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Prius: A Look at Toyota's Hybrid

It will be coming to a driveway near you sometime around the end of the year 2000. The Prius uses a highly efficient powertrain setup that is not only peppy (we merged onto the Southfield Freeway from Ford Rd. with a Prius without a hitch—which as many of you know is no small feat) but energy efficient and is expected to have emissions output that meets the Super Ultra Low Emission Vehicle (SULEV) requirements.
Prius

 

As a practical matter, the currently available electric vehicles (EVs) on the market haven't appealed to a whole lot of people—not even those with an environmental bent. Nor have they seemed to be particularly practical, due largely to such things as a limited range and recharge times that make even the slowest pump at a gas station seem like a torrent of potential energy. Consequently, the number of EVs out there is comparatively miniscule, which has led to a situation wherein there hasn't been a whole lot of concern vis-a-vis producing the vehicles. It is, practically, a matter of craft, not mass, production.

But all of this could change within a couple of years, when Toyota brings its Prius hybrid from Japan to both the U.S. and Europe. According to Chris Hostetter, corporate manager-Strategic Planning, Toyota Motor Sales, USA, the game plan is to sell 20,000 Prius vehicles per year in the U.S. and Europe combined. There's an expectation that about2/3 of that number will be making it to the U.S. At 20,000 per year—in addition to the number sold in Japan (within the first several months of availability starting in December 1997, more than 10,000 were sold)—this is a serious number.

The use of the term "EV" for the Prius isn't precise. Actually, the Prius uses what is called a "hybrid system." Fundamentally, a hybrid system is one wherein the powertrain uses two or more separate power systems.

Hybrids Explained

Generally, there are two main categories of hybrids. One is parallel. An example of this type of powertrain setup has an electric motor used to get the vehicle moving and running along, then an internal combustion (IC) engine kicks in and takes over at higher speeds.

The other primary version is a series hybrid. The essential elements can be the same as in a parallel system, but the arrangement is different. That is, there may be an IC engine that drives a generator; the generator generates electricity that's used by an electric motor to drive the wheels. This approach is called series because things happening in A-B-C manner.

Toyota engineers have come up with a different arrangement from a strict parallel or series approach. Rather, their system, designated the Toyota Hybrid System (THS), is a melding of the two. There is a gasoline engine and a motor. The motor or the engine can power the car at low speeds, which is like a parallel system. Additionally, there is a connection whereby the gasoline engine sends power to a generator, which then can be used by the motor, which makes the arrangement like a series hybrid.

And there's even another option, which is that both the motor and the engine are used at the same time, thereby providing more oomph. That is, when the engine—an in-line, four-cylinder, DOC, 1.5-liter engine—is used alone, the top speed is 88 mph. When the 30-kW permanent magnet motor is added to the mix, the top speed is cranked up to 100 mph.

 

4 cylinder, 1.5-l engine
If you look closely, you can see some interesting things—like a small, 4-cylinder, 1.5-L engine in the front and an array of sealed NiMH batteries in the trunk. The body itself is fabricated with high tensile strength steel. The center pillar reinforcements are heat treated so the material is ultra high strength. Toyota is working to use recycled and recyclable materials in its vehicles. The bumper covers and corner moldings, dashboard, and pillar garnish are made with a recyclable material the company has developed, Toyota Super Olefin Polymer. Behind the dash panel and below the carpeting is Recycled Sound-Proofing Products (RSPP), a material produced with automotive shredder residue.

Splitting the Power

Key to the THS is a power split device that's in the transmission. This is based on a planetary gear system (including a sun gear, ring gear, and planetary carrier with pinion gears). David W. Hermance, general manager, Powertrain, Toyota Technical Center USA (Gardena, CA), speaking of the transmission, says that compared to a conventional transmission, the Prius arrangement is "mechanically simpler." For example, there are no valve bodies or solenoids to make. He admits that it is more expensive to produce—for now. One can only imagine that when the Toyota Production System comes into play in earnest, the consequent efficiencies will help make the economies a whole lot better.

The power split device in the Prius is the thing that diverts the power generated by the IC engine for driving the wheels or for charging the generator. This is like a continuously variable transmission in that it assures the optimal energy split.

In the hybrid transmission, the drive shaft from the engine is connected with the planetary gear carrier. The planetary pinion gears can simultaneously power the outer ring gear, which, in turn, directs power to the wheels, and to the sun gear, which drives the generator. The electricity from the generator can go directly to the motor to help drive the car or it can go through the inverter, where it is converted into direct current and is stored in the nickel-metal hydride (NiMH) batteries. (Hermance provides an amusing description of the battery pack: "It's the biggest Maglite in the world." There are what amounts to 240 D-cells used by the Prius. (Toyota has a battery joint venture with the parent company of Panasonic.))

Another source of power for the batteries is through what's called "regenerative braking." What this means is that when the brakes are applied (or when the driver lifts from the accelerator), the braking system turns the motor into a generator. So as the wheels on the car move, they turn this motor-turned-generator, which results in the generation of electricity, which then goes into the batteries for storage. In Japan, where the miles-per-gallon test mode includes a lot of heavy stop-and-go driving, the fuel economy is on the order of 66 miles per gallon, thanks in no small part to this regenerative braking capability. Hermance quips, "It gets better the worse the driving is." (Consequently, when the Prius arrives in the U.S., the miles per gallon provided will be more along the lines of 50 mpg—which is no small feat in and of itself, given that vehicle measures 168.3 in. long, 66.7 in. wide, 58.7 in. high, and has a wheelbase of 100.4 in.—about the size of a Corolla—but has the interior room approximate to a Camry.)

Making the Engine

Speaking of the engine manufacturing plant, Hermance observes, "There's nothing different there." As might be expected, there's an aluminum block. Although the engine is produced in the way that, well, an engine is typically produced, there are some product differences that should be noted as they affect the manufacturing processes.

 

This is the Toyota Hybrid system (THS) configuration. Depending on the operating conditions, the vehicle is powered by the internal combustion engine, the electric motor, or a combination of the two.

The engine uses what's known as the Atkinson cycle. In it, the expansion and compression strokes are independently set, resulting in high thermal efficiency. That leads to the ability to have small combustion chambers, which are slanted "squish-type" chambers that permit the ignition flame to quickly spread across the chamber, which enhances thermal efficiency. The THS engine also uses Toyota's Variable Valve Timing-intelligent feature, which is available on conventional IC products from the company; it continually varies intake valve timing.

The engine redlines at 4,000 rpm. Low-rev design means that many of the components, compared with conventional four-cylinder engines, are smaller and lighter (e.g., a small-diameter crankshaft; small intake manifold). Presumably, this means that there is comparatively less machining required on these smaller parts, which would have systemic effects throughout powertrain manufacture.

According to Hermance, one of the issues that they're working on is in the area of motor production for the vehicle. As 30-kW permanent magnet motors aren't the kind of thing ordinarily produced by Toyota, they're learning how they can get costs out of the process.

The Japanese Prius is available for about what translates to $16,000 U.S. (although the exchange rate may make that number fluctuate at a high velocity). Although some pundits point out that vehicles like the Prius are being priced artificially low by manufacturers for sociopolitical reasons, we can only think that if any company is able to make a competitively priced non-traditional vehicle, it has to be Toyota.