Partially Premixed Compression Ignition Research Articles

An experimental investigation into the combustion stability and emissions of an n-butanol/diesel blended fueled partially premixed compression ignition (PPCI) engine

Higher cyclic variability in internal combustion engines (ICE) increases driveability problems, specific fuel consumption, and exhaust emissions. Consequently, quantification of the combustion stability in different combustion parameters is essential for the design of closed-loop control for a partially premixed combustion (PPC). This research examines the impacts of a PPC strategy on the spectral features of IMEP combustion parameters for early, and late injection timings of a 4-cylinder CRDI engine using wavelet transforms (WT). Furthermore, operating points were optimized based on the combustion and emissions characteristics of the partially premixed compression ignition engine. Results indicated that the early start of injection (SOI) timing of 12° CA BTDC has a lower standard deviation in the time-series data of IMEP than the late SOI timing. Moreover, when the amount of n-butanol in diesel fuel was increased from 0% to 20% at a load of 2 bar BMEP, COVIMEP decreased from 3.1% to 2.75% at early SOI timing. The WT results reveal that the continuous strong intensity band pattern changes irregularly when the injection timing is advanced. It was also found that at lower cyclic variability conditions (SOI = 12°CA BTDC), CO and smoke emissions decreased, and there was a noticeable increase in NOx emissions.

Development of Two-Step Exhaust Rebreathing for a Low-NOx Light-Duty Gasoline Compression Ignition Engine

The global automotive industry is undergoing a significant transition as battery electric vehicles enter the market and diesel sales decline. It is widely recognized that internal combustion engines (ICE) will be needed for transport for years to come; however, demands on ICE fuel efficiency, emissions, cost, and performance are extremely challenging. Gasoline compression ignition (GCI) is one approach for achieving the demanding efficiency and emissions targets. A key technology enabler for GCI is partially-premixed, compression ignition (PPCI) combustion, which involves two high-pressure, late fuel injections during the compression stroke. Both NOx and smoke emissions are greatly reduced relative to diesel, and this reduces the aftertreatment (AT) requirements significantly. For robust low-load and cold operation, a two-step valvetrain system is used for exhaust rebreathing (RB). Exhaust rebreathing involves the reinduction of hot exhaust gases into the cylinder during a second exhaust lift event during the intake stroke to help promote autoignition. The amount of exhaust rebreathing is controlled by exhaust backpressure, created by the vanes on the variable nozzle turbine (VNT) turbocharger. Because of the higher cycle temperatures during rebreathing, exhaust HC and CO may be significantly reduced, while combustion robustness and stability also improve. Importantly, exhaust rebreathing significantly increases exhaust temperatures in order to maintain active catalysis in the AT system for ultra-low tailpipe emissions. To achieve these benefits, it is important to optimize the rebreathe valve lift profile and develop an RB ON→OFF (mode switch) strategy that is easy to implement and control, without engine torque fluctuation. In this study, an engine model was developed using GT-Suite to conduct steady-state and transient engine simulations of the rebreathing process, followed by engine tests. The investigation was conducted in four parts. In part 1, various rebreathe lift profiles were simulated. The system performance was evaluated based on in-cylinder temperature, exhaust temperature, and pumping work. The results were compared with alternative variable valve actuation (VVA) strategies such as early exhaust valve closing (EEVC), negative valve overlap (NVO), positive valve overlap (PVO). In part 2, steady-state simulations were conducted to determine an appropriate engine load range for mode switching (exhaust rebreathing ON/OFF and vice-versa). The limits for both in-cylinder temperature and exhaust gas temperature, as well as the external exhaust gas recirculation (EGR) delivery potential were set as the criteria for load selection. In part 3, transient simulations were conducted to evaluate various mode switch strategies. For RB OFF, the cooled external EGR was utilized with the goal to maintain exhaust gas dilution during mode switches for low NOx emissions. The most promising mode-switch strategies produced negligible torque fluctuation during the mode switch. Finally, in part 4, engine tests were conducted, using the developed RB valve lift profile, at various low-load operating conditions. The mode switch experiments correlated well with the simulation results. The tests demonstrated the simplicity and robustness of the exhaust rebreathing approach. A robust engine response, low CNL, high exhaust gas temperature, and low engine out emissions were achieved in the low load region.

