Powertrain for Electric and Hybrid Vehicles

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What is a Powertrain?

A powertrain is a system of mechanical parts in a vehicle that first produces energy, then converts it in order to propel it, whether it be an automobile, boat or other machinery. The average person is most familiar with the powertrain of their car, which creates energy in the engine, which is transferred to the transmission. The transmission then takes the power, or output, of the engine and, through specific gear ratios, slows it and transmits it as torque. Through the driveshaft, the engine’s torque is transmitted to the wheels of the car, which, when applied to road, moves the car. Simply put, a powertrain is made up of an engine, a transmission and a driveshaft.

For all intents and purposes, a powertrain is basically made up of a part that produces energy, some sort of a part that converts it to torque, then the part that sends that energy to the element that transfers that power to the road, water, air, etc. In basic explanation of what a powertrain is, one might include vehicles that don’t have wheels as well. It may seem simple, but depending on the vehicle or machine, powertrains can be extremely complicated systems. The engine producing the power may be chemically or manually powered, such as in a gasoline fueled internal combustion engine or a windmill. It may even be nuclear powered.

In a motor vehicle, the term powertrain or powerplant refers to the group of components that generate power and deliver it to the road surface, water, or air. This includes the engine, transmission, driveshafts, differentials, and the final drive (drive wheels, continuous track like with tanks or Caterpillar tractors, propeller, etc.). Sometimes "powertrain" is used to refer to simply the engine and transmission, including the other components only if they are integral to the transmission. In a carriage or wagon, running gear designates the wheels and axles in distinction from the body. A motor vehicle's driveline consists of the parts of the drivetrain excluding the engine and transmission. It is the portion of a vehicle, after the transmission, that changes depending on whether a vehicle is front-wheel drive, rear-wheel drive, four-wheel drive, or the more exotic Sixwheel drive.

In a wider sense, the power-train includes all of its components used to transform stored (chemical, solar, nuclear, kinetic, potential, etc.) energy into kinetic energy for propulsion purposes. This includes the utilization of multiple power sources and non–wheel-based vehicles.

Main components :-

The Engine or powersupply

All cars come equipped with a variety of engines including gas, diesel and electric motors. Some engines include fuel injection or turbochargers designed to increase power.

The Transmission

A transmission or gearbox provides speed and torque conversions from a rotating power source to another device using gear ratios. In British English the term transmission refers to the whole drive train, including gearbox, clutch, prop shaft (for rear-wheel drive), differential and final drive shafts. In American English, however, the distinction is made that a gearbox is any device which converts speed and torque, whereas a transmission is a type of gearbox that can be "shifted" to dynamically change the speed:torque ratio, such as in a vehicle. The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are also used on pedal bicycles, fixed machines, and anywhere else rotational speed and torque needs to be adapted. Electric car transmissions typically have only one forward gear.

The Drivetrain

Power forwarded through the transmission to the drivetrain helps turn the wheels. Power is sent to the front, rear or all four wheels depending on the design of the vehicle.It includes the driveshaft , axles and wheels .

Driveshaft -

A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque

and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement between them. Drive shafts are carriers of torque: they are subject to torsion and shear stress, equivalent to the difference between the input torque and the load. They must therefore be strong enough to bear the stress, whilst avoiding too much additional weight as that would in turn increase their inertia.

Axle - An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or
fixed to its surroundings, with the wheels rotating around the axle. An axle that is driven by the engine is called a drive axle.Modern front wheel drive cars typically combine the transmission and front axle into a single unit called a transaxle. The drive axle is a split axle with a differential and universal joints between the two half axles. Each half axle connects to the wheel by use of a constant velocity (CV) joint which allows the wheel assembly to move freely vertically as well as to pivot when making turns.



wheel

Other Important Components

Powertrains are sometimes described as including other parts such as the differential, clutch and fuel injectors. On hybrid vehicles, the powertrain also includes the battery pack.

