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Many recent studies have shown that the Reactivity Controlled Compression Ignition (RCCI) combustion strategy can achieve high efficiency with low emissions. However, it has also been revealed that RCCI combustion is difficult at high loads due to its premixed nature. To operate at moderate to high loads with gasoline/diesel dual fuel, high amounts of EGR or an ultra low compression ratio have shown to be required. Considering that both of these approaches inherently lower thermodynamic efficiency, in this study natural gas was utilized as a replacement for gasoline as the low-reactivity fuel. Due to the lower reactivity (i.e., higher octane number) of natural gas compared to gasoline, it was hypothesized to be a better fuel for RCCI combustion, in which a large reactivity gradient between the two fuels is beneficial in controlling the maximum pressure rise rate. The multi-dimensional CFD code, KIVA3V, was used in conjunction with the CHEMKIN chemistry tool and a Nondominated Sorting Genetic Algorithm (NSGA-II) to perform optimization for a wide range of engine operating conditions. Engine design parameters that were controlled by the genetic algorithm include the fraction of total fuel that is premixed (methane), the timing of the two diesel injections, the amount of diesel in each injection, the diesel fuel injection pressure, and the EGR percentage. The objective of the optimization was to simultaneously minimize soot, NOx, CO, and UHC emissions, as well as ISFC and ringing intensity. A broad load/speed range was investigated; six operating points from 4 to 23 bar IMEP and 800 to 1800 rev/min were optimized. These load/speed combinations represent typical heavy-duty engine conditions. Using the stock compression ratio of 16.1, it was determined that operation up to 13.5 bar IMEP could be achieved with no EGR, while still maintaining high efficiency and low emissions. The study also examined the sensitivity of RCCI combustion at high load to injection system parameters. The results emphasize that precise injection control is needed for combustion control.

The focus of the present study was to characterize the fuel reactivity of high octane number fuels (i.e., low fuel reactivity), namely gasoline, ethanol, and methanol when mixed with cetane improvers under lean, premixed combustion conditions. Two commercially available cetane improvers, 2-ethylhexyl nitrate and di-tert-butyl peroxide, were used in the study. First, blends of the primary reference fuels iso-octane and n-heptane were port injected under fixed operating conditions. The resulting combustion phasings were used to generate effective PRF number maps. Then, blends of the aforementioned base fuels and cetane improvers were tested under the same lean premixed conditions as the PRF blends. Based on the combustion phasing results of the base fuel and cetane improver mixture, the effective PRF number, or octane number, could be determined. In all three base fuels it was found that 2-ethylhexyl nitrate is more effective at increasing fuel reactivity compared to di-tert-butyl peroxide. However, 2-ethylhexyl nitrate has a potential disadvantage due its nitrate group, which can manifest itself as NOx emissions. The relationship between the fuel-bound nitrate group and the engine-out NOx emissions was extensively characterized in the present study. It was also observed that methanol’s response to cetane improvers was better than that of ethanol, in spite of the fact that they have similar octane numbers in their neat form. Once the reactivity of the base fuels was characterized, two mixtures of methanol and cetane improvers were selected and compared to diesel fuel as the high reactivity fuel (i.e., direct injected) for RCCI combustion.

The focus of the present study was to characterize Reactivity Controlled Compression Ignition (RCCI) using a single-fuel approach of gasoline and gasoline mixed with a commercially available cetane improver on a multi-cylinder engine. RCCI was achieved by port-injecting a certification grade 96 research octane gasoline and direct-injecting the same gasoline mixed with various levels of a cetane improver, 2-ethylhexyl nitrate (EHN). The EHN volume percentages investigated in the direct-injected fuel were 10, 5, and 2.5%. The combustion phasing controllability and emissions of the different fueling combinations were characterized at 2300 rpm and 4.2 bar brake mean effective pressure over a variety of parametric investigations including direct injection timing, premixed gasoline percentage, and intake temperature. Comparisons were made to gasoline/diesel RCCI operation on the same engine platform at nominally the same operating condition. The experiments were conducted on a modern four cylinder light-duty diesel engine that was modified with a port-fuel injection system while maintaining the stock direct injection fuel system. The pistons were modified for highly premixed operation and feature an open shallow bowl design. The results indicate that the authority to control the combustion phasing through the fuel delivery strategy (e.g., direct injection timing or premixed gasoline percentage) is not a strong function of the EHN concentration in the direct-injected fuel. It was also observed that NOx emissions are a strong function of the global EHN concentration in-cylinder and the combustion phasing. In general, NOx emissions are significantly elevated for gasoline/gasoline+EHN operation compared with gasoline/diesel RCCI operation at a given operating condition.

