Bradley D., Russell D.W. (eds.) Mechatronics in Action: Case Studies in Mechatronics - Applications and Education
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Drive-by-wire
Many modern aircraft have now made the transition to “fly-by-wire” which
involves the replacement of mechanical linkages to the control flaps by electronic
signals generated by multiple computers. Cars have already started to move in this
direction with electrical power steering, parking brakes, communications buses
and wire-controlled throttles. The trend is expected to continue, however, there are
clear safety concerns over removing the physical links to such items as the main
brakes. An extension of this concept, the ‘smart wheel’ as developed by Siemens,
is shown in Figure 6.2. [4].
Fig. 6.2 Siemens eCorner ‘smart wheels’ future concept. The hub motor (2) is located inside the
wheel rim (1) and the electronic wedge brake (3) uses pads driven by electric motors. An active
suspension (4) and electronic steering (5) replace conventional hydraulic systems
6.1.2 Drivers for Change
It is interesting to consider why, after a hundred years of car production, the
mechatronic revolution took hold in the way that it did. It is possible to identify a
series of powerful drivers that coincided at that time:
Reliability
It is generally accepted that the modern motor car is now more reliable than ever
before. This is partly due to pressure from the car-buying public and competition
between manufacturers. However, a key enabling technology is the widespread
adoption of electronic ignition and fuel injection. By far, the majority of problems
associated with poor starting or bad running were related to the mechanical wear
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of components in the ignition and fuel delivery systems. Modern electronic engine
management systems largely cured these problems.
Economy and Performance
It was traditionally recognised that a car’s engine can be optimised for either
economy in terms of fuel consumption or performance in terms of power output.
The standard approach for increasing performance was to increase the flow of
air/fuel mixture through the engine by means of larger and multiple carburettors
which invariably had a deleterious effect on fuel consumption.
Paradoxically, the first commercial adoption of electronic fuel injection was on
high performance ‘GTI’ models where the main emphasis was on further increases
in power. However, it was quickly realised that the much enhanced control over
the fuel/air mixture that electronic fuel injection afforded could also be exploited
for significant benefits in economy. More recently, this technology has enabled a
major transformation in both the economy and performance of diesel engines.
Security
Pressure from insurance companies and the police led to a demand for significant
improvements in vehicle alarm and immobilisation systems.
Environmental Protection
Once it became clear that there was a valid technical solution, legislation was
introduced by governments to reduce harmful emissions from vehicles. Apart
from the complete abolition of lead additives in petrol, the main emphasis was on
the reduction of carbon monoxide from exhaust gases.
Exemptions were introduced for older vehicles because traditional mechanical
carburettor systems are not capable of delivering the low levels demanded by the
legislation. Thus, manufacturers were forced to move to mechatronic engine
management.
Safety
Customer demand for increased passenger safety led to the widespread adoption
of stability enhancing technologies such as ABS brakes and traction control as
well as measures to mitigate the effects of a collision such as air bags and seat belt
tensioners.
Enhanced Functionality
Mechatronics has provided manufacturers with the ability to offer enhanced
functionality across many aspects of the motor car. Manufacturers have used this
opportunity to distinguish basic models from top-of-the-range models and to
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generate additional income from the provision of factory-fitted extras. As specific
systems have become more cost effective, competition between manufacturers has
been an important driver in moving enhanced features down the model range.
6.2 Engine Basics
Before considering three automotive mechatronic systems in detail, it is necessary
to understand the basic operation of the internal combustion engine. The rotating
crankshaft at the bottom of the engine is connected to the piston and causes it to
rise and fall in the cylinder. The valves at the top of the engine open and close at
appropriate times in order to admit fuel to the cylinder and to allow burnt exhaust
gasses to be expelled. The most common car engine uses the four-stroke Otto
cycle [5].
(a) (b) (c) (d)
Fig. 6.3 The four-stroke Otto cycle, (a) induction stroke, (b) compression stroke, (c) power
stroke, and (d) exhaust stroke
The cycle starts with the induction stroke of Figure 6.3 (a). With the piston
moving downwards in the cylinder, the inlet valve opens and the air/petrol mixture
is sucked into the cylinder. When the piston is at the bottom, the inlet valve closes.
The piston then starts to move upwards, compressing the fuel mixture to about
a tenth of its original volume. This is the compression stroke of Figure 6.3 (b).
