The test engine used for this program was a production 2018 model year Cummins X15 engine with a 500 hp production calibration. The engine retained the production air handling system, Exhaust Gas Recirculation (EGR) system, internal components, and fuel system. The engine, as shown in Figure 1, is an inline six-cylinder with a bore-to-stroke ratio of 0.8:1, displacement of 15 L, rate power of 373 kW at 1,800 rpm, and a peak torque of 2,500 Nm at 1,000 rpm.

The engine is expected to run mainly in two different operating modes, commonly referred to in this work as thermal management (TM) and fuel economy (FE) modes. The authors attributed the transition between these operating modes of the engine to being triggered based on the downstream primary SCR temperature. As the names suggest, the engine runs on the TM mode when the downstream primary SCR temperature is low. During the TM mode, the engine generates more heat leading to low EO NO_x and higher fuel consumption and vice versa in the case of the FE mode.

The AT system used in this program was a Full Useful Life (FUL) hydrothermally aged system. An accelerated aging protocol (Zavala et al., 2020) was used to age the AT system by thermal exposure to FUL of 435,000 miles, similar to previous works (Harris et al., 2021; Matheaus et al., 2021; Zavala et al., 2022). These aged catalysts are commonly referred to as “Development Aged” end-of-life catalysts (Sharp et al., 2021). These Development Aged catalysts were not exposed to chemical aging such as sulfur and lubricant poisoning.

Figure 2A shows a schematic of the following production-intent AT system in several industry architectures. The AT system consists of a conventional DEF doser, LO-SCR, a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), and a compact mixer followed by SCR and Ammonia Slip Catalyst (ASC). Generally, such an advanced arrangement for the downstream conventional portion of DOC-DPF-SCR-ASC represents a 2022 production system. This combination of LO-SCR and conventional downstream AT is considered the “baseline” in this demonstration (Meruva et al., 2022). Unlike previous studies that used a heated DEF doser upstream of the LO-SCR (Harris et al., 2021; Matheaus et al., 2021; Sharp et al., 2021; Zavala et al., 2022), this study uses a standard DEF doser in both locations. Catalyst specifications are shown in Table 1.

The focus of this work was to evaluate the addition of an EHM system upstream of the LO-SCR, with the remaining AT being the same as the baseline, as shown in Figure 2B. The actual setup is pictured in Figure 3. In this setup, the conventional mixer upstream of LO-SCR was replaced with the EHM system. The purpose of adding the EHM system was to heat up and maintain the LO-SCR at an optimal temperature for efficient NO_x reduction, particularly for rapid heat up during cold start and/or during low-temperature drive cycles.

The EHM system delivers heat to DEF droplets injected in the exhaust pipe, accelerating their conversion to the desired ammonia reductant. It is particularly helpful in lower exhaust temperatures or in low-load cycles while mitigating deposit formation. The EHM system comprises two sections: the EHM and its embedded E-heater. The EHM is the first section of the EHM system, which heats up, enabling DEF injection in exhaust temperatures as low as 130°C. AT system might need additional heat depending on several factors such as engine calibration, cold start, or rapid heat up strategy, AT architecture, system demand or certification target. At these instances, the second section of the EHM system, which is the E-heater, helps in heating the exhaust gas and the downstream catalysts, accelerating catalyst warm-up, especially the SCR catalysts (light-off or primary). The EHM system was positioned upstream of the LO-SCR in this study. In this program, along with the E-heater, the heat generated by the EHM was also leveraged to heat the exhaust gas until the turbo-out temperature reached a certain target, after which the EHM would primarily heat the DEF. During the earlier state, the EHM’s bed temperature would be higher than the latter.

Figures 4A,B show the EHM model and an EHM system model, respectively, each schematically connected to the controller. Figure 4C shows an iso view of a model of the EHM system. The EHM system was designed to operate in 12, 24, or 48 V systems. The demonstration in this work was conducted on a 48 V system, with the first section of the EHM system (Figure 4A) providing 3.8—4 kW and the second section (Figures 4B,C) providing nearly 2 kW, together about 6 kW. Similar to the previous works with E-heaters (Dhanraj et al., 2022; Zavala et al., 2022), the EHM system also contains an E-heater embedded in it. EHM System was used to replace the conventional DEF doser mixer and heated doser as the EHM could heat the DEF with a conventional doser. The swirl plate at the end of the EHM system is used to distribute the flow into the cone upstream of the LO-SCR.

A control algorithm managed the EHM system for heating the AT system. The controller receives signals from the CAN-bus (e.g., mass flow rate, exhaust temperature, DEF injection rate, and NO_x sensor signal). The controller (i.e., in real time) readily manages the EHM system operations to minimize the power consumption while yielding the needed reductant concentration (ammonia), maximizing SCR efficiency, and mitigating deposit formation. In this program, the E-heater was controlled by the Model Based Controller (MBC) and the EHM was controlled by a separate controller.

