Vehicle transportation accounts for nearly 23% of global energy consumption, and global CO2 emissions from vehicles will increase significantly by 2050. Additionally, vehicle emissions are a significant public health danger, especially in densely populated areas. Greater electrification of vehicles is one pathway towards reducing the energy consumption and emissions from transportation. Electrification of vehicles spans the spectrum from the introduction of small motors, generators and batteries in hybrid vehicles, to the larger motors, generators and batteries in plug-in hybrid vehicles, to the complete removal of an internal combustion in pure electric vehicles. V2G-Sim enables users to simulate any of these vehicle types, from conventional vehicles to pure EVs and predict the grid interactions for plug-in vehicles.
To understand the basics of vehicle electrification, several concepts are introduced here:
A powertrain is a collection of systems that convert stored energy into motion to get you places. Traditionally, it refers to an engine, transmission, differential and wheels. However, modern powertrains can have more sophisticated engines, a large battery pack, a motor and/or generator and a connection to the electricity grid, and of course, a very complex control system to run the show. Modern powertrain research spans many disciplines, from thermodynamics to electrochemistry to the design of efficient and reliable gears, and is addressing some of the world's most pressing challenges related to energy and harmful emissions, all while ensuring you can get from Point A to Point B (and Point C on the weekends).
Conventional vehicles use internal combustion engines to convert fuel energy into kinetic energy. Heat release from burning a fuel causes high pressures inside an engine's combustion chamber and this pressure linearly pushes a piston. The linear motion of the piston is converted into rotational motion through a series of linkages, and the rotational motion is ultimately output to the engine's crankshaft. The crankshaft (often attached to a flywheel) couples with the input shaft of a vehicle's transmission. As a transmission changes gears, the relative speed of the output shaft compared with the input shaft changes. The output shaft of the transmission is connected to a differential, which ultimately transmits rotational power to wheels which move the car.
There are many types of engines, including spark-ignited (often running with on gasoline) and diesel engines, each with its own efficiency, power output, and emissions characteristics. Generally speaking, however, engines are most efficient when they are operating at their highest power output operating conditions. While vehicles stand idle (e.g. at a red light), or when they are driving in operating conditions where engines are not highly loaded the engines produce power at efficiency levels much lower than their peak efficiency.
What is a hybrid-electric vehicle (HEV) powertrain?
A hybrid-electric vehicle introduces a motor and/or generator and a battery into the vehicle powertrain. All energy originally comes from the engine burning a fuel, however the motor, generator, and battery work together with the engine to enable higher overall efficiency. In particular, hybrid-electric vehicles enable higher efficiency, and lower fuel consumption, for three important reasons:
The engine, motor and/or generator can be connected together in several different ways depending on the type of hybrid:
Series hybrid: the engine is connected only to a generator, and electric power from the generator can flow to charge the batteries or to the electric motor. Only the electric motor is connected to the wheels.
Parallel hybrid: the engine is connected to the wheels (through the transmission and differential). An integrated motor/generator is positioned either before or after the transmission can also supply power to the wheels. In this manner, the power is supplied to the wheels exclusively from the engine, exclusively from the motor, or from a combination of both the engine and motor. In this manner the engine speed remains coupled to the vehicle speed (through the transmission and differential gear ratios), but the motor/generator can supply or dissipate power to help keep the engine near its most efficient load operating point for a given engine speed.
Powersplit hybrid: the engine, motor, and generator are all coupled to a transmission with a planetary gearset which allows different torque and speed of each component to ultimately determine the power delivered to the wheels. The motor and generator are typically separate components. The planetary gearset in a powersplit hybrid also allows the motor and generator to supply or dissipate power to help keep the engine near its most efficient load operating point, however the use of a planetary gearset also allows the engine speed to be independently varied. As a result for a desired amount of engine power output, both the engine load and speed can be varied to enable maximum engine operating efficiency.
The battery pack in a hybrid-electric vehicle is sized to provide the power and energy capabilities to help keep the engine at its greatest efficiency, and to absorb as much regenerative braking as possible while balancing against the high cost of batteries. All energy to move the vehicle originates from fuel that is burned in the engine.
While HEVs use fuel burned in the engine as the only energy source, a PHEV can also use electricity from the grid as the original energy source. To enable all-electric driving (i.e. driving without turning on the engine) for a reasonable range, PHEVs have larger batteries than their HEV counterparts. The powertrain can be configured as any of the three HEV types listed above, however powersplit and series hybrids are most common. While the vehicle drives in all electric mode with the engine remaining off its batteries are gradually depleted, leading this mode to often be called charge depleting mode. Once the batteries are depleted to a certain level the engine is turned on and the vehicle operates as a normal hybrid vehicle while maintaining the battery charge at roughly constant levels, leading this mode to often be called charge sustaining mode. The battery will remain near this constant level until the vehicle is plugged in again to charge from the electricity grid. For some vehicles even while in charge depleting mode the engine may turn on to assist the motor and batteries, for example during high speed or uphill driving. PHEVs also enable regenerative braking, to recover kinetic energy during braking back into the batteries for later use.
An electric vehicle uses electricity from the grid as its only source of energy. These vehicles have substantially larger batteries than the other vehicle types, as they must provide a reasonable driving range only on batteries. To propel the vehicle, power flows from the batteries to the electric motor. The motor is typically connected to the wheels through a reduction gear and the differential, and typically there is no transmission that changes gears. EVs also enable regenerative braking, to recover kinetic energy during braking back into the batteries for later use.