Parámetros de la Evaluación

Experimental analysis was performed under two different loads, 10mg and 15mg, of fuel injection, for investigating the effects of injection timing, ratio and dwell angle on the pre-mixed combustion phase of diesel engines using the PCCI strength factor defined in Section 3.2.

7.1.1 Engine Test Conditions

The investigation was performed using two injections per cycle and by varying the injec-tion ratio and timing. The first injecinjec-tion early in the cycle provides fuel for optimizing air-fuel mixing quality within the combustion chamber, while the second injection closer to the TDC works as a triggering point for the SOC. The engine operates in PCCI com-bustion mode without exhaust gas recirculating back to the inlet manifold and, therefore, early SOC timings are experienced. The engine air and fuel conditions for this analysis are presented in Table 7.1.

Table 7.1: Engine test conditions for PCCI Strength 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°, -40°, -30°CA ATDC Start of second injection -10°, -5°CA ATDC

First injection ratio 50%, 70%, 80%

Three different injection ratios were tested. The first injection ratio is a 50:50 strategy denoted by “A” for 10mg of fuel load and “AA” for 15mg of fuel load. Half of the fuel amount is injected during the first injection, which takes place between -50°and -30°CA ATDC, and the other half is injected closer to the TDC at -10°or -5°CA ATDC. The second strategy applies a 70:30 injection ratio denoted by “B” and “BB” for medium and higher loads, respectively. Finally, the third category is denoted by “C” and “CC”

and is an 80:20 injection ratio strategy. Table 7.2 details the testing performed for each strategy.

Table 7.2: Injection strategies for PCCI strength study.

7.1.2 Effects of Injection Strategy on PCCI Strength and SOC Timing

7.1.2.1 50:50 Injection Ratio

The effect of injection timing on the strength of the PCCI combustion for the 50:50 injection ratio is shown in Figure 7.1.

Figure 7.1: Effects of injection strategy on PCCI strength for 50:50 injection ratio.

It can be seen that the PCCI Strength level increases for higher load cases, as more fuel leads to an earlier SOC, which can be translated to more time available for the pre-mixed combustion to take place. Comparing strategies 1 with 2 and 3 as well as 4 with 5 and 6, it can be seen that the later the first fuel is sprayed into the cylinder, the higher the strength of the premixed combustion. The same can be concluded for the start of the second injection by comparing strategies 1, 2 and 3 with 4, 5 and 6. An early first injection leads to improved air-fuel mixing and reduces the combustion temperatures, which results in lower burning rate during the premixed combustion phase. A late first injection closer to the TDC leads to a more rapid and robust combustion, which, as a result, increases the ratio of the premixed combustion compared to the diffusion combustion.

Figure 7.2: SOC timing for 50:50 injection ratio strategies.

Figure 7.2 presents the SOC timing for all 50:50 injection ratio cases. Clearly, the cases with higher load tend to have an earlier SOC due to the higher amount of fuel injected, leading to increased premixed combustion as shown in Figure 7.1. In addition, the SOC timing for cases with the second injection taking place at -5°CA ATDC is delayed for approximately 1°CA compared to the cases with the second injection taking place at -10°CA ATDC. However, this delay seems to be shorter for the cases where the first injection occurs closer to the TDC. It can be concluded from Figure 7.2 that early SOC timing increases the available time for the premixed combustion phase to take place before the injection of the remaining fuel and the start of diffusion combustion.

7.1.2.2 70:30 Injection Ratio

The strength of the PCCI combustion for the 70:30 injection ratio does not follow the same trend as for the 50:50 ratio. As can be seen from Figure 7.3, the strength of the PCCI combustion is higher for the 70:30 cases compared to 50:50 strategies due to the decrease of the fuel amount injected during the second pulse, which as a result reduces

the level of diffusion combustion. In this case, it can still be seen that a second injection far from the TDC leads to higher strength due to the earlier SOC. However, the strength does not exhibit the same trend based on the first injection timing as for the 50:50 ratio case.

Figure 7.3: Effects of injection strategy on PCCI strength for 70:30 injection ratio.

It can be noted from Figure 7.3 that the strength level for BB1 is higher than BB2, although the first injection takes place 10°CA earlier. This can be explained by looking at Figure 7.4 where the SOC timing for case BB1 is approximately 2°CA earlier than in case BB2. The early SOC was achieved due to the earlier first injection and the higher fuel amount compared to cases with the 50:50 ratio. The same occurs for case BB4 compared to BB5 and BB6. In addition, in many cases the strength of PCCI combustion is higher for the cases with low load conditions compared to high load. This mainly happens in cases with the second injection taking place at -10°CA ATDC where the SOC timing of the low load cases occurs very close to the start of the second injection. Therefore, the newly introduced fuel contributes to the strengthening of the premixed combustion, and this can be clearly seen in case B3 (in circle), where the premixed combustion ratio is higher than the ratio of the fuel injected during the first injection pulse.

