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Quantitative planar laser-induced fluorescence (PLIF) of gaseous acetone as a fuel-tracer has been used in an optically accessible engine, fueled by direct hydrogen injection. The purpose of this article is to assess the accuracy and precision of the measurement and the associated data reduction procedures. A detailed description of the acetone seeding system is given as well. The key features of the experiment are a high-pressure bubbler saturating the hydrogen fuel with acetone vapor, direct injection into an optical engine, excitation of acetone fluorescence with an Nd:YAG laser at 266 nm, and detection of the resulting fluorescence by an unintensified camera. Key steps in the quantification of the single-shot imaging data are an in-situ calibration and a correction for the effect of local temperature on the fluorescence measurement.

The soot distribution as function of ambient O₂ mole fraction in a heavy-duty diesel engine was investigated at low load (6 bar IMEP) with laser-induced incandescence (LII) and natural luminosity. A Multi-YAG laser system was utilized to create time-resolved LII using 8 laser pulses with a spacing of one CAD with detection on an 8-chip framing camera. It is well known that the engine-out smoke level increases with decreasing oxygen fraction up to a certain level where it starts to decrease again. For the studied case the peak occurred at an O₂ fraction of 11.4%. When the oxygen fraction was decreased successively from 21% to 9%, the initial soot formation moved downstream in the jet. At the lower oxygen fractions, below 12%, no soot was formed until after the wall interaction. At oxygen fractions below 11% the first evidence of soot is in the recirculation zone between two adjacent jets.

High-load HCCI operation is typically limited by rapid pressure-rise rates (PRR) and engine knock caused by an overly rapid heat-release rate (HRR). Exhaust gas recirculation (EGR) is commonly used in HCCI engines, and it is often stated in the literature that charge dilution with EGR (or high levels of retained residuals) is beneficial for reducing the PRR to allow higher loads without knock. However, EGR/retained-residuals affect other operating parameters such as combustion phasing, which can in turn influence the PRR independently from any effect of the EGR gases themselves. Because of the multiple effects of EGR, its direct benefit for reducing the PRR is not well understood. In this work, the effects of EGR on the PRR were isolated by controlling the combustion phasing independently from the EGR addition by adjusting the intake temperature. The experiments were conducted using gasoline as the fuel at a 1200 rpm operating condition.

The potential of boosted HCCI for achieving high loads has been investigated for intake pressures (Piⁿ) from 100 kPa (naturally aspirated) to 325 kPa absolute. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) equipped with a compression-ratio 14 piston at 1200 rpm. The intake charge was fully premixed well upstream of the intake, and the fuel was a research-grade (R+M)/2 = 87-octane gasoline with a composition typical of commercial gasolines. Beginning with Piⁿ = 100 kPa, the intake pressure was systematically increased in steps of 20 - 40 kPa, and for each Piⁿ, the fueling was incrementally increased up to the knock/stability limit, beyond which slight changes in combustion conditions can lead to strong knocking or misfire. A combination of reduced intake temperature and cooled EGR was used to compensate for the pressure-induced enhancement of autoignition and to provide sufficient combustion-phasing retard to control knock.

Low-temperature gasoline combustion (LTGC), based on the compression ignition of a premixed or partially premixed dilute charge, can provide thermal efficiencies (TE) and maximum loads comparable to those of turbo-charged diesel engines, and ultra-low NOx and particulate emissions. Intake boosting is key to achieving high loads with dilute combustion, and it also enhances the fuel's autoignition reactivity, reducing the required intake heating or hot residuals. These effects have the advantages of increasing TE and charge density, allowing greater timing retard with good stability, and making the fuel ϕ- sensitive so that partial fuel stratification (PFS) can be applied for higher loads and further TE improvements. However, at high boost the autoignition reactivity enhancement can become excessive, and substantial amounts of EGR are required to prevent overly advanced combustion.

The downsizing of internal combustion engines to increase fuel economy leads to challenges in both obtaining ignition and stabilizing combustion at boosted intake pressures and high exhaust gas recirculation dilution conditions. The use of non-thermal plasma ignition technologies has shown promise as a means to more reliably ignite dilute charge mixtures at high pressures. Despite progress in fundamental research on this topic, both the capabilities and operation implications of emerging non-thermal plasma ignition technologies in internal combustion engine applications are not yet fully explored. In this work, we document the effects of using a corona discharge ignition system in a single cylinder gasoline direct injection research engine relative to using a traditional inductive spark ignition system under conditions associated with both naturally aspirated (8 bar BMEP) and boosted (20 bar BMEP) loads at moderate (2000 rpm) and high (4000 rpm) engine speeds.

Reactivity Controlled Compression Ignition (RCCI) is an approach to increase engine efficiency and lower engine-out emissions by using in-cylinder stratification of fuels with differing reactivity (i.e., autoignition characteristics) to control combustion phasing. Stratification can be altered by varying the injection timing of the high-reactivity fuel, causing transitions across multiple regimes of combustion. When injection is sufficiently early, combustion approaches a highly-premixed autoignition regime, and when it is sufficiently late it approaches more mixing-controlled, diesel-like conditions. Engine performance, emissions, and control authority over combustion phasing with injection timing are most favorable in between, within the RCCI regime.

