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Hybrid Vehicles

Vehicle Energy Use - Conventional Vehicle, Urban Cycle
Vehicle Energy Use - Hybrid Vehicle, Urban Cycle
Series Hybrid 
Parallel Hybrid 
Dual-Mode Hybrid
Charge Sustaining and Charge Non-Sustaining Hybrids
Why put a combustion engine in an electric vehicle?
What about emissions?
Fuel Cells
Flywheels

Toyota hybridThere are currently many different hybrid-electric system designs utilizing either clean diesel engines, alternative fuels engines, gas turbines or fuel cells in conjunction with batteries. There are even some new technology concepts in the developmental stage that utilize storage devices such as flywheels and ultra capacitors in place of the battery. Use of two different energy sources defines a hybrid.

There are many HEV configurations and design options that can be grouped in three categories: series (range-extending HEVs), parallel (power assist HEVs), and dual-mode HEVs.

The ultimate goal of the hybrid electric vehicle is to provide the equivalent power, range, cost and safety of a conventional vehicle while reducing fuel costs and harmful emissions. At present a HEV is able to operate nearly twice as efficiently as traditional internal combustion vehicles.

Conventional internal combustion engines convert the liquid fuel energy into shaft energy. All energy from the combustion process centers around the crankshaft with the exception of that lost in the form of heat A typical ICE vehicle only uses approximately 16% of the liquid fuel energy to move the vehicle. The heat emitted in the combustion process wastes the majority of the energy while frictional losses from the hundreds of moving parts in the engine, transmission and the mechanical connection to the drive wheels consumes the rest. HEV's on the other hand are designed with energy efficiency in mind.

The main source of energy used in the most common HEV's today are batteries. A battery contains no moving parts. The only energy wasted is a very small amount of heat during the course of a discharge cycle. As previously mentioned, hybrid electric vehicles utilize two different energy sources. Batteries are usually the main energy supplier for the vehicle and an auxiliary engine that burns gasoline, diesel fuel, or alternative fuels such as methanol, ethanol or compressed natural gas provides the auxiliary power. In some cases, the reverse is true with the batteries providing auxiliary power during times of high energy demand. The diagrams below(courtesy of the DOE - HEV Program) compare a conventional vehicle's energy use to that of a hybrid electric vehicle.

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Energy use of a conventional vehicle

Notice the absence of the idling losses and the large reduction in engine losses in the hybrid vehicle shown below.

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Energy use of a hybrid vehicle

As shown by the chart below, a hybrid electric vehicle will travel twice the distance of a conventional vehicle on the same amount of energy. An internal combustion engine is inefficient not only because of the amount of energy loss incurred in the transfer of energy from the liquid state to the drive-train, but it also becomes more inefficient when the vehicle is not moving but the engine is still consuming energy.

Energy Source/Sink
Hybrid Electric Vehicle

Internal Combustion Engine

Fuel
100
100
Transmission Losses
-6
-6
Idling Losses
0
-11
Accessory Loads
-2
-2
Engine Losses
-30
-65
Regenerative Braking
+4
0
 
Total Energy Remaining
66
16

A hybrid electric vehicle is able to utilize the energy produced by it's auxiliary engine, even when the vehicle is not moving, by storing the energy produced during idling in the battery pack. Additionally HEV's recover 10% or more of the energy consumed in propelling the vehicle during deceleration by reversing the direction of current flow from the drive motors. The motors become generators and energy is placed back into the battery by a process known as regenerative braking. This also aids in prolonging the life of the braking system.

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Series Hybrid

Only the electric motor is connected to the wheels in a series hybrid. In a “classical” series HEV, an electric generator, coupled with an engine, supplies electricity for the battery which, in turn, feeds the electric motor. Generally, the engine/generator set keeps the battery charged between 60-80%. When the battery reaches the lower limit, the engine starts. Similarly, when the battery reaches the upper limit, the engine will shut off. However, in some series HEVs, electric power to the motor can come from both the batteries and the engine/generator set. Since only the electric motors are connected to the wheels, the engine can run at optimum performance greatly reducing emissions.

Parallel battery
Entire drive power transmitted electrically
May require larger batteries
Requires on-board charging
Requires some off-board charging
Optimization by separating engine speed from vehicle speed
Engine never idles, thus reduces overall emissions
May not require a transmission
Requires heavy-duty motor
S
E
R
I
E
S

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Parallel Hybrid

Parallel hybrids have mechanical connections to the wheels from both the electric motor(s) and the engine allowing the vehicle to accelerate faster than a series HEVs. These vehicles do not need a dedicated generator and are connected to the electric grid for recharging the batteries (Although the electric motor could be used as a generator to recharge the batteries via a clutch). In a parallel HEV, the electric motor assists the engine during start-up and acceleration.

