autospeed - electronic engine management, part 1.pdf

7/27/2019 AutoSpeed - Electronic Engine Management, Part 1.pdf

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utoSpeed - Electronic Engine Management, Part 1

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Issue: 19 Section: Technical Features 2 March

Electronic

Engine

Management,

Part 1

It's good to have an idea of

how the engine

management system works

before you start modifying

the engine!

By Julian Edgar

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All engines require that the combustion air and fuel get mixed in the right proportion

and that each cylinder's spark plug is fired at the right time. Those are very importan

points to remember because it's easy to lose sight of these basics when making

modifications or considering complex engine management systems.

Remember: it all comes back to:

q how much fuel

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q and when does the spark fire.

The amount of fuel that is mixed with the air is described as the air/fuel ratio. A 'rich

mixture (like 12:1) uses lots of fuel and is good for power, while a 'lean' mixture (like

17:1) uses less fuel and so is more economical. The 10:1 air/fuel ratio shown on the

meter here is very rich indeed! The ignition timing refers to point during the rotation

the engine's crankshaft that the sparkplug fires. This needs to vary because at high

revs there is less time available for combustion to occur, and so the spark needs tostart the burn earlier. Changes in timing are also needed at different engine loads,

different temperatures and on different octane fuels.

Quick Overview

The first part of an engine management system is the fuel delivery system. This s

off at the high pressure fuel pump (pictured) that is mounted in or near the tank. The

fuel line from the pump passes through a filter before it runs forward to the engine b

The fuel line connects to a fuel rail that feeds each of the injectors. At the end of the

is a fuel pressure regulator, with surplus fuel heading back to the tank in the return

The air supply system starts at the airbox. A pick-up collects air from the atmosph

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and channels it through a filter. From there the air heads off to the engine. On some

engines, an airflow meter is located close after the airbox. The throttle body (butterf

controls how much air enters the plenum chamber. So that idle speed can be regula

there is a bypass around the throttle body. The amount of airflow flowing through th

bypass is regulated by a computer-controlled valve, which here can be seen on top

the throttle body. Once into the plenum (the aluminium box in the engine bay), the a

distributed evenly to each intake runner, flowing down the runner to the intake valve

Just before the air reaches the inlet valve it meets the squirting fuel injector. The air

mixture then passes into the combustion chamber when the valve is open.

The electronics system of the engine management uses sensors. Many times eac

second they tell the computer what's happening to the engine. The computer is thebrains behind the system, and in most cars it is called an ECU - an Electronic Contr

Unit. It is programmed by the factory to give the right outputs when it receives the ri

inputs. One example of the outputs it controls are the injectors.

The Inlet Sensors

In order that the ECU makes the right decisions about how much fuel to inject and

when to fire the spark, it needs to know precisely what the engine is doing. Is the ca

full throttle up a long hill at 3000 rpm on a stinking hot day? Or is the cold engine id

down to the shop after it's just been started? It's the input sensors that give this sort

information to the ECU.

The Coolant Temperature Sensortells the ECU how hot or cold the engine is. It is

positioned on the thermostat housing on most cars - in this picture it's the sensor wi

the yellow plug. The sensor has different resistances to electricity at different

temperatures. This means that as it get hotter, the sensor resists the flow of electric

less and less. The sensor is fed a regulated voltage by the ECU, and looks at the

changing voltage coming back to it. From this, the ECU can work out the temperatu

of the coolant.

The temperature of the intake air is also measured. The sensor can be located on th

airbox (as shown here) or on an intake runner. Just like the coolant temp sensor, th

Intake Air Temp Sensoris a variable resistor. This particular sensor has a resistan

of 3555 ohms at 20 C and just 475 ohms at 70 C. What? Your car wouldn't see an

intake air temp of 70 ? It sure does on a hot day....

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The ECU must know what the engine load is if it's to squirt in the right amount of pe

and fire the spark at just the right moment. Some cars use an Airflow Meterto

measure engine load. The amount of power being developed depends on how muc

the engine is breathing. If the engine is drawing in a lot of air (because the throttle is

fully open at 5000 rpm up a hill, for example) then lots of fuel will need to be injecte

keep the air/fuel ratio correct.

There are a couple of different types of airflow meter available. This airflow meter is

Hot WireAirflow Meter. The air rushing into the engine flows around a platinum w

that's been heated by electricity passing through it. The airflow cools down the wire

The ECU tries to keep the wire at the same hot temp at all times, and can work out

much electricity it needs to pump through the wire to do this. The more electricity it

needs to use, the more air that must be being breathed by the engine! Hot wire metautomatically compensate fro intake air temperature variations, measure mass flow

just volume), and cause little restriction to the intake air. In the US a management

system using this approach is sometimes called MAF - mass airflow.