Co-optimization of piston bowl and injector for light-duty GCI engine using CFD and ML

In this study, the piston bowl geometry and injector design of a light-duty GCI engine were co-optimized using computational fluid dynamics (CFD) and advanced machine learning (ML) techniques to maximize the capability of a GCI technology. This study was performed at low- (6 bar), mid- (11 bar) and high-load (22 bar) indicated mean effective pressure (IMEP) conditions. The 3-D CFD setup was first validated against experimental data. Then, the injector and the piston bowl were simultaneously co-optimized. In total, 13 (ten piston- and three injector-related) design parameters were considered. At each load condition, 128 DoE cases were generated, and the features and performance of the top three designs were analyzed. The best DoE solution was selected by defining weighted merit values to provide one best design across all load conditions. The simulated dataset was used for further optimization using advanced ML techniques. It was found that a flatter design with low center height, wide and shallow bowl, and the low lip was preferable in the low- and high-loads. At the low-load partially premixed compression ignition (PPCI) mode, spray targeting the upper lip and divided into the bowl and squish zones for enhanced mixing is preferred. At the high-load mixing-controlled diffusion combustion mode, where injection occurs near the top dead center (TDC) and diffusion burn is the dominant combustion mode, targeting the lower lip is favorable. At the mid-load with a high premixed ratio, the combustion was close to homogeneous charge compression ignition (HCCI), and the piston bowl design had limited effect. Regarding the optimum injector parameters, larger number nozzles with smaller diameters were favorable at low-load to control partially premixed charge. At high-load, larger and fewer number nozzles are recommended. Lastly, the optimum ML design provided similar performance at mid-load but a 3.8–4.5 % reduction in fuel consumption compared to the baseline cases at low- and high-load conditions.

Effect of air and exhaust gas dilutions on ultra-fine particulate emissions in different combustion modes

The effect of air and exhaust dilution ratios on the characteristics of ultra-fine particulate matter (PM) and combustion process in different combustion modes at low loads were investigated based on experimental and numerical simulation in this study. The combustion modes included partially premixed compression ignition (PPCI), diffusion combustion (DC), and modulated kinetics combustion (MKC). The air and exhaust gas dilution ratio varied from 0 % to 40 % and 0 % to 55 % respectively. Additionally, the effect of dilutions and combustion modes on PM characteristics is precisely analyzed by computational fluid dynamics. The NOx and Soot emission are reduced in PPCI and DC mode of combustion with the increase of air and exhaust gas dilution rate. However, NOx emission is decreases while Soot is increases in MKC mode. The concentration of particulate number increases in case of both PPCI and DC mode and achieved highest value as 3.3 × 107 and 1.4 × 107 N/cc respectively. Although, concentration of particulate number (PN) in MKC mode first starts climbing and after attaining the highest level of 1.3 × 107 N/cc it falls down. PN concentration in PPCI, MKC, and DC modes decreases as the exhaust gas dilution rate increases. The study of nuclear and accumulate mode of PM was performed separately where the diameter of particles is <1000 nm. Under variable air and exhaust gas dilution ratios, the value of PN is always lies between 40 and 90 % in the nuclear mode PM domain for PPCI and DC modes while for MKC mode, the proportion of PN fall under 13–57 % at exhaust gas ratio above 30 %. The proportion of nuclear mode particulate mass in PPCI mode is >10 % while its values is <1 % for DC and MKC modes.

Study on control-oriented emission predictions of PPCI diesel engine with two-stage fuel injection

Model predictive control (MPC) is an effective technology used to improve the control accuracy of low temperature combustion engines. The main purpose of this study is to establish a control-oriented emission prediction model based on fuel injection parameters, which provides the basis for the development of MPC controller for PPCI engines. In this study, the combustion model and backpropagation (BP) neural network emission prediction model of a partially premixed compression ignition (PPCI) diesel engine were established, through which the real-time combustion and emissions can be realized by inputting two-stage fuel injection parameters. The Wiebe function linearization method was proposed for the combustion model to achieve high-precision reconstruction of combustion at different fuel injection parameters. The nitrogen oxide (NOx) and particulate matter (PM) emissions were predicted using the combustion characteristics calculated by the combustion model. The results show that the established combustion model can accurately reconstruct the heat release rate and cylinder pressure of PPCI combustion, and the maximum error between the calculated and test values of the combustion parameters is less than 9%. The three-layer BP neural network can achieve high-precision predictions of NOx and PM emissions, with a prediction accuracy greater than 95%. Under the 300th continuous transient seconds, the cumulative prediction error of NOx was 2%, and the cumulative prediction error of PM was 4.9%, indicating high accuracy. This provides the basis for developing MPC controllers for PPCI engines.