Dual mode battery
Electric motor and engine both coupled directly to wheels
Can operate with smaller batteries
Requires off-board charging
Accelerates faster due to dual power sources
Engine idles
Packaging of components less flexible
May not require a transmission
Requires medium-duty motor
P
A
R
A
L
L
E
L

An example of a parallel hybrid vehicle is the HIMR bus (HINO Motors) in Japan.

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Dual-Mode Hybrid

Dual-mode hybrids are basically parallel hybrids with a separate generator that also allows recharging of the batteries. In normal driving conditions, the engine moves both the wheels and the generator, which in turn supplies power to the electric motor and the batteries. During full-throttle acceleration or under heavy load, the motor gets a power boost from the battery.

Dual mode battery
Engine can fuel batteries as well as drive wheels
Requires medium - large batteries
Requires on and off-board charging
Accelerates faster due to dual power sources
Engine idles
Packaging of components less flexible
May not require a transmission
Requires heavy-duty motor
D
U
A
L
-
M
O
D
E

Perhaps the most successful dual- mode hybrid is the Toyota Prius. In the Prius, when the vehicle starts moving, the engine shuts down and only the electric motor drives the wheels, drawing its power from the batteries.

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Charge Sustaining and Charge Non-Sustaining Hybrids

Additionally, hybrids can be defined as "charge sustaining" or "charge non-sustaining". In a "charge sustaining" HEV system, the hybrid power source is capable of providing sufficient energy, independent of the storage device(usually a battery), to drive the vehicle just like it was a conventional vehicle. As long as the hybrid has fuel for the engine, the vehicle will operate.

The hybrid power source in a "charge non-sustaining" HEV is only able to provide recharging energy and cannot supply the necessary energy to drive the vehicle by itself. If an HEV requires an instantaneous 120 kW to accelerate, and the hybrid power source is only capable of supplying 60 kW, the HEV is considered a "charge non-sustaining" system because the engine/generator cannot produce the required energy necessary to accelerate the vehicle. This system must have additional energy from the storage device (battery) to meet the energy needs of the vehicle. A "charge non-sustaining" system is often referred to as a "range extender" because its intended to extend the range of the vehicle.

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Why put a combustion engine in an electric vehicle?

You may be asking yourself why someone would go to all the trouble of building an electric vehicle then turn around and put an internal combustion engine into it. There are many reasons why this is done, some are by choice and others by necessity. Current battery technology does not provide EV's with a range that is acceptable to consumers. An average commute to work is around 40 miles. Currently, EV's are forecasted to have a range in the 80-100 mile range using advanced battery technology such as NiMH. Surveys indicate this range to be borderline for what commuters desire.

Transportation needs go beyond the typical family car. There are trucks, buses and other commercial vehicles that can also benefit from EV technology, but the size these types of vehicles further reduces the range that current battery technology can supply. This is where the hybrid concept becomes very valuable. Because there are two energy sources in a HEV, the auxiliary power source which is typically an internal combustion engine, can be greatly reduced in size. A good example of this is the hybrid electric HMMWV, a military applications vehicle designed and built by PEI Electronics as part of a DARPA grant. The original engine in the vehicle was approximately 9 Liters, and has been replaced by a 288 volt lead acid battery and a 2 liter auxiliary engine.

Hybrid electric auxiliary power sources have several advantages over traditional internal combustion engines. To begin with, alternative fuels (such as natural gas, propane, liquefied natural gas and compressed natural gas)are neither corrosive nor toxic, they possess a high ignition temperature, are lighter than air, and have a narrow flammability range, making alternative fuels inherently safe fuels as compared to other fuel sources. Alternative fuels do not contaminate soil or water. Alternative fuels will always rise to the atmosphere out of doors, unlike other fuels, which are heavier than air and can pool, either as a liquid or a vapor, upon the ground. Alternative fuels contain a distinctive odorant, which allows for detection at 0.5% concentration in air, well below levels, which can cause drowsiness due to inhalation, and well below the weakest concentration that can support combustion.

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What about emissions?

These fuels further reduce harmful emissions. Because the engine is sized to some average load, with the battery providing energy for peak loads, the engine can be tweaked or computer controlled to operate at its maximum efficiency. The engine does not need to speed up or slow down as the load varies.

Natural gas is the cleanest burning alternative fuel. Exhaust emissions from natural gas powered engines are much lower than those from equivalent gasoline-powered vehicles. For instance, natural gas emissions of carbon monoxide are approximately 70 percent lower, non-methane organic gas emissions are 89 percent lower, and oxides of nitrogen emissions are 87 percent lower. In addition to these reductions in pollutants, natural gases also emit significantly lower amounts of greenhouse gases and toxins than do gasoline vehicles.

Natural gas produces little or no evaporative emissions during fueling and use. For gasoline vehicles, evaporative and fueling emissions account for at least 50 percent of a vehicle's total hydrocarbon emissions. Using natural gas as an alternative fuel in a HEV can reduce carbon dioxide exhaust emissions by almost 20 percent.

