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Engine knock is a major challenge that limits the achievement of higher engine efficiency by increasing the compression ratio of the engine. To address this issue, using a higher octane number fuel can be a potential solution to reduce or eliminate the propensity for knock and so obtain better engine performance. Methanol, a promising alternative fuel, can be produced from conventional and non-conventional energy resources, which can help reduce pollutant emissions. Methanol has a higher octane number than typically gasolines, which makes it a viable option for reducing knock intensity. This study compared the combustion characteristics of gasoline and methanol fuels in an optical spark-ignition engine using multiple spark plugs. The experiment was carried out on a single-cylinder four-stroke optical engine. The researchers used a customized metal liner with four circumferential spark plugs to generate multiple flame kernels inside the combustion chamber.

In this study, a new system of assessment method was developed to evaluate the characteristics of urban buses based on remote online monitoring. Four types of buses, including China V emission standards diesel bus, lean-burn CNG bus, air-fuel equivalence ratio combustion CNG bus and gas-electric hybrid bus, were chosen as samples to analyze the emission characteristics of urban buses with different engine types in urban scenario. Based on the traffic conditions in Beijing, the actual emission characteristics of buses under newly-built driving conditions were analyzed. Moreover, the emission factor database of urban buses in Beijing was established to analyze the characteristics of excess emission. The research results are shown as follows. 1) Compared with other types of buses, NOX emission factor and emission rate of lean-burn CNG bus are much higher.

Lean NOx trap (LNT) is a great potential NOx abatement method for lean-burn gasoline engines in consideration of exhaust aftertreatment cost and installation space. NOx firstly is adsorbed on storage sites during the lean-burn period, then reduced to N2 under catalysis of the catalyst sites in the rich-burn phase. There must be a spillover of NOx species between both types of sites. For a better understanding of this spillover process of NOx species between Pt (as the catalytic center) and BaO sites (as storage components in commercial catalyst), this work focused on the vital first step of spillover, the adsorption of NOx on clean substrate surface (γ-Al2O3 (110) surface) and Ba\Pt cluster supported by the surface. Based on first principles software VASP (Vienna Ab-initio Simulation Package), the most stable adsorption structures of NO with Pt3 clusters and (BaO)3 clusters on carrier γ- Al2O3 (110) surface were confirmed and the adsorption energy of these structures were compared.

Two-stroke engines have to face the problems of insufficient charge for short intake time and the loss of intake air caused by long valve overlap. In order to promote the power of a two-stroke poppet valve diesel engine, measures are taken to help optimize intake port structure. In this work, the scavenging and combustion processes of three common types of intake ports including horizontal intake port (HIP), combined swirl intake port (CSIP) and reversed tumble intake port (RTIP) were studied and their characteristics are summarized based on three-dimensional simulation. Results show that the RTIP has better performance in scavenging process for larger intake air trapped in the cylinder. Its scavenging efficiency reaches 84.7%, which is 1.7% higher than the HIP and the trapping ratio of the RTIP reaches 72.3% due to less short-circuiting loss, 11.2% higher than the HIP.

Liquefied natural gas (LNG) engine could provide both reduced operating cost and reduction of greenhouse gas (GHG) emissions. Stoichiometric operation with EGR and the three-way catalyst has become a potential approach for commercial LNG engines to meet the Euro VI emissions legislation. In the current study, numerical investigations on the knocking tendency of several combustion chambers with different geometries and corresponding performances were conducted using CONVERGE CFD code with G-equation flame propagation model coupled with a reduced natural gas chemical kinetic mechanism. The results showed that the CFD modeling approach could predict the knock phenomenon in LNG engines reasonably well under different thermodynamic and flow field conditions.

In decade years, Spark Ignition-Controlled Auto Ignition (SI-CAI) hybrid combustion, also called Spark Assisted Compression Ignition (SACI) has shown its high-efficiency and low emissions advantages. However, high dilution causes the problem of unstable initial ignition and flame propagation, which leads to high cyclic variation of heat release and IMEP. The instability of SI-CAI hybrid combustion limits its dilution degree and its ability to improve the thermal efficiency. In order to solve instability problems and expand the dilution boundary of hybrid combustion, micro flame ignition (MFI) strategy is applied in gasoline hybrid combustion engines. Small amount of Dimethyl Ether (DME) chosen as the ignition fuel is injected into cylinder to form micro flame kernel, which can stabilize the ignition combustion process.

