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Methanol is one of the most promising fuels for the decarbonization of the off-road and transportation sectors. Although methanol is typically seen as an alternative fuel for spark ignition engines, mixing-controlled compression ignition (MCCI) combustion is typically preferred in most off-road and medium-and heavy-duty applications due to its high reliability, durability and high-efficiency. In this paper, the potential of using ignition enhancers to enable methanol MCCI combustion was investigated. Methanol was blended with 2-ethylhexyl nitrate (EHN) and experiments were performed in a single-cylinder production-like diesel research engine, which has a displacement volume of 0.83 L and compression ratio of 16:1. The effect of EHN has been evaluated with three different levels (3%vol, 5%vol, and 7%vol) under low- and part-load conditions. The injection timing has been swept to find the stable injection window for each EHN level and load.

Low-temperature gasoline combustion (LTGC) engines can provide high efficiencies with very low NOx and soot emissions, but rapid control of the combustion timing remains a challenge. Partial Fuel Stratification (PFS) was demonstrated to be an effective approach to control combustion in LTGC engines. PFS is produced by a double-direct injection (DI) strategy with most of the fuel injected early in the cycle and the remainder of the fuel supplied by a second injection at a variable time during the compression stroke to vary the amount of stratification. Adjusting the stratification changes the combustion phasing, and this can be done on cycle-to-cycle basis by adjusting the injection timing. In this paper, the ability of PFS to control the combustion during wide engine load sweeps is assessed for regular gasoline and gasoline doped with 2-ethylhexyl nitrate (EHN). For PFS, the load control range is limited by combustion instability and poor combustion efficiency at low loads.

The influence of early induction stroke direct injection on late-cycle flows was investigated for a lean-burn, high-tumble, gasoline engine. The engine features side-mounted injection and was operated at a moderate load (8.5 bar brake mean effective pressure) and engine speed (2000 revolutions per minute) condition representative of a significant portion of the duty cycle for a hybridized powertrain system. Thermodynamic engine tests were used to evaluate cam phasing, injection schedule, and ignition timing such that an optimal balance of acceptable fuel economy, combustion stability, and engine-out nitrogen oxide (NOx) emissions was achieved. A single cylinder of the 4-cylinder thermodynamic engine was outfitted with an endoscope that enabled direct imaging of the spark discharge and early flame development.

To cope with regulatory standards, minimizing tailpipe emissions with rapid catalyst light-off during cold-start is critical. This requires catalyst-heating operation with increased exhaust enthalpy, typically by using late post injections for retarded combustion and, therefore, increased exhaust temperature. However, retardability of post injection(s) is constrained by acceptable pollutant emissions such as unburned hydrocarbon (UHC). This study provides further insight into the mechanisms that control the formation of UHC under catalyst-heating operation in a medium-duty diesel engine, and based on the understanding, develops combustion strategies to simultaneously improve exhaust enthalpy and reduce harmful emissions. Experiments were performed with a full boiling-range diesel fuel (cetane number of 45) using an optimized five-injections strategy (2 pilots, 1 main, and 2 posts) as baseline condition.

Mixture formation in a hydrogen-fueled heavy-duty engine with direct injection and a nearly-quiescent top-hat combustion chamber was investigated using laser-induced fluorescence imaging, with 1,4-difluorobenzene serving as a fluorescent tracer seeded into hydrogen. The engine was motored at 1200 rpm, 1.0 bar intake pressure, and 335 K intake temperature. An outward opening medium-pressure hollow-cone injector was operated at two different injection pressures and five different injection timings from early injection during the intake stroke to late injection towards the end of compression stroke. Fuel fumigation upstream of the intake provided a well-mixed reference case for image calibration. This paper presents the evolution of in-cylinder equivalence ratio distribution evaluated during the injection event itself for the cylinder-axis plane and during the compression stroke at different positions of the light sheet within the swirl plane.

Nitric Oxide (NO) can significantly influence the autoignition reactivity and this can affect knock limits in conventional stoichiometric SI engines. Previous studies also revealed that the role of NO changes with fuel type. Fuels with high RON (Research Octane Number) and high Octane Sensitivity (S = RON - MON (Motor Octane Number)) exhibited monotonically retarding knock-limited combustion phasing (KL-CA50) with increasing NO. In contrast, for a high-RON, low-S fuel, the addition of NO initially resulted in a strongly retarded KL-CA50 but beyond the certain amount of NO, KL-CA50 advanced again. The current study focuses on same high-RON, low-S Alkylate fuel to better understand the mechanisms responsible for the reversal in the effect of NO on KL-CA50 beyond a certain amount of NO.

Autoignition enhancing additives have been used for years to enhance the ignition quality of diesel fuel, with 2-ethylhexyl nitrate (EHN) being the most common additive. EHN also enhances the autoignition reactivity of gasoline, which has advantages for some low-temperature combustion techniques, such as Sandia’s Low-Temperature Gasoline Combustion (LTGC) with Additive-Mixing Fuel Injection (AMFI). LTGC-AMFI is a new high-efficiency and low-emissions engine combustion process based on supplying a small, variable amount of EHN into the fuel for better engine operation and control. However, the mechanism by which EHN interacts with the fuel remains unclear. In this work, a chemical-kinetic mechanism for EHN was developed and implemented in a detailed mechanism for gasoline fuels. The combined mechanism was validated against shock-tube experiments with EHN-doped n-heptane and HCCI engine data for EHN-doped regular E10 gasoline. Simulations showed a very good match with experiments.

