1. Introduction
As the global community’s concern about climate change and environmental sustainability grows, so does the demand for cleaner and greener transportation options. The most energy-intensive and environmentally dangerous element of a vehicle is its power plant. One of the ways to improve the energy efficiency and environmental performance of a vehicle power plant is the use of alternative types of fuel. Fuels such as hydrogen, natural gas, biofuels, ethanol, dimethyl ether, propane–butane mixtures, and electricity can significantly reduce greenhouse gas and air pollutants emissions compared to conventional fossil fuels. Alternative fuels can diversify the energy sources used for transportation, reducing dependence on imported oil and increasing energy security. This is especially relevant for countries that are largely dependent on oil imports for their transportation needs. The authors [
1] investigated methods to evaluate the efficiency of vehicles with traditional and alternative power plants to ensure a rational choice of transport technology under given conditions. Despite the fact that there are problems, for example, the development of infrastructure for the production, distribution and refuelling of hydrogen, the authors proved that its application is a promising option for sustainable transport.
Hydrogen can be used as a main fuel and as an additive in gasoline, diesel, and gas engines and can form mixtures in different ways. One common way is to use a hydrogen generator. The engine will burn less fuel by adding hydrogen simultaneously, releasing fewer toxic components. A smaller amount of the toxic components is emitted with more complete fuel combustion [
2]. The incomplete combustion of petroleum fuels is the main cause of HC and CO emissions in diesel and petrol engines.
The use of hydrogen in vehicle power plants as an additive to the main motor fuel is currently subject to a great deal of discussions. This is due to the different methods to obtain such fuel and its physical and chemical properties [
3]. The fact is that the addition of hydrogen is not only able to replace the energy resource of the decreasing part of diesel or petrol fuel; the action of hydrogen is more interesting, as hydrogen has a high diffusion rate and therefore can form a homogeneous mixture in the combustion chambers of the engine in a shorter period [
4].
It should also be noted [
5] that the flame expansion rate (burning speed) of hydrogen is significantly higher than the flame expansion rate of similar mixtures based on diesel or petrol fuel. Based on the results obtained in [
5], the characteristics of the studied fuels are presented (
Table 1).
The combustion time of the fuel mixture is significantly reduced with minimal additions of hydrogen [
6]. This is because hydrogen effectively ignites the fuel mixture in the entire cylinder volume, mixing with the entering their mixture. This is because hydrogen molecules can act as initiating centres during the combustion of hydrocarbon fuels [
7]. When the engine is running, it means ignition can be delayed by adding hydrogen and the amount of the main fuel to be used can be reduced.
Hydrogen is the lightest gas; for example, it is 14.5 times lighter than air. If the mass of the molecules is less, it will have a higher speed at the same temperature. Thus, hydrogen molecules move faster than the molecules of any other gas and therefore can transfer heat from one body to another faster [
8]. It follows from this that hydrogen has the highest thermal conductivity among gases and, for example, its thermal conductivity is approximately seven times higher than the thermal conductivity of atmospheric air.
Considering that the purpose of using hydrogen in road transport is to improve the economic and environmental performance of cars, the issues of the physical and chemical properties of hydrogen, and the methods for choosing economically profitable and environmentally safe production and storage methods of hydrogen will be extremely important [
9].
The following methods of hydrogen storage are fundamentally possible at present [
10]:
In a gaseous state under pressure in containers;
In the solid-phase bound state in metal hydrides;
In a chemically bound state in liquid media;
In a chemically liquid state in cryogenic tanks.
The following ways have been the most adopted [
11]: in a liquefied state in cryogenic tanks, in the compressed gas in high-pressure cylinders, and in a bound state in metal hydride batteries. Based on the results [
11], a unified approach to choosing an acceptable method of storing hydrogen in cars is determined (
Table 2).
Despite the obvious advantages of hybrid tanks over metal cylinders, the major disadvantage of hybrid tanks is the significant cost of materials for the manufacture of such a tank and their still quite significant weight [
12]. In addition, the slow release of hydrogen from the hydride presents the problem of obtaining the required amount of hydrogen at various engine operating modes. Another big problem for all methods of storing hydrogen on a vehicle board is the small number of hydrogen refuelling stations. However, the creation of a developed hydrogen infrastructure requires a huge amount of financial cost and is a complex technical task. Therefore, a promising direction is the development and use of technical solutions to produce hydrogen on a carboard.
