Analysis of powertrain design on effective waste heat recovery from conventional and hybrid electric vehicles

Applied Energy xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles q A. Shabashevich, N. Richards ⇑, J. Hwang, P.A. Erickson Hydrogen Production and Utilization Laboratory, Department of Mechanical and Aerospace Engineering, University of California, 2132 Bainer Hall, One Shields Avenue, Davis, CA 95616, USA

h i g h l i g h t s  Exhaust flow variation with drive cycle must decrease for an effective EER system.  EER systems recover exhaust energy with an increasing degree of hybridization.  The ICE is most tolerant to increased back pressure at higher intake pressures.  EER improves the thermal efficiency at higher intake and exhaust pressures.

a r t i c l e

i n f o

Article history: Received 14 August 2014 Received in revised form 15 February 2015 Accepted 19 February 2015 Available online xxxx Keywords: Waste Heat Recovery Hybrid Electric Vehicles Powertrain configuration

a b s t r a c t The growing need for efficient vehicles has led many researchers to explore various ways to utilize waste heat from the Internal Combustion Engine (ICE) to improve vehicle fuel economy. Past efforts in Waste Heat Recovery (WHR) have focused primarily on recovering waste heat from Conventional Vehicles (CV), which dissipate more than two-thirds of the fuel energy as waste heat. In general, WHR has always been considered as a secondary component to the vehicle powertrain and as a result it has had little success, particularly in light-duty vehicles. This investigation explores WHR from a broader perspective to better understand the possibilities and limits of WHR from CVs to future highly hybridized vehicles. Fuel energy distribution in the ICE is used to evaluate the sources of waste heat and identify exhaust energy recovery as the most promising method for improving ICE thermal efficiency. Fundamental analysis of conventional and hybrid powertrain design is used to investigate how they impact the availability of exhaust energy from the ICE. Models and simulations of several engines and vehicles are used to validate the theory presented for effective WHR from light-duty vehicles. The analysis focuses on how available exhaust energy changes for different vehicles, how it varies with drive cycles, and how the induced back pressure from an exhaust energy recovery system affects ICE performance. The results indicate that effective WHR is difficult to achieve in light-duty conventional vehicles, but is viable in a highly hybridized vehicles, where ICE thermal efficiency can be increased by at least 15 percent over various drive cycles. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In order to utilize the waste heat from a Conventional Vehicle (CV), it is important to understand how the chemical energy of the fuel is distributed. Over 60% of the fuel energy distribution in

q This paper is included in the Special Issue of Clean Transport edited by Prof. Anthony Roskilly, Dr. Roberto Palacin and Prof. Yan. ⇑ Corresponding author. Tel.: +1 (530)754 5352. E-mail addresses: [email protected] (N. Richards), [email protected] (J. Hwang), [email protected] (P.A. Erickson).

a light-duty vehicle, operating over a typical city drive cycle, is lost through the thermal engine losses [1]. While many of the other sources of losses can be significantly reduced with a more advanced powertrain design, heat losses, primarily as exhaust, will still be the greatest source of inefficiency as long as the ICE combustion process can be modeled as an Otto cycle or even a Miller cycle. Fundamentally, there will always be waste heat produced by the ICE. Therefore, as recently shown by several authors [2–4], there is great potential to improve ICE thermal efficiency if the wasted heat from the exhaust and coolant can be effectively recovered and converted into a more useful form of energy.

http://dx.doi.org/10.1016/j.apenergy.2015.02.067 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067

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A. Shabashevich et al. / Applied Energy xxx (2015) xxx–xxx

Nomenclature CI CV CVT DOH EER EM

Compression-Ignition Conventional Vehicle Continuously Variable Transmission Degree of Hybridization Exhaust Energy Recovery Electric Motor

The temperature in the coolant is generally around 90 °C with minor variation with engine speed or load [5]. However, the exhaust temperatures can be as high as 900 °C for SI engines and around 500 °C for CI engines [6]. Therefore, the significant difference in temperature between the coolant and the exhaust gases means that Waste Heat Recovery (WHR) from the exhaust will be more efficient, even though liquid coolant has greater heat capacity and thermal conductivity than the exhaust gases. A similar conclusion was also reached by other researchers investigating WHR from vehicles [7,8].

