Aspects of combustion in diesel engine at hydrogen use-a theoretical approach

The use of alternative fuel may be a viable solution in order to ameliorate the engine performance especially in terms of pollutant emissions. Among the alternative fuels that can be use to fuel internal combustion engines hydrogen can be a viable alternative fuel especially due to the advantage of reducing the carbon emission at its use as alternative fuel even for partial substitution of classic fuel. Hydrogen has good combustion properties like higher Lower Heating Value, large inflammability limits, higher combustion speed, which may has a benefic influence on combustion process. The use of hydrogen to diesel engine bring few important issues that must be solved in order to assure the normal engine operation, starting with the fuelling system and engine with the control of the combustion process. The paper presents some results obtained during the theoretical modeling of the in-cylinder process at a diesel engine fuelled with classic fuel and hydrogen. The diesel fuel is energetically substituted by hydrogen in percent’s of 25% and 30%. The influences of hydrogen use on in-cylinder maximum pressure, maximum pressure rise rate, heat release rate, combustion temperature, indicated thermal efficiency and nitrogen oxides and smoke emission levels are shown and analyzed.


Introduction
The substitution of the classic fuels with alternative ones may be a viable method in order to ameliorate the level of the engine gas emissions.From this point of view hydrogen can be use fuel to fuel diesel engines with the advantage of reducing the carbon emission at its use as alternative fuel even for partial substitution of classic fuel.
Hydrogen has good combustion properties like superior Lower Heating Value, large inflammability limits, superior flame speed comparative to diesel fuel, which may has a good influence on combustion process.Hydrogen use as alternative fuel in diesel engine is starting to come to the attention of the specialists in the field, which are trying to continuously improve the energetic and functional-constructive formula of the hydrogen fuelled diesel engine.For diesel engines, hydrogen fuelling brings few important issues that can be solved in order to assure the engine running, starting with the adequate fuelling system use for the combustion process management.Because of a low value of the hydrogen cetane number the use of classic fuel pilot injection to initiate the ignition of the airfuel mixture is required.If diesel fuel is used as substitute fuel, the engine being fuelled with diesel and hydrogen in dual fuel mode, the substitution of diesel with hydrogen must be established taking into consideration energetically and mechanical operation aspects.1303 (2024) 012016 IOP Publishing doi:10.1088/1757-899X/1303/1/012016 2 Some researchers use hydrogen as fuel for engines, good perspectives results being shown.Shahpouri [1] develops o study based on a simulation with one-dimensional model used for hydrogen fuelling to predict the level of the pollutant gases to an ICE.Shahpouri observes the increase of the incylinder temperature, which may affect the NOx level in the exhaust gases, which may require a special tune for amelioration of the NOx and soot emissions level.In general, at hydrogen fuelling, the NOx emission level is higher, depending on fuels cyclic quantities, but emission levels for soot, PM, CO, unburn HC and CO2 are decreased by hydrogen.The thermal efficiency is increased [1].Anticaglia [2] develops a one dimensional CFD model to study the possibilities to increase of the brake thermal efficiency and to decrease the NOx emission level at internal combustion engines fuelled by hydrogen.Anticaglia [2] presents the influences of the hydrogen on combustion process reflected in the thermal efficiency rise.Hydrogen use to the advantages of higher break mean effective pressure at lean mixture use [2].Krebs [3] develops a multi-zone model for hydrogen combustion modelling and shows the increase of maximum pressure during combustion.Also the peak value of the pressure tends to appears sooner per cycle.The rise of combustion temperature peak appears also at the increase of the mixture combustion speed as presence of hydrogen [3].Also the combustion starts earlier but the combustion duration and speed is influenced by the air-fuel ratio [3].The increase of the in-cylinder water mole fraction at hydrogen combustion leads to the decrease of the NOx level in the exhaust gases, but the decrease in also related to the air-fuel ratio [3].The maximum of NOx appears sooner per cycle since hydrogen ratio increase, especially for lean dosages area, but for stoichiometric conditions the influences are not so obvious [3].Michl [4] uses a model to study the influence of hydrogen combustion, that substitute the diesel fuel, on heat transfer and he conclude that during hydrogen combustion peak values for temperature appears for short periods of time, the heat release being higher and shorter.Maghbouli [5] shows that hydrogen use as gaseous fuel in the engine admission in percent of 1% to 7% volume, assures the rise of pressure and temperature, for 7% of hydrogen, the rate of the pressure rise and the rate of the heat release being accelerated, knocking combustion being favoured.Ragupathy [6] uses a basic hydrogen fuel and observe the decrease of the injection duration decrease and the heat release during combustion is higher.Hydrogen combustion favours the rise of the in-cylinder pressure, the maximum value of the pressure per cycle occurs farther from Top Dead Centre (TDC) versus classic fuelling.A similar tendency appears for rate of heat release, HRR, the peak of heat release rate being increased once with the rise of the hydrogen content and the angle of maximum HRR appears closer to TDC, the reduction of the combustion duration being shown [6].Molina [7] shows that higher percentages of the hydrogen assure the acceleration of the combustion flame speed for all range of air-to-fuel ratio, the peak pressure and the HRR being increased.The pressure diagrams allure tends to be very different al hydrogen use, the peak pressure appears closer to TDC with almost 5 CAD [7].The maximum pressure is increased from 110 bar till 148 bar at hydrogen maximum share and the pressure rise rate is increased from 2 to 3.4 bar/crank angle degree, (°CA).The total duration of the burning is shortened, and the rapid stage of combustion appears with almost 8 degrees sooner for 50% hydrogen and with 6 degrees for 100% hydrogen.The main phase of combustion occurs with 12 degrees sooner for 100% hydrogen versus diesel fuel [7].The maximum of HRR appears with 10 degrees sooner at hydrogen maximum share comparative to classic fuelling [7] The indicated efficiency decreases with almost 2.5% for =1.5 for maximum hydrogen content comparative to classic fuelling.But for area of =1.5…2 the indicated efficiency increase with 2.9% at hydrogen use and the lowest indicated specific fuel consumption being achieved at hydrogen fuelling [7].In the exhaust gases, the CO2 level is lower at hydrogen fuelling especially at lean mixture use [7].Cameretti [8] shows that hydrogen assures a more rapid combustion process, the heat release rate peak being increased and achieved sooner with 5 °CA per cycle versus classic fuelling.Also, the combustion law at hydrogen use is superior to methane or to diesel fuel [8].The maximum pressure increases from 52 bar to 56 bar at hydrogen use but the angle of peak pressure appears at a similar position versus Top Dead Centre [8].Hydrogen may influence the final combustion stage, an increased second peak in the curve of the HRR may appears at hydrogen use comparative to classic fuelling, leading to a superior and more vertical combustion law [8].This aspects concord with the increase of the combustion temperature from 2400 K for classic fuelling till 2600 K at hydrogen use [8].The moment of peak pressure tends appears sooner during combustion, with 3 crank angle degrees sooner per cycle for hydrogen fuelling [8].The level of the HC emission level drops from 14900 ppm to ~1300 ppm [8].The level of the NOx emission is superior, 2.6 ppm for H2, comparative to classic fuelling, 0.4 ppm and the solution of EGR use is proposed [8].The CO2 emission level is around 32000 ppm for diesel fuel, but at H2 fuelling the level drops till 17900 ppm and remains steady comparative to classic fuelling [8].Monemian [9] uses different percent of hydrogen to fumigate a diesel engine and observes large amounts of hydrogen can assure to the reduction of the combustion duration and of the dispersion between values of the indicated mean effective pressure (IMEP) [9].Hydrogen provides a large share of the premixed burning phase share, to the increase of the maximum pressure and of the heat release rate, the peak angles being closer to TDC [9].The indicated efficiency is increased at hydrogen use up till 2.2% [9].The NOx emission level slightly decrease for small amount of hydrogen, till 10%, but for large hydrogen percent a slightly increase in NOx emission level appears [9].Soot, HC and CO2 levels in the exhaust gases are lower for H2 versus diesel fuel, the soot emission being drastically reduced [9].After its experiment Monemian develops a modelling process and comperes experimental result with the model result and the similar influences of hydrogen on engine performances being obtained [9].The model results show that a large H2 quantity assure the rise of the peak temperature from around 1740 K to 1850 K [9].Santoso [10] shows that hydrogen use in large quantities, around 49 l/min, affects the thermal efficiency.Small hydrogen flows into the inlet air assure the increase of the combustion pressure above 70 bar for 21 l/min and higher hydrogen flows 36…49 l/min leads to lower maximum pressures, around 62 bar [10].Also, for lower hydrogen flow the maximum pressure rise rate is higher [10].The HRR is decreased for large H2 flows, and its peak values appears later during combustion process [10].Simulation results show similar tendency the combustion being retarded [10].Helldorff [11] uses a CFD model to investigate the hydrogen combustion in diesel engine and its main conclusion shows that hydrogen peroxide product assures to the reduction of the burning duration, rise of heat release rate and maximum pressure.Almutairi [12] shows the rise of the in-cylinder temperature, HRR and NOx emission level because of the higher flame temperature at hydrogen-air mixture combustion.
The paper presents some results obtained during the theoretical modelling of the in-cylinder process at a CFR engine fuelled with diesel fuel and hydrogen.The classic fuel is energetically replaced by hydrogen in percent's of 25% and 30%.The influences of hydrogen use on in-cylinder maximum pressure, maximum pressure rise rate, heat release rate, combustion temperature, indicated thermal efficiency and NO and smoke emission levels are shown and analysed.Hydrogen use may lead to the increase of the combustion parameters, like gases pressure, rate of pressure rise, rate of heat release, gases temperature, thermal efficiency and the NO and smoke emission levels can be ameliorated.

