Diesel engine aftertreatment warm-up through early exhaust valve opening and internal exhaust gas recirculation during idle operation

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1 Standard Article Diesel engine aftertreatment warm-up through early exhaust valve opening and internal exhaust gas recirculation during idle operation International J of Engine Research 1 16 Ó IMechE 2017 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: / journals.sagepub.com/home/jer Dheeraj B Gosala 1, Aswin K Ramesh 1, Cody M Allen 1,MrunalCJoshi 1, Alexander H Taylor 1, Matthew Van Voorhis 1, Gregory M Shaver 1, Lisa Farrell 2, Edward Koeberlein 2, James McCarthy Jr 3 and Dale Stretch 3 Abstract A large fraction of diesel engine tailpipe NOx emissions are emitted before the aftertreatment components reach effective operating temperatures. As a result, it is essential to develop technologies to accelerate initial aftertreatment system warm-up. This study investigates the use of early exhaust valve opening (EEVO) and its combination with negative valve overlap to achieve internal exhaust gas recirculation (iegr), for aftertreatment thermal management, both at steady state loaded idle operation and over a heavy-duty federal test procedure (HD-FTP) drive cycle. The results demonstrate that implementing EEVO with iegr during steady state loaded idle conditions enables engine outlet temperatures above 400 C, and when implemented over the HD-FTP, is expected to result in a 7.9% reductionintailpipe-outnox. Keywords Aftertreatment thermal management, variable valve actuation, early exhaust valve opening, internal exhaust gas recirculation, negative valve overlap, heavy-duty federal test procedure, drive cycle, thermal management Date received: 16 May 2017; accepted: 19 July 2017 Introduction Stringent emission regulations and consumer demand for better fuel economy continue to drive advances in diesel engine technology. Diesel engines are thermally efficient but emit harmful criteria pollutants including oxides of nitrogen (NOx), particulate matter (PM) and unburnt hydrocarbons (UHC). Pathways for reduction in vehicle emissions include reductions in engine-out emissions and improvements in aftertreatment performance. Diesel engine aftertreatment systems typically include a diesel oxidation catalyst (DOC), diesel particulate filter (DPF) and selective catalytic reduction (SCR) system. The DOC converts UHC to carbon dioxide and water, the DPF traps PM and the SCR system reduces NOx to N 2. The DOC lights off around 200 C, 1,2 while the SCR requires temperatures above 250 C for efficient NOx conversion. 3 7 The DPF requires temperatures in excess of 500 C to perform a regeneration to oxidize the accumulated PM. 8 Thermal management of the aftertreatment namely, the ability to bring the aftertreatment components to effective operating temperatures and maintain the temperatures is of paramount concern in modern diesel engine systems. The engine outlet gas temperature and flow rate, together, directly affect the aftertreatment component temperatures. Gehrke et al. 9 classified exhaust thermal management strategies as based on either the air or fuel path. Mayer et al. 10 highlighted the role of excess airflow in 1 Ray W. Herrick Laboratories, Department of Mechanical Engineering, Purdue University, West Lafayette, IN, USA 2 Cummins Technical Center, Columbus, IN, USA 3 Eaton, Marshall and Galesburg, MI, USA Corresponding author: Gregory M Shaver, Ray W. Herrick Laboratories, Department of Mechanical Engineering, Purdue University, 177 S. Russell Street, West Lafayette, IN 47906, USA. gshaver@purdue.edu

2 2 International J of Engine Research 00(0) reducing engine outlet gas temperatures at low loads an undesirable result for thermal management. Ding et al. 11 and Ramesh et al. 12 demonstrated that cylinder deactivation (CDA) improved thermal management during idle and high-speed conditions, respectively, through a reduction in airflow. Garg et al. 13 achieved up to 50 C higher engine outlet temperature via intake valve modulation at 1200 r/min, 2.5 bar brake mean effective pressure (BMEP), also via a reduction in airflow. Bouchez and Dementhon 14 observed a 150 C increase in engine outlet temperature by opening the turbocharger wastegate to reduce air-to-fuel ratio. Fuel path strategies, however, achieve aftertreatment thermal management through reductions in engine efficiency. Parks et al. 15 highlighted the influence of fuel injection timing on engine outlet gas temperature via injection of additional fuel early in the cycle, late in the cycle and increasing fueling in only one cylinder. Hydrocarbon dosers can also be used to inject fuel upstream of the DOC. 16,17 Roberts et al. 18 experimentally observed a 30 C 80 C increase in engine outlet temperatures, and predicted up to a 100 C increase, via early exhaust valve opening (EEVO), at the cost of reduced brake thermal efficiency (BTE), and predicted that the engine outlet temperatures of many low load operating conditions can be increased above 250 C. Bharath et al. 19 concluded from simulations that EEVO and exhaust cam phasing yield higher DOC conversion efficiencies with a deterioration in fuel economy. Blumenroder et al. 20 determined that internal exhaust gas recirculation (iegr) via exhaust gas reinduction which yields higher engine outlet temperatures. Although the aforementioned air path strategies generally show lower fuel consumption and better DOC warm-up through lower air-to-fuel ratios, these strategies are not as effective in warming up the SCR system. This is the result of the SCR system typically being the last component before the tailpipe, 6 which results in the preference for both elevated engine outlet temperatures and flow rates for accelerated warm-up. As a consequence, fuel path approaches are pursued in this article to achieve improved thermal management of the SCR system. Loaded idle (800 r/min, 1.3 bar BMEP) is the operating condition of focus in this study given the significant amount of time that engines spend at idle in practice. As an example, consider Figure 1 which groups all the operating points from the heavy-duty federal test procedure (HD-FTP) into eight modes based on time spent in each mode. As shown, approximately 43% of the HD-FTP is spent in the idle region. Conventional engine operation yields low engine outlet temperatures at these operating conditions, and as such, thermal management during idle operation is of paramount concern. In this study, the merits of implementing EEVO and/or iegr during loaded idle operating conditions (800 r/min, 1.3 bar BMEP) are demonstrated and compared with the conventional fuel economy and thermal management strategies on the engine, both at steady state and over the HD-FTP drive cycle: Figure 1. The engine operates at/around idle for approximately 43% of the time over the HD-FTP. 1. Engine experiments at loaded idle illustrate that EEVO together with iegr can be used to increase engine outlet temperatures in excess of 400 C, as compared to ;255 C with conventional operation, while still realizing elevated exhaust flow rates, and viable fuel consumption and engine outlet NOx levels. 2. The aforementioned experimental data is used to predict the relative merits of EEVO and a combination of EEVO with iegr for aftertreatment warmup, using first law energy balance and normalized heat transfer analyses. 3. Experimental HD-FTP engine data (fuel consumption, engine outlet temperature, SCR catalyst bed temperature and engine outlet NOx) is obtained for various thermal management drive cycles to demonstrate that implementation of a combination of EEVO and iegr during loaded idle is expected to decrease the tailpipe-out NOx by up to 7.9% over the HD-FTP. Experimental setup The experiments described in this article were performed on a six-cylinder Cummins diesel engine equipped with a camless variable valve actuation (VVA) system. The engine is coupled to an alternating current (AC) dynamometer. The engine incorporates cooled high-pressure exhaust gas recirculation (EGR), variable geometry turbine (VGT) turbocharging, air-to-water charge air cooling (CAC) and high-pressure common rail fuel injection. Figure 2 shows a schematic drawing of the air handling system of the engine. The engine outlet location is immediately downstream the turbocharger turbine. The phrases engine outlet temperature and turbine outlet temperature (TOT) are used interchangeably through the remainder of the article.