This is a Vane Airflow Meter. This type of meter works by having a door across the

inlet air path. When air passes into the engine, the door is pushed open (as is being

manually done here). The wider the door is blown open, the more volume of air that

being consumed. The door is connected to an adjustable electrical device called a p

The pot has a variable electrical resistance, and it's connected so that the resistanc

the pot changes at different airflows. The vane airflow meter always causes a restric

to the inlet air because there's the vane in the way. Because it measures only air

volume, vane airflow meters have a built-in temperature sensor. With both volume a

temperature inputs, the ECU can work out the mass of air being breathed.



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Some cars don't use any form of airflow meter. These cars use the input of three

sensors to work out how much air is being breathed by the engine. One is the inlet

temp sensor, the other is a rpm sensor (we'll get to that in a moment), and the final

is a manifold vacuum sensor. This sensor is called a MAP (Manifold Absolute Press

sensor. A MAP Sensorconstantly measures the vacuum in the intake manifold. Th

amount of vacuum depends on revs and throttle opening. In the US this approach is

sometimes "speed density".

If the MAP sensor registers a low manifold vacuum and the rpm sensor indicates th

the engine is pulling high revs, the ECU knows that lots of fuel is needed. High man

vacuum at high orlow revs means that the throttle is closed - so that's a low load

situation. A MAP sensor is tee'd into the intake after the throttle butterfly and doesncause any intake restriction at all. Because the same MAP sensor can be used on a

naturally aspirated engine, it's the type of load sensor used with programmable

management systems like MoTeC.

A big exhaust will make good power on any car. But an EFI car with airflow meterin

will measure the extra air needed and add the correct amount of extra fuel. A MAP-

sensed car with a new exhaust may run lean enough to actually drop power, becau

the ECU doesn't know that there's the extra airflow. All it knows is that there's a cer

manifold vacuum at a certain rpm with a certain inlet air temp. The other thing to be

aware of with a car running a MAP sensor is that if you fit a hot cam, the car will run

poorly at light loads. This is because the light load vacuum signal is changed by the

cam and so the ECU will get confused. This makes ECU modification vital when a n

cam is fitted to a MAP-sensed engine.

Measuring engine load is fine, but at times the ECU also needs to know how far op

the throttle actually is. A Throttle Position Sensortells the ECU this information,

which is used during acceleration enrichment, for example. Most throttle sensors us

variable pot, but some sensors are just two position switches - one position for idle a

the other for full throttle. The sensor shown here uses a pot and so the ECU always

knows exactly how much throttle opening is being used.

The ECU needs to know how fast the engine's spinning and where the crankshaft is

its rotation. This lets the ECU fire the spark and injectors at the right time. There are

couple of different types ofCrankshaft Position Sensors. This Nissan Optical

Crankshaft Position Sensoruses a light shining through slots cut in a circular plat

The plate is attached to the end of the camshaft and is whizzed around past a LED

provides the light. A sensor on the other side of the disc registers when it sees the l

shining through one of the slots, with the ECU then counting the light pulses. Some

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optical sensors use 360 slots in the disc, allowing very fine resolution of engine spe

Different shaped slots positioned elsewhere on the disc allow the ECU to calculate

crank position.

This is a Hall Effect Crankshaft Position Sensor. It uses a chopper disc that spin

around inside the dizzy. Every time the metal vane comes between a magnet and a

special Hall sensor, the Hall sensor switches off. The computer looks at the speed o

the pulsing and works out engine rpm and where the crank is in its rotation.

The purpose of the Vehicle Speed Sensoris dead simple - it tells the ECU how fas

the car is going down the road. The sensor can be mounted on the gearbox (as her

or in the speedo. The sensor shown here is driven by a gear on the gearbox output

shaft. Speed sensing is used by the ECU in some cars to limit top speed and also to

improve fuel economy and drivability.

The Oxygen Sensoris located in the exhaust close to the engine. It tells the ECU

whether the car is running rich or lean at part throttle. The sensor generates its own

voltage output, just like a battery. When the air/fuel ratio is lean, the sensor generate

very low voltage output, like 0.2 volts. When the mixture is rich, the voltage output is

higher, like 0.8 volts. The ECU uses the output of this sensor to keep mixtures arou

14.7:1 a lot of the time.

The Knock Sensoris like a microphone listening for the sounds of knocking

(detonation). It's screwed into the block and is designed to separate out the special

noise which means that knocking is occurring. Many EFI engines run ignition timing

very close to knocking, meaning that this sensor is vital if engine damage isn't to oc

on a bad batch of fuel or on a very hot day. Some cars (especially those with V



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engines) run two knock sensors.

Engine Management, Part 2 - The Outputs

Engine Management, Part 3 - The ECU

More of our most popular Technical Features.

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The best guide to the flows of different injectors that we can find

Airflow Meter SwapsTechnical Features - 3 Sep, 2003

Recalibrating the factory management to suit.

Project EXA - Part 3 - DIY Boost Control.Technical Features - 26 Jan, 1999

An in-cabin boost control that's simple, cheap and effective.

Using a Multimeter to Make Your Car Go HarderTechnical Features - 25 Apr, 2003

Higher performance from LCD digits

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