In-cylinder processes modelling
The processes are simulated with a one-zonal model developed by authors and applied for a CFR diesel engine which has the characteristics: 85 mm bore, 115 mm stroke, 13 compression ratio, 0.652 dm 3 swept volume, 13 °CA start of injection, standard diesel fuel consumption 13 cm 3 per minute classic fuel consumption, operating at 900 rev/min.The modelling is achieved for classic fuel use, diesel fuel, and for diesel fuel and hydrogen use when the energetic replacement ratio of diesel fuel by hydrogen, XE, is 25% and 30%.
In-cylinder fluid state is evaluated from the relations of energy and mass conservation, applied for intake, compression, burning, expansion and exhaust.The equation of energy balance [13,14]  uses the internal energy, dU/d, the heat released during burning, dQcomb/d, the mechanical work, dL/d, the heat transferred to the walls, dQwall/d for a d calculation step, [13,14].Internal energy variation [13,14]: depends on the in-cylinder mass, nM , determined from the fuel molecular mass, Mf , and cyclic fuel dose mf [13,14]: where the fuel quantity: uses the density of the inlet fluid 0 determinate from the state equation at p0, T0 condition, [13,14].
During combustion, the simulation two burning mechanisms, preformed and diffusive, are taken into consideration by Vibe laws [13,14]: where the fraction of the heat released during the premixed burning stage is Rc , the kinetic parameters are mp* and md* and the relative angles for the beginning and share of the premixed and diffusive phases, xr and xd [13,14].
In order to estimate the temperature variation, the mechanical work [13,14]: the heat lost to the walls [13,14]: the combustion heat release [13,14]: are used.The Q is the reaction heat which is equal with the LHV of the fuel for >1.A double Vibe law defines the combustion characteristic d/d: where f1 and f2 are burning parameters of the first two stages of the burning process [13,14].Finally based on previous relation and the energy balance equation, the in-cylinder temperature variation is evaluated by the relation [13,14]: the current temperature being evaluated from the previous value, in the =1...3 °CA: The determination of the global temperature provide the determination of the pressure, [13,14]: ) Also, highly important process of expansion is evaluated in similar mode, the use of energy balance equation for the expansion evolution, [13,14]: with the amount of gas in the cylinder: offers the in-cylinder pressure variation in this specific process, [13,14].
In terms of pollutant emission, the smoke and NOx are important for diesel engine, an estimation of their levels being of interest.
The Hiroyasu formula is used for estimation of the soot mass formation speed [14][15][16]: and the Morel-Karibel formula is used for evaluation of the soot oxidation speed [14][15][16]23]: the fuel mass, mtf ,the combustion pressure and the temperature, p and T, being taking into consideration.From the ratio of the mass of the oxidized soot subtracted from the mass of soot formed, the estimated mass of soot is obtained, as an estimation of the soot comparative level [14][15][16].

