Emissions and fuel economy for a hybrid vehicle

Fuel 115 (2014) 812–817

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Emissions and fuel economy for a hybrid vehicle Imdat Taymaz ⇑, Merthan Benli Department of Mechanical Engineering, University of Sakarya, 54187 Adapazari, Turkey

h i g h l i g h t s  The simulation code developed can be used a research or design tool for the evaluation of mixed hybrid electric vehicles.  All mixed hybrid vehicles are more economical than the conventional vehicles although they produce almost the same power.  CO2 emission is theoretically reduced nearly 30% by mixed hybrid electrical vehicles.

a r t i c l e

i n f o

Article history: Received 26 February 2013 Received in revised form 16 April 2013 Accepted 18 April 2013 Available online 9 May 2013 Keywords: Hybrid vehicle Vehicle performance Fuel consumption CO2 emissions

a b s t r a c t Conventional vehicles play a big role in city transportation all over the world. These vehicles run on energy obtained from fossils fuels such as petroleum oils that pollute environment with the gases that are emitted after burning. In addition, the cost of this fuel type will increase because of decreasing reserves; therefore, these petroleum oils must be used very efficiently. Due to environmental and financial problems, the development of clear and efficient city transportation has accelerated. Hence, hybrid electrical vehicles gain significant importance because they are environmentally friendly and efficient in fuel usage. In this study, a conventional commercial vehicle was chosen for design to a mixed hybrid systems. A simulation program was created for road simulation of these vehicles and with acceleration included; the consumption and emission values were also approximately calculated. As a result, it was seen that the mixed hybrid vehicles possess the same performance values with low fuel and CO2 emission. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction With the increased interest in global environmental issues in recent years, the demands on technology to reduce automobile fuel consumption have further increased. Attention in the area of vehicle bodies is currently focused on reducing size and weight. In the area of the engines, efforts are underway to develop direct injection gasoline and diesel engine technologies to improve fuel efficiency. In the entire field of vehicle technology, hybrid systems are attracting attention, and some hybrid vehicles have already been released to the market [1]. A hybrid vehicle is a vehicle which uses two or more kinds of propulsion. Most hybrid vehicles use a conventional gasoline engine as well as an electric motor to provide power to the vehicle. An important characteristic of hybrid electrical vehicles is the influence of low ambient temperatures on fuel consumption and pollutant and CO2 emissions [2]. The definite advantages of the hybrid electrical vehicles are to extend greatly the original electrical vehicle driving range by two to four times and to offer rapid refueling of liquid gasoline or diesel [3]. ⇑ Corresponding author. Tel.: +90 2642955861; fax: +90 2842955601. E-mail addresses: [email protected] (I. Taymaz), [email protected] (M. Benli). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.04.045

Nowadays some manufacturers produce new vehicles for using a hybrid system but some of the other manufacturers transform their current models into hybrid system. However, some problems have to be solved during transformation. One of the most important problems is the design of the controller. For the design of the controller, vehicle simulation must be done. Controller design could be changed according to the used engine type and battery type in the simulation. Vehicle characteristics can be seen in different compositions. The most efficient design can be created only by this method [4]. A search of the recent literature reveals that a number of computer software simulations are available specifically for hybrid electrical vehicles. These simulation tools have varying abilities to predict vehicle performance in one or more areas, such as fuel economy, emissions, acceleration, and grade sustainability. Some of the more prominent tools are Matlab/Simulink based The Advanced Vehicle Simulator (ADVISOR) [5], A Hybrid Vehicle Evaluation Code (HVEC) [6], CarSim [7], SIMPLEV [8], CSM HEV [9], Elph/V-Elph [10]. In this study, a simulation program was created for road simulation of the vehicles and using different amounts of traffic flow information are compared in terms of fuel economy and CO2 emissions over common city and motorway drive cycles. In