HEV ICE SI TC TEG WHR

Hybrid Electric Vehicle Internal Combustion Engine Spark Ignition Turbo-Compounding Thermo-Electric Generator Waste Heat Recovery

ICE thermal efficiency, either its overall system efficiency has to increase two to three times or the exhaust availability has to increase so that mass flow conditions are more compatible with the requirements of current EER systems. The major focus of this investigation is to analyze different methods for shifting the location of the ICE operating points to improve the availability of exhaust energy without impacting vehicle performance.

2. Analysis of EER in CVs from the powertrain perspective 1.1. Recovering exhaust energy from ground vehicles Over the years numerous types of WHR systems in transportation have been investigated and developed. The most common are Turbo-Compounding (TC), Thermoelectric Generators (TEG), and a Rankine Bottoming Cycle. Turbo-compounding is one of the most successful methods of Exhaust Energy Recovery (EER) primarily for large diesel ICEs, given their moderately high engine loads and relatively constant drive-cycles [9–11]. However, a major consequence of TC is the increased back pressure on the engine as the expansion ratio of the power turbine increases. Therefore, there is a maximum limit to how much energy can be extracted from the exhaust gases of a turbocharged CI ICE before the total power output of the ICE and the TC system begins to decrease. Furthermore, the increasing demand for complex diesel after-treatment systems means that tolerable levels of back pressure may be reached before turbocompounding is even implemented. Another common WHR system is the Thermoelectric Generator. However, experiments utilizing TEG have shown poor performance due to the small temperature difference across the thermoelectric modules [12–16]. The results indicated that current efficiencies of the thermoelectric materials and the TEG are not high enough to have any significant improvement on fuel economy, if any at all. The Rankine bottoming cycle is another option for WHR. Although efficiencies for the Rankine cycle tend to be low due to heat rejection and conversion losses, it can utilize both low and medium temperature waste heat [9,10,17]. This makes the Rankine bottoming cycle effective at all operating conditions, regardless of ICE load or drive cycle. Unfortunately, a major control issue for the Rankine bottoming cycle is its response to transients. It takes a finite amount of time for the working fluid to reach appropriate temperature and pressure to maximize efficiency, therefore the power produced by the Rankine bottoming cycle is not available immediately and fuel economy is not instantaneously minimized [10,18]. Although many researchers have realized the problems of recovering exhaust and coolant energy from a CV, the problems with frequent variation of the exhaust gases with drive cycle were often reduced simply by focusing solely on long-haul heavy-duty trucks [10,18,19]. Most Exhaust Energy Recovery (EER) systems tend to have low efficiencies, particularly at low engine loads and speeds. Therefore, to make EER a viable method for improving

Alternative powertrain designs might be able to help minimize exhaust flow variation by actively controlling the operating conditions of the ICE.

2.1. Relationship between ICE operating characteristics on exhaust energy, mass flow rate and temperature Fig. 1 shows the fuel energy distribution for a turbocharged CI and naturally aspirated SI engine at low-load and high-load. Although actual data will vary from one engine to the next, the general trends will be the same. In the SI engine, the increase in engine power causes a larger percentage of fuel energy to leave with the exhaust, rather than be transferred to the coolant. Therefore, operating the SI ICE at high load and speed makes EER very effective. Unfortunately CVs do not operate at those high load conditions very often. The average operating points of the ICE are in a region where the exhaust energy quality is poor and a large portion of the thermal energy from the combustion is transferred to the coolant. This helps explain why many of the experiments discussed in the literature on this topic were unsuccessful at recovering exhaust energy over a drive cycle. In order to maximize EER from an ICE, the exhaust mass flow rate and temperature have to be as high as possible. Exhaust mass flow rate is directly related to the torque and speed of the ICE, and the temperature of the exhaust gases will depend on two primary factors. The first is the torque produced by the ICE, which is related to the combustion process itself, equivalence ratio, heat transfer inside the combustion chamber, and many other engine specific parameters [1]. The second factor is the heat transfer outside the ICE. For the SI engine at full load, the exhaust temperatures remain relatively high across the entire exhaust system due to high exhaust gas densities and mass flow rates. The operation of the ICE at high load has a significant impact on the heat transfer rate and temperature of the exhaust gases in the exhaust system. It also increases the possible locations for an EER system, because the exhaust temperatures remain relatively high even halfway down the exhaust system. This is not the case for part load operation, however, where the exhaust temperatures drop immediately after leaving the ICE [19].

Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067

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Fuel Energy Distribution at Low-Load 50% 40% 30% 20% 10% 0% Brake Power

Coolant

Exhaust

Misc.

