In the engine community, gasoline compression ignition (GCI) engines are at the forefront of research and efforts are being taken to commercialize an optimized GCI engine in the near future. GCI engines are operated typically at Partially Premixed Combustion (PPC) mode as it offers better control of combustion with improved combustion stability. While the transition in combustion homogeneity from convectional Compression Ignition (CI) to Homogenized Charge Compression Ignition (HCCI) combustion via PPC has been comprehensively investigated, the physical and chemical effects of fuel on GCI are rarely reported at different combustion modes. Therefore, in this study, the effect of physical and chemical properties of fuels on GCI is investigated. In-order to investigate the reported problem, low octane gasoline fuels with same RON = 70 but different physical properties and sensitivity (S) are chosen. Fuels with comparable sensitivity and RON are chosen to study the impact of physical properties on GCI. On the other hand, by keeping the same RON and physical properties, the effect of sensitivity on GCI is investigated. In this regard, three test fuels such as RON 70 gasoline (S=0.7), PRF 70 (S=0) and RON 70 gasoline (S=7) are chosen in the present study. Herein, RON 70 gasoline (S=0.7) and PRF 70 have similar RON and sensitivity but different physical properties; however, RON 70 gasoline (S=0.7) and RON 70 gasoline (S=7) have the same RON and physical properties but different sensitivity. These test fuels were tested in a heavy-duty CI engine at a compression ratio of 17.8 under different combustion modes. The experimental investigation reveals that RON 70 gasoline (S=0.7) and PRF 70 (S=0) behaves the same in terms of combustion behavior (combustion phasing, ignition delay, in cylinder pressure and rate of heat release) regardless of the difference in physical properties. While nitrogen oxide (NOX) and soot emissions are comparable between RON 70 gasoline (S=0.7) and PRF 70 at all combustion modes, the hydrocarbon (HC) and carbon monoxide (CO) emissions are slightly higher for PRF 70 when compared to RON 70 gasoline (S=0.7) at HCCI mode but not at PPC and CI modes due to the impact of physical properties. On the other hand, due to higher sensitivity, the reactivity for RON 70 gasoline (S=7) is improved to advance the combustion phasing at HCCI combustion mode when compared to RON 70 gasoline (S=0.7). At HCCI mode, the HC emissions are lower for high sensitive gasoline when compared to low sensitive gasoline whereas they are comparable at PPC and CI combustion modes. The NOX and soot emissions are comparable at HCCI modes whereas high sensitivity gasoline shows slightly decreased NOX and increased soot emissions, respectively, at PPC and CI combustion modes when compared to low sensitive gasoline.
An, Yanzhao; Shi, Hao; Vallinayagam, R; Sim, Jaeheon; Chang, Junseok; Johansson, Bengt(SAE Technical Paper Series, SAE International, 2019-04-02)[Conference Paper]
Partially premixed combustion (PPC) is a low-temperature combustion concept, which is between conventional diesel compression ignition (CI) and homogeneous charge compression ignition (HCCI). In PPC mode, the start of injection timing (SOI) is earlier than that of CI and later than that of HCCI and stratified in-cylinder fuel/air mixture can be formed to control the auto-ignition by the fuel injection timing. Gasoline fuel is beneficial for PPC mode because of its superior resistance to auto-ignition, which can enhance fuel-air charge mixing process with longer ignition delay time. The scope of this study is to investigate in-cylinder auto-ignition, combustion evolution, combustion stratification, and engine-out emissions at PPC operating mode under lean and low load engine conditions with different injection timings. Primary reference fuel PRF77, was selected as the low-octane test fuel. Fuel-tracer planar laser-induced fluorescence (PLIF) imaging and high-speed color imaging based on natural flame luminosity were performed to visualize fuel injection, spray-wall interaction, and subsequent combustion evolution. Based on the intensity of high sped combustion images, combustion stratification and dynamic flame tracing were evaluated to gain insights into the combustion evolution. Combustion stratification analysis indicated that more inhomogeneous low-temperature combustion was achieved at earlier fuel injection timings along with decreased natural flame luminosity and increased soot emission. Fuel-trapping in piston crevice zone was visualized by fuel-tracer PLIF. Fuel-trapping in squish zone and crevice zone was measured and linked to the formation of unburned hydrocarbon when stronger spray-wall interaction occurs under PPC operating mode. Injector dribbling during the late stage of combustion was found to be as an important source of soot formation through high-speed color imaging and dynamic flame tracing analysis.
