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Fuel injection has come a long way in the past 30 years.

Where once the carburettor ruled the engine, now thefuel injection system reigns. But while much is written

about the electronics and the power of the software used,

comparatively little is reported about the fuelling systems themselves

and how they are used.

Mention the words Fuel Injection to most and they will conjure

up an image of electronic boxes and digital systems but back in the

day fuel injection was never like that. For many years constant ow

mechanical systems were the nature of the beast. Instead of injecting

discreet amounts of fuel in response to an electronic pulse, these

systems continuously pump fuel into the intake manifold. That this

type of system is available even today says much about its effectivenessand suitability for certain tasks.

The problem in fuelling any spark ignition engine is to introduce

the fuel as a vapour into the charge air of the engine in a combustible

form and at the optimum air to fuel

ratio. But before we launch into

the complexity of the task in the

context of fuel injection it is worth

looking at the principles behind

the pinnacle of mixing technology

for many years, the xed-jet

carburettor.

The carburettor

When fuelling any variable

speed, gasoline engine, once up

to running temperature there are

four conditions that need to be

addressed to give adequate control

engine idle, progression, what

I will call normal running and

acceleration. Apart from these we

have issues such as cold start,which every engine has to do at

some time or other, and hot re-

starts, which can be particularly

challenging for some applications.

But for all gasoline engines, even those for racing, those are the four

main areas of operation, during which the carburettor has to function

satisfactorily.

During normal running, air passes through an auxiliary venturi

placed in the engine intake air stream, and a fuel-air mixture is

pulled through by the partial vacuum thus created. The fuel is initially

metered by a main jet from the oat chamber, while the air in the fuel-

air pre-mix is regulated by an air-correction jet. In between, the air is

mixed into the fuel using emulsion tubes.

Without any correction of this nature, as the engine speed (andhence airow) increased, the fuel-air mixture entering it would tend

to move towards rich. While the main jet limits the amount of fuel

metered according to the depression created in the venturi, the air-

correction jets try to control this progressive enrichment and therefore

maintain a constant air-fuel ratio. The emulsion tube has a number

of holes at various heights along it that alter the primary fuel-air

mixing according to the level of fuel in the oat chamber, which itself

depends on the fuel demand coming from the engine.

In essence, therefore, although quite simple in its approach, the

internal workings of the xed-jet carburettor are quite complex

and may take considerable ne tuning. Factor in enrichment foracceleration, a separate circuit for engine idle and another when

just moving away from idle (progression) and you have a complex

instrument that can be temperamental.

30

Of all changes in engine technology

over the past 30 years, fuel injection

is arguably the most significant, as

John Coxonexplains

Picking the mixing

Figure 1 The fixed jet carburettor

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FOCUS : FUEL INJECTION

Lucas mechanical injection

Although it is perhaps hard to believe now, fuel injection systems

in the higher echelons of motor sport were normally mechanical

as recently as the early 1980s. One such system, still in regular use

every weekend at historic events, is the Lucas mechanical system.Still recognised as one of the most sophisticated and easy-to-use

fuelling systems of its time, it uses a moving shuttle to meter the fuel to

individual cylinders [Figure 3].

The shuttle operates between a xed stop and a movable stop to

meter the ow. A rotor geared to half engine speed and spinning

around the metering sleeve times the delivery according to transfer

port angle. With a constant fuel pressure of about 100-110 psi, the

sole function of the injector is therefore to atomise the fuel. So fuel

delivery at the normal wide-open throttle use of racing engines is

proportional to engine speed, consequently it is only an approximate

(but perfectly satisfactory) estimation of the fuelling requirement.More than this, however, the system has a method of altering part-

throttle fuelling using a 3D cam prole mechanically linked directly to

the throttle. Crude by 21st-century standards, the system was popular

with engine builders because engine fuelling characteristics could be

changed very easily with only limited test-bed running. Nevertheless,

the fact that fuelling was timed to inject just at the correct time

minimised fuel losses through the exhaust valve and gave surprisingly

good fuel economy.

Although an electric fuel pump was needed for starting, once it was

running the engine would idle very easily with negligible bore wash.

