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Control of the timing and magnitude of heat release is one of the biggest challenges for premixed compression ignition, especially when attempting to operate at high load. Single-fuel strategies such as partially premixed combustion (PPC) use direct injection of gasoline to stratify equivalence ratio and retard heat release, thereby reducing pressure rise rate and enabling high load operation. However, retarding the heat release also reduces the maximum work extraction, effectively creating a tradeoff between efficiency and noise. Dual-fuel strategies such as reactivity controlled compression ignition (RCCI) use premixed gasoline and direct injection of diesel to stratify both equivalence ratio and fuel reactivity, which allows for greater control over the timing and duration of heat release. This enables combustion phasing closer to top dead center (TDC), which is thermodynamically favorable. However, the main control mechanism in RCCI is the ratio of the two fuels, and the diesel fraction typically reaches zero before full load is achieved. We propose a new strategy that effectively combines the benefits of RCCI and PPC by injecting both gasoline and diesel directly, enabling control over the in-cylinder distribution of both fuels. We present a comparison of RCCI, PPC, and our new strategy, direct dual fuel stratification (DDFS) at a nominal gross mean effective pressure of 0.9 MPa. DDFS allowed for combustion phasing near TDC with reduced combustion noise. Cyclic combustion instability was reduced significantly with the new strategy and approached levels typical of conventional diesel combustion. Compared to RCCI, there was a reduction in noise and required exhaust gas recirculation (EGR) while maintaining similar efficiency. Compared to PPC, there was a reduction in noise and an increase in efficiency. The new strategy therefore combines the efficiency advantage of RCCI with the load advantage of PPC, while reducing EGR and combustion instability.

Premixed charge compression ignition (PCI) strategies offer the potential for simultaneously low NOx and soot emissions with diesel-like efficiency. However, these strategies are generally confined to low loads due to inadequate control of combustion phasing and heat-release rate. One PCI strategy, dual-fuel reactivity-controlled compression ignition (RCCI), has been developed to control combustion phasing and rate of heat release. The RCCI concept uses in-cylinder blending of two fuels with different auto-ignition characteristics to achieve controlled high-efficiency clean combustion. This study explores fuel reactivity stratification as a method to control the rate of heat release for PCI combustion. To introduce fuel reactivity stratification, the research engine is equipped with two fuel systems. A low-pressure (100 bar) gasoline direct injector (GDI) delivers iso-octane, and a higher-pressure (600 bar) common-rail diesel direct-injector delivers n-heptane. A sweep of the common-rail injection timing creates a range of fuel reactivity stratification. A high-speed digital camera provides images of ignition and combustion luminosity, composed primarily of chemiluminescence. A quantitative laser-induced fuel-tracer fluorescence diagnostic also provides two-dimensional measurements of the mixture distribution prior to ignition. The injection timing sweep showed that the peak heat-release rate is highest for either early or late common-rail injections of n-heptane, and displays a minimum at mid-range injection timings near 50° BTDC. At very early injection timings, the optical data show that the charge is well-mixed and overall fuel lean, so that it ignites volumetrically, resulting in rapid energy release. Conversely, when the injection timing is late in the cycle (near TDC), the mixing time is relatively short and much of the fuel-air mixture in the n-heptane jet is fuel-rich. Such mixtures that are near stoichiometric or richer have similar ignition delays, so that the charge ignites nearly instantaneously throughout the n-heptane jets. For the mid-range injection timings, at the minimum in the peak energy release rate, ignition occurs in the downstream portion of the n-heptane jet in localized auto-ignition pockets generated by the common-rail injection of n-heptane. The subsequent combustion process then progresses upstream toward the centrally mounted common-rail injector at a slower rate than either the early or late injection timings. In agreement with the observed combustion zone progression from the bowl-wall toward the injector, the fuel concentration measurements show that the fuel reactivity generally decreases from the bowl-wall toward the common-rail injector.