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813

Nomenclature n x g m cw A

qa qya Df gear(x) Drim Wwheel twheel Pelec(n) Pice(n) Telec(n) Tice(n)

engine speed (rpm) gear position gravity, 9.81 m/s2 vehicle mass (kg) coefficient of air friction front area of the vehicle (m2) air density, 1.2 kg/m3 density of fuel (kg/m3) differential gear gear ratio diameter of rim (inch) wideness of wheel (mm) thickness of wheel (mm) power of electrical engine (HP) power of ICE (HP) torque of electrical engine (Nm) torque of ICE (Nm)

order to develop a fair comparison between the technologies conventional vehicles and hybrid vehicles that matches the performance characteristics of the baseline intelligent vehicle is used. Furthermore, the values are compared with the conventional vehicle values. The flow chart of the simulation program is shown in Fig. 1. The road simulations show that the new designed mixed hybrid vehicles are as fast as conventional vehicles and mixed hybrid vehicles have low fuel and CO2 emission values than conventional ones. Low volume internal combustion engine has an equal or better performance than the conventional vehicle and the electrical motor.

2. Hybrid electrical vehicles In hybrid electrical vehicles, there are more than two energy sources. Mostly, one of them is electrical energy and the other is fossil energy. There exist three mainly types of hybrid systems in the market: series, parallel and mixed type. In series hybrid drive systems, there are no mechanical connections between the internal combustion engine and the wheels. First, all thermal energy is converted into mechanical energy in a thermal engine and then converted into electrical energy by a generator driven by the thermal engine. Additionally, there is an electric traction motor to drive the wheels. Hence a decoupling of energy source operation from the required traction power is possible. In most cases, the energy source, also called auxiliary power unit (APU), will act as base power unit delivering power to the battery or directly to the electric traction motor. The battery acts as peak power unit or energy buffer while driving. The series hybrid has the advantage of operating a thermal engine in a selected optimal operating field, for instance, with low specific fuel consumption in the torque-speed operating area but it must be equipped with big-size batter pack and generator [11]. In parallel hybrid drive systems, internal combustion engine and electrical engine are both connected to the wheels. The electrical engine may also be used as a generator to charge the battery by either regenerative braking or absorbing excess power from the internal combustion engine when its output is greater than that required to move the wheels [12]. One advantage of the parallel hybrid electrical vehicle over the series type is that parallel type requires a smaller internal combustion engine and electrical engine to provide similar performance, but the internal combustion engine works in variational cycles similar to the conventional

BSFC(n) specific fuel consumption Ct CO2 emission, gr a slope (°) Vw wind speed, km/h Rwheel radius of wheel, m Cwheel circumference of wheel, m V(n) velocity of vehicle, km/h P(n) total power, HP T(n) total torque, Nm Ft(n) total force at wheels, N Ff(n) force of wheel friction, N Fa(n) force of air friction, N Fc(n) climbing force, N Fnet(n) net force at wheels, N a(n) acceleration, m/s2 th(n) acceleration at a moment, m/s2 FC(n) fuel consumption at a moment, lt

vehicles; therefore, fuel consumption is higher than the series hybrid system [13]. Mixed hybrid system, shown in Fig. 2, has advantages and disadvantages of series and parallel hybrid systems. High efficiency can be obtained in a mixed hybrid vehicle, a concept that combines both the series and parallel systems. The internal combustion engine power can be used for both vehicle and electric alternator drives [14]. The advantage of this system is obtained from the optimized internal combustion engine operating strategy. Generally, operating the internal combustion engine under low load causes inevitably bad efficiencies, especially for gasoline engines. Such engine map areas can be avoided by increasing the load and resulting the power output. The excess power can be used for powering the electrical engine and charging the batteries [15]. 3. Vehicle subsystems and models For this purpose, two mixed hybrid conventional commercial vehicle models with identical electrical equipment but different internal combustion engines were designed. And they were compared with two conventional commercial vehicles. Conventional vehicles have five gear ratios but hybrid vehicles have three gear ratios because of their higher power. These vehicles parameters used in this study are listed in Table 1. 3.1. Vehicle main parameters Radius of wheel, Rwheel described as follows:

Rwheel ¼

  2P W wheel  t wheel  þ Drim  1:27 100 1000

ð1Þ

where Wwheel is wideness of wheel, twheel is thickness of wheel, Drim is diameter of rim. Then circumference of wheel Cwheel is can be evaluated by:

C wheel ¼ 2  P  Rwheel

ð2Þ

Speed of the vehicle V(n) can be evaluated by:

VðnÞ ¼

3:6  n  C wheel 60  Df  gearðxÞ

ð3Þ

In this equation, Df is differential gear ratio, x is gear position and n is motor speed.

