Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

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240 Fuel Injection

carbon formation, which builds up fairly rapidly around the boundary of the

sprays, causes a rapid deterioration in the quality of combustion.

With smaller engines of up to about 300-mm bore, it is usually sufﬁcient

to rely on the passage and recirculation of the pumped fuel itself to achieve

the necessary cooling. The injector clearances may, in fact, be eased slightly to

promote such circulation. This back leakage, normally about 1 per cent of the

pumped fuel, is led away to waste, or via a monitoring tank to the fuel tank.

For larger engines it is usually necessary to provide separate circuits speciﬁ-

cally for a coolant ﬂow, either of treated water, light fuel or lubricating oil.

All of these considerations lead to a relatively bulky injector and thus to

tricky compromises with the available space within the cylinder head on single

piston engines, where space is also needed to permit the largest possible valve

areas, cages and cooling. Nonetheless, there are performance advantages in

highly rated engines in reducing the inertia of the components directly attached

to the needle of the injector; and this leads fuel injection equipment manufac-

turers to try to ﬁnd space for the injector spring at the bottom rather than at the

top of the injector, as in Figure 8.5.

Fuel lIne

Figure 8.6 shows a typical fuel line pressure diagram and also the needle lift

diagram for an engine having a camshaft speed of 300 rev/min, both at high

load (Figure 8.6a) and at low load (Figure 8.6b). Figure 8.6a shows a very brief

partial reopening of the nozzle, or secondary injection, after the main opening

period. In Figure 8.6 the breaks in each trace are made to indicate intervals of

3 cam degrees.

The fuel line has to preserve as much as possible of the rate of pressure

rise, the maximum pressure and the brevity of injection that the fuel pump has

been designed to achieve. To do this, the fuel line must have the lowest practi-

cal volume and the maximum possible stiffness against dilation. There is also a

limit to the minimum bore because of pressure loss considerations.

The only real degree of freedom left to the designer is to make the fuel line

as short as possible. Even so, a given molecule of fuel may experience two or

three injection pulses on its way from the pump chamber to the engine cylinder

in a medium-speed engine running at full load, rising to 30 or more at idling.

(b)(a)

FIgure 8.6 Fuel line pressure (lower line) and needle lift diagrams. (a) at high load.

(b) at low load (delphi bryce)

If the fuel pump is mounted at a greater distance from the injector, the elas-

tic deformation of the longer pipe and compression of the larger volume of fuel

will dissipate more and more of the pressure developed at the fuel pump and

prolong the period of injector needle lift compared with the pump delivery. In

all medium-speed engines, this compromises on injection characteristics, and

its adverse effect on fuel economy (and heat ﬂux in combustion chamber com-

ponents) is considered unacceptable, and each cylinder has an individual fuel

pump with a HP fuel line as short as possible.

The smaller the engine, the more manufacturers (in fact all manufacturers

of automotive engine sizes) opt for the production convenience of a camshaft

pump (i.e. the pump elements serving all or several cylinders are mounted

in one housing with their own camshaft) and accept any compromise in per-

formance. In such engines, a given molecule of fuel may endure 20 or more

injection pulses while migrating along the HP pipe at full load. In such cases,

the elastic characteristics of the HP pipe (and the trapped fuel) come to equal

the pumping element as a major inﬂuence on the qualities and duration of

injection.

Naturally, all injection pipes must have identical lengths to equalize cylin-

der behaviour. On a 150-kW auxiliary engine of approximately 150-mm bore

running at 1200 rev/min, the effect of a 600-mm difference in pipe length, due

to running pipes each of different length directly from pump to the cylinder it

served, was 8° in effective injection timing, and consequently about 25-bar dif-

ference in ﬁring pressure.

Camshaft or block pumps are, of course, much cheaper, and for a higher

speed engine, one maker (Deutz) adopted the innovative solution of grouping

up to three or four pump elements in one housing, each housing being mounted

close to the cylinders concerned.

The HP fuel lines are made of very high-quality precision-drawn seamless

tube almost totally free from internal or external blemishes. They must, more-

over, be free of installation stress or they risk fracture at the end connections,

and they must be adequately clipped, preferably with a damping sleeve, if there

is any risk of vibration.

Fuel line failures due to pressure are not as frequent as they once were,

thanks to improvements in the production of seamless tube. Nevertheless,

current regulations insist that in marine use fuel lines are sleeved or enclosed

to ensure that in the event of a fracture, escaping fuel is channelled to a tank

where an alarm may be ﬁtted. This is to avoid the ﬁre hazard of fuel ﬁnding a

surface hot enough to ignite it, or of accumulating somewhere where it could

be otherwise accidentally ignited. Most designers now also try to ensure that

leaking fuel cannot reach the lubricating oil circuit via the rocker gear return

drainage or by other routes.