A two-dimensional mesoscopic approach has been developed to investigate the flow and soot loading in the micro-channels of diesel particulate filter. Soot particle size examined is in the range of 10 nm to 10 μm. The flow is solved by an incompressible lattice Boltzmann model and the transport of solid particle is described in a Lagrangian frame of reference by cell automation probabilistic model. The lattice Boltzmann-cell automation probabilistic model (LB-CA model) is validated with the results of previous studies. The heterogeneous porous wall of DPF is generated by quartet structure generation set (QSGS). The effects of porous wall on the pressure field and velocity field are investigated. The distribution and deposition of soot particles with different sizes in clean channels are simulated. The dynamic evolution of solid boundary in soot particle capture process is investigated and the effects of the deposited soot particles on flow field are evaluated.

As the prime after-treatment device for diesel particulate matter (PM) emission control, Diesel Particulate Filter (DPF) has been widely used for its high particle capture efficiency. In order to study the particle motion and deposition distributions in the DPF inlet channel, a 2-D wall flow DPF microcosmic channel model is built in this paper. The motion trajectories of particles with different sizes are investigated considering the drag force, Brownian motion, gravity and Saffman lift. The effects of the space velocity on particle motion trajectories and deposition distributions inside the inlet channel are evaluated. These results demonstrate that the particle motion trajectories are highly dependent on particle sizes and influenced by the space velocity. The effect of the Brownian motion is obvious for fine particles and suppressed when the space velocity is raised.

Methane as an attractive alternative fuel offers the most potential in clean combustion and low CO2 emissions. In this work, combustion characteristics of methane/gasoline dual-fuel were investigated in a spark-ignited engine with port-injection of methane and direct-injection of gasoline, allowing for variations in methane addition and excess air coefficient. Engine experimental results showed that under low load conditions, as methane mass rate was raised, there was a promotion in methane/gasoline dual-fuel combustion, and this became more obvious at lean conditions. Similar observations were also obtained when the engine was operated at intermediate load conditions, but a prolonged combustion duration was found with the methane addition. Further analysis showed that the promotion of methane/gasoline dual-fuel combustion with methane addition mainly occurred in the early stage of combustion, especially for lean conditions.

Controlled Auto-Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), has the potential to improve gasoline engines’ efficiency and simultaneously achieve ultra-low NOx emissions. Two of the primary obstacles for applying CAI combustion are the control of combustion phasing and the maximum heat release rate. To solve these problems, dimethyl ether (DME) was directly injected into the cylinder to generate multi-point micro-flame through compression in order to manage the entire heat release of gasoline in the cylinder through port fuel injection, which is known as micro-flame ignited (MFI) hybrid combustion.

For uniflow scavenged two-stroke marine diesel engines, the main function of scavenging process is to replace the burned gas with fresh charge. The end state of scavenging process is integral to the subsequent compression and combustion, thereby affecting the engine’s fuel economy, power output and emissions. In this paper, a complete working cycle of a large marine diesel engine was simulated by using the 3D-CFD software CONVERGE. The model was validated by mesh sensitivity test and experiment data. Based on this calibrated model, the influences of swirl ratio and exhaust valve closing (EVC) timing on the scavenging process were investigated. The parameters evaluating the performance of scavenging process were introduced. The results show that, by adjusting the swirl orientation angle(SOA) from SOA=10° to SOA=30°, different swirl ratios are generated and have obvious differences in flow characteristics and scavenging performance.

The dual-fuel Reactivity Controlled Compression Ignition (RCCI) combustion could achieve high efficiency and low emissions over a wide range of operating conditions. However, further high load extension is limited by the excessive pressure rise rate and soot emission. Polyoxymethylene dimethyl ethers (PODE), a novel diesel alternative fuel, has the capability to achieve stoichiometric smoke-free RCCI combustion due to its high oxygen content and unique molecule structure. In this study, experimental investigations on high load extension of gasoline/PODE RCCI operation were conducted using late intake valve closing (LIVC) strategy and intake boosting in a single-cylinder, heavy-duty diesel engine. The experimental results show that the upper load can be effectively extended through boosting and LIVC with gasoline/PODE stoichiometric operation.

The evolution of surface functional groups (SFGs) and the graphitization degree of soot generated in premixed methane flames are studied and the correlation between them is discussed. Test soot samples were obtained from an optimized thermophoretic sampling system and probe sampling system. The SFGs of soot were determined by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) after removing the soluble impurities from the soot samples, while the graphitization degree of soot was characterized by Raman spectrum and electron energy loss spectroscopy (EELS). The results reveal that the number of aliphatic C-H groups and C=O groups shows an initial increase and then decrease in the sooting history. The large amount of aliphatic C-H groups and small amount of aromatic C-H groups in the early stage of the soot mass growth process indicate that aliphatic C-H groups make a major contribution to the early stage of soot mass growth.

