A novel ship propulsion concept employs natural gas to reduce ship emissions and improve overall ship propulsion efficiency. This concept proposes a serial integration of Solid Oxide Fuel Cell (SOFC) and a natural gas engine, while anode-off gas (gas at the fuel cell exhaust) is used in the natural gas engine. This study focusses on SOFC-gas engine integration by experimentally analyzing the effects of adding hydrogen, which is the main combustible component of the fuel cell anode-off gas, in marine natural gas engines. The overall challenge is to employ the anode-off gas to improve the performance of marine natural gas engines. To study the effects of anode-off gas combustion in natural gas engines, experiments with hydrogen addition in a marine natural gas engine of 500 kW rated power were performed. Natural gas was replaced with 10 % and 20 % of hydrogen, by volume, without any penalties in terms of output power. We found that the high combustion rate of hydrogen improved combustion stability, which allowed for better air-excess ratio control. Thus allowing leaning to higher air-excess ratios and extending the, otherwise, limited operating window. Hydrogen addition also improved brake thermal efficiency by 1.2 %, while keeping NO x emissions below the maritime emission regulations. The improvement in engine efficiency with a larger operating window may help improve the load-taking capabilities of marine natural gas engines.

Natural gas is traditionally considered as a promising fuel in comparison to gasoline due to the potential of lower emissions and significant domestic reserves. These emissions can be further diminished by using noble gases, such as argon, instead of nitrogen as the working fluid in internal combustion engines. Furthermore, the use of argon as the working fluid can increase the thermodynamic efficiency due to its higher specific heat ratio. In comparison to pre-mixed operation, the direct injection of natural gas enables the engine to reach higher compression ratios while avoiding knock. Using argon as the working fluid increases the in-cylinder temperature at top dead center and enables the compression ignition of natural gas. In this numerical study, the combustion quality and ignition behavior of methane injected into a mixture of oxygen and argon has been investigated using a three-dimensional transient model of a constant volume combustion chamber. A dynamic structure large eddy simulation model has been utilized to capture the behavior of the non-premixed turbulent gaseous jet. A reduced mechanism consists of 22-species and 104-reactions were coupled with the CFD solver. The simulation results show that the methane jet ignites at engine-relevant conditions when nitrogen is replaced by argon as the working fluid. Ignition delay times are compared across a variety of operating conditions to show how mixing affects jet development and flame characteristics.

Heavy-duty compression-ignition (CI) engines converted to natural gas (NG) spark ignition (SI) operation have the potential to increase the use of NG in the transportation sector. A 3D numerical simulation was used to predict how the conventional CI combustion chamber geometry (i.e., re-entrant bowl and flat head) affects the combustion stability, performance and emissions of a single-cylinder CI engine that was converted to SI operation by adding a low-pressure gas injector in the intake manifold and a spark plug in place of the diesel injector. The G-equation based 3D CFD simulation investigated three different combustion chamber configurations that changes the size of the squish region at constant compression ratio and clearance height. The results show that the different flame propagation speeds inside and outside the re-entrant bowl can create a two-zone combustion phenomenon. More, a larger squish region increased flame burning speed, which decreased late-combustion duration. All these findings support the need for further investigations of combustion chamber shape design for optimum engine performance and emissions in CI engines converted to NG SI operation.

Experiments were conducted on a large bore, medium speed, single cylinder, diesel engine to investigate operation with substitution ratio of natural gas varying from 0 to 93% by energy. As reported in a previous publication [1], these data were used to validate an analytical methodology for predicting performance and emissions under a broad spectrum of energy substitution ratios. For this paper, these experimental data are further analyzed to better understand the performance and combustion behavior under natural gas substitution ratios of 0%, 60% and 93%. These results show that by transitioning from diesel to 60% dual-fuel (60% NG substitution ratio), an improvement in the NOx-efficiency trade-off was observed that represented a ∼3% improvement in efficiency at constant NOx. Further, the transition from 60% dual-fuel to 93% dual-fuel (93% NG substitution ratio) resulted in additional efficiency improvement with a simultaneous reduction in NOx emissions. The data suggest that this improvement can be attributed to the premixed nature of the high substitution ratio. Furthermore, the results show that high cycle-to-cycle variation was observed for the 93% dual-fuel combustion tests. Further analysis, along with diesel injection rate measurements, show that the observed extreme sensitivity of the combustion event can be attributed to critical parameters such as diesel fuel quantity and injection timing. Results suggest a better understanding of the relative importance of combustion system components and operating conditions in controlling cycle-to-cycle variation of combustion process.