Several manufacturers shorten and segregate the HP line by leading it to

the injector through, rather than over, the cylinder head. Some others incor-

porate the pump and the injector in a single component, avoiding the HP pipe

altogether (see the next section).

Fuel line 241

242 Fuel Injection

PumP

The ﬁnal item for consideration is the fuel injection pump, which is the heart of

the fuel injection system. Essentially, the pumping element is a robust sleeve or

barrel, which envelops a close-ﬁtting plunger. Each one is produced to a very

ﬁne surface ﬁnish of 1–2 in and to give a clearance of approximately 7–10 m

(microns) depending on the plunger diameter.

In its simplest form (Figure 8.7) the barrel has a supply port at one side

and a spill port at the other. (In early practice these ports were combined.) The

plunger is actuated by the fuel cam through a roller follower. Before, and as the

plunger starts to rise (Figure 8.7a), the chamber is free to ﬁll with fuel and to

spill back into the gallery in the pump housing outside until the rising top edge

of the plunger closes the supply and spill ports (Figure 8.7b). Thereafter the

fuel is pressurized and displaced through the delivery valve towards the injec-

tors. The initial travel ensures that the plunger is rising fast when displacement

commences, so that the pressure rises sharply to the desired injection pressure.

When the plunger has risen far enough, a relieved area on it uncovers

the spill port, the pressure collapses, and injection ceases (Figure 8.7c). The

relief on the plunger has a helical top (control) edge so that the rotation of the

plunger by means of the control rod varies the lift of the plunger during which

the spill port is closed, and therefore the fuel quantity injected and the load car-

ried by the engine, as shown in Figure 8.7c–e. At minimum setting the helical

edge joins the top of the plunger, and if the plunger is rotated so that this point,

or the groove beyond it, coincides with the spill port, the latter is never closed.

In that case no fuel is pumped and the cylinder cuts out (Figure 8.7f). The area

of the plunger between the top and the control edge is termed ‘the land’.

There are, of course, many complications of this simple principle. The ﬁrst

is that a delivery valve, essentially a non-return valve, is needed at the pump

outlet to keep the fuel line full, as shown in Figure 8.8. The delivery valve usu-

ally has an unloading collar to allow for the elasticity of the pipe and fuel, and to

help eliminate the pressure pulsations, which cause secondary injection. Some

types of delivery valve, however, sense and unload the line pressure directly.

The second cause of complication is that, when the spill port opens and

a pressure of anything up to 1600 bar is released, cavitation and/or erosion is

a b c d e f

Bottom of Spill port Spill: end as (c) part as (c) idling Shut down

stroke closure of injection. load

Full load

FIgure 8.7 Principle of operation of the fuel injection pump (delphi bryce)

likely to occur, affecting both the housing directly opposite the port, and the

plunger land, which is still exposed to the port at the instant of release. Modern

designs seek to eliminate these effects by using, respectively, a sacriﬁcial (or

in some cases a specially hardened) plug in the housing wall, and by special

shaping of the ports.

Another complication is the ﬁlling process. When the pump spills, further

plunger lift displaces fuel through the spill port. As it falls on the descend-

ing ﬂank of the cam, the opposite effect applies until the falling control edge

closes the spill port. Further fall, under the action of the plunger return spring,

has to expand the fuel trapped above the plunger until the top edge uncovers

the supply port again. It can do this only by expanding trapped air and fuel

vapour, and by further vaporization, so that a distinct vacuum exists, and this is

exploited to reﬁll the pump.

There is also the effect of dilation due to pressure, the need for lubrication,

and the need to prevent fuel from migrating into spaces where it could mingle

with crankcase oils.

Early pumps could get by without undue complications: the fuel itself pro-

vided adequate plunger lubrication, and fuel which had leaked away in order to

do this was simply rejected to a tank which was emptied to waste.

Today, grooves are provided on the plunger or in the barrel wall. The ﬁrst

groove collects the leaking fuel. Where there are two grooves, the second will

spread lubrication, but there is sometimes an additional groove between them

to collect any further migration of either fuel or lubricant. Figure 8.9 shows a

section of a modern heavy-duty fuel pump incorporating most of the features

described earlier.

The pump is actuated by a follower that derives its motion from the cam.