Powertrain Control Module
A powertrain control module (PCM), also known as the engine control unit (ECU) or module (ECM), is an electronic device that regulates many of a vehicle's important functions and has a direct impact on how well the car runs. It takes in information about various systems from sensors and other sources and makes necessary adjustments to optimize performance and efficiency. Some of the functions governed by the powertrain control module include the fuel mixture, ignition timing, and idle speed. It also monitors emissions and other systems and indicates a problem by sending out a signal that activates a warning light. Frequently called the car's "computer," the powertrain control module is like a car's brain. It takes in information from a variety of sensors that monitor factors including oxygen levels, coolant temperature, and throttle position. The PCM then analyzes the information and makes adjustments when necessary to keep the readings within specified normal ranges, i.e., parameters. This helps the engine operate with the desired performance and efficiency.

Power Transmission System of the Electric Vehicle:
A power transmission system of the electric vehicle uses the power developed by the electric motor for directly rotating the driving wheels, without power loss in a torque converter connected to the engine. When the vehicle travels by the power of the engine (HEV’s), an input clutch is engaged so that the rotation of an engine output shaft is transmitted to a two gear shift device via the torque converter, the input clutch and an intermediate shaft, whereby driving wheels are driven by a differential unit. While powered by an electric motor, the input clutch is released and the rotation of the electric motor is preferably changed in the shift device before being transmitted to the driving wheels.

Figure: Hybrid Transmission System

Figure 2: Torque Converter.

A torque converter is a modified form of fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. Like a basic fluid coupling, the torque converter normally takes the place of a mechanical clutch, allowing the load to be separated from the power source. As a more advanced form of a fluid coupling, however, a torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear.

Electric Powertrains
An electric vehicle (EV) is a vehicle that is powered, at least in part, by electricity. EV configurations include battery eLectric vehicles (BEVs) which are powered by 100% electric energy, various hybrid-electric vehicles (IIEVs), and plug-in hybrid electric vehicles (PIIEVs). The basic EV configurations are :-

Battery Electric Vehicles
A battery electric vehicle (REV) is a vehicle that is powered entirely on electric energy, typically a large electric motor and a large battery pack. Based on the type of transmission; the use of a clutch, gearbox, differential, and fixed gearing; and the number of battery packs and motors there are many Variations on the BEV design. However, a basic BEV system is shown in Figure .

Figure 1: Schematic of a battery electric vehicle (REV) powertrain

Mild Hybrid Electric Vehicles
Unlike a REV, a hybrid electric vehicle (11EV) relics on two energy sources, usually an internal combustion engine and an electric battery and motor/generator. A Mild Hybrid is the least electrified type of 11EV. A Mild Hybrid is a conventional internal combustion engine (ICE) vehicle with an oversized starter motor that can also be used as a generator, usually called an integrated starter-generator (ISG) or a belted alternator starter (I3AS), and an oversized battery that powers and is recharged by the motor. A simple Mild Hybrid system is shown in Figure 2. In a Mild Hybrid, the engine must always be on while the vehicle is moving. however, the motor/generator can be used to enable idle stop in which the engine is turned off while the vehicle is at idle. The motor/generator can be used at high loads to assist the engine and increase vehicle performance. At low loads, it increases load on the engine and recharges the electric battery.

Figure 2: Schematic of a Mild Hybrid powertrain

Series Hybrid Electric Vehicles
In a Series Hybrid there is a single path to power the wheels of the vehicle, but two energy sources. As shown in figure 3, the fuel tank feeds an engine which is coupled to a generator to charge the battery which provides electrical energy to a motor/generator to power the wheels through a transmission although a direct coupling can also be used. The motor/generator is also used to recharge the battery during deceleration and braking.
The Series Hybrid can operate in the following seven modes: •Engine only traction •Electric only traction •Hybrid traction •Engine Traction and Battery Charging •Battery Charging and No Traction •Regenerative Braking •Hybrid Battery Changing

Figure 3: Schematic of a Series Hybrid powertrain

Although most Series Hybrids use an ICE, it is also possible to design a Series Hybrid using a Fuel Cell powered by hydrogen, creating a Fuel Cell Electric Vehicle (FCEV).