Multi-objective optimization of dual-fuel engine performance in PPCI mode based on preference decision

In order to obtain the best emission performance and fuel economy of the dual-fuel engine in PPCI (partial premixed compression ignition) mode while responding to different decision requirements. LSTM (long short-term memory) neural network is used to build a response model that can accurately reflect the relationship between input parameters and output performance of the dual-fuel engine. A clever combination of LSTM and NSGA-II is used to construct a multi-objective performance optimization framework for the dual-fuel engine. The control parameters including MIT (main injection timing), PIT (pre-injection timing), PIQ (pre-injection quantity), EAC (excess air coefficient), SRL (substitution rate limit), and IP (injection pressure) are integrated and optimized under each load condition according to the propulsion conditions. The introduction of decision-maker (DM) preference information is creative, which allows the optimization process to include the intention of the DM, which can guide the evolutionary direction of the population. An organized combination of reference point coordinates R, weight vector w of weighted Euclidean distance, and preference strength δ is the critical one. The interactive optimization process effectively reduces the DM’s reliance on empirical knowledge in preference parameter setting. The experimental results show that all solutions at preference low emission meet the IMO Tier-III limit for NOx, and the average NOx emission at full operating conditions is 1.22 g/(kW·h), which is 78.9% lower than the original engine, along with a 4.76% reduction in BSFC compared to the original engine. When low BSFC is preferred, the average BSFC is 8.44% lower than the original engine and 3.68% lower than when low emissions are preferred. It can be found that the lower BSFC is obtained at the expense of the environment, and the pursuit of higher fuel economy worsens emissions dramatically.

Effects of split injection strategy on the combustion, emission and economic performance of the low-octane gasoline fueled MPCI engines

Application of diesel fueled partially premixed compression ignition (PPCI) engines are limited due to its excessive pressure rise rate at high load. Multiple premixed compression ignition (MPCI) engines fueled with low octane number gasoline (LON) has the potential to operate at both low and high load, while effects of split injection strategy on performance of such engines have not been discovered yet. In the present study, comparisons of LON and commercial diesel are first conducted in a heavy-duty single cylinder engine. Then, pilot injection timing and main injection timing are varied individually to reveal their effects on the combustion, emission, and economic performance of the MPCI engine. Comparing with diesel, MPCI engines with LON merits a low pollutants emission due to the long ignition delay and good volatility of LON. Split injection strategy can reduce the NOx and PM emissions simultaneously for MPCI engine with LON, and also reduce fuel consumption.

An Improved Prediction of Pre-Combustion Processes, Using the Discrete Multicomponent Model

An improved heating and evaporation model of fuel droplets is implemented into the commercial Computational Fluid Dynamics (CFD) software CONVERGE for the simulation of sprays. The analytical solutions to the heat conduction and species diffusion equations in the liquid phase for each time step are coded via user-defined functions (UDF) into the software. The customized version of CONVERGE is validated against measurements for a single droplet of n-heptane and n-decane mixture. It is shown that the new heating and evaporation model better agrees with the experimental data than those predicted by the built-in heating and evaporation model, which does not consider the effects of temperature gradient and assumes infinitely fast species diffusion inside droplets. The simulation of a hollow-cone spray of primary reference fuel (PRF65) is performed and validated against experimental data taken from the literature. Finally, the newly implemented model is tested by running full-cycle engine simulations, representing partially premixed compression ignition (PPCI) using PRF65 as the fuel. These simulations are successfully performed for two start of injection timings, 20 and 25 crank angle (CA) before top-dead-centre (BTDC). The results show good agreement with experimental data where the effect of heating and evaporation of droplets on combustion phasing is investigated. The results highlight the importance of the accurate modelling of physical processes during droplet heating and evaporation for the prediction of the PPCI engine performance.