Hydrogen practically does not occur in nature in a pure form and must be extracted from other compounds using various methods [
13]. A huge number of substances containing hydrogen and a variety of methods for producing hydrogen are the two main advantages of hydrogen energy.
The most common ways to produce hydrogen are the following [
14]:
Steam methane conversion and its homologues are the main industrial method of producing hydrogen. The process is as follows: steam reacts with natural gas at low pressures and high temperatures and in the presence of a catalyst. Depending on further use, hydrogen is produced under different pressures: from 1.0 to 4.2 MPa. Using steam conversion, hydrogen can be produced in varying purities, typically 95–98%. The raw material (light oil fractions or natural gas) is heated to 350–400 °C in a heat exchanger or convection oven and enters the desulphurisation apparatus. After the stages of low-temperature and high-temperature conversion, the gas is supplied for the adsorption of CO
2 and CO. The result is hydrogen of up to 98.5% purity containing 1–5% methane [
15].
In case it is necessary to obtain especially pure hydrogen, an adsorption gas separation unit is used. The gas mixture containing CH
4, CO
2, H
2O, and CO is cooled to remove water and sent to adsorption devices with zeolites. All impurities are adsorbed in one step at ambient temperature. The pressure of the produced hydrogen is 1.5–2.0 MPa. The result is hydrogen with a purity of 99.99% [
16].
Currently, approximately half of the global hydrogen market is produced using this method. The inconvenience of using this method on a car is that the reactor to produce hydrogen is large and cannot be placed on a transport base [
17]. But this method cannot be completely excluded from consideration for cars because in the future it is possible to develop a more compact reactor.
During autothermal reforming, a mixture of natural gas, steam, and oxygen is supplied to the reaction catalytic volume in proportions in which one part of the methane reacts with water vapor, and the other burns in oxygen, producing hydrogen and carbon oxides. The partial combustion of natural gas provides the high temperature required for the shift reaction [
18].
Natural gas is heated to temperatures above 1000 °C in the process of methane cracking at which the process of decomposition of the methane molecule into carbon and hydrogen occurs. The process has a 2-fold lower yield of hydrogen from methane but with a high degree of methane decomposition; it allows hydrogen to be isolated at a lower cost. This process is promising because the reaction produces two valuable products: carbon and hydrogen. Unlike the oxidative transformations of methane, the resulting hydrogen does not require purification from impurities, in particular CO
2 and CO [
19].
The production of hydrogen via the electrolysis of water is the simplest method. Two electrodes are placed in water and a voltage is applied to them. Since pure water is a poor conductor of an electric current, electrolytes (potassium hydroxide KOH, etc.) are added to it. Oxygen is released at the anode during electrolysis and an equivalent amount of hydrogen is released at the cathode. The hydrogen produced using electrolysis is very pure, except for the admixture of small amounts of oxygen, which, if necessary, can be easily removed by passing the hydrogen over suitable catalysts, for example over palladium. However, in installations operating on this principle, 4–5 kWh of electricity is required to produce one cubic metre of hydrogen, which is quite expensive and difficult to provide on a carboard [
20].
A significant number of automobiles, especially commercial ones, use diesel internal combustion engines. This requires the high cost of expensive diesel fuel which is characterised by a very significant toxicity of exhaust gases, in particular nitrogen oxides. It requires the use of increasingly complex and expensive equipment to reduce the toxicity of exhaust gases [
21]. Modern diesel engines have practically exhausted their potential for improving environmental performance, as the cost of onboard automotive equipment to neutralise toxic components becomes unreasonably expensive.
In this regard, a significant number of car manufacturers have announced their plans to stop the production of cars with engines running on diesel fuel already in the coming years. Therefore, it is obvious that it is necessary to find ways to transfer piston engines from diesel fuel to other alternative types of fuel, the main characteristics of which should be better environmental characteristics and a cost that will be lower than traditional petroleum fuels [
22]. It should also be noted that diesel engines have a large resource before major repairs. Therefore, even after the production of new cars with diesel engines is stopped, already-produced cars with diesel engines will continue to drive for a long time. Therefore, the problem of converting already-manufactured cars with diesel engines to other alternative fuel types of fuel will be an urgent issue.
