## 5 EGH Optimization  
From the results presented above, one can realize that a 12 kW EGH, such as the one tested in this paper, is extremely disproportionate considering that this power should come from an on-board generator (i.e., alternator). Although this huge amount of power provides a great increase in \(T_{a\_EGH}\) with initial values over 260 \({}^{\circ}\)C, if we consider the manufacturer SCR efficiency curve represented in Figure 9, it can be observed that heating exhaust gases above 220 \({}^{\circ}\)C has no sense. The SCR presents a maximum efficiency point of 95% at \(T_{a\_EGH}=220\)\({}^{\circ}\)C. Temperatures above this value will not bring any improvement in NOx reduction but will cause the alternator to consume more fuel to feed the EGH. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Test**} & \multirow{2}{*}{\
\begin{tabular}{c} **Engine** \\ **Regime (rpm)** \\ \end{tabular} } &
\begin{tabular}{c} **EGH Energy** \\ **Consumption** \\ **(kWh)** \\ \end{tabular} & \multirow{2}{*}{**Duration (s)**} & \multirow{2}{*}{**Average EGH Power**} & \multirow{2}{*}{**NOx**} & \multirow{2}{*}{**Urea**} \\  & & & & & & **Consumption (kW)** \\ \hline
1 & 1000 & 0.191 & 340 & 2.02 kW & 87.97 & 69.67 \\ \hline
2 & 1250 & 0.098 & 300 & 1.18 kW & 83.46 & 19.59 \\ \hline
3 & 1500 & 0.213 & 260 & 2.95 kW & 89.46 & 1.31 \\ \hline \end{tabular}
\end{table} 
Table 2: EGH consumption and NOx reduction results. Figure 8: Results from Test 3 including NOx values, FTPP and gas temperatures at 1500 rpm. From the experimental data presented in the previous section it has been possible to draw the experimental SCR efficiency curve. From Figure 9, it can be seen that when the EGH is active the curve maintains the efficiency above 90% for temperatures below 220 \({}^{\circ}\)C. Note that the points within the circle cannot be taken into account since urea was not being injected at that moment. This can be explained by the fact that the urea injection decision is made from the temperature of the substrate, and not from the temperature of the gases going into the SCR, \(T_{SCR\_IN}\), used in Figure 9.  
With \(T_{a\_EGH}\) value set at 220 \({}^{\circ}\)C, \(T_{b\_EGH}\) and exhaust mass flow rate known from the previous tests it is possible to determine the theoretical power consumption to achieve 95% of SCR conversion efficiency. Figure 10 shows the optimized EGH power that guarantees a SCR efficiency of 95%. Note that these values consider that all the electrical energy consumed by the EGH was exchanged as thermal energy into the exhaust gases. It can be observed that its consumption is lower than the EGH power used in previous tests. At 1000 rpm, there is a top consumption peak of 5 kW and it is remarkably lower than the 11 kW achieved during the test 1. Its consumption value is zero only at the end of the test when \(T_{b\_EGH}\) is over 220 \({}^{\circ}\)C and no extra energy is needed. At 1250 rpm, the theoretical EGH power reach a peak of 2.2 kW during the first 50 s of testing and remains zero after \(T_{b\_EGH}\) exceeds the 220 \({}^{\circ}\)C target. Finally, in test 3 it can be seen a slightly different scenario as EGH power consumption remains below 0.5 kW. Note that the power required by the EGH depends on the exhaust gas conditions (i.e., mass flow rate and temperature), which fluctuate depending on the FTPP, engine regime and other ECU parameters that control EGR valve aperture, for instance. At this point, it is interesting to quantify the energy that a conventional HDV will require on a real operation profile. To do so it is necessary to determine the time a HDV spends at low engine regimes, typically during urban or suburban driving. Considering that the major part of the NOx emitted occur at low-speed, see Figure 6, this analysis will focus on the most demanding EGH regime of 1000 rpm. Figure 11 presents a real mission profile of a long-haul heavy-duty vehicle.