Dual-fuel Reactivity Controlled Compression Ignition Research Articles

Impacts of pine oil and hydrogen induction with hemp oil methyl ester on dual fuel reactivity controlled compression ignition combustion in diesel engine

AbstractThe current research focuses on the impacts of pine oil injection and hydrogen induction separately with hemp oil methyl ester (HOME) in the single cylinder diesel engine in dual fuel‐reactivity controlled compression ignition (DF‐RCCI) combustion mode. The engine was tested under a DF‐RCCI mode for the different energy shares of 10% pine oil with HOME (10P‐HOME), 30% pine oil with HOME (30P‐HOME), 3‐lpm hydrogen with HOME (3‐lpmH2‐HOME), and 6‐lpm hydrogen with HOME (6‐lpmH2‐HOME) separately at 345 °CA bTDC of low reactivity fuel (pine oil and hydrogen) and 23°C bTDC injection timing of high reactivity fuel (HOME). The results showed a higher Brake Thermal Efficiency (BTE) of 7.44%, 5.32%, 5.72%, and 2.46% for 6‐lpmH2‐HOME, 3‐lpmH2‐HOME, 30P‐HOME, and 10P‐HOME fuel shares, respectively, over the conventional diesel combustion (CDC) at full load. 30P‐HOME, 3‐lpmH2‐HOME, and 6‐lpmH2‐HOME fuel combinations recorded 4.08% 4.42%, and 5.69% lower brake specific fuel consumption (BSFC), respectively, at full load. When comparing DF‐RCCI combustion to CDC, an increase in the heat release rate (HRR) of 2.89%–26.50% and a rise in peak cylinder pressure of 0.77%–12.99% were observed. The 30P‐HOME, 3‐lpmH2‐HOME, and 6‐lpmH2‐HOME emit less smoke in DF‐RCCI combustion mode by 13.06%, 4.84%, and 7.26%, respectively at full load condition. When using 30P‐HOME the exhaust gas temperature (EGT) decreased by 3.50% at full load condition. At part and full load conditions, the 30P‐HOME fuel share reduced oxides of nitrogen (NOX) emissions by 3.93% and 5.26%, respectively.

Analysis of the Vibrational Behavior of dual-fuel RCCI combustion in a Heavy-Duty Compression Ignited Engine fueled with Diesel-NG at Low Load

In the field of internal combustion engines, the Low Temperature Combustions (LTC) appear to have the potential to reduce the formation of both soot and nitrogen oxides. One of the most promising LTC is Reactivity Controlled Compression Ignition (RCCI) which is based on the combustion of a lean low reactivity fuel-air mixture generated in the intake manifold and autoignited by small injections of high reactivity fuel introduced at high pressure in the combustion chamber. By the combination of net-zero natural gas and biodiesel, such LTC methodology might represent a suitable solution moving toward zero-emissions in transportation sector.Despite the potential to reduce pollutant emissions, Low Temperature Combustion strategies face a challenge in controlling the angular position where the combustion takes place which can be overcome by a proper management of the high-pressure injections.One potentially interesting application is related to trucks, mainly because they have long periods of idling, since emissions can be drastically reduced by means LTC. A single cylinder research engine for heavy duty application is operated under steady state conditions at low load and speed to analyze the possibility of controlling the engine behavior in dual fuel RCCI mode. The results indicate that the combustion mode switches from the dual-stage to gaussian within a narrow angular range. A further advance of the start of injection can generate misfires and significant variations in typical combustion indexes, while a delayed start of injection can cause impulsive combustion that rises the cylinder temperature and results in high-frequency pressure oscillations inside the combustion chamber. These oscillations are related to the combustion chamber typical resonance frequency, and if relevant in amplitude and persist for a long time, they might generate a potential source of failures.

