Reactivity Controlled Compression Ignition Engine Research Articles

A numerical study of piston bowl geometry and diesel injection timing in a heavy-duty diesel/syngas RCCI engine

The current numerical study aims to examine the single and combined effects of various piston bowl geometries, diesel main Start of Injection (SOI) timing, and syngas energy ratio on the combustion characteristics, specific emissions (g/kW·h), energy distribution, and exergy of a heavy-duty off-road Reactivity-Controlled Compression Ignition (RCCI) diesel engine. The examined geometries include the stock chamber (baseline combustion chamber), wide-shallow chamber, dual-swirl chamber, stepped-lip chamber. The main SOI timing was varied from −20 to −5 Crank Angle (CA) After Top Dead Center (ATDC) with 5 CA steps. Also, syngas energy ratio including 0% Conventional Diesel Combustion (CDC) mode, 20%, 40%, and 60% of total fuel energy per cycle. To accomplish this goal, the SAGE combustion model coupled with a reduced chemical kinetic mechanism consists of 360 reactions, and 72 species was applied to conduct this investigation. The numerical findings showed that the use of the wide-shallow chamber extended the engine operable range under diesel-syngas combustion operating conditions. In comparison to the baseline CDC case, the application of the dual-swirl chamber under CDC conditions, and main SOI timing of −15 CA ATDC led to reduction of Carbon Monoxide ([Formula: see text]), Hydro-Carbon ([Formula: see text]), Particle Matter ([Formula: see text]), Carbon Dioxide ([Formula: see text]), Indicated Specific Energy Consumption (ISEC), Gross Indicated Efficiency (GIE), and exergy efficiency but with a Nitrogen Oxides ([Formula: see text]) penalty rate. In general, an increase in the bore to bowl diameter ratio caused a shorter ignition delay period and combustion duration. The utilization of the stepped-lip chamber at both CDC and diesel-syngas combustion operating modes resulted in a shorter ignition delay period, but a longer combustion duration. Nonetheless, the stepped-lip combustion chamber reduced the diffusive flame temperature. Also, it is noteworthy that the utilization of the stock chamber along with the main SOI timing at −5 CA ATDC under diesel-syngas 60% combustion mode is a suitable technique to pass EURO-VI [Formula: see text] and [Formula: see text] emissions regulations.

Experimental investigation on effects of ethanol fumigation in dual fuel reactivity controlled compression ignition engine

In recent years, in order to face the challenge provided by the more strict emission rules, a vast number of researchers have researched the notion of low temperature combustion, commonly known as LTC. Because LTC may simultaneously lower the emissions of NOx and PM from internal combustion engines, the theory of LTC has gained a lot of attention from people all around the world. RCCI is a promising combustion approach that can replace up to 80% of the total fuel energy with hydrous ethanol, resulting in good thermal efficiency as well as minimal emissions of NOx and soot from the engine. In recent years, a vast number of research have focussed on studying a variety of techniques to optimize the combustion process, as well as performance and emissions. The usage of hydrous ethanol engines with a high water content has the potential to considerably enhance the amount of energy utilized throughout the course of bio ethanol's life cycle as well as its CO2 emissions.We employed a Reactivity Controlled Compression Ignition (RCCI) engine in our project. We did this by applying the approach of low temperature combustion in order to cut down on the amount of soot that was produced as well as the levels of emissions such as NOx, PM, and CO emissions, etc. We put 100% DIESEL, 90% Diesel with 10% Ethanol, 80% Diesel with 20% Ethanol, 70% Diesel with 30% Ethanol, and 70% Diesel with 30% Hydrous Ethanol through their paces in a variety of loads ranging from 0 to 100, as well as 25, 50, and 100 cycles each. As a result of our study and observations, we have come to the conclusion that the combination of HYDROUS ETHANOL (30% Energy Fraction) and DIESEL (70%Energy Fraction) produces the best results when compared to other combinations of fuels. As a result, this combination results in a reduction in the creation of soot and also results in a reduction in the emission of NOx, CO, and PM. The RCCI engine utilizes this particular blend of fuel because it generates superior outcomes.

