Search Results

Two fundamental aspects of HCCI engine combustion have been investigated using a single-zone model with time-varying compression and the full chemical-kinetic mechanisms for iso-octane, a representative liquid-phase fuel. This approach allows effects of the kinetics and thermodynamics to be isolated and evaluated in a well-characterized manner, providing an understanding of the selected fundamental processes. The computations were made using the CHEMKIN-III kinetic-rate code for an 1800 rpm operating condition. The study consists of two parts. First, low-load HCCI operation was investigated to determine the role of bulk-gas reactions as a source for HC and CO emissions. The computations show that as fueling is reduced to equivalence ratios of 0.15 and lower (very light load and idle), the bulk-gas reactions do not go to completion, leading to inefficient combustion and high emissions of HC and CO.

A phenomenological description, or “conceptual model,” of how direct-injection (DI) diesel combustion occurs has been derived from laser-sheet imaging and other recent optical data. To provide background, the most relevant of the recent imaging data of the author and co-workers are presented and discussed, as are the relationships between the various imaging measurements. Where appropriate, other supporting data from the literature is also discussed. Then, this combined information is summarized in a series of idealized schematics that depict the combustion process for a typical, modern-diesel-engine condition. The schematics incorporate virtually all of the information provided by our recent imaging data including: liquid- and vapor-fuel zones, fuel/air mixing, autoignition, reaction zones, and soot distributions.

A combined experimental and modeling study has been conducted to investigate the sources of CO and HC emissions (and the associated combustion inefficiencies) at low-loads. Engine performance and emissions were evaluated as fueling was reduced from knocking conditions to very low loads (ϕ = 0.28 - 0.04) for a variety of operating conditions, including: various intake temperatures, engine speeds, compression ratios, and a comparison of fully premixed and GDI (gasoline-type direct injection) fueling. The experiments were conducted in a single-cylinder engine (0.98 liters) using iso-octane as the fuel. Comparative computations were made using a single-zone model with the full chemistry mechanisms for iso-octane, to determine the expected behavior of the bulk-gases for the limiting case of no heat transfer, crevices, or charge inhomogeneities.

Chemiluminescence imaging has been applied to investigate the naturally occurring charge stratification in an HCCI engine. This stratification slows the pressure-rise rate (PRR) during combustion, making it critical to the high-load operating limit of these engines. Experiments were conducted in a single-cylinder HCCI engine modified with windows in the combustion chamber for optical access. Using this engine, chemiluminescence images were obtained from three different view angles. These included both single-shot images with intensified CCD cameras and high-speed (20kHz) sequences with an intensified CMOS video camera. The engine was fueled with iso-octane, which has been shown to be a reasonable surrogate for gasoline and exhibits only single-stage ignition at these naturally aspirated conditions. The chemiluminescence images show that the HCCI combustion is not homogeneous but has a strong turbulent structure even when the fuel and air are fully premixed prior to intake.

Combustion phasing is one important issue that must be addressed for HCCI operation. The intake temperature can be adjusted to achieve ignition at the desired crank angle. However, heat-transfer during induction will make the effective intake temperature different from the temperature measured in the runner. Also, depending on the engine speed and port configuration, dynamic flow effects cause various degrees of charge heating. Additionally, residuals from the previous cycle can have significant influence on the charge temperature at the beginning of the compression stroke. Finally, direct injection of fuel will influence the charge temperature since heat is needed for vaporization. This study investigates these effects in a systematic manner with a combination of experiment and cycle simulation using WAVE from Ricardo.

Numerical simulations were conducted to systematically assess the effects of different spray models on the ignition delay predictions and compared with experimental measurements obtained at the KAUST ignition quality tester (IQT) facility. The influence of physical properties and chemical kinetics over the ignition delay time is also investigated. The IQT experiments provided the pressure traces as the main observables, which are not sufficient to obtain a detailed understanding of physical (breakup, evaporation) and chemical (reactivity) processes associated with auto-ignition. A three-dimensional computational fluid dynamics (CFD) code, CONVERGE™, was used to capture the detailed fluid/spray dynamics and chemical characteristics within the IQT configuration. The Reynolds-averaged Navier-Stokes (RANS) turbulence with multi-zone chemistry sub-models was adopted with a reduced chemical kinetic mechanism for n-heptane and iso-octane.

Gasoline-ethanol-methanol (GEM) blends, with constant stoichiometric air-to-fuel ratio (iso-stoichiometric blending rule) and equivalent to binary gasoline-ethanol blends (E2, E5, E10 and E15 in % vol.), were defined to investigate the effect of methanol and combined mixtures of ethanol and methanol when blended with three FACE (Fuels for Advanced Combustion Engines) Gasolines, I, J and A corresponding to RON 70.2, 73.8 and 83.9, respectively, and their corresponding Primary Reference Fuels (PRFs). A Cooperative Fuel Research (CFR) engine was used under Spark Ignition and Homogeneous Charge Compression Ignited modes. An ignition quality tester was utilized in the Compression Ignition mode. One of the promising properties of GEM blends, which are derived using the iso-stoichiometric blending rule, is that they maintain a constant octane number, which has led to the introduction of methanol as a drop-in fuel to supplement bio-derived ethanol.

