Marine Dual-fuel Engine Research Articles

Prediction and optimization of dual-fuel marine engine emissions and performance using combined ANN with PSO algorithms

With the increasingly stringent environmental issues and regulations, there are higher requirements for improving engine performance and reducing pollution. Combining artificial neural network and particle swarm optimization algorithm to optimize the fuel consumption and emissions for micro-ignition dual-fuel engines. A model-based calibration scheme is maintained to reduce the number of experimental points by employing space-filling and V optimization design, to save the experimental cost and improve efficiency. The experimental data used to establish an RBF neural network prediction model that achieves a perfect mapping of engine input and output parameters. Controllable variables such as speed, torque, main injection timing, pilot injection timing, pilot injection quantity, rail pressure, excess air coefficient, and substitution rate limit parameters are input as neural networks. Subsequently, the combination of control parameters was optimized through PSO, thereby to achieve fuel consumption and emissions trade-off. Matching experiment results show actual emissions of NOx, THC, and CO decreased by 20.5%, 30.3%, and 43.1%, respectively, and the BSFC declined by an average of 2.1% contrasted with the original data. It achieves the optimum of emission and fuel consumption at the same time.

Thermodynamic Modeling of a Turbocharged Diesel–Hydrogen Dual-Fuel Marine Engine

In this work, a thermodynamic model is used to analyze the influence of some usual engine parameters such as compression ratio, ambient temperature, turbocharger compressor pressure ratio, equivalence ratio, and engine speed on the performances of a diesel–hydrogen dual-fuel marine engine. The model takes into consideration the change in the composition of the working fluid resulting from the combustion cycle as well as the reactive mixture. This model considers also the dependence of the working fluid specific heat on temperature. Results show that the increase in the equivalence ratio yields to an improvement of the brake specific fuel consumption, the brake power output, the brake thermal, and exergy efficiencies with an increase in the exhaust gas temperature. Nevertheless, it produces a drop in volumetric efficiency. The engine speed has a slight effect on the brake thermal efficiency, exergy efficiency, and the brake specific fuel consumption. The engine speed affects the engine power output mainly at higher values of the fuel–air equivalence ratio due to the reduction in heat losses and the fuel energy consumed by the engine. The increase in the turbo-compressor pressure ratio has a significant effect on the improvement of engine performance. However, it has an insignificant effect on the exhaust gas temperature. The exergy destroyed in the engine dominates. It accounts for 88.2% of the total exergy destroyed in the overall system. The remaining components (turbo-compressor, intercooler, mixer, catalytic converter, and turbine) are responsible for only 11.2%.

Advanced exergo-economic schemes and optimization for medium–low grade waste heat recovery of marine dual-fuel engine integrated with accumulator

Facing the multiple pressures of energy reserves decline, ecological environment deterioration and transportation industry prosperity, in order to enhance the marine performance, improve the enterprises economic benefits, and achieve the purpose of energy conservation and emission reduction, the revolution of green and efficient waste heat recovery technology is imperative. Therefore, the exergy-based configuration design of marine cascade waste heat recovery system is proposed, including supercritical Brayton cycle, organic Rankine cycle, organic flash cycle, organic trilateral cycle and absorption refrigeration cycle. Firstly, the mathematical models of three cycle-configurations are established based on thermodynamic equations, and the authority of simulation results is preliminarily verified. Then, the relationship between working-fluid, matching mode and operation parameters with cycle-configuration performance is analyzed in turn. After the multi-objective optimization, the thermodynamic performance and economic benefits of the cycle-configuration are comprehensively evaluated. Finally, the positive influence of the accumulator on the refrigeration system is explored. The numerical results prove that the proposed cycle-configuration has significant thermodynamic performance and economic benefits, reflects the good performance of energy saving and emission reduction, and can eliminate the negative impact of latent heat fluctuation on the refrigeration system. It provides a feasible idea for researchers and manufacturers who are committed to comprehensively improve the performance of cascade waste heat recovery, and fills the research gap in the latest investigation.