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START

ADJUSTING PARAMETRES

READ NEXT SPEED DEMAND FROM Fİ LE

READ COMPONENT POWER OUTPUTS

CALCULATING PERFORMANCE OF VEHICLE

CALCULATING FUEL CONSUMPTION OF VEHICLE

CALCULATING EXHAUST EMISSION OF VEHICLE

FIND NEXT COMPONENT

NO

FINAL COMPONENT IN DRIVE-TRAIN? YES NO LAST STEP IN DRIVE CYCLE? YES OUTPUT RESULTS

END Fig. 1. Flow chart of simulation program.

Table 1 Properties of hybrid and conventional vehicles.

Type

Fuel Tank

Ice

Generator (EE2)

Transmission

Elec Eng (EE1)

Batteries

Mechanical connection Electrical connection Fig. 2. Mixed hybrid drive system.

Fuel IC engine Max power (HP) Max torque (Nm) Total weight (kg) Frontal area (m2) Air friction coefficient Differential ratio 1. Gear ratio 2. Gear ratio 3. Gear ratio 4. Gear ratio 5. Gear ratio

A

B

C

D

Mixed hybrid Gasoline 1.2 l 8 V 60 102 1450 2.3 0.36

Conventional

Conventional

Gasoline 1.6 l 16 V 100 148 1245 2.3 0.36

Mixed hybrid Diesel 1.3 l 8 V 70 180 1550 2.3 0.36

Diesel 1.9 l 16 V 105 200 1310 2.3 0.36

4.2 2.3 1.4 0.72 – –

3.867 3.90 2.15 1.48 1.12 0.89

4.2 2.3 1.4 0.72 – –

3.93 4.27 2.24 1.44 1.03 0.77

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3.2. Vehicle dynamic model

Table 2 Properties of electrical engines.

All the vehicles are considered like a moving mass subjected to the total force for traction Ft(n). The road loads include, force of wheel friction Ff(n), force of air friction Fa(n), climbing force Fc(n). These forces can be described as follows:

F t ðnÞ ¼

TðnÞ  gearðxÞ  Df R

ð4Þ

   VðnÞ F f ðnÞ ¼ 0:01  m  g  1 þ 160

ð5Þ

F a ðnÞ ¼ 0:0386  qa  cw  A  ðVðnÞ þ V w Þ2

ð6Þ

F c ðnÞ ¼ m  g  sinðaÞ

ð7Þ

F net ðnÞ ¼ F t ðnÞ  F f ðnÞ  F a ðnÞ  F c ðnÞ

ð8Þ

where Vw is the speed of wind, vehicle mass is represented by m, g is gravitational acceleration constant, cw is air friction coefficient, a is slope of the road, qa is air density and A is the front area of the vehicle. 3.3. Internal combustion engines (ICEs) In conventional commercial vehicles, 1.6 l gasoline and 1.9 l diesel engines were used. The power and the torque diagrams of these engines are shown in Figs. 3 and 4, respectively. 1.6 l gasoline ICE is four-cylinders gasoline engine with the rated power of 100 HP at 5750 rpm and the peak torque of 148 Nm at 4000 rpm. 1.9 l diesel engine produces 105 HP at 4000 rpm and 200 Nm at 1750 rpm. By downsizing in mixed hybrid electrical vehicles, these engines give their place to 1.2 l gasoline engine with the rated power 60 HP at 5500 rpm and 102 Nm at 3500 rpm and 1.3 l diesel engine with the rated power 70 HP at 4000 rpm and 180 Nm at 1750 rpm.

