Espumantes

Based on vehicle architecture HEVs are classified as (Chan, 2007)

 Series HEV

 Parallel HEV

 Series-parallel HEV and

 Complex HEV

Figure 2-1: Common architecture of HEV (Chan, 2007)

Front

T

Rear

14 A detailed classification, including benefits and limitation of hybrids is found in Ehsani et al

(2004); a brief summary is given here.

2.2.3.1 Series HEV

In a series HEV, the vehicle is always driven by the electric motor like in a (pure) electric

vehicle. The IC engine is not coupled to drive the wheels directly. This gives the freedom to

operate the engine independent of the vehicle speed and only at the most efficient region.

The output from the engine is used to charge the battery or to provide the propulsive

power to the wheels through the motor. In PHEVs, as the engine is used only to extend the

range it is also called as range extender or range extended electric vehicle (REEV). Series

HEV is the simplest architecture as shown in Figure 2-1(a). In the figure, the electric

machine M (traction motor) can be operated both as motor and generator. The electric

generator G is primarily operated as generator and also can operate as motor to crank the

engine E. Single power plant to drive the wheels simplifies control strategy. However a big

electric machine M is required since it is the only source of the driven wheels. But this in

turn can make multi gear transmission unnecessary due to ideal torque – speed

characteristics of electric machine (Ehsani et al., 2004). This results in simpler architecture,

drivetrain control and easy packaging. Simple architecture and limited power plant control

options leads to the simplest control strategy.

In a series hybrid, the engine mechanical power is converted to electric power and then

again to mechanical power which introduces energy conversion losses. From the Figure 2-2,

minimum losses can be 25%; Efficiency of motor x battery charge x battery discharge x

Introduction to Hybrid Electric Vehicle and Energy Management System

15 Figure 2-2: Typical HEV efficiency (Miller et al., 2003)

Hence series hybrids are suitable as urban vehicles for short trip and not suitable for long

trip due to limited battery range and energy conversion losses. Series architecture is

commonly used in heavy vehicles, military vehicles and buses (Ehsani et al., 2007).

2.2.3.2 Parallel HEV

In a parallel HEV (Figure 2-1(b)) both the engine and the motor can directly drive the

wheels. Excess power from the engine is absorbed by the electric machine engaged in

generator mode to charge the battery. Also if the vehicle power demand is higher than the

engine output power, the electric motor can supply the difference, called as the motor

boost (or electric boost). Similarly use of supplementary engine power along with motor

output power is called the engine boost. Hence control strategy of parallel hybrids is more

complex than series as both power plants can drive the vehicle directly and also

simultaneously.

Unlike in a series HEV, the engine speed is dictated by the vehicle speed. Parallel hybrids

have comparatively smaller electric machine and battery than series HEV, which may

reduce the potential for regenerative braking. For long trips, parallel hybrids are suitable as

the engine can drive the vehicle directly without energy conversion losses which in turn

improves fuel economy. Passenger cars such as Honda Insight and Ford Escape use parallel

architecture.

Usually any EMS demonstration using parallel architecture is equally applicable to

16 majority of the researchers’ (Kutter and Baker, 2010, Wirasingha and Emadi, 2011, Tulpule

et al., 2009, Langari and Jong-Seob, 2005) including this author preferred parallel hybrids.

2.2.3.3 Series-parallel HEV

A series-parallel architecture (Figure 2-1(c)) incorporates the features of both series and

parallel HEV. Hence they combine the advantages of series and parallel architectures.

However they require an additional mechanical link in comparison to series HEV and also

an additional generator G in comparison to parallel HEV. Series-parallel HEVs are expensive

and complicated. In power-split hybrids, the vehicle behaves as a series or parallel hybrid

depending on the control action and planetary gears like in Toyota Prius. In regular series-

parallel hybrids it is achieved by engagement/disengagement of one or two clutches. The

salient feature of series-parallel over parallel HEV is the ability to operate the engine

partially independent of the vehicle speed. In this way, the engine speed can be adjusted to

its optimal region. In addition, in comparison to parallel architecture, the engine operation

can be less transient (Çagatay Bayindir et al., 2011). Series – parallel HEVs are commonly

used in passenger cars such as Toyota Prius and GM Volt.

2.2.3.4 Complex HEV

Vehicle architecture discussed so far are implemented for single axle propulsion; either

front or rear wheels. Complex hybrids are designed for dual axle propulsion (Kumar et al.,

2013). Complex hybrid architecture is similar to series-parallel. The key difference is in the

bidirectional power flow of the electric machine in the complex HEV and the unidirectional

power flow of the generator in the series-parallel HEV. The bidirectional power flow

feature gives additional flexibility in operating modes. Complex HEV is adopted in

production as shown in Figure 2-1(d). With three propulsion power (one engine and two

Introduction to Hybrid Electric Vehicle and Energy Management System

17 wheels which can significantly improve vehicle performance and fuel economy. However

like series-parallel, complex hybrids are complex in structure and expensive (Chan, 2007).

All architectures have advantages and disadvantages. In a study (Freyermuth et al., 2007)

considering series, parallel and power-split PHEVs over urban and extra urban driving

cycles; Urban driving dynamometer schedule (UDDS) and highway fuel economy driving

cycle (HWFET) was investigated. Power split architecture fuel economy was superior to

parallel over UDDS and marginally inferior over HWFET. For both driving cycles series

hybrid fuel economy was inferior to other architectures.