The present study experimentally examines the low-temperature autoignition area of isooctane within the in-cylinder pressure-in-cylinder temperature map. Experiments were run with the help of a Cooperative Fuel Research (CFR) engine. The boundaries of this engine were extended so that experiments could be performed outside the domain delimited by research octane number (RON) and motor octane number (MON) traces. Since homogeneous charge compression ignition (HCCI) combustion is governed by kinetics, the rotation speed for all the experiments was set at 600 rpm to allow time for low-temperature heat release (LTHR). All the other parameters (intake pressure, intake temperature, compression ratio, and equivalence ratio) were scanned, such as the occurrence of isooctane combustion. The principal results showed that LTHR for isooctane occurs effortlessly under high intake pressure (1.3 bar) and low intake temperature (25 °C).

This paper studies autoignition characteristics and HCCI engine combustion of ketone fuels, which are important constituents of recently discovered fungi-derived biofuels. Two ketone compounds, 2,4-dimethyl-3-pentanone (DMPN) and cyclopentanone (CPN), are systematically investigated in the Sandia HCCI engine, and the results are compared with conventional gasoline and neat ethanol. It is found that CPN has the lowest autoignition reactivity of all the biofuels and gasoline blends tested in this HCCI engine. The combustion timing of CPN is also the most sensitive to intake-temperature (Tin) variations, and it is almost insensitive to intake-pressure (Pin) variations. These characteristics and the overall HCCI performance of CPN are similar to those of ethanol. In contrast, DMPN shows multi-faceted autoignition characteristics. On the one hand, DMPN has strong temperature-sensitivity, even at boosted Pin, which is similar to the low-reactivity ethanol and CPN.

The blending of ethanol with PRF (Primary reference fuel) 84 was investigated and compared with FACE (Fuels for Advanced Combustion Engines) A gasoline surrogate which has a RON of 83.9. Previously, experiments were performed at four HCCI conditions but the chemical effect responsible for the non-linear blending behavior of ethanol with PRF 84 and FACE A was not understood. Hence, in this study the experimental measurements were simulated using zero-dimensional HCCI engine model with detailed chemistry in CHEMKIN PRO. Ethanol was used as an octane booster for the above two base fuels in volume concentration of 0%, 2%, 5% and 10%. The geometrical data and the intake valve closure conditions were used to match the simulated combustion phasing with the experiments. Low temperature heat release (LTHR) was detected by performing heat release analysis.

The effect of ethanol blended with three FACE (Fuels for Advanced Combustion Engines) gasolines, I, J and A corresponding to RON 70.3, 71.8 and 83.5, respectively, were compared to PRF70 and PRF84 with the same ethanol concentrations, these being 2%, 5%, 10%, 15% and 20% by volume. A Cooperative Fuel Research (CFR) engine was used to understand the blending effect of ethanol with FACE gasolines and PRFs in spark-ignited and homogeneous charge compression ignited mode. Blending octane numbers (BON) were obtained for both the modes. All the fuels were also tested in an ignition quality tester to obtain Blending Derived Cetane numbers (BDCN). It is shown that fuel composition and octane number are important characteristics of all the base fuels that have a significant impact on octane increase with ethanol. The dependency of octane number for the base fuel on the blending octane number depended on the combustion mode operated.

Future internal combustion engines demand higher efficiency but progression towards this is limited by the phenomenon called knock. A possible solution for reaching high efficiency is Octane-on-Demand (OoD), which allows to customize the antiknock quality of a fuel through blending of high-octane fuel with a low octane fuel. Previous studies on Octane-on-Demand highlighted efficiency benefits depending on the combination of low octane fuel with high octane booster. The author recently published works with ethanol and methanol as high-octane fuels. The results of this work showed that the composition and octane number of the low octane fuel is significant for the blending octane number of both ethanol and methanol. This work focuses on toluene as the high octane fuel (RON 120). Aromatics offers anti-knock quality and with high octane number than alcohols, this work will address if toluene can provide higher octane enhancement.

This study investigates the potential of partial fuel stratification for reducing the knocking propensity of intake-boosted HCCI engines operating on conventional gasoline. Although intake boosting can substantially increase the high-load capability of HCCI, these engines would be more production-viable if the knock/stability load limit could be extended to allow higher loads at a given boost and/or to provide even higher thermal efficiencies. A technique termed partial fuel stratification (PFS) has recently been shown to greatly reduce the combustion-induced pressure-rise rate (PRR), and therefore the knocking propensity of naturally aspirated HCCI, when the engine is fueled with a φ-sensitive, two-stage-ignition fuel. The current work explores the potential of applying PFS to boosted HCCI operation using conventional gasoline, which does not typically show two-stage ignition. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) at 1200 rpm.