Block pumps incorporate all of these components in a common housing for

several cylinders, while individual pumps used on some larger engines incor-

porate the roller. This not only makes servicing more straightforward, but also

ensures uniﬁed design and manufacturing responsibility.

Pump 243

Unloading collar

(a)

(b)

FIgure 8.8 delivery valves (a) volume unloading (b) direct pressure unloading

(delphi bryce)

244 Fuel Injection

The roller is a very highly stressed component, as is the cam, and close

attention to the case hardening of the cam and the roller transverse proﬁle is

essential to control Hertzian (contact) stresses. The roller is sometimes barrel-

led very slightly, and it must bear on the cam track parallel to the roller axis

under all conditions. On the ﬂanks of the cam, this is a function of slight free-

dom for the follower assembly to rotate, while on the base circle and the top

dwell of the cam, it depends on precise geometry in the assembly.

Extended stud

for lifting attachment

Controlled-lift

delivery valve

Easy-lift high-pressure seals

Suspended pump barrel

Phased fill and spill ports

228mm diameter

Special high-grade spring

High duty lead–bronze,

steel-backed bearing,

pressure fed with lube oil

Safety/locating key

Lead–bronze thrust bearing

(for direct reversing engines)

Spragging

mechanism

655mm

Positive lube oil feed to

plunger and tappet assembly

Fuel reservoir

transfer port

FIgure 8.9 Cross-section of bryce FFoar-type ange-mounted unit pump suitable

for four-stroke engines with crankshaft speeds of up to 450 rev/min (delphi bryce)

The roller pin requires positive lubrication, and the cam track must at least

receive a deﬁnite spray. Lubrication channels have to be provided from the

engine system through the pump housing to the roller surface, its bearings, and

the lower part of the plunger.

timing

The desired timing takes into account all the delays and dynamic effects

between the cam and the moment of ignition. The desired timing is that which

ensures that combustion starts and continues while the cylinder pressure gener-

ated can press on the piston and crank with the greatest mechanical advantage,

and is complete before the exhaust valve opens at 110–130° after top dead cen-

tre. The enginebuilder always speciﬁes this from his/her development work,

but it usually means that the only settable criterion, the moment of spill port

closure (SPC), is about 20–25° BTDC.

In earlier practice this was done by barring (or manually turning over) the

engine with the delivery valve on the appropriate cylinder removed, and with

the fuel circulating pump on (or more accurately with a separate small gravity

head) and the plunger rotated to a working position. When the fuel ceased to

ﬂow, the spill port had closed. Sometimes this criterion was used to mark a line

on a window in the pump housing through which the plunger guide could be

seen, to coincide with a reference line on the guide.

The accuracy of this method is, however, disappointing. There are differ-

ences between static and dynamic timings, which vary from pump to pump,

and many makers now rely on the accuracy of manufacture and specify a jig

setting based on the geometry of the pump and the SPC point to set the cam

rise or follower rise at the required ﬂywheel (crank) position.

Some makers allow users to make further adjustments of the timing accord-

ing to the cylinder pressure readings, but this places a premium on the accu-

racy, not only of the calibration of instruments, but of the method itself. It

should be borne in mind that neither the instruments nor the method is as accu-

rate as the pump manufacture by an order of magnitude.

If the cam is ﬁxed, it is usually possible, within stated limits, to adjust the

height of the tappet in relation to the cam. Raising the cam has the effect that

the plunger cuts off the supply spill port (and starts injection) sooner for a given

cam position, and vice versa (Figure 8.10). The limit is set at one extreme by

the need for there to be ‘top clearance’ at the top of the follower lift (i.e. of the

plunger stroke) between the top of the plunger and the underside of the delivery

valve in order to avoid damage. Too small a clearance also means that injection

is commencing too near the base of the cam, and therefore at too low a veloc-

ity to give the required injection characteristics. On the other hand, too large a

clearance (to compensate for an over-advanced cam by lowering the plunger)

risks interfering with the proper spill function, which terminates injection.

The cam with its follower may be designed to give a constant velocity

characteristic throughout the injection stroke. Recent designs with a high initial

Pump 245

246 Fuel Injection

velocity aim to give a constant pressure characteristic, but many manufactur-

ers ﬁnd that the performance advantage or stress advantage over the constant

velocity design is too small to warrant the greater production cost.

uniformity

Bearing in mind the random way in which ignition occurs during each injec-

tion, and perhaps also because the ﬁlling process of the pump chamber is

largely dependent on suction, it is not surprising that successive combustion

cycles in the same cylinder at the same steady load are not identical. A draw

card taken slowly to show a succession of cycles will show this visibly, and a

maximum pressure indicator will show by feel, or otherwise, that maximum

pressures vary from cycle to cycle, perhaps by 5 bar or more. (Any governor

corrections will have an effect, too, but progressively over several cycles.)

