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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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