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IIT Kanpur Kanpur, India (208016)
Alternative Fuels & Advance in IC Engines
Course Instructor Dr. Avinash Kumar Agarwal
Professor Department of Mechanical Engineering
Indian Institute of Technology Kanpur, Kanpur
Fuel Injection Systems
Introduction
Fuel injection is a system for admitting fuel into an internal combustion engine. It
has become the primary fuel delivery system used in automotive engines, having
replaced carburetors during the 1980s and 1990s. A variety of injection systems
have existed since the earliest usage of the internal combustion engine.
The primary difference between carburetors and fuel injection is that fuel injection
atomizes the fuel through a small nozzle under high pressure, while a carburetor
relies on suction created by intake air accelerated through a Venturi tube to draw
the fuel into the airstream.
Modern fuel injection systems are designed specifically for the type of fuel being
used. Some systems are designed for multiple grades of fuel (using sensors to adapt
the tuning for the fuel currently used). Most fuel injection systems are for gasoline
or diesel applications.
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Introduction
The modern digital electronic fuel injection system is more capable at optimizing
these competing objectives consistently than earlier fuel delivery systems (such as
carburettors).
Benefits of fuel injection include smoother and more consistent transient throttle
response, such as during quick throttle transitions, easier cold starting, more
accurate adjustment to account for extremes of ambient temperatures and changes
in air pressure, more stable idling, decreased maintenance needs, and better fuel
efficiency.
Fuel injection also dispenses with the need for a separate mechanical choke, which
on carburetor-equipped vehicles must be adjusted as the engine warms up to
normal temperature. Furthermore on spark ignition engines (direct) fuel injection
has the advantage of being able to facilitate stratified combustion which have not
been possible with carburetors.
Introduction As emission regulations continued to tighten, the demands placed on fuel systems
increased further and it was not sufficient to simply provide flexibility in injection
timing control. Additional drivers that pushed the evolution of diesel fuel injection
systems included:
Maintaining accuracy of timing and fuel metering over the expected life of the engine placed increased demands on the repeatability of timing and injection quantity and on injector durability.
Injection pressures increased to maintain engine thermal efficiency and to enable some reduction in exhaust emissions.
Injector response times became faster to allow predictable injection of small injection quantities. This was an important feature to enable multiple injection events.
Better control over the opening and closing of the injection nozzle to avoid uncontrolled secondary injections and provide a sharp end of injection. This was also important for enabling multiple injections.
Improved mechanical efficiency of the injection system to contribute to the overall goal of improving engine efficiency.
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Common Rail Type Fuel Injection System Electronic control common rail type fuel injection system drives an integrated fuel pump at an ultrahigh pressure to distribute fuel to each injector per cylinder through a common rail. This enables optimum combustion to generate large horsepower, and reduce PM and fuel consumption.
Multi-point injection systems
In multi-point injection systems, the fuel pressure regulator has an inlet connection from the fuel rail, and an
outlet that lets fuel return to the tank.
A control diaphragm and pressure spring determines the exposed opening of the outlet, and the amount of
fuel that can return. So the strength of the pressure spring determines fuel pressure in the fuel rail, and
keeps it at a fixed value.
However, the pressure in the intake manifold varies considerably with changes in engine speed, and with
load, so the pressure drop across the injector must also be taken into account.
For any injection duration, if fuel is held at constant pressure, then, as manifold pressure varies so does the
amount of fuel delivered. That means fuel pressure must be held constant above manifold pressure.
This is done by sealing the spring housing of the pressure regulator, and letting it sense manifold pressure
via a connecting hose. Then, when manifold pressure alters, so does the fuel pressure.
When manifold pressure is low, as at idling, fuel pressure is low. As manifold pressure rises, towards open
throttle, so does fuel pressure.
Since the injectors are all subjected to the same pressure, they all inject an equal amount of fuel.
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Multi-point injection systems
The quantity of fuel delivered is thus controlled very accurately by the pulse width of the injector.