Percentage of Fuel Energy

Percentage of Fuel Energy

A. Shabashevich et al. / Applied Energy xxx (2015) xxx–xxx

Fuel Energy Distribution at High-Load 50% 40% 30% 20% 10% 0% Brake Power

Coolant

Exhaust

Misc.

CI

26%

49%

24%

1%

CI

37%

27%

29%

7%

SI

19%

42%

21%

18%

SI

28%

18%

44%

10%

Fig. 1. Energy distribution for CI and SI engines at low-load and high-load values adapted from Ref. [20,21].

2.2. Relationship between specific fuel consumption and exhaust energy There are technologies that improve light-duty vehicle fuel economy by shifting the ICE operating points closer to the minimum specific fuel consumption region. These technologies include downsizing and turbocharging of the ICE, using a Continuously Variable Transmission (CVT), and hybridizing the vehicle. The advantage of a CVT is its ability to essentially allow the ICE to operate independently of the vehicle speed within the specified ratio range of the transmission [22,23]. In practice, however, this is nearly impossible because the ICE is significantly oversized compared to the typical road load. At lower engine speeds, the ICE has to be throttled to meet the required road load. Another way is through hybridization, where the ICE is downsized and the loss in power is replaced by the Electric Motor (EM) and batteries [24–26]. Fundamentally, the downsizing of the ICE simply shifts the operating points toward higher thermal efficiency. The EM and batteries provide the necessary power needed to maintain vehicle performance [27–32]. Hybrid Electric Vehicles (HEVs) allow several possibilities for energy management strategies ranging from new powertrain configurations [33,34], battery technologies [34], system control [26,35], and overall degree of hybridization [26]. With regards to EER, careful control of the ICE operating points is crucial since it increases the available exhaust energy [36]. This can be achieved by careful management of the vehicle powertrain controls.

alternator losses, reducing transmission losses, reducing weight, and utilizing regenerative braking [25,37–39]. However, none of these methods improve the availability of the exhaust energy, nor will they have a significant impact on the EER system. Although a modern HEV with a CVT has the potential to effectively utilize an EER system, further improvement for EER is possible with greater DOH. In order to fully understand how EER and hybrid vehicle powertrains are related, it is important to look at the theory of hybrid powertrain design to understand the full potential of EER from HEVs. Current HEVs utilize only minor ICE downsizing or shifting of the ICE operating points. Therefore, the variation in ICE operating points with drive cycle remains relatively the same as with a CV. One exception to this general trend of HEV powertrain design is the Toyota Hybrid System, which does in fact significantly shift the location of the ICE operating points. Toyotas system is based on a power-split powertrain configuration that uses an electricCVT to control the load placed on the ICE [40]. While the Toyota Prius powertrain does in fact significantly shift the location of the ICE operating points, it is still a strong ICE dominant HEV with an ICE that is oversized compared to the average road load [40]. This also helps explains the poor exhaust quality of the Toyota Prius as discussed by Arias et al. [8]. To improve the exhaust quality, it is important to downsize the ICE even further to shift the average operating points to regions of even lower specific fuel consumption. 3. Methodology and modeling EER from various vehicles

2.3. Classification of hybrid vehicles The choice of the powertrain control strategy will impact how often and under what load the ICE will operate. One way to classify and compare hybrid vehicle powertrain configurations is with a measure called the Degree of Hybridization (DOH). This is used to relate the proportion of peak power between the two energy converters. For this investigation, the DOH will be based on the peak power of the ICE and the EM defined by Eq. (1), where EMPmax is the peak EM power, and ICEPmax is the peak ICE power.

DOH ¼

EMPmax EMPmax þ ICEPmax

ð1Þ

All the vehicle simulations were performed with Advanced Vehicle Simulator (ADVISOR) software, while a thermodynamic equilibrium model called ChemWorks was used to model the ICE. A Microsoft Excel model of an electric TC system was developed to determine how much electrical energy could be generated from the exhaust heat. ADVISOR was used to simulate how much exhaust energy is produced by the test vehicles, and how it varies with drive cycle and different control strategies. ChemWorks was developed at the University of California Davis to run in MATLAB and calculate ideal gas processes for various combustion reactions. It uses the chemical equilibrium program STANJAN and CHEMKIN libraries to model the ideal internal combustion engine cycle [41,42].

