06-02-2012, 04:59 PM
COMBUSTION STABILITY IN I.C. ENGINES
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Introduction
CNG as an alternative vehicle fuel :
Protection of the environment and energy issues have become increasingly important world-wide concerns with regard to internal combustion engines. Natural gas, being a clean burning and plentiful resource, is in many ways a good alternative fuel to meet these current and future requirements. Typically, emissions of carbon monoxide, reactive (non-methane) hydrocarbons and particulate matter are low, but emissions of oxides of nitrogen have been seen to be relatively high. The nature of a compressed natural gas (CNG) fuel system, being sealed and utilising a gaseous fuel, tends to avoid problems associated with evaporative emissions and cold start enrichment seen in gasoline engines. Additionally, the total emissions of reactive organic gases from fuel storage and refuelling associated with the use of CNG vehicles have been shown to be low . In comparison to gasoline CNG has a low energy density, intake of air is reduced (due to the feed of gaseous fuel into the intake manifold), and the flame speed is also lower.
Exhaust Gas Recirculation (EGR) for NG engines :
It has been noted that natural gas fuelled engines can produce undesirably high NOx emissions. However, NOx production is a by-product of good, efficient combustion.
Thus, the concept explored in this study involves the use of relatively high levels of EGR dilution as a means of emissions reduction. The effect of EGR is to reduce both flame temperature and flame speed. Oxides of nitrogen form at high temperatures from nitrogen and free oxygen
Hydrogen enrichment of Natural Gas :
The concept explored in this paper utilises hydrogen enrichment to extend the tolerance of the natural gas engine to EGR, thus allowing operation at high levels of EGR dilution whilst maintaining combustion stability. It is suggested that the high flame speed and low flammability limit of hydrogen have the effect, when added to the inlet charge, of increasing flame speed, improving combustion stability and reducing burn duration in a stoichiometric NG engine with high levels of EGR.
Fuel Reforming :
The use of hydrogen enrichment requires a suitable source of hydrogen on-board the vehicle. The concept tested here proposes a system of on-board fuel reforming to generate a hydrogen-rich reformed EGR stream.
Work at the Jet Propulsion Laboratory using a partial oxidation, process, or a combination of partial oxidation and steam reforming processes, demonstrated the use of forms of catalytic hydrogen generator to provide a hydrogen-rich gas for the enrichment of gasoline fuelled engines ( Here rich combustion of a part of the hydrocarbon fuel provides heat for the endothermic reaction. ).