Results and discussions
The pressure diagrams for classic fueling, XE=0, and for diesel fuel and H2 use, XE=0.25 and 0.30 are presented in the figure 1.The raised LHV and burning speed of hydrogen associated with a shorter duration of the burning process provides to the increase of the maximum pressure during combustion, the peak values being 5.29 MPa for XE=0.25 and 5.48 MPa for XE=0.3, with 33.2% and 38%, respectively higher comparative to XE=0.The maximum rate of pressure rise (MPRR) increase from 0.147 MPa/°CA till 0.251....0.269MPa/°CA, in the normal rage for diesel engine, but higher with 70.7% at XE=0.25 and with 82.3% at XE=0.30 versus XE=0.The reduction of the burning duration at hydrogen use is in corelation with the increase of the MPRR.The acceleration of the burning process, especially of the rapid phase leads to a closer angle of peak pressure to TDC.The angle of peak pressure is 377°CA for diesel fuel use, 373°CA for XE=0.25, 372°CA for XE=0.30which represents an acceleration of rapid phase combustion at hydrogen use.Similar results and tendency variations for in-cylinder pressure were obtained by other researchers [3,5,6,7,8,9,10,11].The burning speed and LHV of H2 superior to diesel fuel assures to the increase of the HRR with 6%, figure 2, the first HRR peak being closer to TDC due to the increase of the share of rapid combustion phase.Similar results and tendency variations for heat release rate were obtained by other researchers [5,6,7,8,9,11,12] Figure 3. In-cylinder global temperature at XE=0 and for H2 use, XE=0.25,XE=0.30.
A higher heat release rate at hydrogen use assures to the rise of the in-cylinder global temperature once with the rise of the hydrogen cyclic percent, figure 3. The maximum temperature at diesel fuel use is 1559K, the value being increased with 5.1% at XE=0.25 and with 5.8% at XE=0.30 at dual fuelling.In correlation with the pressure variation, figure 1, the angle of maximum temperature appears closer to TDC once hydrogen quantity increase, closer with 10 degrees for XE=0.25 and with 11 degrees for XE=0.30which may represent the increase of isocore combustion share.Similar results and tendency variations for in-cylinder temperature were obtained by other researchers [1,4,5,8,9,12]   The speed of soot oxidation speed increase with 35.4% for XE=0.25 and with 36.5% for XE=0.30,versus XE=0.Due to hydrogen higher burning speed, the presence of hydrogen in the air-fuel mixture accelerate the soot oxidation, figure 6.A faster burning process, reflected in HRR acceleration, figure 2, is also visible in allure of the soot oxidation speed, figure5.
By integrating the speeds of soot formation and soot oxidation, associated laws for the formation and oxidation of soot are obtained.The soot balance, realized as the difference between the obtained laws, allows the evaluation of the smoke emission, which is reduced by 3.44% at XE=0.25 and by 5.92% at XE=0.30, figure 7.
The level of the soot starts to decrease at H2 use, in conditions that hydrogen provides lower soot quantities which are faster oxidation by the hydrogen-diesel fuel flames.Soot reduction was achieved also by others researchers [1,9].Combustion of the air-hydrogen mixture leads to the rise of the HRR, pressure and temperature of the in-cylinder gases which increase the peak of the NO formation speed, but in condition of a lower NO quantities due to reduction of the classic fuel amount that which burn into the rapid stage of combustion once with the decrease of the diesel cyclic dose at the XE increase.The NO forming speed, figure 8, is with 64% for XE=0.25 and with 94% for XE=0.30, in correlation with the rise of burning rate, figures 2, 5 and 6.
From the integration of the NO formation speeds, laws of variation of the NO emission are obtained, noting that NO is formed by 37.2% less at XE=0.25 and by 33.1% less at XE=0.3, figure 9.The reduction in the emission of nitrogen oxides can be attributed to the reduction in the amount of oxygen available per cycle, along with the increase in the dose of hydrogen, noting that the lambda is reduced from 1.987 for the classic fuel to the value 1.561 for XE=0.25 and to the value 1.493 for XE =0.3.Similar results of nitrogen oxides reduction were obtained by other researchers [1][2][3]9].
From the point of view of reduction perspectives of soot and nitrogen oxides the use of small quantities of H2 injected into the engine inlet can represent a viable solution.

