow more design freedom to improve the air's path into the engine.
MPFI SYSTEM:
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MPFI stands for multipoint fuel injection system
In this system there is a separate supply of fuel to each individual cylinder by
the help of fuel injector housed either near to the intake valve or into the intake
port depending upon whether it is a direct injection into the engine cylinder or it
is an indirect injection.
Earlier systems were using the mechanical means to meter the fuel but recent
developments in the engine control unit or engine control module has resulted in
the electronically based fuel metering and fuel supply system. The EFI systems
possess the ECU which calculates the pulse width for the injectors i.e. the time
for which the injectors are to be kept open.
The process of determining the necessary amount of fuel, and its delivery into
the engine, are known as fuel metering. It is done by the ECU or ECM.
The typical components of an electronically controlled multipoint fuel injection
system are:-
-Injectors
-Fuel Pump
-Fuel Pressure Regulator
-ECM - Engine Control Module; includes a digital computer and circuitry to
communicate with sensors and control outputs.
-Wiring Harness
-Various Sensors
In case of MPFI system the pulse width of the injector is calculated by the ECM
and determines the amount of fuel to be injected in the cylinder. The air supply
should also be in matching and in accordance with the fuel injected in order to
meet the conditions of stoichiometry. The air supply is calculated by the throttlebody and position of throttle determines the air inflow.
FUNCTIONAL DESCRIPTION:
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Central to an EFI system is a computer called the Engine Control Unit (ECU),
which monitors engine operating parameters via various sensors. The ECU
interprets these parameters in order to calculate the appropriate amount of fuel
to be injected, among other tasks, and controls engine operation bymanipulating fuel and/or air flow as well as other variables. The optimum
amount of injected fuel depends on conditions such as engine and ambient
temperatures, engine speed and workload, and exhaust gas composition.
The electronic fuel injector is normally closed, and opens to inject pressurized
fuel as long as electricity is applied to the injector's solenoid coil. The duration
of this operation, called the pulse width, is proportional to the amount of fuel
desired. The electric pulse may be applied in closely controlled sequence with
the valve events on each individual cylinder (in a sequential fuel injection
system), or in groups of less than the total number of injectors (in a batch firesystem). The solenoid is energized as the electric supply is provided to it; this in
turn lifts the pintle or nozzle valve of its seat. When the electric supply to the
injector is disconnected, the restraining spring holds the pintle in its original
position. Fuel metering can be very effectively controlled by the solenoid
operated fuel injector. The prerequisite for the fuel injector is that the opening
and closing of the fuel injector should be purely instantaneous without causing
any dribbling of fuel droplets inside the engine cylinder.
Since the nature of fuel injection dispenses fuel in discrete amounts, and since
the nature of the 4-stroke engine has discrete induction (air-intake) events, the
ECU calculates fuel in discrete amounts. In a sequential system, the injected
fuel mass is tailored for each individual induction event. Every induction event,
of every cylinder, of the entire engine, is a separate fuel mass calculation, and
each injector receives a unique pulse width based on that cylinder's fuel
requirements. It is necessary to know the mass of air the engine "breathes"
during each induction event. This is proportional to the intake manifold's air
pressure/temperature, which is proportional to throttle position. The amount of
air inducted in each intake event is known as "air-charge", and this can be
determined using several methods. (See MAF sensor, and MAP sensor).
The three elemental ingredients for combustion are fuel, air and ignition.
However, complete combustion can only occur if the air and fuel is present in
the exact stoichiometric ratio, which allows all the carbon and hydrogen from
the fuel to combine with all the oxygen in the air, with no undesirable polluting
leftovers. Oxygen sensors monitor the amount of oxygen in the exhaust, and the
ECU uses this information to adjust the air-to-fuel ratio in real-time.
