43 Penske/Detroit Diesel electronic unit injection

Diesel injection equipment and systems



233



Fuel in



Fig. 6.49 The Penske Corporation have been producing this electronically controlled

unit injector with a solenoid-actuated spill valve



injector and fuel system in the event of the needle’s being fouled by debris

and failing to shut completely.



6.44



The Cummins PT system



Cummins has been using this system since 1924. The initials PT, standing

for pressure–time, imply that the quantity of fuel flowing through the orifice

into the injector cup is determined by its pressure and the time the orifice



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The Motor Vehicle

Inputs

Timing

reference

Synch

reference



Outputs

PROM



Command

pulse

Injectors



Throttle

position

Turbo-boost

pressure



EDU

Feedback

Electronic

control

module



Oil temperature



Diagnostic data

link (DDL)



Oil pressure

Stop engine light



Coolant level



Check engine light



–



+



Battery



Fig. 6.50 This is the first generation DDEC electronic control system for the GM unit

injectors. It differs from the second generation system in that the command pulse and

feedback are directed to and from the injectors through an EDU instead of directly.

The EDU (electronic distributor unit) functions as a high current switching unit for

energising the solenoids



remains open. The layout of the system is illustrated diagrammatically in

Figs 6.51 and 6.52. Fuel is drawn from the tank, through a filter to a gear

type pump and thence into the governor, whence it passes through a throttle

valve and a shut-down valve, to the pipeline that delivers it to the injectors.

Of these components, all between the pipelines from the tank and to the

injectors are actually grouped in a single unit, Fig. 6.53, into which both the

spin-on filter may be screwed and the drive taken, either directly or in

tandem with another auxiliary such as the compressor, from the engine to the

gear type pump. Delivery pressure from the fuel pump will be subsequently

boosted to the injection pressure by the cam and rocker mechanism, so it

does not have to be more than 1750 kN/m2 as compared with well over

70 000 kN/m2 for injectors in which the valves have to be opened by hydraulic

pressure supplied from an external pump.

The governor, which is of the rotating twin bob-weight type, regulates

only maximum and idling speeds. It does this by moving a spool valve

axially between stops to limit the rate of supply of fuel at its two extreme

positions. From zero load up to maximum speed at any load, the driver

effects control through the accelerator pedal, which actuates the throttle in

the fuel delivery line. When maximum speed is attained at full load (maximum



power output), the throttle valve lever is in the maximum fuel position, so the



pressure, and therefore quantity of fuel delivered, is at its maximum. If the

load is then increased, the engine speed and, with it, the fuel pressure from

the gear type pump will fall. This fall in speed causes the mechanical governor

to relax its axial pressure on its return spring, called the torque spring, thus



Diesel injection equipment and systems

1

2

3

4

5

6

7



235



Fuel from tank

Gear type pump

Governor/pressure regulator

Hydraulic throttle

Shut-down valve

Injector

Cam, roller follower and pushrod

for actuating the injector



5



7

6



4



3



2

1



Fig. 6.51 Diagram of the Cummins PT injection system hydraulics



allowing the spool valve to move to the left, in Fig. 6.51, to reduce the

quantity of fuel recirculating back to the induction side of the pump.

Consequently, more fuel is delivered through the driver-controlled throttle in

the delivery line to the injectors. Another, but natural, consequence of a fall

in engine speed is that the duration of opening of the injector orifice increases,

so more fuel can enter the injector cup. Both effects increase the engine

torque as the speed and power fall off.

The shut-down valve simply cuts off the fuel supply. It is actuated either

electrically, pneumatically or manually.

For turbocharged engines, an air–fuel control (AFC) valve is introduced

into the main control unit, Fig. 6.53. This is a spool valve actuated by a



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The Motor Vehicle



Fig. 6.52 Diagram showing layout of Cummins PT system



diaphragm exposed to the boost pressure, and it is interposed between the

throttle and shut-down valves. If the accelerator pedal is suddenly depressed,

and throttle valve in the fuel supply system thus opened, the passage on to

the injectors is restricted by the AFC valve which, progressively opening,

limits the rate of increase of flow to match that of the boost pressure. This

avoids the emission of black smoke while the turbocharger is accelerating to

catch up to supply enough air for combustion for coping with the extra load.