High-load HCCI combustion has recently been demonstrated with conventional gasoline using intake pressure boosting. The key is to control the high combustion heat release rates (HRR) by using combustion timing retard and mixture stratification. However, at naturally aspirated and moderately boosted conditions, these techniques did not work well due to the low autoignition reactivity of conventional gasoline at these conditions. This work studies a low-octane distillate fuel with similar volatility to gasoline, termed Hydrobate, for its potential in HCCI engine combustion at naturally aspirated and low-range boosted conditions. The HCCI combustion with fully premixed and partially stratified charges was examined at intake pressures (Pin) from 100 to 180 kPa and constant intake temperature (60°C) and engine speed (1200 rpm).

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.

Previous work has shown that conventional diesel ignition improvers, 2-ethylhexyl nitrate (EHN) and di-tert-butyl peroxide (DTBP), can be used to enhance the autoignition of a regular-grade E10 gasoline in a well premixed low-temperature gasoline combustion (LTGC) engine, hereafter termed an HCCI engine, at naturally aspirated and moderately boosted conditions (up to 180 kPa absolute) with a constant engine speed of 1200 rpm and a 14:1 compression ratio. In the current work the effect of EHN on boosted HCCI combustion is further investigated with a higher compression ratio (16:1) piston and over a range of engine speeds (up to 2400 rpm). The results show that the higher compression ratio and engine speeds can make the combustion of a regular-grade E10 gasoline somewhat less stable. The addition of EHN improves the combustion stability by allowing combustion phasing to be more advanced for the same ringing intensity.

Long chain alcohols possess major advantages over the currently used ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. The rapid developments in biofuel technology have made it possible to produce C 4 -C 5 alcohols cost effectively. These higher alcohols could significantly expand the biofuel content and potentially substitute ethanol in future gasoline mixtures. This study characterizes some fundamental properties of a C 5 alcohol, isopentanol, as a fuel for HCCI engines. Wide ranges of engine speed, intake temperature, intake pressure, and equivalence ratio are investigated. Results are presented in comparison with gasoline or ethanol data previously reported. For a given combustion phasing, isopentanol requires lower intake temperatures than gasoline or ethanol at all tested speeds, indicating a higher HCCI reactivity.

A planar temperature imaging diagnostic has been developed and applied to an investigation of naturally occurring thermal stratification in an HCCI engine. Natural thermal stratification is critical for high-load HCCI operation because it slows the combustion heat release; however, little is known about its development or distribution. A tracer-based single-line PLIF imaging technique was selected for its good precision and simplicity. Temperature-map images were derived from the PLIF images, based on the temperature sensitivity of the fluorescence signal of the toluene tracer added to the fuel. A well premixed intake charge assured that variations in fuel/air mixture did not affect the signal. Measurements were made in a single-cylinder optically accessible HCCI research engine (displacement = 0.98 liters) at a typical 1200 rpm operating condition. Since natural thermal stratification develops prior to autoignition, all measurements were made for motored operation.

Chemiluminescence imaging has been applied to a parametric investigation of diesel autoignition. Time-resolved images of the natural light emission were made in an optically accessible DI diesel engine of the heavy-duty size class using an intensified CCD video camera. Measurements were obtained at a base operating condition, corresponding to a motored TDC temperature and density of 992 K and 16.6 kg/m3, and for TDC temperatures and densities above and below these values. Data were taken with a 42.5 cetane number blend of the diesel reference fuels for all conditions, and measurements were also made with no. 2 diesel fuel (D2) at the base condition. For each condition, temporal sequences of images were acquired from the time of first detectable chemiluminescence up through fully sooting combustion, and the images were analyzed to obtain quantitative measurements of the average emission intensity.

To gain a better understanding of how the onset of incomplete bulk-gas reactions changes with engine speed and fuel-type, a parametric study of HCCI combustion and emissions has been conducted. The experimental part of the study was performed at naturally aspirated conditions and included fueling sweeps at four engine speeds (600, 1200, 1800 and 2400 rpm) for research grade gasoline, pure iso-octane and two mixtures of the primary reference fuels (i.e. n-heptane and iso-octane) with octane numbers of 80 and 60. Additionally, single-zone CHEMKIN computations with a detailed mechanism for iso-octane were conducted. The results show that there is a strong coupling between the ignition quality of the fuel and the required intake temperature to phase the combustion at TDC. There is also a direct influence of intake temperature on the completeness of combustion. This is the case because the CO-to-CO2 reactions are highly sensitive to the peak combustion temperatures.