The conversion of diesel-powered vehicles to gas fuels deserves special attention. Technologies for the conversion of diesel engines to methane or liquefied petroleum gas (propane–butane) are known [
23]. However, these gases are a non-renewable resource that can be used in a limited way. This applies especially to such a resource as liquefied petroleum gas.
Several studies have been conducted on the benefits of introducing hydrogen in small quantities to diesel engines. In these studies, diesel fuel was injected as a pilot fuel in the usual way [
24]. The results showed a decrease in toxic components in exhaust gases, with carbon emissions decreasing from 34 to 31.5 ppm and CO emissions decreasing from 0.09 to 0.045%. Furthermore, there was an increase in engine power by 29%. Another article [
25] presents experimental data on the use of hydrogen as a fuel for a diesel engine. The study used different amounts of hydrogen supply in the range of 18–34% to diesel fuel. The results showed that CO
2 emissions were reduced by up to 20%, NOx emissions were reduced by up to 50%, and smoke was reduced by up to 74% compared to monodiesel fuel. The higher calorific value and burning rate of hydrogen also provided an increase in power up to 27%.
The analysis shows that hydrogen and its mixtures are a promising type of automotive fuel but the methods of storing hydrogen or obtaining it on board the car are currently quite expensive and technically difficult. Therefore, the purpose of the proposed work is theoretical and experimental research on the conversion of diesel engines to alternative mixed hydrogen gas fuels is needed.
The tasks of this work are as follows:
To theoretically investigate the possibilities of converting diesel engines to alternative mixed hydrogen gas fuels based on methanol with hydrogen generation on the carboard;
To experimentally investigate the main energy and environmental characteristics of the D21 diesel engine converted to operate on alternative mixed hydrogen gas fuels based on methanol.
3. Results
The research of the hydrogen reactor operation for methanol conversion included determining the zone of stable operation in terms of a hydrogen gas mixture, checking its performance, assessing the productivity of the main components of the synthesised mixture, and taking the main characteristics that prove the relationship between the set of the products obtained in the hydrogen reactor, the degree of hydrogen conversion, and the temperature regime of the hydrogen conversion process.
One of the largest problems for the normal functioning of the hydrogen conversion system is providing it with the correct amount of energy and a given level of temperatures in the hydrogen reaction space to achieve the most in-depth hydrogen thermochemical conversion process and the maximum rate of methanol conversion (Methanol Conversion—MC). At MC < 100%, methanol products at their outlet from the hydrogen reactor, along with H2 and CO, contain vapours of unreacted liquefied methanol.
Table 5 shows the temperature area of the experimental hydrogen reactor.
The important parameters of a hydrogen conversion reactor include the dependence of the rate of conversion on the temperature in the catalytic reactor. Such characteristics can only be reached on the basis of the experiment (
Figure 8). It was experimentally found that almost full conversion (MC > 80%) of methanol is achieved in the hydrogen reactor at 545–550 K.
The chosen medium contributes to the process of methanol transformation already at a temperature of 505–510 K, making it possible to convert methyl alcohol with the release of a hydrogen gas in small load modes of the engine with a temperature shortfall.
Figure 8 shows the dependence of the H
2 and CO receiving on the temperature
T in the hydrogen reactor. Experiments have shown that the maximum CO content is 35% and the maximum possible H
2 content is close to 65%. In general, analysing the results of an experimental study of the parameters of the hydrogen conversion reactor, the temperature level, above which the CH
3OH decomposition reaction can be considered complete, corresponds to 580–590 K.
The test dependences of specific fuel consumption
ge on the crankshaft rotation
n of the experimental engine D21 working on diesel fuel, hydrogen fuel mixtures, and methanol fuel are shown in
Figure 9.
It was found through analysing the tested power that for diesel fuel the minimal specific fuel consumption at crankshaft rotation n = 1250 min−1 was 214 g/(kWh); specific fuel consumption at the nominal crankshaft rotation n = 1850 min−1 was 252 g/(kWh). The minimum specific fuel consumption at crankshaft rotation n = 1250 min−1 was 464 g/(kWh) for methanol; specific fuel consumption at nominal crankshaft rotation was 552 g/(kWh). The minimum consumption of specific hydrogen fuel mixtures was 384 g/(kWh); specific hydrogen fuel mixtures consumption at nominal crankshaft rotation was 452 g/(kWh).
