Recent Advanced Injection Strategy on Biodiesel Combustion
Introduction
Many researchers have tried biodiesel-diesel blends to improve performance and reduce emissions. However, this method often results in reduced engine performance and can damage various automotive components (Mohsin et al. 2014). Hence, there is a need for investigation of injection parameters, namely fuel injection pressure (FIP) and fuel injection timing (FIT) effects on biodiesel performance, combustion, and emission characteristics. Many researchers have found that injection pressure and timing are vital operating parameters that are found to affect engine characteristics.
Suryawanshi and Deshpande (2005) investigated the effect of advanced and retarded fuel injection timing on diesel engine characteristics. They reported that delayed injection timing effectively decreased the NOx, CO, and HC emission with a negligible rise in the total fuel consumption. At a delayed injection time, BTE and exhaust temperature followed a similar trend as baseline fuel. Nevertheless, ignition delay and peak cylinder pressure were decreased. Nwafor and Ogbonna (2000) described that advanced injection timing increases the ignition delay period of rape- seed oil in a Cl engine with a penalty of higher fuel consumption. Also, they found that NOx emissionwas enhanced for the fuel injection advancement by 3 °CA, and it decreased significantly for the retardation of fuel injection by 3 °CA from the standard injection timing. Gumus Sayin, and Canakci (2012) found that standard injection timing gained a better performance result in the brake thermal efficiency (BTE),brake- specific fuel consumption (BSFC), and brake-specific energy consumption (BSEC). Both retarded and advanced injection timing slightly deteriorate the performance characteristics. The overall investigation reported that engine performance and emissions vary in terms of the fuel properties; higher emissions were also reported for both advanced and retarded fuel injection timing. Hence, further investigation is needed to obtain suitable fuel injection timing for different kinds of fuels.
Sayin and Gumus (2011) studied the impact of nozzle opening pressure (NOP) on the Cl engine combustion characteristics for the biodiesel-diesel blends. The increase in NOP provides higher BTE and lower BSFC and BSEC compared to lower NOP due to the breakup of fuel into smaller droplets. The smaller diameter of the fuel droplets provides higher homogeneity in the air-fuel ratio, and the higher surface area of the fuel droplets also leads to better combustion. Smoke, HC, and CO emissions are significantly reduced with a penalty of higher NOx emission at higher NOP for all fuel blends. Sukumar et al. studied the influence of higher NOP ranges from
220 bar to 240 bar for the linseed biodiesel used in a Kirloskar diesel engine. They found that smoke and CO emission reduced, and NOx and HC emissions increased with the boost in NOP. Also, the study reports that too-high NOP improves the fuel spray movement at a higher velocity and increases the vaporization rate, which significantly reduces the NOx emission (Puhan et al. 2009). Purushothaman and Nagarajan (2009) studied the combustion behavior of orange skin powder biodiesel blend (B30) with varying NOPs in the range of 215 bar to 255 bar. The results exhibited that CO emission decreased up to 235 bar and it further increased when increase in the NOP above 255 bar. At higher NOPs up to 235 bar, the BTE and heat release rate (HRR) were increased due to the higher homogeneity of the air-fuel charge, and also it resulted in lower CO emission due to complete combustion. At a too-large NOP, the combustion duration was increased, which led to a lower heat release rate and a longer combustion duration, resulting in higher CO emission.
The simultaneous lowering of diesel engine soot and NOx emissions is needed to meet rigorous exhaust emission, but it seems very difficult to reduce the NOx emissions without increasing soot emissions through varying fuel injection timing. The explanation is that NOx and soot emissions often contradict each other when the timing of the fuel injection is retarded or advanced. An optimized split injection strategy proved to be an effective method for simultaneously reducing soot and NOx emissions for diesel engines. It is known as splitting the main single injection profile between the injections into two or more injection pulses with a definite delay dwell.
In diesel engines, multiple injection strategies typically include one main injection and two pilot injections of a small quantity of fuel before the main injection and after top dead center. More than three injections are usually used in car engines. Mendez and Thirouard (2009) studied the effect of the double injection on diesel engine noise and fuel economy and found that it is possible to simultaneously decrease the noise and fuel consumption using a double injection strategy. Moreover, the pilot injection strategy raises the in-cylinder temperature at the premixed combustion and enhances the atomization and evaporation (d’Ambrosio and Ferrari 2015).