These variations are a fact of life, and not worth worrying about unless they are

20 bar or more. It does mean that if indicator cards are being taken as a check

on cylinder performance, several cycles per cylinder should be taken to ensure

that a properly representative diagram is obtained.

Differences will also occur between cylinders for reasons of component toler-

ance (unless compensated adequately by the setting method), and because of the

dynamic behaviour of the camshaft, which deﬂects in bending due to cam loading

and torsionally due to the interaction of the cam loadings of adjacent cylinders.

These will characteristically fall into a pattern for a given engine range and type.

While enginebuilders should (and usually do) allow for variations in cyl-

inder behaviour to occur, it is obviously good practice to minimize these to

achieve maximum economy with optimum stresses. However, it would be

wise to bear in mind that it is inherently very difﬁcult to make pressure and

Top clearance

Z necessary fo

correct timing

Top clearance

Z necessary for

correct timing

(a) (b) (c) (d)

FIgure 8.10 adjusting timing by top clearance. Conguration of cam, tappet and

pump at commencement of injection (a) and (c), and maximum cam and follower

lift (b) and (d). In each case the pump is set correctly for the required injection

point. Part (a) shows the case for an over-retarded cam, Part (c) that for an over-

advanced cam within permitted limits of top clearances. In either case Y (the residual

lift)  Z  the timing dimension of the pump, t (geC ltd)

temperature measurements of even the average behaviour of combustion gases

in the cylinders.

To achieve, without laboratory instrumentation techniques, an accuracy

which comes within an order of magnitude of the standard of accuracy to

which fuel injection components are manufactured and set is impossible. This

is particularly true of exhaust temperature as a measure of injection quantity,

notwithstanding that this practice has been generally accepted by many years

of use. Fuel injection settings on engines should not therefore be adjusted with-

out good reason, and then normally only on a clear indication that a change

(not indicative of a fault) has occurred.

Fuel quantity imbalance is far from being the only cause of a variation in

the pattern of exhaust temperatures, nor timing of ﬁring pressure. Consider, for

instance, variations in compression ratio, charge air mass, scavenge ratio, not

to mention calibration errors and the accuracy of the instruments used.

Fuel injection pumps (and injectors) have their own idiosyncrasies. The

relationship between output and control rod position (the calibration) is not

absolutely identical in different examples of pumps made to the same detail

design. The output of a family of similar pumps can be made sensibly identical

at only one output, namely, at the chosen balance point. At this output a stop,

or cap, on the control rod is adjusted to ﬁt a setting gauge (or, in older designs,

a pointer to a scale), so that it can be reproduced when setting the pumps to the

governor linkage on the engine.

Obviously, the calibration tolerances will widen at outputs away from the

balance point, which is usually chosen to be near the full load fuelling rate.

As the tolerances will be physically widest at idling, and will also constitute a

relatively larger proportion of the small idling quantity (perhaps 35 per cent),

the engine may not ﬁre equally well on all cylinders at idling, but this sort of

difference is not sufﬁcient to cause any cylinder to cut out unless another fault

is present.

When a pump is overhauled, it is advisable that its calibration should be

checked, and the balance point reset. This ideally entails the use of a suitable

heavy duty test rig, but in the largest sizes of pump, it is sometimes more prac-

tical to rely on dimensional settings. (D.A.W.)

develoPments and trends

Fuel injection systems have a signiﬁcant inﬂuence on the combustion process

and hence a key role to play in improving engine fuel consumption and reduc-

ing noxious exhaust emissions. The following characteristics of an injection

system are desirable in achieving these goals:

l Injection pressures during the whole process should be above 1000–

1200 bar for a good spray formation and air–fuel mixture; a tendency in

practice to 1600–1800 bar and higher is noted.

l Total nozzle area should be as small as possible in relation to cylinder

diameter for good combustion, particularly at part load.

developments and trends 247

248 Fuel Injection

l Total injection duration should be 20° of crank angle or less for achiev-

ing a minimum burning time in order to exploit retarded combustion for

reduced NOx emissions without loss in efﬁciency. A high compression

ratio is desirable.

l High pressures at the beginning of injection promote reduced ignition

delay, while increased mass ﬂow can result in overcompensation and

increased pressure gradients. Consequently, rate shaping is necessary in

some cases, particularly with high-speed engines.

l High-aromaticity fuels cause increased ignition delay in some cases.

