Search Results

Comparing with port-fuel-injection (PFI) engine, the fuel sprays in spark-ignition direct-injection (SIDI) engines play more important roles since they significantly influence the combustion stability, engine efficiency as well as emission formations. In order to design higher efficiency and cleaner engines, further research is needed to understand and optimize the fuel spray atomization and vaporization. This paper investigates the atomization and evaporation of n-pentane, gasoline and surrogate fuels sprays under realistic SIDI engine conditions. An optical diagnostic technique combining high-speed Mie scattering and Schlieren imaging has been applied to study the characteristics of liquid and vapor phases inside a constant volume chamber under various operating conditions. The effects of ambient temperature, fuel temperature, and fuel type on spray atomization and vaporization are analyzed by quantitative comparisons of spray characteristics.

The continuous improvement of gasoline direct injection (GDI) engine is largely attributed to the enhanced understanding of air-fuel mixing and combustion processes. This work investigates the transient behavior of the ambient flow fields of hexane spray using the combined diagnostics of fluorescent particle image velocimetry (FPIV) and mie scattering. A hybrid analysis approach is proposed to investigate the residual effect of spray injection on ambient flow fields, including flow similarity measurement, entrainment velocity calculation, and vortex strength detection. The work investigates the residual effect under different injection durations, injection pressure, and flash-boiling extent of the spray, and unveils correlation between vortex strength and the endurance of the residual effect.

Thermal efficiency comparisons are made between the low temperature combustion and the conventional diesel cycles on a common-rail diesel engine with a conventional diesel fuel. Empirical studies have been conducted under independently controlled exhaust gas recirculation, intake boost, and exhaust backpressure. Up to 8 fuel injection pulses per cylinder per cycle have been applied to modulate the homogeneity history of the early injection diesel low temperature combustion operations in order to improve the phasing of the combustion process. The impact of heat release phasing, duration, shaping, and splitting on the thermal efficiency has been analyzed with zero-dimensional engine cycle simulations. This paper intends to identify the major parameters that affect diesel low temperature combustion engine thermal efficiency.

Conventionally, the diesel fuel ignites spontaneously following the injection event. The combustion and injection often overlap with a very short ignition delay. Diesel engines therefore offer superior combustion stability characterized by the low cycle-to-cycle variations. However, the enforcement of the stringent emission regulations necessitates the implementation of innovative diesel combustion concepts such as the low temperature combustion (LTC) to achieve ultra-low engine-out pollutants. In stark contrast to the conventional diesel combustion, the enabling of LTC requires enhanced air fuel mixing and hence a longer ignition delay is desired. Such a decoupling of the combustion events from the fuel injection can potentially cause ignition discrepancy and ultimately lead to combustion cyclic variations.

Research studies have suggested that changes to the ignition system are required to generate a more robust flame kernel in order to secure the ignition process for the future advanced high efficiency spark-ignition (SI) engines. In a typical inductive ignition system, the spark discharge is initiated by a transient high-power electrical breakdown and sustained by a relatively low-power glow process. The electrical breakdown is characterized as a capacitive discharge process with a small quantity of energy coming mainly from the gap parasitic capacitor. Enhancement of the breakdown is a potential avenue effectively for extending the lean limit of SI engine. In this work, the effect of high-power capacitive spark discharge on the early flame kernel growth of premixed methane-air mixtures is investigated through electrical probing and optical diagnosis.

This work investigates the suitability of n-butanol for enabling PCCI, HCCI, and RCCI combustion modes to achieve clean and efficient combustion on a high compression ratio (18.2:1) diesel engine. Systematic engine tests are conducted at low and medium engine loads (6∼8 bar IMEP) and at a medium engine speed of 1500 rpm. Test results indicate that n-butanol is more suitable than diesel to enable PCCI and HCCI combustion with the same engine hardware. However, the combustion phasing control for n-butanol is demanding due to the high combustion sensitivity to variations in engine operating conditions where engine safety concerns (e.g. excessive pressure rise rates) potentially arise. While EGR is the primary measure to control the combustion phasing of n-butanol HCCI, the timing control of n-butanol direct injection in PCCI provides an additional leverage to properly phase the n-butanol combustion.

This study investigates neat n-butanol, as a cleaner power source, to directly replace conventional diesel fuels for enabling low temperature combustion on a modern common-rail diesel engine. Engine tests are performed at medium engine loads (6∼8 bar IMEP) with the single-shot injection strategy for both n-butanol and diesel fuels. As indicated by the experimental results, the combustion of neat n-butanol offers comparable engine efficiency to that of diesel while producing substantially lower NOx emissions even without the use of exhaust gas recirculation. The greater resistance to auto-ignition allows n-butanol to undergo a prolonged ignition delay for air-fuel mixing; the high volatility helps to enhance the cylinder charge homogeneity; the fuel-borne oxygen contributes to smoke reduction and, as a result, the smoke emissions of n-butanol combustion are generally at a near-zero level under the tested engine operating conditions.