Characterisation of emission-performance paradigm of a reactivity-controlled compression ignition engine using artificial intelligent based multi-objective response surface methodology model fuelled with ternary fuel

Petroleum reserves all over the world have been severe to the point of depletion in the previous several decades. Modern transportation and industry employ diesel-fuelled engines because of their higher thermal efficiency. This study investigates the reactivity-controlled compression ignition engine characteristics utilising n-octanol and Moringa oleifera oil blended with diesel. Tests are carried out at maximum load with different bowl geometries to examine the influence of operating parameters, namely injection pressure (500–1000 bar) and static injection timing (23–27°bTDC). Box–Behnken Forecasting input variables like injection pressure and timing with different piston bowls is done using the response surface approach. Maximum brake thermal efficiency and brake power are achieved with this ideal model, and the emission of unburned HC and NO x is decreased. The effects of several variables, including piston bowls, fuel injection timing (FIT) and fuel injection pressure, are examined for an reactivity-controlled compression ignition engine running on Moringa oleifera oil/1-octanol fuel using an L27 orthogonal array. Various models were created and verified using experiment findings as the basis.

Comparison of direct and port injection of methanol in a RCCI engine using diesel and biodiesel as high reactivity fuels

In this work, methanol dual-direct injection (DI2 mode) was compared with the port injection of methanol (RCCI mode) when biodiesel and diesel fuel were employed as HRFs (High Reactivity Fuels). This study presents an innovative dual-direct injection strategy as an alternative method to the commonly utilized port injection strategy in RCCI engines, and evaluates its impact on both engine performance and pollutant emissions. The engine was operated at a constant speed of 2400 rpm by a single-cylinder diesel research engine, and BMEP of 1.57, 3.15, and 4.72 bar (20 %, 40 %, and 60 % of max torque) with methanol was injected using both the DI2 and RCCI modes while diesel fuel and biodiesel were injected directly. In reactivity-controlled compression ignition (RCCI) mode, NOx emissions were reduced by 60 % and 62 % for diesel fuel and biodiesel, respectively, while the reduction accounted as 92 % and 80 % in smoke opacity. However, CO and unburned HC emissions were measured as higher for advanced modes than conventional mode using diesel fuel and biodiesel. According to experimental results obtained for DI2, unburned HC and CO emissions were reduced (up to 63 % and 22 %) when compared with RCCI mode. In addition, for DI2, NOx emissions were slightly higher than in RCCI mode, mainly due to the high injection pressure (50 bar) of DI2 mode. In comparison to conventional diesel injection (CDI), cylinder pressure and heat release rate (HRR) decreased as the premixed ratio (Rp) increased at 20 % load, while cylinder pressure and HRR increased with load and Rp increased. In both diesel and biodiesel experiments, ignition delay increased as Rp increased. The ignition delay was found to be longer in RCCI mode than in DI2 mode. It was concluded that DI2 was a beneficial way to control inefficient combustion during RCCI operation fuelled by diesel fuel and biodiesel.

A Review on Recent Developments of RCCI Engines Operated with Alternative Fuels

Environmental concerns over automotive exhaust emissions and consumer demand for higher fuel efficiency have led to the development of low-temperature combustion concepts. The reactivity-controlled compression ignition (RCCI) engine is one among them and has the potential to reduce NOx and smoke emissions simultaneously. In this concept, a low-reactivity fuel is injected into the intake port and another high-reactivity fuel is injected into the cylinder directly. This results in reactivity stratification and provides more control over the rate of heat release. However, operating parameters such as reactivity of fuels, premixing ratio, injection strategies, exhaust gas recirculation ratio, piston bowl geometry, and compression ratio influence emissions formation. The article reviews recent developments on the effect of the above operating parameters on the performance and emission characteristics of RCCI engines operated with alternative fuels. The combustion strategies used to extend the RCCI mode to higher loads are also reviewed. Applications of computational fluid dynamics (CFDs) to design the combustion chamber for RCCI engines are discussed. The need for further improvements in the CFD models for RCCI engines is explained. After presenting a thorough review of recent literature, directions for future research on RCCI engines are proposed.