2.4. Modern ICE-dominant hybrid powertrain design 3.1. Vehicle simulations in ADVISOR In addition to shifting the location of the ICE operating points, current HEVs increase fuel economy by lowering the coefficient of drag and rolling resistance, eliminating idling, reducing

The vehicles under consideration were a mid-sized CV and several parallel HEVs with different DOH. None of the test vehicle

Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067

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A. Shabashevich et al. / Applied Energy xxx (2015) xxx–xxx

parameters were optimized and were left as default. To normalize all the test vehicles, the total power of all the test vehicles was set by the peak ICE power of the CV. Four different drive cycles typically used in vehicle testing; UDDS, HWFET, US06, and US06-HWY [43]; were selected to get an accurate representation of the operating characteristics of the ICE for different powertrain configurations over various road conditions. The UDDS and HWFET drive cycles represent the most common drive cycles used in the United States for city and highway driving, respectively. However, these drive cycles are modest with regard to acceleration and maximum speed, therefore US06 and US06-HWY can also be used to represent more aggressive city and highway driving. All the test vehicles were simulated over all the drive cycles and the data on the available exhaust energy, vehicle performance, fuel economy, and ICE operating points were collected and analyzed. A CV is modeled as a mid-sized sedan equipped with a 160 kW, 3.0L SI V6 engine mated with a five-speed manual transmission, with a total weight of 1769 kg. The engine displacement and peak power were chosen because they are representative of current engines for mid-sized sedans. The parallel HEVs used in this investigation was a pre-transmission parallel mid-sized sedan with a total peak power output of 160 kW. In the pre-transmission architecture, the EM is placed before the transmission which allows it to operate more efficiently and places less demand on the batteries [44]. The DOH for the different HEV powertrains was spaced out evenly at 0, 0.1, 0.3, 0.5, and 0.7. The proportion of peak EM power and peak ICE power were adjusted to achieve the desired DOH, while maintaining the same total peak power as for a CV. Table 1 shows the different HEV configurations with different DOH values. The mass of the vehicles were adjusted accordingly by ADVISOR to account for the weight of the different components. These estimates are within reason [34,45] and therefore were not optimized. The total powertrain peak power was not adjusted to account for the increased weight of the different HEVs, because the added component weight does not directly impact vehicle performance, fuel economy, or emissions as a CV [44,46]. This is mainly because the engine operates at higher efficiency and kinetic energy is recovered with regenerative breaking. Plus, the operating characteristics of EMs are ideal for ground vehicles due to the high low-end torque [37]. Therefore, vehicle performance is not immediately hindered by the added weight as it would be in a CV. Since the focus of this investigation is on hybrid powertrains and the increased weight is not as crucial to HEV efficiency as CV, vehicle weight was not minimized. The control strategy selected for these simulations operates the ICE at high engine load, thereby maintaining the desired high

Table 1 Components for different test vehicles. DOH factor ICE displacement (L) ICE peak power (kW) EM peak power (kW) Total powertrain power (kW) Battery energy (Ah) Battery voltage (V) Battery module peak power (kW) Number of modules Batter pack voltage (V) Battery pack peak power (kW) Excess battery pack power (%) Battery module weight (kg) Battery pack weight (kg) Vehicle weight (kg)