Energy, exergy and exergoenvironmental analyses on gas-diesel fuel marine engine used for trigeneration system

This study has addressed a trigeneration system driven by dual fuel marine engine. The marine engine manufacturers have developed the dual fuel (DF) version to improve the combustion and reduce the pollutant gaseous emissions. The aim was evaluated the performances of gas diesel fuel and full diesel based on exergy and exergoenvironmental analyses. The energy, exergy and exergoenvironmental balances were conducted considering the effect of pollutant emissions. The specific exergy cost (SPECO) approach was applied, which use the fuel and product definition. The diesel marine engine original produced 6.90 MW of electrical power, 28.16 kW of cooling water and 278.2 and 588 kW of heating. The findings revealed that the effect of gas diesel fuel has reduced the energy and exergy efficiencies by 87% in relation to full diesel mode. The diesel engine has the highest exergy destruction with 3051 kW. The NO emission has a portion of 92% of environmental impact rate associated to pollutant formation. The lowest energy and exergetic efficiencies and the highest environmental impact rates of components were investigated. The environmental impact rate per exergy unit of electricity power, cooling water of absorption chiller and heating water were presented. The reduction of environmental impact rate of pollutant formation is the main responsible for the better environmental performance of gas diesel fuel marine engine. Despite the reduction in energy and exergy efficiencies, the dual fuel (DF) technology was able to controlling pollutant emissions and improved the exergoenvironmental performance without great modification into engine. Further recommendations were proposed.

Measurements of Emissions to Air from a Marine Engine Fueled by Methanol

Emissions of exhaust gases and particulate matter from a dual fuel marine engine using methanol as fuel with marine gasoil as pilot fuel have been examined for a ferry during operation. The emission factor for nitrogen oxides is lower than what is typically found for marine gasoil but does not reach the tier III limit. The emissions of particulate matter are significantly lower than for fuel oils and similar to what is found for LNG engines. The main part of the particles can be found in the ultrafine range with the peak being at around 18 nm. About 93% of the particles are evaporated and absorbed when using a thermodenuder, and thus a large majority of the particles are volatile. Methanol is a potential future marine fuel that will reduce emissions of air pollutants and can be made as a biofuel to meet emission targets for greenhouse gases.

Numerical analysis of methane slip source distribution in a four-stroke dual-fuel marine engine

We present the results of a numerical model which has been developed for estimating the contribution to the methane slip from different sources in a four-stroke dual-fuel marine engine running on natural gas. The model is a thermodynamic three-zone zero-dimensional full engine cycle model and considers methane slip contributions from short-circuiting, crevices and wall quenching. The model is applied to analyze the methane slip from a four-stroke dual-fuel medium speed marine engine using natural gas as primary fuel. At low loads, wall quenching is found to be the dominant contribution to the methane slip. At full load, the wall quenching contribution is comparable to the level of the short-circuiting and crevice contributions which only vary relatively little with load. At 75% load, the contribution from short-circuiting is highest. In addition, we found that in-cylinder post-oxidation of unburned fuel remaining after the main combustion is negligible.

Effects of fuel reforming on large-bore low-speed two-stroke dual fuel marine engine combined with EGR and injection strategy

Large-bore low-speed two-stroke dual fuel engines have become an urgent need to relieve energy crisis and reduce emissions. In this study, the effects of fuel reforming on large-bore low-speed two-stroke dual fuel marine engine combined with EGR and injection strategy were investigated. With increase of injection advance angle, the ISFC increases slightly, and also NOx emissions increase, which indicate that the optimal injection timing is 12.5°CA BTDC in consideration of emission and fuel economy. A 10% EGR rate is selected for NOx reduction and pressure rise rate suppression, which might increase fuel consumption in lower combustion temperature. Subsequently, fuel reforming is applied for further fuel consumption optimization. Compared with the base case, ISFC is reduced by 7.80% by introducing 1.29% syngas, but NOx emissions are higher than IMO Tier Ⅲ limit emissions standards. After combined optimization, the indicated thermal efficiency could be increased from nearly 50% (base case) to 55% by 12.5 BTDC_E10_2.25% (pilot fuel injection timing is 12.5°CA BTDC, EGR rate is 10%, and γreforming is 2.25%). Compared with the base case, the NOx emissions increase, but still lower than the limit value of IMO Tier Ⅲ emission standards, while the ISFC reduce 9.45% by 12.5 BTDC_E10_2.25%. Base on the above discussion, it can be concluded that the collaborative strategy has great potential in improving the fuel economy and also controlling emissions standard of low-speed dual fuel marine engine.

