It’s pretty amazing that it has taken over 20 years for hybrid electric vehicles to generate truly significant interest. Yet, that’s the story today as many who are interested in electrification have decided to try a gas-electric hybrid first to sate their appetite for an electrified vehicle. It’s an easy choice since there is no real downside to a hybrid – great fuel efficiency, no range anxiety, and a more affordable price of entry compared to a fully electric vehicle. But how do they work? This article, which ran in Green Car Journal a dozen years ago, explained hybridization in an easy-to-understand way that still resonates today. We’re sharing it here just as it originally ran in Green Car Journal’s Summer 2012 issue.
Excerpted from Summer 2012 Issue: The term ‘hybrid vehicle’ covers a lot of territory. Motivated by two or more different power sources, a hybrid electric vehicle (HEV) uses an internal combustion engine (ICE) and one or more electric motors with batteries that store electrical energy. The ICE is usually a gasoline engine, but diesel engines can be used.
In the future, we will see hydrogen fuel cell hybrids where a fuel cell replaces the ICE. Then, there are hydraulic hybrids, now found in large trucks and buses. Here, energy in the form of high pressure hydraulic fluid is stored in accumulators and reservoirs rather than batteries, and hydraulic pressure rather electric motors drive the wheels.
There are both series hybrids and parallel hybrids, with the latter configuration currently far more popular in automotive applications. Cars like the Chevrolet Volt and Fisker Karma are series hybrids. Here, the ICE’s sole or primary job is to drive a generator that supplies electric energy to the battery or directly to an electric motor, or motors, that power the wheels. The engine in a series hybrid can operate at an optimum speed for best fuel economy since its focus is generating electricity rather than providing mechanical power to the wheels.
In a parallel hybrid, both the ICE and electric motor(s) can power the wheels together or individually. The ICE can also keep the battery charged. The ICE in parallel hybrids can be smaller and more fuel efficient since their electric motors can supply supplemental power for peak loads.
Then there are mild hybrids and full hybrids. In a mild hybrid, the ICE and motor/generator operate in parallel, with the motor/generator used for regenerative braking, stop-start capability, and battery charging. While the ICE provides most of the propulsion power, the electric motor can supply additional power, such as during acceleration and hill climbing. A mild hybrid cannot travel solely on its electric motor. The Chevrolet Malibu Eco, Buick eAssist, and BMW ActiveHybrids are examples of mild hybrids.
A full hybrid adds the ability to operate on electric power alone, at least for short distances. Sometimes a full hybrid is called a series-parallel hybrid since, like a series hybrid, its ICE and motor/generator can charge the battery that in turn powers the wheels. Examples include Toyota, Lexus, and Nissan hybrids, including the Prius with its Hybrid Synergy Drive (HSD) and Ford’s Fusion and C-Max hybrids.
Microhybrids are not really hybrids according to the above definition since they save fuel simply by shutting off the engine when a vehicles stops, such as at traffic lights. Their advantage is that microhybrids can deliver a 5 to 10 percent improvement in fuel economy with only minor modifications to a powertrain, while adding only a small amount to a vehicle’s cost. They do require more robust and powerful starters to handle the greater number of starts, plus more capable batteries to keep the air conditioning, radio, and other electronics running during the stop-and-start process when the engine is shut down. . As expected, maximum fuel economy comes in stop-and-go urban driving with no savings achieved during long-distance highway drives.
Often, stop-start is combined with regenerative braking for further fuel savings. This adds complexity since the braking system must have the ability to recoup braking energy and convert it to electricity that’s used to keep batteries charged. Virtually every mild and full hybrid features stop-start and regenerative braking. In fact, these two systems are what help hybrids achieve greater EPA estimated fuel economy in city driving compared to driving on the highway, where steady speeds have traditionally resulted in much better mpg than when driving in stop-and-go traffic.