To achieve stoichiometry, the air mass flow into the engine is measured and
multiplied by the stoichiometric air/fuel ratio 14.64:1 (by weight) for gasoline.The required fuel mass that must be injected into the engine is then translated to
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the required pulse width for the fuel injector. The stoichiometric ratio changes
as a function of the fuel; diesel, gasoline, ethanol, methanol, propane, methane
(natural gas), or hydrogen.
Deviations from stoichiometry are required during non-standard operating
conditions such as heavy load, or cold operation, in which case, the mixture
ratio can range from 10:1 to 18:1 (for gasoline). In early fuel injection systems
this was accomplished with a thermo time switch.
Pulse width is inversely related to pressure difference across the injector inlet and
outlet. For example, if the fuel line pressure increases (injector inlet), or the manifold
pressure decreases (injector outlet), a smaller pulse width will admit the same fuel. Fuel
injectors are available in various sizes and spray characteristics as well. Compensation
for these and many other factors are programmed into the ECU's software.
WORKING REPRESENTATION OF MPFI SYSTEM
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CALCULATIONS FOR PULSE WIDTH OF AN INJECTOR:
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The pulse width of a fuel injector is the time for which the fuel injector is kept open.
Thus the amount of fuel entering into the engine cylinder is directly proportional to the
pulse width which is calculated by the electronic control module or electronic control
unit for each individual cylinder for a particular operating condition.
Minutes / Revolution is the reciprocal of engine speed (RPM)
The term Revolutions / Stroke = 1 / 2, whether it is a four-stroke or a two-stroke
engine.
Mass Fuel / Mass Air is the desired mixture ratio, usually stoichiometric, but often
different depending on operating conditions.
1 / (Mass Fuel / Minute) is the flow capacity of the injector, or its size.
TWO- TYPES OF MPFI SYSTEM:
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The D-MPFI system is the manifold fuel injection system. In this type, the vacuum in
the intake manifold is first sensed. In addition, it senses the volume of air by its density.
As air enters into the intake manifold, the manifold pressure sensor detects the intake
manifold vacuum and sends the information to the ECU. The speed sensor also sends
information about the rpm of the engine to the ECU. The ECU in turn sends commandsto the injector to regulate the amount of gasoline supply for injection. When the injector
sprays fuel in the intake manifold the gasoline mixes with the air and the mixture enters
the cylinder.
L-MPFI SYSTEM
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The L-MPFI system is a port fuel injection system. In this type the fuel metering is
regulated by the engine speed and the amount of air that actually enters the engine. This
is called air mass metering or airflow metering.
As air enters into the intake manifold, the air flow sensor measures the amount of airand sends information to the ECU. Similarly the speed sensor sends the information
about the speed of the engine of the ECU. The ECU processes the information received
and sends appropriate commands to the injector, in order to regulate the amount of
gasoline supply for injection. When injection takes place, the gasoline mixes with the
air and the mixture enters the cylinder.
SINGLE POINT INJECTION SYSTEM VERSUS MPFI SYSTEM
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In the single point injection system or throttle body injection there is either one
or two fuel injectors mounted over the top of the throat of the throttle body. The
fuel gets mixed with the air and the air- fuel mixture is discharged to the various
cylinders.
Whereas in the case of a multipoint fuel injection system there is an individual
injector for each individual cylinder spraying either into the intake port or into
the engine cylinder depending on whether it is a direct injection or the indirect
injection. The figure showing the direct as well as indirect injection is given
below:
Single-point injection, called Throttle-body injection (TBI) by General Motors
and Central Fuel Injection (CFI) by Ford, was introduced in the 1940s in large
aircraft engines (then called the pressure carburettor) and in the 1980s in theautomotive world. The SPI system injects fuel at the throttle body (the same
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location where a carburettor introduced fuel). The induction mixture passes
through the intake runners like a carburettor system, and is thus labelled a "wet
manifold system". Fuel pressure is usually specified to be in the area of 10-15
psi. The justification for single-point injection was low cost.
