Theoretical and experimental research on the use of hydrogen in the automotive spark ignition engine

Reducing the level of pollutant emissions of internal combustion engines requires the adaptation of solutions that improve the combustion process in the cylinder but those solutions may increase their constructive complexity. For this reason, the use of hydrogen is an effective solution, both for replacing fossil fuels and for reducing the concentration of pollutant emissions. The use of hydrogen as an additional fuel for the automotive spark ignition engines offers the possibility of obtaining increased energy performance, decreased pollutant emissions level and decreased concentration of greenhouse gas emissions. The paper presents the results of the some theoretical and experimental investigations carried on an automotive spark ignition engine, analysing the influence of the hydrogen on combustion and implicitly on energy performance and pollution. The simulation of thermo-gas-dynamic processes in the cylinder of the engine fed successively with gasoline/gasoline and hydrogen, allowed the authors to develop a specific and efficient procedure for conducting experimental investigations. The results of the theoretical and experimental research obtained and presented in the paper highlight the advantages of using hydrogen as an additive to gasoline fuelling in the automobile spark ignition engine: the improvement of combustion (reduction of combustion duration, increase of indicated thermal efficiency, reduction of specific energy consumption) and reduction of the level of polluting emissions and of the emission of greenhouse gases. An engine adjustment strategy for the sharp reduction of the concentration of nitrogen oxides is also presented.


Introduction
In the current context of the increase in the need for energy, the price of its production and the tightening of pollution norms, discovering of new efficient solutions for fuelling the spark ignition engine is an important goal.Considering the consumption of fossil fuels and the possible exhaustion of their resources, a very good solution for fuelling the spark ignition engine is represented by non-conventional fuels.Among them, fuelling the spark ignition engine with hydrogen is very promising for increasing the effective break efficiency, reducing fuel consumption and reducing the level of pollutant emissions.Hydrogen is the most widespread element in the Universe, representing over 70% of the matter of the Universe.On Earth, it can be found in the air (in small quantities), combined with oxygen as a constituent part of water and combined with carbon as a constituent part of hydrocarbons.
One of the main advantages of using hydrogen as a fuel for spark ignition engines, besides the important advantages regarding the improvement of combustion, is represented by the various production methods, which can be both carbon free and from fossil fuels [2,3,4,5].The main methods of hydrogen production are: steam reforming of methane, partial oxidation of hydrocarbons, gasification of biomass, coal and waste, thermal cracking and, last but not least, electrolysis of water [2,3,4,5].Most of these methods, with the exception of water hydrolysis and thermal cracking, may produce carbon dioxide.

