Progress on the studies about NOx emission in PFI-H2ICE
Introduction
The current development of human economy and society is believed as sitting on ‘vehicles’, the depletion of fossil fuels and the deterioration of ecological environment are the inevitable cost [1]. Researchers have found that most serious environmental problems (like global warming, acid rain, photochemical pollution, and so on) are related to the increasing harmful gases and inhalable particles into the atmosphere (as listed in Table 1), and the combustion emissions from internal combustion engines of vehicle are the major sources to such environmental pollutants [[1], [2], [3]]. Renewable fuels with cleaner combustion emissions have become ‘medicinal’ to ‘the sick vehicles’, hydrogen has been widely considered as one best alternative fuel for its superior combustion performance, infinite H2–H2O reaction cycle and the cleanest combustion products.
Within the recent decades, the improvements brought by hydrogen on both combustion performance and combustion emissions have been pointed out in some internal combustion engines and other practical thermal power equipment [[4], [5], [6]]. Verhelst [7] had reviewed the progress of hydrogen as the fuel for internal combustion engines, H2ICEs (hydrogen fuelled internal combustion engines) have been proven have high power performance and zero-carbon emission in the 100 km operation test [[8], [9], [10]]; therefore, both scholars and vehicle manufactures show great enthusiasm on the research and development of H2ICEs. Albeit the ideal reaction of H2 and O2 just products H2O without any conventional carbon-kind emissions (like CO, CO2, HC and soot), the high combustion temperature and the nitrogen gas in the cylinder will realise the reaction N2+O2→NOx. Therefore, H2ICEs still has the issue about combustion emissions but the emission could be regarded as just NOx, which is great simpler than any other fuels fuelled internal combustion engines. In fact, some scholars (like Das [11], Ma [12], and so on) had found that high-load conditions have serious NOx emissions on their H2ICEs experimental benches. Therefore, suppressing the generation of NOx is particularly important to H2ICEs.
To traditional fossil fuels fuelled internal combustion engines, since the reduction of NOx and HC hardly be realized at the same time for the different formation mechanism, and post-treatment technologies are very popular to reduce NOx emissions for the simple operation. TWC (three way catalyst) is one major post-treatment technology employed on vehicles, even some catalysts (like Nano-catalytic converter [13], electro-catalytic honey comb catalytic converter [14]) could realise a 84.3% NOx reduction even zero NOx emission, the actual improvement effects are closely related to the types of catalysts employed [15]. However, to H2ICEs, it is unnecessary to still rely on TWC to reduce NOx emission for two main reasons: (a) the cost of TWC, including the extra load cost on vehicles; and (b) NOx emission is the only combustion emission need to be solved. Therefore, controlling even eliminating the formation of NOx emission during the combustion process within the cylinder of H2ICEs is valuable to study.
H2ICE has two injection modes: direct injection (DI) and port fuel injection (PFI). Compared to DI- H2ICE, PFI-H2ICE has a more abundant mixing time, the injection parameters are more optimized, and it is more convenient to retrofit on the existing conventional internal combustion engines, and thus PFI-H2ICEs is a more common type of H2ICEs, the present article will focusing on the NOx emissions in PFI-H2ICE and systematically review the progress on related studies with the hope to guide researchers understand the current technologies on controlling NOx emissions in H2ICE.
Section snippets
Chemical reaction mechanism
In recent years, PFI-H2ICE is developing rapidly, researchers could obtain the variation rules of gas state parameters in the cylinder under different working conditions via both the means of experiments and numerical simulation. However, these objective laws and parameters could not comprehensively analyse the generation mechanism of NOx in H2ICE; therefore, more and more scholars begun to explore the generation mechanism of NOx upon chemical kinetics, and try to find the optimal control
Effects of equivalence ratio on NOx emission
As reviewed in the above, both the generation path and the amount of generation of NOx is closely related to equivalence ratio, and thus the controlling of equivalence ratio would be one important method for reducing NOx emission. Based on PFI-H2ICE, many scholars had carried out researches on the effect of equivalence ratio on NOx emissions, as listed in Table 2.
Sun et al. [22] carried out a performance test on a PFI-H2ICE and obtained the variation rule of NOx emission with the equivalence
Sumerries
Hydrogen fuelled engine is one promising alternative power device to vehicles for the remarkable potentials of higher thermal efficiency and lower exhaust emissions, but the existence of NOx emissions hardly makes hydrogen fuelled engines be a real ‘zero-emission’ engines. For providing sufficient understandings on the NOx emissions within hydrogen fuelled engines to solve the mentioned issue, this article first reviewed the studies about the formation of NOx emissions upon chemical dynamic
Acknowledgements
This work was supported by Fundamental Research Funds for the Central Universities under award 2018JBM044. Sincere thanks to Miss Wei-Qing WANG (Beijing Jiaotong University) and Mr Yang WANG (Beijing Jiaotong University) for their helps during the manuscript. Sincere thanks also to all the editors and all the reviewers for their works on this article.
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