0

0.1

0.3

0.5

0.7

3 160 0 160

3 144 16 160

3 112 48 160

1.9 80 80 160

1 48 112 160

0 0 0 0 0 0 0

28 6.7 1.6 11 74 17.6 9

45 13.7 3.3 16 219 52.8 9

45 13.7 3.3 27 362 89.1 10

60 13.4 4.9 26 348 127.4 12

3.6 39.6 1781

8.4 134.4 1824

8.4 226.8 1928

11.6 301.6 1945

0 0 1769

thermal efficiency of the ICE. The operating point is shifted from one power level to the next to meet the steady-state road load and maintain the state of charge of the battery. Most of the acceleration and deceleration is taken care of by the motor. It simply lets the ICE throttle down to a 60 percent of peak ICE thermal efficiency, and only uses the EM to maintain ideal operating line operation when the road load drops below 60 percent of peak ICE thermal efficiency [27,28,47]. 3.2. ICE simulations in ChemWorks Since it was determined that EER from CI engines is less favorable compared to SI engines in light-duty vehicles, simulations of CI engines were not performed in this investigation. All of the simulations with ChemWorks were made for a 3.0L SI engine with a compression ratio of 9.5, and a polytropic compression and expansion ratio of 1.35 and 1.29, respectively. These polytropic ratios are the default values in the software and they match well to published values for SI engines [1]. The equivalence ratio for all SI engine simulations was kept constant at one and iso-octane was used as the fuel. A mixed or dual combustion cycle was used where 20 percent of the fuel was burned at constant volume and the rest burned at constant pressure. This type of cycle relates more to an actual combustion process than the Otto cycle. 3.3. Electric TC simulations in excel To quantify how much energy can be recovered from the exhaust of a 1.0L SI ICE, a static model of an electric TC system was developed using Microsoft Excel. Based on the ICE operating conditions, the model predicts how much electrical energy can be generated from the exhaust gases. The electric TC model is based on a thermodynamic turbocharger model connected with a generator. It assumes the generator is attached either directly to the turbocharger shaft or simply to the turbine shaft, if the compressor is not used. Due to the high temperature of the exhaust gases, the expansion process for the turbine was modeled in ChemWorks to determine the enthalpy of the exhaust gases out of the turbine. For the compressor, the temperature of the compressed air was determined through an isentropic process with constant specific heats. ChemWorks was then used to determine the enthalpy of the air before and after the compressor. The efficiency of the turbine was set to 60 percent, the efficiency of the compressor to 70 percent, the mechanical efficiency of the electric TC system to 95 percent, and the efficiency of the motor/generator was set to 90 percent. For all experiments, the turbine outlet pressure was kept at 1.05 atm to account for some pressure drop in the exhaust system. The ICE model was simulated for different intake and exhaust pressures at 3000 RPM and 5000 RPM. 4. Results and discussion on EER from ground vehicles Given that the values used for the models were not optimized, the results presented in this discussion are of an approximate magnitude that would be expected from an actual vehicle. The intent is to quantitatively demonstrate the theory and trends discussed thus far on EER, and estimate how much ICE thermal efficiency can be improved if exhaust energy was recovered. 4.1. Drive cycle impact on EER potential Fig. 2 shows a summary of the exhaust mass flow rate, power and temperature for all the tested drive cycles. Overall, the average temperature decreases with more aggressive drive cycles where

Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067

5

300

30

250

25

200

20 150 15 100

10

50

5

0

0 UDDS

HWFET

US06

45.0 40.0

Thermal Efficiency (%)

35

Minimum/Maximum (g/s)

Average/Standard Deviaon (g/s)

A. Shabashevich et al. / Applied Energy xxx (2015) xxx–xxx

US06_HWY

15 10 5 0 US06_HWY

900 800 700 600 500 400 300 200 100 0

900 800 700 600 500 400 300 200 100 0 US06

Minimum/Maximum (degC)

Average/Standard Deviaon (degC)

(b) Exhaust Power

HWFET

US06_HWY

(c) Exhaust Temperature Average

Standard Deviaon

15.0 10.0

Minimum

Maximum

Fig. 2. Exhaust conditions from a 30.L ICE in a CV over various drive cycles.

ICE power requirements are higher. This is due to the expansion cooling of the exhaust gases as the engine load changes. On average the exhaust temperatures still remain above 700 °C immediately after leaving the ICE, which is still sufficiently high for effective EER. Conversely, the mass flow rate behaves almost exactly the same as the exhaust power over a drive cycle, in that they both increase with the more aggressive drive cycles. This suggests that the exhaust mass flow rate has a greater effect on available exhaust power than exhaust temperature. Therefore, to maximize the available exhaust power, the intake air mass flow rate has to be maximized since it is directly related to exhaust mass flow rate. The magnitude and variation in intake air flow rate is the result of the throttle response in a SI engine, which results from torque requests from the driver in response to the drive cycle. From the design point of view for an EER system, this rapid variation in exhaust energy with a drive cycle requires an EER system which is adaptable to many driving conditions. If high vehicle speeds are considered, average exhaust power can reach over 100 kW from a 3.0L SI ICE. However at low vehicle speeds, average exhaust power can be as low as 10 kW from the same ICE. This verifies why most EER systems discussed in literature experienced difficulty recovering exhaust energy over a drive cycle, even if they could adapt to the changes in exhaust flow conditions. The variation in exhaust flow conditions has to decrease before an EER system can be effectively implemented into a light-duty vehicle.

1.5

1

2

3

2.5

3.5

4

4.5

Engine Exhaust Pressure (atm) Fig. 3. 3.0L ICE thermal efficiency at various intake and exhaust pressures at 3000 RPM.