Although diesel engines have higher output torque, lower fuel consumption, and lower HC pollutant emissions, larger amounts of NOx and PM are emitted, compared with equivalent gasoline engines. The diesel particulate filters (DPF) have proved one of the most promising aftertreatment technologies due to the more stringent particulate matters (PM) regulations. In this study, the computational fluid dynamics (CFD) model of DPF was built by utilizing AVL-Fire software code. The main objective of this paper was to investigate the pressure drop and soot regeneration characteristics of hexagonal and conventional square cell DPFs with various inlet mass flow rates, inlet temperatures, cell densities, soot loads and ash loads. Different cell geometry shapes of DPF were evaluated under various ash distribution types.

Although diesel engines offer higher thermal efficiency and lower fuel consumption, larger amounts of Particulate Matters (PM) are emitted in comparison with gasoline engines. The Diesel Particulate Filters (DPF) have proved one of the most promising technologies due to the “particle number” emissions regulations. In this study, the Computational Fluid Dynamics (CFD) multi-channel model of DPF was built properly by utilizing AVL-Fire software code to evaluate the pressure drop and soot accumulation characteristics of DPF. The main objective of this paper was to investigate the effects of soot (capacity and deposit forms) and ash (capacity and distribution factors) interaction on DPF pressure drop and soot accumulation, as well as the effects of DPF boundary conditions (inlet mass flow rate and inlet temperature) on pressure drop.

This research presents an experimental study of the laminar burning combustion and emission characteristics of premixed methane -dissociated methanol-air mixtures in a constant volume combustion chamber. All experiments were conducted at 3 bar initial pressure and 373K initial temperature. The dissociated methanol fractions were from 20% to 80% with 20% intervals, and the equivalence ratio varied from 0.6 to 1.8 with 0.2 intervals. The images of flame propagation were visualized by using a schlieren system. The combustion pressure data were measured and exhaust emissions were sampled with a portable exhaust gas analyzer. The results show that the unstretched laminar burning velocities increased significantly with dissociated methanol enrichment. The Markstein length decreased with increasing dissociated methanol fraction and decreasing equivalence ratio.

The combustion in low-speed two-stroke marine diesel engines can be characterized as large spatial and temporal scales combustion. One of the most effective measures to reduce NOx emissions is to reduce the local maximum combustion temperature. In the current study, multi-dimensional numerical simulations have been conducted to explore the potential of Miller cycle, high compression ratio coupled with EGR (Exhaust Gas Recirculation) and WEF (water emulsified fuel) to improve the trade-off relationship of NOx-ISFC (indicated specific fuel consumption) in a low-speed two-stroke marine engine. The results show that the EGR ratio could be reduced combined with WEF to meet the Tier III emission regulation. The penalty on fuel consumption with EGR and WEF could be offset by Miller cycle and high geometric compression ratio.

In order to further study the effects of air and EGR dilution on the fuel economy improvement of natural gas engines under the premise of meeting EU6 legislation, a comparison between stoichiometric operation with EGR and lean burn operation with and without EGR has been conducted at 1600rpm 50% and 75% load. The conversion efficiencies of the catalysts for both NOx and CH4 emissions are assumed at 90% for lean burn operation. Experiment results indicate that under the condition of meeting both NOx and CH4 predetermined engine-out emissions limits for EU6 legislation, lean operation with a small fraction of EGR dilution enables more advanced combustion phasing compared to pure lean operation, which results in much better fuel economy, thus further improvement compared to stoichiometric operation is achieved.

The main objective of this paper was to investigate the pressure drop characteristics of ACT (asymmetric cell technology) design filter with various inlet mass flow rates, soot loads and ash loads by utilizing 1-D computational Fluid Dynamics (CFD) method. The model was established by AVL Boost code. Different ratios of inlet to outlet channel width inside the DPF (Diesel Particulate Filter) were investigated to determine the optimal structure in practical applications, as well as the effect of soot and ash interaction on pressure loss. The results proved that pressure drop sensitivity of different inlet/outlet channel width ratios increases with the increased inlet mass flow rate and soot load. The pressure drop increases with the increased channel width ratio at the same mass flow rate. When there is little soot deposits inside DPF, the pressure drop increases with the bigger inlet.

The exhaust aftertreatment strategy is one of the most fundamental aspects of spark ignition engine technologies. For various types of engines (e.g., carburetor engine, PFI engine and GDI engine), measuring, purifying, modeling, and control strategies regarding the exhaust aftertreatment systems vary significantly. The primary goal of exhaust aftetreatment systems is to reduce the exhaust emission levels of NOx, HC and CO as well as to lower combustion soot. In general, there is a tradeoff among different engine performance aspects. The exhaust catalytic systems, such as the three way catalyst (TWC) and lean NOx trap (LNT) converters, can be applied together with the development of other engine technologies (e.g., variable valve timing, cold start). With respect to engine soot, some advanced diagnosing techniques are essential to obtain thorough investigation of exhaust emission mechanisms.