Squish Volume Research Articles

Optical investigation of methane, ethane, or propane and diesel at low load RCCI conditions

Natural luminosity (NL) imaging diagnostics were employed to investigate the effects of natural gas (NG) components on reactivity-controlled combustion, using an ultra-low sulfur diesel fuel and three NG major components (methane, ethane, and propane). The experiments were performed in a single-cylinder heavy-duty optical compression-ignition (CI) engine at 500 bar diesel injection pressure, 1000 RPM, ∼ 6.6 bar IMEP, and ∼ 64% energy-based diesel substitution ratio. NL confirmed the premixed nature of the combustion, which started inside the squish area and propagated towards the center of the combustion chamber. The high intensity NL regions that appeared towards the end of the premixed combustion were the result of the diffusive combustion of end-of-injection fuel dribble or of the diesel fuel that condensed on the bowl window surface or accumulated in the bowl straight corners (typical of an optical engine piston). The time it took for the spatially integrated NL (SINL) to decrease from its maximum to its minimum was similar for all investigated cases, suggesting that the gas addition did not change the fraction of the diesel fuel that participated in the diffusion-controlled combustion. Differences in oxidation properties between NG components affected the RCCI combustion at these low load conditions. Propane RCCI was characterized by 1 crank angle (CA) to 3 CA more advanced combustion phasing (despite an increased ignition delay) and higher SINL compared to methane and ethane RCCI. However, as NL for the propane RCCI was lower at similar combustion phasing, it suggests that propane addition created a more stratified diesel-gas mixture, with a richer mixture inside the squish volume, supported by faster and more intense combustion compared to the other two RCCI cases. As a result, the flame speed slowed as it progressed from the squish volume towards the center of the combustion chamber, therefore the delay in capturing the premixed combustion in the NL images. The amount of fuel that burned after the premixed combustion completely engulfed the piston bowl was relatively low compared to fraction of the fuel that burned before it. Finally, the results suggest the importance of tailoring the low–high fuel reactivity ratio of the dual-fuel combustion based on the operating condition.

Effects of piston shape and nozzle specifications on part-load operation of natural gas–diesel dual-fuel RCCI engine and its application to high load extension

• Effects of piston shape and multi-angle nozzle were evaluated for DF RCCI operation. • Enhanced air utilization w/multi-angle nozzle reduced combustion and HT losses. • The highest GIE of RCCI improved as 51.8 % by hardware modifications. • Intake air amount could be increased by lowering CR (from 17 to 15) at the maximum load. • The maximum load of RCCI could be extended from 81 (reference) to 101 kW of BP. In this study, the effects of modifications of both the piston shape and injector nozzle are experimentally investigated for high load extension of a natural gas–diesel dual-fuel reactivity-controlled compression ignition (RCCI) engine. First, a bathtub shape with a reduced compression ratio is applied to the modified piston which was reported to improve the flame propagation in squish volume, the cooling loss, and the air induction rate under the same limit of in-cylinder pressure from previous studies. In addition, a multiangle two-layer injector is used to extend the maximum load, which has been proven to be effective for unburned hydrocarbon emissions and gross indicated efficiency in part-load operation in our previous study. The experiments are conducted under both 1200 rpm part-load operation (approximately 1950 J/cylinder per cycle of fuel input energy) and 1800 rpm maximum load operation. The objectives are to comprehensively evaluate the effects of the modifications on natural gas–diesel RCCI operation and verify the effects of all load conditions. The experimental results reveal that the changes in the piston—reduction in the compression ratio and a bathtub piston shape—are effective for both NOx and smoke emissions as the diesel injection timing is advanced. Although the multi-hole double layer (combined narrow and wide injection) has less effect on emissions compared to that of the change in piston, it affects the mixing process inside the cylinder, thus retarding the combustion phasing, prolonging the combustion duration, and eventually reducing the cooling loss. Both modifications are highly effective for advanced diesel injection timing corresponding to RCCI operation, which extends high-load operation from 80.9 to 101 kW of brake power, relatively 24.8 % increase, by adopting both the piston and injector modifications. Based on the modifications, the mass fraction of natural gas is suggested to be approximately 80 %. A higher fraction causes the variation in the maximum in-cylinder pressure to be extremely sensitive to allow robust operation. In contrast, if it is lower, the amounts of both NOx and smoke emissions exceed the emission standard levels (0.4 g/kWh and 15 mg/kWh for NOx and smoke emissions, respectively, corresponding to Stage-V of non-road engine operation).

