13 Types of Electric Vehicle in Use Today

Developments of ideas from the 19th and 20th centuries are now utilised to produce a new range of electric vehicles that are starting to make an impact.

There are effectively six basic types of electric vehicle, which may be classed as follows. Firstly there is the traditional battery electric vehicle, which is the type that usually springs to mind when people think of electric vehicles. However, the second type, the hybrid electric vehicle, which combines a battery and an IC engine, is very likely to become the most common type in the years ahead. Thirdly there are vehicles which use replaceable fuel as the source of energy using either fuel cells or metal air batteries. Fourthly there are vehicles supplied by power lines. Fifthly there are electric vehicles which use energy directly from solar radiation. Sixthly there are vehicles that store energy by alternative means such as flywheels or super capacitors, which are nearly always hybrids using some other source of power as well.

Other vehicles that could be mentioned are railway trains and ships, and even electric aircraft. However, this book is focused on autonomous wheeled vehicles.

1.3.1 Battery electric vehicles

The concept of the battery electric vehicle is essentially simple and is shown in Figure 1.5. The vehicle consists of an electric battery for energy storage, an electric motor, and a controller. The battery is normally recharged from mains electricity via a plug and a battery charging unit that can either be carried onboard or fitted at the charging point. The controller will normally control the power supplied to the motor, and hence the vehicle speed, in forward and reverse. This is normally known as a 2 quadrant controller, forwards and backwards. It is usually desirable to use regenerative braking both to recoup energy and as a convenient form of frictionless braking. When in addition the controller allows regenerative braking in forward and reverse directions it is known as a 4 quadrant controller.4

There is a range of electric vehicles of this type currently available on the market. At the simplest there are small electric bicycles and tricycles and small commuter vehicles. In the leisure market there are electric golf buggies. There is a range of full sized electric vehicles, which include electric cars, delivery trucks and buses. Among the most important are also aids to mobility, as in Figure 1.4 and Figure 11.2 (in the final chapter), and also delivery vehicles and electric bicycles. Some examples of typical electrical vehicles using rechargeable batteries are shown in Figures 1.6 to 1.9. All of these vehicles have a fairly

Figure 1.5 Concept of the rechargeable battery electric vehicle

4 The 4 "quadrants" being forwards and backwards acceleration, and forwards and backwards braking.

Figure 1.6 The classic electric car, a battery powered city car (Picture of a Ford Think® kindly supplied by the Ford Motor Co. Ltd.)

limited range and performance, but they are sufficient for the intended purpose. It is important to remember that the car is a very minor player in this field.

1.3.2 The IC engine/electric hybrid vehicle

A hybrid vehicle has two or more power sources, and there are a large number of possible variations. The most common types of hybrid vehicle combine an internal combustion engine with a battery and an electric motor and generator.

There are two basic arrangements for hybrid vehicles, the series hybrid and the parallel hybrid, which are illustrated in Figures 1.9 and 1.10 In the series hybrid the vehicle is driven by one or more electric motors supplied either from the battery, or from the IC engine driven generator unit, or from both. However, in either case the driving force comes entirely from the electric motor or motors.

In the parallel hybrid the vehicle can either be driven by the IC engine working directly through a transmission system to the wheels, or by one or more electric motors, or by both the electric motor and the IC engine at once.

In both series and parallel hybrids the battery can be recharged by the engine and generator while moving, and so the battery does not need to be anything like as large as in a pure battery vehicle. Also, both types allow for regenerative braking, for the drive motor to work as a generator and simultaneously slow down the vehicle and charge the battery.

Figure 1.7 Electric bicycles are among the most widely used electric vehicles

The series hybrid tends to be used only in specialist applications. For example, the diesel powered railway engine is nearly always a series hybrid, as are some ships. Some special all-terrain vehicles are series hybrid, with a separately controlled electric motor in each wheel. The main disadvantage of the series hybrid is that all the electrical energy must pass through both the generator and the motors. The adds considerably to the cost of such systems.

The parallel hybrid, on the other hand, has scope for very wide application. The electric machines can be much smaller and cheaper, as they do not have to convert all the energy.

Figure 1.8 Delivery vehicles have always been an important sector for battery powered electric vehicles
Figure 1.9 Series hybrid vehicle layout

There are various ways in which a parallel hybrid vehicle can be used. In the simplest it can run on electricity from the batteries, for example, in a city where exhaust emissions are undesirable, or it can be powered solely by the IC engine, for example, when traveling outside the city. Alternatively, and more usefully, a parallel hybrid vehicle can use the IC engine and batteries in combination, continually optimising the efficiency of the IC

Electric motor, works as a generator when used as regenerative brake

Transmission

Controller

Transmission

Electric motor, works as a generator when used as regenerative brake

Controller

cables

Figure 1.10 Parallel hybrid vehicle layout cables

Figure 1.10 Parallel hybrid vehicle layout engine. A popular arrangement is to obtain the basic power to run the vehicle, normally around 50% of peak power requirements, from the IC engine, and to take additional power from the electric motor and battery, recharging the battery from the engine generator when the battery is not needed. Using modern control techniques the engine speed and torque can be controlled to minimise exhaust emissions and maximise fuel economy. The basic principle is to keep the IC engine working fairly hard, at moderate speeds, or else turn it off completely.