As the name implies, the plug-in hybrid electric vehicle (PHEV) operates as a conventional hybrid but can also be plugged into the electric grid to recharge its batteries. This is in contrast to conventional hybrids that recharge only by their onboard generator and regenerative braking. PHEVs, which have a larger battery pack than standard hybrids so they can be driven longer on battery power alone, may never need a drop of gasoline if driven relatively short distances. Longer drives use a combination of battery and internal combustion engine power. Examples include the Toyota Prius Plug-In, Ford Fusion Energi, and C-Max Energi hybrids.
An Extended Range Electric Vehicle (EREV), sometimes called a Range-Extended Electric Vehicle (REEV), is designed for battery electric driving. It creates its own on-board electricity when batteries are depleted to extend all-electric driving range. EREVs can have either series or parallel hybrid configurations. The series hybrid Chevrolet Volt and Fisker Karma are high-profile examples that travel 25 to 50 miles on battery power and then hundreds of miles more with on-board generated electricity. Other similarly-powered extended range electric vehicles are on their way. The upcoming BMW i3, for example, will have a REx option with a small ICE that extends its nominal 100 mile all-electric range.
Here’s an advanced propulsion system that sought to answer a question not yet asked. As Toyota looked forward in the mid-1990s, it launched an inspired program to engineer an all-new powerplant that would be highly fuel efficient, offer extremely low tailpipe and carbon emissions, and feature unheard of environmental performance. The Toyota Hybrid System – now Toyota’s Hybrid Synergy Drive – was the result that debuted in the all-new Prius that hit the world stage in 1997 and emerged on our shores in 2000. It has been refined over the years to deliver more power and even greater efficiency, eventually making its way to a great many Toyota and Lexus models today. This article is reprinted just as it ran in Green Car Journal’s Winter 2004 issue, sharing our perspective 20 years ago on how important a breakthrough this innovative propulsion technology represented at the time, and why it continues to resonate in the automotive market today.
Excerpted from Winter 2004 Issue: Years ago, as automakers struggled to engineer electric vehicles that could offer practical driving range between charges, more pragmatic developers proposed overcoming the battery EV’s range limitation with a ‘range extender.’ Simply, this concept would add a small on-board gasoline engine to keep batteries charged and supplement electric propulsion when more power was needed.
While no longer a true zero emission vehicle – a key goal of electric vehicle enthusiasts – the concept promised cars that would appeal to a mass market. It would provide significantly higher fuel economy than conventional automobiles and achieve near zero emissions levels, all the while offering performance, functionality, and affordability similar to that of the familiar internal combustion engine vehicles we’ve driven for many decades. This concept has evolved into today’s gasoline-electric hybrid vehicle (HEV).
Toyota and Honda can be credited with first producing HEVs that appealed to wide spectrum of vehicle buyers. Toyota introduced its first-generation Prius hybrid in 1997 to the Japanese market. North America saw its first hybrids with the debut of Honda’s two-seat Insight as an early 2001 model, shortly followed by the introduction of the Toyota Prius to American roads.
Toyota uses its sophisticated Hybrid Synergy Drive system to power today’s Prius, a follow-on to the first-generation Toyota Hybrid System. Both automakers are now offering their second generation hybrid vehicles. In 2003, Honda introduced the five-passenger Honda Civic Hybrid, which offers a more powerful adaptation of its Integrated Motor Assist (IMA) hybrid system. A completely redesigned and more powerful Prius appeared as a 2004 model.
Both the Toyota and Honda hybrids are parallel configurations, with wheels driven by both their internal combustion engine and electric motor. In detail, however, they work quite differently. The Honda IMA system’s electric motor/generator supplies additional power to the gasoline engine when needed for acceleration or when driving demands are greater, such as when climbing grades, thus the designation ‘motor assist.’ The Honda gasoline engine always provides propulsion.
Things are reversed with Toyota’s Hybrid Synergy Drive, which finds the Prius starting out on battery electric power. The gasoline engine seamlessly starts up to provide additional power during acceleration, at higher speeds, or when driving up grades. This ability to run at times on battery power alone is an important distinction to some folks, since this means Toyota’s hybrids are actually zero emission vehicles during the time they’re electrically driven. Honda’s hybrids cannot do this.