Hydrogen properties.
The main properties of hydrogen can be found in the table 1, compared with the characteristics of gasoline and diesel fuel.Since the density of hydrogen is very low, hydrogen develops a tendency to quickly occupy all the space it has available, therefore it is possible that the air admitted to the engine will be quantitatively reduced, therefore the power of the engine could suffer a reduction, thus specific methods of fuelling the engine must be used, or hydrogen could be used as an additional fuel.The diffusion speed of hydrogen is approximately 7...8 times higher in gasoline vapours, which results in facilitating rapid mixing with air, even at low dosages and high engine speeds, which is an advantage for reducing the cyclic variability of the engine and increasing the stability of its operation.The boiling and critical points having very low temperatures translate into difficulties in storing hydrogen on board of the vehicle, among all the methods storing in metal hydrides being a very good choice.The combustion speed of hydrogen is higher than that of conventional hydrocarbons, therefore, when used in a spark ignition engine, the rapid combustion phase is shortened, which can have the effect of increasing the maximum pressure inside the cylinder and its rate of pressure rise.Due to the very low minimum ignition energy, there may be a very high tendency to self-ignition from hot surfaces such as the exhaust valve, spark plug, areas on the piston head, scale deposits or hot gases.The laminar burning speed, which is 7...10 times higher than that of hydrocarbons, reduces the initial phase of combustion, and the turbulent burning speed, which is approximately 2 times higher than that of hydrocarbons, can reduce the rapid burning phase.In the case of using stoichiometric mixtures, the burning speed of hydrogen may become excessive, the combustion may have knock tendencies, and a solution to avoid this phenomenon is the use of lean hydrogen-air mixtures.From the point of view of the combustion kinetics, the increase in the weight of the OH species in the combustion gases of the spark ignition engine powered by hydrogen, can be considered a precursor to the occurrence of knock [6].Also, the intensity of the knock can be modified by other factors such as: the initial temperature of the mixture inside the cylinder [7]; coefficient of excess air lambda; replacing a quantity of air from the intake manifold by using EGR [8,9]; compression ratio; hot spots in the combustion chamber, the degree of swirl inside the cylinder [10].In order to reduce the heat of reaction in the case of fuelling the spark ignition engine with hydrogen, lean mixtures can be used or the ignition timing can be changed in the direction of its decrease, which moves the combustion towards expansion stroke.However, a considerable reduction of the spark advance when has negative effects on the overheating of the exhaust valves.To reduce the effect of increasing the intensity of knock due to the high temperature of the EGR, EGR cooling can be used, a phenomenon that also mitigates the effect of diluting the intake air.
Regarding the lower heating value of hydrogen, it is higher than that of conventional hydrocarbons, which is a beneficial aspect for increasing the effective efficiency of the engine and for reducing its effective specific consumption.The temperature of the adiabatic flame of hydrogen being 2318 K [11] may indicate an increase in the global temperature in the cylinder, which may have an effect on the increase in the emission of nitrogen oxides [11].Considering the correlation between increased flame speed and the possibility of burning lean mixtures, when fuelling the spark ignition engine with the addition of hydrogen, the level of carbon monoxide emission decreases, in the cylinder the conditions for complete carbon oxidation are accomplished [11,12].The engine's smoke emission level decreases, (especially under the conditions of using direct injection for engines that are fuelled with hydrogen in dual mode, hydrogen being injected into the intake manifold of the engine and gasoline being injected directly into the cylinder), because by increasing the speed of combustion decreases the possibility of fuel drops reaching the cold areas of the combustion chamber and thus decreases the possibility of them thermally cracking [11,13].

Fuelling methods
The spark ignition engine can be fuelled with hydrogen by indirect injection or direct injection.The advantage of indirect injection near the valve stem is that the feeding method can be easily applied to already built engines.Also, the classic gasoline supply system can be kept and thus the engine can operate in dual mode, with gasoline through the classic system and with hydrogen through the system added in parallel.In the case of fuelling the spark ignition engine with hydrogen by the direct injection method, the injectors must be mounted in the combustion chamber, and the engine requires considerable modifications, the method lends itself especially to engines under the design phase.An important advantage of direct injection is that filling the cylinder with air is no longer affected as in the case of indirect injection.The research carried out in the works [14,15] on a spark ignition engine equipped with an electronically controlled hydrogen indirect injection system revealed an increase in the maximum pressure in the cylinder, an increase in the speed of pressure rise and an increase in the maximum temperature in the cylinder.Due to the higher burning speed of hydrogen, a reduction in the total duration of combustion was recorded proportional to the amount of hydrogen injected, the minimum duration of combustion being recorded at the maximum amount of hydrogen used.An increase in maximum pressure was also recorded in the paper [16].In articles [17] and [18], the authors fuelled the spark ignition engine with gasoline and hydrogen through the method of indirect injection, using dual fuelling, the gasoline being injected through the classic fuel system.According to the authors, the improvement of the combustion process was ensured by the optimal positioning of the point of maximum pressure immediately in the vicinity of the top dead centre, due to the higher burning speed of hydrogen, which increased the overall burning speed in the cylinder [17,18].