Modeling study of the ignition characteristics of directly injected 1,3-Cyclohexadiene in surrogate Gasoline/Air under RCCI combustion conditions

In this work a validated chemical kinetic model of the combustion of multicomponent gasoline surrogate fuels containing 680 reactions of 152 species has been used to simulate conditions reminiscent of those of Reactivity Controlled Compression Ignition (RCCI) in order to evaluate 1,3-cyclohexadiene (CYC6H8) as the high reactivity directly injected fuel and a gasoline surrogate fuel as the low reactivity premixed fuel. A single-zone modeling approach was adopted using a modified Cantera example describing the simulation of a gaseous Diesel-type internal combustion engine. The adaptable chemical structure of CYC6H8 to cis-1,3,5-hexatriene (lC6H8), load, start of injection and premixed fuel mass fraction were varied in order to analyze the potential of CYC6H8 as an ignition enhancer. Results have been compared to n-heptane and show that CYC6H8 and its isomer lC6H8 have the potential to be used as the high reactivity directly injected fuel in dual-fuel RCCI combustion, especially at high load conditions. For increased load, the ignition promoting effect is more pronounced at lower premixed fuel mass fractions and less affected by a delayed start of injection. The ignition promoting effect of lC6H8 is more pronounced than CYC6H8 for a decreasing premixed fuel mass fraction. Results from the kinetic analysis indicate that at increased concentrations of lC6H8, the thermal effects are becoming more significant.

The influence of different fuels and injection methods of RCCI and DCI in hybrid ICE-Battery vehicle performance

The incorporation of two recent technologies of using the dual-fuel reactivity controlled compression ignition (RCCI) combustion engine within the hybrid electric vehicle (HEV) is practiced to show how this combination can reduce the emission and enhance the thermal efficiency of the system. In particular, the heat transfers from the engine wall and the exhaust heat flow from the engine under different injection modes and fuels are of interest. The study in terms of thermal performance, fuel consumption, and battery state of charge (SOC) focuses mainly on the comparison between three cases of D100 (pure diesel) as the reference (baseline conventional direct pure diesel injection) case, D80H20 (80% diesel, 20% hydrogen) direct co-injection (DCI), and D80H20 RCCI (port + direct dual fuel injection). The NOx emission and engine power in the simulated drive cycle are investigated where the battery capacity and D50M50 (direct co-injection of 50% diesel with 50% methanol) are the additional cases. The findings indicate that the Battery SOC is preserved in better condition when the RCCI mode engine is coupled in the hybrid vehicle. The piston wall heat flux for D80H20 in DCI increases by 45.2% and for the RCCI increases by 60.5% compared to baseline diesel injection mode. It is also proved that the HEV releases considerably lower NOx compared to DCI and more NOx compared to D100 and D50M50.

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.

An experimental and modeling study on autoignition of 2-phenylethanol and its blends with n-heptane

2-Phenylethanol (2-PE) is an aromatic alcohol with high research octane number, high octane sensitivity, and a potential to be produced using biomass. Considering that 2-PE can be used as a fuel additive for boosting the anti-knocking quality of gasoline in spark-ignition engines and as the low reactivity fuel or fuel component in dual-fuel reactivity controlled compression ignition (RCCI) engines, it is of fundamental and practical interest to understand the autoignition chemistry of 2-PE, especially at low-to-intermediate temperatures (<1000 K). Based upon the experimental ignition delay time (IDT) results of neat 2-PE obtained from our previous rapid compression machine (RCM) investigation and the literature shock tube study, a detailed chemical kinetic model of 2-PE is developed herein, covering low-to-high temperature regimes. Besides, RCM experiments using binary fuel blends of 2-PE and n-heptane (nC7) are conducted in this work to investigate the nC7/2-PE blending effects, as they represent a dual-fuel system for RCCI operations. Furthermore, the newly developed 2-PE model is merged with a well-validated nC7 kinetic model to generate the current nC7/2-PE binary blend model. Overall, the consolidated model reasonably predicts the experimental IDT data of neat 2-PE and nC7/2-PE blends, as well as captures the experimental effects of pressure, equivalence ratio, and blending ratio on autoignition. Finally, model-based chemical kinetic analyses are carried out to understand and identify the controlling chemistry accounting for the observed blending effects in RCM experiments. The analyses reveal that nC7 enhances 2-PE autoignition via providing extra ȮH radicals to the shared radical pool, while the diminished nC7 promoting effect on 2-PE autoignition with increasing temperature is due to the negative temperature coefficient characteristics of nC7.

