Clean combustion and emissions strategy using reactivity controlled compression ignition (RCCI) mode engine powered with CNG-Karanja biodiesel

Highlights

• Karanja biodiesel-CNG used as alternate fuel in novel RCCI engine.

• Combustion phasing and knock limit of RCCI engine.

• Reduction in NO_x and smoke emissions.

• Improved BTE of RCCI engine.

Abstract

Heterogeneous combustion in a diesel engine is noisier, uncontrolled and more polluting. This can be achieved with a strategic approach of a reactivity-controlled compression ignition (RCCI) mode engine that operates with low and high reactive fuel combinations. In the present work, a diesel engine is operated in RCCI mode with gaseous fuels viz. CNG as a primary fuel and a blend of diesel and Karanja biodiesel (BD20) as pilot fuel. This research aims to determine the operating limits of CNG fuel for less noisy combustion and clean exhaust.

Further, relative air-fuel ratio (λ), cycle to cycle variations, combustion noise and emissions were studied for full load operation. The CRDI engine is optimized for diesel operation with a split injection strategy. The knock limits for CNG as the primary fuel are obtained. The combustion noise increases at a higher energy share by CNG. Also, higher values of HC and CO emissions are observed. This may be due to higher energy share values, flame speed and octane number of CNG fuel.

Further, NO_x emissions and smoke are decreased. The CNG induction of 10 ms with 90% ES can be noted as a knock limit for 3.5 kW power. The highest BTHE of 24.2% and least BSFC 0.3 kg/kWhr reported by 60%ES of LRF is better than diesel and KBD20 fuel. An optimum 60% energy share of CNG is observed for clean combustion and emissions strategy using the RCCI mode of a modified diesel engine.

Introduction

Compression-Ignition (CI) and Spark Ignition (SI) engines are popular in the transportation industry and widely used for industrial, agricultural, and small-scale applications. CI engine meets high power requirements with better fuel economy [1,2]. CI engines are available from a few watts of power to very high power required for railway and marine applications [3]. CI engines are limited in this journey due to their high level of emissions, mostly NO_x and soot. The NO_x is nothing but NO and NO₂ together are known as oxides of nitrogen [4,5]. A significant difficulty in understanding NO_x formation mechanism or kinetics is that combustion is highly heterogeneous and transient in diesel engines. NO_x is a combination of nitric oxide (NO) and nitrogen oxides (NO₂). These are formed from the oxidation of nitrogen present in the atmosphere (fresh air supplied as a charger) at high temperatures during the post-flame combustion process. Though both gasses are considered toxic, there are several differences between these two gasses. NO is an odorless, colorless gas, while NO₂ is reddish-brown and with a pungent odor. Although NO₂ is primarily found from NO oxidation, controlling of NO before and after combustion is challenging [6].

Conversely, smog or smoke is due to impurities from fuel, air, and their mixture, leading to increased particulate matter. Also, invariable change in fuel and air ratio causes smoke [6]. Besides, this PM, soot, opacity or smoke with fog causes smog. This smog issue has caused severe problems to New Delhi (Haryana, UP and north Indian states), India. The continual efforts are made by the government and researchers. New emissions norms like BS-VI have been imposed through the country from April 2020. This has created a challenge for automotive sectors and researcher to present technology which will meet emission reduction technology at a competitive cost. Amongst popular technology for emissions reductions are electronic control management (ECM) of fuel, fuel-air ratio, diesel particulate filter (DPF), advanced catalytic converters like air/urea injection, and two-staged catalytic converters have emerged. These ECM applications in newer technology like HCCI, PCCI, and RCCI have become researcher interest.

Homogeneous charge compression ignition (HCCI) is advanced combustion technology, potentially reducing NO_x and PM with high efficiency similar to diesel engines. In HCCI, engine combustion occurs due to spontaneous auto-ignition at multiple points throughout the charge volume. These unique characteristics of HCCI allow the combustion of very lean and dilute mixture resulting in relatively lower bulk temperatures and localized combustion temperature, which significantly reduce engine-out NO_x emissions. Unlike CI engines' heterogeneous mixture, HCCI allows homogeneous combustions. Absence of fuel-rich zones in the HCCI engine significantly reduces PM formations [7]. In pre-premixed compression (PCCI) ignition, a correct A:F ratio before combustion can reduce PM, and a high EGR rate and a lean mixture of A:F reduces NO_x. During the pre-premixed charge CI engines, such as ignition power, homogeneous mixture forming, excess wall impingement, etc. Few more drawbacks must be solved because of its higher inflammability and lower diesel volatility [7].

