22 GEARSJuly2008

In April’s issue of GEARS, we reviewed basic safety procedures for working with hybrid vehicles.

This month we’ll look at the different kinds of hybrids on the market today from the technician’s point of view. Specifically, we’ll examine the power flow from a hybrid’s engine and elec-tric motor or motors, as it relates to the vehicle’s transaxle or transmission.

Hybrid powertrains are often described as full hybrid, mild hybrid, or strong hybrid. These are general terms used by the public, the media, and marketing departments. As there are no agreed definitions of such terms, it’s more practical for us to classify hybrids with logical and long-accepted engineering terms such as series, par-allel, or series-parallel, which refer to the power path or paths through the vehicle’s powertrain.

What is a Hybrid?A hybrid vehicle uses two different

kinds of stored energy to propel itself. A Toyota Prius, with its gasoline-fueled engine and battery-powered electric motors, is a hybrid; a vehicle with two engines or two fuel tanks isn’t. A con-ventional diesel-electric locomotive, which uses a diesel engine to rotate a generator that, in turn, powers electric traction motors, isn’t a hybrid, because it only uses one form of stored energy

(diesel fuel).There are three basic hybrid archi-

tectures: series, parallel and series-par-allel. These terms refer to the flow of power through the vehicle’s power-train.

Series HybridsIn a series hybrid, the flow of

power follows a single path, just as it does in a series circuit. An inter-nal-combustion engine (ICE) drives a generator to produce an electrical cur-rent that is used to charge the vehicle’s battery pack (figure 1). The engine-

Basic Hybrid Architectures by Jack Rosebro

of Perfect Sky

Figure 1: Concept view of Chevrolet Volt series hybrid, which uses a small engine to charge the battery pack that powers its electric drivetrain. The Volt is scheduled

for release in late 2010. Illustration courtesy General Motors Corporation.

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24 GEARSJuly2008

Figure 2: Overhead view of Honda Civic hybrid, showing location of powertrain components. Note that the vehicle’s electric motor is positioned between the engine and transaxle. Illustration courtesy American Honda Motor Company.

generator assembly is often referred to as a genset. The generator is also employed as a motor to start the engine. Electric machines such as this, which can produce either torque or charge, are referred to as motor-generators.

The battery pack supplies direct current (DC) to an inverter. The invert-er delivers an alternating current (AC) to the vehicle’s traction motor. AC motors are generally more efficient than DC motors, and almost all mass-produced hybrid and electric vehicles

use AC motors in their drivetrains. During deceleration, the traction motor produces its own alternating current, which the inverter rectifies back to direct current to charge the battery.

In a series hybrid, motive power has only one path to take through the drivetrain: genset to battery to inverter to traction motor, and back again from traction motor to inverter to battery. A series hybrid can roughly be described as an electric vehicle coupled to an onboard generator.

Although some buses and heavy-duty commercial vehicles are series hybrids, no mass-produced, light-duty hybrid uses a series powertrain today. However, it’s important to understand series hybrid operation before studying series-parallel hybrids. In addition, sev-eral automakers are developing series hybrids. The upcoming Chevrolet Volt is the first of a planned global line of series hybrids to be developed over the next decade as part of GM’s E-Flex platform.

Parallel HybridsA parallel hybrid uses a battery

pack to power an electric motor-gen-erator that propels the vehicle in par-allel with the power provided by the internal combustion engine (figure 2). The motor-generator can be positioned

between the vehicle’s engine and trans-mission, mounted inside the transmis-sion itself, or coupled to the engine with a belt drive.

A parallel hybrid can also be designed to power one axle with an electric motor, while using an inter-nal combustion engine to power the remaining axle. The motor can function as a generator during deceleration, idle, or cruise to maintain battery charge.

Honda’s Insight, Civic, and Accord hybrids each have a low-power electric motor-generator sandwiched between the engine and transmission. Some General Motors hybrids use a Belt Alternator Starter (BAS) system, in which the vehicle’s alternator can also function as a starter motor, using two belt tensioners and a specially con-structed serpentine belt.

Series-Parallel HybridsA series-parallel hybrid com-

bines the operational characteristics of both series and parallel hybrids. Many designs are capable of simultaneously dividing output between series and par-allel modes. Ford, Nissan, Lexus, and Toyota hybrids all use a series-parallel architecture.