In cylinder processes modelling.
In order to model the in cylinder processes, a one-dimensional model has been chosen based on Vibe formal combustion laws.The general assumption of this model is that the charge in the cylinder is considered a chemically and thermally homogeneous perfect gas mixture [1].Combustion of fuel is assimilated with formal Vibe type laws.The calculation was carried out by the iterative method, starting from the end of the intake process, dividing the motor cycle into intervals of known size (1 degree CRA), then on each interval separately calculating the state variables.The calculation was completed when the exhaust valve was opened.The balance equation for the one-dimensional model is [1]: where: • dU dα is the change in internal energy; • dQ a dα is the variation in the amount of heat released by combustion; • dL dα is the mechanical work variation; • dQ p dα is the variation in the amount of heat transferred to the walls, [1].
To evaluate the heat transfer to the walls the Woschni 1978 model has been chosen.

Theoretical investigations results
The above presented model has been applied to simulate the in cylinder processes for a A15MF-1.5 DOHC turbocharged spark ignition engine fuelled with gasoline and hydrogen.The specifications of the engine are presented in the table 2. The chosen working regime was 2500 rpm and 50% engine load, and one case of hydrogen fuelling has been investigated, the case where the gasoline was 5% energetically substituted with hydrogen.The substitution ratio of gasoline with hydrogen is given by the following equation, [1].The calculated in cylinder pressure evolution is presented in figure 1.It can be observed that due to the presence of hydrogen in the combustion chamber of the engine, the maximum pressure is higher than in the standard case of fuelling only with gasoline.This can be explained by the higher flame speed of hydrogen and by its higher caloric heating value, which enhances the combustion process.The maximum rate of pressure rise also increases in the case of hydrogen fuelling, figure 3, which indicates that in the presence of hydrogen the combustion became faster.The in cylinder temperature also increases in the case of hydrogen fuelling, figure 2, because of a higher flame temperature.The pressure and the maximum pressure rise versus the substitute ratio of gasoline with hydrogen, the maximum pressure rise is presented relative to the standard case of gasoline fuelling.

Experimental investigations
The experimental investigations have been carried out on the same engine presented in the second subchapter, the A15MF-1.5 DOHC, with the specifications in the table 2. The test bed diagram is presented in figure 4.
.  Before starting the experimental procedure all the equipment were calibrated.The fuelling system designed for hydrogen use is the hydrogen supply system is made up of a hydrogen bottle, which has a pressure of 150 bar, this being provided with a pressure regulator that allows the adjustment of the working pressure in the range of 10-150 bar.The hydrogen bottle is connected to a mass flowmeter used to determine by direct measurement the flow of hydrogen consumed by the engine.Between the flowmeter and the hydrogen bottle, a second pressure regulator is mounted that allows the pressure in front of the flowmeter to be adjusted in the range of 10... .0.5 bar.A flame arrestor is mounted between the hydrogen flowmeter and the engine's fuelling system.The fuelling system is provided with a system of electronically controlled gas injectors, which injects gas into the intake valves port, being used for hydrogen supply.With the help of the open-type electronic control unit, Dastek unichip Q, and through the Dastek program, when opening the hydrogen injectors, the opening time of the gasoline injectors is reduced so as to maintain the same value of the engine's effective power.Through the same procedure, by reducing the opening duration of the gasoline injectors, the air-fuel mixture is leaned.

The working procedure.
The fuelling method used to fuel the spark ignition engine with hydrogen was the indirect injection, in cylinder being admitted a homogeneous mixture of hydrogen and air, and further being mixed with the gasoline-air mixture, the gasoline being injected by the standard indirect gasoline injection system.First was determined the reference, fuelling the engine only with gasoline, then the gasoline was 5% partially substituted with hydrogen, aiming to maintain the same working regime and the same power and torque as in the standard case.The engine working regime was 2500 rpm and 50% engine load.The energetic substitution ratio of gasoline with hydrogen is given by the equation (2.3).