Figure 7.4: SOC timing for 70:30 injection ratio strategies.

By looking at Figures 7.3 and 7.4, it can be concluded that SOC timing plays a key role in the strength of premixed combustion. An early SOC increases the available time for the premixed combustion phase to take place and, therefore, the PCCI strength level.

On the other hand, a late SOC close to or after the start of the second fuel injection can also increase the PCCI strength with the contribution of freshly introduced fuel.

7.1.2.3 80:20 Injection Ratio

Figure 7.5 illustrates the PCCI strength levels for cases with an 80:20 injection ratio.

Figure 7.5: Effects of injection strategy on PCCI strength for 80:20 injection ratio.

It can be seen that the PCCI strength for case C1 is higher than for C2. This can be explained by the very early SOC, almost 16°CA BTDC as shown in Figure 7.6. The high amount of fuel injected during the first pulse and the high in-cylinder temperature led to an advanced SOC, which resulted in high levels of premixed combustion. However, the SOC for the CC1 case is a few degrees later, which could be attributed to lower in-cylinder temperature caused by the higher amount of cold fuel entering the combustion chamber and the air-fuel homogeneity level.

From Figure 7.6, it can be also seen that that cases CC2, C3 and CC3 (in circle) exhibit a PCCI strength level higher than 80%, which is the amount of the first fuel injection. It is obvious that all three cases have a SOC timing after or near the start of the second injection, and the premixed combustion is enhanced by the freshly introduced fuel. This is not the case for strategies 4, 5 and 6, where the second injection takes place at -5°CA ATDC.

Figure 7.6: SOC timing for 80:20 injection ratio strategies.

From the figures above, the level of PCCI strength varies with injection ratio, in-jection timing and fuel load. It can be concluded that the strength of the premixed combustion is highly affected by the fuel amount of the first injection, the start of the second fuel injection and SOC timing.

7.1.3 Effects of PCCI Strength on Performance

7.1.3.1 50:50 Injection Ratio

The effects of PCCI strength level on the performance of the diesel engine for the 50:50 fuel injection ratio are shown in Figure 7.7.

Figure 7.7: Effects of PCCI strength level on engine performance for 50:50 injection ratio.

It can be noted from Figure 7.7 that a lower level of premixed combustion leads to higher power output in most of the fuel injection strategies. A low premixed combustion at an early stage before the TDC will reduce the opposing forces applied on the cylinder while it is moving upwards towards the TDC and will reduce the pumping losses. At the same time, the increased diffusion burning rate at a later stage will improve the performance and power output of the engine.

Strategies with the first injection taking place at -30°CA ATDC (A3, AA3, A6 and AA6) exhibit higher power outputs compared to the other strategies, although their PCCI strength level is higher. The power output of the engine is directly influenced by the injection strategy followed. A late first injection overcomes one of the main disadvantages of PCCI combustion, which is small power limits due to lean flammability.

However, as can be seen in the following sections, a very late first injection reduced the premixed charge level and this is reflected in engine emissions.

7.1.3.2 70:30 Injection Ratio

Figure 7.8 presents the power rating over the PCCI strength level for the 70:30 injection ratio strategies. It can be immediately seen by comparing Figures 7.7 and 7.8 that the power rating for the strategies with 30% of the fuel injected during the second pulse is significantly reduced compared to the cases where a higher amount of fuel is injected close to the TDC.

Figure 7.8: Effects of PCCI strength level on engine performance for 70:30 injection ratio.

It can also be noticed that strategies with the same SOI for the second pulse and en-gine load show increased power output for lower premixed combustion phases. However,

strategies 3 and 6 for both fuel loads do not follow the same trend. In the same way as in the 50:50 injection ratio cases, strategies BB3, B6 and BB6 have a high power output, although their PCCI strength level is high. Strategy B3, which exhibits the highest PCCI strength level, has a lower power output than strategy B1 due to the extremely high level of premixed combustion.

7.1.3.3 80:20 Injection Ratio

Injection strategies with an 80:20 fuel ratio follow a similar trend compared to the other two injection ratio strategies as shown in Figure 7.9.

Figure 7.9: Effects of PCCI strength level on engine performance for 80:20 injection ratio.

The power output for cases C1 and C2 is very similar, with C2 being slightly higher due to the slim difference in the PCCI strength level. This difference is similar to

the power output for case C5, which is higher than C4, which has an increased PCCI strength level. However, for cases C3 and CC3, an increased PCCI strength does not lead to reduced power output. The power rating for case C3 is higher than C2 and C1 and for case CC3 is much higher than CC1 and CC2. The injection timing plays a significant role in the power output characteristics. Cases C3 and CC3 supply an increased amount of fuel at -30°CA ATDC. The increased fuel amount over the first injection pulse extends the first injection duration and reduces the dwell angle between the end of the first and start of the second injections. The short dwell angle leads to extremely high premixed phase and combustion characteristics similar to single injection strategies. This can be concluded by the power output of these two cases and the emission characteristics shown in the following sections.