In this work, an innovative piston bowl design that physically divides the combustion chamber into a central zone and a peripheral zone is employed to assist the control of the ethanol-diesel combustion process via heat release shaping. The spatial combustion zone partition divides the premixed ethanol-air mixture into two portions, and the combustion event (timing and extent) of each portion can be controlled by the temporal diesel injection scheduling. As a result, the heat release profile of ethanol-diesel dual-fuel combustion is properly shaped to avoid excessive pressure rise rates and thus to improve the engine performance. The investigation is carried out through theoretical simulation study and empirical engine tests. Parametric simulation is first performed to evaluate the effects of heat release shaping on combustion noise and engine efficiency and to provide boundary conditions for subsequent engine tests.

Flash boiling spray has been proven to be a useful method in providing finer fuel droplet and stronger evaporation in favor of creating a homogeneous fuel-air mixture. Combustion characteristics of flash boiling spray are thus valuable to be investigated systematically for aiding the development of efficient internal combustion system. An experimental study of flash boiling spray combustion in a SIDI optical engine under early injection has been conducted. The fuel, Iso-octane, was used across all tests. Three fuel spray conditions experimented in the study: normal liquid, transitional flash boiling and flare flash boiling sprays, within each case that Pa/Ps ratio was set in (>1), (0.3~1), and (<0.3) respectively. A small quartz insert on the piston enables optical access for observing combustion process; non-intrusive measurements on flame radicals has been carried out using a high-speed color camera.

The majority of transportation systems have continued to be powered by the internal combustion engine and fossil fuels. Heavy-duty applications especially are reliant on diesel engines for their high brake efficiency, power density, and robustness. Although engineering developments have advanced engines towards significantly fewer emissions and higher efficiency, the use of fossil-derived diesel as fuel sets a fundamental threshold in the achievable total net carbon reduction. Dimethyl ether can be produced from various renewable feedstocks and has a high chemical reactivity making it suitable for heavy-duty applications, namely compression ignition direct injection engines. Literature shows the successful use of DME fuels in diesel engines without significant hardware modifications.

Extensive experiments were conducted on a low emission DI diesel engine by using Dimethyl Carbonate (DMC) as an oxygenate fuel additive. The results indicated that smoke reduced almost linearly with fuel oxygen content. Accompanying noticeable reductions of HC and CO were attained, while a small increase in NOx was encountered. The effective reduction in smoke with DMC was maintained with intake charge CO2, which led to low NOx and smoke emissions by the combined use of oxygenated fuel and exhaust gas recirculation (EGR). Further experiments were conducted on an optically accessible combustion bomb and a thermal cracking set-up to study the mechanisms of DMC addition on smoke reduction.

As a renewable energy source, the ethanol fuel was employed with a diesel fuel in this study to improve the cylinder charge homogeneity for high load operations, targeting on ultra-low nitrogen oxides (NOx) and smoke emissions. A light-duty diesel engine is configured to adapt intake port fuelling of the ethanol fuel while keeping all other original engine components intact. High load experiments are performed to investigate the combustion control and low emission enabling without sacrificing the high compression ratio (18.2:1). The intake boost, exhaust gas recirculation (EGR) and injection pressure are independently controlled, and thus their effects on combustion and emission characteristics of the high load operation are investigated individually. The low temperature combustion is accomplished at high engine load (16~17 bar IMEP) with regulation compatible NOx and soot emissions.

A number of cylinder-pressure derived parameters including the crank angles of maximum pressure, maximum rate of pressure rise, and 50% heat released are considered as among the desired feedback for cycle-by-cycle adaptive control of diesel combustion. For real-time computation of these parameters, the heat release analyses based on the first law of thermodynamics are used. This paper intends to identify the operating regions where the simplified heat release approach provides sufficient accuracy for control applications and also highlights those regions where its use can lead to significant errors in the calculated parameters. The effects of the cylinder charge-to-wall heat transfer and the temperature dependence of the specific heat ratio on the model performance are reported. A new computationally efficient algorithm for estimating the crank angle of 50% heat released with adequate accuracy is proposed for computation in real-time.

The flash boiling phenomenon occurs at some operating conditions when fuel is directly injected into the cylinder of a homogeneous charge spark ignition direct injection (SIDI) engine due to the higher temperature of the injected fuel and lower back pressure. A flash boiling spray has significantly different characteristics from a conventional DI gasoline spray. In this paper, the planar laser-induced exciplex fluorescence (PLIEF) technique with two specially designed dopants of the fluorobenzene (FB) and the diethyl-methyl-amine (DEMA) in n-hexane was implemented to investigate the liquid and vapor phases of sprays from a multi-hole injector. A vapor phase calibration was carried out to quantitatively correlate the fluorescence signal with vapor concentration. The quantitative vapor concentration distribution is then obtained by applying the calibration.

