Matthew J. Hall

The Rotating Liner Engine (RLE) is a design concept where the cylinder liner of a heavy-duty Diesel engine rotates at about 2-4 m/s surface speed to eliminate the piston ring and skirt boundary friction near the top and bottom dead center. Two single cylinder engines are prepared using the Cummins 4BT 3.9 platform, one is RLE, the other is baseline (BSL), i.e. conventional. In 2022, we published the test results of the RLE under load, but we lacked detail test data for the baseline. In this new set of experiments, we compare the RLE performance at idle and under load of up to about 7 bar IMEP (indicated mean effective pressure) to the baseline under similar conditions. It has been proven that the elimination of metallic contact between the compression rings and cylinder wall takes place with a liner speed of 1.5-2.3 m/s surface speed (283-426 rpm for the 102 mm bore) for the 850-1280 rpm crankshaft speed. The RLE FMEP is substantially reduced under load, which is a trend opposite to standard engines. The total reduction of FMEP for idle and medium load is measured to be 0.4 and 0.8 bar respectively. When the above results are applied to complete rather than the modified single cylinder engine application, the combined fuel efficiency benefit is approximated to a fuel consumption reduction of 33 % at idle and up to 10 % for medium loads and speeds. Minimization of cylinder and piston ring wear is expected. One significant observation from the research is that the piston rings and skirt boundary friction is a dominating factor in the friction losses of the modern diesel engine. We have not yet operated the two engines under forced air induction, but we expect the RLE benefit to be approximately double the 0.8 bar measured benefit of the naturally aspirated engines. Extrapolating the experimental results to a 20 bar BMEP bring fuel economy improvement to over 7 %.

We investigate the potential of enhancing small (<20 kg) drone endurance by exploiting the high energy density of hydrocarbons using a prototype generator based on commercial-off-the-shelf (COTS) thermoelectric energy conversion technology. A proof-of-concept prototype was developed to vet design and engineering challenges and to bolster validity of resultant conclusions. The combination of the prototype performance and modeling suggests that endurance augmentation remains a difficult technical challenge with no clear immediate remedy despite many expectant alternatives. Across a sample of representative drones including ground- and air-based, multicopter and fixed wing drones, we report the following: from their current maximum values of 12%, thermoelectric (TE) generator module efficiencies must increase by over two times to achieve endurance parity with lithium batteries for VTOL multicopters. On the other hand, current TE efficiencies can compete with lithium batteries for some low power fixed wing and ground-based drones. Technical contributors for these results include weight of non-energy contributing components, low specific power and the associated tradeoff between specific power and specific energy due to fuel mass fraction, and lastly, low efficiencies.

A computational fluid dynamics (CFD) model was coupled with an advanced statistical strategy combining the response surface method (RSM) and the propagation of error (PoE) approach to optimize and test the robustness of the co-firing of ammonia (NH3) and coal in a fluidized bed reactor for coal phase-out processes. The CFD model was validated under experimental results collected from a pilot fluidized bed reactor. A 3k full factorial design of nine computer simulations was performed using air staging and NH3 co-firing ratio as input factors. The selected responses were NO, NH3 and CO2 emissions generation. The findings were that the design of experiments (DoE) method allowed for determining the best operating conditions to achieve optimal operation. The optimization process identified the best-operating conditions to reach stable operation while minimizing harmful emissions. Through the implementation of desirability function and robustness, the optimal operating conditions that set the optimized responses for single optimization showed not to always imply the most stable set of values to operate the system. Robust operating conditions showed that maximum performance was attained at high air staging levels (around 40%) and through a balanced NH3 co-firing ratio (around 30%). The results of the combined multi-optimization process performance should provide engineers, researchers and professionals the ability to make smarter decisions in both pilot and industrial environments for emissions reduction for decarbonization in energy production processes.