3 Gosala et al. 3 Figure 2. Schematic drawing of the air handling system of the engine with the location of sensors having been labeled. Figure 3. Schematic of the variable valve actuation setup. Kistler 6067C and AVL-QC34C pressure transducers are mounted on each of the six cylinders to log incylinder pressure through an AVL 621 Indicom module and crank angle referenced using an AVL 365C crankshaft position encoder. Fresh airflow is measured using a laminar flow element. Fuel flow rate is measured gravimetrically using a Cybermetrix Cyrius Fuel Subsystem (CFS) unit. CO 2 concentration is measured in the intake manifold as well as the exhaust pipe using Cambustion NDIR500 fast CO 2 analyzers. NOx is measured in the exhaust pipe downstream of the turbocharger using a Cambustion fnox400 analyzer. CO 2, NOx and UHC concentrations are also measured using California Analytical Instruments (CAI) 600-series non-dispersive infrared (NDIR), heated chemiluminescent detector (HCLD) and heated flame ionization detector (HFID) analyzers, respectively. PM in the exhaust stream is measured using an AVL483 photo-acoustic transient analyzer. The coolant, oil and gas temperatures at various locations are measured using thermocouples. All data are monitored and logged through a dspace interface. The engine control module (ECM) is connected to the dspace system through a generic serial interface (GSI) link that allows for cycle-to-cycle monitoring and control of fueling and various other engine operating parameters. VVA The VVA system allows for fully flexible, cylinder independent, cycle-to-cycle control of the valve events of the engine. The system includes 12 electro-hydraulic linear actuators, one for each intake and exhaust valve pair on a cylinder. The system is powered by a hydraulic pump, which maintains oil supply pressure at Figure 4. The aftertreatment system consists of a DOC, DPF and SCR with temperatures being measured at the inlet and outlet of each catalyst psi. Position feedback for each valve is measured using a linear variable differential transformer (LVDT). A real-time controller is run via dspace to control the linear actuators. A schematic of the actuators used in the VVA system is shown in Figure 3. Aftertreatment system A Cummins aftertreatment system, equipped with a DOC, DPF and SCR, is connected to the outlet of the engine, as illustrated in Figure 4. The aftertreatment is used in passive mode, that is, the exhaust gas is passed through the system without active urea injection, for measurement of catalyst bed temperatures. Thermocouples and thermistors are installed at the inlet and outlet of DOC, DPF and SCR to measure the temperature of gas entering and exiting each aftertreatment component. A differential pressure sensor is installed in the DPF to identify if the DPF is excessively clogged so that an active regeneration may be performed. Methodology Steady state tests were performed at loaded idle (800 r/min, 1.3 bar BMEP) to evaluate the benefits of EEVO and iegr over conventional engine operation. All experiments were subject to strict mechanical and