In parallel hybrid systems it is useful to define a variable called the 'degree of hybridisation' as follows:

electric motor power DOH = F

electric motor power + IC engine power

The greater the degree of hybridisation, the greater the scope for using a smaller IC engine, and have it operating at near its optimum efficiency for a greater proportion of the time. At the time of writing the highly important California Air Resources Board (CARB) identifies three levels of hybridisation, as in Table 1.1. The final row gives an indication of the perceived 'environmental value' of the car, and issue considered in Chapter 10.

Because there is the possibility of hybrid vehicles moving, albeit for a short time, with the IC engine off and entirely under battery power, they can be called 'partial zero emission vehicles' (PZEVs).

Hybrid vehicles are more expensive than conventional vehicles. However there are some savings which can be made. In the series arrangement there is no need for a gear box, transmission can be simplified and the differential can be eliminated by using a pair of motors fitted on opposite wheels. In both series and parallel arrangements the conventional battery starter arrangement can be eliminated.

Table 1.1 CARB classification of hybrid electric vehicles, as

in April 2003

Level 1:

Level 2:

Level 3: high-voltage

low-voltage HEV

high-voltage HEV

high-power HEV

Motor drive voltage

<60 V

>60 V

>60 V

Motor drive peak power

>4kW

>10kW

>50kW

Regenerative braking

Yes

Yes

Yes

Idle stop/start

Yes

Yes

Yes

10 year/150 kmile battery

Yes

Yes

Yes

warranty

ZEV program credit

0.2

0.4

0.5

Figure 1.11 The Toyota Prius (Pictures reproduced by kind permission of Toyota.)

There are several hybrid vehicles currently on the market, and this is a sector that is set to grow rapidly in the years ahead. The Toyota Prius, shown in Figure 1.11, is the vehicle which really brought hybrid vehicles to public attention. Within two years of its launch in 1998 it more than doubled the number of electric vehicles on the roads of Japan.5 The Prius uses a 1.5 litre petrol engine and a 33 kW electric motor either in combination or separately to produce the most fuel-efficient performance. A nickel metal hydride battery is used. At start up or at low speeds the Prius is powered solely by the electric motor, avoiding the use of the internal combustion engine when it is at its most polluting and least efficient. This car uses regenerative braking and has a high overall fuel economy of about 56.5 miles per US gallon (68 miles per UK gallon).6 The Prius has a top speed of 160km/h (100mph) and accelerates to 100km/h (62mph) in 13.4 seconds. The Prius battery is only charged from the engine and does not use an external socket. It is therefore refueled with petrol only, in the conventional way. In addition, it seats four people in comfort, and the luggage space is almost unaffected by the somewhat larger than normal battery. The fully automatic transmission system is a further attraction of this car that

5 Honda brought out its parallel hybrid Insight model in 1998. This has somewhat better fuel economy and lower emissions. However, it is only a two-seater, the luggage space is much more limited, and its market impact has not been so great.

6 Further details in Table 11.6 of the final chapter.

has put electric cars well into the realm of the possible for ordinary people making the variety of journeys they expect their cars to cope with.

The Toyota Prius mainly has the characteristics of a parallel hybrid, as in Figure 1.10, in that the IC engine can directly power the vehicle. However, it does have a separate motor and generator, can operate in series mode, and is not a 'pure' parallel hybrid. It has a fairly complex 'power splitter' gearbox, based on epicyclic gears, that allows power from both the electric motor or the IC engine, in almost any proportion, to be sent to the wheels or gearbox. Power can also be sent from the wheels to the generator for regenerative braking.

Most of the major companies are now bringing out vehicles that are true parallel hybrids. The Honda Insight, shown in Figure 8.14, and whose performance figures are given in Table 11.5, is a good example. There is also now a parallel hybrid electric version of the popular Honda Civic available.