<div class="section abstract"><div class="htmlview paragraph">A multi-dimensional cathode spot generation model is proposed to study the interaction between the plasma arc and cathode surface of a spark plug during the ignition process. The model is focused on the instationary (high current) arc phase immediately following breakdown, and includes detailed physics for the phenomena during spot formation such as ion collision, thermal-field emission, and metal vaporization, to simulate the surface heat source, current density and surface pressure. The spot formation for a platinum cathode is simulated using the VOF (volume of fluid) model within FLUENT, where the local metal is melted and deformed by pressure differences on the surface. A random walk model has been integrated to consider the movement of the arc center, resulting in the formation of different types of spots. The simulation results show: it takes approximately 100 ns for the arc to discharge the electric charge of the spark plug side capacitance and form the spot in the instationary arc phase; the ion collisions are the dominant heating source for the spot generation, and thermal-field emission of electrons is the dominant process for current density and surface cooling rather than conduction to the metal cathode. The moving radius in the random walk model determines the different spot types and the surface profiles. The effects of different working conditions (pressure and temperature) are presented from simulation results while keeping breakdown voltage the same, and pressure has a very strong influence on spot formation. A preliminary estimation for the erosion rate due to oxidation is provided in the paper and its importance in different phases during ignition is discussed.</div></div>

The cold start process for a gasoline direct injection (GDI) engine was studied through multidimensional simulations using a Fractal Engine Simulation (FES) model integrated into CONVERGE CFD. The simulations were focused on the very first firing cycle in the cold start process, as most of the hydrocarbon emissions derive from this cycle. The dramatically changing engine speed and low wall temperatures in the cylinder present challenges for simulation. It turned out that the FES model was able to predict the in-cylinder pressure traces for all four cylinders for the first firing cycle, and gave good agreement with the experimental measurements under these extremely transient conditions. More comprehensive analysis demonstrated the causes for the different behavior of the combustion in different cylinders: more injected fuel and a higher induced tumble ratio in cylinder 3, the first to fire, led to a higher average equivalence ratio and better fuel distribution, which resulted in the highest peak pressure; the worse fuel distribution from the lower tumble ratio, together with the lower normalized turbulence, caused weaker combustion in cylinder 4; the peak pressures were similar and on the low side for cylinders 2 and 1, mainly determined by the low average equivalence ratio. An improved strategy with multiple injections was proposed, with the first two in the intake stroke and two later injections in the compression stroke rather than one injection during each stroke. Although the total injected fuel was the same as the baseline strategy, more fuel evaporated due to the enhanced fuel evaporation from both droplets and wall films, resulting in a higher average equivalence ratio in the cylinder. The peak cylinder pressure and IMEP rose by 33.3% and 8.4%, respectively, using the 4-injection strategy compared to the baseline 2-injection strategy, and the 4-injection strategy was verified by the experiments, as well.

<div>Laminar flame properties embody the fundamental information in flame chemistry and are key parameters to understanding flame propagation. The current study focuses on two parameters: the unstretched laminar flame speed (LFS) and <i>ϕ<sub>m</sub> </i> (the equivalence ratio at which the LFS reaches its maximum). Most existing correlations for LFS are either only applicable within a narrow range of conditions or built on a large number of coefficients. Few correlations are available for <i>ϕ<sub>m</sub> </i>. Thus, the objectives of the current study are to provide accurate, while concise, correlations for both properties for a wide range of working conditions in internal combustion (IC) engines, including dilution effects. The original results were obtained for iso-octane and gasoline surrogates from one-dimensional (1D) simulations for a range of 300–950 K for unburned temperature, 1–120 bar for system pressure, 0.6–1.4 for equivalence ratio, and 0–0.5 for diluent mass fraction, and then were correlated using an improved power law method and an improved Arrhenius form method. Comparisons with the literature show that the predicted LFSs from both methods and <i>ϕ<sub>m</sub> </i>s are close to the experimental measurements for a wide range of conditions. The predicted dilution factor has a similar trend with others, but fewer coefficients are needed. Overall, the improved Arrhenius form is recommended to calculate the LFS for future use, considering its lower standard errors. The experimental measurements at very high temperatures and pressures are limited, and thus the predictions under these conditions need further validation.</div>

