Split Injection Strategy

Minimum engine emission and better performance were achieved by employing a split injection strategy in diesel engines. Lee and Reitz (2003) investigated the effect of the split injection strategy on a high-speed direct injection (HSDI) diesel engine equipped with a common rail direct injection (CRDI) system. Various parameters like exhaust gas recirculation (EGR), start of injection, inlet boost pressure, nozzle opening pressure, and dwell period were involved in the optimization of HSDI engine combustion. Results showed that the oxides of nitrogen (NO_x) and PM emissions were reduced in the split injection strategy, whereas BSFC increased. Yehliu et al. (2010) examined the single and split injection strategies on a CRDI diesel engine fueled by vegetable oil-derived biodiesel, ultra-low sulfur diesel, and Fischer-Tropsch fuel. It was observed that fuel ignition characteristics affected the start of combustion. Fischer-Tropsch fuel obtained minimum NO_x and PM emissions compared to other tested fuels under single and split injection modes.

How et al. (2018) used split injection on a diesel engine to investigate engine characteristics fueled by biodiesel blends. The results showed that the split injection strategy drastically reduced NO emission and improved engine performance. Edara et al. (2019) studied the influence of single retarded injection and split injection strategy with cooled EGR on a CRDI-assisted light-duty diesel engine. Test results revealed that retarded injection with 5% cooled EGR showed higher BTE compared to split injection with 5% EGR at a higher injection pressure of 350 bar. Both single and split injection strategies are recommended for higher-viscosity fuel to obtain better engine characteristics. Sindhu et al. (2018) numerically investigated the effect of the split injection strategy on diesel engine characteristics. Results showed that split injection with a small quantity of pre-injection reduced NO emission for the tested fuels. Park et al. (2018) investigated the effect of single and split injection strategies on diesel engine combustion and emissions characteristics. Results revealed that higher injection pressure with the split injection strategy decreased CO, UBHC, and NO emissions. Higher injection pressure with advanced injection timing lowered the BSFC and CO emissions but increased NO emissions.

Li et al. (2016) carried out research on the split injection strategy with swirl ratio on a single-cylinder diesel engine. The results revealed that split injection with swirl ratio enhanced the air-fuel mixture and accelerated combustion efficiency, causing lower engine emissions and minimum BSFC. Teoh et al. (2018) examined the impact of the start of injection and dwell angle on a turbocharged diesel engine’s performance, combustion, and emissions characteristics. Advanced start of injection and long dwell period influenced combustion efficiency and increased NO emission formation. Proper start of injection and dwell angle under two-stage split injection reduced emissions and improved performance characteristics. Plamondon and Seers (2019) studied the impact of single and double injection strategies on a diesel engine fueled by waste frying oil biodiesel. The results revealed that pilot injection enhanced main injection combustion by providing a better air-fuel mixture. Also, emissions were reduced during double injection when compared to the single injection strategy.

Imperato et al. (2016) noticed that splitting of fuel into small quantities for pilot injection reduced premixed combustion and lowered cylinder pressure. Also, pilot injection reduced ignition delay. An increase in the dwell period reduced NO emission, and increased fuel consumption. Jain et al. (2017) examined the influence of split injection and EGR on diesel engine characteristics. Results showed that premixed charge compression ignition combustion relatively lowered in-cylinder temperature when compared to conventional diesel engines. Advancement of main and post-injection timing improved the performance of the engine and reduced emissions. However, retarded main and post-injection timing reduced performance due to lower in-cylinder temperature, leading to partial combustion. Similarly, a higher EGR percentage reduced the performance of the engine due to lower in-cylinder temperature and pressure. Fayad et al. (2017) studied the effect of a post-injection strategy on a modern CRDI system-assisted diesel engine equipped with diesel oxidation catalyst and fueled by butanol-diesel blends. The results showed that the post-injection strategy reduced particulate matter and NO emission, but CO and UBHC emissions increased. This modern injection strategy reduced emissions and improved engine fuel economy.

Multiple Injection Strategy

Currently, advanced injection strategies, including split and multiple injection strategies, are used in diesel engines to further reduce engine exhaust emissions and improve engine performance characteristics (Mohan et al. 2013). Li et al. (2015) examined the effect of the multiple injection strategy on diesel engine characteristics under different modes. The results revealed that minimum premixed and diffusion combustions were observed in the pilot-main injection strategy. In addition, main-post injection favors soot oxidation by decreasing particle diameter. Xining et al. (2015) employed a multiple injection strategy on diesel engine under different modes. Various injection modes are single main injection, pilot-main injection, and main-post injection. Minimum premixed and diffusion combustion was observed in the pilot-main injection strategy compared to the single main injection strategy. In addition, main-post injection favors soot oxidation, leading to decreased particle diameter.

Suh (2014) investigated the impact of the twin-pilot injection strategy on a diesel engine. The results showed that the exhaust emissions of CO, UBHC, and soot were reduced in the twin-pilot injection strategy compared to the single injection and the one-pilot injection strategy. FIT of 20-10°CA proved to be the best timing for lower CO emission formation due to the shorter ignition delay and the higher in-cylinder pressure, which enhanced the CO oxidation rate. Jing et al. (2016) reported the effect of the two injection strategy associated with pilot injection on diesel engine performance and emissions. It was observed that faster flame development was identified for the double injection strategy compared to the single injection strategy. Soot and NO emissions were reduced in the double injection strategy compared to the single injection strategy due to low combustion temperature. Jaykumar and Ramesh (2018) examined diesel engine performance and combustion characteristics fueled by butanol-diesel blends. Test results showed improvement in BTE and low'er smoke emission in the main injection plus post injection (MPI) modes. However, there was a drop in BTE and an increase in smoke emission in the pilot injection plus main injection (PMI) mode.