Optical and Numerical Investigation of Flame Propagation in a Heavy Duty Spark Ignited Natural Gas Engine With a Bowl-in-Piston Chamber

Abstract Increasing the natural gas (NG) use in heavy-duty engines is beneficial for reducing greenhouse-gas emissions from power generation and transportation. However, converting compression ignition (CI) engines to NG spark ignition operation can increase methane emissions without expensive aftertreatment, thereby defeating the purpose of utilizing a low carbon fuel. The widely accepted explanation for the low combustion efficiency in such retrofitted engines is the lower laminar flame speed of natural gas. In addition, diesel engine's larger bowl size compared to the traditional gasoline engines increases the flame travel length inside the chamber and extends the combustion duration. Optical measurements in this study suggested a fast-propagating flame developed even at extremely lean operation. A three-dimensional numerical simulation showed that the squish region of the bowl-in-piston chamber generated a high turbulence intensity inside the bowl. However, the flame propagation speed reduced by 55% when transiting from the bowl to the squish region, due to the large decrease in turbulence intensity inside the squish region. Moreover, the squish volume trapped an important fuel fraction, which experienced a slow and inefficient burning process during the expansion stroke. This resulted in increased methane emissions and reduced combustion efficiency. Overall, it was the specifics of the combustion inside a bowl-in-piston chamber not the methane's slow laminar flame speed that contributed to the low methane combustion efficiency for the retrofitted engine. The results suggest that optimizing the chamber shape is paramount to boost engine efficiency and decrease its emissions.

Effect of Piston Crevices on the Numerical Simulation of a Heavy-Duty Diesel Engine Retrofitted to Natural-Gas Spark-Ignition Operation

Three-dimensional computational fluid dynamics internal combustion engine simulations that use a simplified combustion model based on the flamelet concept provide acceptable results with minimum computational costs and reasonable running times. Moreover, the simulation can neglect small combustion chamber details such as valve crevices, valve recesses, and piston crevices volume. The missing volumes are usually compensated by changes in the squish volume (i.e., by increasing the clearance height of the model compared to the real engine). This paper documents some of the effects that such an approach would have on the simulated results of the combustion phenomena inside a conventional heavy-duty direct injection compression-ignition engine, which was converted to port fuel injection spark ignition operation. Numerical engine simulations with or without crevice volumes were run using the G-equation combustion model. A proper parameter choice ensured that the numerical results agreed well with the experimental pressure trace and the heat release rate. The results show that including the crevice volume affected the mass of a unburned mixture inside the squish region, which in turn influenced the flame behavior and heat release during late-combustion stages.

Lean-Burn Characteristics of a Heavy-Duty Diesel Engine Retrofitted to Natural-Gas Spark Ignition

Increased utilization of natural gas (NG) in the transportation sector can decrease the use of petroleum-based fuels and reduced greenhouse gas emissions. Heavy-duty diesel engines retrofitted to NG spark ignition (SI) can achieve higher efficiencies and low NOX, CO, and hydrocarbon (HC) emissions when operated under lean-burn conditions. To investigate the SI lean-burn combustion phenomena in a bowl-in-piston combustion chamber, a conventional heavy-duty direct-injection CI engine was converted to SI operation by replacing the fuel injector with a spark plug and by fumigating NG in the intake manifold. Steady-state engine experiments and numerical simulations were performed at several operating conditions that changed spark timing (ST), engine speed, and mixture equivalence ratio. Results suggested a two-zone NG combustion inside the diesel-like combustion chamber. More frequent and significant late-burn (including double-peak heat release rate) was observed for advanced ST. This was due to the chamber geometry affecting the local flame speed, which resulted in a faster and thicker flame in the bowl but a slower and thinner flame in the squish volume. Good combustion stability (COVIMEP < 3%), moderate rate of pressure-rise, and lack of knocking showed promise for heavy-duty CI engines converted to NG SI operation.

Optimization of the injection parameters of a diesel/natural gas dual fuel engine with multi-objective evolutionary algorithms

In this study, the injection parameters of a High-Pressure Direct-Injection (HPDI) natural gas engine was optimized by coupling a multi-dimensional computational fluid dynamics (CFD) code with the genetic algorithm (GA). Moreover, the analysis of the combustion and emission characteristics of a diesel/natural gas dual fuel engine are performed in this paper with the introduction of the diesel/natural gas dual fuel injection model into the CFD code. The result reveals that the developed gas fuel injection model is capable to predicate the penetrations of the natural gas injection. For the dual fuel engine with simultaneous reduction of NOx and soot, the optimized diesel injection timing is at −15.2 °CA ATDC, the gas injection timing is around −6.0 °CA ATDC, and the circumferential offset angle between the diesel and natural gas nozzle hole is around 8.8°. The lowest ISFC condition is achieved when the injection interval between the diesel and natural gas is 1.38 °CA. For the high soot emissions cases, the gas injection timings are later compared with the other cases. In the dual fuel engine, most of the NO emissions are generated through the combustion of the direct injected natural gas. Soot formation region is mainly located in the squish volume close to the cylinder walls, where the temperature is low which leads to a lower soot oxidation rate.