Parallel Hybrid Electric Vehicles
In a Parallel Hybrid, there are two parallel paths to power the wheels of the vehicle: an engine path and an electrical path, as shown in figure 4. The transmission couples the motor/generator and the engine, allowing either, or both, to power the wheels. Control of a Parallel Hybrid is much more complex that for a Series hybrid because of the need to efficiently couple the motor/generator and engine in a way that maintains driveability and performance.

Figure 4: Schematic of a parallel hybrid powertrain

The Parallel Hybrid can operate in the following five modes:

•Engine only traction •Electric only traction •Hybrid traction •Regenerative Braking •Battery charging from the engine

Series-Parallel Hybrid Electric Vehicles
A Series-Parallel HEV has both Series and Parallel energy paths. As shown in figure 5, a system of motors and/or generators that sometimes includes a gearing or power split device couples allows the engine to recharge the battery. Variations on this configuration can be very complex or simple, depending on the number of motors/generators and how they are used. These configurations can he classified as Complex hybrids (such as the Toyota Prius and Ford Escape hybrids), Split-Parallel hybrids, or Power-Split hybrids.

Figure 5: Schematic of a series-parallel hybrid powertrain

Plug-in Hybrid Electric Vehicles
A plug-in hybrid electric vehicle (PHEV) is an HEV that can be plugged-in or recharged from wall electricity. PHEVs arc distinguished by much larger battery packs when compared to other HEVs. The size of the battery defines the vehicle’s All Electric Range (AER), which is generally in the range of 30 to 50 miles. PHEVs can be of any hybrid configuration. Although no PHEVs are available on the market today, a number of companies have begun to sell conversion kits and services to convert a standard 11EV into a P11EV by adding additional battery capacity and modifying the vehicle controller and energy management system. Comparison of Hybrid Cars
Manufacturer Model Price MPG (city/hwy) Toyota Prius $20,875+ 61/50 Honda Civic $20,650+ 48/47 9.3 Honda Accord $29,990 30/37 9.1 Ford Escape $26,780+ 36/31 8.9 Lexus RX 400h $49,060 31/27 9.4 Toyota Highlande r $33,595 $39,855 33/28 9.9

Edmunds Rating and 9.3 Links (10 highest)

Safety: brakes, air bags, crash test rating

Antilock brakes, two air bags; good/excellent

Antilock brakes, three air bags; good

Antilock brakes, Antilock brakes, four air bags; two air bags; good good/excellent

Antilock brakes with brake assist; front dual-stage airbags; no test ratings yet

Front-seat side airbags, 1st, 2nd row head curtain airbags; fourwheel antilock disc brakes with BrakeAssis t; no test ratings yet 9/10 268 hp @ 5600 rpm June, 2005 Terrific accelerati on, smooth ride, comfortab le cabin with simple controls and solid materials

Environmental (greenhouse rating) Performance Release Date

10/10 76 hp @ 5000 rpm 2000

10/10 93 hp @ 5700 rpm 2002

8/10 255 hp @ 6000 rpm January, 2005

8/10 133 hp @ 6000 rpm December, 2004

9/10 268 hp @ 5600 rpm April, 2005

Reviews

Great mileage, environmental features, high tech

Great mileage, smooth driving, very quiet

A great performance car, excellent mileage

Swift Quiet, peppy, acceleration, great mileage, plush ride good performance quality, elegant interior

Fuel consumption – A comparison

On-road efficiency for conventional vehicles is 24.6 miles per gallon while hybrid drivetrains achieve 37.9 mpg on gasoline. PHEV electrical efficiency is 3.2 mi/kWh and 49 percent of the PHEV miles are using stored grid electricity.