Furthermore, the design of the fuel injection nozzles produced little orno dripping on engine shutdown, preventing fuel from slowly dripping

into the engine and potentially hydraulicing with disastrous results

the next time the engine was cranked. But perhaps most important is

the fact that throttle response was more or less instantaneous, which

partly explains why drivers often preferred this system to any other

contemporary designs in its heyday [Figure 4].

The introduction of a feedback loop, whereby any slight deviation

from the desired air fuel ratio is corrected, is also particularlychallenging.

With these shortcomings in even the most sophisticated carburettors,

it is little wonder that engineers have looked to other solutions that

physically inject the fuel.

Development of the rst gasoline injector systems goes as far

back as that of the carburettor itself, and even until comparatively

recently they were predominately mechanical. But the introduction

of air pollution legislation spurred the development of a more precise

fuelling technology thats essential for three-way catalyst control.

In response to signals from the intake manifold and engine speed,

fuel was injected directly into the intake manifold via a solenoid-controlled injector. Early systems were extremely expensive and time

consuming to set up, so fuel injection on road vehicles was initially

tted only to executive vehicles and expensive sports cars.

However as emission legislation became more widespread, systems

were developed that were mechanically-hydraulically controlled,

continuously injecting fuel into the intake manifold in a very similar

manner to that of a carburettor. As emission control legislation became

tougher these systems were gradually replaced by fully digitally

controlled electronic systems but even so it was the mid eighties

before fully programmable systems became the norm in Formula One.

For a long time top level motorsport relied upon purely mechanicalsystems. The most prolic of these at the top level was the Lucas

system [Figure 2].

t

Figure 2 The Lucas mechanical fuel injection system

Figure 3 The Lucas metering/timing system

Figure 4 The wide open throttle fuelling from the Lucas system is very much a compromise.

Minimum fuel for best torque (MBT) is derived from dyno running and the fuel delivery rate

selected as the best fit selected to avoid running too weak or too rich at any one particular WOT

condition. Note how the engine runs very weak at less than 2000 rpm. Fortunately race engines

dont run full load at these speeds

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the pump speed by 50% will

increase its output by a similar

amount. In practice, however,

at very low as well as very high

speeds, the effects of internalleakage, pumping friction losses

and simple clearance issues

mean this may not be the case.

But in essence, when run directly

off the engine via some kind of

mechanical drive, the fuel delivery

will be proportional to engine

speed.

To tailor the ow more precisely,

a main bypass jet is introduced

to the return feed going back tothe fuel tank. The larger this jet the greater the return feed and the

less the fuel going to the engine. This is the most basic adjustment in

a constant-ow fuelling system and, when supplemented by a small

poppet valve and spring, the additional restriction will help maintain

the fuel pressure at low engine speeds and idle. At higher engine

speeds, when the engine might be experiencing a loss of volumetric

efciency, the fuel curve may also need to be restricted by introducing

a high-speed bypass valve. Similar to the main bypass valve assembly

the addition of a poppet and spring assembly together with a jet is

used to alter the position at which the fuel is leaned off (see Figure

6). Normally set to function somewhere around 500-1000 rpm abovepeak torque, clever adjustment of this system, together with a richer

main jet, can get closer to the ideal fuelling curve necessary for

maximum power all through the engine speed range.

Constant-flow injection systems

Predating the Lucas system but still very popular even today are

constant-ow designs. Simple and rugged, such systems are easily

tuneable and avoid all the complexities associated with carburettor

tuning. Indeed, in some circumstances, the absence of a carburettor

oat chamber made this kind of system the natural selection. For

instance, where high-g loads are experienced on the track, when

cornering or moving in a straight line, the fuel level in the oat

chamber can be quite erratic. Since the quality of the fuelling depends

very much on the precise fuel level in the chamber, any undue

alteration in the level can have a deleterious effect.Constant-ow systems of this type use a mechanical pump to control

the supply of fuel to the injection unit according to the engines rpm.

This variable ow generates a back-pressure in the fuel system against

the xed orices of the main bypass jet and nozzle, while idle and

part-load conditions are moderated using a barrel valve assembly.

