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The possibility to operate current diesel engines in dual-fuel mode with the addition of an alternative fuel is fundamental to accelerate the energy transition to achieve carbon neutrality. The simulation of the dual- fuel combustion process with 0D/1D combustion models is fundamental for the performance prediction, but still particularly challenging, due to chemical interactions of the mixture. The authors defined a novel data-driven workflow for the development of combustion reaction mechanisms and used it to generate a dual-fuel mechanism for Ammonia and Diesel Primary Reference Fuels (DPRF) suitable for efficient combustion simulations in heavy duty engines, with variable cetane number Diesel fuels. A baseline reaction mechanism was created by merging the detailed ammonia mechanism by Glarborg et al. with reaction pathways for n- hexadecane and 2,2,4,4,6,8,8-heptamethylnonane from a well-established multi-component fuel mechanism.

We propose a novel dual-fuel combustion model for simulating heavy-duty engines with the G-Equation. Dual-Fuel combustion strategies in such engines features direct injection of a high-reactivity fuel into a lean, premixed chamber which has a high resistance to autoignition. Distinct combustion modes are present: the DI fuel auto-ignites following chemical ignition delay after spray vaporization and mixing; a reactive front is formed on its surroundings; it develops into a well-structured turbulent flame, which propagates within the premixed charge. Either direct chemistry or the flame-propagation approach (G- Equation), taken alone, do not produce accurate results. The proposed Dual-Fuel model decides what regions of the combustion chamber should be simulated with either approach, according to the local flame state; and acts as a “kernel” model for the G- Equation model. Direct chemistry is run in the regions where a premixed front is not present.

As the global economy grows, so does the demand for heavy-duty commercial vehicles, both on-road and off-road. Currently, these vehicles are powered almost entirely by diesel engines. There is an imminent need to reduce the greenhouse gases (GHG) from this growing sector, but alternatives to the internal combustion engine face many challenges and can increase GHG emissions. For example, through simple analysis, this work will show that a Class 8 long haul on-highway truck powered entirely by battery electrics and charged from the average US electrical grid, yields significantly higher CO2 emissions per ton-mile as compared to an engine using alternative fuels. Thus, the most pragmatic and impactful way to reduce GHG emissions in commercial vehicles is using low carbon alternative fuels, such as ethanol made from renewable sources.

At recent American Society for Testing and Materials (ASTM) Subcommittee D02.01 meetings, committee members and attendees from the petroleum industry have reiterated a longstanding desire to see precision improvements to the ASTM D613 Standard Test Method for Cetane Number of Diesel Fuel Oil. The existing ASTM D613 precision limits were calculated using ASTM National Exchange Group (NEG) monthly test data from the mid-1970s through the early 1990s. Over the past few decades, many detailed studies were performed to identify and better understand the shortcomings of the cetane method (both engine equipment and instrumentation). Many of these studies concluded that inconsistent combustion is the main contributing factor behind the lack of precision in the cetane number method, followed by shortcomings in the instrumentation used to measure ignition delay.

A new generation of low heat capacitance Thermal Barrier Coatings (TBCs) has been developed under U.S. Dept. of Energy / Advanced Research Projects Agency - Energy (ARPA-E) sponsored research. The TBCs developed under this project have significantly lower thermal conductivity of < 0.35 W/m-K, thermal heat capacitance of < 500 kJ/m3-K, and density of <0.35 g/cm3. Two different binder types were used for thermal barrier coatings applied by High Velocity Low Pressure (HVLP) spraying to the piston, cylinder head, and valve combustion surfaces of a small natural gas engine. The effects of thermal barrier coatings on engine efficiency and knock characteristics were studied in a small, high compression ratio, spark-ignition, internal combustion engine operating on methane number fuels from 60 to 100. The new TBCs with low thermal conductivity and low thermal heat capacities have been shown to increase overall engine efficiency through reduced heat transfer to the piston and cylinder head.