4 4 International J of Engine Research 00(0) Table 1. Mechanical and thermal constraints. Mechanical parameter Units Limit Turbine inlet temperature C 760 Compressor outlet temperature C 230 Turbo speed kr/min 126 Peak cylinder pressure bar 172 Exhaust manifold pressure kpa 500 Pressure rise rate bar/ms 100 Figure 6. Energy enters the cylinders via fuel and is distributed into indicated work, energy of the gas and in-cylinder heat loss. Figure 5. Energy enters the engine via fuel and is distributed into useful brake work, exhaust gas energy, in-cylinder heat loss, EGR cooler heat rejection, friction, turbocharger losses, manifold losses, CAC heat rejection and so on. thermal constraints, as shown in Table 1, so as to not cause any hardware damage to the engine and to make sure that the findings are relevant to real-use scenarios. An engine-cycle efficiency analysis was used to understand the fuel consumption performance of the EEVO and iegr strategies. Closed cycle efficiency (CCE), open cycle efficiency (OCE) and mechanical efficiency (ME) contribute to the BTE, as shown in equation (1). 6 The CCE captures the impact of the incylinder combustion energy release and heat transfer on the engine efficiency. Delay in heat release and increase in heat loss from the cylinder results in lower CCE. The OCE quantifies the effectiveness of the cylinder gas exchange process. OCE is lower when greater work is required by the engine to pump gas through the cylinders, generally resulting in greater difference between the intake and exhaust manifold pressures. ME quantifies the impact of friction and accessory loads on the engine efficiency. Additional information about cycle efficiency analysis can be found in Stanton 6 BTE = OCE 3 CCE 3 ME ð1þ A first law-based energy analysis is also implemented to evaluate the thermal management performance of various strategies by considering the entire engine system as a control volume, as illustrated in Figure 5. Energy enters the engine in the form of fuel injected into the cylinders and is then distributed as useful work done on the piston, energy of the exhaust gas, heat loss from the cylinder, heat loss through the EGR cooler, and various other mechanical and thermal losses including manifold losses, turbocharger losses, friction, CAC heat rejection and auxiliary losses. Brake power and exhaust heat rate can be mathematically described by equations (2) and (3), respectively. In-cylinder heat loss can be calculated by performing an energy balance while considering the cylinders as a control volume, as shown in Figure 6, and represented by equations (4) (7). Heat loss through the EGR cooler is calculated using the temperature of the gas entering and exiting the EGR cooler, and the mass flow rate of EGR, as shown by equation (8). The other thermal and mechanical losses described above are lumped and calculated as shown by equation (9). The variables used in the equations are described in Table 2 _W brake = T brake 3 2pN 60 _Q exh = _m exh C p, exh (TOT T ref ) _m air C p, air (T amb T ref ) _W indicated = NIMEP 3 V d 3 N 120 _W pumping = PMEP 3 V d 3 N 120 _Q charge = _m charge, out C p, charge, out (T EM T ref ) _m charge, in C p, charge, in (T IM T ref ) _Q cyl = _m fuel LHV fuel _W indicated _W pumping _Q charge _Q EGR = _m EGR C p, EGR DT EGRcooler _Q other = _m fuel LHV fuel _W brake _Q cyl _Q EGR Description of strategies ð2þ ð3þ ð4þ ð5þ ð6þ ð7þ ð8þ ð9þ Four strategies, three for thermal management, one for fuel efficiency, at loaded idle (800 r/min, 1.3 bar BMEP), are compared. This section describes the methodology for achieving each of the strategies, and the subsequent Steady state results section quantitatively compares them.figure 7 illustrates the representative intake and exhaust valve profiles, injection timings and heat release rates of the four strategies. Table 3

5 Gosala et al. 5 Table 2. Definition and description of variables used in equations (2) (9). Symbol C p, air C p, charge, in Description Specific heat constant of fresh air Specific heat constant of charge entering the cylinders C p, charge, out Specific heat constant of charge exiting the cylinders C p, exh Specific heat constant of exhaust gas LHV fuel Lower heating value of the fuel (diesel) _m air Mass flow rate of fresh air entering the engine _m charge, in Mass flow rate of charge entering the cylinders _m charge, out Mass flow rate of charge exiting the cylinders _m EGR Mass flow rate of exhaust gas recirculation _m exh Mass flow rate of exhaust gas leaving from the engine _m fuel Mass flow rate of fuel entering the engine N Engine speed (r/min) NIMEP Net indicated mean effective pressure PMEP Pumping mean effective pressure _Q charge Rate of heat transfer to the in-cylinder charge _Q cyl Rate of heat loss from the cylinders _Q EGR Rate of heat rejection through the EGR cooler _Q exh Rate of heat transfer to the exhaust stream _Q other Rate of other mechanical/thermal energy loss T amb Ambient air temperature T brake Brake torque produced by the engine T EM Exhaust manifold temperature T IM Intake manifold temperature T ref Reference temperature, taken as 25 C DT EGRcooler Temperature drop of the gas across the EGR cooler TOT Turbine outlet temperature V d Displaced volume _W brake Brake power developed by the engine Indicated power developed by the cylinders _W indicated EGR: exhaust gas recirculation. summarizes the valve timings, injection strategy, VGT settings and NOx mitigation philosophy for the strategies described in this section. Fuel efficiency baseline (FE baseline) Conventional engine operation at loaded idle incorporates two injection pulses near top dead center (TDC) to achieve high fuel efficiency and elevated EGR fraction to reduce engine-out NOx. Figure 7(a) shows the location of the injections with respect to TDC and the corresponding heat release rate. The nominal valve profiles used by the engine are also shown. This operation is hereafter referred to as FE baseline. Thermal management baseline (TM baseline) The conventional thermal management mode, referred to as the TM baseline, uses a fuel path thermal management strategy incorporating four late injections (per Figure 7(b)) and a mostly closed VGT nozzle. Nominal intake and exhaust valve profiles are used. The late injections result in delayed heat release, as shown in Figure 7(b). The mostly closed VGT nozzle increases the exhaust manifold pressure, elevating the pumping work. The delayed injections and elevated pumping work result in inefficient engine operation, requiring more fuel to maintain torque at loaded idle, thereby increasing the TOT. In addition, the delayed injection strategy (a NOx reducer) eliminates the need for elevated EGR fractions to constrain NOx, such that both air and exhaust flow rates are higher. Elevated TOT and higher exhaust flow rate make TM baseline favorable for aftertreatment thermal management, as will be described in the Steady state results section. EEVO The implementation of EEVO, together with the same injection and VGT settings as the TM baseline strategy, further reduces engine efficiency, thereby increasing the required fueling, through a reduction in the effective expansion ratio. The result is an additional increase in engine outlet temperature. As shown in Figure 7(c), the exhaust valve opening (EVO) was advanced, while the exhaust valve closing (EVC) remained unchanged, increasing exhaust valve duration. The nominal intake valve profile was retained. Engine-out NOx emissions were controlled through modulation of the EGR fraction. Results from two EEVO settings with different air-to-fuel ratios (AFRs) are presented in the Steady state results section in Table 3. Summary of the four strategies described in this work at loaded idle. S.No. Strategy Intake valve Exhaust valve Injection strategy VGT position NOx mitigation 1 FE baseline Nominal Nominal Two injections Partially open High EGR fraction near TDC 2 TM baseline Nominal Nominal Four late injections Fully closed Low EGR fraction (due to late injections) 3 EEVO Nominal Early EVO Nominal EVC Four late injections Fully closed Higher EGR fraction than TM baseline 4 EEVO + iegr (NVO) Phase-delayed Early EVO Early EVC Four late injections Open High internal EGR, no external EGR VGT: variable geometry turbine; FE: fuel efficiency; TDC: top dead center; EGR: exhaust gas recirculation; TM: thermal management; EEVO: early exhaust valve opening; EVO: exhaust valve opening; EVC: exhaust valve closing; NVO: negative valve overlap.