Figure 5 shows a representative control where the EHM and its embedded E-heater are controlled independently to raise the LO-SCR temperature to 130°C, where DEF can then be injected. Once this is achieved, the power demand reduces. Actual power traces for this work are shown in the test results section. In this example, the E-heater was targeting 130°C at the SCR temperature, and it shuts off once it is achieved. The maximum power capacity of the EHM is 4 kW, and its embedded E-heater is 2 kW.

The EHM system is powered up by an external power supply in the engine test cell. The power was calculated by recording the voltage and current transmitted to them. This total power was fed to the dynamometer as a parasitic load onto the engine in real time, assuming an 80% generator efficiency consistent with previous works (Matheaus et al., 2021). This additional load was not accounted for in the cycle work, but the emissions resulting from this load were included in the test result analysis. Eq. 1 shows the formula used for this calculation: T o t a l   E n g i n e   T o r q u e   ( N m ) = C y c l e   T o r q u e   ( N m ) +   E H M   P o w e r   ( k W ) + E − h e a t e r   P o w e r   ( k W ) 0.8 ∗   9548.8 E n g i n e   S p e e d   ( r p m ) . (1)

This program used a model-based controller (MBC) for the upstream and downstream SCR catalysts (Sharp et al., 2017; Rao et al., 2020) to control the DEF injection, which is consistent with the baseline work (Meruva et al., 2022). The model tracks ammonia storage in each of the SCR bricks and has a target ammonia storage based on temperature. This work adapted the MBC to control the thermal management strategies to power on the flow heating capabilities of E-heater using the average LO-SCR temperature as the feedback.

Conventional DEF dosers were used during this work. The DEF dosing minimum temperature was set to 180°C throughout the system for the baseline work (Meruva et al., 2022). The EHM system upstream of LO-SCR allowed the DEF dosing to begin at a lower temperature, down to 130°C for the LO-SCR. The primary downstream SCR maintained a 180°C dosing temperature consistent with previous work (Sharp et al., 2021). The DEF dosing trigger temperature was set to the average gas temperature of LO-SCR and the average gas temperature of the first primary SCR for the respective dosing. In this work, a conventional doser coupled with an EHM system was used to replace the heated DEF doser.

Dosing at low temperatures (130°C) can be useful for increasing ammonia storage in the SCR catalyst early in the cycle. It has been demonstrated on both bench testing and engine testing (Masoudi et al., 2022a; Masoudi et al., 2022b) that EHM strongly promotes both ammonia storage and increased SCR NO_x conversion efficiency.

With an advantage of better DEF atomization with the EHM system when compared to the baseline, the DEF injection strategy upstream of LO-SCR was modified to be more aggressive at low temperatures and less aggressive at high temperatures, assuming the primary SCR to be hot enough to help with the NO_x conversion. This dosing strategy helps reduce NO_x at lower temperatures while reducing the risk of ammonia slip from the primary SCR during high exhaust temperature operation.

Raw exhaust measurements complying with a Code of Federal Regulations Part 1065 were used in this work, which included the following:

• A raw Horiba MEXA 7000 series each for the EO and TP emissions sampling.

The NO_x and CO₂ measurement variability are ∼ ± 0.001 and ∼ ± 2 g/hp-hr, respectively. This is true for all the test results discussed in this work.

This section describes the different drive cycles used to evaluate the performance of the different AT configurations.

The FTP, a regulatory drive cycle in the United States, was tested, also referred to as the heavy-duty transient cycle, which includes a cold and hot cycle. The CARB 2027 NO_x regulatory standard for the FTP drive cycle drops by 90% from 0.2 to 0.02 g/hp-hr (California adopts strong new regulation to further reduce smog-forming pollution from heavy-duty diesel trucks, 2022).

The LLC is a real-world drive cycle consisting of data collected from different applications. This cycle is a new regulatory cycle that is approximately 1.5 h. This cycle focuses on sustained low load, short and long idles, high accelerations after a pronounced cooling period, and low-speed cruise with motoring (MSPROG, 2022). The CARB NO_x regulatory limit is at 0.05 g/hp-hr for the year 2027.

The Beverage Cycle is a subset of the LLC. This cycle is derived from a food service delivering truck (MSPROG, 2022). The cycle power is less than the LLC, representing a sustained low load condition. It is only 800 s in length, much shorter than the LLC. As a result, this cycle was repeated four times in succession for thermal conditioning, and the last two cycles were later analyzed for stable results.

The Stay Hot Cycle focuses on the AT performance after a prolonged period of cooling by idling the engine for 40 min. This cycle involves operating at a preset speed and load preceding the long idle for thermal conditioning of the engine and the AT system until the temperatures attain a steady state. The test then drops to idle for a 40 min time period where the effect of the AT cooling and NO_x reduction can be assessed.