Dual-fuel reactivity controlled compression ignition (RCCI) combustion is a promising method to achieve high efficiency with near-zero NOx and soot emissions; however, the requirement to carry two fuels on board limits practical application. Advancements in catalytic reforming have demonstrated the ability to generate syngas (a mixture of CO and hydrogen) from a single hydrocarbon stream. This syngas mixture can then be used as the low reactivity fuel stream to enable single fuel RCCI combustion. The present effort uses a combination of engine experiments and system level modeling to investigate reformed fuel RCCI combustion. The impact of reformer composition is investigated by varying the syngas composition from 10% H2 to approximately 80% H2. The results of the investigation show that reformed fuel RCCI combustion is possible over a wide range of H2/CO ratios.

Engine experiments were carried out on a heavy-duty single-cylinder engine to investigate the effects of Gasoline Compression Ignition on emissions and performance of a heavy-duty engine operating at a high load condition. Comparisons between gasoline fueled operation and diesel fueled operation are presented using a single, near top dead center injection. Although the fuel’s cetane numbers are very different, the combustion characteristics of the two fuels at high load are similar, with the gasoline-fueled case showing less than two crank angle degree longer ignition delay. Gasoline operation showed lower soot production at similar levels of NOx, initiating study of the impact of exhaust gas recirculation which spanned a range of NOx levels covering the range from minimal urea dosing to high urea dosing. A conventional soot-NOx tradeoff was found to exist with gasoline as exists with diesel.

Gasoline compression ignition (GCI) combustion is a promising solution to address increasingly stringent efficiency and emissions regulations imposed on the internal combustion engine. However, the high resistance to auto-ignition of modern market gasoline makes low load compression ignition (CI) operation difficult. Accordingly, a method that enables the variation of the fuel reactivity on demand is an ideal solution to address low load stability issues. Metal engine experiments conducted on a single cylinder medium-duty research engine allowed for the investigation of this strategy. The fuels used for this study were 87 octane gasoline (primary fuel stream) and diesel fuel (reactivity enhancer). Initial tests demonstrated load extension down to idle conditions with only 20% diesel by mass, which reduced to 0% at loads above 3 bar IMEPg.

Advancements in catalytic reforming have demonstrated the ability to generate syngas (a mixture of CO and hydrogen) from a single hydrocarbon stream. This syngas mixture can then be used to replace diesel fuel and enable dual-fuel combustion strategies. The role of port-fuel injected syngas, comprised of equal parts hydrogen and carbon monoxide by volume was investigated experimentally for soot reduction benefits under a transient load change at constant speed. The syngas used for the experiments was presumed to be formed via a partial oxidation on-board fuel reforming process and delivered through gaseous injectors using a custom gas rail supplied with bottle gas, mounted in the swirl runner of the intake manifold. Time-based ramping of the direct-injected fuel with constant syngas fuel mass delivery from 2 to 8 bar brake mean effective pressure was performed on a multi-cylinder, turbocharged, light-duty engine to determine the effects of syngas on transient soot emissions.

A genetic algorithm (GA) optimization methodology is applied to the design of the combustion system of a heavy-duty (HD) Diesel engine fueled with dimethyl ether (DME). The study has two objectives, the optimization of a conventional diffusion-controlled combustion system aiming to achieve US2010 targets and the optimization of a stoichiometric combustion system coupled with a three way catalyst (TWC) to further control NOx emissions and achieve US2030 emission standards. These optimizations include the key combustion system related hardware, bowl geometry and injection nozzle design as input factors, together with the most relevant air management and injection settings. The GA was linked to the KIVA CFD code and an automated grid generation tool to perform a single-objective optimization. The target of the optimizations is to improve net indicated efficiency (NIE) while keeping NOx emissions, peak pressure and pressure rise rate under their corresponding target levels.

