Hydrogen fuelling a passenger car spark ignition engine for a cleaner environment

The tightening of the restrictions on polluting emissions in the last decade and the possibility of the disappearance of the internal combustion engine in the field of transport must motivate the researchers in the field of internal combustion engines to find new solutions to fuel the spark ignition engine, aiming to reduce the negative impact of polluting emissions on the environment. Unconventional fuels represent a very good choice to fuel the spark ignition engines, based on their excellent combustion properties and hydrogen stands out among them. This paper presents experimental results of fuelling a car spark ignition engine with mixtures of gasoline and hydrogen. The experimental investigations have shown an improvement in the combustion process of the engine, a reduction in the energetic fuel consumption, an increase in the brake thermal efficiency and a decrease in the pollutant emissions level.


Introduction
The remarkable performances achieved by internal combustion engines in recent years have been possible both due to the sustained interest of researchers in the field and due to the requirements given by international regulations regarding polluting emissions and increasing efficiency, especially in the current context of fossil fuel saving requirements.With the increasing restrictions related to pollution standards in accordance with the regulations of the European Commission on the use of internal combustion engines for cars, new viable methods must be found to supply them with alternative fuels so that the thermal efficiency increases and the level of polluting emissions is sharply reduced compared to of the classic case of fossil fuel supply.A viable solution for meeting these goals is to fuel IC engines with hydrogen.Hydrogen may be used both as a single fuel and as an additional fuel, ensuring the reduction of the global consumption of fossil fuels and the reduction of the level of carbon dioxide emission, which has a real potential greenhouse effect.Hydrogen fueled engines produce a lower level of particulate and smoke emissions without requiring major changes to their design.Hydrogen fueling solutions can be successfully applied both to new internal combustion engines in the design stage and to internal combustion engines already in operation.Thus, by optimizing the operating regimes and by optimizing the settings of internal combustion engines fueled by hydrogen, older engines can be brought to levels closer to modern ones in terms of emissions and in terms of brake thermal efficiency and specific energy consumption.
Hydrogen is an element which can be found in the Universe in large quantities and on Earth it is found in the air, as a part of water and as a part of hydrocarbons.The principal methods of hydrogen production are: methane steam reforming, partial oxidation of hydrocarbons, biomass gaseification, coal and waste, thermal cracking and electrolysis of water [2,3,4,5].The elements of difficulty in using hydrogen to fuel spark ignition engines are represented by the calibration of fueling systems and their adjustments to obtain the best results in terms of the level of pollutant emissions, thermal efficiency and in terms of specific fuel consumption.In a study carried out in papers [6,7,8,9], very good results were obtained in terms of the performance and level of pollutant emissions of the hydrogen fueled spark ignition engine.A key property of in-cylinder combustion that has contributed to improved energy and pollution performance is the higher burn rate of hydrogen compared to gasoline.Thus, through its higher burning speed, hydrogen ensured the placement of the moment of maximum pressure near the TDC of the engine and in the case of combustion of very poor air-fuel mixtures, obtaining the pronounced reduction of nitrogen oxides.
A major problem for modern spark-ignition engines equipped with direct fuel injection systems is the level of particulate emission that is higher than in the case of engines equipped with indirect fuel injection systems.The level of particle emission can be reduced in the case of hydrogen fueling in a dual system, the gasoline being partially energetically substituted with hydrogen, because first of all it reduces the content of carbon atoms in the mixture and decreases the risk of thermal cracking of the fuel droplets and secondly, the higher combusting speed of hydrogen reduces the risk of fossil fuel droplets depositing on the internal walls of the combustion chamber [10].