Evaluation of the sample size requirements of machine learning models used in engine design and research

Machine Learning (ML) techniques have been effectively used to learn the intricate relationships between variables that play a significant role in the field of engine design. However, there are two challenges to this approach – (1) Identifying ML regression models that could capture the trends of a response variable, given a non-parametric training set of relatively small size, with an acceptable accuracy and response time, and (2) identifying the size of the dataset, with respect to the input parameters, to be used for training and validation of the ML models. There is not enough information in the literature to reach a consensus on the sampling size to be used for an engine design problem that would yield an acceptable measure of goodness-of-fit. This is evident from the varied size of the training/validation data size used within the engine research community, as will be elaborated on in the following sections. The objective of this paper is to provide an insight into the sampling size required by the different ML models to achieve an acceptable fit between the model and the data, to be used in three types of engine design/optimization problems – (1) conventional diesel combustion (CDC) engine performance over a wide range of speed and load, (2) cold-start operation of a direct-injected spark-ignition (DISI) engine, and (3) high-load performance of a dual-fuel reactivity-controlled compression ignition (RCCI) 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.

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).

Comparative experimental analysis on dual fuel with biodiesel-acetylene in reactivity controlled compression ignition engine

ABSTRACT In the latest Compression Ignition (CI) engines, a gaseous fuel plays a significant role because they have the advantage of a low knock-creating tendency and lower carbon emissions. In B20 Mahua Oil Methyl Ester (MOME)-acetylene dual-fuel Reactivity Controlled Compression Ignition (RCCI) engines, an effort has been made to use gaseous fuel acetylene as a Low Reactivity Fuel (LRF). RCCI engines are well known for their low particulate matter and Nitrogen Oxides (NOx). In the current study, all the aforementioned technologies are concurrently used to attain better performance, emission and combustion characteristics. The test engine is properly modified to allow acetylene to be injected at an angle of 60° to stimulate better turbulence intake air at different flow rates, from 1 to 5 litres per minute (LPM). The result shows an increase in acetylene mass flow rates, increases the Brake Thermal Efficiency (BTE) for B20 MOME. A maximum BTE of 32.6% is achieved at the RCCI B20 MOME-acetylene operation with acetylene flow rate of 4 LPM, which is marginally lower with Conventional Diesel Engine (CDE) operation. In B20 MOME- 4 LPM Acetylene RCCI operation, the NOx emission, the smoke density, Hydro Carbon (HC) and Carbon Monoxide (CO) are reduced by 11.8%, 10.4%, 5.3% and 14.3%, respectively compared to B20 MOME in CDE operation.

Development of a Diesel/Natural Gas Mechanism Model for the CFD Simulation of Dual-Fuel Engine

Diesel/natural gas (NG) can effectively improve the performance and reduce emissions of the reactivity-controlled compression ignition (RCCI) engine. In this work, n-hexadecane was used to characterize diesel and the methane/ethane/propane mixture was used to characterize NG, and a simplified diesel/NG mechanism containing 645 reactions and 155 species was established. We used brute force sensitivity analysis to optimize the key dynamic parameters of the mechanism and, through the laminar flame velocity, the substance concentration in the jet-stirred reactors and the ignition delay in the shock tube to verify the optimized n-hexadecane/NG mechanism and found that this mechanism can better respond to diesel/NG. Finally, the mechanism was coupled with the computational fluid dynamic (CFD) to study the effect of different diesel injection timings (DITs) on the combustion performance of RCCI engines. The results show that as the DIT advanced, the temperature distribution in the cylinder became uneven. Also, when the temperature was lower, the content of unburned methane in the cylinder increased. When the DIT was 45° crank angle (CA) before the top dead center (BTDC), the temperature and equivalent in the cylinder were more evenly distributed than in the cylinder and the unburned methane content was lower and diesel/NG exhibited a better combustion effect. The diesel/natural gas mechanism model can be better applied to the CFD simulation of dual-fuel RCCI engines.

