Important acronyms:
ICE: Internal Combustion Engine – The standard drive train for automobiles in the 20th century.
PHEV: Plug-in Hybrid Vehicle – A car or truck with an ICE and a battery pack that can be charged directly from a typical outlet.
VVT – Variable Valve Timing – A system that 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, eliminating “compression braking” from the system.
Looking to buy a hybrid car and wondering about your options? Or maybe you already have one and want to know more about it. Maybe you’re 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 used, “mild hybrid”, “full hybrid”, “series hybrid”, “parallel hybrid”, “plug-in hybrid”, and the list goes on. What do these words mean? Which one is better for you? Read on, intrepid researcher, and I’ll try to find a way through the jargon for you.
The terms “mild hybrid” and “full hybrid” are defined differently by different people and organizations, depending on the information they are trying to convey. The terms are used more by marketing departments, less so by technically oriented people. Generally speaking, a mild hybrid uses a small engine and battery pack to provide a modest amount of additional power and/or efficiency to an internal combustion engine (ICE)-dominated drivetrain. There are some large trucks that are sold with optional mild hybrid drivetrains, such as the Chevrolet Silverado. The benefits of a mild hybrid include a small increase in fuel economy, the ability to turn off the engine when the car is stopped (such as at a traffic light), and the ability to run power tools and other electrical/electronic devices from energy. stored in the battery pack.
A full hybrid vehicle can produce a significant amount of motive power from its electric drivetrain components. Most people limit this category to cars that can drive at least a short distance on electric power alone.
The terms “series hybrid” (or “serial hybrid”) and “parallel hybrid” are clearly defined, with meanings agreed upon and accepted by virtually everyone familiar with electric cars. In a series hybrid, the electric motor is connected directly to the driveline. The output shaft of the engine drives the transmission, which drives the vehicle’s wheels. The ICE, on the other hand, is NOT directly connected to the transmission line. It’s hooked up only to a generator that produces electricity, just like the old generator Uncle Earl uses to run his beer cooler when he goes camping. Instead of cooling the beer, however, a series hybrid’s generator uses electricity to charge the car’s batteries and power the engine.
The recently unveiled 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 approximately forty miles in electric-only mode, after charging the battery from a plug connection. The Volt is a good example of a “typical” series-parallel application where the vehicle relies heavily on its electric drive system. The gasoline engine only comes into play when the batteries are dead.
A parallel hybrid includes an ICE that IS directly connected to the powertrain. All hybrid vehicles sold by Honda fall into this category. We can simplify the concept of a parallel hybrid transmission as a “standard” ICE transmission with an electric motor inserted, which provides additional power to the overall transmission system. The ICE is usually, but not necessarily, also connected to an electrical generator, which produces electricity that is used to power the engine and charge the batteries. All parallel hybrids available today get most of their power from ICEs, with smaller electric motors and battery packs providing additional power during acceleration.
Some parallel hybrid drivetrains allow the ICE to be mechanically disconnected from the rest of the driveline on occasion. In recent years, this architecture has come to be called “serial/parallel hybrid”. The Toyota Prius is an example of this design. The details of Prius 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, CalCars group technical lead and the main source of engineering expertise for their Prius+ plug-in hybrid (PHEV) project. Gremban explained the basics of the Prius transmission to me. He told me that the Prius has two electric motors/generators and a gasoline engine. All three units are connected to a planetary gear system, which Toyota calls a “power split device.” At any one time, two or three of these units may be spinning simultaneously, so the larger motor/generator can power the car while the ICE is not running. Alternatively, the ICE can power the wheels along with the larger motor, or it can provide electrical power to charge the batteries.
I had also heard that the Prius can depressurize the ICE cylinders to decrease mechanical losses throughout electrical operation, but Gremban did not know the details of this feature. Undeterred, I called Peter Nortman, president of EnergyCS, a company developing a kit that will allow Prius owners to retrofit their cars to convert to PHEVs. Norman 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 occur. Since there is no compression, the engine spins freely, with very little friction. This comes into play during periods of rapid deceleration, when the engine The generator is spinning fast. If the engine was not allowed to spin as well, the smaller engine-generator would spin too far past its 10,000 RPM redline and burn out.”
Now that we know the basic definitions of parallel, series, and series/parallel hybrid transmissions, 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 to put into production for a large automotive company. Simply connect an electric motor to an existing drivetrain, add a battery pack and controller, and PRESTO! Substantial gains in both performance and fuel economy are easy to achieve. However, to drive a car with a parallel-only drivetrain, the ICE must be running 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 driveline. But these benefits come at a cost in terms of complexity. There are more mechanical connections to the driveline, and the different power sources need complicated electromechanical controls to work together effectively. This added complexity creates additional weight and additional areas where mechanical or electronic issues could arise.
By contrast, a series hybrid is remarkably simple. For starters, an electric vehicle transmission has far fewer moving parts than an ICE-powered transmission. Now add an ICE that doesn’t need any messy transmission or torque converter; all you need is an output shaft connected to an electrical generator. Simplicity incarnate! Unfortunately, this simplicity does not equate to efficiency.
“Hang on a minute!” now you could say, “I thought electric cars were more efficient than ICE-powered cars!” And you would be right if you did! However, to calculate the overall efficiency of a series hybrid, we must look at the product of all the inefficiencies in the system. Let’s look at the “downstream” powertrain of the ICE, assuming we are using very efficient components at all times, an engine operating at 90% efficiency, a generator/battery charging system also operating at 90% efficiency, and components powertrain mechanics operating at 85% efficiency.
0.9*0.9*0.85 = 0.69 = 69% total system efficiency.
A non-hybrid powertrain typically runs at or near 80% efficiency of a standard transmission, again, looking at the components downstream of the ICE. With the added low-end torque and other benefits of a parallel hybrid system, efficiency could be increased even further. Clearly, once the ICE comes into play in a series hybrid system using “typical” components now available to automakers, efficiency drops to levels substantially below some other options. Of course, inefficiencies coming from the ICE aren’t a factor when a series hybrid is in electric-only mode. If the ICE is used only rarely, the efficiency numbers become quite impressive, related only to motor and battery head losses.
So what is the ultimate answer to our automotive needs? Well…both parallel and series/parallel hybrids could go a long way in reducing our reliance 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, stock hybrids will emerge as the better option. They are the simplest option that would allow us to get the vast majority of our transportation-related energy from the utility grid. Also, efficiency issues may be eliminated in the very near future. Certain modern single speed transmissions are used in drives with claimed efficiencies of up to 97%. If such a transmission were used with a state-of-the-art engine running at 95% efficiency and a similarly efficient generator/battery charging system, the overall system efficiency, not considering ICE efficiency, would be:
0.95*0.95*0.97 = 88%!
Furthermore, such a serial architecture could allow the use of an Atkinson cycle ICE operating only at full power. Such an ICE could have an average efficiency of 35% to 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 that runs on biodiesel. Some diesel engines have achieved peak efficiencies in the 50% range!
Yes, series hybrids seem 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 likes to say, “Perfect shouldn’t be the enemy of good!” Cars like the Toyota Prius or the Honda Civic Hybrid are wonderful examples of engineering ingenuity and are available today at your local dealerships. Don’t wait for some point in the hazy future to buy a car that’s as green as you can imagine; get the greenest car that is available right now.