Fuel metering is achieved by sensing the engine speed and throttle

angle. The speed is sensed using a positive displacement pump,

generally of the gear-rotor variety which, when used within their

designed operating envelope, have theoretically linear ow rate

characteristics according to the shaft speed. In other words, increasing


t

Figure 5 Kinsler constant flow schematic

Figure 6 The high speed bypass circuit can enable closer fuelling to the ideal by allowing a rich

main jet and then a high speed circuit to follow the engine requirement

Figure 7 Constant flow nozzle

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efciency (as a result of the fuel vaporising and displacing air) is

greater than the benet for combustion, this option is probably bestforgotten. As a rule, many race engine manufacturers try a number of

injector positions before signing off any new engine design.

When it comes to fuel control, however, digital fuel injection can

take a number of different directions. The rst and simplest is very

similar in some ways to the earlier mechanical systems. Monitoring

only the engine speed and throttle angle can give the basis for

a very simple and highly effective fuel control system, and this

method is sometimes referred to as Alpha-N. Since gasoline engines

are very tolerant of variations in air-fuel ratio from 6% rich to

3% lean with only small changes to the power delivered such a

system will outperform even the best mechanical system. But day-today variations in atmospheric pressure and ambient temperature

will change the density of the intake air so, since electronic

components are not expensive, more sophisticated methods have

been developed.

By substituting throttle angle as an approximation of airow for a

calculated value derived from the gas laws, the actual airow can

be inferred from pressure and temperature measurements inside

the intake manifold. Known as the Speed-Density method, if the

volumetric efciency is therefore known at all the engine operating

conditions of engine speed and manifold air pressures, a much more

accurate calculation of the airow rate and hence air-fuel ratio canbe produced.

For the ultimate in control, air ow meters have been developed

to measure the actual ow rate of the air passing into the engine.

Designed around hot wire, hot lm or bubble technology, these are

best positioned away from the heavily pulsing airow and, as such,

when used, they tend to be found upstream of the throttle in the cold

air feed pipe that leads to the entrance of the air plenum. With so many

spec engine formulae today, where no modication to the engine or its

systems is allowed, this is an increasingly familiar option.

Injector sequencingIn a typical port-injected four-cylinder engine, once the appropriate

injector pulse width had been calculated, all four injectors could be

red at once. This would deliver the appropriate amount of fuel to

each cylinder. In practice though we tend to inject all four cylinders

At wide-open throttle, once the fuel has passed through the barrel

assembly it will go directly to the nozzles in each intake tract before

being sprayed into the intake air stream [Figure 7]. Much like the

carburettor, however, to improve vaporisation with naturally aspirated

engines the fuel is mixed with a small amount of air immediatelybefore delivery. This air is usually bled from the clean side of the

manifold nearer the intake.

For part-throttle operation the barrel valve comes into play.

Connected directly to the throttle assembly using a mechanical link,

when the throttle begins to close so does a spool in the barrel valve.

At wide-open throttle the passageway through the barrel is effectively

a large notch cut in the side of the spool and all the fuel ows directly

to the nozzle, unrestricted. As the throttle is closed this spool rotates,

and the ramp ground onto the side reduces the passageway to restrict

the ow of fuel to match the engines reduced air requirement. This

opening will be at a minimum at engine idle.At about half throttle another port is opened that leads to a

secondary bypass valve, and from here back to the fuel tank. At an

opening of about 40-20 this port is progressively opening and takes

the excess fuel back to the tank. This secondary bypass valve normally

has a higher pressure than the main bypass system so that when the

engine returns to idle, this poppet valve closes and allows the spring

and poppet valve in the main jet to regulate the idle fuel pressure.

Once mastered, injection systems of this type are easily set up and

are still very popular, although they do need day-to-day ne tuning

to compensate for changes in atmospheric pressure and ambient

temperatures.

Electronic fuel injection

The sophistication, simplicity of build and ease of calibration

makes modern electronic fuelling systems the only option for many.

