Along with the excitement and growth around any rapidly developing (and selling!) technology such as hybrid vehicles inevitably comes confusion. New terms applied to new technologies tend to be applied differently, and consumers and manufacturers alike need to get on the same page.
Herewith, a short field guide for understanding and classifying the different types of hybrids. (Click on diagram at right to enlarge. Based on material from Technical University of Berlin and Argonne National Laboratory.)
Hybrid vehicles combine two or more power sources that provide propulsion. A conventional car has only a single source of propulsion: either a diesel or gasoline internal combustion engine. In Hybrid Electric Vehicles (HEVs), at least one power source delivers electrical energy. There are a wide range of possible configurations for an HEV—which is where the confusion comes in. They all are hybrids, but can vary dramatically in terms of the capability of the electric motor, cost, performance and other benefits.
The principle factor in determining the class of HEV is how the electric motor contributes to the propulsion of the vehicle and in what proportion. There are four primary types of hybrids I'll list here:
Micro hybrid. These cars have electric motors that do not supply additional torque when the ICE is running. (In other words—no driving power comes from the electric motor.) In micro hybrids, the electric motors provide functions such as auxiliary power, starter/generator, managing engine stop/start and the use of regenerative braking to charge the battery. Fuel savings are in the 5% to 15% range. The new GM Silverado hybrid is a micro-hybrid. Ford is working on a commercial diesel micro-hybrid project in Europe.
Mild hybrid. The electric motor provides supplementary torque to the gasoline or diesel engine, but is never the sole source of driving power. The system also supports features such as regenerative breaking, start/stop and so on. (Ricardo, an international auto engineering consultancy, defines a mild hybrid as providing up to 10% of the maximum engine power.) The Honda Civic Hybrid and the upcoming Accord Hybrid are current examples of mild hybrid configurations. There are numerous other mild hybrids in the development pipeline.
Although the literature I reviewed pegs fuel savings for mild hybrids between 15% to 25%, Honda for one has achieved greater savings with other advances in the transmission and engine management. This just highlights that the hybrid powertrain is only one element of the overall vehicle that determines fuel efficiency, emissions and performance.
Full hybrid. Unlike the micro or the mild hybrids, the full hybrid vehicle can be driven by the electric motor or the engine independently or together. This capability extends to a full electric launch; i.e., you can start driving the car just using the electric motor. The electric motor and the gasoline (or diesel) engine provide different levels of power—Ricardo suggests that a full hybrid electric motor typically provides around 40% of the maximum engine power as additional torque. Full hybrids are also called parallel hybrids because the electric motor and the engine are hooked up in parallel to the same transmission. Because full hybrids are more complex than the preceding types and also require a more powerful motor and battery system, the vehicles are more costly. The Prius is the best known example of a full hybrid. The new Ford Escape is also a full hybrid.
Fuel savings with a full hybrid can be quite high. Ford is suggesting 50-56% over a standard Escape. The Prius as much as doubles fuel economy from a “standard” sedan. Actual fuel savings varies with the specific use of the vehicle and driving style of the driver.
Series hybrid. Also called a serial hybrid, or sometimes a range extender. The series hybrids are electric cars with support from a small ICE. Only the electric motor propels the vehicle. (Hence, series. The engine sits behind and provides electricity to the motor that turns the wheels.) The fuel-burning engine drives an alternator that generates electricity that either flows to the electric motor or to a battery for storage. When the car is running solely on batteries, the engine turns on when they drain to a certain level and begins to recharge them.
Fuel Cell Vehicles—including the much discussed hydrogen fuel cell vehicle—are advanced series hybrids. A separate power source (the fuel cell) generates the electricity that drives the motor or is stored for later use.
Most of the action in the market during the next few years will be with mild and full hybrids. It is important to stress that the ultimate capabilities of the vehicle depend on a very large number of factors, not just the configuration of the drivetrain. These factors include:
Power and quality of the electric motor
Efficiency and type of the fuel engine (e.g., gasolkine, diesel, HCCI)
Quality of the engine and powertrain management software
Capabilities of the battery
- Hybrid Electric Vehicles—The Technology of the Near Future. Professor Dietrich Naunin, Technical University of Berlin and President, German Electric Vehicle Association.
- An Overview of Hybrid Vehicle Technologies. Robert P. Larsen, Director Center for Transportation Research, Argonne National Laboratory
- Hybrid-Electric Vehicle Design Retail and Lifecycle Cost Analysis, UCD-ITS-RR-03-01, Institute of Transportation Studies, University of California
- EV Configurations, University of Michigan
- A Systems Approach to the Mild Hybrid Powertrain, Ricardo/Valero
- Hybrid Electric Vehicles, US DOE EERE Clean Cities Program