The Prius uses a four-cylinder, 1.5-liter Atkinson cycle engine. The four-stroke Atkinson cycle, invented by James Atkinson in 1882, is different than the Otto cycle engine we’re used to driving in very distinct ways. Compared to the Otto cycle, where the intake valve is closed near bottom-dead-center, the Atkinson cycle does not close the intake valve at BDC, but leaves it open as the piston rises on the compression stroke. What this means is that some of the air/fuel charge is pushed back out and into the intake manifold and is used in other cylinders. This reduces the volume of the air/fuel mixture that’s compressed and combusted without severely restricting the throttle opening. Restricting throttle opening results in large pumping losses and greatly reduced efficiency. This method of reducing power output without incurring large pumping losses makes the Prius engine much more efficient than a conventional Otto cycle engine under most operating conditions. Effectively, the use of the Atkinson cycle allows the Prius engine to operate quite efficiently at relatively low power levels while still having sufficient power for climbing hills at freeway speeds.
Prius uses the same basic 1.5 liter engine as the Toyota Echo, an engine rated at 108 horsepower at 6000 rpm. The Atkinson cycle allows the engine to be downsized to 76 horsepower at 4600 rpm while still being as efficient, or perhaps more so, than the Echo variant. Also, adding a supercharger to the Atkinson cycle results in the Miller cycle like that used in the Mazda Millenia.
Variable intake valve timing (VVT-I) reduces cylinder pressure to eliminate knocking, important because the engine has a 13:1 compression ratio. A high compression ratio, while good for performance and efficiency, can lead to pre-ignition (knocking), which can damage an engine if unchecked. The aluminum, dual overhead camshaft (DOHC) 16-valve engine produces 76 horsepower at 5000 rpm and 82 lbs-ft of torque at 4200 rpm. Because the engine speed is limited, it can use smaller and lighter components for improved fuel economy. The engine earns an Advanced Technology Partial Zero Emission Vehicle (AT-PZEV) rating, is a Super Ultra Low Emission Vehicle (SULEV), and has an EPA rating of 60 mpg city/51 mpg highway, for a combined estimated 55 mpg fuel economy rating.
Toyota’s HSD also takes special measures to address cold start emissions. Since combustion is not as efficient when an engine is cold and a catalytic converter must reach operating temperature before it can treat exhaust gases, cold starts result in greater emissions levels. The HSD system stores hot coolant in a three-liter vacuum bottle and dumps this into the engine during a cold start to help remedy this.
The permanent magnet, AC (alternating current) synchronous motor produces 67 horsepower (50 kilowatts) at 1200-1540 rpm. Most importantly, it produces 295 lbs-ft of torque at 0-1000 rpm, more than enough to get the car going without help from the gasoline engine. A sealed nickel-metal-hydride (NiMH) battery is used.
An inverter converts the battery’s DC (direct current) to AC for use by the electric motor and generator, and vice-versa. Precise current and voltage control is assured by an intelligent power module. A built-in transformer converts some of the hybrid battery’s power into 12 volts DC to operate vehicle accessories. In the latest generation Prius, the high voltage converter system increases battery voltage from 202 volts to 500 volts for driving the electric motor. This reduces power loss by up to 25 percent because electricity can be supplied at lower current, ensuring large amounts of electricity to the motor for significantly greater output while allowing for a smaller battery.
The Prius’ transaxle contains a planetary gear that adjusts and blends the amount of torque from the engine and motor as it’s applied to the front wheels. It also functions as a continuously variable transmission (CVT) with drive ratio controlled by varying the rpm of the generator that also runs off the planetary gear. This Power Split Device allows the engine to operate in its most efficient load and speed range most of the time. The planetary gear system connects the engine, generator, and motor together, allowing operation in a parallel hybrid mode with the electric motor and gasoline alone or together powering the car. It can also operate like a series hybrid when the gasoline engine operates independently of the vehicle speed to charge the battery or provide power to the wheels. Finally, it allows the generator to start the engine so a separate starter is not needed.