Characteristics of non-methane hydrocarbons and methane emissions in exhaust gases under natural-gas/diesel dual-fuel combustion

In dual-fuel combustion, the levels of total hydrocarbon (THC) emissions have increased, unlike the neat diesel compression ignition (CI) combustion. Intuitively, it can be inferred that most THC emissions are produced by the crevice effect of low-reactivity fuels (high-volatility fuels) under dual-fuel combustion. Additionally, it is possible that high levels of THC emissions are produced by dual-fuel combustion. If reactivity-controlled compression ignition (RCCI) combustion is applied by advancing the diesel injection timing to improve the premixing conditions, the occurrence of the wall-wetting effect may induce the production of high levels of THC emissions owing to incomplete combustion. Thus, in this study, the characteristics of non-methane hydrocarbons (NMHC) and methane emissions under general dual-fuel combustion conditions (diesel injection timings before top dead center (BTDC) 0 and 30 crank angle degree (CAD)) and RCCI conditions (diesel injection timings at 60 and 84 CAD BTDC) were investigated under a low-load in a 6-L heavy-duty engine.The natural gas fraction varied from 50% to the maximum level considering the combustion stability (coefficient of variance of indicated mean effective pressure). The results revealed that the overall THC emissions from the general dual-fuel combustion were higher compared with those of RCCI combustion, owing to the methane emissions from the squish area and the lower average in-cylinder temperature. Notably, methane emissions were dominant in the THC emissions (52–87%) under the general dual-fuel combustion condition, while NMHC emissions were a major part of the in THC emissions (61–87%) under the RCCI regimes. Hence, combustion inefficiency is a major concern in dual-fuel combustion. In this study, this problem was resolved by advancing the diesel injection timings. The findings in this study can be useful in the design of oxidation catalysts, regardless of whether the combustion regime is a general dual-fuel combustion or RCCI combustion considering the portion of methane emissions in the exhaust gas. Additionally, it is known that the RCCI combustion is superior to the general dual-fuel combustion with respect to higher thermal efficiency and low harmful THC emissions.

Effect of injection strategies on a single-fuel RCCI combustion fueled with isobutanol/isobutanol + DTBP blends

In recent years, improved combustion controllability through in-cylinder reactivity stratification by using two different fuels have led to introduction of dual-fuel reactivity controlled compression ignition (RCCI) strategy. In conventional RCCI, gasoline or natural gas can be used as the low-reactivity fuel, and diesel or biodiesel can be used as the high-reactivity fuel. This strategy has the potential to operate with a single low-reactivity fuel and direct injection (DI) of the same fuel blended with a small amount of cetane improver. In the present study, numerical simulations have been carried out to study injection strategy in a single-fuel RCCI engine fueled with isobutanol – isobutanol + 20% di-tert-butyl peroxide (DTBP). Firstly, the effects of start of injection (SOI) timing, injection pressure (pinj), spray cone angle (SCA), and DI fuel ratio were explored. Then, the effect of DI fuel ratio was discussed in each best case in order to decrease the high DI requirement. The results indicate that SOI = −88° ATDC, pinj = 1400 bar, and SCA = 45° can improve the single-fuel RCCI engine performance and emissions compared to the baseline case (SOI = −58° ATDC, pinj = 600 bar, SCA = 72.5°). Moreover, it is shown that by advancing the SOI timing to −88° ATDC, a 20% reduction in DI ratio, 3.3% increase in gross indicated efficiency (GIE) together with reductions in CO, and NOx emissions by 3.56 g/kW-h and 0.254 g/kW-h, could be achieved, respectively.

A comprehensive parametric, energy and exergy analysis for oxygenated biofuels based dual-fuel combustion in an automotive light duty diesel engine

Presently methanol based dual fuel reactivity controlled compression ignition (DFRCCI) has more and more attention in fuel and engine research field because of its potential in reducing the engine-out harmful emissions and evading the dependency on fossil fuel resources. However, while using methanol along with diesel in DFRCCI combustion, a substantial increase in incomplete combustion loss was observed at lower loads due to lower fuel stratification, global reactivity and in-cylinder temperature. Hence, to address this limitation, diesel fuel was replaced by a biodiesel, and a detailed experimental study was performed with energy and exergy analysis to show the potential of biodiesel over diesel. In this study, a modified automotive diesel engine was tested under CDC and DFRCCI combustion strategy using Methanol/ Diesel (M/D) and Methanol/ Biodiesel (M/BD) fuel combinations at 2100 rpm, 3.4 bar BMEP load condition. The experimental parametric study shows that 60% of methanol energy can be effectively utilised with an advanced direct injection and a moderate level of exhaust gas recirculation. Energy analysis clearly indicated the reason for the improvement in brake thermal efficiency in DFRCCI combustion. Reduced energy loss in the exhaust and coolant, increased brake thermal efficiency was observed in DFRCCI compared to CDC. However, increased incomplete combustion loss was observed in M/D DFRCCI combustion compared to CDC. Whereas, reduced incomplete combustion loss was observed while using biodiesel instead of diesel due to increased fuel stratification caused by the physical properties of biodiesel and in-cylinder temperature. Further, lower exergy destruction was observed in M/BD DFRCCI compared to M/D DFRCCI combustion.