6 6 International J of Engine Research 00(0) Figure 7. At loaded idle: (a) fuel efficiency (FE) baseline uses nominal valve timings and has two injection pulses close to TDC, (b) thermal management (TM) baseline uses nominal valve profiles and has four late injections, (c) EEVO strategy has an early EVO and nominal EVC, with four late injections and (d) iegr strategy has EEVO (same as (c)) and EEVC, combined with LIVO and LIVC, with four late injections. order to illustrate the trade-off between AFR, PM, NOx, exhaust flow rate and fuel efficiency. EEVO with iegr via negative valve overlap Per Figure 7(d), negative valve overlap (NVO) was implemented in addition to EEVO, to achieve internal EGR (iegr) for NOx control without needing external EGR. The aim with this strategy is to further increase engine outlet temperatures via elimination of heat loss from the external EGR path. The valve profiles, injection timings and heat release of the implemented strategy are shown in Figure 7(d). The EVO was kept the same as that of the previously described EEVO strategy and combined with early EVC (EEVC). The intake valve profile was phase-delayed, such that the duration of the intake valve profile remained unchanged, to avoid blowdown of the re-compressed exhaust gas into the intake manifold. External EGR was eliminated by shutting the EGR valve. The use of uncooled iegr requires more dilution to control NOx, resulting in a low AFR, elevated engine

7 Gosala et al. 7 Figure 8. From the log P log V plot at loaded idle (800 r/min, 1.3 bar BMEP): (1) FE baseline has smaller power and pumping loops, to improve fuel efficiency, (2) TM baseline has a bigger pumping loop to make engine operation inefficient, (3) EEVO shows early blowdown and a bigger pumping loop and (4) iegr via NVO and EEVO has a smaller pumping loop, and shows early blowdown due to EEVO. outlet temperatures and reduced exhaust flow rates. The VGT nozzle was opened further to make the engine relatively more fuel efficient to avoid AFRs that are too low. The fuel injection strategy remained unchanged from TM baseline and EEVO strategies, as shown in Figure 7(d). Two iegr settings with different AFR are presented in the Steady state results section. Different AFRs were achieved through modulation of the amount of iegr in order to illustrate the trade-off between AFR, PM, NOx, exhaust flow rate and fuel efficiency. Steady state performance Steady state testing was performed at loaded idle (800 r/min, 1.3 bar BMEP), to quantify the impact of the three aforementioned thermal management strategies. The primary focus was on obtaining higher TOT and exhaust gas energy in order to accelerate aftertreatment component warm-up. Figure 8 compares the pressure volume diagrams. The FE baseline strategy has the smallest power and pumping loops due to the combination of early injections, open VGT and open EGR valve. The TM baseline strategy has a larger pumping loop as a result of the mostly closed VGT position which increases the exhaust manifold pressure. Late injections result in higher in-cylinder pressures at the end of the power stroke, which drive higher exhaust gas temperatures. The pumping loop for the EEVO strategies is slightly bigger than TM baseline due to blowdown of highpressure exhaust gas into the exhaust manifold. The iegr strategies result in an increase in in-cylinder pressure during the exhaust stroke, due to re-compression of the trapped residual gas. The combination of EEVC with late intake valve opening (LIVO) results in a gas spring effect during the initial portion of the intake stroke, thereby reducing the pumping loop for the iegr strategy. Late intake valve closing (LIVC) results in reduction of effective compression ratio, which results in lower in-cylinder pressure during the compression stroke for the iegr strategy. Figure 9(a) illustrates that the FE baseline exhibits a TOT of 140 C, which is notably lower that the desired aftertreatment component temperatures (. 250 C) and consumes 37% less fuel than TM baseline. The TM baseline achieves a TOT of 255 C, which is close to the desired aftertreatment temperatures. The EEVO and iegr strategies were implemented with AFRs of 22 and 24 to illustrate the trade-off between AFR, TOT, CCE, BTE, PM and NOx. The EEVO strategies reach TOTs of 317 C and 346 C, and consume 42% and 56% more fuel than TM baseline, respectively. The iegr strategies enable TOTs in excess of 380 C, at the cost of 34% and 42% higher fuel consumption than TM baseline. The EEVO and iegr strategies, owing to their higher steady state TOTs and comparable exhaust flow rates, demonstrate potential for faster aftertreatment component warm-up compared to the conventional TM baseline strategy. Figures 9(b) (d) compare the BTE, and open and closed cycle efficiencies of the four strategies. The FE baseline has 60% higher BTE than TM baseline, as a result of 52% higher OCE and 5.4% higher CCE. Lower OCE of TM baseline can be explained by the increased pumping work (consistent with larger pumping loop in Figure 8) as a result of a mostly closed VGT position and lower CCE is a result of the late injection strategy. The EEVO strategy with lower AFR (AFR = 22) has 36% lower BTE than TM baseline, as a result of a 6% lower OCE due to a bigger pumping loop, and 30% lower CCE due to early blowdown. EEVO with higher AFR (AFR = 24) has 30% lower BTE than TM baseline as a result of 5% lower OCE and 25% lower CCE. The iegr strategy with lower AFR (AFR = 22) has 29% lower BTE than TM baseline. The OCE is 42% higher, given the open VGT position and the gas spring effect caused by the re-compressed exhaust gas during the intake stroke; both of which, despite heat loss during the re-compression event, result in a smaller pumping loop. The CCE is 50% lower than TM baseline as a result of early opening of the exhaust valve which is used as a part of the approach. Furthermore, the in-cylinder pressure after blowdown, during the expansion stroke, is lower for the iegr strategy due to lower exhaust pressures, as a result of which the CCE for the iegr strategy is lower than the EEVO strategy. Similarly, the iegr strategy with higher AFR (AFR = 24) has 25% lower BTE than TM baseline, as a result of 43% higher OCE and 48% lower CCE. In contrast to the TM baseline and EEVO strategies which rely on reduction in OCE to drive elevated TOT,

