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

Revised November 2011

XR3 Hybrid three-wheel on-road vehicle


Electric cars have been around since the inception of the automobile. But in the early race for dominance, the internal combustion engine (ICE) quickly won out as the best power system for cars. Although the electric power train was superior in many respects, as a source of energy, the battery was no match for the high energy content, ease of handling, and cheap and abundant supplies of petroleum motor fuel. Today, nearly a century after the electric vehicle (EV) was forced into near oblivion, EVs may actually become the ultimate winner. As easily-recoverable petroleum deposits dwindle, automobile populations soar, and cities become choked with combustion by-products, the ICE is increasingly becoming the victim of its own success. Automobiles must become cleaner and more energy efficient. This document explores the benefits and challenges of clean and efficient electric powered automobiles.

EV Energy Efficiency

Most researchers agree that a switch to EVs would reduce the total primary energy consumed for personal transportation. However, many do not agree on the precise amount of energy that might be saved. The divergence in estimations is mainly due to the fact that energy use comparisons between battery-electric vehicles (BEVs), hybrid-electric vehicles (HEVs), and conventional vehicles (CVs) are affected by a number of variables and necessary assumptions. Vehicle mass, performance, range requirements, system configuration, operating schedule, and the upstream losses of converting source fuels into useable energy and delivering it to the end user all affect system-wide energy use. And a realistic baseline EV performance profile is difficult to define, primarily because the technology is relatively undeveloped and rapidly changing.

Considering only the vehicle itself, EVs are more energy efficient than CVs. A BEV operates at roughly 46% efficiency, whereas a CV operates at about 18% efficiency. In other words, approximately 46% of the electrical energy taken from the wall plug to charge EV propulsion batteries is delivered to the drive wheels as useful work. In contrast, only about 18% of the energy dispensed into the fuel tank as liquid motor fuel ends up at the drive wheels of a CV. In order to determine system-wide energy efficiency (from source fuel to drive wheels), the upstream losses of refining and delivering motor fuel and the losses of generating and delivering electricity must be factored in.

The losses of converting source fuels into electrical energy (conversion losses) and delivering the energy to a local electrical outlet are far greater than the losses of extracting, refining, and delivering petroleum motor fuel. However, petroleum fuel-chain efficiency does not include conversion losses, as does the electrical energy chain. Conversion of liquid motor fuel into useable power takes place in the vehicle and is therefore considered a component of CV energy efficiency. Specifically, about 83% of the energy contained in crude oil arrives at the service station as gasoline, whereas only 20% to 27% of the primary energy used to generate electricity (depending on the source fuel and conversion efficiency) arrives at the electrical outlet ready to charge EV batteries. When the entire energy chain is considered, studies generally conclude that battery-electric cars are roughly 10% - 30% more energy efficient than conventional gasoline cars, depending on the particular assumptions of vehicle energy use and energy chain efficiency. Comparisons between HEVs and CVs are more diverse because of the many design variables of the hybrid power system. HEVs are generally considered slightly more efficient to significantly more efficient than CVs - again, depending on the assumptions used in the comparison.

Due to the variables and the significant differences between electric and conventionally powered vehicles, precise energy use comparisons are difficult to achieve, and conclusions are often open to debate. Comparisons may not fully account for the differences in engineering and performance between baseline EVs and CVs. For example, comparison EVs may be better engineered or more poorly engineered than their gasoline-fueled counterparts. And studies often include an indirect energy penalty for EVs in the form of greater vehicle mass. Looking to the future, however, improved batteries and hybrid systems will likely reduce the mass disadvantage that exists today with EVs.

It is important to csider that EV/CV comparisons are comparisons between a highly developed power system and a new power system in the early stages of development.  Significant improvements can be expected as EV technology evolves. Regardless of whether EV energy efficiency, based on today's technology, would save 10-, 20-, or 30-percent, an electric powered personal transportation system will become more efficient over time. In addition, generating plant conversion efficiency is steadily improving, and the total primary energy consumed by EVs will decline in step with more efficient plants. Moreover, the benefits of electric powered transportation extend far beyond the prospect of saving energy. Electrical generation plants can use a number of alternative fuels that are not easily adaptable to mobile power systems, and emissions are more easily and effectively controlled at the relatively fewer fixed sites than with millions of individual systems on independent vehicles.