Conclusions
Following the analysis of the modelling results, it can be stated that: -hydrogen use in percent's of 25% and 30% assures the rise of peak pressure with 33.2% and 38%, respectively higher comparative to the value allocated to diesel fuel; the maximum of the pressure rise rate increases from 0.147 MPa/°CA till 0.251 MPa/°CA....0.269MPa/°CA, in the normal rage for diesel engine, but higher with 70.7% at XE=0.25 and 82.3% at XE=0.30 versus XE=0 and can be related with the reduction of the burning duration due to higher burning speed of air-H2 mixture; the angle of maximum pressure is 377°CA for diesel fuel use, and get closer to TDC at 373°CA for XE=0.25 and 372°CA for the XE=0.30.
-the heat release rate increases with 6% and the first pick occurs closer to TDC which is in correlation with the faster burning of air-H2 mixture and increased share of rapid combustion phase; IOP Publishing doi:10.1088/1757-899X/1303/1/01201611 -increase of the in-cylinder global temperature once with the rise of H2 percent, the value being increased with 5.1% at XE=0.25 and with 5.8% at XE=0.30 at dual fuelling.
-the indicated efficiency increases till 0.425 for XE=0.25 and 0.426 for XE=0.30., values that are superior to the diesel fuel use with 3.15% and 3.39%, respectively; -the increase of H2 percent leads to the reduction of the soot forming speed, the peak of soot forming speed being with 2.8% lower for XE=0.25 and with 8.17% for XE=0.3, comparative to diesel fuel fuelling, in correlation with the decrease of the diesel fuel which assure the decrease of the carbon percent in the air-fuel mixture leading to the decrease of the speed of soot formation; the speed of soot oxidation increases with 35.4% for XE=0.25 and with 36.5% for XE=0.30versus diesel fuel; the soot balance shows that the smoke emission is reduced by 3.44% at XE=0.25 and by 5.92% at XE=0.30, in conditions that hydrogen provides lower soot quantities which are faster oxidation by the hydrogendiesel fuel flames; -the increase of the maximum speed of NO formation is favoured by H2 amount increase, but in condition of a lower NO quantities due to reduction of the classic fuel amount which combust into the rapid stage once with the decrease of the diesel percent at the increase of H2 quantity; the NO forming speed is higher with 64% for XE=0.25 and with 94% for XE=0.30, in relation with the rise of the burning rate in general; the laws of NO variation show that NO is formed by 37.2% less at XE=0.25 and by 33.1% less at XE=0.3; the reduction in the emission of nitrogen oxides can be attributed to the reduction in the amount of oxygen available per cycle, along with the increase in the dose of hydrogen, noting that the lambda is reduced from 1.987 for the classic fuel to the value 1.561 for XE=0.25 and to the value 1.493 for XE =0.3.
-thermal efficiency increase, soot and NO reduction perspectives can define the use of H2 injection into the inlet air as a viable solution.

Figure 6 .
Figure 6.Speed of soot oxidation speed of the soot at different XE.

Figure 7 .
Figure 7. Relative soot emission level at different energetic substitute ratios.

Figure 8 .
Figure 8.The NO formation speed at different energetic substitute ratios.

Figure 9 .
Figure 9.The NO in-cylinder level at different XE.