Effect of gasoline additive on combustion and emission characteristics of an n-butanol Partially Premixed Compression Ignition engine under different parameters

The experiments were conducted on a modified two-cylinder diesel engine to investigate the effects of excess-air coefficient (λ) and intake temperature (Tin) of different blending ratios (volume ratio of gasoline in the blends) on the combustion and emission characteristics of a Partially Premixed Compression Ignition (PPCI) engine. The results show that with the increase of gasoline blending ratio, the peak in-cylinder pressure (Pmax), the peak in-cylinder temperature (Tmax) and the peak heat release rate (HRRmax) of four test fuels all increase first and then decrease. When gasoline volume fraction is 10%, HC and CO emissions are the lowest. In addition, intake temperature (Tin) has a significant effect on the n-butanol/gasoline PPCI engine. With the increase of Tin, the in-cylinder Pmax and HRRmax of four test fuels gradually increase, the combustion phase advances and HC and CO emissions decrease, while NOx emissions increase slightly. Furthermore, as λ increases, the Pmax, Tmax and HRRmax of the four test fuels show monotonously reducing trend. At the same time, mixture concentration has basically no effect on start of combustion (CA10), the combustion duration (CD) gradually extends, and HC and CO emissions increase.

Impact analysis of partially premixed combustion strategy on the emissions of a compression ignition engine fueled with higher octane number fuels: A review

This paper reviews the effect of a partially premixed combustion strategy on the exhaust emission of a diesel engine fuelled with high research octane number fuels. For the past few years, researchers have been aiming to decrease the soot and NOx emission simultaneously and maintain high engine efficiency in diesel engines by improving the combustion which is achieved by optimizing the fuel injection tactics. Single fuel and split fuel (pilot/main) injection tactics are used in the partially premixed combustion (PPC) strategy of the diesel engine to achieve stable combustion and less soot and NOx emission. The PPC strategy in diesel engine can be easily achieved with high octane number fuel resulting in prolonged ignition delay. The air/fuel mixing would be improved by virtue of longer ignition delay (ID) and as a result, soot emission decreases in the partially premixed compression ignition (PPCI) engine. Additionally, higher octane number fuels (i.e., Ethanol, Methanol and Butanol) improve the charge cooling effect, resulting in reduced NOX emissions. Thus, the combined use of PPC mode diesel engine along with high octane number fuels in modern CI engine could be a favourable upcoming solution to meet the strict exhaust emission legislation and reduce the dependence on fossil fuels.

Development and optimization of a novel solid oxide fuel cell-engine powering system for cleaner locomotives

Due to the advantages of high efficiency and fuel flexibility, solid oxide fuel cells have become a prominent option to power the future of heavy-duty transportation. Providing and selecting several solid-oxide fuel cell-based powering systems as options for the transportation industry have become an important task. For this reason, the purpose of this paper is to propose a novel powering integrated system for cleaner rail transportation. A partially-premixed compression ignition engine is integrated for the first time with a solid oxide fuel cell instead of a simple compression ignition engine to evaluate the integrated system performance for locomotives. A detailed thermodynamic model based on energy and exergy analyses is developed and used to evaluate the new powering system. The power split between the fuel cell and the engine effects on the overall exergy efficiency and total space requirements are explored for the first time in an integrated solid oxide fuel cell-based system through a newly developed optimization procedure using a sequence of multi-objective optimization methods. At the reference case, the overall energy and exergy efficiencies are 80.1% and 77.6%, respectively, which are around 15% more efficient than a simple solid-oxide fuel cell-gas turbine powering system, because of the new fuel cell-engine integration. The overall exergy efficiency is 78.98% and the total space requirement is 30.73 m3 at the optimum operating point. Moreover, the further originality of this paper is to develop and optimize the performance and sizing of the present integrated system for a limited-space application, namely locomotives.

Optimization of the direct injection natural gas engine under different combustion modes

The objective of this study is to optimize the performance of a direct injection natural gas engine by using the newly developed many-objectives optimization genetic algorithm NSGA-III. A total number of 11 parameters are optimized simultaneously with respected to four optimization objectives, including the ISFC, NO, CH4 and Soot emissions. The main findings are as follows: More than half of the calculated cases in the Pareto front can meet the EURO V emission regulation. In addition, a larger EGR rate is needed in order to achieve a lower NO emission. The optimized natural gas spray angles are around 80° and 70°; the diesel spray angles are around 60°, 70° and 75°; the EGR rates are around 28% and 14%; the swirl ratios are around 1.6 and 0.5; the compression ratios are around 16.5 and 18.5; the azimuth angles are around −3° and 5°. The combustion modes of the optimized cases can be divided into three categories based on the difference between the natural gas and diesel injection timings. The high-temperature combustion regions in the NG PPCI (Partial Premixed Compression Ignition) combustion mode with open combustion chamber are widely distributed, which result in an obviously higher NO emission than the other combustion modes. However, its Soot emission is the lowest. For diesel PPCI and HPDI (High Pressure Direct Injection) combustion mode, the natural gas is mainly consumed through diffusion combustion. Therefore, the Soot emissions are relatively higher. However, as the ignition timing is postponed, their NO emissions are relatively lower.