Heavy-duty compression-ignition (CI) engines converted to natural gas (NG) operation can reduce the dependence on petroleum-based fuels and curtail greenhouse gas emissions. Such an engine was converted to premixed NG spark-ignition (SI) operation through the addition of a gas injector in the intake manifold and of a spark plug in place of the diesel injector. Engine performance and combustion characteristics were investigated at several lean-burn operating conditions that changed fuel composition, spark timing, equivalence ratio, and engine speed. While the engine operation was stable, the reentrant bowl-in-piston (a characteristic of a CI engine) influenced the combustion event such as producing a significant late-combustion, particularly for advanced spark timing. This was due to an important fraction of the fuel burning late in the squish region, which affected the end of combustion, the combustion duration, and the cycle-to-cycle variation. However, the lower cycle-to-cycle variation, stable combustion event, and the lack of knocking suggest a successful conversion of conventional diesel engines to NG SI operation using the approach described here.

Natural gas as an alternative fuel in engine applications substantially reduces both pollutant and greenhouse gas emissions. High pressure dual fuel direct injection of natural gas and Diesel pilot has the potential to minimize methane slip from gas engines and increase the fuel flexibility, while retaining the high efficiency of a Diesel engine. Speed and load variations as well as various strategies for emission reduction entail a wide range of different operating conditions. The influence of these operating conditions on the ignition and combustion process is investigated on a rapid compression expansion machine. By combining simultaneous Shadowgraphy and OH* imaging with heat release rate analysis, an improved understanding of the ignition and combustion process is established. At high temperatures and pressures the reduced pilot ignition delay and lift-off length minimize the effect of natural gas jet entrainment on pilot mixture formation. A simple geometrical constraint was found to reflect the susceptibility for misfiring. At the same time natural gas ignition is delayed by the early pilot ignition close to the injector tip. The shape of heat release is only marginally affected by the operating conditions and mainly determined by the degree of premixing at the time of gas jet ignition. Luminescence from the sooting natural gas flame is generally only detected after the flame extends across the whole gas jet at peak heat release rate. Termination of gas injection at this time was confirmed to effectively suppress soot formation, while a strongly sooting pilot seems to intensify soot formation within the natural gas jet.

Diesel engines have been widely used due to the higher reliability and superior fuel conversion efficiency. However, they still generate significant amount of carbon dioxide (CO 2 ) and particulate matter (PM) emissions. Natural gas is a low carbon and clean fuel that generates less CO 2 and PM emissions than diesel during combustion. Replacing diesel by natural gas in internal combustion engines help reduce both CO 2 and PM emissions. Natural gas – diesel dual fuel combustion is a practical and efficient way to replace diesel by natural gas in internal combustion engines. One concern for dual fuel combustion engines is the diesel injector tip temperature increase with increasing natural gas fraction. This paper reports an experimental investigation on the diesel injector tip temperature variation and combustion performance of a natural gas – diesel dual fuel engine at medium and high load conditions. The natural gas fraction was changed from zero to 90% in the experiment. The results suggest that the injector tip temperature increased with increasing natural gas fraction at a given diesel injection timing or with advancing the diesel injection timing at a given natural gas fraction. However, the injector tip temperature never exceeded 250 °C in the whole experimental range. The effect of natural gas fraction on combustion performance depended on engine load and diesel injection timing.

Because of the potential to reduce NO x and PM emissions simultaneously and the utilization of biofuel, diesel/compressed natural gas (CNG) dual-fuel combustion mode with port injection of CNG and direct injection of diesel has been widely studied. While in comparison with conventional diesel combustion mode, the dual-fuel combustion mode generally leads lower thermal efficiency, especially at low and medium load, and higher carbon monoxide (CO) and total hydrocarbons (THC) emissions. In this work, n-butanol was blended with diesel as the pilot fuel to explore the possibility to improve the performance and emissions of dual-fuel combustion mode with CNG. Various pilot fuels of B0 (pure diesel), B10 (90% diesel/10% n-butanol by volume basis), B20 (80% diesel/20% n-butanol) and B30 (70% diesel/30% n-butanol) were compared at the CNG substitution rate of 70% by energy basis under the engine speeds of 1400 and 1800 rpm. The experiments were carried out by sweeping a wide range of pilot fuel start of injection timings based on the same total input energy including pilot fuel and CNG. As n-butanol was added into the pilot fuel, the pilot fuel/CNG/air mixture tends to be more homogeneous. The results showed that for the engine speed of 1400 rpm, pilot fuel with n-butanol addition leads to a slightly lower indicated thermal efficiency (ITE). B30 reveals much lower NO x emission and slightly higher THC emissions. For the engine speed of 1800 rpm, B20 can improve ITE and decrease the NO x and THC emissions simultaneously relative to B0.