Other components in the main control unit include a magnetic screen

between the gear type pump and the governor, to take out any particles of

metal that might damage or impair the operation of the unit injectors; a

pulsation damper to smooth out the delivery from the pump; and a spiral

gear for driving a tachometer. A screw on the end remote from the bobweights on the governor shaft limits the axial movement of the governor

sleeve away from it, for setting the idling speed.

The injectors are illustrated in Fig. 6.54. At the beginning of the upstroke,

in preparation for the next injection, fuel from the low pressure manifold

enters at A, passes through the inlet orifice B, and on down through a series

of drilled holes, turns up to pass through a check valve F, and then down

again to an annular groove in the top end of the injector cup, whence it flows

up yet again through passage D into the waisted portion of the stem of the

injector. From there it flows out and up through passage E on its way back

to the tank. This fuel flow cools the injector and tends to warm the fuel in the



Shut-down valve

Fuel to injectors

Pulsation damper

Tachometer shaft

Filter screen

Fuel inlet

Gear pump

Air-fuel control barrel

Main shaft

Drive coupling

Throttle shaft

Idle speed adjusting screw

By-pass ‘button’

Governor plunger

Torque spring

Idle spring pack

Governor weights



Diesel injection equipment and systems



1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Fig. 6.53 The combined control, governor and pump unit of the Cummins PT system



237



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The Motor Vehicle



A

B

E



E



F



D



F



D



D



C



C



Startupstroke

(fuelcirculates)



Upstrokecomplete

(fuelentersinjectorcup)



Downstroke

(fuelinjection)



Fuel at low pressure enters

injector at (A) and flows

through inlet orifice (B),

internal drillings, around

annular groove in injector

cup and up passage (D) to

return to fuel tank. Amount

of fuel flowing through

injector is determined by

fuel pressure before inlet

orifice (B). Fuel pressure

in turn is determined by

engine speed, governor

and throttle.



As injector plunger moves

upward, metering orifice

(C) is uncovered and fuel

enters injector cup. Amount

is determined by fuel pressure.

Passage (D) is blocked,

momentarily stopping

circulation of fuel and

isolating metering orifice

from pressure pulsations.



As plunger moves down

and closes metering orifice,

fuel entry into cup is cut

off. As plunger continues

down, it forces fuel out of

cup through tiny holes at

high pressure as fine spray.

This assures complete

combustion of fuel in

cylinder. When fuel passage

(D) is uncovered by plunger

undercut, fuel again begins

to flow through return

passage (E) to fuel tank.



Fig. 6.54 Sequence of operations of Cummins unit injector: (left) start;

(centre) upstroke; (right) downstroke



tank, thus helping to prevent wax formation in very cold weather. The quantity

of fuel flowing is a function of its pressure which, in turn, is primarily a

function of engine speed but modified by the restrictions imposed by the

governor, throttle valve and, in the case of a turbocharged engine, the AFC

valve.

As the upstroke is completed, the metering orifice C is uncovered, and the

circulation back to the tank is interrupted by the closure of the passage D.

Pulsations in the supply from the fuel pump are absorbed by the pulsation

damper in the control unit so, with the closure of passage D, the flow through

orifice C is steady. Therefore the quantity of fuel passing through this orifice

into the injector cup is a function of its pressure. Any back-flow will close

the check valve F.

On the next injection stroke the downwardly moving plunger first shuts

off the fuel supply coming through the metering orifice C and thus traps the

metered quantity of fuel in the injector cup. Since no more fuel can subsequently

pass in from the metering orifice, there is no possibility of dribbling through

the injector holes after the injection stroke has been completed.

Continuing down, the plunger pressurises the fuel in the cup and forces it



Diesel injection equipment and systems



239



out through tiny holes in the nozzle, spraying it into the combustion chamber.

