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A hybrid vehicle, abbreviated HEV , is one that uses uses both an internal combustion engine ( ICE ) and an electric motor to propel the vehicle. Most hybrids use a high-voltage battery pack and a combination electric motor and generator to help or assist a gasoline engine. The ICE used in a hybrid vehicle can be either gasoline or diesel, although only gasoline-powered engines are currently used in hybrid vehicles. An electric motor is used to help propel the vehicle, and in some designs, capable of propelling the vehicle alone without having to start the internal combustion engine.
Background In the early years of vehicle development, various propulsion systems were used, including steam-, gasoline-, and electric-motor-powered. Early electric vehicles ( EVs ), also called battery electric vehicles ( BEV ) used lead-acid batteries, an electric traction motor, and a mechanical controller. A traction motor is an electric motor used to rotate drive wheels and propel the vehicle, which moves as a result of traction between the wheel and the road surface.
The controller allowed different voltages to be applied to the electric motor, depending on the needs of the driver.
Assume that an early electric vehicle used six 6-volt batteries. If the batteries are connected in series, the negative terminal of one battery is connected to the positive terminal of the second battery. Two 6-volt batteries in series result in 12 volts, but the same current as one of the batteries. Two 6-volt 500-amp/hour batteries in series, then, would output 12 volts and 500-amp/hours. Batteries connected in parallel increase the current, but the voltage remains the same. Two 6-volt 500-amp/hour batteries in parallel, would be six volts and 1,000 amp/hours. Old electric vehicle mechanical controllers were able to switch all six batteries in combinations of series and parallel configurations to achieve lower voltage for slow speeds and higher voltages for higher speeds.
Electric vehicles did not have a long range and needed to have the batteries charged regularly. Electric vehicles were almost more popular than steam power in 1900, when steam had 40% and electric had 38% of the sales. Gasoline-powered cars represented only 22% of the vehicles sold.
NOTE: Due to the oil embargo of 1973 and increased demand for alternative energy sources, Congress enacted Public Law 94-413, the Electric and Hybrid Vehicle Research, Development, and Demonstration Act of 1976, which was designed to promote new technologies.
In the late 1990s, several manufacturers produced electric vehicles, using electronic controllers to meet the demands for zero emission vehicles specified California law. Electric vehicles were produced by Ford, Toyota, Nissan, and General Motors. Legislation was passed in California that included the zero emissions vehicle ( ZEV ) mandate.
As a direct result of the California zero emissions vehicle ( ZEV ) mandate originally calling for 10% ZEV, GM developed the Electric Vehicle 1, known as EV1, leased to customers in California and Arizona.
Figures 64–1 and 64-2 Views of the components of the General Motors electric vehicle (EV-1). Many of the features of this vehicle, such as regenerative braking and DC-to-DC converters currently used on hybrid vehicles, were first put into production on this vehicle.
Driving a hybrid electric vehicle is the same as driving any other conventional vehicle. In fact, many drivers and passengers are often not aware they are driving or riding in a hybrid electric vehicle. Some unique characteristics that the driver may or may not notice:
After the internal combustion engine achieves normal operating temperature and other conditions are met, the engine will stop when the vehicle slows down and stops. This may concern some drivers, who, thinking the engine has stalled, try to restart it.
The brake pedal may feel different, especially at slow speeds, when slowing to a stop. At low speeds the brake system switches from regenerative braking to applying brake force to the mechanical brakes. A slight surge or pulsation may be felt.
The power steering works even when the engine stops because all hybrid electric vehicles use an electric power steering system.
Some hybrid electric vehicles are able to propel the vehicle using the electric motor alone, resulting in quiet, almost eerie operation.
If a hybrid electric vehicle is being driven aggressively and at a high rate of acceleration, there is often a feeling that the vehicle is not going to slow down when the accelerator pedal is first released. This is caused by two factors:
The inertia of the rotor of the electric motor attached to the crankshaft of the ICE results in the engine continuing to rotate after the throttle has been closed.
The slight delay that occurs when the system switches the electric motor from powering the vehicle to generating (regenerative braking). While this delay would rarely be experienced, and is not dangerous, for a fraction of a second it gives a feeling the accelerator pedal did not react to a closed throttle.