This paper compares the greenhouse gas (GHG) emissions attributed to driving a popular production vehicle powered by an internal combustion engine (ICE), as well as a hybrid electric vehicle (HEV), with GHG emissions associated with walking, running and bicycling. The purpose of this study is to offer a different perspective on the problem of global warming due to anthropogenic causes, specifically on transportation and eating patterns. In order to accurately estimate emissions, a full life cycle of food has been considered coupled with energy expenditures of the aforementioned activities obtained from several different sources and averaged for more reliable results. The GHG emissions were calculated for Sweden, the UK, and the US. Depending on the availability of certain data, the methodology for different countries was altered slightly. The question whether walking, running or taking a bicycle is better for the environment than driving a car cannot be answered uniquely. This study demonstrates that the answer depends on several factors, such as diet composition, the number of people commuting, vehicle powertrain, as well as the country analyzed. The conclusion is that if one has an eco-friendly diet and travels alone the preferred modes of transport would be bicycling, walking and running, the cleanest of which by far is bicycling. However, if the diet has a higher CO2 footprint, as in the case of diets containing a large amount of meat and/or imported products, then the preference shifts towards cars, among which the most environmentally friendly are hybrid electric vehicles. The same conclusion applies to the cases where the number of people commuting together exceeds two-three persons.
A new approach for investigating combustion behavior of practical fuels under homogeneous charge compression ignition (HCCI) conditions was developed with the help of a cooperative fuel research (CFR) engine. The method uses a set of two pressure-temperature diagrams and two charts, each with an octane number scale based on primary reference fuels (PRF), created from experimental results by sweeping the intake temperature. The two pressure-temperature diagrams report conditions leading to the start of the low temperature combustion and the start of the main combustion, respectively. Additional two charts - required compression ratio and fraction of low temperature heat release charts - describe global combustion behavior and the importance of the low temperature combustion. Each diagram and chart, together with their respective octane number scale, allow to examine the combustion behavior of practical fuels by comparing their combustion behavior with those of the PRFs. Finally, octane numbers representing the various combustion behaviors of a practical fuel can be rated. Application of the method with two low-octane number surrogate fuels led to the following main results. The required compression ratio chart provides a quick description of the combustion behavior. The pressure-temperature diagrams indicate the ease with which a fuel ignites under low temperature combustion and main combustion regimes. An extra pressure-temperature diagram reports start and end of the negative temperature coefficient regime and highlights that this regime is independent of the fuel. Accordingly, each combustion regime is clearly defined in the pressure-temperature diagram. The fraction of low temperature heat release finally indicates how low temperature combustion vanishes. Finally, octane numbers for each practical fuel were rated from each diagram and chart. Rated octane numbers suggest that a single PRF cannot reflect the entire combustion behavior of a practical fuel; but multiple PRFs are required for HCCI combustion.
