Premixed Charge Compression Ignition Analysis
7.2 Emission Optimization using Statistical Methods
The effects of splitting the first injection of the PCCI strategy into two injection pulses followed by a final triggering injection closer to TDC have been investigated in this section under two engine loads: 10mg and 15mg of fuel. An experimental analysis has been carried out to simultaneously optimize diesel engine fuel and air operating parameters for low exhaust emissions using the Taguchi method. The three parameters investigated are the first and second fuel injection ratios while keeping constant the final fuel injection amount, the valve opening time for the exhaust ports and the exhaust back pressure.
Three parameters were modified and tested under various operating conditions. The first parameter investigated is the first and second fuel injection ratio while keeping the final fuel injection amount constant. It was shown earlier that the injection ratio and timing can have a significant effect on emission formation and engine performance. The second tested parameter is the valve opening time for the exhaust ports. Exhaust valve timing can have a significant effect on the exhaust gas extrusion process and, therefore, change the amount of exhaust gases trapped in the cylinder working as internal EGR.
The final parameter is the exhaust back pressure. The EBP is commonly met in the state-of-the-art diesel engines where modern after-treatment and advanced EGR systems are used. The EBP can have negative effects on engine performance and fuel consumption due to high pumping losses, but at the same time can contribute to the optimization of some harmful emissions due to the restricted exhaust gas extrusion from the cylinders.
7.2.1 Engine Test Conditions
The investigation was performed by varying the first injection ratio from 0 to 50%, while the final triggering injection was kept constant at a 50% ratio of the total fuel amount injected. This high amount of fuel at the final stage of the strategy was selected for high power output as seen previously in Chapter 7.1, while the further splitting of the first injection contributes to the improvement of air-fuel homogeneity and reduction of
emissions.
The EVO time was varied between 94°and 64°CA BBDC to investigate the effects of valve timing on the exhaust gas trapping and emissions formation of the engine. Finally, the EBP was varied between 1 and 1.3 bars by adjusting the closure of the exhaust valve fitted on the Hydra’s exhaust system from 0 to 95% in order to analyse the effects of EBP on engine performance and emissions. The maximum EBP that can be reached for a single-cylinder engine is relatively small compared to modern multi-cylinder engines with advanced EGR and after-treatment systems. Table 7.3 analytically presents the engine test conditions for this study.
Table 7.3: Engine test conditions for Taguchi study.
Intake air temperature 300K
Intake air pressure 1 bar
Fuel temperature 350K
Fuel injected 10mg and 15mg per cycle Injection pressure around 1,200 bar Start of first injection -50°CA ATDC Start of second injection -30°CA ATDC Start of third injection -5°CA ATDC First injection ratio 0-50%
Second injection ratio 0-50%
First injection ratio 50%
EVO timing 94°- 69°CA BBDC
Exhaust back pressure 1 - 1.3 bar
7.2.2 Taguchi Method of Optimization
The Taguchi process starts by defining the goals that need to be identified and the parameters that need to be controlled. In this case, the parameters under investigation are the first/second fuel injection ratio, the EVO time and the EBP. Each parameter
will be tested under five different levels. Based on the number of parameters and their levels, the appropriate experimental layout of L25 orthogonal array is used. Table 7.4 presents the process parameters and their levels used in this study. The level of each factor used in the 25 experiments performed based on the L25 orthogonal array is shown in the Appendix B section.
C Exhaust valve position % of closure (≈ bars)
The 25 different air and fuel injection strategies for both engine loads were performed, and the results are fully listed in the Appendix C. Three trials for each experiment were conducted, and the results obtained were used to calculate the S/N ratios shown in Table 7.6.
7.2.3 Optimal Factors
The optimum set of parameters is determined by choosing the levels with lowest S/N ratio as our target to reduce emissions and BSFC. Before selecting the parameters, an analysis of variance (ANOVA) was performed for understanding the significance of each of the tested parameters on the output characteristics. The ANOVA experiment was carried out using Minitab 17 software and its results are listed in Table 7.5.
Table 7.5: ANOVA experiment results.
The contribution of the parameters on the emissions and BSFC changes for different fuel loads is clear. For low load conditions, NOx emissions are highly influenced by the EBP levels, while in the higher load conditions all parameters have a significant effect on the emission output. The opacity levels are highly influenced at all testing levels under both fuel load conditions. Finally, the EBP level highly affects the BSFC at high load conditions while at lower fuel loads BSFC is influenced by all tested conditions.
The optimum parameters for low NOx, opacity and BSFC were selected using Table 7.6 .
Table 7.6: S/N ratios for NOx, opacity and BSFC (in blue font are the parameters counted for choosing the optimum levels for low NOx, opacity and BSFC).