Effect of Polyoxymethylene Dimethyl Ethers-Diesel Blends as High-Reactivity Fuel in a Dual-Fuel Reactivity Controlled Compression Ignition Combustion

Effect of Polyoxymethylene Dimethyl Ethers-Diesel Blends as High-Reactivity Fuel in a Dual-Fuel Reactivity Controlled Compression Ignition Combustion

Natural gas-diesel reactivity controlled compression ignition with negative valve overlap and in-cylinder fuel reforming

Dual-fuel reactivity controlled compression ignition combustion offers potentially superior overall efficiency and ultra-low nitrogen oxides and soot emissions. Using natural gas as the low reactivity fuel also provides high knock-resistance and carbon dioxide emission reduction. However, the concept suffers from relatively low combustion efficiency at low engine loads, causing unacceptable methane slip. This study tackles this issue, applying numerical simulations to investigate the application of negative valve overlap to improve combustion efficiency of reactivity controlled compression ignition at low engine loads. The objective is modification of in-cylinder thermal and chemical state before combustion, by varying timing and amount of fuel injected directly into the recompressed hot exhaust gases. The study uses TNO's multi-zone, chemical kinetics-based combustion model with variable valve actuation functionality. The simulation is based on two experimentally validated cases: an uncooled exhaust gas recirculation strategy and a lean burn concept. In both cases, negative valve overlap elevates in-cylinder temperature and cuts methane emissions by 15%, without combustion optimization. Crucially, it enables peak exhaust recompression temperatures above 850 K, sufficient for diesel reforming/oxidation. The lean RCCI strategy takes greater advantage of fuel reforming than the exhaust gas recirculation case. Optimum conditions give almost 99% combustion efficiency and ultra-low methane emissions. Net indicated efficiency is 40.5% (@15% load), despite negative valve overlap’s substantial pumping losses. Low-load net efficiency is 5.5 percentage points above the lean strategy baseline and 3 pp. better than the exhaust gas recirculation baseline. This strategy is considered applicable on state-of-the-art dual-fuel gas engines without hardware changes.

Thermal efficiency improvement of PODE/Gasoline dual-fuel RCCI high load operation with EGR and air dilution

In recent years, polyoxymethylene dimethyl ethers (PODE) is used for reactivity gradient enlargement in direct injection engines due to its high Cetane Number (CN). In the current study, thermal efficiency improvement of PODE/Gasoline dual-fuel reactivity controlled compression ignition (RCCI) high load operation with exhausted gas recirculation (EGR) and air dilution is experimentally investigated, together with a zero-dimension (0-D) thermodynamic analysis. The experimental result shows that 15% gross indicated thermal efficiency (ITEg) improvement can be achieved through both air dilution and EGR dilution. Air dilution under fixed intake pressure shows significant effects on ITEg improvements. EGR dilution under fixed equivalence ratio (Φ) slightly reduces the ITEg. Further study with fixed the total charge heat capacity reveals that at lean conditions, the total charge heat capacity has greater influence on the combustion processes and ITEg than equivalence ratio. Comparing the experimental results with the thermodynamic modelling results, it indicates that combustion efficiency is an important factor that determines the ITEg with air/EGR dilution. Further analysis reveals that air dilution greatly improves the CO oxidation owing to the increased oxygen concentration, which obviously improves the combustion efficiency and ITEg.