When the piston is close to the top, the spark plug ignites the fuel, causing
combustion and the expanding gasses to drive the piston downwards in the power
stroke as shown in Figure 6.3 (c). The final stage is the exhaust cycle of Figure 6.3
(d). The piston rises again and the exhaust valve opens to allow the burnt gases to
escape. The cycle then repeats. Thus, the crankshaft makes two revolutions in
each cycle.
The four-stroke engine can have from one to twelve cylinders, though the
commonest number for passenger cars is four.
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It can be concluded from the above that three key system requirements are:
1. The spark plug ignition must be precisely timed in relation to the movement of
the piston. Sparking normally occurs just before the piston reaches the top (said
to be advanced), though the precise position varies with fuel type.
2. The ratio of air/petrol fuel mixture must be carefully controlled. The ‘ideal’
mixture is known as the stochiometric ratio and is 14.68 parts of air to one part
of fuel by weight. A slightly lower ratio (12.6:1) is known to give maximum
power and a higher ratio (15.4:1) is best for economy [6].
3. The valves must open and close at exactly the right stage of the cycle. This is
one element of the system that is still under purely mechanical actuation (a
camshaft driven by a belt or chain from the crankshaft). However, electronic
valve control is an active research area and promises even more benefits in
terms of economy and performance in future vehicles.
6.3 The Mechanical Solution for Ignition Timing
and Fuel Delivery
6.3.1 Traditional Mechanical Ignition Timing
The main component in traditional mechanical ignition timing is the distributor of
Figure 6.4 (a) [7]. The purpose of the distributor is to trigger a release of high
voltage electricity from the coil and distribute it to the appropriate spark plug. The
triggering is performed by the contact breaker or points shown in Figure 6.4 (b).
An upper central shaft in the distributor is turned by a lower shaft connected to the
engine which causes a multi-sided cam to revolve inside the body of the
distributor. The cam separates the circuit breaker each time a spark is required.
The rotor-arm on top of the rotating shaft then makes contact with the appropriate
plug lead terminal to send the high voltage to the appropriate spark plug. The
condenser (or capacitor) is present to prevent the initial spark jumping across the
contact breaker gap and eroding the contact breaker surfaces.
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Vacuum
advance
High voltage
input from coil
High voltage output
to spark plug
Drive shaft
from engine
Contact
breaker
points
Condenser
(a) (b)
Fig. 6.4 A 4-cylinder distributor, (a) external view, and (b) internal view
The initial ignition timing is set by rotating the body of the distributor around
the central shaft so that the cams break the points at the correct time. However, for
maximum efficiency and performance, the spark plug needs to fire increasingly in
advance of the piston reaching the top of the cylinder (Top Dead Centre or TDC)
as the engine speeds up. This is achieved by the incorporation of centrifugal
weights that vary the angular relationship between the upper and lower shafts. The
aim is for the spark plug to fire so as to give the relatively slow combustion
process enough time to develop maximum pressure just as the piston is starting to
descend. If the spark timing is too early, there is the danger that the maximum
cylinder pressure is developed whilst the piston is still rising. This produces huge
stresses in the engine and is known as knocking. Paradoxically higher octane fuels
burn slower and so the ignition timing needs to be more advanced to allow more
time for combustion. The danger however is that if the owner fills-up with lower
octane fuel, there is the likelihood of knocking.
Yet another refinement is the vacuum advance. At idle, when little air/fuel
mixture is available to the engine, the inlet manifold will be at negative pressure.
This is used via a diaphragm to slightly rotate the plate that supports the contact
breaker points to again advance the spark.
As previously stated, many reliability problems with cars originated with the
ignition system. The contact breaker points in particular are subject to both wear
and the accumulation of dirt. They require routine replacement and adjustment to
keep the engine running smoothly and efficiently.
6.3.2 Fuel Delivery – the Carburettor
The carburettor was invented in 1895 by Karl Benz and was still in use a hundred
years later. Its basic function is to deliver the optimum mix of air and petrol to the
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cylinder prior to combustion. Referring to Figure 6.5, the basic operating principle
is as follows [8]:
Fig. 6.5 The basic carburettor
1. When a piston descends in the cylinder of an engine in the induction cycle, the
inlet valve is open. This sucks in a volume of air/fuel mixture roughly equal to
the displaced volume of the cylinder.
2. This action pulls in air through a filter and into the body of the carburettor. This
is in the form of a Venturi tube in order to accelerate (and hence reduce the
pressure) of the incoming air. The tube also contains a butterfly valve and a
fuel jet. The butterfly valve, which determines the volume of the air flow, is
connected to the accelerator pedal and is the means by which the driver
controls the output of the engine and hence the speed of the car.