Toyota’s Hybrid Synergy Drive is presently packaged in the sleek, aerodynamic, and efficient five-door Prius hatchback that’s officially classified as a mid-sized car, quite a leap forward from the compact and somewhat quirky first generation Prius. This advanced hybrid vehicle shares virtually nothing with other Toyota models. Features include a throttle-by-wire and an electric air compressor for the air conditioning.
Hybrid Synergy Drive is quite scalable, so expect to see it used in other Toyota and Lexus models. For example, it will be used in the 2006 Lexus RX 400h luxury SUV that will go on sale this coming April 15, along with the Toyota Highlander Hybrid that will debut later in the year. Both models are expected to be mated to a 3.3-liter V-6 engine with front and optional rear motors, in a package producing 270 horsepower. Other Toyota hybrid models will be sure to follow.
With Nissan and Ford already HSD licensees and other automakers reportedly investigating this acclaimed hybrid system for their own models, Toyota has clearly gambled big with its huge investment in this technology, and won big as well. We’ll surely be seeing a lot of Toyota’s Hybrid Synergy Drive in the years ahead.
Green Car Journal recently experienced driving what Toyota proudly says is its greatest Camry Hybrid achievement to date. The rather posh, redesigned Camry Hybrid approaches the combined fuel efficiency of Toyota’s Prius, with the Camry HV LE + achieving 53 highway and 51 city mpg while comfortably seating five adults. Yet, there’s much more to this efficient Camry model than initially meets the eye. Hybrid or not, this variant arguably delivers the best overall drive and ride experience in the 2018 Toyota Camry lineup.
In today’s highly scrutinized auto market, nothing is more important to the successful launch of a new car than its visual first impression, followed by a satisfying walk-around and driving experience. Toyota’s all-new 2018 Camry Hybrid accomplishes all these, presenting a very refined and well-designed package with an intuitive driver-to-car interface, enveloped in a sporty body design that rivals many European offerings. Happily, it’s also a kick to drive!
Comfortable, Refined, and Tech-Rich
During our recent test drive, the Camry was virtually silent as we exited the driveway in electric-only mode, exhibiting that quiet, electric-only drive characteristic that some modern hybrids do so well. This is one of them. As the Camry’s four-cylinder gasoline engine kicked in, we pushed the accelerator aggressively and launched onto the two-lane, finding the combined gasoline-electric horsepower and torque impressive, and the interior quiet.
The 2018 Camry Hybrid produces impressive combined torque and horsepower while sipping gasoline, thus reducing emissions. Its drivetrain technology is borrowed from the Prius and does an excellent job of presenting V-6-like torque while achieving four-cylinder fuel efficiency
Featuring MacPherson struts up front with a redesigned and much-improved double wishbone suspension at the rear, this four-door, five-place sedan is quick off the line and handles with the best of the segment.
One forgets it’s a hybrid being driven within minutes of taking the wheel. In fact, having just exited the 306 horsepower Camry XLE moments earlier, Toyota’s mainstream hybrid sedan surprisingly delivers just as dynamic a driving experience as the high-output V-6 XLE.
Great Attention to Detail, Performance, and Price
Acceleration is seamless thanks to the Camry Hybrid’s redesigned, electronically-controlled continuously variable transmission (ECVT). Sequential shift mode allows for a select-shift feel, plus there’s a choice of four drive modes to tailor the driving experience. Braking and steering provide a natural feeling. We like the feel of the hybrid thanks to a lower center of gravity facilitated by positioning the Camry Hybrid’s higher density, compact battery module below the second row seat. This battery placement does not impede the function of the Camry’s 40/60 split and fold-down rear seat, affording unobstructed access to a rather spacious and well-finished trunk, a first for hybrids of this type in the auto industry.
From where we sit, Toyota borrowed a design cue or two from its upscale Lexus brethren, sized it down a tad, and injected it into the most visually-dynamic Camry offering to date. In the case of the 2018 Camry Hybrid, at a base MSRP of $27,800 you get a car that drives as good as it looks. Kudos to Toyota since that's not an easy accomplishment in the bread-and-butter mid-size car segment that’s historically driven by cost effective, price-sensitive, and fuel efficient imperatives.
