ELECTRIC AND HYBRID VEHICLES

Hybridization Ratio Some new concepts have also emerged in the past few years, including full hybrid, mild hybrid, and micro hybrid. These concepts are usually related to the power rating of the main electric motor in a HEV. For example, if the HEV contains a fairly large electric motor and associated batteries, it can be considered as a full hybrid. On the other hand, if the size of the electric motor is relatively small, then it may be considered as a micro hybrid.

Typically, a full hybrid should be able to operate the vehicle using the electric motor and battery up to a certain speed limit and drive the vehicle for a certain amount of time. The speed threshold is typically the speed limit in a residential area. The typical power rating of an electric motor in a full hybrid passenger car is approximately 50–75 kW.

The micro hybrid, on the other hand, does not offer the capability to drive the vehicle with the electric motor only. The electric motor is merely for starting and stopping the engine. The typical rating of electric motors used in micro hybrids is less than 10 kW. A mild hybrid is in between a full hybrid and a micro hybrid.

An effective approach for evaluating HEVs is to use a hybridization ratio to reflect the degree of hybridization of a HEV. In a parallel hybrid, the hybridization ratio is defined as the ratio of electric power to the total powertrain power.

For example, a HEV with a motor rated at 50 kW and an engine rated at 75 kW will have a hybridization ratio of 50/(50+75)kW=40%. A conventional gasoline-powered vehicle will have a 0% hybridization ratio and a battery EV will have a hybridization ratio of 100%. A series HEV will also have a hybridization ratio of 100% due to the fact that the vehicle is capable of being driven in EV mode.

Interdisciplinary Nature of HEVs HEVs involve the use of electric machines, power electronics converters, and batteries, in addition to conventional ICEs and mechanical and hydraulic systems. The HEV field involves engineering subjects beyond traditional automotive engineering, which was mechanical engineering oriented. Power electronics, electric machines, energy storage systems, and control systems are now integral parts of the engineering of HEVs and PHEVs.

The general nature and required engineering field by HEVs

In addition, thermal management is also important in HEVs and PHEVs, where the power electronics, electric machines, and batteries all require a much lower temperature to operate properly, compared to a non-hybrid vehicle’s powertrain components. Modeling and simulation, vehicle dynamics, and vehicle design and optimization also pose challenges to the traditional automotive engineering field due to the increased difficulties in packaging the components and associated thermal management systems, as well as the changes in vehicle weight, shape, and weight distribution.

State of the Art of HEVs In the past 10 years, many HEVs have been deployed by the major automotive manufacturers. It is clear that HEV sales have grow significantly over the last 10 years.

Breakdown of HEV sales by model Breakdown of HEV sales by model** in the United States in 2009 (in thousands)

The Toyota Prius (2010 model)

Partial list of HEVs available in the United States

The powertrain layout of the Toyota Prius (EM, Electric Machine; PM, Permanent Magnet)

The powertrain layout of the Honda Civic hybrid

The Ford Escape hybrid SUV

The Chrysler Aspen two-mode hybrid

Challenges and Key Technology of HEVs HEVs can overcome some of the disadvantages of battery-powered pure EVs and gasoline-powered conventional vehicles. These advantages include: optimized fuel economy, reduced emissions when compared to conventional vehicles, increased range reduced charging time, reduced battery size (hence reduced cost) when compared to pure EVs.

However, HEVs and PHEVs still face many challenges: including higher cost when compared to conventional vehicles, electromagnetic interference caused by high-power components, safety and reliability concerns due to increased components and complexity, packaging of the system, vehicle control, power management.

CONCEPT OF HYBRIDIZATION OF THE AUTOMOBILE Vehicle Basics Constituents of a Conventional Vehicle Present-day engine-propelled automobiles have evolved over many years. Today’s automobiles initially started with steam propulsion and later transitioned into ones based on the internal combustion engine (ICE).

Cutaway view of an ICE

The engine has a chamber where gasoline or diesel is ignited, which creates a very high pressure to drive the pistons. A piston is connected through a reciprocating arm to a crankshaft. The crankshaft is connected to a flywheel which is then connected to a transmission system. The purpose of the transmission system is to match the torque speed profile of the engine to the torque speed profile of the load. The shaft from the transmission system is ultimately connected to the wheels through some additional mechanical interfaces such as differential gears.

Transmission system and engine connected together

Vehicle and Propulsion Load The power generated from the engine is ultimately used to drive a load. In an automobile this load includes the road resistance due to friction, uphill or downhill drive related to the road profile, and the environmental effect of, for example, the wind, rain, snow, and so on. In addition, some of the energy developed in the vehicle is wasted in overcoming the internal resistance within the vehicle’s components and subsystems, none of which are 100% efficient. Examples of such subsystems or components include the radiator fan, various pumps, whether electrical or mechanical, motors for the wipers, window lift, and so on. These items are just a few examples from a whole list of vehicular loads. The energy lost in these devices is released eventually as heat and expelled into the atmosphere.