Comprehensive analysis of the air quality impacts of switching a marine vessel from diesel fuel to natural gas

New environmental regulations are mandating cleaner fuels and lower emissions from all maritime operations. Natural gas (NG) is a fuel that enables mariners to meet regulations; however, emissions data from maritime operations with natural gas is limited. We measured emissions of criteria, toxic and greenhouse pollutants from a dual-fuel marine engine running either on diesel fuel or NG as well as engine activity and analyzed the impacts on pollutants, health, and climate change. Results showed that particulate matter (PM), black carbon (BC), nitric oxides (NOx), and carbon dioxide (CO2) were reduced by about 93%, 97%, 92%, and 18%, respectively when switching from diesel to NG. Reductions of this magnitude provide a valuable tool for the many port communities struggling with meeting air quality standards. While these pollutants were reduced, formaldehyde (HCHO), carbon monoxide (CO) and methane (CH4) increased several-fold. A health risk assessment of exhaust plume focused on when the vessel was stationary, and at-berth showed the diesel plume increased long-term health risk and the NG plume increased short-term health risk. An analysis of greenhouse gases (GHGs) and BC was performed and revealed that, on a hundred year basis, the whole fuel cycle global warming potential (GWP) per kWh including well-to-tank and exhaust was 50% to few times higher than that of diesel at lower engine loads, but that it was similar at 75% load and lower at higher loads. Mitigation strategies for further reducing pollutants from NG exhaust are discussed and showed potential for reducing short-term health risks and climate impacts.

A framework for evaluating and optimizing the cascade utilization of medium-low grade waste heat in marine dual-fuel engines

Waste heat recovery technology performs a significant function in energy conservation and pollution reduction. It is also an effective means for alleviating the current global pollution and energy shortage. For this purpose, an combined system, which includes a supercritical carbon dioxide Brayton cycle, a low-temperature Rankine cycle, an absorption refrigeration system, and a capillary seawater desalination system, is established. The accuracy of the system is primarily verified by comparing its results with those found in existing literature. Some sensitive parameters that affect system performance are subsequently analyzed step by step, and the performance of the subsystem under the maximum continuous operational load condition is determined. The electricity production, fresh water mass flow, and cold energy are found to be 123.97 kW, 138.82 kg/h, and 86.48 kW, respectively. Considering the contribution of the low-temperature waste heat recovery device, the equivalent electricity production of the combined system is optimized by genetic algorithm. After obtaining the optimal operational condition, the thermodynamic performance, economic performance, and energy evaluation indexes are calculated. The results demonstrate that low-temperature waste heat recovery devices can operate in a wide range of load conditions, and the electricity production, energy efficiency, and exergy efficiency of the combined system are 222.31 kW, 23.19%, and 75.29%, respectively. Compared with the original engine system, the thermodynamic and emission reduction performance significantly improve, hence, this design is economically feasible and is an effective technology for energy saving, emission reduction, and energy cascade utilization.

Thermo‐economic analysis and optimization of a combined Organic Rankine Cycle ( ORC ) system with LNG cold energy and waste heat recovery of dual‐fuel marine engine

A combined Organic Rankine Cycle (ORC) system with liquefied nature gas (LNG) cold energy and dual-fuel (DF) marine engine waste heat utilization was proposed. Engine exhaust gas and engine jacket cooling water were adopted as parallel heat sources. Thermo-economic analyses of the proposed system with 32 working fluids combinations were performed. Two objective functions covering thermal efficiencies and economic index were employed for performance evaluation. Afterward, the effects of operation pressure on the objective functions were investigated. Finally, the optimal conditions were obtained from the Pareto front with the Non-dominated Sorting Genetic Algorithm-II (NSGA-II) method. The results show that the proposed ORC system has better energy recovery performances than the parallel ORC system. R1150-R600a-R290, R1150-R601a-R600a, and R170-R601-R290 are determined as the three most promising working fluids combinations. Under optimized conditions, the output power range is 199.97 to 218.51 kW, the energy efficiency range is 13.64% to 15.62%, and the exergy efficiency range is 25.29% to 27.3%. The payback period ranges from 8.36 to 8.74 years. The working fluids selection helps to reduce the exergy destruction of intermediate heat exchanger, which could be up to 30.59%.

