engine menagment2.pdf

fast tech / TECH / ENGINE MANAGEMENT /

fast tech

CIRCUIT

Words: Stewart Sanderson

Engine

management

systems (even

’80s Ford ones)

are much more

involved than

you might

think. Here’s

what you need

to know.

looked at the

majority of sensors to be found

within a modern car’s engine

bay. The multitude of those

we discussed are fitted to the

engine with the sole purpose

of feeding information back

to the engine management

system, so that it has all the

information it needs to perform

its vital tasks. This month, we’re

going to look at what the most

important tasks performed by

the management computer really

are and how it actually performs

them after utilising all the

information provided to it by the

input sensors.

As briefly mentioned last

month, there are two different

systems commonly in use on

your Ford: the Speed Density

system (Weber Cosworths and

Fiesta Turbos), and the Mass Air

Flow system (pretty much all

the rest!). Let’s start with the old

speed density systems because

these are the most basic of all but

use virtually all the same sensors

as the Mass Air Flow system bar

one main one.

Modern ECUs constantly monitor what the

engine is doing, including revs, air and

coolant temperatures and air/fuel ratio

the amount of fuel to inject into

an engine’s cylinder by using

both engine speed and air density

as the two main factors. Engine

speed is quite simple to measure

and calculate, because it uses

the speed signal coming from the

crankshaft speed sensor. But air

density? How does it work that

out?

Well, it uses the output from

the MAP sensor to tell it what

the manifold absolute pressure

is at any given time, and it also

uses the output from the air

temperature sensor to keep

it fully up to speed with the

temperature of the air in the

plenum chamber. That means

it knows both the pressure and

temperature of the air about to

enter the cylinders.

Using these two pieces of

information, it can calculate

the actual density of the air

very accurately indeed. The air

density relates to the amount of

oxygen likely to be present within

the plenum at that particular

instance, and thus has a bearing

on the engine’s fuel and spark

advance requirements.

Speed density systems are very

fast, very accurate, and because

they measure the characteristics

of the intake charge actually

inside the inlet manifold, they

give good, reliable and repeatable

results. However, they still suffer

from one very serious flaw. The

actual temperature and, more

importantly, the actual pressure,

of the intake charge in the

combustion chamber is never

going to be exactly the same as it

is in the inlet manifold.

The air charge must pass

from the plenum, where it’s

measured, through an inlet tract

in the cylinder head and then

past an inlet valve or two before

it even reaches the combustion

chamber. Along this journey it will

also be subject to resonances

and other complex factors that

can both reduce or increase

(but mostly reduce) the actual

amount of air that ever makes it

into the cylinder itself on each

intake stroke. In order to be able

to account for these differences

in air flow, a speed density-based

ECU also needs one further

piece of vital information before

it can accurately perform its

SPEED DENSITY

Speed density takes its name

from the fact that it calculates

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DECEMBER 2006 FAST FORD

FAST FORD JANUARY 2007

0111

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also affect idle speed, as these

add a huge rotational load, and

sometimes the engine speed is

kept falsely high on overrun until

a vehicle is stationary, because

this reduces emissions.

Lambda sensors let the ECU know if the car is running

richer or leaner than ideal, so it can adjust accordingly

OTHER THINGS?

There are a whole host of other

things that can be operated by

the ECU. The radiator fan is a

common item switched on and

off by the ECU, and in turbo

cars the boost level is usually

fully ECU-controlled. Automatic

transmission control is often

integrated into the engine

management system too, not

to mention seat, environment

and headlight controls in many

systems. Virtually all fuel-

injected cars have the fuel pump

activation done by the ECU

through a relay, not to mention

the management light and auto

diagnostics that are an important

injector open for half the time has

a 50 per cent duty cycle, while if

it’s open for three-quarters of the

available time, it has a 75 per cent

duty cycle. At maximum power in

a standard engine, the injectors

might have an 85 per cent duty

cycle. That means the injectors

are flowing at 85 per cent of their

full capacity.