Natural gas is widely used in sequentially port fuel injection engine to meet stringent emission regulation. Lean burn operation is one of the ways to improve spark-ignition engine fuel economy. The instability in the combustion process of the lean burn engine is one of the major challenges for engine research. In this study, the performance and combustion characteristics of a lean burn sequential injection compressed natural gas (CNG) engine were investigated numerically using computational fluid dynamics (CFD) modeling over a wide range of air/fuel equivalence ratio. A detailed chemical kinetic mechanism was used for natural gas combustion along with laminar flame speed model to capture lean burn operating condition within the combustion chamber. Combustion pressure, indicated mean effective pressure (IMEP), and heat release were analyzed for performance analysis, whereas flame development angle (CA 10), combustion duration, thermal efficiency were taken for combustion analysis. The results show that on increasing air/fuel equivalence ratio at a given spark timing, IMEP decreases as the lean burn mixture produces less amount of gross power output due to insufficient available energy. Moreover, lower burning velocity characteristic of natural gas extends the combustion duration, where a substantial amount of total energy released after top dead center. It is also seen that optimum spark timing (MBT) for maximum IMEP advances with an increase in air/fuel equivalence ratio due to late ignition timing under lean burn condition. CFD model successfully captures the effect of dilution to illustrate the considerations to design future combustion engine for spark ignited natural gas engine.

Non-premixed combustion of directly-injected natural gas offers diesel-like performance and efficiency with lower fuel costs and reduced greenhouse gas emissions. To ignite the fuel, a separate ignition source is needed. This work reports on the initial development of a new hot-surface based ignitor, where a small quantity of natural gas is injected and ignited by a hot element. This generates a robust pilot flame to ignite the main gas injection. A series of experimental tests were conducted to evaluate the sensitivity of the pilot flame formation process to hot surface temperature and geometry and to gas pilot injection geometry. Tests were conducted in a constant-volume combustion chamber at up to 6 bar with hot surface temperatures up to 1750 K. Reacting-flow computational fluid dynamics (CFD) evaluation is used to help interpret the results and to extrapolate to engine-relevant pressures. The results show that hot surface temperatures around 1500 K can minimize the pilot ignition time. An injector geometry where the pilot gas jets are angled such that they impinge on the hot surface but retain sufficient momentum to convect mass into the main chamber helps to ensure rapid and stable ignition. The CFD results indicate that, at engine pressures, a stable gas pilot flame could be established within 1–2 ms using the proposed injector geometry. These results will be used to underpin further development activities on this concept.

Lean-burn engines are important due to their ability to reduce emissions, increase fuel efficiency, and mitigate engine knock. In this study, the surface roughness of spark plug electrodes is investigated as a potential avenue to extend the lean flammability limit of natural gas. A nano-/micro-morphology modification is applied on surface of the spark plug electrode to increase its surface roughness. High-speed Z-type Schlieren visualization is used to investigate the effect of the electrode surface roughness on the spark ignition process in a premixed methane-air charge at different lean equivalence ratios. In order to observe the onset of ignition and flame kernel behavior, experiments were conducted in an optically accessible constant volume combustion chamber at ambient pressures and temperatures. The results indicate that the lean flammability limit of spark-ignited methane can be lowered by modulating the surface roughness of the spark plug electrode.

