Electric and Hybrid Vehicles
An Overview of the Benefits, Challenges, and Technologies
by Robert Q. Riley

Electric vehicles are divided into two general categories: battery-electric vehicles and hybrid-electric vehicles, which represent the design orientation of the vehicles' power system. Battery-electric vehicles, or BEVs, are vehicles that use secondary batteries (rechargeable batteries, normally called storage batteries) as their only source of energy. A hybrid-electric vehicle, or HEV, combines an electrical energy storage system with an onboard means of generating electricity or augmenting the energy stores of the battery, normally through the consumption of some type of fuel. Each type of EV has its own operating characteristics and preferred design practices, as well as advantages and disadvantages.

A primary technical advantage with EVs of either category is the inherent bi-directionality of their energy/work loop.  An EV power train can convert energy stores into vehicle motion, just like a conventional vehicle, and it can also reverse direction and convert vehicle motion (kinetic energy) back into energy stores through regenerative braking. In contrast, combustion engine vehicles cannot reverse the direction of the onboard energy flow and convert vehicle motion back into fuel. The significance of regeneration becomes apparent when one considers that approximately 60 percent of the total energy spent in urban driving goes to overcoming the effects of inertia, and theoretically, up to half of this energy can be reclaimed on deceleration.

Other technical advantages center on the superiority of the EV's electro-mechanical power train. In comparison to the internal combustion engine, an electric motor is a relatively simple and far more efficient machine. Moving parts consist primarily of the armature (dc motors) or rotor (ac motors) and bearings, and motoring efficiency is typical on the order of 70- to 85-percent. In addition, electric motor torque characteristics are much more suited to the torque demand curve of a vehicle.

A vehicle needs high torque at low speeds for acceleration, then demands less torque as cruising speed is approached. An electric motor develops maximum torque at low rpm, then torque declines with speed, mostly in step with a vehicle's natural demand. In contrast, an ICE develops very little torque at low rpm, and must accelerate through nearly three-quarters of its rpm band before it can deliver maximum torque. A multi-ratio transmission is therefore necessary in order to correctly match ICE output characteristics to the vehicle demand curve. Due to the more favorable output curve of the electric motor, an EV drive train usually does not require more than two gear ratios, and often needs only one. Moreover, a reverse gear is unnecessary because the rotational direction of the motor itself can be reversed simply by reversing the electrical input polarity. These advantages lead to a far less complex and more efficient power train, at least on a mechanical level.

The mechanical simplicity of the EV power train is somewhat offset by increased complexity on an electronic level. Electrical power is delivered to the wall outlet in the form of alternating current, and must be converted into direct current in order to charge EV batteries. In the case of EVs powered by dc motors, electricity from the battery must then be "chopped" into small bursts of variable duty cycle in order to control the speed and torque of the motor. With EVs using ac motors, the direct current from the battery must undergo complex power condition in order to deliver alternating current and provide control over motoring output. Power conditioning systems have traditionally been large and expensive devices. In recent years, however, electronic control technology has improved and costs and size have declined. With increased demand, the technology should continue to improve, and economies of scale will come into play.

The main disadvantage of BEVs is limited energy stores due to the limitations of the secondary battery, and HEVs tend to be plagued by increased mass and costs due to the increased complexity of the power system.

BEVs use rechargeable batteries as a source of electrical energy. A BEV's batteries do not store electrical energy in the same sense that a fuel tank stores liquid fuel. Instead, they are essentially self-contained electrochemical reactors in which the by-products are retained within the battery housing. During recharge, these by-products are reconstituted into their original state where they are ready for another electrochemical reaction cycle.

Secondary batteries are limited in their capacity to produce electrical energy by the accumulation of by-products, and by the limited quantity of reactants they can contain. And because the recharge cycle does not fully reverse the changes that take place during discharge, waste products accumulate, the reacting components degrade, and the battery's ability to produce electrical energy steadily declines until it is no longer serviceable. In comparison, a tank of gasoline contains roughly 100 times more energy than an equal mass of lead/acid batteries. Moreover, part of the IC-engine's reactants are taken from the air, by-products are continuously discharged, rather than retained and reconstituted, and the storage and conversion system is largely unaffected by the process. The task of designing a BEV that will match the conventional vehicle's specific energy profile is enormously challenging because of the inherent limitation of its electrochemical energy system.

The most promising replacement for the lead/acid battery appears to be the Lithium-based batteries. Lithium-based batteries can store four or five times as much energy. However, these advanced batteries are very costly. Table T2 provides a performance comparison between different battery couples.

Table T2 Battery Comparison

Hybrid-Electric Cars

The idea of a hybrid-electric vehicle naturally evolves from the inherent limitations of the storage battery. As first conceived, a hybrid vehicle would employ an onboard means of generating electricity in order to augment the limited energy available from the battery. The vehicle might then run on battery energy alone when range is within the capability of the battery's energy stores, then use the heat-engine when range requirements exceed the energy stores of the battery.