Fuel economy will be higher compared to a similar-type vehicle. In city-type driving conditions, engine stop and regenerative braking really add to the efficiency of a hybrid electric vehicle. Range of the hybrid version may be about the same as the conventional version of the same vehicle because the hybrid version usually has a smaller fuel tank capacity.
The hybrid version will cost and weigh more than the conventional version. The increased cost is due to batteries, electric motor(s), and controllers used plus additional components needed to allow operation of heating and air conditioning systems during idle stop periods. While the cost is offset in part by returning improved fuel, it may take many years of operation before the extra cost is offset by cost savings from the improved fuel economy. Many owners purchase a hybrid electric vehicle for other reasons, including a feeling that they are helping the environment and love of the high technology involved.
The typical starter motor used on a conventional gasoline or diesel engine rotates the engine 100 to 300 revolutions per minute (RPM). Because the typical engine idles at about 600 to 700 RPM, the starter motor is rotating the engine at a speed slower than it operates. This makes it very noticeable when starting because the sound is different when cranking compared to when the engine actually starts and runs. When the motor-generator of a hybrid-electric vehicle rotates the engine to start it, the engine is rotated about 1000 RPMs, which is about the same speed as when it is running. As a result, engine cranking is just barely heard or felt. The engine is either running or not running, which is a truly unique sensation to those not familiar with the operation of hybrid electric vehicles (HEVs). How Fast Does the Motor Generator Turn When Starting the Engine?
Types of hybrid-electric vehicles include series, parallel, and series-parallel designs.
CLASSIFICATIONS OF HYBRID-ELECTRIC VEHICLES In a series - hybrid design, the engine turns a generator, which can charge batteries or power an electric motor that drives the transmission. Figure 64–3 A drawing of the power flow in a typical series-hybrid vehicle. The internal combustion engine never powers the vehicle directly.
Figure 64–4 This diagram shows the components included in a typical series-hybrid design. The solid-line arrow indicates the transmission of torque to the drive wheels. The dotted-line arrows indicate the transmission of electrical current.
In a series-hybrid design, sole propulsion is by a battery-powered electric motor. Energy for the batteries comes from another on-board energy source, such as an internal combustion engine.
The engine is only operated to keep the batteries charged, thus the vehicle could be moving with or without the internal combustion engine running. Series-hybrid vehicles also use regeneration braking to help keep the batteries charged. An advantage of a series-hybrid design is that no transmission, clutch, or torque converter is needed. A disadvantage of a series-hybrid design is the added weight of the internal combustion engine — actually a heavy, on-board battery charger — to what is basically an electric vehicle. The electric motor and battery capacity have to be large enough to power the vehicle under all operating conditions.
Yes, using a diesel engine instead of a gasoline engine in a hybrid electric vehicle is possible. While the increased efficiency of a diesel engine would increase fuel economy, the extra cost of the diesel engine is the major reason this combination is not currently in production. Is a Diesel - Hybrid Electric Vehicle Possible?
In a parallel - hybrid design, multiple propulsion sources can be combined, or one energy source alone can drive the vehicle. The battery and engine are both connected to the transmission.
Figure 64–5 The power flow in a typical parallel-hybrid vehicle. Continued The vehicle can be powered by internal combustion alone, by electric motor alone, (full hybrids), or a combination. In most cases, the electric motor is used to assist the internal combustion engine.
Among the advantages of using a parallel-hybrid design is that by using an electric motor or motors to assist the internal combustion engine, the engine itself can be smaller than normally needed. One disadvantage of a parallel-hybrid design is that complex software is needed to seamlessly blend electric and ICE power. Another concern about the parallel-hybrid design is that it had to be engineered to provide proper heating and air-conditioning system operation when the ICE stops at idle. NOTE: A parallel-hybrid design could include additional batteries to allow for plug-in capability, to extend the distance the vehicle can travel using battery power alone. Continued
Figure 64–6 Diagram showing the components involved in a typical parallel-hybrid vehicle. The solid-line arrows indicate the transmission of torque to the drive wheels, and the dotted-line arrows indicate the flow of electrical current.