An efficient computational fluid dynamics model for predicting high pressure dual-fuel combustion is one of the most essential steps in order to improve the concept, to reduce the number of experiments and to make the development process more coste-efficient. For Diesel and natural gas such a model developed by the authors is first used to analyze the combustion process with respect to turbulence chemistry interaction and to clarify the question whether the combustion process is limited by chemistry or the mixing process. On the basis of these findings a reduced reaction mechanism is developed in order to save up to 35% of computing time. The prediction capability of the modified combustion model is tested for different gas injection timings representing different degrees of premixing before ignition. Compared to experimental results from a rapid compression expansion machine, the shape of heat release rate, the ignition timing of the gas jet and the burnout are well predicted. Finally, misfiring observed at different geometric configurations in the experiment are analyzed with the model. It is identified that in these geometric configurations at low temperature levels the gas jet covers the preferred ignition region of the diesel jet. Since the model is based on the detailed chemistry approach, it can in future also be used for other fuel combinations or for predicting emissions.

Since the beginning of this century, Liquefied Natural Gas (LNG) has been attracting more and more attention as a cleaner energy alternative to other fossil fuels, mainly due to the possibility to transport it over longer distances than natural gas in pipelines and lower environmental impact than other liquid fuels. It is expected that this trend in the use of LNG will lead to steady increases in demand over the next few decades. At present, in the automotive sector, natural gas is employed as fuel in spark-ignited (SI) engines in the gas phase (CNG) adopting port-fuel injection system (PFI) in the intake manifold, with the main result of reducing CO 2 emissions by up to 20%, compared with gasoline operation. However, SI engines which are operated in this manner suffer loss of peak torque and power due to a reduction in volumetric efficiency. Direct-Injection (DI) inside the cylinder can overcome this drawback by injecting CNG after intake valve closure. Another strategy could be the injection of natural gas in the liquid phase, both in PFI or DI mode. The injected fuel evaporation cools down the intake air; increasing the charge density with a substantial improvement in the engine volumetric efficiency and delivered power. However, at present, injection systems dedicated to cryogenic injection of natural gas are still in the prototype state. In the present study, the volumetric efficiency and performance of a turbocharged, LNG fuelled SI-ICE were numerically analysed both in the cases of DI and PFI modes and compared with the results of a conventional CNG system. Various fuel injection timings and injector position were analysed. The engine performance was evaluated by means of a one-dimensional model developed with the simulation program GT-Power, while the verification of the LNG-air mixture characteristics was carried out with the commercial code Aspen HYSIS. The numerical activity has shown that gaseous DI, before inlet valves closing, gives the worst result since methane, once injected into the cylinder, expands hindering the entry of air. On the other side, liquid PFI represents the best configuration to maximize the volumetric efficiency and therefore the engine power. All the technological issues related to a cryogenic liquid methane injection system were not taken into consideration in this study.

Following the successful commercial use of a 9.0L, V-8 automotive-derivative engine for stationary power generation, a new 4.5L, four-cylinder engine has been developed utilizing a modular family design approach. Substantial commonality of power cylinder components has been achieved including the complete power cylinder and cylinder head. This paper describes the design and development approach to the engine family. These spark-ignited engines are typically used for standby emergency power and demand response applications utilizing commercial grade natural gas or propane. Driving a synchronous electrical generator operating at 60 HZ or 50Hz, engine speeds are either 1800 rpm/3600 rpm or 1500 rpm/3000 rpm respectively, depending upon selection of either a 2-pole or 4-pole alternating current generator. Designed for stoichiometric combustion, the engine configurations can include naturally-aspirated, turbocharged or turbocharged and after-cooled versions. Depending upon end-use applications, exhaust emissions technology and regulatory compliance can be met solely through engine calibration or inclusion of a 3-way catalyst with active air-fuel ratio control. Since the 9.0L engine version was successfully introduced in 2012, significant efforts have been undertaken to achieve commonality of desired features between the existing veeengine and the future in-line versions, including optimization of performance characteristics in consideration of future power rating structures. Starting from 9.0L commercial introduction, the content herein specifically describes the development of the new 4.5L engine with regard to design and analysis.

Natural gas/diesel dual fuel engines used in oil and gas drilling operations must be able to meet NO x emissions limits across a wide range of substitution percentage, which affects the air to natural gas ratio or gas lambda. In a dual fuel engine operating at high substitution, premixed, propagating natural gas flames occur and the NO x formed in such premixed flames is known to be a strong function of gas lambda. Consequently there is interest in understanding how NO x formation in a dual fuel engine is affected by gas lambda. However, NO x formation in a dual fuel engine is complicated by the interaction with the non-premixed diesel jet flame. As a result, previous studies have shown that enriching the air-fuel ratio can either increase or decrease NO x emissions depending on the operating conditions investigated. This study presents multi-dimensional combustion simulations of an air-fuel ratio sweep from gas lambda 2.0 to 1.5 at 80% substitution, which exhibited a minimum in NO x emissions at a natural gas lambda of 1.75. Images from the simulations are used to provide detailed explanations of the physical processes responsible for the minimum NO x trend with natural gas lambda.