A single-cylinder light-duty diesel engine was used to investigate dual fuel reactivity controlled compression ignition (RCCI) operated with two different fuel combinations: gasoline/diesel fuel and methanol/diesel fuel. The engine was operated over a range of conditions, from 1500 to 2300 rpm and 3.5 to 17 bar gross IMEP. Using the stock re-entrant piston bowl geometry, both fuel combinations were able to achieve low NOx and PM emissions with a peak gross indicated efficiency of 48%. However, at light load conditions both gasoline and methanol yielded poorer combustion efficiencies. Previous studies have shown that the high-levels of piston induced mixing that are created by the stock piston are not required, and in fact are detrimental due to increased heat transfer losses, for premixed combustion. Thus a modified piston featuring a shallow, flat piston bowl with nearly no squish land was also investigated. Using the modified piston, the gross indicated efficiency of RCCI combustion was significantly improved at light loads due to increases in combustion efficiency and decreases in heat transfer losses. At higher loads the modified piston also performed better than the stock piston, but the improvements were not as significant. Over the entire load and speed range, the modified piston yielded low NOx and PM emissions with a peak gross indicated efficiency of nearly 51%.

A novel exhaust-runner soot diagnostics has been developed and tested in a skip-fired diesel optical engine. Crank-angle-resolved soot emissions are measured during the cylinder blowdown and exhaust processes by multi-pass extinction of coherent, visible (635 nm) light as it passes through an optical exhaust runner. To evaluate diagnostic accuracy, comparisons are made of soot volume fraction measured by extinction in the optical exhaust runner and by a conventional smoke meter. The diagnostic exhibits excellent sensitivity with soot volume fraction detection limits of better than 0.2 parts per billion (ppb). The experiments also employed an engine exhaust particle sizer (EEPS) to characterize particle size distributions from the skip-fired optical engine. The diagnostic has been employed in this work to assess skip-fired cycle variations in soot emissions for two- and six-hole production diesel fuel injectors as oxygen concentration (i.e., dilution) is varied. As oxygen concentration is reduced for the two-hole tip, both mean soot volume fraction and cycle variations increase, with the coefficient of variation (COV) of extinction as high as 46%. At low intake oxygen (O₂) concentrations, soot apparently leaves the cylinder relatively early in the exhaust process. As O₂ concentration increases and soot volume fraction declines, smaller soot particles (as measured by the EEPS) leave the cylinder later in the exhaust stroke. Trends for the six-hole injector are similar, but cycle variations are significantly lower. The reduction in the cyclic variation of soot volume fraction as the number of orifices increases from two to six provides evidence that soot formed in a burning jet is a relatively stochastic event for which the COV is inversely proportional to the square of the number of distinct combustion plumes. The persistence of 27% exhaust soot COV for the six-hole tip at 14% intake O2 suggests that improved control and optimization of the spray formation and combustion event could significantly reduce the average engine-out PM emissions.

The potential of low temperature combustion to yield low NOx and soot while maintaining diesel-like thermal efficiencies has been demonstrated through countless studies. Methods of achieving low temperature combustion are just as numerous and they range from using high cetane number fuels, like diesel, with large amounts of exhaust gas recirculation, to completely premixing a high octane number fuel, like gasoline, and approaching an HCCI-like condition. The potential of operating a heavy-duty compression ignition engine fueled with conventional gasoline in a partially premixed combustion mode to have high thermal efficiency and low emissions has been demonstrated in this study. The objective of this work was to optimize the engine using computational tools. The KIVA3V-CHEMKIN code, a multi-dimensional engine CFD model was coupled to a Nondominated Sorting Genetic Algorithm (NSGA II), which is a multi-objective genetic algorithm. Two engine operating conditions were investigated in this study, a mid-load and a high-load point, 11 bar and 21 bar IMEP, respectively. The goal of the optimization study was to simultaneously reduce six objectives, which are soot, NOx, unburned hydrocarbons, carbon monoxide, indicated specific fuel consumption, and ringing intensity, which is related to maximum pressure rise rate. The genetic algorithm was allowed to vary eight engine design parameters that included pilot injection parameters, main injection parameters, injector included angle, number of injector nozzle holes, and swirl ratio. A non-parametric regression analysis tool was used to post-process the optimization results in order to illustrate the effects of the design parameters on the objectives. The results show that gross indicated thermal efficiencies of 50% with low emissions and low ringing intensity are possible at the mid-load condition. The high-load condition yields low NOx emissions, and an efficiency of 50% as well, but indicates that meeting soot emissions and ringing intensity constraints will be a challenge.

Many studies have demonstrated ability of low temperature combustion to yield low NOx and soot while maintaining diesel-like thermal efficiencies. Methods of achieving low temperature combustion are numerous and range from using high cetane number fuels, like diesel, with large amounts of exhaust gas recirculation, to completely premixing a high octane number fuel, like gasoline, and approaching an HCCI-like condition. Both of the aforementioned techniques have relatively short combustion duration that results in very a rapid rate of heat release, and hence very rapid rates of pressure rise. This has been one of the major challenges for premixed, low temperature combustion at mid and high load. Reactivity Controlled Compression Ignition (RCCI) has been introduced recently, which is a dual fuel partially premixed combustion concept. In this strategy in-cylinder fuel blending is used to develop fuel reactivity gradients in the combustion chamber that result in a broad combustion event and reduced pressure rise rates. RCCI has been demonstrated to yield low NOx and soot with high thermal efficiency in a heavy-duty engine using a compression ratio of 16.1 at loads up to 15 bar gross IMEP. However, extension to full-load operation has proven to be difficult with a high compression ratio. The objective of this work was to optimize the engine with a low compression ratio of 11.7 using computational tools. The KIVA3V-CHEMKIN code, a multi-dimensional engine CFD model was coupled to a Nondominated Sorting Genetic Algorithm (NSGA II), which is a multi-objective genetic algorithm. Three engine operating conditions were investigated in this study, a low-load, mid-load, and a high-load point, 4, 9, and 23 bar gross IMEP, respectively. The goal of the optimization study was to simultaneously reduce six objectives, which are soot, NOx, unburned hydrocarbons, carbon monoxide, indicated specific fuel consumption, and the maximum pressure rise rate. The genetic algorithm was allowed to vary six engine design parameters, namely percent premixed gasoline, EGR fraction, and diesel direct injection parameters.