A good-quality multimeter can

be used to read off the injector

duty cycle by back-probing the

injector plug when it’s connected

to the injector. It’s worth noting

that it’s not recommended to

exceed 85 per cent duration for

reliability reasons.

variable as the amount of fuel

to be injected. If you missed it,

it may be worth reading our in-

depth feature on spark advance

in issue 244/September 2006 for

more details.

IDLE SPEED CONTROL

Idle speed control is carried out

by changing the amount

of air that can bypass the

closed throttle butterfly.

In all cases, we have

a throttle plate

bypass

channel

installed. It’s

essentially

blocked by

the idle valve,

so when it’s closed we

have no air bypassed,

and when it’s open we

have lots of air bypassed,

and hence a very high idle speed.

Some cars use a pulsed valve

(a bit like an injector in the way it

switches on and off) to regulate

the amount of air that can get

past the throttle body. If the

idle speed needs to be raised,

the duty cycle of the valve is

increased and more air squeezes

through. Often, the valve is only

used to its full potential when

the engine’s cold, because

they have an increased idle

speed requirement when

temperatures are lower. In

these scenarios the duty cycle

to the valve is high when the

engine’s cold and gradually

decreases as the engine warms

up until, when the engine is

fully warm, very little (or none,

depending on the system) air

is allowed to pass through the

valve, and so the idle speed

remains low.

Other times at which

idle speed compensation is

commonly required are when

there’s high electrical load – it

compensates for the large draw

of power from the alternator,

which will slow the engine slightly.

Air conditioning compressors

closed throttle butterfly.

In all cases, we have

On speed density-managed cars,

altering the breathing capabilities

means your ECU will require a remap

calculations – something known

as a volumetric efficiency map.

This is simply a reference that

enables the ECU to calculate how

much of that dense air will flow

into the cylinders themselves

with any given throttle input

at any given engine speed.

Essentially, it’s a map that relates

to how efficient the engine itself

actually is.

However, the problems

associated with using a

volumetric efficiency map

are twofold. Firstly, there’s no

sensor you can use to measure

VE directly because it varies

depending on load and other

factors. Instead,

you must run

the engine

at each

load point,

and then manually adjust it

until you get the air-to-fuel ratio

perfect using exhaust gas and

temperature sensors.

Secondly, every time you do

anything at all to the engine that

changes its breathing ability, you

instantly change the VE across

the range. Therefore, you must

once again recalculate all the

VE points if the management

is to do its job correctly. As the

engine ages, it accumulates

carbon deposits on the pistons

that increase the compression

ratio, the cam lobes wear so

the cams open a little less, and so

on and so forth. This means that

the volumetric efficiency of the

engine is constantly changing,

and the state of tune is constantly

deteriorating as a direct result.

From an emissions point of view,

this really is very poor indeed.

So remember folks, if you do

something to make your speed

density-managed system flow air

differently to standard, or even

to when you last had your new

performance chip written, expect

problems with it sooner rather

than later. The ECU is totally blind

to your meddling and has not

added the extra fuel you should

now need to stop the engine

melting down like a Mars bar in

a furnace. Adding turbos, cams,

headwork, bigger throttle bodies,

anything like that at all, will 100

per cent require a remap if you

want to avoid disaster. So

beware out there, please...

the power of the speed density

unit, but which could actually

compensate for any changes

in volumetric efficiency (VE) all

by itself? A system that could

see the air density and, more

importantly, the actual air flow,

and automatically adjust the fuel

delivered for it? You’ve probably

already guessed by the name that

this is exactly what a mass air

flow system does – it measures

air flow as well as density.

Apart from this, the rest of the

system works much the same as

a speed density one, and simply

monitors the engine sensors,

makes decisions about what to

do with ignition, idle and fuelling,

and then sends out the relevant

signals to get the job done. But

what jobs are we talking about

exactly? Let’s take a look at the

main hardware activation and

control circuits, and then we can

look at the various operating

modes and features.

switches them on for. When

they’re switched on, fuel squirts

out of them in a fine spray. The

pulsing on and off of the injectors

is the clicking noise that you can

hear if you listen really closely to

a fuel-injected engine.