The multi-point fuel injection system can be either pulsed or continuous
injection system. In the pulsed injection system fuel is supplied intermittently in
pulses whereas in the case of continuous injection system the amount of fuel to
be injected is a function of the pressure of the fuel line. In a continuous
injection system, fuel flows at all times from the fuel injectors, but at a variable
flow rate. This is in contrast to most fuel injection systems, which provide fuel
during short pulses of varying duration, with a constant rate of flow during each
pulse. Continuous injection systems can be multi-point or single-point, but not
direct.The most common automotive continuous injection system is Bosch's K-
Jetronic introduced in 1974.
In general for MPFI system controlled electronically, an ECU in is controlled by
the data input from a set of SENSORS located all over the Engine and its
Auxiliaries. These detect the various operating states of the Engine and the
performance desired out of it. Such Sensors constantly monitor : 1) Ambient
Temperature, 2) Engine Coolant Temp., 3) Exhaust/manifold temp., 4) Exhaust
O2 content, 5) Inlet manifold vacuum, 6) Throttle position, 7) Engine rpm, 8)
Vehicle road speed, 9) Crankshaft position, 10) Camshaft position, etc. Based
on a programmed interpretation of all this input data, the ECU gives the
various commands to the Engines fuel intake and spark ignition timing
systems, to deliver an overall satisfactory performance of the Engine from start
to shut down, including emission control.
The sensors are basically the thermistor type and send the signal in the form of
varying voltage to the electronic control unit. The oxygen sensor mounted in the
exhaust manifold is the important sensor determining the nature of combustion
i.e. whether it is complete or incomplete combustion by tracing out the amount
of oxygen contained in the exhaust from the engine.
ADVANTAGES OF MPFI SYSTEM:
Improved Fuel Consumption--Vehicles with carburettors do not get nearly thefuel economy of those with multi-point fuel injection. The underlying reason is
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that fuel delivery systems of these older vehicles are less precise. A multipoint
fuel injection system, which uses one fuel injector for each cylinder of the
engine, delivers just the right amount of gasoline to each cylinder. Thus,
gasoline is not wasted in the process. Over time, the gasoline saved with a
multi-point fuel injection system saves the vehicle owner loads of money.
Emissions--Emissions test results are an important factor today. A car from this
century emits a small fraction of what a vehicle emitted even a few decades ago.
Multi-point injection systems are better for the environment because the
emissions of hazardous chemicals being released when fossil fuels are burned
are minimized. As mentioned above, the more precise delivery of fuel to the
engine means that fewer toxious by products are released when the fuel
combusts within the engine. The implements within the engine meant to cleanthe exhaust have been fine-tuned in a multi-point system to work more
efficiently. Therefore, the engine--and the air--is cleaner as a result of multi-
point systems.
The ECU stores a default code for each sensor; thus in the case of any
malfunction or fault in the sensor, the default value is used in the worst case
conditions so that the engine continues to run satisfactorily.
Better Performance--The performance of an engine suffers with the use of a
carburettor, but multipoint fuel injection allows for far better engine
performance. This is due to a few factors. Instead of allowing for additional air
intake, multi-point injection atomizes the air that is taken through a small tube.
Because multipoint injectors are usually controlled by computers, each function
of a carburettor is performed by a different system component. These systems
also improve the cylinder-to-cylinder distribution of an engine, which allows it
to conserve energy.
1) More uniform A/F mixture will be supplied to each cylinder; hence the
difference in power developed in each cylinder is minimum. Vibration from the
engine equipped with this system is less; due to this the life of enginecomponents is improved.
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(2) No need to crank the engine twice or thrice in case of cold starting as
happens in the carburettor system.
(3) Immediate response, in case of sudden acceleration / deceleration.
(4) Since the engine is controlled by ECM* (Engine Control Module), more
accurate amount of A/F mixture will be supplied and as a result complete
combustion will take place. This leads to effective utilization of fuel supplied
and hence low emission level.
(5) The mileage of the vehicle will be improved.
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