Technology developments in directional drilling and hydraulic fracturing have led to increased natural gas reserves. Development of these unconventional resources is an energy intensive process. Prime-movers of unconventional well development were previously identified to be over-the-road trucks, drilling engines, and hydraulic stimulation engines. Diesel engines dominate these markets but industry is attempting to cut costs by using dual fuel and dedicated natural gas engines. On-road engines are subject to the transient FTP cycle for certification and off-road engines are subject to the 5-mode ISO 8178 D-2 cycle. It is well known that in-use activity can differ from certification activity. Significant in-use activity data for each prime-mover were collected and a Markov-Chain Monte-Carlo Simulation with a genetic algorithm was used to develop test cycles for each. The developed test cycles allowed for operation of a smaller yet similar engine within a controlled laboratory environment. Laboratory tests utilized a Cummins 8.9L ISL-G to analyze the emissions of new cycles compared to certification cycles and to examine the effects of fuel quality on emissions. The ISL-G is a spark-ignited engine used for heavy-duty trucks and could see market penetration in fleets serving the well development industry. It is similar in technology to the Waukesha LI7044, which is used in drilling operations — both employ air fuel ratio control and three-way catalysts. For the case of “pump” quality fuel, compressed natural gas was used. The developed OTR truck cycle produced higher brake-specific emissions of CO 2 , CO, NO x , and lower HC emissions compared to the FTP. The drilling and fracturing cycles tended to have lower CO 2 and HC emissions but higher CO emissions when compared to the D-2 cycle. Two additional fuel blends were used on the new cycles and represented blends with higher ethane and propane fractions — which are common to shale gases that could fuel prime-movers in the future. The minimum recommended methane number for this engine was 75 and additional fuel blends had methane numbers of 75.5 (propane blend) and 75.3 (ethane blend). As expected, CO 2 emissions increased with increased alkane concentration, while opposite trends were shown for THC and CH 4 . NO x emissions also tended to decrease with higher ethane and propane blends, across all cycles. For all cycles and fuels, HC emissions were predominately CH 4 - 94–97%. Variations in activity and the effects of different fuels should be addressed when estimating emissions since using standard certification or emissions factors may not be representative of in-use emissions.

Two-stroke engines are often used for their low cost, simplicity, and power density. However, these engines suffer efficiency penalties due to fuel short-circuiting. Increasing power density has previously been an area of focus for performance two-stroke engines — such as in dirt bikes. Smaller-displacement engines have also been used to power remote controlled cars, boats, and aircraft. These engines typically rely on gasoline or higher-octane liquid fuels. However, natural gas is an inherently knock-resistant fuel and small natural gas engines and generators could see increased market penetration. Power generators typically operate at a fixed frequency with varied load, which can take advantage of intake and exhaust system tuning. In addition, stationary engines may not be subject to size restrictions of optimal intake and exhaust systems. This paper examines methods to improve combustion stability, efficiency, and power density of a 29cc air-cooled two-stroke engine converted to operate on natural gas. Initial conversion showed significant penalties on delivery ratio, which lowered power density and efficiency. To overcome these issues a tuned intake pipe, two exhaust resonators, and a combustion dome were designed and tested. The engine was operated at 5400 RPM and fueling was adjusted to yield maximum brake-torque (MBT). All tests were conducted under wide-open throttle conditions. The intake and exhaust systems were designed based on Helmholtz resonance theory and empirical data. The engine utilized a two-piece cylinder head with removable combustion dome. The combustion dome was modified for optimal compression ratio while decreasing squish area and volume. With all designs incorporated, power increased from 0.22 kW to 1.07 kW — a factor of 4.86. Efficiency also increased from 7% to 12%. In addition to these performance gains, the coefficient of variation (COV) of indicated mean effective pressure (IMEP) decreased from just above 11% to less than 4%.