As well as the parallel hybrid arrangement shown in Figure 1.10, in which the IC engine and electric machine sit side by side, there is an almost infinite number of other possible arrangements. The Honda vehicles mentioned above have the electric machine sitting in line with the crankshaft, in the place of the flywheel in a conventional IC engine. Other notable hybrids appearing on the market, such as hybrid versions of the popular sports utility vehicle (SUV) in the USA, have the IC engine and the electric machines connected to different axles, as in Figure 1.12. Here the electric system drives the rear wheels, and the IC engine the front. This is a true parallel hybrid, and the road can be thought of as the medium that connects the two parts of the system, electric and engine. This arrangement has many attractions in terms of simplicity of packaging, and that it is a very neat way of giving the vehicle a four wheel drive capability. The battery will be mainly charged by regenerative braking, but if that is insufficient, at times of low speed travel the rear wheels could be electrically braked thus charging the battery, and the front driven harder to maintain speed. This transfers energy from the IC engine to the battery, using the road.

Figure 1.12 Parallel hybrid system with IC engine driving the front axle, and electric power to the rear wheels

Front wheels, electrically driven (and

Figure 1.13 Yet another possible parallel hybrid arrangement: electric power to the front wheels, IC engine to the rear

Front wheels, electrically driven (and braked) ^/'^^¿N

Figure 1.13 Yet another possible parallel hybrid arrangement: electric power to the front wheels, IC engine to the rear

A variation on this idea is shown in Figure 1.13. Many SUVs are rear wheel driven by the IC engine, and these can be made into parallel hybrids by providing electrical power to the front axle. This has a slight advantage over the arrangement in Figure 1.12, in that more regenerative braking power is available on the front axle, due to the weight transfer to the front wheels under braking.

Even more variety in the arrangement for hybrid vehicles becomes apparent when we note that, with all types of hybrid, the battery could be charged from a separate electrical supply, such as the mains, while the vehicle is not in use. This would only be worthwhile if a larger battery was used, and this could allow the car considerable 'battery only' range. There are no manufacturers with vehicles of this type planned for launch soon, but it might be a development in years ahead.

Despite the huge variety in detail that is possible with IC engine/battery hybrids, the major technological components are essentially the same: electric motors, batteries, and controllers. So we do not have a chapter dedicated to hybrids, which their importance could justify, because the underlying technology is explained the chapters covering these topics.

1.3.3 Fuelled electric vehicles

The basic principle of electric vehicles using fuel is much the same as with the battery electric vehicle, but with a fuel cell or metal air battery replacing the rechargeable electric battery. Most of the major motor companies have developed very advanced fuel cell powered cars. Daimler Chrysler for example have developed fuel cell cars based on the Mercedes A series, fitted with Ballard fuel cells, one of which is shown in Figure 1.14. This fuel cell runs on hydrogen which is stored in liquid form.

Figure 1.14 The Necar 4 fuel cell car from 1999. This was the first fuel cell car to have a performance and range similar to IC engine vehicles. The top speed is 144 kph, and the range 450 km. The hydrogen fuel is stored as a liquid. (Photograph reproduced by kind permission of Ballard Power Systems.)

Figure 1.14 The Necar 4 fuel cell car from 1999. This was the first fuel cell car to have a performance and range similar to IC engine vehicles. The top speed is 144 kph, and the range 450 km. The hydrogen fuel is stored as a liquid. (Photograph reproduced by kind permission of Ballard Power Systems.)

Although invented in about 1840, fuel cells are an unfamiliar technology for most people, and they are considered in some detail in Chapter 4.

As we shall see in later in Chapter 5, a major issue with fuel cells is that, generally, they require hydrogen fuel. This can be stored on board, though this is not easy. An alternative is to make the hydrogen from a fuel such as methanol. This is the approach taken with the Necar 5, a further development of the vehicle in Figure 1.14, which can simply be refuelled with methanol in the same way as a normal vehicle is filled up with petrol. The car has a top speed of 150 kph, an overall fuel consumption of 5l/100km of methanol. It is shown in Figure 5.5.

Another fuel cell vehicle of note is the Honda FCX shown in Figure 1.15, which was the first fuel cell vehicle in the USA to be registered officially as a zero emission vehicle (ZEV) with the environmental protection agency (EPA).

Public service vehicles such as buses can more conveniently use novel fuels such as hydrogen, because they only fill up at one place. Buses are a very promising early application of fuel cells, and an example is shown in Figure 1.16.

Zinc air batteries produced by the Electric Fuel Transportation Company have been tested in vehicles both in the USA and in Europe. The company's stated mission is to bring about the deployment of commercial numbers of zinc-air electric buses, in this decade. During the summer of 2001 a zero emission zinc-air transport bus completed tests at sites in New York State, and later in the year was demonstrated in Nevada. In Germany, a

Figure 1.15 The Honda FCX was the first fuel cell car to be certified for use by the general public in the USA in 2002, and so theoretically become publicly available. This four seater city car has a top speed of 150 kph and a range of 270 km. The hydrogen fuel is stored in a high-pressure tank (Photograph reproduced by kind permission of Ballard Power Systems.)