Reactivity enhancement of natural gas/diesel RCCI engine by adding ozone species

Reactivity controlled compression ignition (RCCI) is an alternative combustion strategy with the potential of significant advantages in terms of increasing thermal efficiency together with reduction of NOx and soot emissions. The RCCI engines have been encountered not to be applicable broadly since they suffer from some difficulties mainly contributed to the controllability of combustion and limited operating ranges. The difficulties become even worse where low-reactivity fuels like natural gas (NG) are applied. Ozone gas, a chemical species with an extreme level of reactivity, can improve the combustion efficiency along with the advancement of combustion phasing in the RCCI engine. In this study a multidimensional computational fluid dynamic (CFD) which is coupled with proper detailed chemical kinetic mechanisms is employed to investigate the influence of ozone addition (i.e., 10, 100 and 1000 ppm) to air-fuel mixture with different initial conditions (e.g., intake temperature and equivalence ratio), on the performance and emissions characteristics of the RCCI engine. The results imply that the addition of even low concentrations of ozone (10 ppm) have considerable influences on the RCCI combustion characteristics. Addition of ozone by a distinct quantity not only can enhance the combustion phasing controllability, but also can extend the operating range of the RCCI engine in both lower intake air temperature and the lower fraction of the high-reactive fuel. From the results it is conceived that by adding 1000 ppm ozone into the air-fuel mixture, it is possible to reduce the diesel fuel fraction from 20% to 10% and intake air temperature from 355 K to 335 K, respectively.

Numerical investigation of natural gas (NG) as low reactivity fuel in a reactivity controlled compression ignition (RCCI) engine model

Natural Gas (NG) with its high octane number and low C/H ratio is beneficial for reactivity controlled compression ignition (RCCI) engines in controlling the rate of pressure rise and lower the emissions, given that these engines still suffer from these two problems. Therefore, in this study, a detailed 3D combustion model was developed in ANSYS-Forte environment on the scanned image of a real single-cylinder diesel engine for the application of NG to RCCI engines and then validated with the experimental data of the same engine. Afterward, the effect of NG premixed ratio (Rp, 15%, 30% and 45%) on the combustion and emission characteristics of the RCCI engine were analyzed at 20% load and 2400 rpm. Results showed that by increasing the Rp, the heat release rate and cylinder pressure decreased while the ignition delay (ID) period increased. Moreover, NOx and HC emissions were observed to be increased whereas CO and C2H2 emissions tended to be declined as Rp increased. CO emissions were decreased up to 29% when RCCI mode was applied. Also, C2H2 emissions were reduced by about 97% by RCCI compared to CDM. However, engine efficiency started to decrease as Rp increased.

Performance optimization of RCCI engines running on landfill gas, propane and hydrogen through the deep neural network and genetic algorithm

The present study is to optimize the performance of the reactivity controlled compression ignition engines taking landfill gas plus propane or hydrogen or a combination of both as low reactivity fuels. A heavy duty, 2.44 L caterpillar 3401E single-cylinder diesel engine is employed for the numerical simulation, and a number of the most effective variables on the engine performance are determined as inputs. The research purposes are to procure the maximum energy efficiency (i.e. the maximum thermodynamics first and second law efficiencies, and the minimum indicated specific fuel consumption), and to fulfill the Euro VI engine emissions standards (i.e. CO, UHC, and NOX maximum emissions 1.5, 0.13, and 0.4 gr/kW-hr, respectively). The maximum peak pressure and peak pressure rise rate are restricted to 150 bar and 10 bar/crank angle deg., respectively. The deep neural networks are utilized to obtain the engine mathematical model and the genetic algorithms are used to attain the intended optimization. Based on the outcomes, injecting a combination of propane and hydrogen to the premixed charge by 3.4 and 6.6 % by volume, respectively, and adopting an engine equivalence ratio of 0.31, an intake pressure of 1.983 bar, and a diesel fuel injection timing of about 75 deg. before the top dead center provide the maximum performance for the considered engine. In the optimized state, a 6.6 % hydrogen by volume takes 2.5 % of the total fuels energy. This research has proven that numerical modeling in combination with deep neural networks and genetic algorithms is an effective methodology to optimize the performance of the reactivity controlled compression ignition engines. Moreover, a dimensional analysis made shows the dimensionless peak pressure is function of the dimensionless engine heat release and the ratio of the engine cumulative heat release to the primary inlet mixture flow work. In the optimized case, the dimensionless ratio of cumulative heat release to the inlet flow work of fluid is 11.5.