The time that they’re switched

on and open for is called their

pulse width. The percentage of

time that the injectors are open

for is called the duty cycle. An

SPARK PLUG FIRING

In most systems, the ECU does

not normally control the ignition

coil (or coils) directly. Instead, an

ignition module is used to switch

the power to the coil on and off,

with the ECU actually telling the

ignition module exactly when it

needs to do this switching so that

the spark is delivered at just the

right time.

Spark time is taken from

various spark tables within the

ECU control map and is just as

so when it’s closed we

have no air bypassed,

and when it’s open we

have lots of air bypassed, part of the modern-day ECU.

Fuel injectors are the most important

component that the ECU controls

part of the modern-day ECU.

The diagnostic systems not only

indicates to us humans when

there’s an engine management

problem, but can also be used

to communicate to us what the

problem is actually likely to be.

STRATEGIES

The ECU has various strategies

built into it for performing various

modes of operation, such as

idling, cruising, accelerating and

decelerating, to name just a few.

Let’s take a quick look at how

these can be expected to work.

used to its full potential when

sensor you can use to measure

VE directly because it varies

depending on load and other

factors. Instead,

you must run

the engine

at each

load point,

ratio, the cam lobes wear so than later. The ECU is totally blind

to your meddling and has

added the extra fuel you should

now need to stop the engine

melting down like a Mars bar in

a furnace. Adding turbos, cams,

headwork, bigger throttle bodies,

anything like that at all, will 100

per cent require a remap if you

want to avoid disaster. So

beware out there, please...

All engine management set-ups use

a rev limiter, either cutting the spark

or fuel at a pre-determined limit

CLOSED LOOP

Regulars will know that this is a

favourite of ours, and Motorsport

Developments went to the

trouble of developing a system

for Cosworths, so they can run

closed loop perfectly even on

very large injectors. The system

works quite simply on paper. The

ECU is programmed to keep the

air/fuel ratio close to 14.7:1 – this

is the mixture at which the engine

gives the most power for the

least fuel and producing the least

amount of exhaust byproducts.

The oxygen sensor (or Lambda

sensor, as it’s often known)

MASS

AIR FLOW

SYSTEMS

Considering

all the above,

wouldn’t it be

nice if we could

have a management

system that had all

Speed density systems use air pressure and temperature to

calculate air density (and thus oxygen level) inside the plenum

(above), as this affects the engine’s fuel and spark advance needs

have a management

system that had all

FUEL INJECTOR

OPERATION

Arguably the most important

components that the ECU control

are the fuel injectors. The amount

of fuel that flows through the

injectors is determined by the

length of time that the ECU

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cars the boost level is usually

fully ECU-controlled. Automatic

transmission control is often

integrated into the engine

fast tech

/ TECH / ENGINE MANAGEMENT /

sends a voltage signal back to

the ECU, telling it whether the car

is running richer or leaner than

14.7:1. If the engine’s running

a little rich, the ECU will lean it

out. If it’s a bit lean, the ECU will

richen it up.

The oxygen sensor constantly

feeds back the relevant AFR as

a rich/lean signal and the ECU

makes adjustments to keep it

perfect – hence the name ‘Closed

Loop’. Closed loop running

on most cars occurs when

the engine’s up to operating

temperature, the throttle opening

is neither large nor small, and is

operating within a predetermined

engine speed range.

The ECU can, of course, switch

back out of closed loop running

instantaneously. You can be

driving along gently in closed loop

and then slam the throttle to the

floor. The ECU instantly switches

out of closed loop, ignoring the

output of the oxygen sensor and

substantially richening mixtures

to a safer and more power

productive level. None of this can

be felt by the driver.

Closed loop running relies on

the oxygen sensor being in good

condition. If it’s defective, the car

won’t go into closed loop, and

both fuel economy and emissions

will suffer.

reached the maximum

allowable revs. These

soft-cut limiters mean

that the car can be used

right to the rev limit

without a worry. Some

systems even have both

in case you somehow

manage to ignore the

soft limiter.