This work explores how the high-load limits of HCCI are affected by fuel autoignition reactivity, EGR quality/composition, and EGR unmixedness for naturally aspirated conditions. This is done for PRF80 and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) with a CR = 14 piston installed. By operating at successively higher engine loads, five load-limiting factors were identified for these fuels: 1) Residual-NOx-induced run-away advancement of the combustion phasing, 2) EGR-NOx-induced run-away, 3) EGR-NOx/wall-heating induced run-away 4) EGR-induced oxygen deprivation, and 5) excessive partial-burn occurrence due to EGR unmixedness. The actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the EGR quality level (where high quality refers to the absence of trace species like NO, HC and CO, i.e. simulated EGR), the level of EGR unmixedness, and the selected pressure-rise rate (PRR).

This work explores the high-load limits of HCCI for naturally aspirated operation. This is done for three fuels with various autoignition reactivity: iso-octane, PRF80, and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liter displacement), mostly with a CR = 14 piston installed, but with some tests at CR = 18. Five load-limiting factors were identified: 1) NOx-induced combustion-phasing run-away, 2) wall-heating-induced run-away, 3) EGR-induced oxygen deprivation, 4) wandering unsteady combustion, and 5) excessive exhaust NOx. These experiments at 1200 rpm show that the actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the selected CA50, and in some cases, the tolerable level of NOx emissions. For iso-octane, which has the highest resistance to autoignition of the fuels tested, the NOx emissions become unacceptable at IMEPg = 473 kPa.

Negative valve overlap (NVO) is a valve strategy employed to retain and recompress residual burned gases to assist HCCI combustion, particularly in the difficult regime of low-load operation. NVO allows the retention of large quantities of hot residual burned gases as well as the possibility of fuel addition for combustion control purposes. Reaction of fuel injected during NVO increases charge temperature, but in addition could produce reformed fuel species that may affect main combustion phasing. The strategy holds potential for controlling and extending low-load HCCI combustion. The goal of this work is to demonstrate the feasibility of applying two-wavelength PLIF of 3-pentanone to obtain simultaneous, in-cylinder temperature and composition images during different parts of the HCCI/NVO cycle. Measurements are recorded during the intake and main compression strokes, as well as during the more challenging periods of NVO recompression and re-expansion.

Partially premixed low-temperature combustion (LTC) using exhaust-gas recirculation (EGR) has the potential to reduce engine-out NOx and soot emissions, but increased unburned hydrocarbon (UHC) emissions need to be addressed. In this study, we investigate close-coupled post injections for reducing UHC emissions. By injecting small amounts of fuel soon after the end of the main injection, fuel-lean mixtures near the injector that suffer incomplete combustion can be enriched with post-injection fuel and burned to completion. The goal of this work is to understand the in-cylinder mechanisms affecting the post-injection efficacy and to quantify its sensitivity to operational parameters including post-injection duration, injection dwell, load, and ignition delay time of the post-injection mixture.

Ethanol and ethanol/gasoline blends are being widely considered as alternative fuels for light-duty automotive applications. At the same time, HCCI combustion has the potential to provide high efficiency and ultra-low exhaust emissions. However, the application of HCCI is typically limited to low and moderate loads because of unacceptably high heat-release rates (HRR) at higher fueling rates. This work investigates the potential of lowering the HCCI HRR at high loads by using partial fuel stratification to increase the in-cylinder thermal stratification. This strategy is based on ethanol's high heat of vaporization combined with its true single-stage ignition characteristics. Using partial fuel stratification, the strong fuel-vaporization cooling produces thermal stratification due to variations in the amount of fuel vaporization in different parts of the combustion chamber.

A single-cylinder, light-duty, diesel engine was used to investigate the effect of changes in intake pressure (boost) on engine performance and emissions in low-temperature combustion (LTC) regimes. Two different LTC strategies were examined: a dilution-controlled regime characterized by high rates of exhaust gas recirculation (EGR) with early-injection (roughly 30° BTDC), and a late-injection (near TDC) regime employing moderate EGR levels. For both strategies, moderate (8 bar IMEP) and low (3 bar IMEP) load conditions were tested at intake pressures of 1.0, 1.5, and 2.0 bar. For both LTC strategies, increased intake pressure reduces emissions of unburned hydrocarbons (UHC) and CO, with corresponding improvements in combustion efficiency and indicated specific fuel consumption (ISFC), particularly at high load. Depending on the operating condition, UHC and CO emissions can stem from either over-lean or over-rich mixtures.