Normally “load” can be related to the amount of opposing force or torque. But a more scientific definition of load comes from the fact that it is not defined by a single number or numerical value. Load is a collection of a set of numbers defined by the speed–torque or speed–force characteristics in the form of a table or graph, that is, through a mathematical equation relating speed and torque. Similarly the engine is also defined by speed–torque characteristics in the form of a table or graph, that is, through a mathematical equation. The operating point of the combination of the engine and the load system together will then be at the intersection of these characteristics.

Load and engine characteristics of a vehicle

A complete vehicle or automotive system has various loads A complete vehicle or automotive system has various loads. Some of these are electrical devices, and others are mechanical devices. The electrical loads are normally run at a low voltage (nominally 12 V). The reason for this, i.e. running the non-propulsion loads at low voltage, is primarily related to safety issues. There is an existing manufacturing base for many of these non-propulsion loads (as indicated below), where it is easier to take advantage of the situation and use the existing low-voltage components, rather than transform the voltage system.

Examples of these loads are: brakes – mechanical (hydraulic or low-voltage electrically assisted); air-conditioner – generally mechanical; radiator fan – can be belt driven mechanically (or low-voltage electrical); various pumps – can be mechanical (or low-voltage electrical); window lift – electrical; door locks – electrical; wipers – electrical; various lights – non-motor load, low-voltage electrical; radio, TV, GPS – non-motor, low-voltage electrical; various controllers – for example, engine controller, transmission controller, vehicle body controller; and various computational microprocessors – non-motor, low-voltage electrical.

Drive Cycles and Drive Terrain Since a vehicle will be driven through all kinds of road profiles and environmental conditions, to exactly know beforehand about which loads the vehicle will encounter under all circumstances is difficult. It is of course possible for one to perform experiments and place sensors etc. to monitor the speed and torque of a vehicle, but to do so under all circumstances for all vehicle platforms is simply impossible.

Hence, for the engineering studies, a few limited situations have been developed which more or less cover typical road profiles and the terrains one can expect to encounter. Using a few of these profiles, one can create or synthesize various arbitrary road profiles. Such profiles can involve things like driving within a city, on a highway, across some special uphill or downhill terrain, to name but a few.

Drive cycles only provide time, an corresponding speed fluctuations, and labels attached to these tell us what kind of drive cycle it is, for example, city, highway, and so on. So, if a vehicle goes through different driving situations, partly city, partly highway, and so on, then one can obtain speed vs. time data by synthesizing multiple typical drive cycles.

A typical automotive drive cycle

The question then arises about the ways to utilize the drive cycle information. Assume that we want to know about the fuel economy of a particular vehicle X. It is not sufficient to say that vehicle X does 25 MPG. We also need to say under what conditions this was obtained. That is, whether it was under a city drive cycle, or highway drive cycle, and so on. Then one can compare another vehicle Y against X, under similar drive cycle conditions, and make a fair comparison.

As there are different kinds of drive cycles, that of a passenger car cannot be compared against the drive cycle of a refuse truck or a postal mail vehicle, since they have very different kinds of stop and go driving. Similarly the drive cycle of a heavy mining vehicle cannot be compared with the above either.

Finally, it should be noted that a drive cycle concerns the road profile through which a vehicle goes and hence is a situation external to the vehicle. However, the response of a vehicle to a given drive cycle, in terms of fuel economy, will be different depending on whether the vehicle is a regular ICE vehicle, fully electric vehicle (EV), hybrid electric vehicle (HEV), and so on.

Basics of the EV Why EV? Although these days people talk more about HEVs which have become very popular, their underlying system is complex because it has two propulsion sources. A pure EV is relatively simpler since it has only one source of energy, that is, a battery or perhaps a fuel cell. Similarly its propulsion is performed by an electric motor and the need for an ICE is not there. If the ICE is gone, the vehicle will not need any fuel injectors, various complicated engine controllers, and all the other peripherals associated with the engine and transmission. With a reduced parts count and a simpler system, it will be more reliable as well.

In addition, an EV is “virtually” a zero-emission vehicle (since nothing has technically zero emissions in a true global sense). Of course, if one considers the ultimate source of energy, by tracing the path backward from the battery to the utility industry, it will be found that the location of pollution has been essentially shifted from the vehicle to elsewhere. Furthermore, an EV is virtually quiet. In fact it can be so quiet that people have even talked about introducing artificial noise in the vehicle so that they can hear it, which is something important to know from a safety point of view.