Toward the end of the stroke, the passage D is once more uncovered, and the

cooling flow of fuel back to the tank resumed. On completion of injection,

the tapered end of the plunger momentarily remains on its seat, in the bottom

of the cup, until the next metering and injection sequence begins.



6.45



The GM unit injection system



In basic concept, the GM unit injection, Fig. 6.55, bears some similarity to

the Cummins PT system just described, but it differs in many respects. First,

there is no separate unit housing all the control functions: instead, each

injector, Fig. 6.56, houses what is virtually a single element of a jerk pump,

such as that illustrated in Fig. 6.27, and injection is controlled by a multisegment toothed rack that extends the full length of the head from the foremost

to the rearmost injectors.

From the tank, the fuel is lifted by a transfer pump, through first a strainer

and then a fine filter, up to the gallery and on into branch pipes connecting

it to the unit injectors. As the fuel enters each injector, at A, Fig. 6.56, it

passes through an additional, small, filter from which ducts take it down

through B into a sleeve in the casting around the injector barrel and plunger.

Thence it flows through the radial port F in the barrel, into the chamber



Fig. 6.55 Diagram showing layout of the General Motors unit injection system



240



The Motor Vehicle



Fig. 6.56 GM unit injector



below the end of the plunger. As the plunger descends, the fuel beneath it is

forced up the axial hole in it and out through a radial hole into the spill

groove. From the spill groove, it flows through the radial port E, on the left

of the barrel, out into the sleeve in the housing. The return passage from the

housing, delivering to the outlet H, is behind that for the inlet. It is of smaller

diameter than the inlet, so that the fuel in the housing remains always under

pressure. The function of the surplus fuel flow is to cool the unit during its

passage through the barrel.

As the plunger is lifted by the return spring at its upper end, it shuts off



Diesel injection equipment and systems



241



the spill port on the left in Fig. 6.56, and then draws fuel through the radial

hole on the right, in the barrel, into the chamber beneath it. Incidentally,

higher up on the right, there is another hole C sloping upwards, to allow fuel

to run into an annular groove in the bore of the barrel, for its lubrication.

When the cam actuates the rocker mechanism, it pushes the plunger down

again, so that its lower end D first shuts off the inlet hole, after which the

upper edge of its spill groove shuts off the spill port E. The closure of the

latter traps a metered quantity of fuel beneath the plunger which, continuing

down, forces this fuel, at increasing pressure, through hole G in the wall of

the cylindrical housing for the needle return spring, whence it passes into the

nozzle. On the pressure of this fuel reaching a predetermined value, it lifts

the piston on which the needle return spring seats and, with it, the needle

from its conical seat, whereupon the fuel sprays out through the holes in the

nozzle into the combustion chamber.

As the plunger returns, the spiral upper edge of the spill groove in the

plunger uncovers the spill port in the barrel, suddenly releasing any pressure

in the fuel remaining in the nozzle so that, subsequently, there can be no

dribble through its spray holes. The surplus fuel flows back through the axial

and radial holes in the plunger into the spill groove, whence it passes out

through the radial hole, on the left in the illustration, back into the main

housing. On completion of the injection cycle, the plunger comes back up to

its original position, with both the inlet and spill ports open, for resumption

of the cooling flow.

The upper edge of the spill groove around the plunger is of spiral form, so

that the spill timing, and thus the metering of the quantity of fuel injected,

can be regulated by rotation of the plunger, This is done by means of the

previously mentioned rack. To stop the engine, the rack is moved to the

right-hand extreme of its travel, rotating the plunger clockwise to the position

where, as can be seen in the illustration, the spill port is at no point shut off

by any vertical displacement of the plunger between the limits of its operation.



6.46



Common rail injection systems



With the current demand for high injection pressures for satisfying the

regulations on exhaust emissions, interest in the common rail system of

injection has intensified. The basic principle stemmed from a Vickers Patent

of 1913, and a practical system first went into production in the USA by the

Atlas Imperial Diesel Engine Company. However, for meeting the requirements

prior to the introduction of legal limits on emissions and noise, the in-line

and, later, the distributor type pumps were more economical to produce and

posed fewer design problems.