Assessment of relative fuel cost for dual fuel marine engines along major Asian container shipping routes

This study contributes to academic literature on bunkering for liner shipping with the application of dual fuel engines in providing fuel flexibility subjected to limited liquefied natural gas (LNG) bunkering facilities along a subset of the major shipping lanes East of Suez. New LNG bunkering facilities have developed in recent years which is critical to the uptake of LNG as a bunker fuel while with the installation of dual fuel engines, vessels operators will now have a choice regarding the main fuel for propulsion. An additional layer regarding the combinatorial aspect of different fuel types to utilize as well choice to bunker at LNG facilities enroute (not calling port) is incorporated subjected to maintaining weekly schedules by speed adjustments.

Shock tube and kinetic study on ignition characteristics of methane/n-hexadecane mixtures

Ignition characteristics of n-hexadecane/methane mixture under low-pressure high-temperature (P: 2.0 bar, 5.0 bar &T: 1269–1887K) and high-pressure low-temperature (P: 40–140 bar &T: 700–1200 K) conditions were studied by aerosol shock tube and CHEMKIN with LLNL C16 mechanism. The results indicated that at low-pressure high-temperature conditions, increasing initial pressure and decreasing equivalence ratio could reduce ignition delay times (IDT) by 39% and 52% respectively. N-hexadecane’s addition and n-hexadecane’s replacement also shortened IDT by 88% and 96% separately. Raising n-hexadecane’s replacement further reduced IDT. The reduction degree of IDT dramatically decreased when n-hexadecane’s content was high. N-hexadecane was fully consumed earlier than methane. Radicals that formed from n-hexadecane’s decomposition induced H-abstraction of n-hexadecane, which advanced methane’s slow oxidation. H was the crucial radical in n-hexadecane’s H-abstraction reactions. At ultra-high-pressure low-temperature condition, adding n-hexadecane and increasing n-hexadecane content dramatically reduced IDT. With n-hexadecane’s H-abstraction dominating ignition process, n-hexadecane’s decomposition was significantly inhibited at low-temperature condition. OH was the key radical in n-hexadecane’s H-abstraction reactions. The obviously suppressed n-hexadecane’s decomposition was responsible for the same end time point of complete consumption of two fuels. These results are helpful for providing control methods for natural-gas preignition and abnormal combustion triggered by lubricating oil in dual-fuel marine engine.

Optical Characterization of the Combustion Process inside a Large-Bore Dual-Fuel Two-Stroke Marine Engine by Using Multiple High-Speed Cameras

<div class="section abstract"><div class="htmlview paragraph">Dual-fuel engines for marine propulsion are gaining in importance due to operational and environmental benefits. Here the combustion in a dual-fuel marine engine operating on diesel and natural gas, is studied using a multiple high-speed camera arrangement. By recording the natural flame emission from three different directions the flame position inside the engine cylinder can be spatially mapped and tracked in time. Through space carving a rough estimate of the three-dimensional (3D) flame contour can be obtained. From this contour, properties like flame length and height, as well as ignition locations can be extracted. The multi-camera imaging is applied to a dual-fuel marine two-stroke engine, with a bore diameter of 0.5 m and a stroke of 2.2 m. Both liquid and gaseous fuels are directly injected at high pressure, using separate injection systems. Optical access is obtained using borescope inserts, resulting in a minimum disturbance to the cylinder geometry. In this type of engine, with fuel injection from positions at the rim of the cylinder, the flame morphology becomes asymmetric. The optical spatial mapping and tracking method is demonstrated to be well suited for the study of such an asymmetric combustion system. Spatial mapping and tracking of flame position is applied to both engine operating modes; normal diesel operation and dual-fuel operation with diesel pilot ignition of the gas. Similarities and differences between diesel and gas flame shape and development can thus be visualised directly. The effects of changing charge density, gas injection pressure and injection nozzle geometry on the flame geometry and development are also studied.</div></div>

Multi-Objective Thermo-Economic Optimization of a Combined Organic Rankine Cycle (ORC) System Based on Waste Heat of Dual Fuel Marine Engine and LNG Cold Energy Recovery