4.2. Impact on pressure on CV performance 4.2.1. Back pressure on ICE thermal efficiency The ICE thermal efficiency drops off dramatically at part load with an increase in exhaust pressure, as shown in Fig. 3. This verifies that regardless of how much energy is recovered from the exhaust of a CV, if back pressure is increased significantly, any gains with the EER system will be offset by the decrease in ICE thermal efficiency. The ICE thermal efficiency does not drastically change at higher intake pressures with increase in exhaust pressure. The relationship between intake and exhaust pressures, residual gases, and ICE thermal efficiency are important to consider when designing an EER system for vehicular applications. 4.2.2. Impact on exhaust power with equivalent intake and exhaust pressure Based on Fig. 3, the ICE is most tolerant to increased back pressures with higher intake pressures likely due to reduced ICE pump work and the amount of residual gases inside the combustion chamber. Therefore, one possible way to overcome the decrease in power output is to maintain the intake pressure at the same level as the exhaust pressure. This is verified in Fig. 4, which shows the thermal efficiency of the ICE operating under the same conditions. Note that maintaining a high thermal efficiency with increased ICE back pressure is easier with higher intake pressures. This concept, however, poses several problems for CVs operating with SI engines. Not only do the ICE operating conditions change drastically over a drive cycle, but the engine is often operating at part-load with the ICE intake pressure considerably below 1 atm. Furthermore, intake pressures in SI engines can never be 42 41

Thermal Efficiency (%)

20

UDDS

Intake Pressure = 1.5 atm Intake Pressure = 0.95 atm Intake Pressure = 0.75 atm Intake Pressure = 0.5 atm

20.0

0.5

Minimum/Maximum (kW)

Average/Standard Deviaon (kW)

25

US06

25.0

0.0 200 180 160 140 120 100 80 60 40 20 0

30

HWFET

30.0

5.0

(a) Exhaust Mass Flow Rate

UDDS

35.0

40 39 38 37 36 35 34 33 32 1000

Intake Pres. = 0.95 / Exhaust Pres. = 1.05 - Baseline Intake Pres. = 1.5 / Exhaust Pres. = 1.5 Intake Pres. = 2 / Exhaust Pres. = 2

2000

3000

4000

5000

6000

Engine RPM Fig. 4. 3.0L ICE thermal efficiency at equivalent intake and exhaust pressures.

Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067

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A. Shabashevich et al. / Applied Energy xxx (2015) xxx–xxx

Average Exhaust Power (kW)

30 US06-HWY US06 HWFET UDDS

25 20 15 10 5 0 DOH 0

DOH 0.1

DOH 0.3

DOH 0.5

DOH 0.7

Degree of Hybridyzaon (%) Fig. 5. Average exhaust power over a drive cycle.

significantly above 1 atm, unless higher octane fuel is used or the compression ratio is decreased [48]. Otherwise, SI engines experience pre-ignition of the fuel, which leads to power loss, increased emissions, and can even damage the engine over time. This type of system might work in EM dominant HEVs, where the ICE tends to operate at higher loads. 4.3. Hybridization impact on EER potential To illustrate the effects of hybridization graphically, Fig. 5 shows the average exhaust power over the four drive cycles.

As the DOH increases, several interesting characteristics of exhaust gas behavior are revealed. First, the average power of the exhaust gases decreases with the DOH. This is expected since ICE displacement decreases with an increase in DOH, so consequently the average mass flow rate of the exhaust also decreases. Another observation is that the average exhaust power plateaus after DOH of 0.5, and is slightly greater for DOH of 0.7 than for DOH of 0.5. The plateau in exhaust power with DOH is due to the difference in ICE sizes, which results in the ICE operating in different regions for each vehicle. The default control strategy is setup to use the engine first to meet the road load and then use the EM to provide additional power when needed. If the ICE is significantly downsized, it will operate at full load most of the time and the EM will make up for the difference between the desired power and the peak ICE power at that instant. With the increase in DOH for a vehicle, the average exhaust power decreases due to ICE downsizing from 3.0L for a CV to a 1.0L for a HEV. This may appear as a negative characteristic for EER, until the ratio of maximum to average exhaust power and the standard deviation in exhaust power, as shown in Fig. 6a, is also considered. All the parameters decrease with an increase in DOH. For the CV, the average exhaust power is 23.3 kW with a standard deviation of 15.7 kW and a maximum exhaust power that is 7.5 times the average. For the parallel HEV with DOH of 0.7, the average exhaust power is 11.6 kW with a standard deviation of 3.9 kW and a maximum exhaust power that is only 3.2 times the average. This greatly narrows the possible exhaust power from