Effects of Wobbe Index on the Combustion and Emission Characteristics of a Natural Gas/Diesel RCCI Engine

Abstract Due to energy shortage and environmental problems, the application of natural gas in internal combustion engine has attracted extensive attention. Therefore, diesel pilot ignition natural gas engine is a promising technology. However, the different sources of natural gas lead to the change in composition, which has a great impact on engine combustion and emission. In this study, the relationship between Wobbe index (WI) and swirl ratio (SR) of six different natural gas mixtures was studied by numerical simulation method. Besides, reactivity controlled compression ignition (RCCI) combustion strategy was evaluated. The results showed that increasing the WI increased the in-cylinder pressure and temperature, increased the ignition delay, and shortened the combustion duration, the gross indicated efficiency (GIE) of the six gases exceeded 50%. In addition, the increase of WI increased the nitrogen oxide (NOx) emissions and reduced the hydrocarbon (HC) and carbon monoxide (CO) emissions. Moreover, the peak pressure rise rate (PPRR) increased with the rise of WI, which may lead to engine knock. The results also showed that the increase of SR increased the in-cylinder pressure and temperature and improved the PPRR. When the SR was 0.7 and the WI was 51.7, the combustion and emission performance of the RCCI engine was relatively better.

A Numerical Study on the Combustion and Emissions Characteristics of a Heavy Duty Natural Gas/Diesel RCCI Engine

Abstract Due to energy shortages and environmental issues, the application of reactivity controlled compression ignition (RCCI) combustion in internal combustion engines has received extensive attention. Through the verification of the model, RCCI combustion can be accurately simulated. In this study, the combustion and the emission performance of a single-cylinder heavy-duty natural gas/diesel RCCI engine have been optimized through numerical simulation. Six important parameters including start of injection (SOI) timing, intake valve closing temperature, intake valve closing pressure, exhaust gas recirculation (EGR), swirl ratio, and spray angle have been investigated. The goal is to meet the requirements of European VI emission regulations while maintaining a high gross indicated efficiency (GIE). A strategy to achieve clean and efficient combustion of RCCI engine is proposed. The results showed that the addition of EGR can effectively reduce nitrogen oxide (NOx) emissions. SOI had the greatest impact on RCCI combustion and emission performance. Earlier SOI can improve the uniformity of the fuel mixture in the cylinder. Under the combined optimization of six important parameters, NOx, hydrocarbons, and carbon monoxide emissions can meet European VI emission regulations, and fuel consumption can meet Environmental Protection Agency consumption regulations, improving the GIE.

Effects of EGR on combustion and emission characteristics of PODE/methanol RCCI mode at high load

In order to extend the high-load operation range of polyoxymethylene dimethyl ethers (PODE)/methanol dual-fuel reactivity controlled compression ignition (RCCI) combustion mode, a numerical model of in-cylinder combustion was established by coupling PODE/methanol chemical reaction kinetics with CONVERGE software. And then the influence mechanism of exhaust gas recirculation (EGR) addition on in-cylinder combustion process, combustion stability and pollutant formation of the RCCI engine under high load and pilot injection strategy were investigated. The results show that with the increase of EGR ratio, the peak values of the first heat release formed by PODE pilot injection combustion and in-cylinder pressure decrease, the combustion start timing and CA50 phase are gradually delayed. While the peak value of the second heat release formed by PODE main injection combustion at low EGR ratio and the combustion duration remain unchanged. Meanwhile, as the EGR ratio increases from 0 to 30 %, the brake thermal efficiency (BTE) decreases from 46.54 % to 44.40 %. And the in-cylinder equivalent ratio and temperature field distributions become uniform, and both high-temperature region (above 1800 K) and NOx generation area are reduced, which decreases NOx emissions by 42.6 % without increasing soot emissions. The methanol mixture at squish zone is subjected to the high-temperature and high-pressure of pilot injection combustion, resulting in obvious pressure oscillation. While EGR addition decreases the generation of HCO and OH active radicals, ignition points and reaction rate, which effectively inhibits methanol spontaneous combustion and knock phenomenon, and decreases the ringing intensity from 2.82 MW/m2 to 1.50 MW/m2. The pilot injection strategy coupled with 18 % EGR can extend the indicated mean effective pressure to 1.22 MPa. However, the load further extension is mainly limited by the sharp decline of BTE.