120

Power (HP)

EE1

EE2

Highest power Highest torque Nominal power Nominal torque Maximum rpm Weight

67 HP 240 Nm 41 HP 140 Nm 8000 d/d 40 kg

40 HP 140 Nm 24 HP 80 Nm 8000 d/d 29 kg

Table 3 Properties of batteries. Batteries

Lithium-ion

Nominal voltage Energy density Power density Current Volume Weight

3.6 V 310 Wh/lt 880 W/lt 100 A 0.51 lt 1.07 kg

3.4. Electrical engines These mixed hybrid electrical vehicles include two electrical engines which are the EE1 and the EE2. The EE1 is a traction motor and the EE2 is a generator. The EE1 exploits the principle of both electrically propulsion and regenerating the brake energy. The EE1, which sometimes drives the vehicle alone, must be more powerful because it must produce more power and torque than the vehicle needs in a city cycle. The specifications of two electrical engines are listed in Table 2. 3.5. Batteries For mixed hybrid electrical vehicles, lithium-ion battery type is chosen. In the battery pack, a hundred (1 0 0) 44 Amp-hr lithiumion cells are connected series. By the use of this battery pack, nearly 100 km zero emission driving is aimed. The specifications of battery pack are shown in Table 3. 4. Energy management strategy

100 80

A B C D

60 40 20 0 1000

Electrical engine

2000

3000

4000

5000

6000

7000

rpm

In the existing research, mixed hybrid electrical vehicles are designed with four driving modes like as; acceleration, normal driving, deceleration with regenerative braking and battery charging with ICE. The controller of the hybrid vehicles manages the energy flow in subsystem of the vehicle. The controller takes inputs from accelerator and brake pedals. The control points are battery pack and fuel tank of the vehicles. Controller manages transmission, ICE, electrical engines and battery pack by data which collected from input and control points.

Fig. 3. ICE power diagrams of vehicles.

4.1. Acceleration mode

Torque (Nm)

250 200 A B C D

150 100 50 0 1000

2000

3000

4000

rpm

5000

6000

Fig. 4. ICE torque diagrams of vehicles.

7000

In this mode, all the traction power of the vehicle is used. EE1 and ICE drive vehicle which can be described in the following equations:

PðnÞ ¼ P elec ðnÞ þ Pice ðnÞ

ð9Þ

Total power of the vehicle P(n) is sum of total power of electrical power Pelec(n) and internal combustion engine power Pice(n). Total torque can be described in the following equations:

TðnÞ ¼ T elec ðnÞ þ T ice ðnÞ

ð10Þ

where Telec(n) is electrical engine torque and Tice(n) is ICE torque. The length time of the acceleration mode depends on the capacity

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of the battery pack. In this study, mixed hybrid electrical vehicles can be driven nearly 100 km in this mode. In the simulation, acceleration time can be calculated as:

aðnÞ ¼

F net ðnÞ 1:1  m

VðnÞ  Vðn  wÞ thðnÞ ¼ 3:6  aðnÞ

ð11Þ

ð12Þ

where a(n) symbolizes acceleration and th(n) symbolizes acceleration time between V(n) and V(n  w). Key parameter of the study is motor speed because most of the other parameters depend on it. The motor speed increases by steps of w in the simulation program. Total acceleration time is described in the following equation:

tht ¼

X

thðnÞ

ð13Þ

tice;c ¼

Eelec Pice ðnÞ

ð17Þ

4.5. Fuel consumption evaluation Instant fuel consumption of the ICE can be described in the following equations:

FCðnÞ ¼

BSFCðnÞ  T ice ðnÞ  gearðxÞ  Df 36  qya  Rwheel

ð18Þ

where qya is the density of fuel and BSFC(n) is specific fuel consumption of ICE. Total fuel consumption of the vehicle can be calculated by:

FCt ¼

X

FCðnÞ  t ice;c þ

X

FCðnÞ  tice;t

ð19Þ

where tice,t symbolizes working time of ICE for traction. 4.2. Normal driving mode