Singh, Eshan; Hlaing, Ponnya; Shi, Hao; Dibble, Robert W.(SAE Technical Paper Series, SAE International, 2019-04-02)[Conference Paper]
Pre-ignition is an abnormal engine combustion phenomenon where the inducted fuel-air charge ignites before the spark ignition. This premature combustion phenomenon often leads to heavy knocking events. The mixture preparation plays a critical role in pre-ignition tendency for a given load. Literature shows efforts made towards improving pre-ignition-limited-IMEP by splitting the injection pulse into multiple pulses. In this study, two direct injectors are used in a single cylinder research engine. A centrally mounted direct injector was used to inject Coryton Gasoline (RON 95) fuel early in the intake stroke. A second fluid was injected late in the compression stroke to suppress pre-ignition. The fluids used in the second direct injector was varied to see the effects of the molecule and its physical and chemical property on pre-ignition suppression tendency. Methanol, ethanol, water, and gasoline were tested as second fluid. Engine tests were conducted at 2000 rpm and at an intake pressure of 2.1 bar (abs). Although alcohols show high pre-ignition tendency as fuels, they were most effective at pre-ignition suppression when injected later in the compression stroke. The pre-ignition suppression led to a decrease in IMEP and an increase in cycle-to-cycle variation. Water injection was highly effective at maintaining peak IMEP values. Water injection was further explored for pre-ignition suppression. The water injection helped reduce pre-ignition count when injected at two different injection times each in intake, compression and late exhaust stroke.
Singh, Eshan; Dibble, Robert W.(SAE Technical Paper Series, SAE International, 2019-04-02)[Conference Paper]
Pre-ignition in modern engines is largely attributed to oil-fuel mixture droplets igniting before the spark timing. Researchers have also found pre-ignition events to be triggered by high hydrocarbon emissions from the previous cycle as well as late spark timing in the previous cycle. Additionally, an ideally scavenged engine was not found to be limited by pre-ignition. These observations point to a significant role of residuals in triggering pre-ignition events. Current work studies pre-ignition in a probabilistic approach. The effect of residuals and in-cylinder thermodynamic state is studied by varying the exhaust back pressure and intake air temperature respectively. Experiments were performed with a fixed mass flow rate of air + fuel and intake air temperature while the exhaust back pressure was varied. Intake air pressure varied in response to fixed intake temperature. Pre-ignition and super-knock count increased with increasing exhaust back pressure. In the next set of experiments, mass flow rate of air + fuel and intake air pressure were fixed, while the exhaust back pressure was varied. Intake air temperature was varied to fix the intake air pressure constant. Pre-ignition counts generally increased with increasing intake temperature, although the exhaust back pressure decreased. Number of super-knock cycles correlated directly with intake air temperature. Conclusively, the current study shows that probability of a pre-ignition event relies on (a) the likelihood of precursor generation (from fuel impinging the liner), (b) the likelihood of precursors being held back in cylinder (related to exhaust back pressure) and (c) the reactivity of bulk mixture (related to in-cylinder temperature).
Nyrenstedt, Gustav; Alturkestani, Tariq Lutfallah Mohammed; Im, Hong G.; Johansson, Bengt(SAE Technical Paper Series, SAE International, 2019-01-15)[Conference Paper]
Earlier studies on efficiency improvement in CI engines have suggested that heat transfer losses contribute largely to the total energy losses. Fuel impingement on the cylinder walls is typically associated with high heat transfer. This study proposes a two-injector concept to reduce heat losses and thereby improve efficiency. The two injectors are placed at the rim of the bowl to change the spray pattern. Computational simulations based on the Reynolds-Averaged Navier-Stokes approach have been performed for four different fuel injection timings in order to quantify the reduction in heat losses for the proposed concept. Two-injector concepts were compared to reference cases using only one centrally mounted injector. All simulations were performed in a double compression expansion engine (DCEE) concept using the Volvo D13 single-cylinder engine. In the DCEE, a large portion of the exhaust energy is re-used in the second expansion, thus increasing the thermodynamic efficiency. To isolate the heat losses associated with the changed spray pattern of the two-injector concept, effects of the heat release are excluded during the analysis. Results showed that the optimal injection strategy allows a decrease in the temperature close to the walls, leading to heat loss reduction up to 13 % or 2 % of the fuel energy. The residual exhaust energy was increased by 1.5 %-points with the two-injector concept when compared to the reference case. This proved the advantage of the two-injector concept compared to conventional single injector case for the DCEE application.
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