| Small high-speed diesel engines have been playing a crucial role for a long time in the development of rural economy in China. With the increasingly serious energy and environmental issues, emission regulations for non-road compression ignition engines came into being. Swirl chamber diesel engine is in possession of an increasing proportion of single-cylinder diesel engine because of its advantages in emissions and noise. It has become an urgent task for researchers to develop green and energy-saving diesel engines for "three rural issues". Combining the advantages of direct-injection diesel engine and swirl combustion chamber diesel engine, a new direct-injection swirl chamber combustion system (DISCCS) is brought forward creatively.A swirl chamber is set inside the piston of a direct-injection diesel engine. A passage is formed for the swirl chamber by the orientation surface of the throat and the orientation surface of the central pedestal located in the center of the piston. The swirl chamber is located beneath this passage. During the compression stroke, the air is squished into the swirl chamber through the relative narrow passage, and forms a certain longitudinal swirl. After impingement on the combustion chamber wall, partial or all of the spray enters the swirl chamber after the second break-up, and mixes with the air rapidly. After ignition, cylinder pressure increases rapidly, and combustion flame and unburned mixture dash out of the swirl chamber. After ignition, the pressure inside the swirl chamber increases and that makes the combusting flame and unburned mixture rush out of the swirl chamber, and mixes with the air again and combustion completely over the swirl chamber. Therefore, this combustion system has the merits of low emission for swirl chamber, and also the high efficiency for reducing the heat lost through the cylinder head. For the aboved new combustion system, the following research work has been carried out in this paper:(1) Numerical simulations have been conducted using a 3-D CFD code AVL FIRE 2008 on mixture formation and combustion processes with different combustion chambers. Results showe that:The centralpedestal, passage width have great impacts on the combustion and emissions. The airflow intensity has an optimal value to emissions, no the stronger the better. The appropriate airflow can improve the quality of the mixture, while, over strong airflow will lead to poor mixture homogeneity.(2) Experiments have been done to investigate the influence of nozzle holes and the protrusion height on mixture formation and combustion processes. Results show that: For the DISCCS, the emissions with 4-hole nozzle are better than that with 6-hole nozzle. The 4×0.36×140°nozzle with protrusion height of 1.8mm has better performance than that of 2.8mm or 3.8mm. The reduction of nozzle protrusion height results in a larger spray dispersion range, a better air utilization and mixture quality, and such results in a lower Soot emission.(3) The engine performance experiments have been done with swirl combustion system in a 135 single-cylinder direct-injection diesel engine. With a 4×0.36×130°nozzle, whenθfs=8°CA BTDC under 10%～25% and 75%～90% load, NOx and Soot emissions are lower than the original engine. There is an obvious double-peak heat release rate in DISCCS. The first peak of heat release rate is lower than the original engine, while in medium-term of the combustion, due to the effect of combustion swirl and reverse-squish, the second peak of heat release rate is higher than the original engine.(4) In order to study the performance of the new combustion system under supercharging, a simulative supercharging system has been developed. The intake air density is increased by compressing air in a screw-type air compressor. The performance tests have been made using the original combustion chamber and swirl combustion chamber under different boost pressures in the 135 diesel engine. The experimental results show that the constant pressure supercharging can reduce both NOX and Soot emissions simultaneously, and can improve the fuel economy. When the boost pressure is 0.18MPa, with 6×0.3×150°nozzle, and and the original combustion chamber andθfS of 8°CA BTDC, the fuel consumption is 223.1g/kW·h (reduced by 16%) at 90% load, At the same time, the NOX emission is 219×10-6 (reduced by 81%) and Soot is 0.2BSU (reduced by 64%). The ignition delay is shortened because the outlet temperature from the compressor is 15°higher than the ambient temperature, which is equivalent to intake heating.(5) Exhaust gas recirculation is a low-cost technology for reducing NOX emission. The parallel Venture-EGR system has been developed. The venture is used to introduce exhaust gas into the supercharged intake air. In the swirl combustion chamber combustion system, the experimental results show that:EGR can significantly reduce NOx emission, but increase Soot emission. EGR results in a extended ignition delay period and higher first peak of heat release rate.
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