The drive cycle named the World Harmonized Transient Cycle (WHTC) is based on the global pattern of heavy-duty commercial vehicle usage. It is a transient engine dynamometer cycle. Both the cold and hot tests for the WHTC are reported.

Table 2 shows the FTP composite values for the baseline AT along with the added EHM system upstream of the LO-SCR. FTP composite brake-specific emissions are calculated based on 1/7 of the cold test and 6/7 of the hot test. The results reveal that the baseline AT system equipped with the EHM system reduces the composite FTP TP Brake Specific NO_x (BSNO_x) emission by 3.3-fold (from 0.06 to 0.018) compared with the baseline AT, complying with the NO_x regulatory limit of 0.02 g/hp-hr for the year 2027 California regulations, also now in consideration with the US EPA for 2031 and beyond (Summary of EPA proposal, 2022).

The rise in Brake Specific CO₂ (BSCO₂) of ∼2% compared with the baseline AT testing is due to the fuel penalty for powering the EHM system. Further optimization is warranted to improve the NO_x and CO₂ tradeoff.

Table 3 shows that the AT equipped with the EHM system helps reduce NO_x in cold FTP twofold compared with the baseline AT with approximately a 2% fuel penalty. The higher LO-SCR out NO_x for the test with EHM system configuration was because of the change in the DEF injection strategy to support better NO_x conversion at low-temperature conditions and to reduce the ammonia slip at high-temperature conditions. The EHM system energy consumption was 0.47 kW-hr, which comprises EHM and its embedded E-heater consuming 0.235 and 0.235 kW-hr, respectively. Each element of the EHM system uses the same amount of power on the Cold FTP.

The EHM was set to a maximum of approximately 4 kW, and it tries to heat the exhaust gases until the turbine out temperature is at 200°C. Then, it primarily heats the DEF. Its embedded E-heater was set to a maximum of 2 kW, and it targets the LO-SCR average temperature to reach 235°C. Once the target temperatures were reached, the EHM system was turned off.

Figure 6 shows a graphical comparison of the baseline AT and the AT + EHM system configurations using LO-SCR average temperature, primary SCR inlet temperature, EO NO_x, LO-SCR out NO_x, TP NO_x, the heater power consumption, and BSCO₂ as the parameters for the cold FTP cycle.

As observed from the average LO-SCR temperature plot, the catalyst temperature cools down in the baseline run during an idle event in the baseline configuration, which is avoided with the AT + EHM system configuration. This helps the LO-SCR convert more NO_x during the initial 500 s of the cycle by increasing the LO-SCR temperature; however, this also adds to the fuel penalty for powering up the EHM system by about 2%. As mentioned earlier, during 800–1000 s of the cycle, the DEF dosing strategy reduces the DEF injection at the LO-SCR, allowing the primary SCR to convert most of the NO_x as the primary SCRs are hot enough to convert NO_x at these regions.

Table 4 shows the numerical comparison of the baseline AT and AT + EHM system configurations for the hot FTP cycle. The EHM system control strategies were the same between the hot FTP and the cold FTP cycles, except that, in the hot FTP cycle, the heating function of the E-heater targeted 225 °C as the LO-SCR average temperature. The table displays that the AT equipped with an EHM system helps drop the TP NO_x by almost 5.5-fold (from 0.043 to 0.008) with a fuel penalty of approximately 2%. The EHM system energy consumption for the hot FTP was 0.3 kW-hr, which comprises EHM and its embedded E-heater consuming 0.2 and 0.10 kW-hr, respectively. About two-thirds of the energy is used by the EHM, whereas its embedded E-heater contributes the remaining third. Note that even the hot FTP requires power to the EHM system, which is about 36% less than the cold FTP.

Figure 7 compares the baseline AT and the AT + EHM system configurations using LO-SCR average temperature, primary SCR inlet temperature, EO NO_x, LO-SCR out NO_x, TP NO_x, the heater power consumption, and BSCO₂ as the parameters for the hot FTP cycle.

The hot FTP test results show that, unlike the baseline configuration, the AT + EHM system configuration sustains the LO-SCR operational temperatures during the idling events until 400 s. This heat at the LO-SCR helps the primary SCR retain the temperature from the previous cycle enabling improved TP NO_x conversion performance by more than fourfold or more than 80%. The DEF injection strategy was the same across all the AT + EHM system configuration tests, which explains the lower LO-SCR NO_x conversion during 800—1,000 s of the cycle, such as the cold FTP.