Manifold pressure sensing is not required in throttle-body systems, as the injection occurs above the throttle
plate, at atmospheric pressure. So fuel pressure is determined by the force of the regulator spring, acting on
the diaphragm.
The injectors are sealed into the manifold by O-rings that prevent air entering, at that point. The O-rings,
together with plastic caps on the injector nozzles, also act as a barrier to heat being transferred to the injector
body.
For a short time after an engine is switched off engine temperature keeps rising and that can cause vapor in
the fuel lines. This pump stops this with a non return valve on its outlet which maintains the pressure in the
fuel line during that short time.
This pressure will decay over about 20 to 30 minutes, but it ensures effective hot-starting characteristics.
When the engine is running, the circulation of fuel ensures cool fuel is being delivered at all times, and vapor
formation is prevented.
The pump control circuit normally allows the pump to operate for a few seconds only, when the ignition is
switched on.
And thereafter, during cranking.
And when the engine is running above a specified minimum RPM.
ADVANTAGES OF MULTI POINT FUEL INJECTION SYSTEM More uniform air-fuel mixture will be supplied to each cylinder, hence the difference in power
developed in each cylinder is minimum.
The vibrations produced in MPFI engines is very less, due to this life of the engine component is
increased.
No need to crank the engine twice or thrice in case of cold starting as happen in the carburetor
system.
Immediate response, in case of sudden acceleration and deceleration.
The mileage of the vehicle is improved.
More accurate amount of air-fuel mixture will be supplied in these injection system. As a result
complete combustion will take place. This leads to effective utilization of fuel supplied and hence low
emission level.
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MPFI SYSTEMS
Advantages:
o increased power and torque
o improved volumetric efficiency
o more uniform fuel distribution
o more rapid engine response to changes in throttle position
o more precise control of the equivalence ratio during cold-start and engine warm-up
MPFI SYSTEMS
Actual amount of fuel injected per cycle per cylinder is to calculated based on method to
determine the air flow rate. These are
o (1) speed-density system (D-Jetronic),
o (2) air-flow meter to measure air flow directly (the Bosch L-Jetronic) or hot-wire air
mass flow meter
o (3) Mechanical, air-flow-based metering (the Bosch K-Jetronic system)
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Jetronic
Jetronic is a trade name of a fuel injection technology for automotive petrol
engines, developed and marketed by Robert Bosch GmbH from the 1960s
onwards.
Bosch licensed the concept to many automobile manufacturers. There are
several variations of the technology offering technological development and
refinement.
D-Jetronic (1967–1976)
Analog fuel injection, 'D' is from German: "Druck" meaning pressure. Inlet
manifold depression (vacuum) is measured using a pressure sensor located
in, or connected to the intake manifold, in order to calculate the duration of
fuel injection pulses. Originally, this system was called Jetronic, but the
name D-Jetronic was later created as a retronym to distinguish it from
newer versions.
D-Jetronic was a precursor of modern common rail systems as it had
constant pressure fuel delivery to the injectors and pulsed injections, albeit
grouped (2 groups of injectors pulsed together) rather than sequential
(individual injector pulses) as on later systems.
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speed-density system (D-Jetronic)
Speed density electronic multi point port fuel injection system (D-Jetronic)
Fig. (1): Schematic of a speed-density system
Air flow calculation is based on engine speed and manifold pressure and air temperature
The electrically driven fuel pump delivers the fuel through a filter to the fuel line A pressure regulator maintains the pressure in the line at a fixed value (e.g. 270
kN/m2 , 39 Ib/in2, usually relative to manifold pressure to maintain a constant fuel pressure drop across the injectors).
Branch lines lead to each injector and the excess fuel returns to the tank via a second line
The inducted air flows through the air filter, past the throttle plate to the intake manifold
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An electromagnetically actuated fuel-injection valve is located either in the intake manifold tube or the intake port of each cylinder.
The major components of the injector are the valve housing, the injector spindle, the magnetic plunger to which the spindle is connected, the helical spring, and the solenoid coil.