8 8 International J of Engine Research 00(0) Figure 9. Experimental steady state data at loaded idle (800 r/ min, 1.3 bar BMEP), comparing various thermal management and fuel efficiency strategies. (Strategies described in greater in detail in the Description of strategies section). the iegr strategies have high OCE and yield elevated TOT due to lower heat loss (as explained later in this section). ME (not shown) is fairly consistent among these strategies. The higher TOT of the EEVO and iegr strategies can be explained by low AFRs, as shown in Figure 9(e). The AFRs of FE baseline and TM baseline are 40 and 36, respectively. The two points for EEVO and iegr strategies have AFRs of 24 and 22, each, with the lower AFR strategy in each case exhibiting higher TOT. Low AFRs with EEVO and iegr strategies can be explained by higher fuel consumption and higher exhaust gas dilution required to control NOx. An increased amount of dilution is required with the iegr strategies due to uncooled iegr, which is not as effective as cooled EGR in controlling engine-out NOx (described below). Per Figure 9(h), similar engine-out NOx for all strategies was targeted through modulation of the EGR or iegr fractions. The FE baseline requires an elevated EGR fraction (Per Figure 9(g)), due to early fuel injections which promote elevated NOx. The TM baseline strategy is able to achieve similar NOx levels with a 17% lower EGR fraction than FE baseline, due to its late injection strategy. The EGR fraction reduction results in higher fresh airflows and consequently, higher exhaust flow rate (per Figure 9(f)). The EEVO strategies require higher EGR fractions to achieve the same NOx due to higher injected fuel mass, as a result of which the exhaust flow rates are slightly lower. iegr is not quite as effective in reducing engine-out NOx as cooled EGR, as the iegr gas is hot. The iegr strategy with higher AFR, which has lower trapped residual exhaust gas, has higher NOx due to lower dilution of in-cylinder charge, while the iegr strategy with lower AFR has NOx level close to that of TM baseline, as shown by Figure 9(h). Higher residual exhaust gas for the iegr strategies, in addition to LIVO, also reduces the volumetric efficiency, resulting in lower fresh air and exhaust flow rates. PM emissions of the thermal management strategies are higher than the FE baseline strategy, with the TM baseline showing approximately six times higher PM, as illustrated in Figure 9(i). The EEVO and iegr strategies with AFR = 24 have approximately six and eight times higher PM than TM baseline, respectively, while those with AFR = 22 emit ;12 and ;23 times higher PM. The large amount of hot residual exhaust gas in the iegr strategy contributes to the high PM production at steady state. It is shown later in this article that the amount of PM generated during idle is relatively low compared to higher speed and load conditions; therefore, the increase in cumulative PM generated over the iegr drive cycle is not as pronounced. The UHC emissions for the EEVO and iegr strategies are nearly the same as that of the conventional operation, as shown in Figure 9(j).