In this paper, a combined organic Rankine cycle (ORC) system that can effectively utilize the cold energy of Liquefied Nature Gas (LNG) and the waste heat of dual fuel (DF) marine engine was proposed. Particularly, the engine exhaust gas and the jacket cooling water of the DF marine engine were used as heat sources. Firstly, a thorough assessment of thermo-economic performance was conducted for the combined ORC system using 11 environmentally friendly working fluids (WFs). Afterwards, the effects of evaporation and condensation pressures on the net output work, energy efficiency, exergy efficiency, total investment cost and payback period were examined. Furthermore, the thermo-economic performances of the ORC system were optimized via multi-objective optimization with a genetic algorithm. Finally, exergy destructions and investment costs of each component under the optimal operating conditions were analyzed to make suggestions for further improvement. The results show that R1150-R1234yf-R600a and R170-R1270-R152a are the two most promising WF combinations. The exergy destruction of the combined ORC system mainly exists in heat exchangers. Through WF optimization, the exergy destruction in the intermediate heat exchanger was reduced by 18.99%. The proportion of expanders investment cost could be greater than 50% and the payback period of the combined ORC system varies in the range of 7.68–9.43 years. This study demonstrated that the selection of WF and the optimization of operating conditions had important potential to improve thermo-economic performances of ORC systems.

Towards Marine Dual Fuel Engines Digital Twins—Integrated Modelling of Thermodynamic Processes and Control System Functions

This study aims at developing an integrated model that combines detailed engine thermodynamic modelling and the control system functional modelling paving the way towards the development of high-fidelity digital twins. To sufficiently represent the combustion process, a multi-Wiebe function approach was employed, whereas a database for storing the combustion model parameters was developed. The developed model was employed for the systematic investigation of a marine four-stroke dual fuel engine response during demanding transient operation with mode switching and load changes. The derived results were analysed to identify the critical engine components and their effect on the engine operational limitations. The results demonstrate that the developed model can sufficiently represent the engine and its subsystems/components behaviour and effectively capture the engine control system’s functionality. The appropriate turbocharger matching along with the sufficient design of the exhaust gas waste gate valve and fuel control systems are crucial for ensuring the smooth engine operation of dual fuel engines.

Shock tube and kinetic study on ignition characteristics of lean methane/n-heptane mixtures at low and elevated pressures

To acquire ignition control methods for dual-fuel marine engine and HCCI engine, ignition characteristics of lean n-heptane/methane mixture under pressure of 2.0 bar and temperature range from 1241 to 1825 K were studied by shock tube and CHEMKIN with LLNL3.1 mechanism. And ignition processes under temperature range from 700 to 1200 K and pressure range from 40 to 140 bar were investigated by CHEMKIN with NUI mechanism. The results illuminate that at low-pressure high-temperature condition, n-heptane’s replacement and the increase of n-heptane content obviously reduced ignition delay times (IDT). The reduction degree of IDT decreased when n-heptane content was high. N-heptane’s addition also reduced IDT. But this reduction magnitude was less than that of n-heptane’s replacement. Methane’s addition slightly inhibited n-heptane’s auto-ignition. The reaction time of n-heptane was obviously earlier than that of methane. N-heptane decomposition induced radical formation firstly, which triggered subsequent n-heptane’s H-abstraction and the advance of methane’s oxidation. At ultra-high-pressure low-temperature condition, increasing n-heptane’s content enhanced negative temperature coefficient (NTC) behavior. The end time point of complete consumption of two fuels was the same. Low-temperature condition inhibited n-heptane decomposition, with n-heptane’s H-abstraction dominating ignition process.

Comparison of Saturated and Superheated Steam Plants for Waste-Heat Recovery of Dual-Fuel Marine Engines

From the working data of a dual-fuel marine engine, in this paper, we optimized and compared two waste-heat-recovery single-pressure steam plants—the first characterized by a saturated-steam Rankine cycle, the other by a superheated-steam cycle–using suitably developed simulation models. The objective was to improve the recovered heat from the considered engine, running with both heavy fuel oil and natural gas. The comparison was carried out on the basis of energetic and exergetic considerations, concerning various aspects such as the thermodynamic performance of the heat-recovery steam generator and the efficiency of the Rankine cycle and of the combined dual-fuel-engine–waste-heat-recovery plant. Other important issues were also considered in the comparison, particularly the dimensions and weights of the steam generator as a whole and of its components (economizer, evaporator, superheater) in relation to the exchanged thermal powers. We present the comparison results for different engine working conditions and fuel typology (heavy fuel oil or natural gas).

