A Tale of Two Hybrids
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. 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," etc. What do these words mean? Which type 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 used more by marketing departments, less by technically-oriented people. A mild hybrid uses a small motor and battery pack to provide a modest amount of extra power to a drivetrain dominated by an ICE. There are a few large trucks being sold with optional mild hybrid drivetrains, 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 devices from energy stored in the battery pack.
A full hybrid vehicle can produce a significant amount of driving force from its electric motor. Most people limit this category to vehicles that can drive for at least a short distance on electric power only.
Many people consider the terms "series hybrid" (or "serial hybrid") and "parallel hybrid" to be more useful because they are clearly defined. 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 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 charges the car's batteries and powers 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 hybrid vehicle with a large motor and battery pack. The small gas engine only comes into play when the batteries are drained.
A parallel hybrid includes an ICE connected directly to the drive-line. All of the hybrid vehicles sold by Honda fall into this category. We can simplify the concept of a parallel hybrid as having a standard ICE drivetrain with an electric motor inserted, providing additional power to the overall drive system.
The ICE is connected to a 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 ICEs with small 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. This architecture is called a "series/parallel hybrid." The Toyota Prius is one example of this layout. The details of the Prius's design are relatively complex, 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+ 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, either 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 through the smaller motor/generator to charge the batteries.
The Prius can also 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. "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 makes sense 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. It would be possible to make a PHEV with a purely parallel hybrid drivetrain, but you could never operate the vehicle without using at least a bit of gasoline or diesel fuel.
In many ways, a series/parallel layout gives drivers the "best of both worlds." These cars can operate in electric-only mode and this architecture has been used to create fully-functional PHEVs. Additionally, this layout benefits 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 in the drive-line, and the motor(s) and ICE 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 when the ICE is in operation. For the moment, we will assume that the ICE in series and parallel hybrids are all equally efficient. Let us assume that we are using very efficient electric components - 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. In order to calculate overall drive-line efficiency, we multiply the efficiencies of the various sub-systems.
0.9*0.9*0.85 = 0.69 = 69% total system efficiency
69% of the energy created by the ICE in this scenario is turned into useful power that makes the vehicle move. The driveline in a modern, non-hybrid automobile operates at or near 80% efficiency for a standard transmission, not including inefficiencies of the ICE. Adding an electric motor in parallel to a modern ICE could increase drivetrain efficiency even further because the low-end torque of the electric motor could help auto designers simplify the transmission and other mechanical features of the drivetrain. Therefore, the drive-line efficiency of a parallel hybrid, not considering the ICE, is likely to be more than 10% higher than the efficiency of a series hybrid if both are using similar ICEs and commonly-available motor/generators, batteries and other electrical components.
So what is the ultimate answer to our automotive needs? Use of both parallel and series/parallel hybrid drivetrains 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. Series hybrid drivetrains are perfect for simple, efficient PHEVs with large electric motors and tiny ICEs that produce only enough power to make sure a car will not completely drain its batteries on long trips. Using a series PHEV layout would allow cars to get the vast majority of their energy from the utility grid, with liquid fuel needed only occasionally.
Furthermore, drive-line efficiency concerns associated with series hybrids can be eliminated. 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 with similar efficiency, overall system efficiency, not considering the ICE, would be:
0.95*0.95*0.97 = 88%!
Additionally, a series hybrid can use a wide variety of supplemental power plants for additional power, from microturbines to biodiesel engines. Gremban explained to me that some diesel engines have achieved peak efficiencies in the 50% range, and since the supplemental power plant in a series hybrid can almost always be operated at peak efficiency, this 50% efficiency would also be the average efficiency for the power plant. Compare that to the 17%-19% average efficiency of an ICE in a modern car, and the series hybrid begins to look even better!
Yes, series hybrids are 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.
Graphics of hybrid architectures courtesy of Wikipedia.com
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