Figure 1.15 The Honda FCX was the first fuel cell car to be certified for use by the general public in the USA in 2002, and so theoretically become publicly available. This four seater city car has a top speed of 150 kph and a range of 270 km. The hydrogen fuel is stored in a high-pressure tank (Photograph reproduced by kind permission of Ballard Power Systems.)

Figure 1.16 Citaro fuel cell powered bus, one of a fleet entering service in 2003 (Photograph reproduced by kind permission of Ballard Power Systems.)

government-funded consortium of industrial firms is developing a zero emission delivery vehicle based on EFTC's zinc air batteries.

Metal air batteries (described in Chapter 2) are a variation on fuel cells. They are refuelled by replacing the metal electrodes which can be recycled. Zinc air batteries are a particularly promising battery in this class.

1.3.4 Electric vehicles using supply lines

Both the trolley bus and the tram are well known, and at one time were widely used as a means of city transport. They are a cost effective, zero emission form of city transport that is still used in some cities. Normally electricity is supplied by overhead supply lines and a small battery is used on the trolley bus to allow it a limited range without using the supply lines.

It is now difficult to see why most of these have been withdrawn from service. It must be remembered that at the time when it became fashionable to remove trams and trolley buses from service, cost was a more important criterion than environmental considerations and worries about greenhouse gases. Fossil fuel was cheap and overhead wires were considered unsightly, inflexible, expensive and a maintenance burden. Trams in particular were considered to impede the progress of the all-important private motor car. Today, when IC engine vehicles are clogging up and polluting towns and cities, the criteria have changed again. Electric vehicles powered by supply lines could make a useful impact on modern transport and the concept should not be overlooked by designers, although most of this book is devoted to autonomous vehicles.

1.3.5 Solar powered vehicles

Solar powered vehicles such as the Honda Dream, which won the 1996 world solar challenge, are expensive and only work effectively in areas of high sunshine. The Honda Dream Solar car achieved average speeds across Australia, from Darwin to Adelaide, of 85 kph (50 mph). Although it is unlikely that a car of this nature would be a practical proposition as a vehicle for every day use, efficiencies of solar photovoltaic cells are rising all the time whilst their cost is decreasing. The concept of using solar cells, which can be wrapped to the surface of the car to keep the batteries of a commuter vehicle topped up, is a perfectly feasible idea, and as the cost falls and the efficiency increases may one day prove a practical proposition.

1.3.6 Electric vehicles which use flywheels or super capacitors

There have been various alternative energy storage devices including the flywheel and super capacitors. As a general rule both of these devices have high specific powers, which means that they can take in and give out energy very quickly. However, the amount of energy they can store is currently rather small. In other words, although they have a good power density, they have a poor energy density. These devices are considered in more detail in Chapter 3.

A novel electric vehicle using a flywheel as an energy storage device was designed by John Parry, UK. The vehicle is essentially a tram in which the flywheel is speeded up by an electric motor. Power to achieve this is supplied when the tram rests whilst picking up passengers at one of its frequent stations. The tram is driven from the flywheel by an infinitely variable cone and ball gearbox. The tram is decelerated by using the gearbox

Figure 1.17 The Parry People Mover. This electric vehicle uses a flywheel to store energy (Photograph kindly supplied by Parry People Movers Ltd.)

to accelerate the flywheel and hence transfer the kinetic energy of the vehicle to the kinetic energy of the flywheel, an effective form of regenerative braking. The vehicle is illustrated in Figure 1.17. The inventor has proposed fitting both the flywheel and gearbox to a conventional battery powered car. The advantage of this is that batteries do not readily take up and give out energy quickly, whereas a flywheel can. Secondly the arrangement can be made to give a reasonably high efficiency of regeneration, which will help to reduce the battery mass.

Experimental vehicles using ultra capacitors (also considered in Chapter 3) to store power have also been tested; normally they are used as part of a hybrid vehicle. The main source of power can be an IC engine, as with the bus shown in Figure 1.18, or it could be a fuel cell. The MAN bus in Figure 1.18 uses a diesel engine. In either case the purpose of the capacitor is to allow the recovery of kinetic energy when the vehicle slows down, and to increase the available peak power during times of rapid acceleration, thus allowing a smaller engine or fuel cell to power a vehicle.

Energy stores such as capacitors and flywheels can be used in a wide range of hybrids. Energy providers which can be used in hybrid vehicles include rechargeable batteries, fuelled batteries or fuel cells, solar power, IC engines, supply lines, flywheels and capacitors. Any two or more of these can be used together to form a hybrid electric vehicle, giving over 21 combinations of hybrids with 2 energy sources. If 3 or more energy sources are combined there are a further 35 combinations. Certainly there is plenty of scope for imagination in the use of hybrid combinations.

Figure 1.18 Hybrid diesel/electric bus, with electrical energy stored in capacitors (Photograph reproduced by kind permission of Man Nutzfahrzeuge AG.)
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