Source Fuels Flexibility

In essence, BEVs are the ultimate alternative fuel vehicles because their energy comes from the source fuels used to generate electricity. In the U.S., which gets 55% of its electrical energy from coal, battery-electric cars are predominately coal powered cars. About a third of the energy used by BEVs in the U.S. would come from clean-burning natural gas. In Canada, which relies heavily on hydroelectric power, battery-electric cars are powered mainly by the natural energy of water seeking its own level. Over half of the electrical energy in France comes from nuclear plants, which makes French BEVs predominantly nuclear powered cars. In addition, BEVs make it possible to meet transportation energy needs with solar, wind, and geothermal energy, which are already viable options for fixed generation sites, but are not well suited to mobile applications. Source fuel flexibility alone offers significant practical and economic benefits, especially in view of the diversity of regional energy resources. And EVs, both battery-electric and hybrid-electric configurations, are inherently cleaner.

Environmental Benefits

A BEV produces zero vehicular emissions. However, emissions are produced at the generation site when the source fuel is converted into electrical power. The emissions of electric cars therefore depend on the emissions profile of regional generating plants.

Some researchers conclude that, in regions serviced by coal-fired plants, a switch to EVs may actually increase emissions of sulfur oxides (SOx) and particulate matter (PM), and perhaps increase emissions of carbon dioxide (CO2). Conclusions, however, are usually based on the existing mix of coal-fired plants, and often, they do not consider the effect of newer and cleaner plant designs. Studies generally conclude that emissions of SOx, PM, and CO2 are reduced in regions that rely on natural gas, and virtually eliminated in regions supplied by hydroelectric and nuclear power. According to Electric Power Research Institute (EPRI), substituting EVs for CVs would reduce urban emissions of non-methane organic gases (NMOG) by 98%, lower nitrogen oxide (NOx) emissions by 92%, and cut carbon monoxide (CO) emissions by 99%. In addition, EPRI estimates that, on a nationwide basis, EVs in the U.S. will produce only half the CO2 of conventional vehicles.

In another study of six driving cycles in four U.S. cities, BEVs reduced HC and CO emissions by approximately 97%, regardless of the regional source fuels mix. In comparison to large generating plants, conventional cars produce large amounts of HC and CO emissions, mainly because of cold starts and short trips that do not allow vehicles to become fully warmed up.

The environmental benefits of a hybrid-electric vehicle depend on the design of the hybrid power system. Some studies show that optimized hybrid vehicles may be nearly as clean as battery-electric vehicles. Designs using a combustion engine for onboard electrical generation and an operating schedule that is heavily biased toward the engine/generator system (genset) produce the greatest amount of harmful emissions. But even in this worst-case scenario, emission levels are lower than those of a typical CV. This is due to the fact that a hybrid vehicle genset is either switched off, and therefore producing zero emissions, or it is operating at predetermine output where it produces the fewest emissions and achieves the best fuel economy per unit of output (the region of lowest bsfc).

Typically, a hybrid genset is not throttled for variable output, as is the engine in a conventional vehicle. This leads to more effective emission controls because it is technically easier to control combustion-engine emissions when the engine runs continuously and at a constant output. When the hybrid operating schedule is biased more toward the energy storage system (relies more on the battery, rather than the genset), emission levels become more like those of a BEV. And with fuel-cell hybrids, vehicular emissions are virtually eliminated. Table T1 provides an energy efficiency and emissions comparison between electric vehicles and ICE vehicles running on a variety of fuels.

Table T1 Energy Efficiency and Emissions for Mid-Size Automobile

Efficiency Over Fuel Chain (%)
Net Emissions Over Fuel Chain (1) in g/mile (2)
ICE Vehicle



BEV by Source Fuel

Natural Gas
Adv. NG

Fuel Cell Vehicle

Natural Gas










(1) From primary resource extraction through vehicle end-use, except for SO2, NOx, CO2, and HC emissions, which are estimated for fuel/electricity production and vehicle tailpipe only.
    (2) g/mile x 0.621 = g/km.
   (3) Assumes ethanol-derived farm and conversion energy, and a zero net CO2 release from biomass conversion due to the carbon content of the biomass having been adsorbed from the environment during crop growing.
   (4) Assumes hydrogen from natural gas, which releases CO2 during reforming.
* Condensed from "Diverse Choices for Electric and Hybrid Motor Vehicles," OECD paper by John J. Brogan, et al, Director, Office of Propulsion Systems, U.S. Department of Energy (1992)

Electric cars hold the promise of transforming personal transportation into a far more environmentally benign commodity. And by transferring the job of power generation to a more centralized and specialized sector, emission controls become more effective and economical, and source fuel options broaden and become less technically challenging.

Technical Overview of Electric Vehicles

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.

EV Batteries

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

Specific Energy


Specific Power


Energy Efficiency

In Percent

Nickel/Metal Hydride

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.

Types of Hybrids

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). 

Power System Architecture and Control Strategy

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. 

Widescale Use of Electric Cars

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. 


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