EV Basics I - A Tale of Two Hybrids

Important acronyms:braking. "They use VVT, variable valve timing, to open
ICE: Internal Combustion Engine - The standard drivethe valves when compression would normally be
train of cars way back in the 20th Century.occurring. Since there is no compression, the engine
PHEV: Plug-In Hybrid Vehicle - A car or truck with anspins freely, with very little friction. This comes into play
ICE and a battery pack that can be charged straightduring periods of rapid deceleration, when the large
from a typical electrical outlet.motor generator is spinning quickly. If the engine were
VVT: Variable Valve Timing - A system which allowsnot allowed to turn as well, the smaller motor
an ICE to open and close cylinder valves with at leastgenerator would over-spin beyond its 10,000 RPM
some degree of independence from crankshaftredline and burn out."
position. Such systems can be used to depressurizeNow that we know the basic definitions of parallel,
engine cylinders, removing "compression braking" fromseries, and series/parallel hybrid drivetrains, it is logical
the system.to ask the question, "Which is best?" All three
Looking to buy a hybrid car and wondering about yourarchitectures have benefits and problems. Each works
options? Or perhaps you already own one and wantwell under certain conditions but not others.
to find out more about it. Maybe you are just curious...A pure parallel system is the easiest for a major
There are many good reasons to be curious aboutautomotive company to put into production. Simply
hybrids, but learning about them can be a dauntingattach an electric motor to an existing drivetrain, add a
process. There are so many terms being thrownbattery pack and controller, and PRESTO! It is easy to
around, "mild hybrid," "full hybrid," "series hybrid," "parallelachieve substantial gains in both performance and fuel
hybrid," "plug-in hybrid," and the list goes on. What doeconomy. However, in order to drive a car with a
these words mean? Which one is best for you? Readparallel-only drivetrain, the ICE must be operating at all
on, intrepid researcher, and I will try to find you a pathtimes. There is no option to drive on electric power
through the jargon.alone.
The terms "mild hybrid" and "full hybrid" are definedIn many ways, a series/parallel layout gives drivers the
differently by different people and organizations,"best of both worlds." These cars can operate in
depending on what information they are trying toelectric-only mode and the efficiency of their electric
convey. The terms are used more by marketingdrivetrains approaches that of series hybrids.
departments, less by technically-oriented people.Additionally, they benefit from an efficient mechanical
Generally speaking, a mild hybrid uses a small motorconnection between the ICE and the drive-line. But
and battery pack to provide a modest amount ofthese benefits come at a cost in terms of complexity.
extra power and/or efficiency to a drivetrainThere are more mechanical connections to the
dominated by an internal combustion engine (ICE).drive-line, and the different power sources need
There are a few large trucks being sold with optionalcomplicated electromechanical controls in order to
mild hybrid drive trains, such as the Chevrolet Silverado.work together effectively. This added complexity
Benefits of a mild hybrid include a small increase in fuelcreates added weight and additional areas in which
economy, the ability to shut down the engine when themechanical or electronic problems could arise.
car comes to a stop (such as at a traffic light) and theIn contrast, a series hybrid is remarkably simple. For
ability to run power tools and other electric/electronicstarters, an electric vehicle drivetrain has far fewer
devices from energy stored in the battery pack.moving parts than an ICE-powered drivetrain. Now add
A full hybrid vehicle can produce a significant amountan ICE that does not need any messy transmission or
of driving force from its electric drivetrain components.torque converter; all it needs is an output shaft
Most people limit this category to cars that can driveconnected to an electrical generator. Simplicity
for at least a short distance on electric power only.embodied! Unfortunately, this simplicity does not equate
The terms "series hybrid" (or "serial hybrid") andwith efficiency.
"parallel hybrid" are clearly defined, with meanings that"Wait a moment!" you might now say, "I thought
are agreed-upon and accepted by virtually all peopleelectric cars were more efficient than ICE-powered
who are conversant on electric cars. In a series hybrid,cars!" And you would be correct if you did! However,
the electric motor is connected directly to the drive-line.in order to calculate the overall efficiency of a series
The output shaft of the motor drives the transmission,hybrid, we must look at the product of all the
which drives the wheels of the vehicle. The ICE, on theinefficiencies in the system. Let us look at the drivetrain
other hand, is NOT connected directly to the drive-line."downstream" of the ICE, assuming that we are using
It is connected only to a generator which producesvery efficient components throughout, a motor that
electricity, just like the old generator Uncle Earl uses tooperates at 90% efficiency, a generator/battery
run his beer 'fridge when he goes camping. Instead ofcharging system that also operates at 90% efficiency,
cooling beer, however, the generator in a series hybridand mechanical drivetrain components that operate at
uses the electricity to charge the car's batteries and85% efficiency.
power the motor.0.9*0.9*0.85 = 0.69 = 69% total system efficiency!