This study focuses on the development of an autoignition model for diesel sprays that is applicable to phenomenological multi-zone combustion models. These models typically use a single-step Arrhenius expression to represent the low-temperature chemistry leading up to autoignition. There has been a substantial amount of work done in the area of n-heptane autoignition in homogeneous mixtures. Reduced kinetic mechanisms with ten reactions or less have been proposed in the literature to represent the complex low-temperature oxidation of n-heptane. These kinetic models are attractive for multi-zone simulations because of the low number of reactions involved. However, these kinetic mechanisms and the multi-zone treatment of the fuel spray do not account for the effect of turbulence/chemistry interactions on the chemical reaction rate. In this work a correlation has been developed for the total ignition delay time that is a combination of the homogenous ignition delay and dissipation effects. The homogeneous ignition delay is predicted from a chemical reaction mechanism for n-heptane, and the dissipation effects are captured through a phenomenological expression for a characteristic scalar dissipation rate. The characteristic scalar dissipation rate includes effects of injection pressure, ambient density, and injector hole size. The characteristic scalar dissipation rate is compared to a critical scalar dissipation rate to assess the additional delay due to turbulence/chemistry interactions. The autoignition model was implemented into a multi-zone spray model and validated against constant volume ignition delay measurements of diesel sprays.

Reactivity controlled compression ignition (RCCI) has been shown to be capable of providing improved engine efficiencies coupled with the benefit of low emissions via in-cylinder fuel blending. Much of the previous body of work has studied the benefits of RCCI operation using high injection pressures (e.g., 500 bar or greater) with common rail injection (CRI) hardware. However, low-pressure fueling technology is capable of providing significant cost savings. Due to the broad market adoption of gasoline direct injection (GDI) fueling systems, a market-type prototype GDI injector was selected for this study. Single-cylinder light-duty engine experiments were undertaken to examine the performance and emissions characteristics of the RCCI combustion strategy with low-pressure GDI technology and compared against high injection pressure RCCI operation. Gasoline and diesel were used as the low-reactivity and high-reactivity fuels, respectively. GDI injection pressures range from 150 to 200 bar, while the CRI pressures range from 250 to 500 bar. Start of injection (SOI) timings ranged from −35° aTDC and −115° aTDC. The experimental results show comparable engine performance and emissions output, but with slight reductions in overall combustion efficiency when using low-pressure fueling with the stock re-entrant piston. CFD simulations were also performed to aid in visualization of the in-cylinder fuel distributions, which are controlling factors for RCCI combustion. By utilizing an optimized RCCI piston geometry, equivalent RCCI combustion performance can be achieved under low-pressure fueling, at moderate and high loads. The optimized geometry also allows for significant increases in thermal efficiency, with peak efficiencies over 47% observed.

In a recent experimental study the in-cylinder spatial distribution of mixture equivalence ratio was quantified under non-combusting conditions by planar laser induced fluorescence (PLIF) of a fuel tracer (toluene). The measurements were made in a single cylinder, direct injection, light duty diesel engine at conditions matched to an early injection low temperature combustion mode. A fuel amount corresponding to a low load (3.0 bar indicated mean effective pressure) operating condition was introduced with a single injection at −23.6° ATDC. The data were acquired during the mixture preparation period from near the start of injection (−22.5° ATDC) until the crank angle where the start of high-temperature heat release normally occurs (−5° ATDC). In the present study the measured in-cylinder images are compared with a fully resolved three dimensional CFD model, namely KIVA3V-RANS simulations. The impacts of computational grid resolution and of the flow initialization method are discussed as they pertain to the mixture preparation process. The simulation results indicate that a fine grid resolution is required to capture the nominal spray penetration in the experiments, however coarse and fine grids are shown to yield similar overall fuel distributions at the start of combustion, which is ultimately important for predicting combustion characteristics and engine-out emissions. It was found that the simulations do an excellent job at reproducing the experimental observation that the majority of the incomplete combustion products (UHC and CO) stem from overly lean mixtures at the start of combustion, which reside in the upstream portions of the fuel spray, near the injector. Additionally, plume-to-plume variability seen in the experiments is explained by possible irregularities in the fuel injector resulting in slightly different fueling rates per injector hole.