Toyota has added ‘Prime’ to the branding of its second generation plug-in hybrid electric vehicle (PHEV) to emphasize it’s the most technologically advanced, best-equipped Prius ever. Prime is the first Toyota hybrid to feature a dual-mode generator drive system that enables the Hybrid Synergy Drive’s electric motor and generator to both provide power for maximum acceleration. A new 8.8 kWh lithium-ion battery pack delivers up to 22 miles of all-electric driving, double that of the first-generation plug-in Prius. Toyota estimates 120 MPGe or greater or the model, which is expected to be the highest MPGe rating of any PHEV.
Prime features an array of connected and advanced electronics systems including an available 11.6-inch HD multimedia screen. Prius Prime will start appearing in U.S. showrooms in late fall and will be available in all 50 states.
We are all enamored by the advanced technologies at work in vehicles today. And why wouldn’t we be? The incredibly efficient cars we have today, and the even more efficient models coming in the years ahead, are testament to a process that combines ingenuity, market competitiveness, and government mandate in bringing ever more efficient vehicles to our highways.
It’s been a long and evolutionary process. I remember clearly when PZEV (Partial Zero Emission Vehicle) technology was first introduced in the early 1990s, a breakthrough that brought near-zero tailpipe emissions from gasoline internal combustion engine vehicles. That move was led by Honda and Nissan, with others quickly following. Then there were the first hybrids – Honda’s Insight and Toyota’s Prius – that arrived on our shores at the end of that decade. Both technologies brought incredible operating efficiencies that drastically reduced a vehicle’s emissions, increased fuel economy to unexpected levels, or both.
Of course, there were first-generation battery electric vehicles in the mid-1990s that foretold what would become possible years later. That first foray into EV marketing was deemed by many a failure, yet it set the stage for the advanced and truly impressive EVs we have today. Those vehicles may not yet be cost-competitive with conventionally powered vehicles due to very high battery costs, but that doesn’t diminish the genius engineering that’s brought them to today’s highways.
Even conventionally-powered cars today are achieving fuel efficiency levels approaching that of more technologically complex hybrids. Who would have imagined popular cars getting 40 mpg or better, like the Dodge Dart, Chevy Cruze, Mazda3, Ford Fiesta, and many more in a field that’s growing ever larger each year?
VW and Audi have proven that clean diesel technology can also achieve 40+ mpg fuel efficiency while providing press-you-back-in-your-seat performance, and importantly, doing this while meeting 50 state emissions criteria. That’s saying something considering diesel has historically had a tough go of it meeting increasingly stringent emissions standards in California and elsewhere. Yet, with elegant engineering by these automakers and their diesel technology supplier Bosch – plus this country’s move to low-sulfur diesel fuel late last decade – ‘clean’ diesel was born.
I would be remiss if I didn’t mention natural gas vehicles. There was a time when quite a few automakers were exploring natural gas power in the U.S., but that faded and left Honda as the lone player in this market with its Civic Natural Gas sedan. Now others are joining in with dual-fuel natural gas pickups and vans, benefitting from advanced engine technologies, better natural gas tanks, and a sense that with increasing natural gas reserves in the U.S., demand for natural gas vehicles will grow. As Honda has shown with its Civic, it’s possible to operate on this alternative fuel while also netting admirable fuel efficiency.
All this advanced powertrain technology is important. It makes air quality and petroleum reduction goals achievable, even ones like the ethereal 54.5 mpg fleet fuel economy average requirement that looms for automakers by 2025. There’s no doubt that advanced technologies come at a cost and reaching a 54.5 mpg average will require the full range of efficiency technologies available, from better powerplants and transmissions to greater use of lightweight materials, aerodynamic design, and answers not yet apparent. But I’m betting we’ll get there in the most efficient way possible.
Ron Cogan is editor and publisher of Green Car Journal and editor of CarsOfChange.com