The specific fuel consumption of the crankshaft rotation in the entire range of the crankshaft rotation compared to diesel working on 100% of hydrogen fuel mixtures increased by 78–82%. Specific fuel consumption from the crankshaft rotation on average in all ranges of the crankshaft rotation, compared to diesel working on 100% of methanol, increased by 110–115%.
The test dependences of the power parameters of the experimental engine D21 converted for hydrogen fuel mixtures, diesel and methanol fuel are shown in
Figure 10. It was found through analysing tested power that at crankshaft rotation
n = 1850 min
−1 the nominal effective power for the diesel engine was 18.2 kW.
When analysing the test power, it was found that at crankshaft rotation n = 1850 min−1 the nominal effective power for methanol (6.45 kg of air on 1 kg of CH3OH) was 17.2 kW, but on the hydrogen fuel mixtures (6.45 kg of air on 1 kg of mixtures), it was equal to 21.3 kW. The value of the average effective power of the engine in the whole range of crankshaft rotation in comparison to the diesel working on methanol (6.45 kg of air in 1 kg of CH3OH) decreased by 5.5%, and for hydrogen fuel mixtures (6.45 kg of air on 1 kg of mixtures) it increased by 17.0%.
Analysing the change in toxic components in the exhaust gases engine during the transition from diesel to hydrogen fuel mixtures (6.45 kg of air on 1 kg of mixtures) the following can be noted. There is a large decrease in nitrogen oxides in all ranges of the rotation of the crankshaft (
Figure 11). Thus, at
n = 790–810 min
−1 of the crankshaft rotation, the nitrogen oxides decreased from 1140 ppm (8.4 g/kW·h) when the engine was on diesel to 455 ppm (3.4 g/kW·h) when the engine was on the hydrogen fuel mixtures. The decrease in the nitrogen oxides was 60.1%. At
n = 1840–1860 min
−1 the nitrogen oxides decreased from 830 ppm (6.1 g/kW·h) when the engine was on diesel to 310 ppm (2.3 g/kW·h) when the engine was on the hydrogen fuel mixtures. That is, the decrease in the nitrogen oxides was 62.6%. The decrease in the nitrogen oxides during the engine on diesel in comparison with the engine on the hydrogen fuel mixtures is explained through lower thermal dissipation and a lower increase in the combustion pressure of the engine.
There is a large decrease in CO in all ranges of the crankshaft rotation (
Figure 12).
Thus, at the crankshaft rotation n = 790–810 min−1, the carbon monoxide decreased from 0.228% when the engine was on diesel to 0.085% when the engine was running on the hydrogen fuel mixtures. At crankshaft rotation n = 1840–1860 min−1, the carbon monoxide decreased from 0.122% when the engine was on diesel to 0.072% when the engine wa on hydrogen fuel mixtures. The decrease in CO content occurs in the range of 51.6–61.8%.
The hydrocarbon C
nH
m content increases some in all the ranges of crankshaft rotation (
Figure 13).
Thus, at the crankshaft rotation n = 790–810 min−1, the CnHm content varies from 62 ppm when the engine is on diesel to 32 ppm when the engine is on hydrogen fuel mixtures. That is, the decrease in CnHm content was 30 ppm. At crankshaft rotation n = 1840–1860 min−1, the CnHm content increases from 96 pm when the engine is on diesel to 155 ppm when the engine is on hydrogen fuel mixtures. That is, the increase in hydrocarbons was 1.6 times greater.
The experiments showed that the CnHm content for the diesel engine was lower in most modes. However, for this particular engine at close to idle speeds, the CnHm content for the diesel engine was higher. In our opinion, the higher level of CnHm emissions from a diesel engine with a low crankshaft rotation is caused by the incomplete evaporation and combustion of the D21 diesel engine with a mechanical injection system of diesel fuel. Hydrogen fuel mixtures arrive already in a gaseous state, which improves the combustion process.
