1. Introduction

High carbon dioxide emissions can be reduced by increasing the efficiency of the internal combustion engine or reducing the carbon content of the fuel burned. Reasonable use of alternative fuels will allow one to reduce these emissions through the use of both of these factors. Effective reduction of GHG by reducing the number of carbon atoms in the fuel and improving the efficiency of the engine can be achieved thanks to the fuels presented in this study.

The use of alternative fuels in SI engines that are adapted to burn conventional hydrocarbon fuels such as gasoline shows no measurable increase in overall engine efficiency. The biggest impact that the use of an alternative fuel has on the overall efficiency of the engine is a change to its thermal efficiency. Homogeneous and low-temperature methods of combustion of the fuel–air mixture are the most effective methods to achieve high thermal efficiency of the engine. This translates directly into the overall efficiency of the engine and lowering of carbon dioxide emissions ^[1][2].

In the case of gaseous fuels, their combustion in diesel engines is difficult due to the specificity of the construction and operation of such an engine. Direct gas injection into the engine TDC is not yet a proven solution that allows for independent operation of a diesel engine using gas fuel ^[2][5].

A diesel engine can operate effectively using two fuels at the same time. The gaseous fuel burned in this type of engine is ignited by a small dose of diesel fuel (pilot dose), or another liquid fuel that replaces diesel and has similar properties to regular diesel. The current aims when developing alternative engine fuels are continuously increasing engine efficiency and reducing GHG emissions ^[3][6]. The most important greenhouse engine combustion gases are colored yellow in all Table 1 .

24. Low-CO₂ Emission Fuels for a Dual-Fuel RCCI Engine

Certain considerations in the preparation of this article left the authors in doubt about several points, and we address them here.

The work ^[5][61] contains quite specific information on the combustion characteristics of hydrogen mixed with natural gas. It was an original work by one author, which is currently not available among the other works by this author. Other publications by this author, where he appears most often with other authors, do not in most cases concern the topics we discussed, and in the case of one topic related to HCNG, they did not present such bold theoretical theses, but numerical and empirical studies and their results. Although the author’s publication ^[5][61] was a source of very interesting data, which hase have some doubts as to the validity (2012).

Papers looking at specific results of tests on HCNG with a specific composition are very difficult to compare with each other. The composition of fuels should be determined by the regulations governing its chemical and physical parameters. In many countries, fuel such as HCNG is not registered, so there are no regulations governing its composition. The composition itself may therefore be determined by researchers in a way that differs from other researchers, and the method itself may not be disclosed, because showing all information in scientific works is neither recommended nor common practice. Supposed differences in the composition of HCNG gas, which theoretically can be compared with each other, may in practice turn out to be incorrect. For this reason, one should be very careful when comparing results from different papers.

As already described in detail in the article, natural gas itself can significantly differ in chemical composition depending on the source, and the creation of Hythane is based 100% on this base fuel. The problem with the formation of ortho- and parahydrogen in hydrogen fuel has also been described in detail; it also introduces some inaccuracies in the physicochemical parameters of the final fuel. The use of even chemically pure hydrogen can lead to the use of hydrogen with undesirable physical properties for a given application. Hythane is a fuel that not only widely used yet, and it is very strongly dependent on the “component fuels” that create it; therefore, its parameterization is a noticeable problem for researchers, so we are not convinced that the current information on the precisely parameterized composition of HCNG can be relied upon.

There is also a lack of studies in the scientific literature that describe the use of Hythane in a dual-fuel or RCCI engine. This also applies to the possibility of using HVO as a pilot dose. In the context of the HVO, its use in “dual fueling” was also not tested, and that should be done. In a dual-fuel system, the focus is on replacing liquid fuel with gas fuel as much as possible, so the issue of pilot fuel becomes secondary. Nevertheless, HVO should be examined in terms of its ability to initiate combustion in RCCI and the emissions it puts out in this mode of operation. This leads to the conclusion that despite the analysis carried out, further research is needed in regard to the use of this type of low-emission alternative fuel in high-performance dual-fuel engines. Studies heading in this direction can be found. Take ^[6][98] as an example: studies of an engine powered by HCNG and B100 fuel were performed. Additional research is still needed on this subject though. The existing research of this type could not always be linked with the research on emissions of exhaust gas components related to the issues of global warming.