7.1.4 Effects of PCCI Strength on Emissions

The effects of the combustion ratio between the premixed and diffusion combustion stages and the SOC timing on NOx and exhaust smoke are analysed in the following sections.

7.1.4.1 NOx Emissions

Figure 7.10 presents the effects of PCCI strength level on NOx formation of all injection ratio cases. It is clear for strategies with a fuel injection ratio of 50:50 that NO_xformation is increased as the PCCI strength level is enhanced. The premixed combustion phase often leads to rapid and intense in-cylinder pressure and temperature increase, which are mainly responsible for NOx formation due to high in-cylinder temperatures. The higher premixed rates lead to very high temperatures and NO_x levels.

It can also be seen from the 50:50 injection ratio results that strategies A3, AA3, A6 and AA6, with the shortest dwell angle between the two injections, exhibit the highest NO_xemissions. As described in Section 7.1.3.1 where these strategies were found to have the higher power ratings, the very short dwell angle overcomes the lean flammability combustion drawback of PCCI combustion, but at the same time limits its ability to

reduce NOx formation.

Figure 7.10: Effects of PCCI strength level on NOx emissions for all strategies.

The NOx versus PCCI strength trend for 70:30 injection ratio cases does not follow the same trend as in the 50:50 ratio cases. It seems that the PCCI strength does not play a key role in NOx formation. It can be seen from the chart that for the low load cases, NO_x emissions are lower when the first and second injections take place earlier in the cylinder (B1<B2<B3 and B4<B5<B6). However, the same falling trend for NO_x emissions do not occur for the higher load strategies where the cases with the first

injection occurring at -40°CA (BB2 and BB5) show the lowest NOx formation. In the same way as the 50:50 injection ratio cases, strategies with short dwell angle show the higher NOx formation rates.

On the other hand, the 80:20 injection ratio strategies follow an opposite pattern to the 50:50 injection ratio cases where NOxemissions are reduced when the PCCI strength level is increased. A very high first injection ratio leads to combustion that exhibits the benefits of HCCI combustion without a triggering injection close to TDC, which is the high homogeneous mixing of fuel and air that leads to cleaner combustion. For the 80:20 injection ratio cases, the lowest NOxformation is shown in cases with the first injection taking place at -50°CA (C1 and C4) for the medium engine load and at -40°CA (CC2 and CC5) for the higher engine load. Similar to the previous injection ratio strategies and for both engine loads, strategies with the shortest dwell angle have the highest NO_x level.

Figure 7.11 shows the effect of SOC timing on NO_xformation. It can be observed that for cases with injection ratios of 50:50 and 70:30 with low and medium PCCI strength levels, respectively, that NO_x formation is reduced as SOC timing is retarded. This happens due to the shorter time available for the premixed combustion to take place, which directly reduces the period of high temperatures in the combustion chamber and, therefore, NOx emission.

On the other hand, the 80:20 injection ratio strategy with very high PCCI strength levels shows a different trend where early SOC timing exhibits lower NOx emission. In this case, early SOC timing advances the premixed combustion, which is nearly identical to full combustion, to an earlier stage where the in-cylinder pressure and temperatures are lower and, therefore, NO_xformation is reduced. As the SOC timing is retarded, the in-cylinder temperature is increased and the formation of NOx is further enhanced by the injection of the second amount of fuel as shown in Figures 7.10 and 7.11.

Figure 7.11: Effects of SOC timing on NO_x emissions for all strategies.

7.1.4.2 Opacity

Figure 7.12 presents the exhaust gas opacity over the PCCI strength for all injection strategies. It can be noticed that smoke follows a descending trend when the PCCI strength is increased. This can be explained due to the reduction of the diffusion com-bustion, which is a low temperature combustion that lacks oxygen and leads to a slow burn rate rich in soot production. Soot formation mainly takes place during the diffusion

combustion where the soot particles are largely and densely formed due to fuel oxidation and/or pyrolysis at high temperatures. The smoke emission follows the same trend for all injection ratio strategies.

It can be also seen that smoke levels are lower for low load combustion due to the lower amount of fuel injected during the second pulse, and also smoke is reduced as the second injection ratio decreases. The strategy with the 80:20 ratio exhibits the lowest levels of smoke for the majority of the cases. However, strategies AA6 and BB6 show lower smoke levels compared to BB6 and CC6, respectively. This can be explained by the very low premixed combustion rates in both strategies, which lead to a high ratio of diffusion combustion and increased smoke.

Figure 7.12: Effects of PCCI strength level on smoke for all strategies.