10 mg 15mg
Parameter Level NOx Opacity BSFC Optimum NOx Opacity BSFC Optimum
A 1 58.85 4.62 -16.05 62.31 18.02 -15.95
2 53.75 3.09 -13.73 61.50 12.76 -15.57
3 52.84 5.16 -12.92 57.99 16.48 -16.04
4 46.11 5.73 -9.10 A4 56.57 11.52 -14.72 A4
5 48.89 6.75 -11.10 56.14 11.64 -14.11
B 1 52.89 4.80 -12.66 59.58 14.68 -15.33
2 53.30 5.44 -13.78 B2 59.36 14.42 -15.09
3 51.69 5.58 -12.24 56.50 15.07 -15.17 B3
4 52.25 5.39 -12.38 59.31 14.41 -15.01
5 51.71 5.10 -12.54 58.94 13.40 -15.25
C 1 51.51 5.55 -12.68 59.65 13.93 -15.29
2 52.68 5.57 -13.08 59.19 12.85 -15.26
3 51.83 4.81 -12.39 59.53 14.27 -15.11
4 52.75 5.76 -13.26 59.18 15.12 -15.13
5 53.79 4.32 -12.87 C5 56.31 15.03 -15.24 C5
7.2.4 Confirmation Experiments
The final step of the Taguchi analysis for emission optimization is to perform the con-firmation experiments for both fuel loads in order to make sure that the optimal factors contribute to the reduction in harmful emissions and BSFC. The results of optimized engine performance for both fuel loads were compared with the baseline engine, which are cases P1 and PP1 with no EBP, no early pilot injection and EVO at 69°CA BBDC.
Figure 7.13: Comparison of in-cylinder pressure of baseline and optimized engines.
Figure 7.14: Comparison of heat release rate of baseline and optimized engines.
Figure 7.13 shows the in-cylinder pressure variation for the optimized engine com-pared to the baseline for both fuel loads. It seems that for both fuel loads, the baseline
engines exhibit a sharp in-cylinder pressure, which is usually a triggering point for high flame temperatures and NOx formation. The optimized engines show a smoother in-cylinder pressure increase due to the pilot fuel injected early in the in-cylinder, which can significantly contribute to air-fuel mixing improvement and reduction of NOx emission.
This can also be confirmed by Figure 7.14 where the heat release rate for the baseline and optimized cases is shown.
It can be seen from Figure 7.14 that the baseline engines have a more intense and rapid HRR increase at the beginning of combustion, leading to high HRR peak values and PCCI strength levels. On the other hand, the optimized engines show a smoother inclination with lower pre-mixed combustion phases and HRR peak values. This happens due to better air-fuel mixing quality at SOC timing as a result of the early pilot fuel injection, which eliminates any fuel-rich points in the cylinder and prevents high flame temperatures.
Figure 7.15 presents the NOxover the opacity readings for all engine tests at both fuel loads. As can be seen, the optimized engine performance for both fuel loads exhibits a combination of the lowest NOxand soot emissions. It is obvious that emission formation mainly depends on the pilot fuel injection ratio where a 30% fuel injection gives the lowest NOxand soot emissions for both fuel loads. On the other hand, strategies without pilot injection exhibit high NOxemission due to the rapid HRR increase at the SOC as shown before. The small EBP and exhaust valve timing testing regions seem to have little effect on emission formation, which can be confirmed by the ANOVA experiment results in Table 7.5.
Figure 7.15: NOxover opacity for all cases at both fuel loads; values in the legend are the percentage of pilot injection while data labels are the percentage of EBP valve closure.
However, the simultaneous reduction in NOx and soot is expected to negatively impact the BSFC and power output of the engine. Figure 7.16 presents the BMEP over the BSFC for all the engine tests. It can be seen that although the fuel consumption and BMEP values for the optimized engine are relative low, the values achieved are the highest possible for the pilot injection strategy followed. The high contribution value of the pilot injection ratio as shown in Table 7.5 limits the effects of the other parameters
on the reduction of BSFC. However, the ANOVA and Taguchi experimental analysis provided the highest possible BSFC reduction within the allowed limits set by the pilot fuel injection ratio.
Figure 7.16: BMEP over BSFC for all cases at both fuel loads; values in the legend are the percentage of pilot injection.
Unfortunately, due to the implementation of three variables in our analysis, it is not feasible to present results on the effects of each parameter on the NOx, smoke and BSFC.
However, as forecasted by the ANOVA, it is clear that pilot fuel injection has the highest
contribution to emission formation and fuel consumption.
7.3 Summary
In this chapter, experimental analysis was performed to investigate the effects of fuel injection ratio and timing with variable exhaust flow strategies on the combustion char-acteristics of a diesel engine. In order to fully understand combustion performance, the heat release rate of each strategy was calculated based on the in-cylinder pressure, and the ratio of the pre-mixed combustion over the whole combustion process was measured.
From the results, it is clear that the amount of the pre-mixed combustion phase plays a key role in soot formation. An increased pre-mixed combustion phase leads to lower soot formation as a result of the short diffusion combustion phase. On the other hand, NOx formation is increased when the pre-mixed phase is long. However, at very high pre-mixed combustion phases, NOx formation is reduced due to the high homogeneity of the air-fuel mixture providing the benefits of HCCI low NOx combustion without triggering a fuel injection.