Due to the lack of a sufficient amount of research of this type, itwe have decided to also support these efforts with significant research on fuels of this type in single-fuel internal combustion engines, and further research will allow us to conduct a far more in-depth analysis on this subject of interest.

Finally, it is necessary to mention the extensive bibliography related to the subject of the dual-fuel engine itself. There is a lot of information about it. Readers may find the references on this subject helpful, but because the subject of the RCCI engine itself is not the focus of this article, the items related to it have been limited to the minimum necessary, which proves the wealth of literature available on this subject ^[3][7][8][6,79,80].

N₂O emissions are important for global warming, but the N₂O concentrations in IC engines’ exhaust gases in comparison to CO₂ concentrations are insignificant, and the number of researchers focusing on that topic is small. The greater problem is CH₄, the emission of which is proportionally more common, and with the advent of new regulations limiting the emission of harmful exhaust components, compliance with them may become problematic when fueling an engine with fuel based on natural gas.

The latest Euro 7 scenarios assume the introduction of a limitation of methane in the exhaust gas, measured as a separate component of the exhaust gas. Its permitted emission level differs in the two main scenarios envisaged for the new Euro 7 standard. Table 213 and Table 314 show the emission limits for light and heavy-duty vehicles, taking into account Euro 6, which is already in force, and the two scenarios for Euro 7: A and B.

Table 213. Euro 7 emission limits scenarios—LDV in mg/km, #/km ^[9][99].

Table 314. Euro 7 emission limits scenarios—HDV in mg/kWh and #/kWh ^[9][99].

The predicted emission limit for methane is very strict and may prevent the use of CNG and LNG as standalone fuels, as many engines powered by these fuels may have serious problems with meeting such a stringent standard. Depending on which scenario is finally introduced by the EU, alternative fuels such as HCNG and HVO may have a significant impact on the possibility of meeting them, thanks to the possibility of effectively reducing greenhouse gas emissions caused by internal combustion engines.

35. Conclusions

Alternative fuels could be the key to reducing CO₂ emissions and other greenhouse gases in the combustion process.

Researchers regularly write about the advantages of using hydrogen as a fuel for internal combustion engines. For example: “A hydrogen fueled internal combustion engine has great advantages on exhaust emissions including carbon dioxide (CO₂) emission in comparison with a conventional engine fueling fossil fuel” ^[10][100]. One must bear in mind the difficulties of using it on its own. However, fuels rich with it enable reductions in CO₂ emissions.

Both HCNG and CNG are commonly researched and used in IC engines. The amount of CNG research by far exceeds the amount of HCNG research, and the same is true of the number of applications in practice. Both of these fuels have shown high potential for reducing CO₂ emissions in tests. HCNG has been more impressive, and CO₂ emissions decreased with the content of hydrogen in the fuel. There is also a lack of widespread research on N₂O emissions, which indicates further uncertainty about the climate impact of this type of fuel.

The studies that were successfully analyzed do not strictly concern the influences of the analyzed fuels on global warming. The works that treated the analyzed fuels as sources of power for dual-fuel or RCCI engines were also rare. In the case of the fuels selected by the authors, the co-combustion of which would have the greatest potential to reduce CO₂ emissions, it was not possible to obtain a research paper in which both these fuels (HVO and HCNG) were used in RCCI engine emission tests. Despite the lack of consistent reference data, the overall picture of the knowledge available indicates the validity of further work in the context of RCCI with a dual-fuel supply of HCNG and HVO (as a pilot dose). All data on the emissions of the combustion of these fuels in mono-fuel systems report measurable benefits through reduced GHG emissions. Further work should be empirical, or measure engine performance and emissions on laboratory engine beds, in dyno testing, and in tests in real cars travelling in traffic.

ItWe noticed a problem with the parameterization of the physicochemical properties of gaseous fuels. The spread between the various fuels is big; the problem is strongest for NG and HCNG fuels. Big differences in parameters are not only allowed by regulations. Large differences in the parameters of some fuels are not only allowed by the regulations, but also often in cases of new fuels, such as HCNG, are not covered by them at all. This is certainly a fact used by their producers. Even in research, there are noticeable differences in the chemical composition of different HCNG, which also did not agree with the theoretical calculations carried out by the author. For this reason, the properties of natural gas taken from one of the cited works were used in the Table 415 below.

Table 415. Summary table with a comparison of important fuel parameters ^{[11][12][13][14][15][16][17][18]}[11,66,67,68,101,102,103,104].