To comply with increasingly stringent pollutant emissions regulations, diesel engine operation in a catalyst-heating mode is critical to achieve rapid light-off of exhaust aftertreatment catalysts during the first minutes of cold starting. Current approaches to catalyst-heating operation typically involve one or more late post injections to retard combustion phasing and increase exhaust temperatures. The ability to retard post injection timing(s) while maintaining acceptable pollutant emissions levels is pivotal for improved catalyst-heating calibrations. Higher fuel cetane number has been reported to enable later post injections with increased exhaust heat and decreased pollutant emissions, but the mechanism is not well understood. The purpose of this experimental and numerical simulation study is to provide further insight into the ways in which fuel cetane number affects combustion and pollutant formation in a medium-duty diesel engine.

This work is a comprehensive technical review of existing literature and a synthesis of current understanding of the governing physics behind the interaction of multiple fuel injections, ignition, and combustion behavior of multiple-injections in diesel engines. Multiple-injection is a widely adopted operating strategy applied in modern compression-ignition engines, which involves various combinations of small pre-injections and post-injections of fuel before and after the main injection and splitting the main injection into multiple smaller injections. This strategy has been conclusively shown to improve fuel economy in diesel engines while achieving simultaneous NOX, soot, and combustion noise reduction - in addition to a reduction in the emissions of unburned hydrocarbons (UHC) and CO by preventing fuel wetting and flame quenching at the piston wall.

Compliance with future ultra-low nitrogen oxide regulations with diesel engines requires the fastest possible heating of the exhaust aftertreatment system to its proper operating temperature upon cold starting. Late post injections are commonly integrated into catalyst-heating operating strategies. This experimental study provides insight into the complex interactions between the injection-strategy calibration and the tradeoffs between exhaust heat and pollutant emissions. Experiments are performed with certification diesel fuel and blends of diesel fuel with butylal and hexyl hexanoate. Further analyses of experimental data provide insight into fuel reactivity and oxygen content as potential enablers for improved catalyst-heating operation. A statistical design-of-experiments approach is developed to investigate a wide range of injection strategy calibrations at three different intake dilution levels.

The effect of an annular, piston bowl-rim cavity on in-cylinder and engine-out soot emissions is measured in a heavy-duty, optically accessible, single-cylinder diesel engine using in-cylinder soot diagnostics and exhaust smoke emission measurements. The baseline piston configuration consists of a right-cylindrical bowl, while the cavity-piston configuration features an additional annular cavity that is located below the piston bowl-rim and connected to the main-combustion chamber through a thin annular passage, accounting for a 3% increase in the clearance volume, resulting in a reduction in geometric compression ratio (CR) from 11.22 to 10.91. Experiments using the cavity-piston configuration showed a significant reduction of engine-out smoke ranging from 20-60% over a range of engine loads.

Low-temperature gasoline combustion engines can provide high efficiencies with very low NOx and particulate emissions, but rapid control of the combustion timing (50% burn point, CA50) remains a challenge. Partial Fuel Stratification (PFS) was recently demonstrated [2019-01-1156] to control CA50 over a wide range at some selected operating conditions using a regular-grade E10 gasoline. PFS was produced by a double direct injection (D-DI) strategy using a gasoline-type direct injector. For this D-DI-PFS strategy, the majority of the fuel is injected early in the intake stroke, establishing the minimum equivalence ratio in the charge, while the remainder of the fuel is supplied by a second injection at a variable time (SOI2) during the compression stroke to vary the amount of stratification. Adjusting the stratification changes the combustion timing, and this can be done on a cycle-to-cycle basis by adjusting SOI2.

Combustion issued from an eight-hole, direct-injection spray was experimentally studied in a constant-volume pre-burn combustion vessel using simultaneous high-speed diffused back-illumination extinction imaging (DBIEI) and OH* chemiluminescence. DBIEI has been employed to observe the liquid-phase of the spray and to quantitatively investigate the soot formation and oxidation taking place during combustion. The fuel-air mixture was ignited with a plasma induced by a single-shot Nd:YAG laser, permitting precise control of the ignition location in space and time. OH* chemiluminescence was used to track the high-temperature ignition and flame. The study showed that increasing the delay between the end of injection and ignition drastically reduces soot formation without necessarily compromising combustion efficiency. For long delays between the end of injection and ignition (1.9 ms) soot formation was eliminated in the main downstream charge of the fuel spray.