The recently-introduced Chevrolet Volt E-Flex conceptA non-hybrid drivetrain typically operates at or near
car proposes to use a series hybrid architecture.80% efficiency for a standard transmission, once again
According to GM, it will have a large electric motor andlooking at components downstream of the ICE. With
a small ICE. It will be capable of going roughly fortythe additional low-end motor torque and other benefits
miles in electric only mode, after charging the batteryof a parallel hybrid system, the efficiency could be
pack from a plug-in connection. The Volt is a gooddriven up even higher. Clearly, once the ICE comes into
example of a "typical" series parallel application inplay in a series hybrid system using "typical"
which the vehicle relies most heavily on its electriccomponents now available to auto manufacturers, the
drive system. The gas engine only comes into playefficiency drops to levels substantially below some
when the batteries are drained.other options. Of course, inefficiencies coming from the
A parallel hybrid includes an ICE that IS directlyICE are not a factor when a series hybrid is in
connected to the drivetrain. All of the hybrid vehicleselectric-only mode. If the ICE is used only rarely,
sold by Honda fall into this category. We can simplifyefficiency numbers become quite impressive, relating
the concept of a parallel hybrid drivetrain as being aonly to motor and battery charging losses.
"standard" ICE drivetrain with an electric motor inserted,So what is the ultimate answer to our automotive
providing additional power to the overall drive system.needs? Well... both parallel and series/parallel hybrids
The ICE is usually, although not necessarily, connectedcould go a long way to decreasing our dependence on
to an electrical generator as well, which producesliquid fuels. Both could dramatically increase the fuel
electricity used to power the motor and charge theeconomy of the cars and trucks we drive. However,
batteries. All of the parallel hybrids available today getat some point in the near future series hybrids will
most of their power from the ICEs, with smalleremerge as the best choice. They are the simplest
electric motors and battery packs providing extraoption that would allow us to get the vast majority of
power during acceleration.our transportation-related energy from the utility grid.
Some parallel hybrid drivetrains allow the ICE to beFurthermore, efficiency concerns may be eliminated in
mechanically disconnected from the rest of thethe very near future. Certain modern single speed
drive-line at times. In recent years, this architecture hastransmissions are used in drivetrains with claimed
come to be called a "series/parallel hybrid." Theefficiencies of up to 97%. If such a transmission were
Toyota Prius is one example of this layout. The detailsused with a state-of-the art motor operating at 95%
of the Prius's design are relatively complicated, so Iefficiency and a generator/battery charging system
contacted two Prius experts to help fill in the gaps inwhich had similar efficiency, overall system efficiency,
my knowledge. First, I spoke with Ron Gremban,not considering the ICE efficiency, would be:
technical lead for the group CalCars and the primary0.95*0.95*0.97 = 88%!
source of engineering expertise for their Prius+ plug-inAdditionally, such a series architecture could allow for
hybrid (PHEV) project. Gremban explained thethe use of an Atkinson cycle ICE operating only at
fundamentals of the Prius drivetrain to me. He told mepeak power. Such an ICE could have an average
that the Prius has two electric motor/generators andefficiency of 35%-40%, twice the average efficiency
one gasoline engine. All three of these units areof a typical car engine on the road today. Alternatively,
attached to a planetary gear system, which Toyotaa series hybrid could replace the gasoline engine with
calls a 'Power Split Device.' At any point, two or threean engine running on biodiesel. Some diesel engines
of these units can be spinning simultaneously, so thehave achieved peak efficiencies in the 50% range!
larger motor/generator can be driving the car while theYes, series hybrids appear to be the most promising
ICE is not running. Alternatively, the ICE can be drivingcandidate to become the vehicle of the future. But
the wheels along with the larger motor, or it can bedon't let that stop you from making your next car a
providing electrical power to charge the batteries.parallel or series/parallel hybrid. As CalCars founder
I had also heard that the Prius can de-pressurize theFelix Kramer is fond of saying, "The perfect should not
cylinders in the ICE to decrease mechanical lossesbe the enemy of the good!" Cars like the Toyota Prius
during all electric operation, but Gremban did not knowor Honda Civic hybrid are marvelous examples of
the details of this function. Undeterred, I called Peterengineering ingenuity, and they are available today at
Nortman, president of EnergyCS, a company which isyour local dealerships. Don't wait for some point in the
developing a kit which will allow Prius owners to retrofitfoggy future to buy a car that is as green as you can
their cars to make them PHEVs. Nortman was quiteimagine; get the greenest car that is available right
familiar with Toyota's design to eliminate compressionnow.