The pilot injection tends to minimize the noise of combustion, but it can lead to an increase in particulate matter (PM), as it decreases the ignition delay of the diesel that occurs during the main injection (Yadav and Ramesh 2018). Splitinjection eliminates noise, pollution, and the BSFC with its versatile injection approach (Mohan, Yang, and Chou 2013). It was found that pilot injection reduces the smoke emission because of increased air-fuel mixing at premixed combustion in the cylinder (Hotta et al. 2005). The air-fuel mixing is improved by post-pilot injection, which leads to rises the in-cylinder temperature, thus enhancing soot oxidation (Yao et al. 2009). Besides, the post-injection reduces the PM emission with a large effect on fuel consumption and hydrocarbon emission (Chen 2000). Further research also suggested the efficacy of post-injection in soot reduction, with no effect on NOx emissions (Benajes, Molina, and Garcia 2001). Optimization of post-injection timing was important to minimize smoke emission (Yao et al. 2009). Higher smoke and NOx emission were observed when post-injection was below 8°CA offset to the main injection. This is due to elevated temperature during the combustion (Desantes et al. 2007). However, it was found in another study that above 13°CA after top dead center post-injection did not help to lower the smoke level either. Furthermore, the
NOx emission was significantly reduced with three pulse injections compared with a single injection (Desantes et al. 2007).
Single Injection on CRDI-Assisted Diesel Engine
Conventional diesel produces more exhaust emission, which does not meet emission norms. To reduce emission and improve engine performance, a modern electronic injection system was used in the diesel engine instead of a conventional fuel injection system (Agarwal et al. 2014). Agarwal et al. (2015) stated that combustion duration significantly diminished with an increase in NOP due to better atomization. Higher NOP was preferred for higher-viscosity fuel to ensure better atomization. PM was reduced at higher NOP and advanced FIT. More delay and a better air-fuel mixture led to complete combustion. Gumus et al. (2012) found that maximum NO emission occurred at higher NOP due to better atomization and evaporation. At higher NOP, droplet size was very small, resulting in faster combustion, leading to higher NO emission. Mohamed Shameer and Ramesh (2018) examined the influence of FIT and NOP on diesel engine performance and combustion characteristics. Experimental results infer that advanced FIT and higher NOP amplified biodiesel combustion phenomena, resulting in higher performance and lower emissions.
Sayin et al. (2012) examined the effects of NOP on Cl diesel engine combustion characteristics. Results showed that higher NOP increased in-cylinder temperature and pressure, causing higher NO emission. This was due to higher NOP enhancing atomization and reducing fuel droplet size. It resulted in complete combustion, which leads to an increase in cylinder pressure and temperature. Ryu (2013) studied the effect of nozzle opening pressure on diesel engine combustion and emission characteristics. It was noticed that at higher load, unburned hydrocarbon (UBHC) and CO emissions decreased. The reason was that increase in NOP reduced droplet size and improved combustion efficiency. Rajesh Kumar et al. (2016) investigated the effect of NOP on diesel engine performance and combustion characteristics fueled by biodiesel and its blends. It was noticed that BTE increased and BSFC decreased when NOP increased from 180 to 220 bar due to better atomization, leading to faster combustion. Yesilyurt (2019) investigated the influence of fuel injection pressure on diesel engine characteristics fueled by biodiesel blends. Results indicated that biodiesel had lower emissions and performance compared to conventional diesel. Fuel injection pressure improved BTE and lowered emissions, but NO and CO, emissions increased.
Jindal et al. (2010) examined the influence of NOP on diesel engine performance and emission characteristics fueled by Jatropha biodiesel. It was observed that BTE increased with an increase in NOP, while BSFC decreased with increased NOP from 200 to 250 bar. Kannan and Anand (2011) investigated the effect of NOP and FIT on diesel engine characteristics fueled by biodiesel and its blends. The use of pure biodiesel in engines causes issues like injector choking, piston ring sticking, and poor spray characteristics. Results showed that higher NOP and advanced FIT enhanced engine characteristics. Kshirsagar and Anand (2017) suggested higher NOP and advanced FIT for higher-viscosity fuel to achieve better performance characteristics and lower emissions. Higher NOP and advanced FIT improved fuel spray characteristics, resulting in complete combustion. Harish et al. (2019) investigated the effect of fuel injection timing on diesel engine characteristics fueled by bio- diesel-diesel-ethanol blends. Maximum BTE and minimum CO and UBHC emission were revealed at advanced FIT.