Engine experiments were conducted on a heavy-duty single-cylinder engine to explore the effects of charge preparation, fuel stratification, and premixed fuel chemistry on the performance and emissions of Reactivity Controlled Compression Ignition (RCCI) combustion. The experiments were conducted at a fixed total fuel energy and engine speed, and charge preparation was varied by adjusting the global equivalence ratio between 0.28 and 0.35 at intake temperatures of 40°C and 60°C. With a premixed injection of isooctane (PRF100), and a single direct-injection of n-heptane (PRF0), fuel stratification was varied with start of injection (SOI) timing. Combustion phasing advanced as SOI was retarded between -140° and -35°, then retarded as injection timing was further retarded, indicating a potential shift in combustion regime. Peak gross efficiency was achieved between -60° and -45° SOI, and NOx emissions increased as SOI was retarded beyond -40°, peaking around -25° SOI.

Past research has shown that reactivity controlled compression ignition (RCCI) combustion offers efficiency and NOx and soot advantages over conventional diesel combustion at mid load conditions. However, at high load and low speed conditions, the chemistry timescale of the fuel shortens and the engine timescale lengthens. This mismatch in timescales makes operation at high load and low speed conditions difficult. High levels of exhaust gas recirculation (EGR) can be used to extend the chemistry timescales; however, this comes at the penalty of increased pumping losses. In the present study, targeting the high load - low speed regime, computational optimizations of RCCI combustion were performed at 20 bar gross indicated mean effective pressure (IMEP) and 1300 rev/min. The two fuels used for the study were gasoline (low reactivity) and diesel (high reactivity).

Due to the new challenge of meeting number-based regulations for particulate matter (PM), a numerical and experimental study has been conducted to better understand particulate formation in engines fuelled with compressed natural gas. The study has been conducted on a Heavy-Duty, Euro VI, 4-cylinder, spark ignited engine, with multipoint sequential phased injection and stoichiometric combustion. For the experimental measurements two different instruments were used: a condensation particle counter (CPC) and a fast-response particle size spectrometer (DMS) the latter able also to provide a particle size distribution of the measured particles in the range from 5 to 1000 nm. Experimental measurements in both stationary and transient conditions were carried out. The data using the World Harmonized Transient Cycle (WHTC) were useful to detect which operating conditions lead to high numbers of particles. Then a further transient test was used for a more detailed and deeper analysis.

Reactivity Controlled Compression Ignition (RCCI) has been shown to be an attractive concept to achieve clean and high efficiency combustion. RCCI can be realized by applying two fuels with different reactivities, e.g., diesel and gasoline. This motivates the idea of using a single low reactivity fuel and direct injection (DI) of the same fuel blended with a small amount of cetane improver to achieve RCCI combustion. In the current study, numerical investigation was conducted to simulate RCCI and HCCI combustion and emissions with various fuels, including gasoline/diesel, iso-butanol/diesel and iso-butanol/iso-butanol+di-tert-butyl peroxide (DTBP) cetane improver. A reduced Primary Reference Fuel (PRF)-iso-butanol-DTBP mechanism was formulated and coupled with the KIVA computational fluid dynamic (CFD) code to predict the combustion and emissions of these fuels under different operating conditions in a heavy duty diesel engine.

A novel 2-zone combustion chamber concept (patent pending) was developed using multi-dimensional modeling. At minimum volume, an axial projection in the piston divides the volume into distinct zones joined by a communication channel. The projection provides a means to control the mixture formation and combustion phasing within each zone. The novel combustion system was applied to reactivity controlled compression ignition (RCCI) combustion in both light-duty and heavy-duty diesel engines. Results from the study of an 8.8 bar BMEP, 2600 RPM operating condition are presented for the light-duty engine. The results from the heavy-duty engine are at an 18.1 bar BMEP, 1200 RPM operating condition. The effect of several major design features were investigated including the volume split between the inner and outer combustion chamber volumes, the clearance (squish) height, and the top ring land (crevice) volume.