Park et al. (2011) observed the influence of the multiple injection strategy on a diesel engine. The results showed that CO and UBHC emissions decreased but NO emission increased due to a shorter dwell period. Vanegas et al. (2008) observed the influence of the multiple injection strategy on a CRDI diesel engine’s performance and emission. In-cylinder pressure and temperature were reduced due to minimum fuel being burnt at premixed combustion, resulting in lower CO, UBHC, and NO emissions and lower BSFC. Hiwasea et al. (2013) studied the effect of split multiple stage fuel injection on diesel engine performance and emissions characteristics. Split multiple stage fuel injection showed strong combustion and provided controlled temperature and pressure inside the combustion chamber. This strategy significantly reduced NO emission and reduced SFC compared to a conventional continuous fuel injection system. Choi and Reitz (1999) investigated the influence of the multiple injection strategy on diesel engine characteristics fueled by oxygenated fuels. Soot emission was lower in the split injection strategy compared to the single injection strategy. Split injection reduced particulate emission at low loads, particularly with advanced fuel injection timing.

Combination of Split Injection and EGR on Biodiesel Combustion

The NO_x and PM trade-off is the major drawback in Cl engine combustion. The dilemma between NO_x and PM in Cl engine combustion was first reported by Akihama et al. (2001). They conducted experiments using a higher volume of EGR for the trade-off between the NO_x and PM. The study concluded that the addition of EGR with a higher percentage decreases the NO_x and PM significantly. However, a higher volume percentage of EGR deteriorates engine performance and combustion characteristics. The combustion duration increased by 39.8% for increasing the EGR volume percentage from 0 to 30% because of the higher equivalence ratio and temperature (Yu et al. 2013). Hence, a few works have reported that EGR along with advanced injection strategy enhances the combustion and performance characteristics with regard to the trade-off between the NO_x and PM. In the previous study, the single-stage fuel injection with advanced fuel injection was adopted for the low-temperature combustion technology to reduce the NO_x and PM. Advanced fuel injection creates the wall impingement problem, which results in higher HC emission and lower brake thermal efficiency (Hashizume et al. 1998; Neely, Sasaki, and Leet 2004). Hence, the researcher employed the split injection strategy for regulating the in-cylinder condition during the time of the main injection. Fang et al. reported that split injection along with EGR was used for a heavy-duty engine. The study concludes that higher pilot injection quantity with lower EGR effectively controlled NO_x emissions (Fang et al. 2012). Engelmayer et al. (2015) conducted experiments using a 300-kW test engine by adopting the cooled EGR along with multiple fuel injections. The study reported that soot emissions were effectively controlled in the partial load and rated load conditions using high-pressure multiple injections with addition of EGR.

CRDI has been used for high-pressure injection systems since it better meets the demand for lower emissions and specific fuel consumption. Also, CRDI enhances the specific power output and torque, and it injects the fuel at multiple times; the time separation between the injections can be reduced to 0 microseconds. Dober et al. (2008) studied the effect of multiple injections using CRDI along with EGR addition for diesel engines. The study reported that HC and CO emissions are increased in the advanced injection with EGR condition due to low combustion temperature and leanburn conditions. The cooled EGR is preferable for the emission reduction technology due to the higher density of exhaust gas, which occupies a low volume in the combustion chamber. This creates a higher volume for the fresh air entering the combustion chamber, which digests a higher quantity of EGR as compared to hot EGR. Mehrotra et al. reported that around 16.4% and 23.3% reductions of NO_x and PM, respectively, were achieved for the cooled EGR in the same quantity of burnt fuel (Mehrotra et al. 2014). Another study reported that cooled EGR with delayed injection timing improves thermal efficiency even at a low fuel injection pressure (Brijesh, Chowdhury, and Sreedhara 2013). Sarangi et al. (2013) investigates the combined effect of EGR, and 50% of pilot and main injection quantity requires the minimum quantity of EGR as compared to single injection.

Gautam Edara et al. (2019) investigated the effect of single and multiple injections with 5% and 10% EGR additions in light-duty diesel engines. They reported that multiple injections with and without EGR of 5% improve the brake thermal efficiency. The exhaust gas temperature decreased for the multiple injections as compared to single injection, and the addition of 10% EGR decreases the exhaust gas temperature by 10°C compared to a retarded single injection. Bhowmick et al. (2019) investigated the combined effect of cold EGR and split injection for a Calophyllum inophyllum biodiesel blend (BlO)-powered diesel engine. They varied the split injection quantity by 10% pilot and 90% main injection quantities and adjusted the EGR rate by 10% and 20%. The study reported that brake thermal efficiency decreased with EGR addition in the split injection strategy. NO_x emissions significantly decreased witli the addition of 20% EGR along with split injection as compared to split injection without EGR. The addition of EGR decreases the heat release rate and combustion temperature in the split injection condition due to a dilution effect. Because of this, higher CO, HC, and smoke emissions were noted in the EGR addition.