Computational optimization of fuel supply, syngas composition, and intake conditions for a syngas/diesel RCCI engine

By utilizing the promising alternative fuel of syngas, and the syngas/diesel dual-fuel reactivity controlled compression ignition (RCCI) is a potential combustion strategy for internal combustion engines. However, the optimal operating parameters for syngas/diesel RCCI engines under wide operating conditions have not been investigated. In this study, the operating parameters include fuel supply, syngas composition, and intake conditions of a syngas/diesel RCCI engine were optimized under wide load by integrating the KIVA-3V code and the non-dominated sort genetic algorithm II (NSGA-II). The results indicated that nitrogen oxides (NOx) emissions can be controlled in considerably low levels, and the efficient combustion of the premixed syngas in the squish region can be realized with high premix ratio and early pilot injection of diesel. Equivalent indicated specific fuel consumption (EISFC) and ringing intensity (RI) are the major issues for the optimization at low and mid load, respectively. The double diesel injection strategy with the relatively late main injection timing is an effective way to both improve combustion efficiency at the low load and reduce RI at the mid load. For the double diesel injection, the ratio of pilot injection is controlled in a narrow range to provide sufficient high reactivity fuel in the piston bowl and to avoid the local high-temperature combustion region simultaneously. With the restrictions of EISFC and RI, the optimal H2 fraction in the syngas is 60–80%. Based on the optimal fuel supply and intake conditions, a syngas with 75% H2 and the diluent factor C of 0.8 is capable of realizing the high efficiency, moderate combustion, and low emissions for the RCCI engine at full load range.

Sizing a conventional diesel oxidation catalyst to be used for RCCI combustion under real driving conditions

Reactivity controlled compression ignition (RCCI) combustion has demonstrated to be able to avoid the NOx-soot trade-off appearing during conventional diesel combustion (CDC), with similar or better thermal efficiency than CDC under a wide variety of engine platforms. However, a major challenge of this concept comes from the high hydrocarbon (HC) and carbon monoxide (CO) emission levels, which are orders of magnitude greater than CDC, and similar to those of port fuel injected (PFI) gasoline engines. The high HC and CO emissions levels combined with the low exhaust temperatures during RCCI operation could present a challenge for the current exhaust aftertreatment technologies.The objective of this work is to evaluate the potential of a conventional diesel oxidation catalyst (DOC) for light-duty diesel engines when operating under dual-fuel RCCI diesel-gasoline combustion and to define its necessary size to accomplish with the current emissions standards. For this purpose, a 1-D model has been developed and calibrated through gas emissions measurements upstream and downstream the DOC under different engine steady-state conditions. After that, the DOC response in transient conditions has been evaluated by means of vehicle systems simulations under different driving cycles representative of the homologation procedures currently in force around the world. The results show that the HC and CO levels at the DOC outlet are unacceptable considering the different emissions regulations. By this reason, a dedicated study to define the DOC size needed to accomplish the different emissions standards is carried out. The results suggest that, the DOC volume needed to fulfill the type approval regulation limits ranges from four to six times the original volume.

The effects of DI fuel properties on the combustion and emissions characteristics of RCCI combustion

An experimental investigation has been conducted to explore the effects of direct-injection (DI) fuel properties in dual-fuel reactivity-controlled compression ignition (RCCI) combustion on a single-cylinder, heavy-duty diesel engine. The combustion and emission characteristics of four DI fuels with cetane number (CN) ranging from 34 to 100 were compared under different combustion boundary conditions. The results showed that a higher CN DI fuel allows more advanced SOI timing, more EGR, and higher premixed ratio. However, although the operable DISOI timing and EGR regions can be extended, the operable Rp region is narrowed for higher CN fuels. In addition, there is a competing relationship between DI fuel CN and EGR/premixed ratio (Rp) in controlling CA50, PPRR, and HC/CO emissions. With late injection strategy, enlarging the reactivity gradient through increasing CN of DI fuel shows more promising effects on combustion rate and pressure rise rate reduction, which is desirable for high load extension. Comparing the highest indicated thermal efficiency (ITE) cases of different CN fuels, it’s illustrated that the approach of lower EGR (<45%) with high Rp (∼80%) is suitable for low CN fuel (i.e. CN34) with early injection strategy to obtain a lower PPRR and a better ITE simultaneously. However, at late injection condition, more EGR with lower Rp (<80%) is required to control NOx/soot emissions. For high CN fuel (CN ≥ 56), the approach of higher Rp (>80%) with high EGR (∼45%) is suitable with either early or late injection strategy to simultaneously improve the PPRR, ITE and NOx/soot emissions.