In-cylinder and exhaust soot mass were measured in an optically accessible heavy duty diesel engine under various intake oxygen concentrations from 8 vol% to 21 vol% to gain insight into soot formation and destruction processes. Exhaust-gas recirculation (EGR) in the optical engine was simulated by dilution of intake gases with nitrogen. In-cylinder soot, measured by 2-color optical pyrometry, was compared to engine-out soot, measured by a commercial optical smoke meter. Each EGR rate was studied under two separate fueling conditions: (i) constant injection duration, and (ii) constant global equivalence ratio. The in-cylinder and exhaust soot measurements of the current study agree well with findings of previous studies in the literature. Under both fueling conditions, in-cylinder soot temperatures decreased with the reduction of in-cylinder oxygen concentration.

Engine combustion strategies that preserve high cycle efficiency while minimizing engine-out pollutant emissions are the focus of major research efforts around the world. Such high efficiency clean combustion (HECC) strategies typically employ compression ignition of a charge that exhibits an elevated degree of fuel/air premixing and/or dilution with combustion products. Prior studies have shown that a highly dilute, mixing-controlled combustion strategy using a high-cetane, oxygenated fuel can achieve HECC while avoiding the control, high-load knock, and light-load incomplete combustion difficulties that are often experienced with approaches that use a high degree of charge premixing. On the other hand, employing high dilution levels (e.g., by using large amounts of cooled exhaust gas recirculation, EGR) can place excessive burdens on engine heat exchangers and air-handling systems.

Spark plugs instrumented with a ring of optical fibers in the threaded-body region have seen considerable use in the past ten years, and it is expected that their application to unmodified production engines will increase in the years to come. Interpretation of the optical signals obtained with the probe is often difficult, particularly under lean operating conditions where the low luminosity of the flame leads to imprecise flame arrival detection. A systematic look at the optical signals, along with direct imaging of the flame, has been undertaken to calibrate and optimize the determination of flame arrival times. In addition, an evaluation of the different models available for the analysis of the flame arrival data is made. Data fits are compared with real flame images, to determine which model best estimates the convective velocity of the flow and the expansion speed of the flame kernel.

Reaching for higher loads and improving combustion-phasing control are important challenges for HCCI research. Although HCCI engines can operate with a variety of fuels, recent research has shown that fuels with two-stage autoignition have some significant advantages for overcoming these challenges. Because the amount of low-temperature heat release (LTHR) is proportional to the local equivalence ratio (ϕ), fuel stratification can be used to adjust the combustion phasing (CA50) and/or burn duration using various fuel-injection strategies. Two-stage ignition fuels also allow stable combustion even for extensive combustion-phasing retard, which reduces the knocking propensity. Finally, the LTHR reduces the required intake temperature, which increases the inducted charge mass for a given intake pressure, allowing higher fueling rates before knocking and NOx emissions become a problem. However, the amount of LTHR is normally highly dependent on the engine speed.

EGR can be used beneficially to control combustion phasing in HCCI engines. To better understand the function of EGR, this study experimentally investigates the thermodynamic and chemical effects of real EGR, simulated EGR, dry EGR, and individual EGR constituents (N2, CO2, and H2O) on the autoignition processes. This was done for gasoline and various PRF blends. The data show that addition of real EGR retards the autoignition timing for all fuels. However, the amount of retard is dependent on the specific fuel type. This can be explained by identifying and quantifying the various underlying mechanisms, which are: 1) Thermodynamic cooling effect due to increased specific-heat capacity, 2) [O2] reduction effect, 3) Enhancement of autoignition due to the presence of H2O, 4) Enhancement or suppression of autoignition due to the presence of trace species such as unburned or partially-oxidized hydrocarbons.

Knowledge of the concentration field and flammability envelope from a small-scale hydrogen leak is an issue of importance for the safe use of hydrogen. A combined experimental and modeling program is being carried out by Sandia National Laboratories to characterize and predict the behavior of small-scale hydrogen releases. In contrast to the previous work performed by Sandia on large, momentum-dominated hydrogen leaks, these studies are focusing on small leaks in the Froude number range where both buoyant and inertial forces are important or, in the limit, where buoyancy dominates leak behavior. In the slow leak regime buoyant forces affect the trajectory and rate of air entrainment of the hydrogen jet leak and significant curvature can occur in the jet trajectory. Slow leaks may occur from leaky fittings or o-ring seals on hydrogen vehicles or other hydrogen-based systems where large amounts of pressure drop occur across the leak path.

High-speed video imaging in a swirl-supported (Rs = 1.7), direct-injection heavy-duty diesel engine operated with moderate-to-high EGR rates reveals a distinct correlation between the spatial distribution of luminous soot and mean flow vorticity in the horizontal plane. The temporal behavior of the experimental images, as well as the results of multi-dimensional numerical simulations, show that this soot-vorticity correlation is caused by the presence of a greater amount of soot on the windward side of the jet. The simulations indicate that while flow swirl can influence pre-ignition mixing processes as well as post-combustion soot oxidation processes, interactions between the swirl and the heat release can also influence mixing processes. Without swirl, combustion-generated gas flows influence mixing on both sides of the jet equally. In the presence of swirl, the heat release occurs on the leeward side of the fuel sprays.