The knock-suppression effectiveness of exhaust-gas recirculation (EGR) can vary between implementations that take EGR gases after the three-way catalyst and those that use pre-catalyst EGR gases. A main difference between pre-and post-catalyst EGR gases is the level of trace species like NO, UHC, CO and H2. To quantify the role of NO, this experiment-based study employs NO-seeding in the intake tract for select combinations of fuel types and compression ratios, using simulated post-catalyst EGR gases as the diluent. The four investigated gasoline fuels share a common RON of 98, but vary in octane sensitivity and composition. To enable probing effects of near-zero NO levels, a skip-firing operating strategy is developed whereby the residual gases, which contain trace species like NO, are purged from the combustion chamber. Overall, the effects of NO-seeding on knock are consistent with the differences in knock limits for preand post-catalyst EGR gases.

In this work we studied the effects of piston bowl design on combustion in a small-bore direct-injection diesel engine. Two bowl designs were compared: a conventional, omega-shaped bowl and a stepped-lip piston bowl. Experiments were carried out in the Sandia single-cylinder optical engine facility, with a medium-load, mild-boosted operating condition featuring a pilot+main injection strategy. CFD simulations were carried out with the FRESCO platform featuring full-geometric body-fitted mesh modeling of the engine and were validated against measured in-cylinder performance as well as soot natural luminosity images. Differences in combustion development were studied using the simulation results, and sensitivities to in-cylinder flow field (swirl ratio) and injection rate parameters were also analyzed.

With the global adoption of diesel common rail systems and the wide variation in composition of local commercial fuels, modern fuel injection systems must be robust against diverse fuel properties. To bridge the knowledge gap on the effects of compositional variation for real commercial fuels on spray combustion characteristics, the present work quantifies ignition and soot formation/oxidation in three unique, international diesel blends. Schlieren imaging, excited-state hydroxyl radical (OH*) chemiluminescence imaging and diffused back-illumination extinction imaging were employed to quantify vapor penetration, ignition, and soot formation and oxidation for high-pressure sprays in a constant-volume, pre-burn chamber. The three fuels were procured from Finland, Japan and Brazil and have cetane numbers of 64.1, 56.1 and 45.4, respectively.

Φ-sensitivity is a fuel characteristic that has important benefits for the operation and control of low-temperature gasoline combustion (LTGC) engines. A fuel is φ-sensitive if its autoignition reactivity varies with the fuel/air equivalence ratio (φ). Thus, multiple-injection strategies can be used to create a φ-distribution that leads to several benefits. First, the φ-distribution causes a sequential autoignition that reduces the maximum heat release rate. This allows higher loads without knock and/or advanced combustion timing for higher efficiencies. Second, combustion phasing can be controlled by adjusting the fuel-injection strategy. Finally, experiments show that intermediate-temperature heat release (ITHR) increases with φ-sensitivity, increasing the allowable combustion retard and improving stability. A detailed mechanism was applied using CHEMKIN to understand the chemistry responsible for φ-sensitivity.

Low-temperature gasoline combustion (LTGC) engines can provide high efficiencies and extremely low NOx and particulate emissions, but controlling the combustion timing remains a challenge. This paper explores the potential of Partial Fuel Stratification (PFS) to provide fast control of CA50 in an LTGC engine. Two different compression ratios are used (CR=16:1 and 14:1) that provide high efficiencies and are compatible with mixed-mode SI-LTGC engines. The fuel used is a research grade E10 gasoline (RON 92, MON 85) representative of a regular-grade market gasoline found in the United States. The fuel was supplied with a gasoline-type direct injector (GDI) mounted centrally in the cylinder. To create the PFS, the GDI injector was pulsed twice each engine cycle. First, an injection early in the intake stroke delivered the majority of the fuel (70 - 80%), establishing the minimum equivalence ratio in the charge.

The impact of 50 ppm intake seeding of ozone (O3) on performance and emissions characteristics was explored in a single-cylinder research engine operated under lean spark assisted compression ignition (SACI) conditions. Optical access into the engine enabled complementary crank angle resolved measurements of in-cylinder O3 concentration via ultraviolet (UV) light absorption. Experiments were performed at moderate loads (4 - 5 bar indicated mean effective pressure) and low-to-moderate engine speeds (800 - 1400 revolutions per minute). Each operating condition featured a single early main injection and maximum brake torque spark timing. Intake pressure was fixed at 1.0 bar, while intake temperatures were varied between 42 - 80 °C. Moderate amounts of internal residuals (12 - 20%) were retained through the use of positive valve overlap. Ozone addition was to found stabilize combustion relative to similar conditions without O3 addition by promoting end gas auto-ignition.

Engine experiments have revealed the importance of fuel condensation on the emission characteristics of low temperature combustion. However, direct in-cylinder experimental evidence has not been reported in the literature. In this paper, the in-cylinder condensation processes observed in optically accessible engine experiments are first illustrated. The observed condensation processes are then simulated using state-of-the-art multidimensional engine CFD simulations with a phase transition model that incorporates a well-validated phase equilibrium numerical solver, in which a thermodynamically consistent phase equilibrium analysis is applied to determine when mixtures become unstable and a new phase is formed. The model utilizes fundamental thermodynamics principles to judge the occurrence of phase separation or combination by minimizing the system Gibbs free energy.