The Toyota and Ford hybrids are classified as series - parallel - hybrids because they operate using electric motor power alone or with the assist of the ICE. The internal combustion engine may be operating even though the vehicle is stopped if the electronic controller has detected that the batteries need to be charged.
NOTE: The internal combustion engine may or may not start when the driver starts the vehicle depending on the temperature of the engine and other conditions. This can be confusing to some who are driving a hybrid electric vehicle for the first time and sense that the engine did not start when they tried to start the engine. Continued
Figure 64–7 A series-parallel-hybrid design allows the vehicle to operate in electric motor mode only or in combination with the internal combustion engine.
The belt system, called the belt alternator starter ( BAS ), is the least expensive system that can still claim the vehicle is a hybrid. For many buyers, cost is a major concern and the BAS system allows certain hybrid features without an entire redesign of the engine and power train. Consumers can upgrade to BAS hybrids at a reasonable cost and will get a 2% to 5% increase in fuel economy, mostly affecting city mileage. BAS replaces the belt-driven alternator with an electric motor that serves as a generator and motor. When the engine is running the motor, acting as a generator will charge a separate 36-volt battery (42-volt charging voltage).
When the engine needs to be started again after being in idle stop, the BAS motor cranks the engine by taking power from the 36-volt battery pack and applies its torque via the accessory belt
Figure 64–8 This chart shows what is occurring during various driving conditions in a BAS-type hybrid.
NOTE: A BAS system uses a conventional starter motor for starting the ICE the first time, and only uses the motor-generator to start the ICE when leaving idle stop mode. The motor-generator is larger than a standard starter motor so more torque can be generated in cranking mode, also referred to as the motoring mode . The fast rotation of the BAS allows for quicker starts of the engine, and makes the start/stop operation possible. Having the engine shut off when the vehicle is at a stop saves fuel. On extremely small vehicles, the belt alternator starter might nudge a vehicle into the mild hybrid category. Continued
Figure 64–9 The components of a typical belt alternator-starter (BAS) system. The BAS system is used in the Saturn VUE hybrid SUV.
stickers, which are also limited as to how many in the entire state can be issued.
In most locations the answer is yes, but it depends on the type of hybrid vehicle. The high-occupancy vehicle (HOV) lane in many cities is reserved for use by vehicles that are carrying more than one occupant as a way to encourage carpooling and the use of public transportation. In California, only those hybrids classified as being high-fuel-economy models and those meeting certain emission ratings qualify. Those that do qualify, such as the Toyota Prius, are issued stickers that show that they are entitled to be in the HOV lane even if there is just the driver in the vehicle. High-performance hybrids, such as the Honda Accord hybrid, do not meet the specified fuel economy rating to allow the owners to be issued HOV Can Hybrids Use the HOV Lane? Figure 64 – 10 This sticker on a hybrid vehicle allows the driver to use the high-occupancy vehicle (HOV) lanes even if there is only one person in the vehicle as a way to increase demand for hybrid vehicles in California.
Figure 64–11 A reversible starter/alternator is used to provide idle stop function to conventional vehicles. This very limited and low cost system is called a micro-hybrid-drive.
Micro-Hybrid Drive System A major fuel-saving feature of hybrid electric vehicles is idle stop mode. The internal combustion engine is stopped, instead of idling, while in traffic.
A system developed by Valeo, often called a micro - hybrid - drive system, uses a reversible starter/alternator. It is used as a conventional alternator when the engine is running, and as a starter by transmitting power through the drive belt system to start the engine.
Common Features of Most Hybrids that improve fuel economy: Idle stop Turns off engine when the vehicle is stopped. When the brake is released, the engine immediately starts. This ensures the vehicle is not using fuel, nor creating CO2 emissions, when the engine is not required to propel the vehicle.
An assist hybrid-electric vehicle is a term used to describe a vehicle where the electric motor is not able to start moving the vehicle on electric power alone. This type of hybrid would include all mild hybrids (36 to 42 volts), as well as the medium hybrids that use 144- to 158-volt systems. What is an Assist Hybrid? Continued
Regenerative braking When decelerating, the braking system captures energy and stores it in the battery or other device for later use, helping to keep batteries charged. Power assist The electric motor provides extra power using current drawn from the battery to assist ICE during acceleration. This power - assist mode enables the vehicle to use a smaller, more fuel-efficient engine without giving up performance. Engine-off drive-electric vehicle mode The electric motor propels the vehicle at lower speeds. The ICE is not being used during acceleration, no fuel is being used and no emissions are being released. When the hybrid is in this mode, it is essentially an electric vehicle.