Electromagnetic fuel injectors injecting gasoline fuel at anywhere up

to 100 bar (about 1450 psi), and sometimes under closed-loop control

and fully integrated with an ignition system incorporating knock

control, would seem to be about as close to the ideal fuel management

in the traditional port-injected gasoline engine as you can get.

But advances in modern diesel engine technology, particularly

common rail systems, has ensured that a whole new technology ofdirect injection is being developed for the gasoline engine such that in

another ten years port injection for the gasoline engine may be a thing

of the past. For the time being, however, manifold-injected or port-

injected systems call them what you will are the most commonly

available.

Single-point systems were advocated briey in the early days of

electronic injection. But because they combined the cost of fuel

injection with the air-fuel ratio distribution problems of the carburettor

they were rejected in favour of the multi-point system, and are now a

long-forgotten dead end in automotive history. These days virtually all

injection systems use at least one, possibly two injectors per cylinder,injecting the fuel as close as possible to and in many cases onto

the back of the intake valve.

In racing engines the fuel can also be injected centrally into the

bellmouth of the intake runner, although when the fall in volumetric

Figure 8 Typical port fuel injector

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with only half the amount needed once every engine revolution (that

is, every 360 crank degree).

Sometimes referred to as the batch-triggered method, any fuel

that doesnt enter the cylinder during the inlet valve opening period

will remain in the intake port until the next cycle. Its not necessarilyas bad as it sounds, as this fuel will have time to take in heat from

the manifold wall and surroundings and vaporise in time for the next

valve opening event, giving a more homogenous mixture. The longer

injection periods of this method also means that smaller injectors can

be used, giving better overall fuel metering control [Figure 8].

The other option is sequential injection. In this case each injector

is red according to the intake valve events on its corresponding

cylinder. Fired only to coincide with the induction stroke, sequential

systems seem to give better control over batch red systems but do

need larger injectors to inject the fuel into the engine in the shorter

time frames available. Logically, in competition engines, injectionmust have been fully completed by the time the intake valve closes

but in on-road applications at part throttle, injection is more usually

completed immediately before the intake valve opens. Whatever the

intended use, sequential systems have to vary the pulse width and

adjust the start of injection (SOI) to full the above conditions, so this

needs to be freely programmable in any control system.

In terms of out-and-out performance, there is little to choose

between batch-ring and sequential systems. Sequential systems will

inevitably give better driveability and fuel economy through more

precise control of the fuel, but if engine performance is your only goal,

the simplicity of batch-ring may be the best choice. As an alternative,some might say compromise, a group-ring or semi-sequential method

may be used.

On a traditional four-cylinder engine, therefore, instead of triggering

all injectors at once, the cylinders would be grouped into pairs with

each pair injected on alternate revolutions. So on our typical 1-3-4-2

ring cycle, cylinders 1 and 3 could be red together, followed by 2

and 4 on the next revolution. This arrangement allows the injection to

be selected as a function of the engine operating point and minimises

the incidence of fuel hanging around in the port/manifold for long

periods.

Direct injectionIn the past 10-12 years, spurred on by growing fuel economy and

emissions legislation worldwide, engineers have at last been able to

prove the benets of injecting the fuel directly into the combustion

chamber. Referred to as gasoline direct injection, GDI, or sometimes

DFI (Direct Fuel Injection) or even DISI (Direct Injection Spark

Ignition), this technology is slowly nding its way into the race engine

business. Well call it simply DI.

Although banned in Formula One, there cant be any serious vehicle

manufacturer that doesnt have a DI programme of some sort such are

the potential benets. Unlike port injection, however, the challenges

of producing a combustible mixture using DI are signicant, and thefuel has to be both metered and formed into an homogenous mixture

in a much shorter time than any of the methods already mentioned.

For true DI operation, the fuel needs to be injected wholly within

the induction stroke and after the exhaust valve closes before

compression, to ensure full atomisation in advance of combustion

and remember, an engine running at 11,000 rpm has a maximum

injection period of only 1.5 ms!