Exposure to the levels of suspended fine particulate matter found in many U.S. cities has been shown to increase the risk of respiratory illness. Diesel exhaust is under review as a hazardous air pollutant. Engines operating on natural gases produce only tiny amounts of this matter.

Per unit of energy, natural gases contain less carbon than any other fossil fuel, and thus produce lower CO2 emissions per vehicle mile traveled. While natural gas powered engines do emit methane, another principle greenhouse gas, any slight increase in methane emissions would be more than offset by a substantial reduction in CO2 emissions compared to other fuels.

Utilizing natural gas in the auxiliary engine also reduces the emission levels of carbon monoxide (approximately 70 percent lower than a comparable gasoline vehicle) and volatile organic compounds. Although these two pollutants are not themselves greenhouse gases, they play an important role in helping to break down methane and some other greenhouse gases in the
atmosphere, and thus increase the global rate of methane decomposition.

While regular gasoline contains more energy per unit volume, as compared to alternative fuels (shown in table below), the difference in cost between gasoline and alternative fuels makes up for the lost energy on a per mile basis.

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Fuel and Primary or Typical Composition
Energy Available for Power in One Gallon
Factor : Gallons required for same mileage as gasoline
Unleaded Regular Gasoline
(C8H15-18)
114,000 BTU
1.00 Gallon liquid
Natural Gas (CH4)
114,000 BTU
1.00 Gallon liquid
Liquefied Natural Gas (LNG) (CH4)
76,000 BTU
1.50 Gallon liquid
Diesel (C16H34)
128,000 BTU
0.89 Gallon liquid
Propane (HD5)(C3H8)
82,450 BTU
1.38 Gallon liquid
Methanol (CH3OH)
57,000 BTU
2.00 Gallon liquid
M85 (85% methanol,15% gasoline)
65,500 BTU
1.74 Gallon liquid

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Fuel Cells

Fuel Cell DiagramResearch is still taking place in an effort to make hybrid electric vehicles even more efficient as well as more environmentally friendly. The main emphasis at this point is in energy storage methods. Primary vehicle energy storage devices such as the fuel cell have the potential to increase energy efficiency. A fuel cell is an electrochemical device in which a fuel reacts with oxygen to release electrons, producing electricity. The fuel cell's greatest benefit is that it produces zero emissions.

Fuel cell vehicles are an attractive advance from battery powered vehicles. They offer the advantages of battery power, but can be re-energized quickly and could go longer between refueling. There are many different types of fuel cells currently under development and advances are still being made on units that have not made it to the production line. The most common type of fuel cell is known as a PEM fuel cell. These fuel cells have a Proton Exchange Membrane through which the atomically smaller protons migrate through while the free electrons which are larger are conducted through the external circuit as electricity.

Ballard PEM fuel cellShown at rightt, is a Ballard PEM fuel cell. The fuel cell consists of two electrodes, the anode and cathode, separated by a polymer membrane electrolyte. Each side of the electrodes is coated on one side with a thin platinum catalyst layer. The electrodes, catalyst and membrane together form the membrane electrode assembly. Hydrogen fuel dissociates into free electrons and protons(positive hydrogen ions) in the presence of the platinum catalyst and the anode. The free electrons are conducted in the form of usable electric current through the external circuit. The protons migrate through the membrane electrolyte to the cathode. At the cathode, oxygen from the air, electrons from the external circuit and protons combine to form pure water and heat. Individual fuel cells produce about 0.6 volt and are combined into a fuel cell stack to provide the amount of electrical power required.

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Fuel cell stackWithin the fuel cell stack(shown at left), gases(hydrogen and air) are supplied to the electrodes on either side of the PEM through channels formed in the flow field plates. Hydrogen flows through the channels to the anode where the platinum catalyst promotes its separation into protons and electrons. On the opposite side of the PEM, air flows through the channels to the cathode where oxygen in the air attracts the hydrogen protons through the PEM. The electrons are captured as useful electricity through an external circuit and combine with the protons and oxygen to produce water vapor on the cathode side.

One of the major challenges with fuel cells will be infrastructure. There are few fuel stations that sell hydrogen. Using a reformer to convert other fuels to hydrogen adds to the cost of fuel cells and the packaging of the fuel system. Thus, although the development of fuel cells for motor vehicles may be close, normal use may take many years.

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Flywheels

Flywheel cross-sectionAnother promising device which can store energy is the flywheel. Flywheels store energy mechanically. To absorb energy, the flywheel converts electrical energy into kinetic energy using a built in motor, making the flywheel's rotor spin faster. To deliver energy, the kinetic energy stored in the rotor is converted to electrical energy using the same motor with the polarity of the field coils reversed, which causes the energy to flow into the battery.

The flywheel operates on the same principal as the regenerative braking feature, but uses the speed of the rotor rather than the momentum of the vehicle to generate energy.

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