4.6. CO2 emission evaluation

In this study, the traction of the mixed hybrid vehicle is provided by EE1 or ICE in this mode. In city cycles especially speed under 50 km/h, EE1 drives vehicle on its own. When the mixed hybrid vehicle stops, the EE1 does not need energy but in the same conditions ICE needs energy. Therefore, using EE1 in city cycles provides fuel economy. No emission driving range with EE1 depends on battery pack capacity. The mixed hybrid electrical vehicles display no emission range until nearly 100 km. When the capacity of the battery pack decreases fewer than 60% then ICE starts and produces mechanical energy which is converted to electrical energy by EE2. In this mode, total torque can be described in the following equations:

TðnÞ ¼ T elec ðnÞ

ð14Þ

In motorway cycles as over 50 km/h, EE1 shuts down and ICE provides the traction of the mixed hybrid vehicle. This mode especially occurs on fixed speed cycles. When more power is needed, EE1 starts and ICE goes on working at constant speed and it provides fuel economy too. In this mode, total torque can be described in the following equations:

TðnÞ ¼ T ice ðnÞ

ð15Þ

4.3. Deceleration mode If the brake pedal is depressed when the mixed hybrid vehicle is driven by one of the traction engines or both of them, then ICE shuts down and only EE1 charges battery pack with regenerative braking. For charging battery in every condition, the battery pack capacity must be under 90%. The regenerative power is limited under 65% of the total braking power because EE1 is linked to the two wheels of the vehicle.

Total CO2 emission of the vehicle can be calculated by:

Ct ¼

Eelec ¼

t EE1 ðnÞ  Pelec ðnÞ

ð16Þ

where tEE1 is working time of EE1 for tracking the vehicle. Working time of ICE for charging can be described in the following equations:

X

CðnÞ  tice;t

ð20Þ

5. Road simulation with hybrid electrical vehicle simulation program The acceleration values of vehicles are shown in Fig. 5. Acceleration test was made for velocity ranges of 0–50 km/h, 0–100 km/h and 0–130 km/h. The maximum engine speed is chosen 4600 rpm for diesel engines and 7000 rpm for gasoline engines. At 0–50 km/h and 0–130 km/h acceleration tests by high torque of electrical engine, C is the fastest vehicle but in 0–100 km/h tests A is faster than the other vehicles. As a result, mixed hybrid vehicles are faster than conventional vehicles. The fuel consumption values obtained from the simulations for vehicles are shown in Fig. 6. For fuel consumption test in dense city cycle, the velocity is 25 km/h and the distance is 3.2 km. Start/Stop value is 2.5 per kilometer. For the other test in desolate city cycle, the velocity is 65 km/h and the distance is 8.3 km. Start/Stop value is 0.25 per kilometer. For fuel consumption test in motorway cycle, the constant velocities are 88 km/h and 112 km/h. Test distance is 7.5 km. In every cycle hybrid vehicles are nearly 30% economical than conventional ones. In addition, hybrid vehicles’ CO2 emission, as shown in Fig. 7, is less than conventional vehicles in every cycle. In city cycles, it can be understood more clearly. The simulation results show that the CO2 emissions of hybrid vehicle are reduced by 30–60% on average in comparison to conventional gasoline and diesel vehicles.

acceleration time (s)

X

CðnÞ  t ice;c þ

where C(n) is instant CO2 emission of the ICE.

4.4. Charging mode When the capacity of the battery pack decreases under 60% then ICE and EE2 work as a generator until the capacity reaches 90%. The efficiency of the generator system depends on the efficiency of ICE and EE2. Gasoline fueled ICE produces the highest torque at 4000 rpm but diesel fueled ICE produces at 1750 rpm. Hence, these engines work at these speeds when charging. In a cycle energy provided from battery pack can be calculated as:

X

28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

0 -50 km/h 0 -100 km/h 0 -130 km/h

A

B

C

Fig. 5. Acceleration values of vehicles.

