ECEN 2060 Hybrid and Electric Vehicles

3. Series Hybrid Electric Drive Train Figure 3.1 shows configuration of a simple series hybrid electric drive train. Although details are significantly more complicated, Chevrolet Volt is an example of a plug-in hybrid electric vehicle (PHEV) based on the series hybrid architecture. v Fv n1 T 1 Fuel ICE

n2 T 2

VDC

Electric motor/ generator 1

3-phase inverter/ rectifier 1

ED1 Battery charging (alternator)

3-phase inverter/ rectifier 2

Battery Energy storage

Electric motor/ generator 2

nv T v Transmission

ED2 Traction Regenerative braking

ICE starting

Wheels (radius rv)

Figure 3.1. Series hybrid electric drive train architecture. In the Chevy Volt example, the ICE is a 62 kW (83 hp), 1.4 L engine, ED1 is a 55 kW electric drive, ED2 is a 111 kW (149 hp) electric drive, and the battery has a total of 16 kWh capacity out of which 65% (10.4 kWh) is usable. After the battery is fully charged from the electric grid, Volt operates as an electric vehicle with an all-electric range of 40-80 km (25-50 miles). After that, the ICE is engaged to maintain the battery state of charge and Volt operates as a hybrid electric vehicle.

In the series architecture, all system components are connected in series: ICE, electric drive 1 (ED1), battery, electric drive 2 (ED2), and transmission to wheels. Main functions of the system components are as follows: 

The vehicle is propelled by the electric drive 2 (ED2) consisting of the 3-phase inverter/rectifier 2 and the electric motor/generator 2. In the traction mode, at the input of the inverter 2, power comes from the battery and/or the ICE via generator 1 and rectifier 1. From the DC battery voltage VDC, inverter 2 generates variablefrequency 3-phase voltages and currents for motor 2. Motor 2 shaft turns at n2 rpm and produces traction torque T2. A single-gear transmission turns the wheels at nv rpm with torque Tv. The resulting traction force Fv 

Tv rv

(3.1)

propels the vehicle at speed v, where rv is the wheel radius. The traction power propelling the vehicle forward is Pv = Fv v = Tv nv (2/60). The mechanical power at the output of generator 2 is Pm 2  T2 n 2 2 / 60  

Pv

t

(3.2)

where t is the transmission efficiency. Electrical power supplied by the battery and/or rectifier 1 is P2 

Pm 2

 i 2 m 2

where i2 is the efficiency of inverter 2, and m2 is the efficiency of motor 2.

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(3.3)

ECEN 2060 Hybrid and Electric Vehicles



When negative traction power is required, i.e. when the vehicle is decelerating or descending, the power flows in the opposite direction: from the wheels back to the battery. This mode of operation is called regenerative braking.



In the hybrid electric mode, generator 1 takes mechanical Pice = T1 n1 (2/60) and via rectifier 1 produces electrical power P1

power

P1   g1 r1 Pice ,

(3.4)

where g1 and r1 are the efficiencies of generator 1 and rectifier 1, respectively. Electrical power P1 charges the battery and/or supplies a part of or the entire traction power P2. Electric drive 1 (ED1) serves two main functions: controls the battery state of charge (SOC), and sets the operating point of ICE. Note the ICE operating point can be set independent of the traction power requirement. Hence, it is possible to improve efficiency by setting the ICE operating point close to its maximum efficiency point. 

ED1 also serves as an ICE starter. During ICE start-up, power flows in the opposite direction, from battery via inverter 1 and motor 1 to the ICE crankshaft.

A system controller in the series hybrid electric drive train has two main objectives: 1. Control traction and braking based on the driver command. This is accomplished by electronically controlling inverter 2 to deliver the requested torque T2 at the output of motor 2. Upon braking command, ED2 performs regenerative braking as long as the requested braking torque and power are within the capabilities of electric drive 2, and as long as the battery state of charge (SOC) is below an upper limit (SOC)max. Any excess braking is performed by conventional mechanical (hydraulic) brakes. 2. Control battery state of charge so that it remains between target limits ( SOC ) min  SOC  ( SOC ) max .

(3.5)

The limits (SOC)max and (SOC)max are system design parameters, which are decided based on battery size, cost, and battery life trade-offs. This will be discussed in more detail in the section on batteries. The battery SOC control is accomplished via ED1 and ICE. One simple SOC control strategy is to turn ICE on and operate it at the maximum efficiency point whenever SOC drops to (SOC)min, and then turn ICE off whenever SOC reaches (SOC)max. 3.1 Sizing of series hybrid electric drive components This section discusses basic ideas behind sizing components in the series hybrid electric drive, i.e. selecting the power ratings for ICE, ED1 and ED2, as well as the power rating and the energy storage capacity for the battery. To illustrate the ideas, a numerical example is considered in this section based on Example 1.1. Vehicle parameters are as follows: 

Vehicle mass Mv = 1500 kg

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

Front area Av = 2.16 m2



Aerodynamic drag coefficient Cd = 0.26



Tire rolling resistance frr = 0.01.



