Important acronyms:
ICE: Internal Combustion Engine - The standard drive train of cars way back in the 20th Century.
PHEV: Plug-In Hybrid Vehicle - A car or truck with an ICE and a battery pack that can be charged straight from a typical electrical outlet.
VVT: Variable Valve Timing - A system which allows an ICE to open and close cylinder valves with at least some degree of independence from crankshaft position. Such systems can be used to depressurize engine cylinders, removing "compression braking" from the system.
Looking to buy a hybrid car and wondering about your options? Or perhaps you already own one and want to find out more about it. Maybe you are just curious... There are many good reasons to be curious about hybrids, but learning about them can be a daunting process. There are so many terms being thrown around, "mild hybrid," "full hybrid," "series hybrid," "parallel hybrid," "plug-in hybrid," and the list goes on. What do these words mean? Which one is best for you? Read on, intrepid researcher, and I will try to find you a path through the jargon.
The terms "mild hybrid" and "full hybrid" are defined differently by different people and organizations, depending on what information they are trying to convey. The terms are used more by marketing departments, less by technically-oriented people. Generally speaking, a mild hybrid uses a small motor and battery pack to provide a modest amount of extra power and/or efficiency to a drivetrain dominated by an internal combustion engine (ICE). There are a few large trucks being sold with optional mild hybrid drive trains, such as the Chevrolet Silverado. Benefits of a mild hybrid include a small increase in fuel economy, the ability to shut down the engine when the car comes to a stop (such as at a traffic light) and the ability to run power tools and other electric/electronic devices from energy stored in the battery pack.
A full hybrid vehicle can produce a significant amount of driving force from its electric drivetrain components. Most people limit this category to cars that can drive for at least a short distance on electric power only.
The terms "series hybrid" (or "serial hybrid") and "parallel hybrid" are clearly defined, with meanings that are agreed-upon and accepted by virtually all people who are conversant on electric cars. In a series hybrid, the electric motor is connected directly to the drive-line. The output shaft of the motor drives the transmission, which drives the wheels of the vehicle. The ICE, on the other hand, is NOT connected directly to the drive-line. It is connected only to a generator which produces electricity, just like the old generator Uncle Earl uses to run his beer 'fridge when he goes camping. Instead of cooling beer, however, the generator in a series hybrid uses the electricity to charge the car's batteries and power the motor.
The recently-introduced Chevrolet Volt E-Flex concept car proposes to use a series hybrid architecture. According to GM, it will have a large electric motor and a small ICE. It will be capable of going roughly forty miles in electric only mode, after charging the battery pack from a plug-in connection. The Volt is a good example of a "typical" series parallel application in which the vehicle relies most heavily on its electric drive system. The gas engine only comes into play when the batteries are drained.
A parallel hybrid includes an ICE that IS directly connected to the drivetrain. All of the hybrid vehicles sold by Honda fall into this category. We can simplify the concept of a parallel hybrid drivetrain as being a "standard" ICE drivetrain with an electric motor inserted, providing additional power to the overall drive system. The ICE is usually, although not necessarily, connected to an electrical generator as well, which produces electricity used to power the motor and charge the batteries. All of the parallel hybrids available today get most of their power from the ICEs, with smaller electric motors and battery packs providing extra power during acceleration.
Some parallel hybrid drivetrains allow the ICE to be mechanically disconnected from the rest of the drive-line at times. In recent years, this architecture has come to be called a "series/parallel hybrid." The Toyota Prius is one example of this layout. The details of the Prius's design are relatively complicated, so I contacted two Prius experts to help fill in the gaps in my knowledge. First, I spoke with Ron Gremban, technical lead for the group CalCars and the primary source of engineering expertise for their Prius plug-in hybrid (PHEV) project. Gremban explained the fundamentals of the Prius drivetrain to me. He told me that the Prius has two electric motor/generators and one gasoline engine. All three of these units are attached to a planetary gear system, which Toyota calls a 'Power Split Device.' At any point, two or three of these units can be spinning simultaneously, so the larger motor/generator can be driving the car while the ICE is not running. Alternatively, the ICE can be driving the wheels along with the larger motor, or it can be providing electrical power to charge the batteries.