9 Gosala et al. 9 Figure 10. First law analysis at loaded idle shows that: (1) FE baseline has lower exhaust gas energy and lower losses than TM baseline, resulting in lower fuel consumption, with significant incylinder and EGR cooler heat losses, (2) EEVO results in higher exhaust heat rate at the cost of higher fuel consumption and (3) EEVO + iegr (NVO) helps in increasing exhaust heat rate by eliminating EGR cooler losses, despite showing higher incylinder heat losses. Figure 10 describes an alternate approach to understand the fuel consumption, TOT and exhaust flow rate trends, using the first law energy analysis illustrated with Figures 5 and 6 and equations (2) (9). The total fuel consumption of each strategy is shown and categorized into energy expended into brake work, exhaust heat, pumping loss, heat loss through the cylinders, heat rejection to the EGR cooler, and other thermal and mechanical losses. The brake power of all strategies is the same, corresponding to the same operating condition (loaded idle). The exhaust gas energy in FE baseline is 68% lower than TM baseline, due to the lower TOT and exhaust flow rate. The EEVO strategies with high and low AFRs produce 25% and 36% higher exhaust heat than TM baseline, respectively, as a result of elevated TOT, while both the iegr strategies produce 49% higher exhaust heat than TM baseline, and 24% and 13% higher than the EEVO strategies, despite having lower exhaust flow rate. The exhaust flow rates for both the iegr strategies compensate for the differences in TOT to yield the same exhaust heat. As described earlier, the pumping losses are lower for the iegr strategies as compared to the EEVO and TM baseline strategies due to the more open VGT position. The in-cylinder heat losses are higher for the iegr strategies than the EEVO strategies due to the uncooled trapped gases rejecting more heat through the cylinder walls, with the lower AFR iegr strategy showing higher heat loss due to a greater quantity of trapped residual gas. Higher in-cylinder heat losses for the iegr strategies are more than compensated for by the absence of heat rejection to the EGR cooler (due to the absence of external EGR) and lower pumping losses, which make the iegr strategies more fuel efficient than EEVO while maintaining higher exhaust heat rates. Figure 11. Exhaust gas energy is plotted against fuel energy with the dotted lines displaying the fraction of fuel energy converted to exhaust gas energy. EEVO + iegr (NVO) EEVO converts over 20% of fuel energy to exhaust heat followed by TM baseline and EEVO. Figure 11 shows an alternate representation of the first law analysis, where the energy transferred to the exhaust gas is plotted against injected fuel energy. The dotted contours represent the percentage of the injected fuel energy that leaves the engine via the exhaust gas flow. A higher percentage, along with higher fuel energy, yields a higher exhaust gas energy, which is generally preferable for exhaust thermal management. The iegr strategies show the highest conversion of fuel energy to exhaust heat (;22% and 24%), followed by TM baseline (;20%). The conversion efficiency of the EEVO strategies is lower than TM baseline due to higher losses (;18%), although the exhaust energy is higher on account of higher fuel consumption for the EEVO approach. FE baseline, which is most fuel efficient, results in approximately 10% of the injected fuel energy in the exhaust stream heat. Aftertreatment thermal management depends both on TOT and exhaust flow rate. The exhaust gas energy represented in Figures 10 and 11 does not take into account the dynamic temperature variation of the aftertreatment system. The impact of exhaust flow rate and TOT on warm-up of the aftertreatment system can be understood using a normalized heat transfer analysis. 11 The normalized heat transfer analysis, which models heat transfer from the exhaust gas to the catalysts, is an effective tool to understand the aftertreatment warm-up merits for various operating strategies. The DOC, DPF and SCR are treated as a single lumped catalyst bed at a temperature T cat. The heat transfer from the exhaust gas to the catalyst is modeled by equation (10), where m exh is the exhaust flow rate, TOT is the TOT and C is a constant Q = C 3 _m 4 5 exh 3 (TOT T cat ) ð10þ This model yields an approximate heat transfer rate from the exhaust gas to the aftertreatment system, for a given lumped catalyst temperature, as a function of the

10 10 International J of Engine Research 00(0) Table 4. TOT and exhaust flow rate of warm-up strategies, also shown in Figure 9. Operation AFR TOT ( C) Exhaust flow rate (normalized) FE baseline TM baseline EEVO EEVO EEVO + iegr(nvo) EEVO + iegr(nvo) TOT: turbine outlet temperature; AFR: air-to-fuel ratio; FE: fuel efficiency; TM: thermal management; EEVO: early exhaust valve opening; NVO: negative valve overlap. Figure 12. Normalized heat transfer rates from the exhaust gas to the aftertreatment system at different lumped catalyst bed temperatures, for the described thermal management strategies. experimentally measured exhaust flow rate and TOT. A positive heat transfer rate corresponds to catalyst warm-up, as heat is transferred from the exhaust gas to the catalyst when TOT is higher than the catalyst bed temperature T cat. A negative heat transfer rate corresponds to catalyst cool-down, as the heat is transferred from the catalyst to the exhaust gas when T cat is above TOT. The zero crossing occurs when T cat = TOT. High heat transfer rates at lower catalyst bed temperatures are desirable of aftertreatment warm-up strategies. Figure 12 compares the normalized heat transfer rates for the six strategies. The iegr strategies show the highest heat transfer, followed by the EEVO and TM baseline strategies. Both the high and low AFR iegr strategies display similar heat transfer rates at lower catalyst bed temperatures, due to a trade-off between their TOTs and exhaust flow rates, shown in Table 4, although they diverge once the catalyst is hot, since TOT plays a more dominant role for warm-up at higher temperatures. The EEVO strategy with lower AFR shows higher heat transfer than EEVO with higher AFR as a result of its elevated TOT and similar exhaust flow rates. It may therefore be concluded that iegr shows the best potential for accelerating the warm-up of the SCR catalyst owing to its high TOT and comparable exhaust flow rate, with lower fuel consumption than EEVO, at loaded idle operation. The following section demonstrates the implementation of iegr at loaded idle over the HD-FTP drive cycle. Drive cycle performance Methodology Experimental HD-FTPs were run to compare the warm-up characteristics of the previously described strategies. It was concluded from the previous section that the iegr strategy showed best potential for aftertreatment warm-up and was therefore implemented over the drive cycle. The low AFR iegr strategy was chosen as a result of the lower steady state NOx emissions, although both the iegr strategies are expected to display similar aftertreatment warm-up characteristics, as shown in Figure 12. The HD-FTP test procedure consists of a cold-start drive cycle, followed by an identical hot-start drive cycle with a soak period of 20 min between the cycles. The speed and torque profile for the HD-FTP drive cycle is shown in Figure 13. The aftertreatment components was brought to the same initial conditions prior to starting the cold cycle, by flowing fresh air at 25 C from an external source through the aftertreatment system. The drive cycle was started after the inlet and outlet gas temperatures of all the catalysts stabilized at 25 C. The composite drive cycle fuel consumption and emissions are calculated by taking a weighted mean of the fuel consumption and emissions for the cold- and hot-start cycles, normalized with the weighted brake work, with weighting factors of 1/7 and 6/7, respectively, 21 as shown by equations (11) (13) FC composite = NOx composite = PM composite = 1 7 FC cold FC hot 1 7 W brake, cold W brake, hot 1 7 NOx cold NOx hot 1 7 W brake, cold W brake, hot 1 7 PM cold PM hot 1 7 W brake, cold W brake, hot ð11þ ð12þ ð13þ Tailpipe-out NOx is estimated by passing the measured engine-out NOx flow rate through an SCR conversion efficiency curve, as shown in Figure 14. The efficiency curve assumes that the SCR catalyst is presoaked with urea. Therefore, as illustrated, NOx conversion is assumed to start after the SCR exceeds 100 C and reaches a maximum of 96% efficiency between 300 C and 450 C. Using SCR efficiencies corresponding to the measured gas temperatures at the outlet of the SCR, tailpipe-out NOx is predicted. The catalyst bed temperature is expected to be higher than