Wheel radius rv = 0.3 m

The performance objectives are: 

Acceleration time ta = 11 s from 0 to vf = 100 km/h (60 mph).



Maximum speed vmax = 160 km/h (100 mph).



Maximum continuous cruising speed vcmax = 130 km/h (80 mph).



Gradeability: 5% at 100 km/h (60 mph)

Electric motors have a maximum speed n = 5000 rpm, and a maximum to base speed ratio x = nmax/nb = 5. Transmission efficiency is t = 90%. Efficiencies of electric motor/generators and inverter/rectifiers are assumed to be 95% each. Sizing of ED2 components In the series hybrid of Fig. 1, the entire traction power must be provided by ED2. Therefore, motor/generator 2 must be sized according to the maximum traction power requirement, which follows from the acceleration performance specification. In the considered numerical example, assuming single-gear transmission efficiency of t = 0.9, the required power rating for the motor/generator 2 has been found in Example 1.1 to equal 71.5 kW. The US06 driving cycle test resulted in the maximum power requirement of 71.5/0.9 = 79.4 kW. We select Pmg 2  80 kW (107 hp).

(3.6)

The maximum speed of the motor/generator 2 (n2max = 5000 rpm) should match the Since maximum vehicle speed (vmax = 160 km/h = 44.4 m/s). nvmax = (30/)(vmax/rv) = 1413 rpm, the required gear ratio is 5000/1413 = 3.54. The motor/generator 2 base speed is n2b = n2max/x = 1000 rpm. The maximum torque T2max that the motor/generator 2 should produce for 0 < n2 < 1000 rpm is T2 max  (30 /  )

Pmg 2 n2b

 764 Nm .

(3.7)

Inverter/rectifier 2 should be rated at Pmg2/g2, which assuming 95% efficient motor/generator, gives the required inverter/rectifier power rating, Pir 2  (80 kW)/0.95  84 kW .

(3.8)

Sizing of ICE The internal combustion engine does not need to supply the maximum traction power. Instead, ICE should be sized so that the vehicle can meet the maximum (continuous) CU Boulder, DM Fall 2010

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ECEN 2060 Hybrid and Electric Vehicles

cruising speed and the gradeability performance requirements. Using (1.2), the maximum cruising speed vcmax of 80 mph requires traction power of Pv = 20.7 kW. The gradeability (5% at 60 mph) requires Pv = 30.2 kW. Taking the larger of the two, and taking into account the assumed 95% efficiency for the series components (generator 1, rectifier 1, inverter 2, motor 2, transmission), the required ICE power rating is: PICE  (30.2 kW)/(0.9  0.95 4 )  41 kW (55 hp).

(3.9)

Note that the required ICE power rating is significantly lower than the maximum required traction power. The Chevy Volt example (see Fig. 3.1) also illustrates this point. The engine “downsizing” is one of the HEV advantages. It should also be noted that, even though each component in series is assumed to have relatively high efficiency, the cumulative effect of losses in the series HEV drive results in relatively significant increase in the required ICE power rating (from about 30 kW to about 40 kW in this example). This is considered one of disadvantages of the series HEV drive train. Alternative parallel or parallel/series HEV drive configurations will be discussed in a later section. Sizing of ED1 ED1 components are sized based on the ICE power rating, Pmg1 = mg1PICE  PICE, Pir1 = mg1ir1PICE  PICE. HEV Battery sizing In a hybrid electric vehicle (HEV), battery ratings include a power rating Pbat, i.e. the ability of the battery to supply power Pbat while keeping the output voltage VDC above a minimum threshold, and an energy capacity rating Ebat based on the ability to supply (or absorb) energy while its state of charge SOC remains within the limits (SOC)min and (SOC)max. Charging or discharging of the battery depends on the driving cycle and the system control strategy. A simple example is considered here assuming the US06 driving cycle and the same test vehicle as before. The battery power Pb equals the difference between the power P1 supplied by ICE via ED1, and the traction power P2 delivered to the wheels via ED2 and transmission, Pb  P1  P2 .