I had also heard that the Prius can de-pressurize the cylinders in the ICE to decrease mechanical losses during all electric operation, but Gremban did not know the details of this function. Undeterred, I called Peter Nortman, president of EnergyCS, a company which is developing a kit which will allow Prius owners to retrofit their cars to make them PHEVs. Nortman was quite familiar with Toyota's design to eliminate compression braking. "They use VVT, variable valve timing, to open the valves when compression would normally be occurring. Since there is no compression, the engine spins freely, with very little friction. This comes into play during periods of rapid deceleration, when the large motor generator is spinning quickly. If the engine were not allowed to turn as well, the smaller motor generator would over-spin beyond its 10,000 RPM redline and burn out."
Now that we know the basic definitions of parallel, series, and series/parallel hybrid drivetrains, it is logical to ask the question, "Which is best?" All three architectures have benefits and problems. Each works well under certain conditions but not others.
A pure parallel system is the easiest for a major automotive company to put into production. Simply attach an electric motor to an existing drivetrain, add a battery pack and controller, and PRESTO! It is easy to achieve substantial gains in both performance and fuel economy. However, in order to drive a car with a parallel-only drivetrain, the ICE must be operating at all times. There is no option to drive on electric power alone.
In many ways, a series/parallel layout gives drivers the "best of both worlds." These cars can operate in electric-only mode and the efficiency of their electric drivetrains approaches that of series hybrids. Additionally, they benefit from an efficient mechanical connection between the ICE and the drive-line. But these benefits come at a cost in terms of complexity. There are more mechanical connections to the drive-line, and the different power sources need complicated electromechanical controls in order to work together effectively. This added complexity creates added weight and additional areas in which mechanical or electronic problems could arise.
In contrast, a series hybrid is remarkably simple. For starters, an electric vehicle drivetrain has far fewer moving parts than an ICE-powered drivetrain. Now add an ICE that does not need any messy transmission or torque converter; all it needs is an output shaft connected to an electrical generator. Simplicity embodied! Unfortunately, this simplicity does not equate with efficiency.
"Wait a moment!" you might now say, "I thought electric cars were more efficient than ICE-powered cars!" And you would be correct if you did! However, in order to calculate the overall efficiency of a series hybrid, we must look at the product of all the inefficiencies in the system. Let us look at the drivetrain "downstream" of the ICE, assuming that we are using very efficient components throughout, a motor that operates at 90% efficiency, a generator/battery charging system that also operates at 90% efficiency, and mechanical drivetrain components that operate at 85% efficiency.
0.9*0.9*0.85 = 0.69 = 69% total system efficiency!
A non-hybrid drivetrain typically operates at or near 80% efficiency for a standard transmission, once again looking at components downstream of the ICE. With the additional low-end motor torque and other benefits of a parallel hybrid system, the efficiency could be driven up even higher. Clearly, once the ICE comes into play in a series hybrid system using "typical" components now available to auto manufacturers, the efficiency drops to levels substantially below some other options. Of course, inefficiencies coming from the ICE are not a factor when a series hybrid is in electric-only mode. If the ICE is used only rarely, efficiency numbers become quite impressive, relating only to motor and battery charging losses.
So what is the ultimate answer to our automotive needs? Well... both parallel and series/parallel hybrids could go a long way to decreasing our dependence on liquid fuels. Both could dramatically increase the fuel economy of the cars and trucks we drive. However, at some point in the near future series hybrids will emerge as the best choice. They are the simplest option that would allow us to get the vast majority of our transportation-related energy from the utility grid. Furthermore, efficiency concerns may be eliminated in the very near future. Certain modern single speed transmissions are used in drivetrains with claimed efficiencies of up to 97%. If such a transmission were used with a state-of-the art motor operating at 95% efficiency and a generator/battery charging system which had similar efficiency, overall system efficiency, not considering the ICE efficiency, would be:
0.95*0.95*0.97 = 88%!
Additionally, such a series architecture could allow for the use of an Atkinson cycle ICE operating only at peak power. Such an ICE could have an average efficiency of 35%-40%, twice the average efficiency of a typical car engine on the road today. Alternatively, a series hybrid could replace the gasoline engine with an engine running on biodiesel. Some diesel engines have achieved peak efficiencies in the 50% range!
Yes, series hybrids appear to be the most promising candidate to become the vehicle of the future. But don't let that stop you from making your next car a parallel or series/parallel hybrid. As CalCars founder Felix Kramer is fond of saying, "The perfect should not be the enemy of the good!" Cars like the Toyota Prius or Honda Civic hybrid are marvelous examples of engineering ingenuity, and they are available today at your local dealerships. Don't wait for some point in the foggy future to buy a car that is as green as you can imagine; get the greenest car that is available right now.