Effect of Injection Strategies in Diesel/NG Direct-Injection Engines on the Combustion Process and Emissions under Low-Load Operating Conditions

The direct injection of natural gas (NG), which is an important research direction in the development of NG engines, has the potential to improve thermal efficiency and emissions. When NG engines operate in low-load conditions, combustion efficiency decreases and hydrocarbon (HC) emissions increase due to lean fuel mixtures and slow flame propagation speeds. The effect of two combustion modes (partially premixed compression ignition (PPCI) and high pressure direct injection (HPDI)) on combustion processes was investigated by CFD (Computational Fluid Dynamics), with a focus on different injection strategies. In the PPCI combustion mode, NG was injected early in the compression stroke and premixed with air, and then the pilot diesel was injected to cause ignition near the top dead center. This combustion mode produced a faster heat release rate, but the HC emissions were higher, and the combustion efficiency was lower. In the HPDI combustion mode, the diesel was injected first and ignited, and then the NG was injected into the flame. This combustion mode resulted in higher emissions of NOx and soot, with a diffusion combustion in the cylinder. HC emissions significantly decreased. Compared with PPCI combustion, HPDI had a higher thermal efficiency.

Identifying Unregulated Emissions from Conventional Diesel Self-Ignition and PPCI Marine Engines at Full Load Conditions

A study on unregulated emissions of a conventional diesel self-ignition and partial premixed compression ignition (PPCI) marine engine at full load condition was performed, respectively. In this work, PPCI was realized in a marine engine by blending 15% diesel with 85% light hydrocarbons (LHC). Gas chromatography-mass spectrometry (GC-MS) was used to detect and identify unregulated emissions, and the chemical formula and peak area of representative species were obtained. Furthermore, the unregulated emissions were classified and semi-quantitatively analyzed. The results show that the maximum in-cylinder pressure of PPCI is almost 11 bar lower than that of conventional diesel combustion, and the crank angle at that moment is also delayed by 2 °CA. Compared to conventional diesel combustion, the maximum pressure rise rate of PPCI is reduced by 3.5%, while the maximum heat release rate of PPCI increases by 23.5%. Further, PPCI produces fewer species in unregulated emissions, and their chemical formula are less complex than that of conventional diesel combustion. Compared to conventional diesel combustion, the relative concentration of alkane and organic components in PPCI decreases significantly, while ketone and ester increase.

Mechanism of Combustion Noise Influenced by Pilot Injection in PPCI Diesel Engines

Pilot injection combined with exhaust gas recirculation (EGR) is usually utilized to realize the partially premixed compression ignition (PPCI) mode in diesel engines, which enables the simultaneous decrease of nitrogen oxide and soot emissions to satisfy emission regulations. Moreover, the ignition delay of main injection combustion can also be shortened by pilot injection, and then combustion noise is reduced. Nevertheless, the mechanisms of pilot injection impacts on combustion noise are not completely understood. As such, it is hard to optimize pilot injection parameters to minimize combustion noise. Therefore, experiments were conducted on a four-stroke single-cylinder diesel engine with different pilot injection strategies and 20% EGR as part of an investigation into this relationship. Firstly, the combustion noise was analyzed by cylinder pressure levels (CPLs). Then, the stationary wavelet transforms (SWTs) and stationary wavelet packet transform (SWPT) were employed to decompose in-cylinder pressures at different scales, and thus the combustion noise generated by pilot and main combustion was investigated in both the time and frequency domain. The results show that pilot injection is dominant in the high frequency segment of combustion noise, and main injection has a major impact on combustion noise in the low and mid frequency segment. Finally, the effects of various pilot injection parameters on suppressing combustion noise were analyzed in detail.