Introduction to AC Powertrains

From the mid-1980s to the early 1990s, advanced AC powertrain technologies were developed under cost-shared research contracts between DOE and the Ford Motor Company. New high-power microprocessors, semiconductors, and transistors contributed to electronics technologies that overcame previous barriers to the AC powertrain concept. This breakthrough, along with DOE.s EV battery research, enabled EV technology to become competitive in some commercial markets (e.g., for .neighborhood vehicles.). Furthermore, the advanced AC powertrain technologies resulting from DOE.s early research are prominent in the technology base that supports other promising, emerging applications. In particular, DOE and industry are collaboratively developing advanced hybrid-electric and fuel cell technologies for automotive applications, within the government/industry Partnership for a New Generation of Vehicles (PNGV). The Technology An EV propulsion system consists of a battery pack and a powertrain. The powertrain includes the electric motor, associated power electronics to control motor speed and torque, and the transaxle that transfers the mechanical energy of the electric motor to the wheels. In AC powertrains, the power electronics incorporates an inverter to convert DC battery output to the AC energy required for AC motor operation. The vehicle testing laboratory at the Idaho National Engineering and Environmental Laboratory validated the benefits of these drivetrains. Commercialization The Ford Motor Company incorporated an advanced AC powertrain into its Ranger EVs in 1998, and sold over 600 Ranger EVs by 2000. General Motors (GM) also capitalized on the improved technology, leasing more than 600 GM EV1s and 500 Chevrolet S-10 pickup trucks with the AC powertrains in 1998 and increasing the production of each model in 1999. DaimlerChrysler also incorporated the technology into 200 of its EVs.

A case study of Tesla Roadster
Unveiled as a prototype in 2006, Tesla Motors (named after the famous electrical inventor), has released the plug in electric Roadster in 2008 with a 220 mile range – however the vehicle is expensive in comparison with a Lotus Elise (US $109,000), which is due to the high development, manufacturing and component costs as well as low projected sales volume. The initial product – the Roadster – will have just 1,000 customers in the US, and Tesla is planning to sell the Roadster in Europe in spring 2009. The company expects to produce 2,000 cars a year (the first phase is sub contracted to Lotus – the vehicle is based on the Elise platform), with final assembly in California. Tesla Motors has also ’inked’ a technology deal with Daimler, according to remarks reportedly made by Chairman Elon Musk. The second phase will be a four seat luxury car produced in California. The Roadster has a zero-emission, 248hp 100% electric engine powered by 450kg lithium-ion battery pack giving the vehicle a top speed of 210kph (electronically limited) and 0-100 km/h acceleration of only 4 seconds.

Performance Comparison
The vision of replacing many of the cars on the road with clean commuter vehicles has caused most producers of electric cars to build low-end cars with as low a price as possible. But even if a solid argument could be made that electric cars will ultimately be cheaper than equivalent gasoline cars, they will certainly not be cheaper until their sales volume approaches that of a typical gasoline car – many thousands per year at least. Until an electric car manufacturer achieves high enough sales to approach a gasoline car manufacturer’s volume efficiencies, electric cars will need to compete on other grounds besides price. Aside from the obvious emissions advantage, there is another way that an electric car can vastly outperform a gasoline car – in a word, torque. A gasoline engine has very little torque at low rpm’s and only delivers reasonable horsepower in a narrow rpm range. On the other hand, an electric motor has high torque at zero rpm, and delivers almost constant torque up to about 6,000 rpm, and continues to deliver high power beyond 13,500 rpm. This means that a performance electric car can be very quick without any

transmission or clutch, and the performance of the car is available to a driver without special driving skills. With a gasoline engine, performance comes with a big penalty – if you want a car that has the ability to accelerate quickly, you need a high-horsepower engine, and you will get poor gas mileage even when you are not driving it hard. On the other hand, doubling the horsepower of an electric motor improves efficiency. It is therefore quite easy to build an electric car that is both highly efficient and also very quick. At one end of the spectrum, the electric car has higher efficiency and lower total emissions than the most efficient cars. At the other end of the spectrum, the electric car accelerates at least as well as the best sports cars, but is six times as efficient and produces one-tenth the pollution. The chart on the following page compares the Tesla Roadster with several high-performance cars and with several highefficiency cars.

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