In the late 1980s and early 1990s, Fiat and its subsidiaries in Italy developed

a workable system. However, because specialist suppliers could supply a

wide range of manufacturers, and therefore in much larger quantities and at

a lower cost, Fiat decided to drop their own version. The first major producer

in the field for light high speed diesel engines therefore was Bosch. In this

system the common rail serves as the hydraulic accumulator, the compressibility

of the fuel in it catering for injection without significant interference by

pulsation.

Several other common rail schemes have been proposed. For example,

the pressure in the rail can be multiplied by a conventional plunger type unit



242



The Motor Vehicle



injection pump the spill valve of which is controlled electronically by the

ECU. With such a system it is still possible to boost the injection pressure up

to perhaps 2000 bar or more, but it is less compact than the Bosch system

described in the next section. For large engines, a conventional hydraulic

accumulator can be included to supplement the capacity of the common rail.



6.47



The Bosch system



As can be seen from Fig. 6.57, the fuel is lifted by the low pressure pump in

the tank, through a filter to the roller cell type high pressure pump, which

transfers it to the forged steel common rail. This rail, extends approximately

the full length of the cylinder head. Generally about 10 mm diameter and

from 280 to 600 mm long, it serves as a pressure accumulator. For minimum

pressure fluctuation, the rail needs to be as long a practicable but, if too long,

engine starting may be slow. In a well-designed installation, the pressure in

the rail remains virtually constant throughout the injection process, and injection

pressures ranging from 1350 to 1600 bar can be obtained.

From the common rail, a separate pipe takes the fuel to the injector for

each cylinder. The injectors are solenoid controlled, the injection pressure

being nominally that in the common rail. A number of advantages arise out

of this separation of the injection and pressurising functions. First, the injector

in the cylinder head is much more compact than one combining a pump and

injection valve, so there is more room around it for the inlet and exhaust

valves and cooling passages. Second, the injection pressure can be more

easily regulated. Third, two-stage injection is readily effected, simply by

causing the ECU to send signals to the high speed solenoid to open and close

the injection valve twice in rapid succession. In addition to the simplicity

Pre-supply pump

Fuel tank



Pressure control valve

Rail pressure sensor

Common rail



Filters



Pressure

control

valve

Sensors



High

pressure

pump



Four injectors

Air mass sensor



ECU



A



B



C



D



E



F



Fig. 6.57 Principal components of the Bosch common rail injection system. The

sensors A to F are as follows: A Crankshaft position; B Camshaft position;

C Accelerator pedal; D Boost pressure; E Air temperature; F Coolant temperature



Diesel injection equipment and systems



243



and compactness of this system, it has the advantage that, if required, injection

into each cylinder can be varied individually by the ECU to compensate for

slight variations in compression ratio due, for example, to wear. Finally,

there are several ways in which the injection characteristic curve can be

shaped, see the penultimate paragraph of Section 6.49.



6.48



Components of the Bosch system



The ECU is served by sensors as follows: temperature and mass flow of the

air passing through the intake filter; pressure of the fuel in the rail; engine

speed and crank angle, which can be sensed from teeth on the rim of the

flywheel; a sensor in the throttle pedal unit transmits signals indicting throttle

position and rate of change of position; and another senses the temperature

of the coolant in the engine.

Illustrated in Fig. 6.58 is the fuel lift pump, which Bosch term the presupply pump. It is of the roller cell type, although gear type pumps can be

employed. For cars, the pressure of fuel delivered from the lift pump is

boosted to that required for injection by the radial plunger type high pressure



(a)



(b)

Fig. 6.58 (a) A characteristic of the roller cell type pre-supply, or fuel lift, pump is an

output with a lower level of pulsation than the principal alternatives. It is generally

installed in the fuel tank. (b) Diagrammatic representation of the cross-section of the

roller cell assembly, illustrating the progress of the fuel from inlet to outlet