(3.10)

Suppose that the system controller adjusts ICE to provide the average required power, i.e. suppose that P1 = (P2)avg. With the driving cycle starting at t = 0, and ending at t = ttrip, the total energy absorbed by the battery is then ttrip

Eb (t trip ) 

ttrip

 P  P d   P  1

0

2

2 avg



 P2 d 0 ,

(3.11)

0

neglecting electric drive losses. Under the assumptions stated above, the net change in energy stored over the entire driving cycle equals zero. This means that the battery state of charge SOC(ttrip) equals the state of charge SOC(0) at the beginning of the trip. During the driving cycle, however, SOC changes in time as

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ECEN 2060 Hybrid and Electric Vehicles

 E b (t ) 1 SOC (t )  SOC (0)   SOC (0)  Ebat Ebat

 P  t

2 avg



 P2 d .

(3.12)

0

where Ebat is the battery rated energy storage capacity. From (3.12), the maximum change in SOC over the driving cycle is SOC max 

Eb max  Eb min . Ebat

(3.13)

Given a battery specification SOCmax, (3.13) can be used to find the required energy storage capacity Ebat,

Ebat 

Eb max  Eb min . SOC max

(3.14)

Fig. 3.2 shows the waveforms obtained for the test vehicle in the US06 driving cycle: the vehicle speed v, the required vehicle traction power Pv, and the change in energy Eb(t), neglecting electric drive losses. In this case, Ebmax – Ebmin is found to be about 0.5 kWh. Assuming SOCmax = 30% is allowed by the battery system design to ensure sufficiently long battery life, (3.14) gives the required battery capacity rating (in kWh), Ebat 

Eb max  Eb min 0.5  kWh  1.7 kWh . SOC max 0.3

(3.15)

It is interesting to note that this simplified example gives a battery rating requirement close to the actual battery energy storage capacity in Toyota Prius. This battery capacity (3.15) is worth only 1.7 kWh/(12.7 kWh/kg) = 134 g of gasoline. The fact that the battery only supplies the difference between the ICE power and the traction power results in very modest battery size requirements. Importantly, HEVs can achieve sizable efficiency and therefore fuel economy improvements with relatively small batteries. It is instructive at this point to briefly examine the battery capacity requirements in a pure battery electric vehicle (EV), or plug-in HEV (PHEV). Battery sizing in EV or PHEV vehicle In a pure electric vehicle, ICE is not available, and the battery must provide the total traction energy. A block diagram of a pure battery electric vehicle (EV) is the same as the series HEV in Fig. 3.1, except that ICE and ED1 are not present, and a way of charging the battery from the electric grid would be provided. Considering the same test vehicle in the US06 driving cycle, we find that the total traction energy required over the entire cycle is Etrip = 1.2 kWh. The trip distance is ltrip = 8 miles, which means that Etrip/ltrip = 0.15 kWh/mile of traction energy is required. Assuming that the entire traction energy is supplied by the battery, assuming the same SOCmax specification (30%), and neglecting losses in the electric drive train, the vehicle with the battery capacity found in (3.13) would have an all-electric driving range lrange of only

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lrange 

SOC max Ebat 0.5  miles  3.3 miles ( Etrip / ltrip ) 0.15

(3.16)

before the battery reaches the minimum SOC. From (3.16) it is easy to see that the range of a pure electric vehicle could be improved by deeper discharge cycles, i.e. by allowing a larger SOCmax (which would however result in reduced battery life), or by increasing the battery capacity Ebat. Very similar considerations hold for plug-in electric vehicles (PHEV) that are expected to operate as pure electric vehicles over a limited range of X miles (PHEVX), and then have an extended driving range as needed operating as HEV, with the battery recharged from an ICE. In the example above, a PHEV20, i.e. a plug-in hybrid with 20 miles of all-electric range would require about 20/3.3 = 6 times larger battery storage compared to the HEV. The Chevy Volt, which is considered a PHEV40, i.e. a plug-in hybrid electric vehicle with about 40 miles of electric range, has a 16 kWh battery with about 10 kWh of usable energy storage (SOCmax = 65%), which means that it is expected to spend about 0.25 kWh per mile.

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ECEN 2060 Hybrid and Electric Vehicles

100

v [mph]

80 60 40 20 0

0

100

200

300 time [s]

400

500

600

0

100

200

300 time [s]

400

500

600

0

100

200

300 time [s]

400

500

600

100

Pv [kW]

50

0

-50

0.6

E [kWh]

0.4 0.2 0 -0.2

Figure 3.2 Vehicle speed v, traction power Pv, and the required change in stored energy Eb in the series HEV example neglecting electrical drive losses and assuming that the ICE supplies the average traction power at all times.

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