Average Exhaust Mass Flow Rate (g/s)

Average Exhaust Power (kW)

25

20

15

10

Average Exhaust Power St. Dev. in Exhaust Power Maximum/Average Exhaust Power

5

0

DOH 0

DOH 0.1

DOH 0.3

DOH 0.5

35 30 25 20 15 10 5 0

DOH 0.7

DOH 0

DOH 0.1

DOH 0.3

DOH 0.5

DOH 0.7

Degree of Hybridizaon (%) (b) Mass Flow Rate for Different Powertrain Configurations over the US06

Degree of Hybridizaon (%) (a) Exhaust Power for Different Powertrain Configurations over the US06 1000

35

900 30 800 700 600 Maximum Temperature Average Temperature St. Dev. In Temperature

500 400 300 200

Average ICE Efficiency (%)

Average Exhaust Temperature (deg C)

Average Mass Flow Rate St. Dev. in Mass Flow Rate Maximum/Average Mass Flow Rate

25 20 15 US06-HWY US06 HWFET UDDS

10 5

100 0

0

DOH 0

DOH 0.1

DOH 0.3

DOH 0.5

DOH 0.7

DOH 0

DOH 0.1

DOH 0.3

DOH 0.5

DOH 0.7

Degree of Hybridizaon (%)

Degree of Hybridizaon (%)

(c) Exhaust Temperature for Different Powertrain Configurations over the US06

(d) ICE Thermal Efficiency for Different Powertrain Configurations and Drive Cycles

Fig. 6. Average and standard deviation in exhaust power for various drive cycles and different DOH.

Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067

7

efficiency of the ICE. It was noted previously that peak ICE thermal efficiency exists in regions of high exhaust energy, hence why vehicle hybridization was investigated. Fig. 6d shows the average thermal efficiency of the ICE over four drive cycles for different DOH. As shown, the ICE thermal efficiency increases with DOH and its variation with drive cycle tends to decrease.

9 Intake Pressure = 1.5 atm

8

Intake Pressure = 0.95 atm

7

Intake Pressure = 0.75 atm

6

Intake Pressure = 0.5 atm

5 4 3

4.4. Potential impacts of EER with electric TC system

2 1 0 1.05

1.5

2.0

2.5

3.0

3.5

4.0

Power Turbine Exhaust Inlet Pressure (atm)

Electrical Power Generated (kW)

(a) Electrical Power Generated from 1.0L SI with an E-TC System at 3000 RPM 16 Intake Pressure = 1.5 atm

14

Intake Pressure = 0.95 atm

12

Intake Pressure = 0.75 atm

10

Intake Pressure = 0.5 atm

8 6

4.4.1. Potential EER from ICE The amount of electrical power generated by the electric TC system is related to the load placed on the ICE, operating speed of the ICE, and the pressure ratio that is created by the power turbine. Fig. 7 shows how much electricity is generated from the exhaust of a 1.0L SI ICE at 3000 RPM and 5000 RPM. As expected, with higher ICE loads and speeds, more electricity is generated with greater power turbine pressure ratios. There is a dip in the electrical energy generated for the 1.5 atm intake pressure and 1.5 atm exhaust pressure because most of the recovered exhaust energy is used to drive the compressor. Note that at part-load a much smaller fraction of exhaust energy is recovered as exhaust pressure is increased. Plus, there is a peak in how much exhaust energy can be recovered at lower intake pressures.

4 2 0

1.05

1.5

2.0

2.5

3.0

3.5

4.0

Power Turbine Exhaust Inlet Pressure (atm) (b) Electrical Power Generated from a 1.0L SI with an E-TC System at 5000 RPM Fig. 7. Electrical power from an E-TC system from a 1.0L SI ICE at (a) 3000 rpm and (b) 5000 rpm.