From a technical viewpoint, the EV has another benefit From a technical viewpoint, the EV has another benefit. In the ICE, which is a reciprocating engine, the torque produced is pulsating in nature. The flywheel helps smooth the torque which would otherwise cause vibration. In the EV the motor can create a very smooth torque and, in fact, it is possible to do away with the flywheel, thus saving material and manufacturing cost, in addition to reducing weight. And finally, the efficiency of an ICE (gasoline to shaft torque) is very low. The engine itself has about 30–37% efficiency for a gasoline and about 40% for a diesel engine, but by the time the power arrives at the wheel, the efficiency is just 5–10%. On the other hand, the efficiency of the electric motor is very high, on the order of 90%. The battery and power electronics to drive the motor also have high efficiency. If each of these components has an efficiency on the order of 90%, by the time the battery energy leaves the motor shaft, the overall efficiency will be something like 70%. This is still substantially higher than that in the ICE.

Constituents of an EV The complete EV consists of not only the electric drive and power electronics for propulsion, but also other subsystems to make the whole system work. One needs a battery (or a fuel cell) to provide the electrical energy.

System-level diagram of an EV.

Vehicle and Propulsion Loads There is a significant amount of commonality between the loads in an EV and a regular automobile. Hence, just like a regular vehicle, some of these loads are electrical devices and others are mechanical devices. As noted earlier, those loads which are electrical normally run at a low voltage (nominally 12 V), with the exception of the propulsion load, that is, the propulsion motor, which runs at a high voltage (several hundred volts). The reason for this has to do with safety primarily. And, of course, the existing manufacturing base for many of these non-propulsion loads can be used to advantage by using the existing low-voltage components, rather than transforming the voltage system.

Examples of these loads are same as those noted previously: propulsion or traction motor – high-voltage electrical load; brake motor (if a fully or partially electrical brake system is used) – low voltage; air-conditioner motor (if electrical) – low voltage; radiator fan (if electrical) – low voltage; various pumps (if electrical) – low voltage; window lift – low voltage; door locks – electrical; wipers – electrical; various lights – non-motor load, low-voltage electrical; radio, TV, GPS – non-motor, low-voltage electrical; various controllers, for example, engine controller, transmission controller, vehicle body controller; various computational microprocessors, digital signal processors (DSPs) – non-motor, low-voltage electrical.

The above list more or less covers the various loads in the vehicle, including propulsion loads. The propulsion load can be several kilowatts for a mild hybrid vehicle regenerative braking system, up to say 50kW or a few hundred kilowatts for propulsion in a hybrid vehicle. The various pumps and fans can be only a few hundred or less watts, whereas some small motors like door lock motors could be just a few tens of watts. Similarly the lights can range from a few tens to about a hundred watts.

Basics of the HEV Why HEV Basics of the HEV Why HEV? Previously we discussed the architecture of a purely EV. As we saw, the EV propulsion uses an electric motor for propulsion. The energy comes from the battery (or perhaps a fuel cell). The battery bank in a pure EV can be quite large if the vehicle is to go a few hundred miles on one full charge to begin with. The reason for this is that battery technology, as it stands today, does not have a very high energy density for a given weight and size, compared to a liquid fuel like gasoline. Although new batteries like lithium-ion batteries have a much higher energy density than the existing lead acid or nickel metal hydride batteries, it is still much lower compared to liquid fuel.

As noted earlier, the HEV is a complex system since it has two propulsion sources. Comparatively a pure EV is simpler since it has only one source of energy, namely, a battery or perhaps a fuel cell. In the EV the propulsion is produced by only the electric motor and there is no ICE. This removes the need for fuel injectors, complicated engine controllers, and all other peripherals. Hence, with a reduced parts count, the system is simpler and more reliable.

Of course, there is an efficiency improvement in the HEV compared to the ICE, but it will still be lower than in the EV. The overall efficiency will depend on the relative size of the ICE and the electric propulsion motor power.

A variant of the HEV is found in locomotives and in very high powered off-road vehicles. In a number of variants of such systems there is no battery. The ICE is used to drive a generator which creates AC power. This power is translated to DC and then to another AC power required to drive an electric motor. The problem with this system is that the engine has to be run continuously to produce the electricity. The advantage is that it does not need a battery. Furthermore, the ICE can be run at an optimal speed to achieve the best possible efficiency. One problem with this system is that it does not lend itself to regenerative energy recovery during braking. The battery helps regenerative energy recovery by allowing storage and it can also be coordinated more optimally in terms of when the ICE or the electric motor should be run.

Constituents of a HEV As noted earlier, an EV is simpler than a HEV. As we can see, the only difference between this diagram and the one for the EV is that this one has an additional subsystem called IC engine, along with the necessary interface and the controller. Otherwise the two diagrams are identical.

System-level diagram of a HEV.

Basics of Plug-In Hybrid Electric Vehicle (PHEV) Why PHEV?