The term hybrid refers to a type of vehicle. However, there are different levels of “hybridization” on the market. Different vehicle manufacturers use various hybrid technologies.
Continued Mild Hybrid Incorporates idle stop and regenerative braking, but not capable of using the electric motor to propel the vehicle without help from the internal combustion engine. A mild hybrid costs less, but saves less fuel (8% to 15%) compared to a full hybrid vehicle and usually uses a 42-volt electrical motor and battery package (36-volt batteries, 42-volt charging). Examples include the GM Silverado pickup truck and Saturn VUE.
Medium Hybrid Using voltages of about 144 to 158 volts, a medium hybrid provides engine stop/start, regenerative braking, and power assist. Like a mild hybrid, it typically is not capable of propelling the vehicle from a stop using battery power alone. Fuel economy savings are about 20% to 25%. Examples of a medium hybrid vehicle include the Honda Insight, Civic, and Accord.
Full Hybrid Also called a strong hybrid , a full hybrid uses idle stop, regenerative braking, and is able to propel the vehicle using the electric motor(s) alone. Fuel economy savings are about 30% to 50%. Examples include the Ford Escape SUV, Toyota Highlander, Lexus RX400h, Lexus GS450h, Toyota Prius, and Toyota Camry. Each vehicle manufacturer has made its decision on which hybrid type to implement based on assessments of the market niche for a particular model.
Idle Stop Regenerative Braking Motor Assist Engine Off Drive — EV Mode Chevrolet Silverado 1500/GMC Sierra Hybrid Ford Escape and Mercury Mariner Hybrid Honda Accord Civic Hybrid Honda Insight Toyota Prius Saturn VUE X X X X X X X X X X X X X X X X X X X X X X Hybrid Features by Make and Model
EFFICIENCIES OF ELECTRIC MOTORS AND INTERNAL COMBUSTION ENGINES
An electric motor can have efficiency (including controller) of over 90%, while a gasoline engine has efficiency of 35% or less.
An ICE does not have the overload capability of an electric motor, which is why the rated power of an internal combustion engine is usually much higher than required for highway cruising.
Operating smoothly at idle speed produces a much lower efficiency than operating at a higher speed.
Maximum torque of an internal combustion engine is reached at intermediate speed and the torque declines as speed increases further.
There is a maximum fuel efficiency point in the speed range for the ICE, and this speed is optimized by many hybrid vehicle manufacturers by using a transmission that keeps the engine speed within the most efficient range.
Electric Motors Offer ideal characteristics for use in a vehicle because of the following factors:
Constant power over all speed ranges
Constant torque at low speeds needed for acceleration and hill climbing capability
Constant torque below base speed
Constant power above base speed
Only single gear or fixed gear is needed in the electric motor transmission
When a hybrid electric vehicle is operating at low speeds, it is often being propelled by the electric motor alone, sometimes called motoring mode . As a result, the vehicle is very quiet and is said to be operating in quiet mode. During this time, the driver should be aware that the vehicle is not making any sound and should be careful when driving in congested areas. Service technicians should also be extremely careful when moving a hybrid electric vehicle around the shop due to the silence of the vehicle. Watch Out for Motoring Mode
A full or strong hybrid uses a high-voltage system (250–650 volts) and is capable of operating using the electric motor(s) alone and achieves the highest fuel economy improvement of all types of hybrids.
Early in vehicle history, electric vehicles were more popular than either steam- or gasoline-powered vehicles.
Legislation passed in California in 1998, which mandated zero emission vehicles (ZEVs), caused the vehicle manufacturers to start producing electric vehicles. When the law was changed to allow the substitution of other vehicles that produced lower emissions, but not zero, it helped promote the introduction of hybrid electric vehicles (HEVs).
A hybrid vehicle is defined as having two power sources to propel the vehicle.
Electric motors are perfect for vehicle use because they produce torque at lower speed, whereas internal combustion engines need to have an increased speed before they produce maximum power and torque.