3. As the air flows past the jet at reduced pressure, it draws in fuel which divides
into a spray of small droplets and mixes with the air (an aerosol)
2
. The fuel is
supplied from a reservoir built into the body of the carburettor, the level of
which is controlled by a float.
In the simplest form of carburettor, the proportions of the air/fuel mixture are
determined by the jet diameter and its position in the air flow. This works
reasonably well for steady power output at high engine speeds, though there are
problems at other times:
• When the engine is cold the mixture needs to be much richer in order to get the
engine to start.
• When the engine is ticking over at low speed, the air flow through the venture
is not sufficient to produce adequate fuel flow.
• During sudden acceleration of the car, the air can gain speed quicker than the
heavier fuel producing a lean mix when a rich one is required.
Over time, the mechanical complexity of carburettors was increased in order to
address these problems and to attempt to produce an optimum mix for all
conditions. One such refinement is the variable choke carburettor where a damped
2
The same principle as is used in a scent spray
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piston rises and falls in the Venturi tube in an attempt to keep the speed of air flow
across the jet constant [9]. At the same time, a tapered needle connected to the
piston moves in and out of the jet, altering its effective diameter. In addition, some
carburettors use a pump to boost fuel flow during rapid acceleration.
The problem of cold starting was resolved by means of a choke which
simultaneously reduces air flow and increases fuel flow. For many years, these
were hand operated, but eventually became automatic based on some kind of
temperature sensor.
It is clear that the carburettor evolved to become a relatively complex and
expensive piece of precision mechanical engineering. However, even in its most
advanced form, it could not deliver the precision of fuel metering required to meet
the demands of the modern car engine. This is also the case for a new carburettor.
Older carburettors, where the needles and jets have been subjected to wear or poor
adjustment, present even greater problems. Only an electronic fuel injection
system can consistently deliver adequate fuel control in terms of performance,
economy and environmental emissions.
6.4 The Mechatronic Solution to Engine Management
The first component to be replaced by mechatronics was the troublesome contact
breaker points. From the 1970s onwards, these were replaced by a non-contact
sensor inside the distributor that consisted of a rotating toothed armature (one
tooth for each cylinder) that induces a signal from an electromagnetic transponder
each time a tooth passes in front of it [7]. This signal was then sent to an electronic
ignition control unit that triggered the firing of the coil and hence the spark. This
simple innovation produced a stronger and more reliable spark and removed the
need for the replacement and maintenance of points. At this stage, the mechanical
centrifugal and vacuum advance systems remained.
The real revolution came in the mid-1980s when advances in electronic fuel
injection and microprocessor technology enabled complete control over both
ignition and fuel delivery to be contained within a single Engine Control Unit
(ECU). This allows for a much clearer separation between sensing, processing and
actuation in accordance with mechatronic principles. Both the distributor and the
carburettor have now become redundant [6, 10].
6.4.1 Sensors
Crankshaft and camshaft sensors generally consist of toothed wheel armatures
passing an electromagnetic Hall Effect sensor. By counting the pulses, the ECU
can evaluate firstly engine speed in rpm, and secondly, the actual current position
of the pistons and the stage in the four-stroke cycle. The armatures generally
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contain a missing tooth so that the ECU can identify this and synchronise with a
specific piston position. Because the crankshaft rotates twice within each four-
stroke cycle and it is important for the ECU to know the current stage of the cycle,
some indication is also required from the camshaft as to which spark plug is about
to fire.
A knock sensor is essentially a microphone fitted to part of the engine block to
listen for the distinctive sound of engine knocking, indicating that the spark is too
far advanced. The microphone is associated with a bandpass filter to identify the
relevant knocking frequencies and inform the ECU.
A Lamda or oxygen sensor is placed in the exhaust system to measure any un-
burnt oxygen in the waste gasses. They are primarily used to control the air/fuel
mix ratio as richer fuel mixes tend to burn quicker, requiring less ignition advance.
A lambda sensor contains a ceramic layer which produces a current in the
presence of excess oxygen. This current can then be detected by the ECU. They
only work effectively when hot, and hence the ECU may be told to ignore the
output when the exhaust system is cold. In order to reduce the waiting time, some
sensors are fitted with heaters and are known as Heated Exhaust Gas Oxygen
sensors or HEGOs.