CFD-based Data-driven Modeling of Reactivity and Stratification Dynamics for RCCI Engine Control

Reactivity Controlled Compression Ignition (RCCI) is a Low-Temperature Combustion (LTC) regime that provides thermal efficiency and emissions benefits compared to traditional Spark Ignition (SI) and Compression Ignition (CI) regimes. However, it is difficult to control combustion at these engines and run them at optimal conditions due to dependency of the their combustion on air-fuel mixture chemical reactivity and fuel stratification inside the combustion chamber. Modeling of reactivity and stratification can provide new pathways to control combustion in RCCI engines. In this study, a data-driven approach based on Computational Fluid Dynamics (CFD) results and a Linear Parameter Varying (LPV) method is proposed to model reactivity and stratification at RCCI engines. This work is illustrated for a real 2-liter 4-cylinder engine. The results show that the developed data-driven model (DDM) has acceptable prediction accuracy to estimate reactivity and stratification for RCCI engine control. The proposed method can be also implemented in other combustion modes in internal combustion engines.

Cylinder Pressure Feedback Control for Ideal Thermodynamic Cycle Tracking: Towards Self-learning Engines

To meet increasingly strict future greenhouse gas and pollutant emission targets, development time and costs of heavy-duty internal combustion engines will reach unacceptable levels. This is mainly due to increased system complexity and need to guarantee robust performance under a wide range of real-world conditions. Cylinder Pressure Based Control is a major contributor to achieve these goals in advanced, highly-efficient engine concepts. Current Cylinder Pressure Based Control approaches use combustion and air-path parameters as feedback signals. These signals are not directly linked to engine efficiency; therefore, compensation for changing ambient conditions, engine ageing or differences in fuel qualities is a non-trivial problem. Contrary to other methods, the method presented in this paper aims to realise an idealised thermodynamic cycle by directly control of the entire cylinder pressure curve. From measured in-cylinder pressure, a new set of feedback signals is derived using principle component decomposition. With these signals, optimal fuel path settings are determined. The potential of this method is demonstrated for a dual fuel Reactivity Controlled Compression Ignition (RCCI) engine, which combines very high efficiency and ultra low nitrogen oxides and particle matter emission. For the studied RCCI engine, it is shown that the newly proposed optimisation method gave the same optimal fuel path settings as existing methods. This is an important step towards self-learning engines.

Study on Combustion Characteristics of Diesel/Natural Gas/Hydrogen RCCI Engine

Abstract With the increasing environmental and energy problems, the development of new combustion technology has become a hot topic in the research of internal combustion engine. Reactivity controlled compression ignition (RCCI) combustion mode is the most promising combustion mode at present, which can solve the problems of combustion ignition controllability, and expand the engine load range. In this paper, optical diagnostics and numerical simulation are used to study the diesel/natural gas/hydrogen tri-fuel RCCI engine performance. Results show that hydrogen can significantly improve in-cylinder combustion and emission characteristics. With the increase of hydrogen addition, the combustion efficiency is increased accordingly, and the afterburning is prevented, which makes the exhaust energy loss decrease.

Experimental study on combustion characteristics of a fischer‐tropsch diesel/methanol reactivity controlled compression ignition mode engine