Control of the timing and magnitude of heat release is one of the biggest challenges for premixed compression ignition, especially when attempting to operate at high load. Single-fuel strategies such as partially premixed combustion (PPC) use direct injection of gasoline to stratify equivalence ratio and retard heat release, thereby reducing pressure rise rate and enabling high load operation. However, retarding the heat release also reduces the maximum work extraction, effectively creating a tradeoff between efficiency and noise. Dual-fuel strategies such as reactivity controlled compression ignition (RCCI) use premixed gasoline and direct injection of diesel to stratify both equivalence ratio and fuel reactivity, which allows for greater control over the timing and duration of heat release. This enables combustion phasing closer to top dead center (TDC), which is thermodynamically favorable.

The focus of the present study was to characterize Reactivity Controlled Compression Ignition (RCCI) using a single-fuel approach of gasoline and gasoline mixed with a commercially available cetane improver on a multi-cylinder engine. RCCI was achieved by port-injecting a certification grade 96 research octane gasoline and direct-injecting the same gasoline mixed with various levels of a cetane improver, 2-ethylhexyl nitrate (EHN). The EHN volume percentages investigated in the direct-injected fuel were 10, 5, and 2.5%. The combustion phasing controllability and emissions of the different fueling combinations were characterized at 2300 rpm and 4.2 bar brake mean effective pressure over a variety of parametric investigations including direct injection timing, premixed gasoline percentage, and intake temperature. Comparisons were made to gasoline/diesel RCCI operation on the same engine platform at nominally the same operating condition.

In the current work, a series-hybrid vehicle has been constructed that utilizes a dual-fuel, Reactivity Controlled Compression Ignition (RCCI) engine. The vehicle is a 2009 Saturn Vue chassis and a 1.9L turbo-diesel engine converted to operate with low temperature RCCI combustion. The engine is coupled to a 90 kW AC motor, acting as an electrical generator to charge a 14.1 kW-hr lithium-ion traction battery pack, which powers the rear wheels by a 75 kW drive motor. Full vehicle testing was conducted on chassis dynamometers at the Vehicle Emissions Research Laboratory at Ford Motor Company and at the Vehicle Research Laboratory at Oak Ridge National Laboratory. For this work, the US Environmental Protection Agency Highway Fuel Economy Test was performed using commercially available gasoline and ultra-low sulfur diesel. Fuel economy and emissions data were recorded over the specified test cycle and calculated based on the fuel properties and the high-voltage battery energy usage.

While Low Temperature Combustion (LTC) strategies such as Reactivity Controlled Compression Ignition (RCCI) exhibit high thermal efficiency and produce low NOx and soot emissions, low load operation is still a significant challenge due to high unburnt hydrocarbon (UHC) and carbon monoxide (CO) emissions, which occur as a result of poor combustion efficiencies at these operating points. Furthermore, the exhaust gas temperatures are insufficient to light-off the Diesel Oxidation Catalyst (DOC), thereby resulting in poor UHC and CO conversion efficiencies by the aftertreatment system. To achieve exhaust gas temperature values sufficient for DOC light-off, combustion can be appropriately phased by changing the ratio of gasoline to diesel in the cylinder, or by burning additional fuel injected during the expansion stroke through post-injection.

The application of close-coupled post injections in diesel engines has been proven to be an effective in-cylinder strategy for soot reduction, without much fuel efficiency penalty. But due to the complexity of in-cylinder combustion, the soot reduction mechanism of post-injections is difficult to explain. Accordingly, a simulation study using a three dimensional computational fluid dynamics (CFD) model, coupled with the SpeedChem chemistry solver and a semi-detailed soot model, was carried out to investigate post-injection in a constant volume combustion chamber, which is more simple and controllable with respect to the boundary conditions than an engine. A 2-D axisymmetric mesh of radius 2 cm and height 5 cm was used to model the spray. Post-injection durations and initial oxygen concentrations were swept to study the efficacy of post-injection under different combustion conditions.