project maker
09-08-2014, 04:51 PM
COMBUSTION STABILITY IN I.C. ENGINES
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Abstract
A study was carried out to evaluate the potential of hydrogen enrichment to increase the tolerance of a stoichiometrically fuelled natural gas (NG) engine to high levels of dilution by exhaust gas recirculation (EGR). This provides significant gains in terms of exhaust emissions without the rapid reduction in combustion stability typically seen when applying EGR to a methane-fuelled engine. This paper gives the envelope of benefits from hydrogen enrichment. In parallel the performance of a catalytic exhaust-gas reforming reactor was studied in order that it could be used as an onboard source of hydrogen-rich EGR. It was shown that sufficient hydrogen was generated with currently available prototype catalysts to allow the engine, at the operating points considered, to tolerate up to 25% EGR, while maintaining a coefficient of variability of indicated mean effective pressure (IMEP) below 5%. This level of EGR gives a reduction in NO emissions greater than 80% .
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.
However, natural gas has high activation energy in comparison to other hydrocarbon fuels. The high ignition temperature and resistance to self-ignition result in excellent ‘antiknock’ properties; pure methane has an equivalent research octane number (RON) of 130, and so natural gas can safely be used in engines with higher compression ratios than are possible with gasoline. Work at Toyota on a converted ‘Camry’ SI-engined model has shown that many of the problems of low energy density can be overcome, with the range being two-thirds of the petrol fuelled model for an increase in kerb weight of 55kg.
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. ).
Early works looked at the processes of exhaust gas reforming of liquid automotive fuels (gasoline and heptane) by direct contact with simulated engine exhaust gases. The aim there was to study the feasibility of developing a system that would utilise the waste heat in the engine exhaust to drive the endothermic reforming reactions. The results obtained at high exhaust temperatures demonstrated the potential to generate over 30% hydrogen in the reformed fuel and to increase its calorific value by up to 28%.
The work presented here concerned the application of fuel reforming in natural gas engines in view of the aforementioned problems with the use of EGR in such engines. The latter part of this paper presents the specific improvements obtained in combustion quality by hydrogen addition and the potential of exhaust gas fuel reforming to generate the necessary hydrogen.
Conclusions
The use of EGR with addition of reformed fuel could potentially offer significant emissions improvements.
Tolerance of the NG engine to EGR, as measured in terms of combustion stability was shown to be greatly extended by addition of main components ( hydrogen and carbon monoxide) of a reformed fuel.
The proportion of hydrogen or hydrogen – carbon monoxide mixture required to maintain combustion stability has been quantified for a range of operating points.
Certain catalysts can be used to produce a reformed fuel of the required composition (over 20% hydrogen) from exhaust gases with natural gas added.
[attachment=66832]
Abstract
A study was carried out to evaluate the potential of hydrogen enrichment to increase the tolerance of a stoichiometrically fuelled natural gas (NG) engine to high levels of dilution by exhaust gas recirculation (EGR). This provides significant gains in terms of exhaust emissions without the rapid reduction in combustion stability typically seen when applying EGR to a methane-fuelled engine. This paper gives the envelope of benefits from hydrogen enrichment. In parallel the performance of a catalytic exhaust-gas reforming reactor was studied in order that it could be used as an onboard source of hydrogen-rich EGR. It was shown that sufficient hydrogen was generated with currently available prototype catalysts to allow the engine, at the operating points considered, to tolerate up to 25% EGR, while maintaining a coefficient of variability of indicated mean effective pressure (IMEP) below 5%. This level of EGR gives a reduction in NO emissions greater than 80% .
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.
However, natural gas has high activation energy in comparison to other hydrocarbon fuels. The high ignition temperature and resistance to self-ignition result in excellent ‘antiknock’ properties; pure methane has an equivalent research octane number (RON) of 130, and so natural gas can safely be used in engines with higher compression ratios than are possible with gasoline. Work at Toyota on a converted ‘Camry’ SI-engined model has shown that many of the problems of low energy density can be overcome, with the range being two-thirds of the petrol fuelled model for an increase in kerb weight of 55kg.
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. ).
Early works looked at the processes of exhaust gas reforming of liquid automotive fuels (gasoline and heptane) by direct contact with simulated engine exhaust gases. The aim there was to study the feasibility of developing a system that would utilise the waste heat in the engine exhaust to drive the endothermic reforming reactions. The results obtained at high exhaust temperatures demonstrated the potential to generate over 30% hydrogen in the reformed fuel and to increase its calorific value by up to 28%.
The work presented here concerned the application of fuel reforming in natural gas engines in view of the aforementioned problems with the use of EGR in such engines. The latter part of this paper presents the specific improvements obtained in combustion quality by hydrogen addition and the potential of exhaust gas fuel reforming to generate the necessary hydrogen.
Conclusions
The use of EGR with addition of reformed fuel could potentially offer significant emissions improvements.
Tolerance of the NG engine to EGR, as measured in terms of combustion stability was shown to be greatly extended by addition of main components ( hydrogen and carbon monoxide) of a reformed fuel.
The proportion of hydrogen or hydrogen – carbon monoxide mixture required to maintain combustion stability has been quantified for a range of operating points.
Certain catalysts can be used to produce a reformed fuel of the required composition (over 20% hydrogen) from exhaust gases with natural gas added.