Although simple in concept, the task of achieving significant improvements in energy efficiency depends on the correct integration of subsystems within a sophisticated control strategy that continuously monitors and balances the energy flow onboard the vehicle. When approached as a system, a hybrid power system is no longer a simple battery-electric system augmented by a heat-engine. Instead it is an integrated, self-adapting, propulsion system that may ultimately utilize batteries (or ultra-capacitors) as an energy reservoir for load leveling, rather than in their traditional role of supplying total vehicle motive power.

Much of the research today is oriented toward developing the most effective control strategy, the best bias between subsystems, and the correct combination of subsystem types needed to achieve maximum efficiency with a minimum of hardware, mass, and manufacturing costs. The original concept of a heat-engine-augmented BEV is not necessarily incorrect, but the shift in perspective to a systems approach has opened new opportunities for greater efficiency and performance.

Hybrids are normally divided into the subtypes of either series or parallel, which refers to the way in which the engine supplies power to the propulsion system. In the series hybrid, a heat engine powers a generator which charges the battery or supplies power directly to the propulsion circuit and thereby reduces demand on the battery. Town Car (available in a DIY plans package on this site) is a series hybrid. The XR3 Hybrid, also available on this site, is a ground-connected parallel hybrid. In a parallel hybrid, the heat engine delivers mechanical power directly to the drive train, and the generator is eliminated.  With this type, the battery-electric system or the heat engine may be used to propel the vehicle, or they may be used simultaneously for maximum power. The parallel hybrid is more efficient than the series hybrid. The efficiency advantage comes from the fact that parallel hybrids deliver heat-engine power (mechanical power) directly to the power train, rather than converting the power into electricity.

Losses occur whenever power is converted from one form to another.  In the series hybrid, a heat engine runs a generator to produce electricity. This conversion (from mechanical to electrical power) results in a loss of about 20 percent.  Electricity may be delivered to a battery or to a motor to provide motive power.  If it is delivered to a motor, the motor converts electrical energy into mechanical power, but the conversion results in a loss of about 20 percent. If electrical energy is delivered to a battery, an additional 20 to 30 percent of the energy will be lost in the conversion process (putting the charge into the battery and then taking it out again). Since the heat engine produces mechanical power in the first place, it is more efficient to put it to work in its native form rather than making a double conversion - from one form to another form and then back to its original form. But decisions about when to convert and how much to convert are engineering decisions that are made within the context of the total design of the system.

The first hybrids introduced were “mild hybrids”. With a mild hybrid a downsized heat engine is installed to provide the primary motive power. The electric power system is configured to augment the shortfall in torque of the heat engine during periods of acceleration. At cruising speeds the heat engine powers a generator (normally the torque-assist motor is electronically switched into a generation mode) to replenish the electrical energy that was used for acceleration. These types of hybrids are not designed to run on battery power alone. So the battery pack is very small in comparison to that of a conventional BEV – perhaps enough energy stores to drive for two or three blocks. 

The next generation of hybrids will be built on the “plug-in” hybrid (PHEV) architecture. These types of hybrids contain enough energy stores to drive significant distances on battery power alone. Much of the plug-in hybrid’s energy is taken from the grid system (wall-plug electricity) where it is less expensive to produce. Electricity taken from the grid system roughly equates to motor fuel at 50 to 75 cents per U.S. gallon (depending on many variables). 

The best power system architecture for HEVs is still the subject of ongoing investigation. There are many options, and there is no “right way” to design a HEV. 

Traditionally, hybrid control strategy has been dependent on the mission of the vehicle and the particular tradeoffs made by engineers when they define the architecture of the power system. Before designers can proceed, the vehicle mission and power and energy requirements are defined. Typically, energy and power requirements are based on a multiple of driving schedules that include power/duration plots of acceleration, cruise, and total trip energy demands for each driving schedule. The vehicle and its power system is then designed according to the mission profile, which may be comprised of worst-case demand levels from several different driving schedules. Control system strategy is designed according to the characteristics of the subsystems and the power demand curves of driving schedules.

In a system that is heavily biased toward either on-board generation or direct mechanical power from a heat engine, the battery is normally downsized and reconfigured for maximum specific power.  In a series hybrid, for example, average power may be supplied by a genset with the battery serving as a reservoir for regenerative braking energy, and to supply peak power for acceleration and passing. Ultra-capacitors offer many advantages with this type of system. 

Plug-in hybrids offer the ability to change the bias of the power system between an emphasis on battery stores and an emphasis on heat-engine power. Flexible biasing between the two power systems can offer some advantages; the XR3 Hybrid is built around this concept. For trips on the order of 40 miles or less, the XR3 may be operated on battery power alone. For greater range, including range equal to that of a conventional vehicle, the XR3 may be operated on a combination of electrical and heat-engine power, or on heat-engine power alone. 

The roadblocks to widescale use of EVs have included technical, economic, and perceptual disadvantages. Technical problems have traditionally centered on the limitations of the storage battery, which is responsible for today's emphasis on hybrid power systems. But HEVs have also been plague by inherent disadvantages - primarily greater vehicle mass and higher manufacturing costs, which are natural by-products of their inherently greater mechanical and electrical complexity. Industry has been hard at work developing new designs that will reduce manufacturing costs and provide environmentally benign personal transportation products.