When other combustion modes are used, for example stratied, lean

combustion when operating at part load (for example running in the

pit lane), the fuel might be injected much later in the cycle and during

the compression stroke.But whatever the timing of injection, the task is to provide the fuel

ow to match that of the ow of intake air and its distribution within

the cylinder. Injecting against compression pressures means the

injector will therefore need to work at much higher pressures than its

port-mounted alternative.

Injector types

Setting aside for the time being their method of mounting and whether

they are for port or direct systems, injector nozzles fall into three types

pintle, disc and ball.

Owing much of its heritage to diesel technology, the pintle nozzleconsists of a tapered needle sitting in a tapered seat. When energised

this needle is withdrawn, allowing the pressurised fuel to discharge.

Popular with early OE manufacturers because of concerns over

injector fouling with other designs, these have largely been superseded

by disc or ball varieties.

The disc type injector eliminates the pintles armature and the

solenoid acts directly on a at disc through the core of the injector

body. This disc rests on a seat with an orice in it, through which the

fuel is injected. Lighter and with a better response than the ball type,

this design results in less build-up of deposits and hence a longer

service life.Used by GM but now a favourite with Bosch the ball type injector

mechanism uses the same solenoid operating technology as the pintle

but also has a ball and socket arrangement to seal the orice and

control injection more precisely. Designed with multiple holes for a t

Figure 9 Bosch direct injector (Courtesy Bosch Engineering GmbH)

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range of spray angles and patterns,

injection rates can be much

quicker than other designs.

On the electrical side, pulse-

width modulated (PWM) injectorstend to come in two categories

depending on the impedance of

the solenoid windings. Since the

injector driver circuit is part of the

ECU electronics it is important to

match these to the correct type of

injector if the injector is to work as

it should and the circuit not fail.

These two injector driver designs

are peak and hold and saturation.

Peak-and-hold drivers generallywork with low-impedance

injectors. Typically in the 1-4

band, the full battery voltage

is applied across the solenoid windings until the current reaches a

level where the solenoid moves the injector to its maximum opening

position, after which the current is reduced for the rest of the pulse.

Because of the low impedance, the switching currents will be high

and, since heat generated is proportional to the square of the current,

the heat to be dissipated within the driver circuit will also be high

[Figure 11].

Saturation drivers, by contrast, have much higher impedancewindings, typically 10-17 or lower with a ballast resistor, and the

injector driver is fully on during the full injector pulse width. With the

resulting lower currents, the heat build-up will be much less but the

speed of response will suffer.

Since many low-pressure motorsport systems rely on normal

production-based components, most injectors for motorsport are

based around saturation technology, despite their (slightly) poorer


performance. But because the high-pressure injectors in DI and high

pressure port-injected systems need to be able to open and close

rapidly, they use peak-and-hold driver technology.

The precise differences between a high-pressure port injector and a

direct injector suitable for Bosch DI installations are only application-

specic. While the direct injector may require a long, narrow nose

for reasons of packaging in and around the combustion chamber,modern high-pressure injectors are based around the same 12 V

electromagnetic, 200 bar maximum pressure, architecture.

While fuel injector fuel-feed positions can be either bottom feed

sometimes referred to as gallery feed or top feed, the self-venting

capability of top-feed units tends to make them most suited to

motorsport applications.

The fuel circuit

Fuel systems for injected engines are usually divided into high

and low pressure. Any system where the fuel line pressure into the

injector is 10 bar or less is generally considered to be low pressure.Consequently, anything above that, whether direct or port injected,

is considered high. In reality, few low-pressure systems work above

8 bar, whereas high-pressure systems can be 50, 100 or now even

200 bar, although current Formula One fuel systems are limited by

regulation to 100 bar.

In most OE vehicle fuel systems the pump will almost invariably

be in the fuel tank itself. Better fuel pick-up and modular designs

with fewer joints to leak may have advantages in this business but in

competition, for reasons of ease of access, the separate in-line pump is

still king.

In these low-pressure systems the fuel line pressure is typicallybetween 3 and 4 bar (45-60 psi), but because of the need to inject fuel

more quickly in some designs, injectors are often rated up to 8 bar.

Pumps to deliver these pressures at the ow rates required are usually

of a roller cell design. When much higher pressures are needed this

type of pump will be used as a lift pump to prime the fuel system

of another, mechanically-driven unit that takes its drive from the

camshaft.