Reactivity Controlled Compression Ignition combustion (RCCI) has been demonstrated at mid to high loads [1, 2, 3, 4, 5, 6] as a method to operate an internal combustion engine that produces low NOx and low PM emissions with high thermal efficiency. The current study investigates RCCI engine operation at loads of 2 and 4.5 bar gross IMEP at engine speeds between 800 and 1700 rev/min. This load range was selected to cover the range from the previous work of 6 bar gIMEP down to an off-idle load at 2 bar. The fueling strategy for the low load investigation consisted of in-cylinder fuel blending using port-fuel-injection of gasoline and early cycle, direct-injection of either diesel fuel or gasoline doped with 3.5% by volume 2-EHN (2-ethylhexyl nitrate). At these loads, engine operating conditions such as inlet air temperature, port fuel percentage, and engine speed were varied to investigate their effect on combustion. Results show that at the 4.5 bar gIMEP operating condition it was possible to maintain 54% gross indicated thermal efficiency with NOx and PM emissions below US EPA 2010 limits. The results also show that it is possible to operate at a near idle load of 2 bar gross IMEP load with a gross indicated thermal efficiency of 49% at 1300 rev/min and 44% at 800 rev/min.

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.

Single cylinder engine experiments were used to investigate a fuel reactivity controlled compression ignition (RCCI) concept in both light- and heavy-duty engines and comparisons were made between the two engine classes. It was found that with only small changes in the injection parameters, the combustion characteristics of the heavy-duty engine could be adequately reproduced in the light-duty engine. Comparisons of the emissions and performance showed that both engines can simultaneously achieve NOx below 0.05 g/kW-hr, soot below 0.01 g/kW-hr, ringing intensity below 4 MW/m², and gross indicated efficiencies above 50 per cent. However, it was found that the peak gross indicated efficiency of the baseline light-duty engine was approximately 7 per cent lower than the heavy-duty engine. The energy balances of the two engines were compared and it was found that the largest factor contributing to the lower efficiency of the light-duty engine was increased heat transfer losses. Detailed CFD modeling was used to explore options to reduce the heat transfer losses of the light-duty engine. It was found that by reducing the swirl ratio from 2.2 to 0.7, increasing the engine speed from 1900 to 2239 rev/min, and improving the combustion chamber geometry, the heat transfer losses in the light-duty engine could be reduced by the equivalent of 2 per cent of the fuel energy. The modeling showed that light duty engine could achieve 53 per cent gross indicated efficiency, while maintaining near zero NOx and soot, and an acceptable ringing intensity.

This study investigates the potential of controlling premixed charge compression ignition (PCCI and HCCI) combustion strategies by varying fuel reactivity. In-cylinder fuel blending using port fuel injection of gasoline and early cycle direct injection of diesel fuel was used for combustion phasing control at both high and low engine loads and was also effective to control the rate of pressure rise. The first part of the study used the KIVA-CHEMKIN code and a reduced primary reference fuel (PRF) mechanism to suggest optimized fuel blends and EGR combinations for HCCI operation at two engine loads (6 and 11 bar net IMEP). It was found that the minimum fuel consumption could not be achieved using either neat diesel fuel or neat gasoline alone, and that the optimal fuel reactivity required decreased with increasing load. For example, at 11 bar net IMEP, the optimum fuel blend and EGR rate for HCCI operation was found to be PRF 80 and 50%, respectively. Engine experiments using a dual-fuel PCCI strategy with port fuel injection of gasoline and early cycle multiple injections of diesel fuel with a conventional diesel injector (i.e., wide angle and large nozzle hole) were performed. The experimental results confirmed that an extension of the PCCI operating regime is possible when optimized fuel blends are used. At the 11 bar operating point, NOx and soot were ∼0.01 g/kW-hr and ∼0.008 g/kW-hr, respectively. That is, US 2010 heavy duty emissions regulations are easily met without after-treatment while achieving 50% thermal efficiency.