The properties of hydrogen.
The properties of hydrogen are presented in the table 1, presented comparatively with those of gasoline and diesel fuel.It can be observed in the table above that the density of hydrogen is very low compared to fossil fuels and therefore hydrogen quickly take all the available space and this aspect can affect the intake process in the case of hydrogen injecting in the intake plenum.To not affect the rated power of the engine a good choice is to use hydrogen as an additive fuel, the gasoline being partially substituted.The higher combusting speed of hydrogen than that of gasoline is an advantage when used in a SI engine, the first combustion phase becoming shorter, thus it can be obtained an increasing in the pressure inside the cylinder.Because hydrogen has an extremely low ignition energy, it develops a trend to auto-ignite from hot surfaces like the EX valve, spark plug, or hot gases.The laminar combusting speed of hydrogen is 7...10 times higher than gasoline combusting speed and this is a good advantage to reduce the initial combustion phase, and also the turbulent burning speed inside the cylinder is increasing, leading to a reduction in the rapid combustion phase inside the cylinder.To avoid knock when using stoichiometric mixtures a good solution is to make the mixture lean.To avoid the increase in the intensity of knock EGR cooling is preferred and this also helps to reduce the dilution of intake air.
Due to hydrogen's lower heating value higher than value of hydrocarbons, we can obtain an increase in the BTE of the engine and reduce the BSFC.Burning hydrogen produces an adiabatic flame of approximately 2300 K [10] and may increase the temperature in the combustion chamber, which can increase the level of NO emission [10].The smoke emission level of the engine fuelled with hydrogen decreases, especially for the engines with direct gasoline injection when the engines are bifuelled with hydrogen, the second fuel being provided to the intake plenum of the engine and gasoline directly injected, because by making the speed of combustion higher the phenomenon of the occurrence of thermally cracking of fuel drops on the colder areas of the combustion chamber decreases [10,11].

Fuelling methods
The SI engine may be fueled with H2 by indirect injection or direct injection.Because the port injection is common among SI engines, it can be applied without difficulty for engines already in operation.Also, the standard fuelling system can still be used on the vehicle and the engine may run in dual mode.The use of direct hydrogen injection requires serious engines modifications, particularly the mounting of injectors directly inside the combustion chamber.By injecting H2 directly affecting the filling of the cylinder with air is avoided.In the papers [12] and [13], the authors H2 fueled the SI engine by indirect injection, gasoline being provided by the classic fuel system.The burning process was optimal positioned near the TDC of the engine due to greater combustion speed of H2 [12,13].

Experimental investigations
The A15MF supercharged SI engine has been used to carry out the experiments, fuelled with gasoline and hydrogen.The engine ran at 2500 revolutions per minute and 55% engine load with λ=1, and the gasoline was 5% substituted with H2.

The working procedure.
To carry out the experiments at the beginning was determined the reference, supplying the engine only with gasoline, afterwards the gasoline was 5% partially substituted with hydrogen, using the indirect injection method, aiming to conserve the rated power and torque.

Experimental results.
In the figure below is presented the max pressure which increased for the case of fuelling with H2 because of a higher flame speed in the homogeneous mixture of hydrogen and air.The maximum rate of pressure rise also was higher, and is presented relative to the reference case, figure 3.  The BSEC decreased when the engine was supplied with H 2 , due to higher lower heating value of H 2 .In the figure 4 is shown the BSEC.The BTE increased in the case of fuelling with H2, the figure 5 is showing the BTE for the presented cases, relative to standard engine case.
The NO emission level decreases in the case of supplying the engine with H2, figure 6, similar results are presented for the CO2 and unburnt hydrocarbons emissions level, figures 7 and 8.    3. Conclusions 1.The in cylinder pressure and its maximum rate of rise increased when the engine was fuelled with hydrogen with 9% and 29%. 2. The BSEC decreased with approximately 20% for H2 fuelling.3. The BTE increased with 6% in the case of hydrogen fuelling.4. The NO emission level was lower with 18% when the engine was fuelled with H2. 5.The CO2 emission level and the UHC emission level decreased with 40% and 25% respectively.

Figure 2 .
Figure 2. Max pressure presented relative to reference case.

Figure 3 .
Figure 3. Max rate of pressure rise, presented relative to the reference case.

Figure 6 .
Figure 6.The NO emission relative to the reference case.

Figure 7 .
Figure 7.The CO2 emission presented relative to the reference case.

Figure 8 .
Figure 8.The unburnt hydrocarbons relative to the reference case.