AbstractF‐T diesel is an alternative fuel with a high cetane number (CN) for internal combustion engines. Methanol has a low CN and contains oxygen. F‐T diesel and methanol are potential fuels for the reactivity controlled compression ignition (RCCI) engines. In a single‐cylinder 186FA compression‐ignition engine with a combination of methanol inlet pipe injection and F‐T diesel in‐cylinder injection, the combustion characteristics of the F‐T diesel/methanol RCCI mode engine are investigated. The results show that as methanol premixed ratio (MPR) increases, the ignition delay (ID) is prolonged at low load conditions, the maximum in‐cylinder pressure (Pmax) and indicated thermal efficiency (ITE) a decreased. When the MPR increases from 0% to 60%, the ID is extended by 1.7°CA, the Pmax decreases by 8.1%, and the ITE decreases by 1.1%. At high load conditions, MPR rises from 0% to 60%, combustion duration (CD) is shortened by 2.4°CA, Pmax is increased by 8.3%, and the ITE is increased by 1.7%. The fuel injection advance angle (IAA) of F‐T diesel significantly influences the combustion process of the RCCI engine. As the IAA increases, the ID is shortened at high loads, and the Pmax and the ITE are increased. When the MPR is 60% and the IAA is 14°CA, the ITE of the engine is increased by 2.04%, and the equivalent indicated fuel consumption rate(be) is reduced by 5.68% compared with the engine fueled with F‐T diesel under high loads.

Effects of Premixed Ratio on Engine Operation Range and Emissions of a Reactivity Controlled Compression Ignition Engine

Effects of Premixed Ratio on Engine Operation Range and Emissions of a Reactivity Controlled Compression Ignition Engine

Simulation study on effects of EGR ratio and compression ratio on combustion and emission characteristics of PODE/methanol RCCI engine

In order to solve the problems of combustion instability and emission deterioration of reactivity controlled compression ignition (RCCI) engine under high load, the in-cylinder combustion model of polyoxymethylene dimethyl ethers (PODE)/methanol dual-fuel RCCI engine was constructed by CONVERGE software coupling with chemical reaction mechanism, and the influence mechanism of exhaust gas recirculation (EGR) ratio and compression ratio (CR) on engine performance, in-cylinder combustion and emission characteristics of PODE/methanol RCCI mode was systematically studied. Results show that with the increase of EGR ratio, the combustion phase of RCCI mode is gradually delayed, the ignition delay and combustion duration are extended, the CA50 phase is gradually delayed, the combustion temperature decreases, the mixture distribution becomes more uniform, and the combustion process becomes smoother. When the EGR ratio increases from 0 % to 30 %, NOx production is reduced by 47.3 %, ringing intensity (RI) and maximum pressure rise rate (MPRR) decreases by 1.31 MW/m2 and 0.37 MPa/°CA respectively, and brake thermal efficiency (BTE) decreases from 45.45 % to 43.19 %. With the reduction of CR, the combustion start point is gradually postponed, the peak heat release rate and combustion temperature continuously decrease, and both IMEP and BTE show a downward trend. The variation of CR breaks the trade-off relationship between NOx and soot, and improves combustion stability, but it is also limited by the BTE reduction. When the CR decreases from 17.5 to 15.5, NOx and soot production are reduced by 28.58 % and 20.1 %, RI and MPRR are reduced by 1.26 MW/m2 and 0.7 MPa/°CA respectively, and BTE decreases from 45.45 % to 44.52 %. Therefore, the CR of 16.5 is more suitable for stable operation of PODE/methanol dual-fuel RCCI engine at high load.

Experimental assessment and multi-objective optimization on the engine characteristics of reactivity controlled compression ignition engine powered by Ternary fuel

• Multi objective optimisation is adopted to operate the operating parameters with ternary fuel on RCCI engine. • ANOVA, a statistically valid test is used to develop a regression model. • Biodiesel enriched engine resulted in 2.55% higher NHRR and 6.48% higher CP. • HC and NOx emissions decreased by 5.86% and 11.4%, at higher FIP. • Ternary fuel performs effectively at higher FIP. In light of global energy insecurity and other pressing concerns like global warming, stringent emissions, and rapid industrialization, researchers are looking at the potential for alternative fuel research as a fossil fuel substitute. The present research work is carried out to study the influence of injection strategies (i.e., FIP and FIT) by varying the engine load at a constant velocity of 2000 rpm. The effect of several parameters such as engine load, FIT, and FIP on an RCCI engine fuelled with B20/1-pentanol fuel is examined using an L20 orthogonal array for an RCCI engine fuelled with B20/1-pentanol fuel. Various models were constructed and validated with the aid of experimental results. The ideal engine parameters found were 62.06% of engine load, FIP of 438.5 bar, and 27 0bTDC, and under this configuration, the output obtained is as follows: 206.9 ppm, 25.09 ppm, 33.04 J/deg, 29.39% and 2.39 kW for NOx, HC, HRR, BTE and BP emission respectively. Also, the regression model is developed using ANOVA, a statistically reliable test, and the test results suggest that the regression model is acceptable for the following. The following are the R 2 values obtained: NOx − 99.78%, HC– 99.14%, NHRR- 98.5%, BTE-99.93%, and BP-99.96% respectively.

