SummaryIn order to decrease harmful exhaust gas emissions from internal combustion engines, researchers are continuously looking for alternative fuels and combustion techniques, and in this context the use of natural gas as a supplement for conventional fuels has been suggested. Since natural gas has high autoignition temperature, it cannot be used in compression ignition engines as the in-cylinder temperature around the top dead center is simply not high enough to ignite it. Therefore, in such an application natural gas has to be used with high reactivity fuel, usually diesel fuel. In these applications diesel fuel is used solely as a high energy ignition device, while natural gas is used as primary fuel. This type of combustion process is called dual fuel combustion process. This paper presents the effect of operating parameters, such as diesel fuel injection pressure, injection timing, diesel fuel mass, diesel fuel substitution ratio and intake charge conditioning, as well as engine load and speed on efficiency and harmful exhaust gas emissions from dual fuel diesel/natural gas internal combustion engines.

Compressed natural gas can be used in diesel engine with great benefits, but because of its low reactivity it is usually used in a so called dual fuel combustion process. Optimal parameters for dual fuel engines are not yet investigated thoroughly which is the motivation for this work. In this work, a numerical study performed in a cycle simulation tool (AVL Boost v2013) on the influence of different injection timings on fuel slip into exhaust and influence of compression ratio on knock phenomena in port injected dual fuel engine was conducted. The introduction of natural gas into the intake port of a diesel engine usually results in some fuel slipping into the exhaust port due to valve overlap. By analysing the simulation results, the injection strategy that significantly decreases the natural gas slip is defined. The knock occurrence study showed that the highest allowed compression ratio that will result in knock free operation of the presented engine is 18 for ambient intake condition, while for charged intake conditions the compression ratio should be lowered to 16.

Compressed natural gas is in automotive industry recognized as one of the "cleanest" fossil fuels which can be used in internal combustion engines with a number of benefits. Since natural gas has much higher octane rating than gasoline it is expected that higher compression ratios can be used. The goal of the research is to determine the change of performance of spark ignited engine with the increase of compression ratio to values similar to compression ignited engine while keeping the exhaust emissions on the acceptable level and avoiding knock combustion. Measurements are performed at compression ratios 12, 16, and 18 at three different values of air excess ratio. Methane with known composition from a pressure cylinder is used instead of natural gas and the results are comprised of indicating results (in-cylinder and intake pressure in a crank angle space), emissions, temperatures, and mass-flows on various intake and exhaust positions. Analysis of results shows high influence of compression ratio and excess air ratio on combustion, performance, and exhaust gas emissions.

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