4. Discussion
The experiments showed that the fuel-economy indicators during the engine run with a catalytic reactor on a hydrogen mixture at all the modes of engine crankshaft rotation frequency were higher (on average by 15–21%) than when the engine operated on pure methanol without a hydrogen reactor. At the same time, at low crankshaft rotation frequencies (800 to 1000 min
−1), the specific fuel consumption when the engine was running on a hydrogen mixture with a catalytic reactor differed less than the specific fuel consumption when the engine was running on pure methanol. This is obviously explained by the relatively low temperature levels of exhaust gases when the engine is operating at low loads, which is in good agreement with the work of [
39]. The low temperatures of the exhaust gases, in turn, lead to a somewhat lower productivity of the reactor in terms of hydrogen output.
The BSFC of the obtained mixtures has a high value. This is due to the relatively low heat of the combustion of methanol, which is 22.5 MJ/kg compared to 42.5 MJ/kg for diesel fuel. However, at an air/fuel ratio of 6.45/1 for methanol, the calorific value of the converted methanol mixture will be approximately 3950 kJ/m3, compared to 3400 kJ/m3 for the diesel mixture. Therefore, the power of the experimental engine when operating on a mixture of converted methanol during the experiments was higher than when the engine was operating on diesel fuel.
The greatest increase in thermal efficiency and decrease in specific fuel consumption (by 20–21% compared to operation on pure methanol) was observed in the range of engine crankshaft rotation frequency from 1250 to 1300 min−1 (at exhaust gas temperatures in the range of 450–500 °C). This is explained by the fact that in this range of exhaust gas temperatures, the consumed thermal energy of the reactor and the thermal energy of the exhaust gases already become practically the same. Experimental studies have shown that in the regimes of crankshaft revolutions and engine loads, when the temperature of exhaust gases at the entrance to the reactor exceeds 450 °C, the performance of the reactor, taking into account the hydrogen component, reaches the highest value, which maximally reduces the specific fuel consumption.
As confirmed by the authors [
40], the improvement in the fuel economic parameters of the experimental engine with the proposed method of hydrogen fuel formation is due to the effect of two features: the effect of utilisation of waste gas heat and the improvement of kinetic combustion in the fuel due to the presence of hydrogen.
Compared to diesel fuel, the specific fuel consumption of an engine running on hydrogen methanol conversion mixtures has increased by 82%. Since the price of methanol is, on average, 30–35% of the cost of diesel fuel (
Table 6), the conversion of diesel engines to work on hydrogen mixtures of methanol conversion is very profitable. The average cost of diesel in the EU ranges from 1.3 to 1.8 euro/L [
41].
Methanol is currently identified as a transition fuel for industrial decarbonisation; among potential fuel alternatives (with significantly lower carbon content compared to diesel or gasoline), methanol is considered a short- to medium-term solution for decarbonisation [
42]. However, the main powerful advantage of using a hydrogen mixture based on converted methanol in engines is the reduction in CO
2 emissions during the life cycle of methanol. This is because technologies for obtaining methanol directly from CO
2 have been developed today [
43,
44]. CO
2, which is formed during the combustion of methanol in engines, can then be used as a raw material for the production of methanol. The obtained cost of methanol production was USD 565 per ton of produced methanol [
44].
The total cost of engine conversion and the conditions in Ukraine amounted to 1.750 euros. If we take the fuel consumption of a diesel engine in a passenger car of 5 L per 100 km of mileage, savings from using a hydrogen mixture, based on methanol, is 2.5–5 euros per 100 km of mileage. The payback of the system will be from 35 to 70 thousand km. Payback will be faster for trucks with higher diesel fuel consumption.
The decrease in the cost of consumption of alternative hydrogen fuel was accompanied by an improvement in the power and environmental parameters of a diesel engine operating with an on-board hydrogen reactor.
As noted in [
45], the presence of a supplementary amount of H
2 in the influence zone leads to a decrease in NO
x output, since during the combustion of a hydrogen fuel mixture, in addition to fuel oxidation reactions, nitrogen recovery, etc., occurs.
The experiments confirmed that depending on the frequency of rotation of the crankshaft and the load on the engine, the formation of NOx in the exhaust gases decreased by 54–61%, and the reduction in carbon monoxide takes place in the range of 52–62%.