Experimental results.
In figure 5, the maximum in cylinder pressure is presented versus the substitution ratio of gasoline with hydrogen.The maximum pressure inside the cylinder increased for the case of fuelling with hydrogen because of a higher flame speed due to the enhancement of the combustion process.The maximum pressure may increase due to the reduction of the combustion duration at hydrogen use comparative to the classic fuel use case.The maximum rate of pressure rise also increased, in figure 6 being presented relative to the standard case of fuelling only with gasoline.The brake specific energetic fuel consumption decreased for the case of fuelling with hydrogen, due to higher value of the lower heating value of hydrogen comparative to gasoline.In figure 7 is presented the BSEC for the investigated cases.The brake thermal efficiency also increased in the case of fuelling with hydrogen.In figure 8 is presented the BTE evolution for the investigated cases, relative to the case of fuelling only with gasoline.
The nitrogen oxides emission level decreases when the engine was fuelled with hydrogen, figure 9, the same trend has been obtained for the CO2 and unburnt hydrocarbons emissions level, figures 10 and 11.   4. Conclusions 1.The in cylinder maximum pressure and the maximum rate of pressure rise increased when the engine was fuelled with hydrogen with 10% and 23% respectively versus classic fuelling.2. The specific energetic fuel consumption decreased with 15%, till 11075 kJ/kWh, in the case of hydrogen fuelling comparative to gasoline fuelling.3. The brake thermal efficiency increased with 14.4% in the case of hydrogen fuelling versus gasoline use.4. The nitrogen oxides emission level decreased with 25% when the engine was fuelled with hydrogen comparative to classic fuelling.5.The carbon dioxide emission level and the unburnt hydrocarbons emission level decreased with 39% and 22% respectively for the case of fuelling with hydrogen versus gasoline fuelling.6.The operating regime investigated and analyzed in the present research, 50% engine load and speed of 2500 rpm, is a regime frequently used in the exploitation of the automotive engine in urban regime and therefore presents a high interest.The reduction of pollutant emissions, HC decreases with 22%, NOx decreases with 25% and the GHG gases CO2 decrease with 39% and the reduction with 15% of the specific consumption and increase of thermal efficiency with 14.4% at this regime being positive and interesting aspects, in the context of reducing the level of emissions in the urban environment.The study of the operation of the hydrogen fueled engine is extensive and covers a wide range of speeds and engine loads, including the full engine load regime and the nominal speed regime, the results being presented in future research.7. Using substitution degrees as low as 5% presents some advantages such as: maintaining the amount of air per cycle without affecting the volumetric efficiency, limiting the increase of maximum pressure and maximum temperature during combustion to values close to the operation when fueled with gasoline, a small increase in the pressure rise rate (up till 50 MPa and 1.4 bar/crank angle degree), engine reliability is not being affected.The use of small cyclic doses of hydrogen, such as 5%, also has the advantage of maintaining the constructive simplicity of the fuelling system, whilst the use of higher degrees of substitution or only hydrogen use may imply more important constructive changes.A hydrogen fuelling system for the use of small percentages of 5% can be easily adapted to engines already in operation, improving their energetic and pollution performance, offering the advantage of keeping them in operation in the near future.A possible disadvantage of the use of small amounts of hydrogen in the intake air can be constituted by the limitation of the reduction of the level of polluting emissions (NOx, HC) and greenhouse gases (CO2), of the specific energetic fuel consumption, in the idea that more large cyclical amounts of hydrogen would emphasize the reduction of the level of polluting emissions and the increase of thermal efficiency.

Figure 1 .
Figure 1.The calculated in cylinder pressure for the investigated cases.

Figure 2 .Figure 3 .
Figure 2. The calculated in cylinder temperature for the investigated cases.

8 Figure 5 .
Figure 5. Maximum in cylinder pressure for the investigated cases.

Figure 6 .
Figure 6.Maximum rate of pressure rise, relative to the standard case of fuelling only with gasoline.

Figure 10 .
Figure 10.The relative carbon dioxide emission level.