Pre-injection with high injection pressures is necessary and can achieve

non-sensitivity to fuel quality.

l Electronically controlled adjustment of injection timing should be

applied for optimized NOx emissions at all loads, speeds and other

parameters.

l The load from the torque of the injection equipment on the camshaft

and/or the gear train should be as low as possible in order to prevent

unwanted additional stresses and noise.

l For safety reasons, even a total breakdown of electrical and other power

supplies should not result in the engine stopping.

To understand fully the complex hydraulic events during the fuel injection

process, the Swiss-based specialist Duap says that it is essential to appreciate

the function of all the elements (pump, pressure pipe, fuel valve and nozzle)

forming the injection system.

It is commonly believed by non-specialists that the plunger within the

pump pushes the fuel upwards like a pillar, thus affecting the immediate injec-

tion of fuel into the combustion chamber. The reality, however, is that the

plunger moves with such a high speed that the fuel likely to be conducted is

highly compressed locally (elasticity of fuel). The compressed fuel now gener-

ates a pressure wave, which runs through the pipe and valve, causing the nozzle

to open and inject. The pressure wave of the fuel is forced through the system

at a speed of around 1300 m/s. In the same way that sound in the air is reﬂected

by houses and hills, the wave is reﬂected between the fuel valve and the pump.

It may therefore easily run back and forth between those components several

times before the nozzle is actually forced to open and inject.

Duap warns that this process underlines the importance of maintaining the

injection system in an excellent condition and ensuring that the fuel is pro-

perly treated and free of dirt. If, for example, the nozzle spray holes are par-

tially blocked by extraneous elements or carbon particles, the pressure wave

may not be sufﬁciently reduced within the system. This eventually results in

the destruction of the fuel pump cam or other vital parts of the injection system

upon the next stroke.

The task of the injection system is to feed fuel consecutively to each cylin-

der within a very short period of time (0.004–0.010 s, depending on the engine

revolutions per minute). It is also essential that the same amount of fuel is

delivered with each stroke: deviations in the quantity supplied to different cyl-

inders will adversely affect the performance of the engine and may result in

crankshaft damage due to resonance.

All of the key elements of the system must be manufactured to very small

tolerances; the clearance between plunger and barrel, for example, is not larger

than 4–16 m (depending on the plunger diameter). Such a high-precision

clearance also dictates an adequate surface quality (within 0.2–0.5 m), a ﬁnish

which can be achieved only by careful grinding and lapping.

Before the fuel is pushed into the pressure pipe (linking the pump and fuel

valve), it has to pass the pressure valve, which has several tasks. The ﬁrst task

is to separate the pump hydraulically from the pressure pipe after the fuel has

passed the valve; the second is to smooth the pressure wave running back and

forth within the pipe: this calming down is necessary to secure the proper clos-

ing of the fuel valve without having additional and uncontrolled injection; and

the third is to maintain a certain pressure within the pressure pipe for the next

injection stroke (rest pressure). The fulﬁlment of these tasks can be guaranteed

only by using pressure valves manufactured to very tight tolerances (bearing in

mind that the valves play an important role in ensuring that identical amounts

of fuel are delivered to each engine cylinder).

Another highly underestimated component of the fuel injection system is

the pressure pipe. Despite its small size and apparent simplicity, Duap notes

that the pipe has to endure pressure waves of up to 1800 bar; even the smallest

mark may thus lead to fracture.

The last link in the injection system chain is the fuel valve (nozzle holder

assembly and nozzle), which is designed to inject and atomize the fuel into

the combustion chamber when the pressure wave has reached a pre-determined

strength.

unIt InjeCtor versus PumP/PIPe/InjeCtor

A unit injector is more likely to be considered for engines today than in the

past, but the choice between unit injector (Figure 8.11) and pump/pipe/injector

(Figure 8.12) systems can be complex. The UK-based specialist Delphi Bryce

(formerly Lucas Bryce and now part of the Woodward Governor group) cites

the following merits and demerits of the two conﬁgurations:

1. Unit injector positive features: hydraulic volumetric efﬁciency (smaller

plunger), longer nozzle life, fewer components, no high-pressure pipe

and fast end of injection. More complex cylinder head designs and

higher cylinder head loads are negative features.

2. Pump/pipe/injector system positive features: less space required by

injector, facilitates use of residual fuels and water injection, simple

pushrod mechanism, generally easier servicing and higher low load

operating pressure. Large trapped fuel volume and parasitic power loss

are negative features.

unit injector versus pump/pipe/injector 249