"Hybrid vehicles" is a term that covers a wide spectrum of design approaches. Even in its more restricted usage, lately, to mean just gasoline-electric hybrids, it’s not a single type of vehicle. Approaches can be broadly characterized in terms of the relative amount of drive power supplied by electric motors, vs. driveshaft from the IC engine.

On one end of the spectrum are vehicles with integrated starter-alternators that, in terms of the drive train, do little more than restart the engine when the vehicle accelerates from a stop. They can augment engine torque at low speeds, but lack the power to contribute significantly at higher speeds. They provide limited regeneration capability, when the vehicle is braking lightly under engine compression, but aren’t designed to handle the power that would be generated by heavy braking from speed. Because the SA, in this configuration, is connected to the engine flywheel, it cannot function as either a motor or generator unless the engine is turning. There is no way to operate the moving vehicle on electrical power only. This limits the potential for improving fuel economy and reducing emissions to the "ultra-low" range. However, the integrated SA’s ability to boost bottom end torque does allow the engine and gear ratios to be re-tuned for performance toward the top end. An SA-augmented four-cylinder engine can feel like a V-6. Exploiting this feature improves fuel economy beyond the estimated 11% that comes from elimination of idling.

It should be noted that vehicles of this general type are a solid part of the conservative roadmap. Models are on the drawing boards at all of the major auto companies, and will begin to appear in showrooms within the next few years. The transition is being driven, not by any overwhelming consumer demand for more efficient vehicles, but by a combination of factors. Electric power steering and power brakes are not only more efficient, but have lower maintenance and will ultimately be cheaper to manufacture than belt-driven hydraulics. They require a beefier electical system, and an integrated starter-alternator is better engineering solution than a conventional starter motor and more powerful belt-driven alternator. The fact that it's powerful enough to eliminate idling and boost low-end torque is not incidental; it's part of the package that makes it a better engineering solution. But cost and reliability are also part of the package. Fuel economy and reduced emissions are in there too, but they aren't predominant.

Not surprisingly, the first models to hit the showrooms will be SUVs. They will be marketed as hybrids, and sold initially at a premium over the current generation of "conventional" vehicles. However, in the long run, the switch from hydraulic and belt-driven systems to electrically driven systems is expected to reduce manufacturing and vehicle maintenance costs.

All-Electric Drive Train

On the other end of the hybrid vehicle spectrum are designs in which electric motors supply all traction power to the wheels. Electric drive replaces the mechanical components of a conventional drive train: clutch, transmission, differential, and main drive shaft. The only function of the IC engine, if present, is to generate electrical power.

This type of drive train is, of course, required for battery and fuel cell EVs. However, it may be desirable for IC-powered vehicles as well. If the electric drive train components become cheaper than the mechanical components replaced, and prove equally or more reliable, then they will win, hands down. That’s not presently the case, however. A premium cost would currently have to be paid for an all-electric drive train. The question then is how much of a premium is justified by the advantages such a system can offer?

The advantages are significant. Even when battery power transfer and storage capabilities are minimal, an all-electric drive functions as a form of continuously variable transmission (CVT) between the IC engine and the wheels. It yields benefits in performance, fuel economy, and emissions that are associated with CVT operation. When battery power transfer and storage capabilities are uprated, additional advantages follow. The engine can be reduced in size and designed to operate only full on or full off. This binary mode of operation makes the engine very simple, and enables it to avoid a great many design compromises that limit conventional auto engines. It should allow emission standards to be met without a catalytic converter, or with a simpler and cheaper form of converter.

The chief obstacle to this approach is the size and cost of the power controllers required. For lively performance, a mid-size sedan requires 150 kW of power (200 HP) to the drive system. The power electronics required to deliver that much power in precisely controlled pulses are expensive. As a point of reference, the AC150 electric drive system is currently marketed by a company called AC Propulsion, in San Dimas, California. Consisting of a 150 kW power controller and a co-designed 200 HP, 3-phase AC induction motor, it sells for approximately $30,000. AC150 units are custom-built as orders are received, so their cost is not very indicative of what production units for the auto industry would run. On the other hand, a serious fraction of their cost is in the raw BOM for power components. A 50% cost reduction for the same design in mass production is very likely; a 66% reduction is possible; an 80% reduction, however, seems unlikely. Driving the cost of a 150 kW drive system below $5000 will require new technology, or at least a new design approach.