In this paper, a numerical investigation of the ignition process of dual fuel engines is presented. Optical measurements revealed that a homogeneous natural gas charge ignited by a small diesel pilot comprises the combustion phenomena of compression ignition of the pilot fuel as well as premixed flame propagation. The 3-Zones Extended Coherent Flame Model (ECFM3Z) was selected, since it can treat auto-ignition, pre-mixed flame propagation and diffusion flame aspects. Usually combustion models in multi-dimensional computational fluid dynamics (CFD) software packages are designed to handle only one reactive species representing the fuel concentration. In the context of the ECFM3Z model the concept of a multi-component fuel is applied to dual fuel operation. Since the available ignition models were not able to accurately describe the ignition characteristics of the investigated setup, a new dual fuel auto-ignition model was developed. Ignition delay times of the fuel blend are tabulated using a detailed reaction mechanism for n-heptane. Thereby, the local progress of pre-ignition reactions in the CFD simulation can be calculated. The ignition model is validated against experiments conducted with a periodically chargeable constant volume combustion chamber. The proposed model is capable to reproduce the ignition delay as well as the location of the flame kernels. The CFD simulations show the effect of temperature stratification and variations in the injection pressure on the apparent ignition delay of the micro pilot.

The burning of natural gas (NG) in compression ignition dual fuel engines has been highlighted for its fuel flexibility, higher thermal efficiency and reduced particulate matter (PM) emissions. Recent research has reported the significant impact of the introduction of NG to the intake port on nitrogen dioxide (NO 2 ) emissions, particularly at the low loads. However, the research on the mechanism of NO 2 formation in dual fuel engines has not been reported. This research simulates the formation and destruction of NO 2 in a NG-diesel dual fuel engine using commercial CFD software CONVERGE coupled with a reduced primary reference fuel (PRF) mechanism consisting of 45 species and 142 reactions. The model was validated by comparing the simulated cylinder pressure, heat release rate, and nitrogen oxides (NO x ) emissions with experimental data. The validated model was used to simulate the formation and destruction of NO 2 in a NG-diesel dual fuel engine. The formation of NO 2 and its correlation with the local concentration of nitric oxide (NO), methane, and temperature were examined and discussed. It was revealed that NO 2 was mainly formed in the interface region between the hot NO-containing combustion products and the relatively cool unburnt methane-air mixture. NO 2 formed at the early combustion stage is usually destructed to NO after the complete oxidation of methane and n-heptane, while NO 2 formed during the post-combustion process would survive and exit the engine. This was supported by the distribution of NO and NO 2 in the equivalence ratio (ER)-T diagram. A detailed analysis of the chemical reactions occurring in the NO 2 containing zone consisting of NO 2 , NO, O 2 , methane, etc., was conducted using a quasi-homogeneous constant volume model to identify the key reactions and species dominating NO 2 formation and destruction. The HO2 produced during the post combustion process of methane was identified as the primary species dominating the formation of NO 2 . The simulation revealed the key reaction path for the formation of HO 2 noted as CH 4 ->CH 3 ->CH 2 O->HCO->HO 2 , with conversion ratios of 98%, 74%, 90%, 98%, accordingly. The backward reaction of OH+NO 2 = NO+HO 2 consumed 34% of HO 2 for the production of NO 2 . It was concluded that the increased NO 2 emissions from NG-diesel dual fuel engines was formed during the post combustion process due to higher concentration of HO 2 produced during the oxidation process of the unburned methane at low temperature.

In this paper, a multidimensional computational fluid dynamics (CFD) model coupled with detailed chemistry calculations was used to analyze dual-fuel combustion based on high pressure direct injection of natural gas. The main focus was to analyze the capability of predicting pressure curve and heat release rate (HRR) for different injection strategies. Zero-dimensional homogeneous constant volume reactor calculations were used to select a reaction mechanism for the temperature range below 800 K. As the best-performing mechanism, the Chalmers mechanism was chosen. To validate the numerical model, the setup was first split into a single gas injection and a single Diesel injection. They were validated individually using shadowgraphs obtained from a Rapid Compression Expansion Machine (RCEM). Diesel ignition timing and position in the combustion chamber were close to experimental results. Gas direct injection showed good agreement with regard to penetration and mixing. In the dual-fuel setup, the injection timing of natural gas was varied to create a first case with mainly diffusive combustion and a second case with mainly premixed combustion of natural gas. For both setups good agreement with pressure curve and heat release rate were achieved. A qualitative comparison of shadowgraphs with the density field highlights the important points to predict dual-fuel combustion.