11 Gosala et al. 11 Figure 13. Strategies for aftertreatment warm-up thermal management were implemented at the shaded loaded idle portions of the HD-FTP. Table 5. The thermal management strategies were implemented during idle operation, with the remainder of the drive cycle in baseline operation. Drive cycle Idle operation Other operating conditions FE baseline cycle FE baseline FE baseline calibration TM baseline cycle TM baseline TM baseline calibration iegr cycle EEVO + iegr (NVO) TM baseline calibration FE: fuel efficiency; TM: thermal management; iegr: internal exhaust gas recirculation; EEVO: early exhaust valve opening; NVO: negative valve overlap. drive cycle was run with the best fuel consumption settings both at idle and other operating conditions. Table 5 summarizes the operating calibrations used by various drive cycles at idle and non-idle operating conditions. Figure 14. SCR NOx conversion efficiency curve used to predict tailpipe-out NOx emissions over the drive cycle. The SCR catalyst is assumed to be initially soaked with urea. the temperature of the gas exiting the catalyst; therefore, the results present a worst-case scenario for tailpipe-out NOx prediction. iegr was implemented at idle regions throughout the cold-start HD-FTP cycle and during the first 600 s of the hot-start cycle, as represented by the shaded regions in Figure 13, to maximize the SCR catalyst temperature during both the drive cycles. The engine was operated in the stock thermal management calibration over all other operating conditions. The FE baseline Results and discussion The iegr strategy with AFR of 22 has been shown to emit ;23 times higher PM than the TM baseline strategy at steady state loaded idle operation, as illustrated in Figure 9. However, Figure 15 demonstrates that over the HD-FTP drive cycle, the iegr strategy emits only 6.7% higher engine outlet PM than the TM baseline strategy. This is a result of low exhaust flow rates during idle operation, which reduces the impact of elevated idle PM over the drive cycle. Increased engine outlet PM is undesirable; however, it is straightforward to perform an active, or passive regeneration, albeit with a fuel penalty. The DPF can trap PM without reaching elevated temperatures; therefore, engine-out PM will not reach the tailpipe when the aftertreatment system is cold, unlike the SCR system which requires warm-up for efficient NOx conversion. Turbine outlet temperatures achieved over the HD- FTP drive cycle are shown in Figure 16. The iegr strategy results in the highest TOT when implemented at loaded idle, followed by the TM baseline strategy, consistent with the steady state results shown in Figure

12 12 International J of Engine Research 00(0) Figure 15. Implementing EEVO + iegr (NVO) during the idle regions shown, results in 6.7% higher engine-out PM over the HD- FTP drive cycle. Figure 16. iegr via NVO and EEVO, and EEVO at idle show higher TOT than the baseline calibrations during the drive cycle, notably during the idle regions (shaded). 9. The iegr strategy also shows higher TOT at other operating conditions as a result of heating up the engine block, exhaust manifold and turbocharger more quickly. It can be observed that the TOT for the TM baseline strategy reduces during loaded idle between 250 and 300 s of the drive cycle, thereby cooling the engine, while the TOTs for the iegr drive cycle increases toward the steady state TOT and therefore continues to heat up the engine block. As a result, higher TOTs are maintained at subsequent non-idle conditions even though the iegr strategy is not being implemented at all operating conditions. Although the steady state TOT is not reached during any of the loaded idle regions, it may be observed that the TOT increases faster for strategies having higher steady state TOT. Figure 17 demonstrates that the iegr and TM baseline drive cycles outperform the FE baseline with regard to warming up the SCR system. Besides, the SCR temperatures at the start of the hot cycle are higher for the TM baseline and iegr strategies on account of greater heat transfer to the catalyst over the cold cycle. The iegr strategy shows higher SCR temperatures than TM baseline between ;450 and ;750 s in the cold cycle and between ;100 and ;800 s in the hot cycle, as a result of faster aftertreatment component warm-up. Figure 18 shows the predicted SCR conversion efficiency using the measured SCR outlet gas temperature (Figure 17) and the SCR conversion efficiency curve (Figure 14). Peak SCR efficiency is achieved when the SCR temperature reaches 300 C, which occurs around 700 s into the drive cycle. The iegr strategy shows higher SCR conversion efficiency than TM baseline and FE baseline strategies between ;450 and ;750 s in the cold cycle and between ;100 and ;650 s in the hot cycle, at which point the efficiency reaches its maximum value. The measured cumulative engine-out NOx and predicted cumulative tailpipe-out NOx over the drive cycle are shown in Figure 19. The predicted tailpipe-out NOx