The novel method known as RCCI combustion strategy has been currently introduced to lower these issues. The RCCIis a dual fuel strategy that uses different reactivity fuels to improve the combustion process and reduce emissions from a diesel engine [8]. Compression ignition engine under RCCI mode operates with both high reactive fuel (HRF, high cetane number) and low reactive fuel (LRF, high octane number) [9]. The experimental efforts have been taken by Subramanian et al. for comparative exergy analysis of multi-cylinder SI engine operated with different gaseous (CNG, HCNG and H₂) fuels [10]. Ebrahimi et al. [11] investigated RCCI combustion of a heavy-duty diesel engine fueled with landfill gas and diesel engine. The study revealed that the addition of H₂ to landfill gas boosts the methane dissociation rate dramatically. Thus, the combustion duration is reduced considerably.

The landfill gas reduces NO_x slightly. The RCCI is an automatic combustion approach controlled by mixing petrol and D100 fuel in the cylinder [12]. This combustion technique involves compliance with the premixed charging ignition principle since A:F is combined before combustion, whereas the CI fuel reactivity regulated by reactivity differs in the cylinder [13]. The fuel with the high ON is directly supplied into the CC from the intake port, and the fuel with the high CN produces the CC's reactivity. The reactivity of the fuel can be split into global and gradient reactivity. The reactivity of the fuel can be split into global and gradient reactivity. The reactivity of the fuel can be split into global and gradient reactivity [14]. The fuel type and the quantity of fuel injected in the CC define global reactivity. The fuel Injection technique for higher ON and CN fuels will vary the reactivity gradient, such as early and late injection. Together with the fuel injection strategy, both proportions influence the CI combustion regulated by reactivity [15]. The RCCI is an efficient ignition control with higher efficiency and reduced emissions, also demonstrated in recent simulation and experimental studies. Compression ignition-controlled reactivity is more promising than homogeneous charge CI with lower temperature combustion techniques [16]. The reactivity can be varied by adjusting the ratio of high CN and ON of the fuel in RCCI combustion [17].

NO_x emissions were minimized by the introduction of cotton biodiesel in the RCCI ignition combustion. At lower concentrations of cotton biodiesel, HC emissions were reduced, and higher% HC increased. At max. load, 30% of inclusion of cotton biodiesel reduced smoke. Also, BTE increased with the inclusion of cotton biodiesel [18]. Many alternative fuels could be utilized, considering the fuel control system in RCCI engine. Furthermore, oxygenated fuels should be analyzed in the combustion of RCCI [19,20].

In contrast with the traditional single fuel diesel combustion mode, the RCCI engines are helpful for light-duty on-road applications due to the enhanced performance characteristics and the low operating costs for dual fuel diesel natural gas RCCI engines [21]. RCCI displayed lower HRR at low and medium loads as the n-butanol ratio was improved. Increased n-butanol ratio contributes to smoke elimination, and CO and HC emissions increase [22].

The current study reports, management of RCCI engines using fuel strategies like single fuel strategy with additives for cetane improver and two fuel strategies. The review also focuses on engine management like fuel ratio, compression ratio, injection strategies, and EGR. The HRF-LRF and energy shares are playing a vital role in combustion control and emission reduction. Also, reactivity gradient, reactivity stratification, heat release process, combustion phasing and combustion noise control are essential [23,[24], [25], [26], [27]]. Alcohol fuels have high applications in prospects of RCCI because of their high latent heat of vaporization and higher molecular oxygen [28]. Diesel injection timing, in-cylinder fuel blending ratios and EGR rate are vital variables to a stable RCCI engine. RCCI capabilities at two different CR viz 14.4 and 11 are studied. For higher CR required delayed injection strategies and low CR allows to fulfill self-imposed constraints (max RPR25 bar/CA, NO_x<0.4 g/kWh, and soot<0.01 g/kWh from idle to full load [29]. In the cylinder, directly injected n-heptane combined with port injection of ethanol, n-butanol, and n-amyl alcohol are used in a single-cylinder engine. If overall LHVare constant and premixed ratio (RP) increase, NO_x and soot decrease. Also, ethanol RCCI particular high RPs combustion phase is strongly delayed, and NO_x –soot is decreased [30].