Many of the advantages that are offered by the all-electric drive train can be achieved in design approaches that still rely on the IC engine drive shaft to supply most of the vehicle’s traction power, most of the time. To achieve those advantages, electric motor-generators of substantially more power than those employed in "mild" hybrid designs are required, but they remain smaller and less powerful than those required for an all-electric drive train.

The Toyota Prius is the preeminent example of this approach. It uses two independent motor-generators, with a combined power of 44 HP (30 kW). One unit is coupled directly to the drive shaft. The other operates in tandem with the IC engine. The engine and the second motor-generator are yoked through a planetary differential, which delivers a ratioed sum of their rotations to the drive shaft. The combination of IC engine, planetary differential, and two motor-generators allow Toyota to implement a continuously variable electric transmission, with no clutch and no transmission gearbox.

Its operation is described and illustrated with an animated figure on the Toyota Prius page on the How Stuff Works web site.

By varying the speed and load of the tandem motor-generator, any effective gear ratio between the IC engine and the drive wheels can be achieved. That includes an infinite ratio (engine stopped with vehicle moving) and negative ratios (vehicle in reverse). In practice, this effective gear ratio is continuously adjusted according to the vehicle speed, accelerator setting, and battery charge state. The objective is to regulate power, as much as possible, through engine speed, rather than throttling. When the power demand is too low for running the engine at full throttle, the engine is stopped, and the vehicle runs solely on electric drive. This approach helps to maximize fuel efficiency, and minimize emissions.

When the Prius was first introduced, much of its hybrid drive system had to be custom built in small lots. To seed the market, Toyota was offering the vehicle at a price well below its initial production cost. As new manufacturing facilities for hybrid drive components have come on line, production cost has dropped. The price to consumers has not changed, but Toyota recently announced that it no longer subsidizes production. It now makes enough profit on each Prius sold to support the line, and is even expanding it with new hybrid vehicle models. It has also announced an agreement to supply hybrid drive components to Nissan. By 2010, it expects to have converted all of its product lines to hybrid drive.

Another cost-efficient hybrid configuration has been termed a "through the road" configuration. It starts with a conventional two-wheel drive train, and adds electric drive on the other two wheels. The separate systems pull together, when extra power or extra road traction are needed. When power demand is low—at low speeds or low accelerator settings—the electric drive operates alone. The drive from the IC engine is disengaged, through an automatic clutch or automatic shift to neutral, and the IC engine is stopped.

The batteries for the electric drive system in this configuration are regeneratively charged when the vehicle brakes. If normal braking does not supply enough energy to keep them charged, the batteries can be recharged by light braking on the electrically driven wheels, while the IC engine drives the other pair of wheels. This is the "through the road" power transmission that gives the configuration its name.

The appeal of this approach is that it offers the road traction advantages of four-wheel drive and many of the fuel-economy and emission benefits of hybrid drive, at a cost that is below that of a conventional four-wheel drive. The electric drive component need not be particularly powerful. In nearly all situations where four-wheel traction is important, the vehicle speed is very low. It does not take a lot of power for an electric motor to deliver high torque at low speed.

In the purest and simplest "through the road" hybrid configuration, the motor-generator functions as the vehicle’s starter-alternator. That means that the vehicle starts out under battery-supplied electric power going to two wheels, and with the automatic clutch in. The engine is started by letting out the clutch when the vehicle is moving. The inevitable roughness of this starting mode and the difficulties it creates for starting on slippery roads make this "pure" through-the-road configuration unlikely to be used in production.

In practice, the "through the road" configuration is likely to be combined with the "mild" hybrid configuration on the drive sub-train of the IC engine. I.e., the engine will be equipped with its own starter-alternator. That means that it will not be necessary to rely on power transmission between the front and rear wheels for battery charging and engine starting. Handling and road feel should be smoother and more secure. With two separate motor-generators, this configuration somewhat resembles the "balanced" hybrid configuration used by Toyota, but without the inherent CVT functionality. It has the advantage that the motor-generators need not be as powerful, but the disadvantage that an automatic clutch and gearbox are still needed.