Series-parallel hybrids differ from parallel hybrids in that they employ two motor-generators rather than one (fig-

Basic Hybrid Architectures

Although some buses and heavy-duty commercial vehicles are

series hybrids, no mass-produced, light-duty hybrid uses a series

powertrain today.

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26 GEARSJuly2008

ure 3). They differ from series hybrids in that their engine torque can contrib-ute directly to propelling the vehicle.

In a series-parallel hybrid, one motor-generator is permanently cou-pled to the differential and functions as a traction motor. A smaller motor-gen-erator can be used to start the engine, to charge the vehicle’s battery pack, or as a reactive member to enable overdrive. The motor-generators are connected to one another, as well as the vehicle’s engine, through a planetary gearset that is commonly called a power-split device, or PSD.

The interaction of various inputs to the power-split device produces an out-put with continuously variable gearing. Although today’s series-parallel hybrid transmissions and transaxles don’t use belt-and-cone assemblies, they are CVTs nonetheless.

Some series-parallel transaxles, such as the Aisin unit used in the Ford Escape hybrid and its variants, integrate power electronics assemblies such as an inverter in the transaxle case. Other designs require an externally-mounted inverter.

Two-Mode HybridA two-mode hybrid is a variation

of the series-parallel hybrid. Originally developed by General Motors for tran-sit buses, it’s used in the Chevy Tahoe and GMC Yukon hybrid SUVs. BMW, Chrysler and Mercedes-Benz are also developing two-mode hybrids.

The two-mode hybrid adds two planetary gearsets to a conventional series-parallel, power-split transaxle or transmission (figure 4). This expands the unit’s continuously variable gearing range by feeding output through any of four fixed gear ratios. This design is well suited to heavier vehicles that are rated for towing.

Plug-In HybridsAny of the three basic hybrid archi-

tectures can be used to make a plug-in hybrid (PHEV), as long as both battery pack and traction motor are powerful enough to propel the vehicle without assistance from an engine. The battery pack of a plug-in hybrid has much more storage capacity than a conventional hybrid battery pack, and can be charged by a readily available external source such as a 110-volt wall socket.

When fully charged, a typical plug-in hybrid will use a charge-depleting (CD) strategy to propel the vehicle on battery power alone until the battery’s

state of charge (SOC) drops to a pre-determined level. At that point, the vehicle’s engine will start, and the sys-tem will revert to a charge-sustaining (CS) strategy, much as conventional hybrids do.

Although no major manufacturer currently produces a plug-in hybrid, PHEVs are under development by Ford, General Motors, Toyota, and several other manufacturers. Plug-in hybrids are expected to be on sale by 2010.

“Microhybrids”Virtually all production hybrids

employ an idle start-stop strategy that can turn the engine off when the vehicle is at rest as well as restart it on demand within a few tenths of a second. But this feature isn’t exclusive to hybrids. It’s been adapted to many vehicles in the European market, and can be expected to show up in US-market vehicles with-in the next few years.

Although such vehicles are often described as “microhybrids,” their only motive power is a conventional inter-nal-combustion engine. A microhybrid typically uses a high-power starter motor mounted in line with the engine’s crankshaft, or coupled to the crankshaft with a belt drive.

Figure 3: Schematic of Toyota Prius hybrid transaxle components, showing two motor-generators and a planetary gearset, connected to drive axles via a chain. Engine is at right. Illustration courtesy Toyota Motor Corporation.

Basic Hybrid Architectures

GEARS July 2008 27

Further StudyHybrid powertrains are complex

systems. Studying their operation takes time and dedication. OEM service information web sites can be excellent sources of information on hybrids.

Current and proposed emissions and fuel economy regulations are driv-ing more and more automakers toward hybrid powertrains. It’s only a matter of time before these vehicles begin to show up in your service bay.

Jack Rosebro has taught hybrid and plug-in hybrid technology to independent, fleet, government and dealership technicians in five countries. His company, Perfect Sky, is focused on advanced powertrain technology training and curriculum development, and is based in Santa Barbara, California.

Figure 4: Architecture of a two-mode transmission, showing two motor-generators and three planetary gearsets. illustration courtesy General Motors Corporation.