Spray characteristics of a gasoline-diesel blend (ULG75) using high-speed imaging techniques

Partially-premixed compression-ignition (PPCI) is an advanced combustion mode that simultaneously reduces particulate and NOx emissions. Fuels with properties intermediate to that of gasoline and diesel are preferred for PPCI. This paper is a follow-on to a recent work published in Fuel from the authors’ research group on the topic of PPCI combustion with ULG75 (75 vol% gasoline/25 vol% diesel). That paper reported the opportunity of using ULG75 and hot external exhaust-gas-recirculation (EGR) for solving the low-load combustion-stability issue caused by the low reactivity of ULG75. That paper also concluded that ‘It was not possible to obtain stable ULG75 PPCI combustion without using low fuel injection pressure’. Additionally, from the point of ULG75’s volatility and viscosity, it does not require as high injection pressure as diesel. This paper assesses the spray characteristics (spray morphology, penetration length, cone angle and droplet size) of ULG75 at low injection pressures (Pinj = 35/75 MPa). In this work, high-speed macroscopic imaging (0.25 million fps) was carried out at backpressure (BP) of 0.1/4.0 MPa, and ambient temperature (Ta) of 25/125 °C. In addition, ultra-high speed microscopic imaging (1 million fps) was carried out at BP = 4 MPa and Ta = 25 °C. The same tests were conducted for diesel, and the results were used for comparisons. Under preheated ambient condition (Ta = 125 °C), ULG75 showed a noticeable improvement in the spray quality as compared to the non-evaporative condition (Ta = 25 °C), indicated by a shorter spray penetration length, larger spray cone angle and spray area. However, no such changes were observed for diesel spray upon increasing Ta from 25 °C to 125 °C. Both Pinj and BP strongly affected the spray penetration length evolution. However, they showed limited impacts on spray cone angle and spray area. In addition to the experimental study, empirical estimations were conducted to predict the spray penetration length, air entrainment and spray droplet size. The calculations predicted smaller SMD for ULG75 than diesel at all conditions and also indicated ambient temperature as a key factor affecting the spray quality, especially at low injection pressures.

Simultaneous 36 kHz PLIF/chemiluminescence imaging of fuel, CH2O and combustion in a PPC engine

The requirements on high efficiency and low emissions of internal combustion engines (ICEs) raise the research focus on advanced combustion concepts, e.g., premixed-charge compression ignition (PCCI), partially premixed compression ignition (PPCI), reactivity controlled compression ignition (RCCI), partially premixed combustion (PPC), gasoline compression ignition (GCI) etc. In the present study, an optically accessible engine is operated in PPC mode, featuring compression ignition of a diluted, stratified charge of gasoline-like fuel injected directly into the cylinder. A high-speed, high-power burst-mode laser system in combination with a high-speed CMOS camera is employed for diagnostics of the autoignition process which is critical for the combustion phasing and efficiency of the engine. To the authors’ best knowledge, this work demonstrates for the first time the application of the burst-system for simultaneous fuel tracer planar laser induced fluorescence (PLIF) and chemiluminescence imaging in an optical engine, at 36 kHz repetition rate. In addition, high-speed formaldehyde PLIF and chemiluminescence imaging are employed for investigation of autoignition events with a high temporal resolution (5 frames/CAD). The development of autoignition together with fuel or CH2O distribution are simultaneously visualized using a large number of consecutive images. Prior to the onset of combustion the majority of both fuel and CH2O are located in the recirculation zone, where the first autoignition also occurs. The ability to record, in excess of 100 PLIF images, in a single cycle brings unique possibilities to follow the in-cylinder processes without the averaging effects caused by cycle-to-cycle variations.

Effect of Fuel Reactivity on Ignitability and Combustion Phasing in a Heavy-Duty Engine Simulation for Mixing-Controlled and Partially Premixed Combustion