A fractal engine simulation (FES) sub-model was integrated into three-dimensional simulations for modeling turbulent combustion for a gasoline direct injection (GDI) engine. The FES model assumes that the effects of turbulence on flame propagation are to wrinkle and stretch the flame, and fractal geometry is used to predict the surface area increase and thus the turbulent burning velocity. Different formulas for the four sequential stages of combustion in SI engines are proposed to account for the changing effects of turbulence throughout the combustion process. However, most prior studies related to the FES model were quasi-dimensional simulations, with few found in multi-dimensional studies, and none under cold start conditions or stratified charges. This paper describes how the model was implemented into multidimensional simulations in CONVERGE CFD, and what the formulas are in the four sequential stages of combustion in SI engines. The capabilities of the FES model for simulating the cold start cases, under the conditions of the dramatically changing engine speed and mixture stratification in a complex engine geometry, are presented in this study. The FES model was able to not only simulate the steady-state cases with constant engine speed, but also predict the in-cylinder pressure traces in all four cylinders for the very first firing cycle with transient engine speed, and gave good agreement with the experimental measurements under these extremely transient conditions. The uncertain maximum fractal dimension was chosen as 2.37 in this research, and a simple linear correlation with engine speed was used to obtain the coefficient used in calculating the kernel formation time which controls the so-called combustion or ignition delay.

<div class="section abstract"><div class="htmlview paragraph">A multi-dimensional model of the spark ignition process for SI engines was developed as a user defined function (UDF) integrated into the commercial engine simulation software CONVERGE CFD. The model simulates spark plasma movement in an inert flow environment without combustion. The UT model results were compared with experiments for arc movement in a crossflow and also compared with calorimeter measurements of thermal energy deposition under quiescent conditions. The arc motion simulation is based on a mean-free-path physical model to predict the arc movement given the contours of the crossflow velocity through the gap and the interaction of the spatially resolved electric field with the electrons making up the arc. A further development is the inclusion of a model for the thermal energy deposition of the arc as it is stretched by the interaction of the flow and the electric field. A novel feature of this model is that the thermal energy delivered to the gap at the start of the simulation is distributed uniformly along the arc rather than at discrete points along the arc, as is the case with the default CONVERGE CFD ignition models. This feature was found to greatly reduce the tendency of the arc to distort its shape and tangle itself in a non-physical way, as is the tendency when discrete energy input is used. It was found that the tangled distortion of the arc when using discrete energy input was due to perturbations along the arc caused by differential expansion of the gas along groups of adjacent mesh cells that either had energy input or did not. The distributed energy feature also gave arc temperature distributions that were more spatially uniform and had steeper temperature gradients, consistent with experimental arc images. The results are compared with experimental high-speed video images of arc movement for a spark plug of similar geometry and taken over a range of pressures and crossflow velocities in a high-pressure constant volume vessel. There is good agreement between the simulations and experimental images for the arc stretch distance in response to a crossflow. The simulations did not display as much lateral arc dispersion as seen in the experimental results, however, that were perhaps associated with flow recirculation zones downstream of the gap, present in the experiments. The influence of the electric field was shown by turning off the electric field effect in the simulations such that the arc movement was influenced by the flow field alone. The effect of the electric field was found to be more pronounced at lower crossflow velocities of 5 m/s and at lower pressures.</div></div>

A parametric study was carried out for the first firing cycle of a 4-cylinder, 2.0-liter, turbocharged gasoline direct injection (GDI) engine. The primary goal was to see how changes in the fuel injection parameters would affect the GDI engine combustion and emissions for the first four combustion events that constitute the first firing cycle. Experimental studies were carried out with a custom-designed powertrain control system to measure the HC emissions and pressure development for the first firing cycle. The quantitative experimental results were accompanied by simulations of the detailed temporal and spatial fuel concentration profiles using Converge CFD engine simulation software. An alternative calculation method was used to calculate the average combustion equivalence ratio for each of the four cylinders. This method showed that the majority of the cold start HC emissions during the first firing cycle was unburned gasoline and its possible decomposition products, which did not contribute significantly to the combustion and heat release. For the same amount of fuel injected into a cylinder, increased fuel rail pressure resulted in better evaporation and combustion, while slightly increasing the HC emissions during the cold start process. A multiple injection strategy was studied that split the fuel delivery between the intake stroke and the compression stroke with either one or two injections in each of those strokes (two or four injections total). The quadruple injection strategy led to better first cycle combustion, with higher engine IMEP and lower HC emissions. This resulted from a richer fuel mixture in the region near the spark plug due to better fuel evaporation and a better spatial fuel distribution. While increasing fuel rail pressure with either injection strategy failed to significantly lower the HC emissions given the same amount of injected fuel mass, higher rail pressure with the quadruple injection strategy resulted in higher IMEP for the same amount of injected fuel; this may provide the possibility to reduce the total fuel injection mass which may have benefits for both fuel consumption and emissions.