Abstract
Since the pressure development in a combustion chamber is uniquely related to the combustion process, substantial variations in the combustion process on a cycle-by-cycle basis are occurring. To this end, an experimental study of cycle-by-cycle variation in a direct injection spark ignition engine fueled with natural gas- hydrogen blends combined with exhaust gas recirculation at relative air-fuel ratios was conducted. The impacts of relative air-fuel ratios (i.e. λ = 1.0, 1.2, 1.3 and 1.4 which represent stoichiometric, moderately lean, lean and very lean mixtures respectively), hydrogen fractions and EGR rates were studied. The results showed that increasing the relative air-fuel ratio increases the COVIMEP. The behavior is more pronounced at the larger relative air-fuel ratios. More so, for a specified EGR rate; increasing the hydrogen fractions decreases the maximum COVIMEP value just as increasing in EGR rates increases the maximum COVIMEP value. (i.e. When percentage EGR rates is increased from 0% to 17% and 20% respectively. The maximum COVIMEP value increases from 6.25% to 6.56% and 8.30% respectively). Since the introduction of hydrogen gas reduces the cycle-by-cycle combustion variation in engine cylinder; thus it can be concluded that addition of hydrogen into direct injection compressed natural gas engine employing EGR at various relative air-fuel ratios is a viable approach to obtain an improved combustion quality which correspond to lower coefficient of variation in imep, (COVIMEP ) in a direct injection compressed natural gas engine employing EGR at relative air-fuel ratios. Keywords—Coefficient of variation in indicated mean effective pressure (COVIMEP); Direct injection compressed natural gas (DI-CNG); Exhaust gas recirculation (EGR); Hydrogen fractions; Relative air-fuel ratios.
Highlights
An effective approach to overcome these challenge of slow burning velocity and poor lean burn capability of natural gas fuel is to mix natural gas with the fuel that possess fast burning velocity
From the foregoing; it is technically reasonable to say that addition of hydrogen into natural gas is a viable approach to improve engine stability at lean mixture operation
The main results are summarized as follows: 1. COVIMEP increases with increasing in relative air-fuel ratios (λ = 1.0, 1.1, 1.2, 1.3, and 1.4)
Summary
An effective approach to overcome these challenge of slow burning velocity and poor lean burn capability of natural gas fuel is to mix natural gas with the fuel that possess fast burning velocity. Hydrogen is regarded as the best gaseous candidate for natural gas due to its very fast burning velocity; much better lean capability and small quenching distance This combination is expected to improve the lean-burn characteristics and reduces the engine emission. Some approaches like ultra-lean combustion or EGR need to be adopted combining with hydrogen addition to realize low NOX emission in a natural gas-hydrogen fuelled engine [6].
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