EGR is used to control the rate of heat release rate. It was reported that HCCI reduces total combustion duration due to a very high rate of HRR. Enrichment of the fuel-air mixture enhances HRR & location of peak in-cylinder pressure towards BTDC side due to earlier combustion [31]. A premixed low temperature combustion (LTC) engine is known for clean combustion and high efficiency. The change in fuel IT and injection pressure and charge dilution (by hot EGR) causes clean combustion. The engine operation showed a lower rate of pressure rise, fuel consumption near-zero Box soot emissions and improved thermal efficiency [32]. Alcohol fuels can be used in engines for emissions reductions. This may be due to the high latent heat of vaporization and cooling effect of alcohol fuel in combustion, which produces lower cylinder combustion pressure and temperature, resulting in decreased NO_x and opacity emissions [1,12,[33], [34], [35]]. The thermal behavior of nanofluids is dependent on their thermophysical properties [36]. The dispersions of titanium dioxide (TiO₂) nanoparticles in water-ethylene glycol 50 (W-EG50) as the base fluid are a newer approach for thermal properties improvement [37]. The CuO–water nanosuspension shows improvement in thermal conductivity of water as a result of enhancement in thermal properties of nanofluid [38]. The average increase in efficiency when switching from water to Ag-water nanofluid was found to be around 11% is noted [39]. Hybrid nanofluid is a recently developed class of nanofluids having two different types of nanoparticles suspended in the base fluid [40]. Also, nanoparticles made of aluminum oxide (Al₂O₃) and titanium oxide (TiO₂) showed improvement in heat flux [41]. Experimentally studied the thermal performance and heat transfer of n-pentane-acetone and n-pentane-methanol blends [42]. Jet injection of different oxide nanofluids, including water–Al₂O₃, water–CuO and water–TiO₂ under the steady-state condition, enhances heat transfer [43], [44], [45], [46]. The ultra-lean combustion of methane and biogas in porous burner stabilizes flames and fuel flow rate fluctuations [47].

The newer approach, like artificial neural network (ANN), is used to determine the thermal conductivity of Al₂O₃–Cu / EG with an equal volume (50:50) nanofluids [48]. Also, the ANN model approach is used to predict the viscosity of Biodiesel blends [49]. The best ANN model was selected based on statistical criteria of R² and AARD% in properties prediction. Further ANN modeling is used to predict the exhaust emissions and engine performance of alcohol blends at various speed and compression ratios [50]. ANN model is used to optimize off-grid Hybrid wind/hydrogen energy systems [51].

Conversely, an analytical approach and the interpolation-supplemented Lattice-Boltzmann method (ISLBM) were used to quantify convective and diffusive transport during CO₂ dissolution [52]. A numerical framework is proposed based on multiple relaxation time lattices Boltzmann (LB) model and novel discretization techniques for simulating compressible flows [53]. A three-dimensional multiphase lattice Boltzmann model has reported the spontaneous phase transport in complex porous media [54]. Also, axisymmetric DNS, subcritical, vortex and hysteresis are studied [55]. ANOVA techniques are also used to identify the F-value and P-value of the model. These models are helpful to predict thermophysical properties and thermal behavior more accurately [56].

In this present study, CRDI research engine is operated with RCCI mode using CNG as the primary fuel in port injection, and Karanja biodiesel (KBD20) blend as the primary fuel using CRDI split injection strategy. CNG fuel share was maintained from 0 to 100% energy share. However, 0% and 100% CNG share plays a single fuel strategy and finds out base and datum for RCCI operation. The present research aims to highlight optimum RCCI operation with maximum emissions reduction at least combustion noise.