Light-end fuels have recently garnered interest as potential fuel for advanced compression ignition (CI) engines. This next generation of engines, which aim to combine the high efficiency of diesel engines with the relative simplicity of gasoline engines, may allow engine manufacturers to continue improving efficiency and reducing emissions without a large increase in engine and aftertreatment system complexity. In this work, a 1D heavy-duty engine model was validated with measured data and then used to generate boundary conditions for the detailed chemical kinetic simulation corresponding to various combustion modes and operating points. Using these boundary conditions, homogeneous simulations were conducted for 242 fuels with research octane number (RON) from 40 to 100 and sensitivity (S) from 0 to 12. Combustion phasing (CA50) was most dependent on RON and less dependent on S under all conditions. Both RON and S had a greater effect on combustion phasing under partially premixed compression ignition (PPCI) conditions (19.3 deg) than under mixing-controlled combustion (MCC) conditions (5.8 deg). The effect of RON and S were also greatest for the lowest reactivity (RON &gt; 90) fuels and under low-load conditions. The results for CA50 reflect the relative ignition delay for the various fuels at the start-of-injection (SOI) temperature. At higher SOI temperatures (&gt;950K), CA50 was found to be less dependent on fuel sensitivity due to the convergence of ignition delay behavior of different fuels in the high-temperature region. Combustion of light-end fuels in CI engines can be an important opportunity for regulators, consumers, and engine-makers alike. However, selection of the right fuel specifications will be critical in development of the combustion strategy. This work, therefore, provides a first look at quantifying the effect of light-end fuel chemistry on advanced CI engine combustion across the entire light-end fuel reactivity space and provides a comparison of the trends for different combustion modes.

On the effects of fuel properties and injection timing in partially premixed compression ignition of low octane fuels

A better understanding on the effects of fuel properties and injection timing is required to improve the performance of advanced engines based on low temperature combustion concepts. In this work, an experimental and computational study was conducted to investigate the effects of physical and chemical kinetic properties of low octane fuels and their surrogates in partially premixed compression ignition (PPCI) engines. The main objective was to identify the relative importance of physical versus chemical kinetic properties in predicting practical fuel combustion behavior across a range of injection timings. Two fuel/surrogate pairs were chosen for comparison: light naphtha (LN) versus the primary reference fuel (PRF) with research octane number of 65 (PRF 65), and FACE (fuels for advanced combustion engines) I gasoline versus PRF 70. Two sets of parametric studies were conducted: the first varied the amount of injected fuel mass at different injection timings to match a fixed combustion phasing, and the second maintained the same injected fuel mass at each injection timing to assess resulting combustion phasing changes. Full-cycle computational fluid dynamic engine simulations were conducted by accounting for differences in the physical properties of the original and surrogate fuels, while employing identical chemical kinetics. The simulations were found to capture trends observed in the experiments, while providing details on spatial mixing and chemical reactivity for different fuels and injection timings. It was found that differences in physical properties become increasingly important as injection timing was progressively delayed from premixed conditions, and this was rationalized by analysis of mixture stratification patterns resulting from injection of fuels with different physical properties. The results suggest that accurate descriptions of both physical and chemical behavior of fuels are critical in predictive simulations of PPCI engines for a wide range of injection timings.

An experimental investigation on thermal efficiency of a compression ignition engine fueled with five gasoline-like fuels

Partially Premixed Compression Ignition (PPCI) and Multiple Premixed Compression Ignition (MPCI) are two high efficiency and clean combustion strategies. This paper presents an experimental investigation on the efficiency of PPCI and MPCI combustion using five gasoline-like fuels: gasoline, gasoline/ethylhexyl-nitrate (EHN) blends, gasoline/diesel blends, gasoline/polyoxymethylene dimethyl ethers (PODEn) blends, gasoline/diesel/PODEn blends, denoted as G, GE, GD, GP, GDP, respectively. The study was conducted using a single cylinder engine that was retrofitted from a production Euro IV diesel engine. The results showed that in PPCI mode, the order of thermal efficiency from the highest to the lowest was GP, G, GDP, GE, and GD, while the order in MPCI mode was GP, GDP, GE, G, and GD. The thermal efficiency of MPCI mode was higher than that of PPCI mode, except for GP, whose thermal efficiency was the highest of about 48.8% for both modes due to the early CA50 and short combustion duration. In PPCI mode, NOx and soot emissions were below 0.4g/kWh and 0.01g/kWh, respectively, satisfying the Euro VI emission standard. NOx emission in MPCI mode can meet the requirement of Euro VI. However, the soot emission exceeded the limit because the second stage combustion was partially diffusion combustion, implying an extra Gasoline Particle Filter (GPF) was needed to meet the Euro VI standard. This study indicates that PPCI injection strategy alone and MPCI injection strategy with GPF are two potential ways of retrofitting a Euro IV engine to meet the Euro VI emission standard. GPF increases manufacturing cost, but higher thermal efficiency in MPCI mode could reduce the total cost of ownership due to the lower fuel consumption.