the ICE, which simplifies the design requirements of an EER system. Fig. 6b shows very similar characteristics for the average exhaust mass flow rate as with the average exhaust power. With an increase in DOH, the average mass flow rate decreases and becomes more stable over the US06 drive cycle. For a CV, the average mass flow rate is 30.2 g/s with a standard deviation of 21.9 g/s and a maximum exhaust flow rate that is 7.9 times the average over the US06. For the parallel HEV with DOH of 0.7, the average exhaust mass flow rate is 16.9 g/s with a standard deviation of 6.6 g/s and a maximum exhaust power that is only 3.5 times the average over the US06. As stated before, the reduction in exhaust flow variations simplifies the design requirements for an EER system. A slightly different trend emerges for the exhaust temperature shown in Fig. 6c. The average temperature decreased with DOH because the temperature variation is caused by large loading and unloading of the ICE in vehicles with higher DOH. This is also visible in the standard deviation of the exhaust temperature and results from mixed high and low speed driving, where the ICE is cycled ON and OFF often. This also results in higher maximum exhaust temperatures for parallel HEVs with higher DOH, as compared to a CV or parallel HEVs with a lower DOH. With some powertrain controls optimization, it should be possible to reduce temperature variation even further by having better control over how often the ICE is cycled ON and OFF. The ultimate goal of hybridizing a vehicle, and implementing advanced powertrain control algorithms, is to increase the thermal

4.4.2. Potential improvement in ICE thermal efficiency Fig. 8 shows the percent improvement in thermal efficiency for the 1.0L SI ICE with an electric-TC system. The ICE thermal efficiency continues to improve with higher intake and exhaust pressures. This is not the case, however, when operating the ICE at part-load, where maximum thermal efficiency improvement is reached much sooner. At an intake pressure of 0.5 atm, the electric-TC system improves ICE thermal efficiency by only 3.4 percent. For intake pressure of 0.75 atm, the electric TC system improves ICE thermal efficiency by 11 percent. Exhaust pressures have to be relatively high for the EER system to stop improving the ICE thermal efficiency for higher intake pressures. As intake pressure decreases, the improvement in ICE thermal efficiency with electric TC system also declines. Considering that CVs spend most of their time operating at part-load, this shows that very little improvement in SI ICE thermal efficiency is possible with electric-TC or any other EER that induces back pressure in the exhaust system. However, if intake pressures of the ICE can remain high over the drive cycle, significant improvement in ICE thermal efficiency is possible with an electric TC system.

ThermalEfficiencyImprovement (%)

Electrical Power Generated (kW)

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25 Intake Pressure = 1.5 atm

20

Intake Pressure = 0.95 atm Intake Pressure = 0.75 atm Intake Pressure = 0.5 atm

15 10 5 0 1.05

1.5

2.0

2.5

3.0

3.5

4.0

Engine Exhaust Pressure / Power Turbine Exhaust Inlet Pressure (atm) Fig. 8. 1.0L SI ICE thermal efficiency improvement with turbine efficiency of 60% and generator efficiency of 90% at 3000 RPM.

Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067

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5. Conclusions Based on the analysis in this investigation, the following conclusions can be made on WHR from ground vehicles:  The drive cycle has a large impact on the effectiveness of any EER system. This is particularly true for CVs, which operate mostly at part-load with occasional instances of high ICE load and speed. The difference between available waste heat at part-load and full-load operation can be large. For the CV in this investigation, on average the available exhaust power was 12.1 kW with a standard deviation of 5.1 kW over the UDDS and 23.3 kW with a standard deviation of 15.7 kW over the US06. This complicates the design of any EER system, because the mass flow rates and temperatures of the exhaust gases will vary dramatically with drive cycle.  The increase in exhaust pressure due to the EER system reduces the power produced by the ICE. However, the ICE can tolerate more back pressure as the difference between intake and exhaust pressure is minimized, or if the intake pressure is greater than the exhaust pressure. Since this is harder to achieve in SI engines, the intake pressure should be as close to atmospheric pressure as possible to reduce the impact of back pressure on ICE performance. Otherwise the compression ratio of the SI ICE has to decrease to avoid auto-ignition.  The ability of the EER system to effectively recover exhaust energy increases with DOH. This results from the downsizing of the ICE, which forces the average operating points closer to ICE full load. The loss in power due to ICE downsizing is replaced with the EM and batteries, or some another secondary energy converter and energy storage system. By shifting the average ICE operating points to higher loads, more energy is available in the exhaust. Plus, the ICE is more tolerable of increased back pressure due to the higher intake pressures.

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Please cite this article in press as: Shabashevich A et al. Analysis of powertrain design on effective Waste Heat Recovery from Conventional and Hybrid Electric Vehicles. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.067