Table 5 shows the numerical comparison of the baseline AT and AT + EHM system configurations for the LLC. It reveals the AT + EHM system helps reduce the TP NO_x emission by 22-fold, which is a cycle-averaged TP NO_x conversion efficiency of 99.3%, relative to 82.5% in the baseline. The EO NO_x is higher for the AT + EHM system configuration test as the heat generated from the EHM system is gradually transmitted across the AT system heating the primary SCR, and this enables the engine to run in FE mode. A certain threshold of primary SCR temperature helps the engine stay in FE mode for a longer duration, reducing the CO₂ penalty and increasing the EO NO_x, but as the EHM system was powered on for most of the test, the overall CO₂ penalty was still higher by approximately 5%. The EHM system energy consumption was 4.3 kW-hr, which comprises EHM and its embedded E-heater consuming 3.04 and 1.26 kW-hr, respectively. Approximately 70% of the electrical energy is put into the EHM and the remaining into its embedded E-heater for the LLC.

The heating strategy controlled by the MBC targeted the average LO-SCR temperature based on the inlet temperature of the downstream SCR 1, as shown in Table 6. The EHM control strategy was the same as the FTP cycles, except that it tried to heat the exhaust gas until the turbo-out was at 180°C.

Figure 8 compares the LLC between baseline AT and the AT + EHM system configurations. Control parameters are LO-SCR average temperature, primary SCR inlet temperature, EO NO_x, LO-SCR out NO_x, TP NO_x, the heater power consumption, and BSCO₂.

The baseline results show that the LO-SCR was ineffective for most of the cycle in converting the EO NO_x due to the extremely low-temperature range. However, with the AT + EHM system configuration, the average LO-SCR temperature was mostly >200°C over the entire cycle, helping it convert NO_x even after long idles, which normally cools down the AT system (as it did in the baseline run). Furthermore, in the AT + EHM system, the heat from the upstream catalyst was gradually transferred to the downstream SCR, which helped reduce the TP NO_x by 22-fold or an additional ∼96% to a net TP NO_x efficiency of 99.3% in this configuration compared to the baseline helping the system to maintain the TP NO_x within the California 2027 regulatory limit.

Table 7 shows the numerical comparison of the baseline AT and AT + EHM system configurations for the Beverage Cycle. The EHM system control strategy was the same as the LLC. The AT + EHM system reduces the baseline TP NO_x by 38-fold (from 1.669 to 0.044). It produces a cycle-averaged NO_x reduction efficiency of 99.2% versus 58.9% in baseline configuration. Similar to the LLC, the engine runs a lot more in the FE mode in the configuration with the EHM system, which explains the drastic difference in the EO NO_x values. The EHM system energy consumption was 1.76 kW-hr, which comprises EHM and its embedded E-heater, consuming 1.08 and 0.68 kW-hr, respectively. The EHM requires 62% of the electrical energy, whereas its embedded E-heater uses 38%.

Figure 9 compares the Beverage Cycle with baseline AT and the same with EHM system configurations using control parameters LO-SCR average temperature, primary SCR inlet temperature, EO NO_x, LO-SCR out NO_x, TP NO_x, the heater power consumption, and BSCO₂.

The LO-SCR NO_x conversion increases in the AT + EHM system configuration compared to baseline by maintaining the temperature above 200°C for most of the cycle. This heat transferred onto the downstream SCR helped in the overall reduction in TP NO_x compared to baseline. Though the Beverage Cycle is not a regulatory cycle, the TP NO_x emission values were within 0.05 g/hp-hr with the AT + EHM system configuration, even with an increased EO NO_x. The net fuel penalty for powering the EHM system was approximately 10%. The primary SCR temperature shown in Figure 9 is the inlet gas temperature. The actual catalyst bed temperature is expected to be a little hotter, which might help reduce the outlet NO_x from the LO-SCR.

Table 8 shows the numerical comparison of the baseline AT and AT + EHM system configurations for the Stay Hot Cycle. The cycle used the EHM to provide extra heat to the system. This reduced fuel penalty, relying solely on the EHM to heat the AT system. The EHM control strategy is the same as that described in the LLC. As observed in Table 8, the AT + EHM system configuration reduces the TP NO_x by almost eightfold (from 0.238 to 0.031) with a fuel penalty of approximately 5%. The EHM system energy consumption was 1.30 kW-hr, which was consumed completely by the EHM as its embedded E-heater was turned off during this cycle.

Figure 10 compares the baseline AT and the AT + EHM system configurations using LO-SCR average temperature, primary SCR inlet temperature, EO NO_x, LO-SCR out NO_x, TP NO_x, the heater power consumption, and BSCO₂ as the parameters for the Stay Hot Cycle.

Though the EHM is mostly advantageous for converting DEF to reductants, it was used in this Stay Hot Cycle to provide enough heat to retain the LO-SCR temperature for long durations to keep the NO_x conversion active, which does not happen in the case of the baseline, as the LO-SCR temperature drops down to ∼120°C where it likely cannot respond to any incoming EO NO_x.

The WHTC Composite is provided first, followed by details on the WHTC cold and then the hot cycles.