Early Bosch high-pressure pumps for rst-generation DI systems

consisted of three-barrel type pumps driven off an eccentric camshaft

lobe, and equally spaced around the cam. Delivering 50-120 bar,

these have now been replaced by pumps up to 200 bar using a singlepump piston [Figures 14 and 15].

In the case of low-pressure systems, for many years OE vehicle

manufacturers would pump the fuel directly into the injector fuel rail,

with any surplus fuel being diverted back by the pressure regulator

Figure 10 Cross-section of Bosch

EV14 port injector (Courtesy Bosch

Engineering GmbH)

Figure 11 Peak and hold injector characteristics Figure 12 Saturation injector characteristics

Figure 13 Low pressure pump

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to the fuel tank. This had the dual advantages of sending any vapour

vented from the injector back to the tank and re-circulating the fuel tominimise the threat of fuel vapour forming in the rst place. With one

side of the invariably mechanical regulator connected to the intake

manifold pressure, the actual pressure difference across the injector

would remain constant at all times. Thus the pulse width so calculated

would be based on this constant pressure difference across the injector.

Instead of placing the regulator at the far end of the fuel rail, later

low-pressure systems repositioned it together with a pressure sensor

in the fuel tank immediately next to the fuel pump and lter assembly.

With this approach, only the fuel to be injected into the engine was

delivered to the fuel rail, any excess being returned directly to the

tank. With no reference to the manifold pressure, this has to be takeninto account when the injector pulse width is being calculated.

Systems of this type are called returnless and have led to another

development, referred to as demand-controlled. Similar to the

returnless system, demand-controlled systems pump only the fuel

needed for use by the engine. The pressure to the fuel rail is controlled

directly from the engine ECU working to closed-loop control with the

pressure sensor in the rail. Although a pressure relief valve is installed

for safety reasons, a conventional pressure regulator is no longer

needed, and the delivery volume to the rail is controlled by changing

the voltage to the pump. This is undertaken by a clock-operated

module triggered from the engine ECU.Giving better metering precision and having the ability to increase

the fuel pressure under conditions such as hot starts preventing the

formation of fuel vapour, demand-controlled systems can also extend

the operating range of injectors for turbocharged applications. But

despite the many advantages to OE vehicles of the returnless system,

most low-pressure motorsport systems still rely on a recirculating fuel

line incorporating a return fuel line back to the tank.

For gasoline direct systems and high pressure port-injected

variations a low-pressure electric pump is used to supply the fuel

from the tank to the intake of the high-pressure, mechanical pump

at a minimum of 6 bar. Beyond this the fuel can either be sent to theinjectors using a continuous delivery at constant pressure, with the

ow rate controlled by the speed of the piston, or a demand-controlled

system whose ow rate and pressure can be controlled by a separate

function from the ECU.

Piezoelectric injectorsOne of the latest innovations in fuel injection technology is the

introduction of the piezoelectric injector. Developed for diesel engine

applications, these fast-acting units are excellent at creating a greater

number of pulses essential to control the rate shaping combustion

process in a modern diesel engine. But their size and weight, as well

as the exibility of the spray patterns that can be created, make it

most unlikely that the technology will be seen in gasoline-fuelled

motorsport in the near future.

In summary

So there you have it, the simple mechanical systems through to thefar more complex fully electronic digital versions. While the former

still have a role to play the latter with their almost innite possibilities

are much more exacting. With advanced fuel injection you will, of

course, need some form of engine controller. Fortunately there must be

dozens of suitable systems on the market today, one of which will be

ideally suited to your requirements, whatever they are.

Moreover, any system capable of running a sequential port injection

system will also be capable of DI. However, getting the fuel spray

pattern to match the airow characteristics in the combustion chamber

is another matter. n

Figure 14

High-pressure

mechanical fuel pump

(Courtesy Bosch

Engineering GmbH)

Figure 15 High-pressure pump cross-section.

(Courtesy Bosch Engineering GmbH)

Figure 16 2008 Porsche LMP2 engine

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