Experimental and numerical study of the pollution formation in a diesel/CNG dual fuel engine

A dual fuel combustion model comprising a modified turbulent flame speed closure (TFSC) model and a partially premix reactor model is developed and incorporated into the KIVA-3V code to simulate the combustion processes within the dual fuel engine. Numerical results are validated by the cylinder pressures, heat release rates and emissions of the corresponding experimental data. The effects of engine speeds on the combustion process of dual fuel engines are investigated and the spatial and temporal distributions of the in-cylinder temperature and pollutions are compared. The results reveal that most of the pilot fuel vapor remains within the bowl, and the combustion of the pilot fuel forms a high temperature region. Moreover, some of pilot fuel vapor enters the squish volume, and forms a high temperature region above the piston top. NO formation region follows the development of the high temperature field, which is generated by the combustion of the pilot diesel. At low engine speed, the piston crevices are largely responsible for the creation of the unburned CH4 emissions. However, at high engine speed, bulk gas partial oxidation in the cylinder center region is the major source of the CH4 emissions. The flame quench zone in the squish volume close to the cylinder walls is the main source of the CO emission at low engine speed. However, the bulk gas partial oxidation zone in the cylinder center region is the major source of the CO emission at high engine speed.

The influence of fuel injection and heat release on bulk flow structures in a direct-injection, swirl-supported diesel engine

Particle image velocimetry is applied to measure the vertical (r–z) plane flow structures in a light-duty direct-injection diesel engine with a realistic piston geometry. The measurements are corrected for optical distortions due to the curved piston bowl walls and the cylindrical liner. Mean flow fields are presented and contrasted for operation both with and without fuel injection and combustion. For operation with combustion, the two-dimensional divergence of the measured mean velocity fields is employed as a qualitative indicator of the locations of mean heat release. In agreement with numerical simulations, dual-vortex, vertical plane mean flow structures that may enhance mixing rates are formed approximately mid-way through the combustion event. Late in the cycle a toroidal vortex forms outside the bowl mouth. Imaging studies suggest that soot and partially oxidized fuel trapped within this vortex are slow to mix with surrounding fluid; moreover, the vortex impedes mixing of fluid exiting the bowl with air within the squish volume.

Combined measurements of flow structure, partially oxidized fuel, and soot in a high-speed, direct-injection diesel engine

The evolution of bulk flow structures and their influence on the spatial distribution of heat release zones and of partially oxidized fuel and particulate matter (soot) is examined experimentally in a swirl-supported, direct-injection diesel engine. Vector fields describing the bulk flow structures are measured with particle image velocimetry (PIV), while complementary scalar field measurements of partially oxidized fuel and soot are obtained in the same vertical plane using broadband laser-induced fluorescence (LIF) and laser-induced incandescence (LII) techniques, respectively. The two-dimensional divergence of the mean velocity fields is also employed to provide information on the mean locations of heat release. Measurements are performed at a highly dilute, 12% O 2, operating condition characteristic of low-NO x , low-temperature diesel combustion systems. The spatial distributions of unburned fuel rapidly develop a structure characterized by two separate zones of high fuel concentration, an inner zone in the cylinder center and an outer zone in the squish volume. Single-cycle measurements show that this two-zone structure is present on an individual cycle basis, and is not an artifact of averaging distinct, single-zone distributions. For this engine build, the mean flow structures developed do not actively promote mixing of either zone, although bulk flow structures in the upper-central region of the cylinder vary significantly on a cycle-by-cycle basis. The measured spatial distributions of particulates indicate that particulates are formed primarily in the inner zone—and remain un-oxidized late in the cycle.

A Parametric Study of Low-Temperature, Late-Injection Combustion in a HSDI Diesel Engine

A parametric study of automotive diesel combustion in a low-temperature, late-injection combustion regime is described. Injection pressure was varied from 600-1200 bar, swirl ratio from 1.44-7.12, and intake temperature from 30-110°C. In-cylinder pressure records, heat release analysis, spatially-integrated soot luminosity, and images of the spatial distribution of combustion luminosity are employed to study the influence of these parameters on the combustion and soot formation/oxidation processes. Load points of 3 and 6 bar gross IMEP at 1 500 RPM and an O 2 concentration of 0.15 are considered. Increased injection pressure is found to enhance the early mixture formation process, resulting in increased peak apparent heat release, generally decreased soot luminosity, and modestly increased light-load soot oxidation rates. At lower injection pressures, more soot luminosity is observed from the squish volume. In contrast, variation of flow swirl impacts the latter half of the combustion process, and affects the initial combustion only slightly. An optimum Ricardo swirl ratio of roughly 3 is found for best moderate-load efficiency and soot oxidation. A marked reduction in early heat release rates and peak soot luminosity is observed with decreased intake temperature. Nevertheless, significant in-cylinder soot luminosity is observed even at the lowest intake temperatures, indicating that complete suppression of in-cylinder soot formation is difficult with the fuel injection and combustion system characteristics employed.