Effects of piston shape and nozzle specifications on part-load operation of natural gas–diesel dual-fuel RCCI engine and its application to high load extension

• Effects of piston shape and multi-angle nozzle were evaluated for DF RCCI operation. • Enhanced air utilization w/multi-angle nozzle reduced combustion and HT losses. • The highest GIE of RCCI improved as 51.8 % by hardware modifications. • Intake air amount could be increased by lowering CR (from 17 to 15) at the maximum load. • The maximum load of RCCI could be extended from 81 (reference) to 101 kW of BP. In this study, the effects of modifications of both the piston shape and injector nozzle are experimentally investigated for high load extension of a natural gas–diesel dual-fuel reactivity-controlled compression ignition (RCCI) engine. First, a bathtub shape with a reduced compression ratio is applied to the modified piston which was reported to improve the flame propagation in squish volume, the cooling loss, and the air induction rate under the same limit of in-cylinder pressure from previous studies. In addition, a multiangle two-layer injector is used to extend the maximum load, which has been proven to be effective for unburned hydrocarbon emissions and gross indicated efficiency in part-load operation in our previous study. The experiments are conducted under both 1200 rpm part-load operation (approximately 1950 J/cylinder per cycle of fuel input energy) and 1800 rpm maximum load operation. The objectives are to comprehensively evaluate the effects of the modifications on natural gas–diesel RCCI operation and verify the effects of all load conditions. The experimental results reveal that the changes in the piston—reduction in the compression ratio and a bathtub piston shape—are effective for both NOx and smoke emissions as the diesel injection timing is advanced. Although the multi-hole double layer (combined narrow and wide injection) has less effect on emissions compared to that of the change in piston, it affects the mixing process inside the cylinder, thus retarding the combustion phasing, prolonging the combustion duration, and eventually reducing the cooling loss. Both modifications are highly effective for advanced diesel injection timing corresponding to RCCI operation, which extends high-load operation from 80.9 to 101 kW of brake power, relatively 24.8 % increase, by adopting both the piston and injector modifications. Based on the modifications, the mass fraction of natural gas is suggested to be approximately 80 %. A higher fraction causes the variation in the maximum in-cylinder pressure to be extremely sensitive to allow robust operation. In contrast, if it is lower, the amounts of both NOx and smoke emissions exceed the emission standard levels (0.4 g/kWh and 15 mg/kWh for NOx and smoke emissions, respectively, corresponding to Stage-V of non-road engine operation).

Numerical Investigation of the Knocking Combustion Characteristics of the N-Butanol/N-Octanol RCCI Engine

The n-butanol/n-octanol fueled reactivity-controlled compression ignition engine was numerically studied based on the KIVA-CHEMKIN code. First, the knocking combustion characteristics were analyzed while functioning with a premixed n-butanol percentage of 20% (B20), since it exhibited the most severe knocking. Ten local regions were monitored to obtain local data, such as pressure and heat release rate. The local pressure oscillation was quantified by a band-pass filter. Second, the premixed n-butanol percentage and the intake valve close (IVC) timing were varied to investigate their effects on the combustion characteristics and emissions formations, as well as their potential for mitigating knocking. The results showed that a strong pressure oscillation was observed for B20 near the cylinder wall, which indicates severe knocking. This consequence is mainly caused by the low-temperature combustion of the n-octanol/n-butanol/air mixture near the cylinder-wall region. Increasing premixed n-butanol percentage and retarding IVC timing could result in an extended ignition delay, lowered peak pressure, and reduced maximum pressure rise rate (PRR). Condition B80 with an IVC timing of −126 °ATDC could improve the indicated mean effective pressure by 11.7% and reduce the maximum PRR by 63.4% when compared to condition B20.