Hybrid Electric Propulsion Systems Report

2.0 GENERAL HYBRID-ELECTRIC CONCEPTS

In recent years, hybrid vehicles have gained popularity and publicity due to the rising prices of petroleum. These vehicles promise better fuel efficiency and lower emissions when compared to traditional gasoline vehicles. This leads to the question: what exactly is a “hybrid” vehicle? This section will try to answer this question by giving a general overview of the features and concepts that define a hybrid-electric propulsion system.

2.1 Basic Components

A hybrid vehicle is technically defined as a vehicle that has two or more sources of propulsion. However, when most people talk about a “hybrid” or “hybrid-electric” vehicle in modern times, they are referring to a vehicle that is powered by a combination of an electric motor and a petroleum (usually gasoline) engine. Other than the gasoline engine and the electric motor, a typical hybrid system also contains a large battery, an electronic control system, and a regenerative braking system. The battery is used to power the electric motor and store energy that has been recaptured by the regenerative braking system; the electronic control system is in place to manage the power flow of the system, and the regenerative braking system is used to recapture kinetic energy that would usually be lost during the braking process (Westbrook, 2001). A more detailed explanation of the regenerative braking system is provided in the next subsection.

2.2 Basic Features

Other than having the necessary components, all hybrid vehicles also have the following three features:

2.2.1 Idle-Off
The idle-off capability automatically turns off the gasoline engine when the vehicle is stopped, and turn the engine back on when it is needed (Hybrid Center, 2005). This capability allows the system to save fuel that would otherwise be wasted while the engine is idling. This feature is one of the main reasons why hybrid-electric vehicles consume less fuel than traditional gasoline vehicles, especially in stop-and-go driving. However, idle-off is not a defining characteristic for true hybrid-electric propulsion systems, as conventional systems can also be designed to have this capability (Hybrid Center, 2005).

2.2.2 Regenerative Braking
The regenerative braking system is another reason why hybrid-electric vehicles tend to be more fuel efficient than conventional vehicles (Jefferson & Barnard, 2002). During the braking process, the electric motor turns into a generator (Westbrook, 2001). This serves two purposes as it allows the system to convert some of the kinetic energy from the wheels into electrical energy and also the generator provides a resistance to help slow the vehicle (Hybrid Center, 2005). The electrical energy is then sent to the battery for storage. In order for such a system to have a noticeable impact on fuel economy, the electric motor must be powerful enough to efficiently recapture the kinetic energy, and the battery must have enough capacity to store this energy for future use (Hybrid Center, 2005).

2.2.3 Electric Motor Power Assist
The electric motor power assist is the defining feature of hybrid-electric systems, as a vehicle must have this feature in order to be classified as a true hybrid-electric vehicle. To implement this feature, a vehicle must have a powerful enough motor and battery pack to assist the gasoline engine in driving the wheels. The power assist capability allows hybrids to use smaller and more efficient engines to achieve similar performance of vehicles with larger engines while having an advantage in fuel economy (Jefferson & Barnard, 2002).

2.3 Hybrid Classifications

Hybrid-electric vehicles are classified depending on their capabilities. Below are the two common categories of hybrids.

2.3.1 Mild Hybrids
If a hybrid vehicle only has the three features mentioned in the previous subsection, then the vehicle is classified as mild hybrid. Out of the three hybrid configurations discussed in this report, the parallel configuration falls into this category. Mild hybrids have the three basic features that define a hybrid, but they lack the capability to run only on battery power (Jefferson & Barnard, 2002).

2.3.2 Full Hybrids
Full hybrids have all the features of mild hybrids, plus they have the ability to run only on battery power in certain situations. In these situations, such as starting from stop or driving at low speeds, the car is propelled only by the electric motor, which draws power from the battery (Hybrid Center, 2005). Out of the three configurations discussed in this report, both the series and series-parallel systems fall into this category.

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