In terms of NO
x emissions, their reduction is extremely relevant, especially for diesel engines. The values obtained by us are close to the results of other engines that operated on methanol fuel. For example, the reduction in NO
x emissions is recorded in the works [
42,
46]. In our opinion, the reduction in NOx emissions is due to significantly less oxygen in the air/methanol fuel mixture compared to the air/diesel fuel mixture. The air/methanol ratio is 6.45/1 and the air/diesel stoichiometric ratio is 14.7/1, but also, diesel engines also operate with an excess air factor λ greater than unity. Along with the positive result of reducing NO
x emissions, the authors note that for a specific experimental engine for different operating modes, NO
x reduction occurred at values of 2.3–3.4 g/kW·h. The obtained values correspond only approximately to Euro IV requirements for diesel engines, and, therefore, cannot be recommended for immediate implementation but require research to further reduce NO
x emissions to current requirements.
At the same time, as stated in [
47], the presence of hydrogen in fuel combustion counteracts the process of soot release. Hydrogen affects the release of soot at all phases of the formation and combustion of carbon particles. Hydrogen intensifies the processes of burning soot at the expense of water, which acts as an oxidiser. Reducing soot emissions in fuel combustion processes helps reduce heat loss and correspondingly increases engine power.
The experiments confirmed that the engine power, depending on the frequency of rotation of the crankshaft and the engine load, increased by 10–14% on the hydrogen mixture compared to the operation of the experimental engine on diesel fuel.
To ensure the operation of the hydrogen reactor on board the car, it is necessary to remove excess heat, for example, from the exhaust gases of the engine or the cooling system. In our opinion, this energy is most rationally obtained by utilising the thermal energy of exhaust gases. In gasoline and gas engines, energy losses in the overall heat balance reach 30–40% and in diesel engines—25–35%. This corresponds to 12–21 MJ of thermal energy of exhaust gases per 1 kg of fuel consumed by the engine. At the same time, the temperature of the engine exhaust manifold reaches from 750 to 900 K at idling speed to 1100–1300 K at maximum speed and loads. The thermodynamic cycle (Otto cycle) for engines converted from diesel fuel to a hydrogen mixture with spark ignition and heat recovery from exhaust gases can be depicted using a T-S diagram (
Figure 14).
In
Figure 14, heat suitable for regeneration is expressed as a share of the waste heat
QWH, i.e., the degree of regeneration in the thermodynamic cycle is equal to
where
QWH is the heat waste removed in the thermodynamic cycle.
Heat quantity removed per cycle
QWH (
Figure 14) is determined from the expression
where
Cμ.νol is the average molar heat capacity of combustion products at constant volume and
Mpr.com is the number of combustion products at a constant volume.
Endothermic heat conversion quantity
QRH is determined from (8) and from the expression
In other words, the regeneration degree depends on the conversion process temperature Th and it increases with the decrease in Th and this can be determined from the expression.
The regeneration of the hydrogen degree depends on the conversion process temperature in the thermodynamic cycle
Th; it increases with the decrease in
Th in the cycle and it can be determined from
The total thermal energy consumption for the fully completed conversion of 1 kg of methanol reaches 6.9 MJ. The heat removed with exhaust gases for the D21 engine is, on average, 30–40 kJ/s, depending on the operating mode.
If the composition of the conversion products corresponds to the conditions of thermodynamic equilibrium (the complete completion of the conversion process), the conditions for achieving the maximum possible degree of regeneration will be realised. If the endothermic effect of the conversion reaction corresponds to the supply of an equivalent amount of heat into the reaction space from an external source (a heating coolant), which in this case is the engine exhaust gas, these conditions will be realised.
It is obvious, that the requirements for engine exhaust gas temperatures will not be met in all engine operating modes. For example, for an unheated engine, the efficiency of the hydrogen reactor will be reduced [
48]. However, the operating time of the vehicle engines in the warm-up mode is relatively short.
In addition, catalysts make it possible to ensure the operation of on-board hydrogen reactors at lower operating temperatures. For example, the conversion of methanol in a hydrogen reactor will take place at temperatures of 300–400 °C [
49] and these values will determine the minimum possible temperature regimes for the operation of on-board hydrogen reactors. Therefore, we can speak about the possibility of constant operation of on-board hydrogen reactors due to heat recovery of engine exhaust gases.