13 Gosala et al. 13 Figure 17. Higher SCR temperature is predicted when EEVO and iegr via EEVO and NVO are implemented at idle. Figure 18. Higher SCR conversion efficiency is predicted for EEVO and iegr via NVO and EEVO due to higher predicted SCR catalyst temperature. is the same as engine-out NOx until ;450 s in the cold cycle since the SCR has not yet reached sufficient temperatures to operate efficiently, per Figures 17 and 18, after which the cumulative tailpipe-out NOx becomes almost constant due to higher SCR conversion efficiencies. Although the iegr strategy has slightly higher engine-out NOx than TM baseline drive cycle, better SCR warm-up with iegr results in higher conversion efficiency during the drive cycle, as a result of which the predicted tailpipe-out NOx with iegr is the lowest. The tailpipe-out NOx is significantly lower during the hot cycle as the aftertreatment components are already warm. It may therefore be concluded that using iegr at idle operation allows reduction of tailpipe-out NOx to levels not achievable via conventional engine operation, through faster warm-up of the aftertreatment system. Figure 20 shows the additional fuel consumption required to achieve the predicted reduction in tailpipeout NOx, with respect to the TM baseline operation. The iegr drive cycle consumes higher fuel, as expected, with the difference occurring only during the loaded idle regions since the same TM baseline strategy is implemented at all non-idle operating conditions. The FE baseline drive cycle uses a different calibration both at idle and non-idle operations, and therefore, a reduction in fuel consumption is observed throughout both drive cycles. The weighted fuel consumption increase for the iegr drive cycle over the TM baseline cycle is 2.1%. The trade-off curves among cumulative predicted tailpipe-out NOx, fuel consumption and engine outlet PM, weighted over both the cycles, are shown in Figure 21(a). Figure 21(a) demonstrates that the iegr strategy enables tailpipe-out NOx levels lower than that possible via conventional means, with a similar trade-off between NOx and fuel consumption as that achieved by conventional means. Specifically, the TM baseline strategy of delayed injections and mostly closed VGT reduces predicted tailpipe-out NOx by 36% from the fuel-efficient FE baseline operation, by consuming

14 14 International J of Engine Research 00(0) Figure 19. Implementing iegr via NVO and EEVO at idle shows the lowest predicted tailpipe-out NOx despite having highest engine-out NOx, followed by EEVO at idle, due to their superior SCR conversion efficiency. Figure 20. An increase in fuel consumption during idle regions with respect to TM baseline can be observed, as expected, over the HD-FTP when EEVO and iegr via NVO and EEVO are implemented. FE baseline has lower fuel consumption throughout. 5.7% higher fuel. The iegr strategy further reduces tailpipe-out NOx by 7.9%, with consumption of 2.1% more fuel over the drive cycle. Figure 21(b) shows the trade-off between predicted tailpipe-out NOx and engine-out PM over the drive cycle. Summary A combination of EEVO and iegr (NVO) has been shown capable of reducing the tailpipe NOx levels below that achievable via conventional means alone, through faster warm-up of the aftertreatment components. The reduction in tailpipe-out NOx occurs with a fuel consumption penalty, with a trade-off between NOx and fuel consumption similar to that achieved by conventional engine operation. Higher PM emissions require more frequent active or passive regeneration of the DPF. More specifically: 1. EEVO allows a steady state TOT of 346 C at loaded idle, close to 100 C higher than the TM baseline, at the cost of 56% higher fuel consumption and up to 12 times higher PM emissions. Inefficient engine operation caused by the early exhaust gas blowdown allows for injection of greater fuel quantity, resulting in higher energy transfer to the exhaust gas. Increasing AFR above 24:1 reduces PM and fuel consumption but also reduces TOT. 2. Internal EGR, together with no external EGR, when implemented with EEVO, enables a steady state TOT of 403 C, with 42% higher fuel consumption than TM baseline strategy (14% fuel savings w.r.t. EEVO strategy). Absence of external EGR eliminates heat loss through the EGR cooler, thereby better utilizing the fuel energy to warm-up the exhaust gas. PM emissions at steady state are

15 Gosala et al. 15 Figure 21. (a) Trade-off between predicted tailpipe-out NOx and fuel consumption and (b) trade-off between predicted tailpipeout NOx and engine-out PM emissions over the combined HD-FTP drive cycle. up to 24 times higher than the TM baseline strategy. Increasing AFR above 24:1 reduces PM and maintains similar warm-up characteristics but increases engine-out NOx. Higher TOT with similar exhaust flow rates and lower fuel consumption makes EEVO + iegr (NVO) a more favorable aftertreatment warm-up strategy than EEVO. 3. EEVO + iegr (NVO) at loaded idle over the HD-FTP helps achieve higher SCR temperatures over both the cold and hot drive cycles, thereby resulting in a 7.9% reduction in predicted tailpipeout NOx, at the cost of 2.1% higher fuel consumption and 6.7% higher engine outlet PM. 4. VVA strategies at loaded idle help achieve faster SCR warm-up than the baseline strategies. Implementation of similar strategies at other nonidle operating conditions is needed to further accelerate aftertreatment component warm-up. Future work Future work in this direction could involve identification of means by which the PM emissions of iegr can be reduced during steady state operation. Furthermore, a comparison study between different ways of achieving iegr using flexible valve actuation specifically, exhaust reinduction and exhaust trapping may also be undertaken. Acknowledgements The authors would like to thank the shop staff at Herrick Laboratories David Meyer and Ron Evans, for the immense help and support they have extended toward this work. Declaration of conflicting interests The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Purdue shares a nondisclosure agreement with its sponsors Cummins and Eaton, as a result of which absolute data values are not shown. All data are normalized with respect to the baseline calibration. Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The project is funded by Cummins and Eaton. References 1. Stadlbauer S, Waschl H, Schilling A and del Re L. DOC temperature control for low temperature operating ranges with post and main injection actuation. SAE technical paper , Song X, Surenahalli H, Naber J, Parker G and Johnson JH. Experimental and modeling study of a diesel oxidation catalyst (DOC) under transient and CPF active regeneration conditions. SAE technical paper , Koebel M, Elsener M and Kleemann M. Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines. Catal Today 2000; 59(3): Charlton S, Dollmeyer T and Grana T. Meeting the US heavy-duty EPA 2010 standards and providing increased value for the customer. SAE Int J Commer Veh 2010; 3(1): Girard J, Cavataio G, Snow R and Lambert C. Combined Fe-Cu SCR systems with optimized ammonia to