Plug-in Hybrid Technology

A hybrid vehicle has two or more methods for storing energy to move the vehicle. The modern hybrid uses an electric motor in addition to a gasoline or diesel engine. Typically, an electric battery stores and supplies energy for the electric motor and the fuel tank stores energy for the engine. A hybrid vehicle therefore contains all the parts and pieces of a conventional vehicle. It also has an electric motor, batteries and some additional electronics to coordinate between the motor and engine. The transmission in a hybrid can also be different than that of a conventional vehicle.

A hybrid reduces fuel consumption by allowing the engine to operate closer to its maximum efficiency. To do this, part of the load in a hybrid is provided by an electric motor. Motors are very efficient at most speeds and loads. They are also capable of providing high power at low speeds. This capability makes them well matched for the needs of a vehicle. Hybrids also reduce fuel consumption by allowing the engine to be smaller. Since the electric motor is now providing some of the required load, the engine does not need to provide as much power. Engine power increases with size, but engine efficiency typically decreases as the engine gets larger. In a hybrid, the engine is smaller and is operated more efficiently than that of a comparable conventional vehicle.

Hybrid vehicles have two common configurations: parallel and series. In the parallel configuration, the engine and/or the motor can provide power to the transmission at the same time. Electronic controls adjust the balance of power delivery based upon maximum efficiency and battery energy level. Commercial passenger cars available today like the Toyota Prius and Honda Civic Hybrid are considered parallel hybrids. In a series hybrid, only the electric motor actually drives the wheels. The engine turns a small generator that creates electricity that recharges the batteries. Again, electronic controls adjust the engine load as required to optimize efficiency and minimize emissions.

Series	Parallel

There are several ways to send energy to the batteries. In a series configuration, the engine powers a small generator that provides most of the energy to the batteries. In a parallel configuration, the drive motor sometimes acts as a generator. The electronic controls allow some of the engine power to flow to the generator and charge the batteries this way. The controls constantly monitor the status of the batteries and adjust the engine operation to keep the batteries charged.

Hybrids can also charge the batteries through regenerative braking. In a conventional vehicle, when the driver presses the brake pedal, the brake pads are pushed against metal surfaces attached to the wheels. These metal surfaces and the pads themselves get very hot since some of the energy of the moving vehicle has now flowed into those pads. This heat then flows out into the ambient air. Essentially, all that braking energy is wasted. When the driver of a hybrid presses the brake pedal, some of the energy actually flows back into the batteries. Mechanical energy from the transmission flows into the motor (which is now behaving as a generator) and is transformed into electrical energy that enters the batteries. Since generators are hard to turn, this creates a load on the transmission that slows the wheels of the vehicle. Hybrids also have conventional brakes with pads and metal surfaces as well. The electronic controls adjust how much energy is returned to the batteries through regenerative braking.

Hybrids can also be designed such that the vehicle receives some energy from the electric grid. This is known as a plug-in hybrid. Plug-in hybrids have slightly different control software than regular hybrids, a few more batteries, and some method for plugging in to a charging station. The typical practice is to plug in the vehicle when it is parked overnight and electricity is inexpensive. This reduces the fuel consumption even further and drastically improves the fuel economy. Owners of the Toyota Prius can now purchase an aftermarket kit that allows them to convert their standard hybrid into a plug-in.

The control software of a regular hybrid keeps the batteries as fully charged as possible during driving. This type of battery management is known as charge sustaining. Plug-in hybrids can be operated differently. They can operate in charge depleting mode and drain the batteries to perhaps 20 percent capacity. This allows for more of the plug-in hybrid’s energy to come from the batteries. Both standard and plug-in hybrids may operate as all-electric vehicles for some distance, and they can be categorized by this all-electric distance. For example, a hybrid electric vehicle (HEV) with an all-electric range of 10 miles is known as an HEV-10. A hybrid with a 30 mile all-electric range (an HEV-30) would have more electric energy storage capacity.

The school buses developed as part of this project are plug-in hybrids. You can learn more about the project and the benefits from our technical feasibility study found here.