Simulation-based investigation of a marine dual-fuel engine

ABSTRACTRecent developments have rendered the Dual Fuel (DF) engines an attractive alternative solution for achieving cost-efficient compliance to environmental regulations. The present study focuses on the safety investigation of a marine DF engine in order to identify potential safety implications. This investigation is based on an integrated engine model, which was developed in GT-ISE™ software and is capable of predicting both the engine steady-state behaviour and transient response. The model includes the engine thermodynamic simulation module as well as the engine control system functional module; the latter is responsible for implementing the ordered load changes and the operating mode switching. The developed model is first validated against available published data and subsequently used to simulate several test cases with fuel changes, from gas to diesel and diesel to gas with rapid and with delayed wastegate valve operation. The derived simulation results are used to investigate the potential safety implications that can arise during the engine operation. The results demonstrate that the engine–turbocharger matching as well as the wastegate control are critical parameters for ensuring the compressor surge free operation during gas to diesel modes transition.Abbreviations: 0D: zero-dimensional; 1D: one-dimensional; BMEP: brake mean effective pressure; CO2: carbon dioxide; DF: dual fuel; D/G: diesel generator; DTG: diesel to gas fuel modes switching; ECA: emission control area; ECS: engine control system; EEDI: energy efficiency design index; GTD: gas to diesel modes switching; HFO: heavy fuel oil; IMO: International Maritime Organization; LFO: light fuel oil; LNG: liquefied natural gas; MCR: maximum continuous rating; NOx: nitrogen oxides; PHA: preliminary hazard analysis; PI: proportional–integral; SOx: sulphur oxides; TC: turbocharger; WG: wastegate; λ: air–fuel equivalence ratio

Investigation of the Miller cycle on the performance and emission in a natural gas-diesel dual-fuel marine engine by using two zone combustion model

Compared to the standard cycle, the Miller cycle decreases the cylinder maximum combustion temperature which can effectively reduce NOX emissions. In this paper, a zero-dimensional two-zone combustion model is used to establish the simulation model of a marine dual-fuel engine, which is calibrated according to the test report under different loads. Due to the high emissions under part load, the Miller cycle (early intake valve closing method) is used for optimization. By analyzing the cylinder pressure, temperature, heat release rate and NOx emissions under different cases, it can be found that the effective working volume and thermal efficiency decrease with the advance of intake valve closing and improve with the increase of the geometric compression ratio. In all optimization cases, the NOX emissions and fuel consumption are reduced by 72% and 0.1%, respectively, by increasing the geometric compression ratio to 14 and the intake valve closing timing to 510 degree of crank angle (The reference top dead center is 360 degree of crank angle). The simulation results show that the early intake valve closing Miller cycle can effectively reduce the NOX emissions and cylinder peak pressure.

Size and morphology of soot produced by a dual-fuel marine engine

In recent years, liquefied natural gas (LNG) has become a popular alternative to heavy fuel oils for use in marine engines due to its low cost and increasingly stringent emissions regulations from the International Maritime Organization (IMO). While natural gas (NG) combustion produces substantially less soot than other fossil fuels, its soot emissions are not negligible and have not been morphologically characterized. In this study, transmission electron microscopy (TEM) was used to interpret scanning mobility particle sizer (SMPS) distributions of catalytically-denuded particulate matter (soot) from an NG-powered commercial vessel. The vessel studied was powered by a dual-fuel compression ignition engine which uses a diesel pilot only to ignite the NG. Less than 2% of the engine's power is provided by the diesel pilot for engine loads above 50%. For comparison, tests were also run in diesel-only mode. During NG operation, the SMPS size distributions did not follow a lognormal distribution as expected. Instead, the distribution remained roughly constant from 10 nm to 100 nm. High resolution (HR)-TEM revealed particles as small as 30 nm in diameter were soot. From the TEM images, soot primary particle size and projected-area-equivalent diameter were estimated for individual soot particles. These data were then used to infer the effective density of the particles collected. In general, the primary particle size and correspondingly the estimated effective density of soot from this engine were larger than those from the “universal fit” proposed for soot in previous literature. For both operating conditions the source of the soot was likely the diesel pilot however, there were statistically significant differences between particles produced by the two fuelling modes. With LNG fueling, there were far fewer particles, and mean soot aggregate size decreased, but the primary particle size (for a 100 nm aggregate) increased, relative to diesel. Engine load did not have a statistically significant effect on soot morphology.