OVERRUN FUEL

SHUT-OFF

When you lift off the

throttle totally (like when

you’re coming up to a set

of traffic lights) the ECU

switches off the injectors

for economy and ultimate

emissions. The injectors

come back on again

when engine revs drop to

around 1500 rpm so as to

ensure the engine can idle

properly. Injector shut-off

has obvious benefits to both

fuel economy and emissions,

and thus needs no further

explanation. It’s one of the

ECU strategies that’s most

reliant on the input of the

speed sensor.

It’s worth noting that

sometimes these strategies

can cause poor surging at low

speeds when you’re driving at

light throttle. The CVH EFi was

thus afflicted as standard, so

often we remove the feature

altogether in our maps if the car

is affected.

The overrun fuel shut-off facility can cause low-speed

surging problems on EFi-equipped CVHs. It can be ﬁ xed!

another wonder of the modern

engine management system

– the LOS. LOS stands for Limited

Operation Strategy, or ‘Limp-home

mode’ as it’s become known.

Say a sensor was to fail – for

example, the coolant temperature

sensor becomes defective (or the

wire to the sensor is damaged).

The ECU sees that the sensor

is down and substitutes a value

based on the other sensor inputs

instead. It is hoped that this will at

least get you home. Some

ECUs can do without all but a

couple of inputs and still keep the

engine running.

to a sensor calibration failure, as

opposed to a total electrical or

mechanical failure.

That’s enough for this month.

Hopefully now you’ve got a good

idea of what each sensor does

and what jobs the ECU actually

performs in its day-to-day engine-

running tasks.

REV LIMITING

All engine management systems

use a rev limiter. Some of these

completely cut off fuel at the

prescribed engine speed,

withholding it until you’re 500

rpm below the limit. Hitting

this type of rev limiter makes

you think you’ve just broken

something serious! Other rev

limiters cut off the spark (or the

injectors) of individual cylinders

one after the other, so

that you can barely

feel that you’ve

LIMITED OPERATION

STRATEGY

When EFi cars were released,

they were universally accepted as

being the way forward for reliable

fuel delivery, and incredible

reliability was expected. However,

as the system grew more

complex, so did the possibilities

of complex repairs and

diagnosis problems if they

went wrong.

However, they’ve

thankfully proved to be

very reliable indeed,

but for those times

when they do go

wrong we’ll be

unlikely to get

stranded as

we were with

old-fashioned

systems,

thanks to

CONTACT

injectors) of individual cylinders

one after the other, so

that you can barely

feel that you’ve

as the system grew more

complex, so did the possibilities

of complex repairs and

diagnosis problems if they

went wrong.

However, they’ve

thankfully proved to be

very reliable indeed,

but for those times

when they do go

wrong we’ll be

unlikely to get

stranded as

we were with

old-fashioned

systems,

thanks to

Stewart Sanderson

co-owns Motorsport

Developments in Blackpool

01253 508400

www.remapping.co.uk

AUTO DIAGNOSTICS

All engine management systems

of the past 10 years or so have

what’s called a ‘self-diagnosis’

ability. That means you can plug

into them, ask them a question

and they’ll tell you what’s wrong

with them!

For example, imagine the intake

air temperature sensor wire is

broken. If you put the ECU into

self-diagnosis mode (this often

needs an expensive piece of

equipment), the ECU will indicate

that it’s the air temp sensor that

has the problem, so you can

change it and ask the ECU to

retest it, just to confirm your fault

has gone.

It’s worth noting, however, that

they are not always right, and

sometimes they can’t see a fault

that to the driver is blindingly

obvious, but that’s usually due

NEXT MONTH

We’ll look at how the ECU

actually interprets the

sensor inputs and comes up

with the right fuel and

spark requirements. We’ll

discuss how air, water,

throttle and engine speed

affect the spark and fuelling

strategies. For example,

what difference does the

water sensor reporting 20

degrees make to the

engine’s fuelling when it’s

actually very hot and nearer

90 degrees? By the end of

it, you’ll see why sensor

information is so critical.

See you next month!

Modern ECUs like

this one for the

Mk3 Mondeo even

control things like

seating and head-

light settings

0114 JANUARY 2007 FAST FORD

sends a voltage signal back to reached the maximum

allowable revs. These

another wonder of the modern

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