Heavy-duty engine experiments were conducted to explore reactivity controlled compression ignition (RCCI) combustion through addition of the cetane improver di-tertbutyl peroxide (DTBP) to pump gasoline. Unlike previous diesel/gasoline dual-fuel operation of RCCI combustion, the present study investigates the feasibility of using a single fuel stock (gasoline) as the basis for both high reactivity and low reactivity fuels. The strategy consisted of port fuel injection of gasoline and direct injection of the same gasoline doped with a small volume percent addition of DTBP. With 1.75% DTBP by volume added to only the direct-injected fuel (which accounts for approximately 0.2% of the total fueling) it was found that the additized gasoline behaved similarly to diesel fuel, allowing for efficient RCCI combustion. The single fuel results with DTBP were compared to previous high-thermal efficiency, low-emissions results with port injection of gasoline and direct injections of diesel. The comparison between fueling strategies found that the higher volatility of gasoline enabled a reduction in the direct injection pressure from 800 (bar) with diesel to 400 (bar) with gasoline. At the tested conditions, the peak gross indicated based thermal efficiency was over 57%. The emissions trends and magnitudes of the single fuel strategy were also comparable to those of the diesel/gasoline dual-fuel strategy, and both engine-out NOxand PM met EPA HD 2010 emissions mandates without aftertreatment. Also, the decreased low temperature heat release with the single fuel strategy was found to lower compression work and increased thermal efficiency by approximately 1% over the diesel/gasoline case. The results demonstrate that a very small percentage of an appropriate additive can be used to establish a sufficiency large reactivity gradient to match the performance of a dual fuel strategy when operated in the RCCI combustion regime.

Due to the upcoming regulations for particulate matter (PM) emissions from GDI engines, a computational fluid dynamic (CFD) modeling study to predict soot emissions (both mass and solid particle number) from gasoline direct injection (GDI) engines was undertaken to provide insights on how and why soot emissions are formed from GDI engines. In this way, better methods may be developed to control or reduce PM emissions from GDI engines. In this paper, the influence of engine operating parameters was examined for a side-mounted fuel injector configuration in a direct-injection spark-ignition (DISI) engine. The present models are able to reasonably predict the influences of the variables of interest compared to available experimental data or literature. For a late injection strategy, effects of the fuel composition, and spray cone angle were investigated with a single-hole injector. For an early injection strategy, the effects of multi-component fuel surrogates for gasoline, SOI timings and wall temperatures were studied with a six-hole injector. The investigations confirmed the necessity to consider the multi-component fuel composition and also demonstrate how and why wall films significantly contribute to soot emissions from DISI engines.

Reactivity Controlled Compression Ignition (RCCI) is an engine combustion strategy that that produces low NOₓ and PM emissions with high thermal efficiency. Previous RCCI research has been investigated in single-cylinder heavy-duty engines [1,2,3,4,5,6]. The current study investigates RCCI operation in a light-duty multi-cylinder engine at 3 operating points. These operating points were chosen to cover a range of conditions seen in the US EPA light-duty FTP test. The operating points were chosen by the Ad Hoc working group to simulate operation in the FTP test [7-8]. The fueling strategy for the engine experiments consisted of in-cylinder fuel blending using port fuel-injection (PFI) of gasoline and early-cycle, direct-injection (DI) of diesel fuel. At these 3 points, the stock engine configuration is compared to operation with both the original equipment manufacturer (OEM) and custom machined pistons designed for RCCI operation. The pistons were designed with assistance from the KIVA 3V computational fluid dynamics (CFD) code. By using a genetic algorithm optimization, in conjunction with KIVA, the piston bowl profile was optimized for dedicated RCCI operation to reduce unburned fuel emissions and piston bowl surface area. By reducing these parameters, the thermal efficiency of the engine was improved while maintaining low NOx and PM emissions. Results show that with the new piston bowl profile and an optimized injection schedule, RCCI brake thermal efficiency was increased from 37%, with the stock EURO IV configuration, to 40% at the 2,600 rev/min, 6.9 bar BMEP condition, and NOx and PM emissions targets were met without the need for exhaust after-treatment.