A throttle position sensor is a simple rotary potentiometer usually connected to
the end of the butterfly valve in the air induction system. Knowing the degree to
which the valve is open gives the ECU a good indication of the driver’s
intensions. By looking at direction of movement and rates of change of the valve,
the ECU can determine if the vehicle is accelerating, decelerating or cruising; all
of which influence ignition timing and fuel ratio.
The mass air flow sensor (MAF) is contained in the air induction system and
provides information on the mass of air entering the engine which is obviously
key to determining the appropriate amount of fuel to inject. The sensor consists of
a heated wire element that is maintained at constant temperature in the varying air
flow. The current required to maintain this temperature is directly proportional to
the mass of air flowing.
Earlier sensors measured volume of flow, but as the density of air reduces with
temperature, additional temperature information was required to calculate the
mass of air with sufficient accuracy.
A water temperature sensor allows the ECU to detect a cold start and hence
enrich the fuel. The normal fuel mixture is then adopted when the temperature
reaches a pre-set value.
6.4.2 Actuators
Ignition coils still provide the high voltage for the spark plugs, but in the absence
of a distributor, modern systems often use individual coils for each plug or pair of
plugs. The triggering signal comes directly from the ECU.
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Fuel Injectors add a spray of fuel through a nozzle to the incoming air flow in
order to achieve the appropriate air/fuel mix ratio. Current practice is to use one
injector per cylinder, all of which are fed by a constant-pressure fuel line.
Variation in the amount of fuel added is determined by very precise control over
the time that the injector valve is opened on each induction cycle. The opening of
the valve is affected by a signal from the ECU that powers a solenoid within the
injector. Closure is by means of a return spring and the fuel line pressure.
6.4.3 Processing
The ECU generally takes the form of a module such as that of Figure 6.6 within
the engine compartment. The module contains:
• all the electronics for receiving and conditioning the signals from the sensors;
• a powerful processor for interpreting the signals and determining the outputs;
• output circuits and amplifiers for driving the ignition coils and fuel injectors.
In addition, the ECU will contain a fault memory that can be read when the
vehicle is serviced. The essence of determining the most appropriate system
outputs is the reading of values from look-up tables or maps such as that of Table
6.1.
Fig. 6.6 A typical ‘black box’ Engine Control Unit (ECU) module
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Table 6.1 A portion of the fuel map for a high-revving motorcycle engine
Fuel Map
Throttle Position
RPM 0.00 4.00 7.00 11.00 16.00 22.00 29.00
500 3.06 3.65 3.65 4.64 4.80 5.00 5.14
1000 2.08 2.78 3.34 3.34 4.45 4.85 5.12
2000 2.07 2.78 3.34 3.34 4.45 4.85 5.12
3000 1.85 2.78 3.34 3.34 4.45 4.85 5.12
4000 1.75 2.62 3.15 3.15 4.20 4.58 4.83
4500 1.75 2.62 3.15 3.20 4.25 4.71 5.05
5000 1.75 2.62 2.60 2.97 4.04 4.43 4.73
6000 1.72 2.57 2.11 2.64 3.33 3.77 4.14
6500 1.63 2.44 2.31 2.64 3.41 3.80 4.11
7000 1.61 2.42 2.59 2.76 3.62 4.00 4.27
7500 1.60 2.40 2.72 2.43 3.37 3.80 4.15
8000 1.58 1.90 1.90 2.09 3.13 3.61 4.03
9500 4.67 5.08 5.21 5.26 5.10 5.32 5.19
10000 4.67 5.08 5.27 5.33 5.29 5.45 5.19
Injector opening time (µs)
Aftermarket ECUs that are reprogrammable are available for car developers
and in this case, the engine maps will be stored on a separate EPROM chip within
the ECU. Such ECUs are provided with software that can be used in conjunction
with a laptop to tune the engine while running. The basic tables are the ignition
map and the fuel map. In both cases, one axis of the map represents the speed of
the engine (rpm) and the other some indication of engine load. Table 6.1 shows an
example of the fuel map for a motorcycle engine. In this case, the vertical axis is
rpm and the horizontal axis represents the percentage of throttle opening. The
figures in the cells indicate the time in microseconds that the injector should
remain open for.
The data in the above table is often represented as a 3D surface, as shown in
Figure 6.7, which is clearly the origin of the term engine ‘map’. Figure 6.8 then
shows a typical architecture for an engine management system. At the heart is a
powerful processor such as the Motorola MPC 500 32-bit chip [11].