Diesel engines operating in the low-temperature combustion (LTC) mode generally tend to produce very low levels of NOx and soot. However, the implementation of LTC is challenged by the higher cycle-to-cycle variation with heavy EGR operation and the narrower operating corridors. The robustness and efficiency of LTC operation in diesel engines can be enhanced with improvements in the promptness and accuracy of combustion control. A set of field programmable gate array (FPGA) modules were coded and interlaced to suffice on-the-fly combustion event modulations. The cylinder pressure traces were analyzed to update the heat release rate concurrently as the combustion process proceeds prior to completing an engine cycle. Engine dynamometer tests demonstrated that such prompt heat release analysis was effective to optimize the LTC and the split combustion events for better fuel efficiency and exhaust emissions.

The diesel fuel reforming process in an exhaust gas recirculation (EGR) loop of a diesel engine is capable of utilizing the engine exhaust energy to support the endothermic process of hydrogen gas generation. However, the EGR stream commonly needs to be heated to enable the operation of the reformer and thus to sustain higher yield of hydrogen. A central-fuelling and flow-reversal embedment that is energy-efficient to raise the central temperatures of the catalytic flow-bed is therefore devised and tested to drastically reduce the supplemental heating to the EGR reformer. One-dimensional modeling analyses are conducted to evaluate the fuel delivery strategies and temperature profiles of the reformer at various reforming gas flow rates and engine-out exhaust temperatures and compositions. This research attempts to quantify the energy saving by the catalytic flow-reversal and central-fuelling embedment in comparison to a unidirectional flow EGR reformer.

High-dilution stratified EGR combustion system operating at stoichiometric air-fuel ratio (A/F) could offer significant fuel economy saving comparable to the lean burn or stratified charge direct injection SI engines, while still complies with stringent emission standards by using the conventional three-way catalytic converter. The most critical challenge is to keep substantial separation between EGR gas and air-fuel mixture, or to minimize the mixing between these two zones to an acceptable level for stable and complete combustion. Swirl-type stratified EGR and air-fuel flow structure is considered desirable for this purpose, because the circular engine cylinder tends to preserve the swirl motion and the axial piston movement has minimal effect on the flow structure swirling about the same axis. In this study, KIVA3V was used to simulate mixing and combustion processes in a typical pent-roof gasoline engine cylinder during compression and expansion strokes.

Low temperature combustion (LTC) has been proved to overcome the trade-off between NOx and soot emissions in direct injection compression ignition engines. However, the lowered NOx emissions are accompanied by high hydrocarbon and CO emissions. Moreover, the NOx emissions under LTC has much higher NO2 concentrations compared with traditional high temperature combustion conditions. Experimental investigations have been carried out to show the hydrocarbon impact on NOx emissions and NO-NO2 conversion under various engine operation conditions, but the mechanism is less understood. The article includes numerical studies of the impact of hydrocarbons in the in-cylinder conversion of NO to NO2 during low temperature conditions in a compression ignition engine. In the present work, a stochastic reactor model with detailed chemical kinetics is utilized to investigate the reaction pathways during the NOx reduction and NO2 conversion processes.

Port water injection is considered as a promising strategy to further improve the combustion performance of internal combustion engines for its benefit in knock resistance by reducing the cylinder temperature. A thorough investigation of the port water injection technique is required to fully understand its effects on the engine combustion process. This study explores the potential of the port water injection technique in improving the performance of a turbo charged Gasoline Direct Injection engine. A 3D computational fluid dynamics model is applied to simulate the in-cylinder mixing and combustion for this engine both with and without water injection. Different water injection timings are investigated and it is found that the injection timing greatly effects the mass of water which enters the combustion chamber, both in liquid and vapor form.

Engine performance and emission comparisons were made between the use of 100% soy, Canola and yellow grease derived biodiesel fuels and an ultra-low sulphur diesel fuel in the oxygen deficient regions, i.e. full or high load engine operations. Exhaust gas recirculation (EGR) was extensively applied to initiate low temperature combustion. An intake throttling valve was implemented to increase the differential pressure between the intake and exhaust in order to increase and enhance the EGR. The intake temperature, pressure, and EGR levels were modulated to improve the engine fuel efficiency and exhaust emissions. Furthermore, a preliminary ignition delay correlation under the influence of EGR was developed. Preliminary low temperature combustion modelling of the biodiesel and diesel fuels was also conducted. The research intends to achieve simultaneous reductions of nitrogen oxides and soot emissions in modern production diesel engines when biodiesel is applied.