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To counteract the harmful effects of vehicle emissions on humans and the environment, such as global warming due to greenhouse gases, there is a focus on gaseous fuels as an alternative energy source of transportation. Heavy-duty natural gas vehicles are widely used to improve the air quality of urban areas in Korea because natural gas has the advantage of low greenhouse gas emission levels. However, more in-depth study is required in order for clean fuel vehicles to hold a dominant position over well-developed diesel vehicles. It is difficult to meet reinforced emission standards with only a lean combustion strategy without an aftertreatment system in a lean-burn natural gas engine. Hydrogen-natural gas (HCNG) blends have been proposed as an alternative to improve fuel economy and emissions of lean-burn natural gas engines, since they have a wider flammability range and faster burning speed. HCNG blends could also play a role as a technical bridge for the hydrogen era.

Reduction of carbon dioxide (CO₂) emission, which causes global warming, is an important guideline for vehicle engine development. There are two types of methods for reducing the CO₂ emission of a vehicle engine. The first involves improving engine efficiency. The second involves the use of a low-carbon fuel, i.e., fuel with high hydrogen to carbon ratio. Hydrogen-compressed natural gas blend (HCNG) has been researched as a low-carbon fuel. Given that thermal efficiency of an engine cycle increases with its compression ratio (CR), an HCNG engine with high compression ratio not only has high efficiency but also low CO₂ emission. However, unexpected combustion such as knock could occur owing to the increased CR. In this study, we investigated the knock and emission characteristics of an 11-L heavy-duty HCNG engine with a modified CR. A conventional CNG engine was fuelled with HCNG30 (CNG 70 vol% and hydrogen 30 vol%).

Natural gas produced from coal or biomass is known as synthetic natural gas (SNG), which is expected to replace compressed natural gas (CNG). In this study, we used an 11-l heavy-duty CNG engine in a feasibility study of SNG. SNG, which is composed of 90.95% methane, 6.05% propane, and 3% hydrogen, was produced for the experiment and used as fuel to estimate its effects on combustion and emission characteristics. The torque, fuel flow rate, efficiency, fuel consumption, combustion stability, combustion phase, and emissions characteristics obtained using SNG were compared to those obtained using CNG in an engine speed range of 1,000-2,100 rpm under full load conditions. In addition, an engine fueled with SNG was given an overall evaluation using the World Harmonized Stationary Cycle (WHSC) emission test. The engine's knock characteristic was analyzed at 1,260 rpm under a full load condition. The results showed that there was no difference in power output.

Efforts have been made to apply biogas to an IC engine for power generation as a way to cope with the energy crisis as well as to reduce greenhouse gas. However, due to its gas component variations by origin and low energy density, using biogas in the engine applications and achieving a steady power generation is not an easy task. One way to overcome these deficiencies is to add hydrogen in biogas. Because of the excellent combustion characteristics of hydrogen, use of hydrogen-biogas blend fuel can allow not only accomplishing stable in-cylinder combustion, but also reducing the harmful emissions such as THC and CO. Despite several advantages of this approach, there exists a major drawback~a significant increase in NOx emission caused by high adiabatic combustion temperature of hydrogen.

An 11L heavy duty LPG MPI engine has been developed using the liquid phase LPG injection system, which is one of the next generation LPG fueling technologies, since the LPG MPI engine can achieve the higher power and efficiency, and lower exhaust emissions than the conventional mixer type system. Two prototypes - a natural aspiration(NA) engine and a turbocharged inter-cooler(TCI) engine - were developed in this work and tested to measure the performance and emissions. For a NA type engine, in order to achieve the low emissions, the stoichiometric air/fuel ratio was adapted with a three-way catalytic converter. Whereas, for a TCI type, the lean burn technology was introduced to minimize the thermal loading due to an increase of the engine power. The results in this work demonstrated that the LPG MPI engines have the higher engine performance and lower exhaust emissions than the base diesel engine.

Influence of the inter-ring crevice, the volume between the top and second piston rings, on unburned hydrocarbon (UHC) emission was experimentally and numerically investigated. The ultimate goal of this study was to estimate the level of UHC emission induced by the blow-up of inter-ring mixture, i.e., unburned gases trapped in the inter-ring crevice. In the experiments, the inter-ring mixture was extracted to the crankcase during the late period of expansion and the early period of exhaust stroke through the engraved grooves on the lower part of cylinder wall. Extraction of the mixture resulted in the significant reductions of UHC emission in proportion to the increments of blowby flow rate, without any losses in efficiency and power. This experimental study has confirmed the importance of inter-ring crevice on UHC emission in an SI engine and established a relationship between the inter-ring mixture and UHC emission.