D

Fuel Consumption (lt/100km)

I. Taymaz, M. Benli / Fuel 115 (2014) 812–817

14

25 km/h city cycle

12

65 km/h desolate city cycle

10

88 km/h motorway cycle 112 km/h motorway cycle

8 6 4 2 0

A

B

C

D

CO2 Consumption (kg/100km)

Fig. 6. Fuel consumption values of vehicles.

22 20 18 16 14

although they produce almost the same power. The simulation results which indicate the economical feasibility of this approach show that the fuel and CO2 emissions are theoretically reduced nearly 30% by mixed hybrid electrical vehicles. As hybrid vehicles are introduced into the passenger transportation market, the share of conventional vehicles decreases proportionally, leading to a decrease in CO2 emissions. The permanent increase of fuel costs and the demand for more environmental vehicles, which have a sustainable impact on the environment, will lead to significant higher production numbers of mixed hybrid electrical vehicles, supported by a decrease of the additional component costs. For a further work, an optimization tool for different drive cycles can be built and more effective amortization scenarios, for a certain region and certain driving habits, can be developed. By optimizing the whole system, engine design can be better suited for operation that can be very different than for conventional vehicles.

25 km/h city cycle

References

65 km/h desolate city cycle 88 km/h motorway cycle 112 km/h motorway cycle

12 10 8 6 4 2 0

817

A

B

C

D

Fig. 7. CO2 emission values of vehicles.

6. Conclusions The simulation code developed can be used a research or design tool for the evaluation of mixed hybrid electric vehicles. Although they were initially intended for the sole purpose of simulating hybrid vehicle, there is no apparent limitation to the powertrains which the simulation software can model. In this study, conventional and mixed hybrid electrical vehicles are compared by this software. The mixed hybrid vehicles are nearly 260 kg heavier than conventional vehicles because of additional systems. All mixed hybrid vehicles are more economical than the conventional vehicles

[1] Aoyagi S, Hasegawa Y, Yonekura T, Abe H. Energy efficiency improvement of series hybrid vehicles. JSAE Rev 2001;22:259–64. [2] Alvarez R, Weilenmann M. Effect of low ambient temperature on fuel consumption and pollutant and CO2 emissions of hybrid electric vehicles in real-world conditions. Fuel 2012;97:119–24. [3] Chau KT, Wong YS. Overview of power management in hybrid electric vehicles. Energy Convers Manage 2002;43:1953–68. [4] Westbrook MH. The electric and hybrid electric car. London: SAE; 2001. [5] Wipke K, Cuddy M, Burch S. ADVISOR 2.1: a user-friendly advanced powertrain simulation using a combined backward/forward approach: national renewable research laboratory, NREL/JA-540-26839; 1999. [6] Aceves SM, Smith JR. A hybrid vehicle evaluation code and its application to vehicle design. SAE Techical Paper; 1995. p. 950491. [7] Cuddy M. A comparison of modelled and measured energy use in hybrid electric vehicles. SAE Techical Paper; 1995. p. 950959. [8] Cole GH. SIMPLEV: a simple electric vehicle simulation program, version 2.0. EG&G Idaho Inc.; 1993. [9] Braun C, Busse D. A modular Simulink model for hybrid electric vehicles. SAE Techical Paper; 1996. p. 961659. [10] Butler K, Stevens K, Ehsani M. A versatile computer simulation tool for design and analysis of electric and hybrid drive trains. SAE Techical Paper; 1997. p. 970199. [11] Husian I. Electric and hybrid electric vehicle design fundamentals. New York: CRC Press; 2003. [12] Jefferson CM, Barnard RH. Hybrid vehicle propulsion. Boston: WIT Press; 2002. [13] Chan CC, Chau KT. Modern electric vehicle technology. New York: Oxford University Press; 2001. [14] Joeri Van M, Peter Van den B, Gaston M. Models of energy sources for EV and HEV: fuel cells, batteries, ultracapacitors, flywheels and engine-generators. J Power Sources 2004;128:76–89. [15] Gutmann G. Hybrid electric vehicles and electrochemical storage systems – a technology push–pull couple. J Power Sources 1999;84:275–9.