When the solenoid is not excited, the solenoid plunger of the magnetic circuit is forced, with its seal, against the valve seat by the helical spring and closes the fuel passage.
When the solenoid coil is excited, the plunger is attracted and lifts the spindle.
Air mass flow meter system (L-Jetronic)
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Air mass flow meter system (L-Jetronic)
o It uses air-flow meter to measure air flow directly which is placed upstream of the throttle.
o The meter measures the force exerted by the flowing air stream on a plate as it is displaced and it provides a voltage proportional to the air flow rate
Advantages of direct air-flow measurement are: • Automatic compensation for tolerances, combustion chamber deposit
buildup, wear and changes in valve adjustments; • The dependence of volumetric efficiency on speed and exhaust backpressure
is automatically accounted for; • Less acceleration enrichment is required because the air-flow signal precedes
the filling of the cylinders; • Improved idling stability; and • Lack of sensitivity of the system to EGR since only the fresh air flow is
measured.
L-Jetronic
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The mass of air inducted per cycle to each cylinder, m,, varies as
o Thus the primary signals for the electronic control unit are air flow and engine speed.
o The pulse width is inversely proportional to speed and directly proportional to air flow.
o The engine block temperature sensor, starter switch, and throttle valve switch provide input signals for the necessary adjustments for cold Start, warm-up, idling, and wide-open throttle enrichment.
K-Jetronic
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K-Jetronic
o Mechanical, air-flow-based metering, continuous injection systems o Air is drawn through the air filter, flows past the air-flow sensor, past the throttle valve,
into the intake manifold, and into the individual cylinders o The fuel is sucked out of the tank by a roller-cell pump and fed through the fuel
accumulator and filter to the fuel distributor o A primary pressure regulator in the fuel distributor maintains the fuel pressure constant.
Excess fuel not required by the engine flows back to the tank. o The mixture-control unit consists of the air-flow sensor and fuel distributor. It is the
most important part of the system, and provides the desired metering of fuel to the individual cylinders by controlling the cross section of the metering slits in the fuel distributor.
o Downstream of each of these metering slits is a differential pressure valve which for different flow rates maintains the pressure drop at the slits constant.
o Fuel-injection systems offer several options regarding the timing and location of each injection relative to the intake event.
K-Jetronic
The K-Jetronic mechanical, injection system injects fuel continuously in
front of the intake valves with the spray directed toward the valves.
Thus about three-quarters of the fuel required for any engine cycle is stored
temporarily in front of the intake valve, and one quarter enters the cylinder
directly during the intake process.
With electronically controlled injection systems, the fuel is injected
intermittently toward the intake valves and fuel-injection pulse width to
provide the appropriate mass of fuel for each cylinder air charge varies from
about 1.5 to 10 ms over the engine load and speed range.
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K-Jetronic
In crank angle degrees this varies from about 10" at light load and low speed to
about 300" at maximum speed and load. Thus the pulse width varies from being
much less than to greater than the duration of the intake stroke.
To reduce the complexity of the electronic control unit, groups of injectors are often
operated simultaneously.
In the Bosch L-Jetronic system, all injectors are operated simultaneously. To
achieve adequate mixture uniformity, given the short pulse width relative to the
intake process over much of the engine load-speed range, fuel is injected twice per
cycle; each injection contributes half the fuel quantity required by each cylinder per
cycle. (This approach is called simultaneous double-firing.)
In the speed-density system, the injectors are usually divided into groups, each
group being pulsed simultaneously
K-Jetronic
o Sequential injection timing, where the phasing of each injection pulse relative to its
intake valve lift profile is the same, is another option.
o Engine performance and emissions do change as the timing of the start of injection
relative to inlet valve opening is varied.
o Injection with valve lift at its maximum, or decreasing, is least desirable.
o For example, for a six-cylinder engine, two groups of three injectors may be used. Injection for each group is timed to occur while the inlet valves are closed or just starting to open.
o The other group of injectors inject one crank revolution later
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