Aspects of the combustion variability analysis at an automotive engine fuelled with hydrogen

Over the last decades, the use of the alternative fuels was one of the main research activities for specialists in the field of internal combustion engines. The development of the modern automotive engines is constantly challenged by the more severe emission legislation. The engine emissions levels and the fuel efficiency are directly influenced by the engine operation, reproduction of the combustion phases from one cycle to other, cyclic dispersion during combustion process being important. In general, the use of alternative fuels in internal combustion engines provides an improvement of the energetic and pollution performance, or just a slight improvement of them, but the study of the combustion process must be completed with aspects regarding the cyclic variability. In particular, using this alternative fuel, a study of cyclic variation of the combustion process would be necessary in order to establish if the normal operation of the engine can be ensured. The paper presents some aspects of the analysis of the cyclic variability at a spark ignition engine fuelled with gasoline and hydrogen. During the engine operation at the regime of 2500 rev/min speed and 55% engine load, a number of 250 consecutive combustion cycles was recorded for classic fuel use and for hydrogen use. The coefficient of cyclic variation (CCV) or the coefficient of variation (COV) is determined for different combustion parameters such as maximum pressure, maximum pressure rise rate and mass fraction burned, defined by angles at which the conventional fractions of 10%, 50% and 90% of the heat of reaction is released. Thus, the values of the COV for maximum pressure (COV)pmax, maximum pressure rise rate (COV)dp/dα, angles of 10, 50 and 90% heat release as (COV)10%, (COV)50% and (COV)90% were calculated and compared with the admissible limit of 10%. The combustion variability analysis establishes the limits of the normal operation of the spark ignition engine fuelled with gasoline and hydrogen compared with the classic fuelling method.


Introduction
Hydrogen is a crucial component in alternative fuel research and is considered to be the most environmental friendly energy source.Recent research indicates that hydrogen fuel has great potential to be used in internal combustion engines and can achieve zero carbon emissions without radical changes [1].The pollutant emissions of the hydrogen fueled engine, compared with fossil fuel, are quite reduced, especially carbon emission, but the problem is the relatively high NOx emission due to the high flame temperature of hydrogen, so it`s necessary to study the low-temperature combustion and lean-burn 1303 (2024) 012017 IOP Publishing doi:10.1088/1757-899X/1303/1/012017 2 regimes.Because of the difficulty of storing large quantities on-board and the power loss at stoichiometric ratio, using pure hydrogen to fuel the engine is not feasible, so hydrogen blended fuels can be used as an alternative in the transition phase for automotive use.According to [2], cyclic variations or cycle-to-cycle variation (CCV), have a direct effect on vehicle drivability.CCV can deteriorate the engine`s output torque, resulting in combustion instability, engine noise and vibration.The main method to evaluate cyclic variation is the coefficient of variation in indicated mean effective pressure (COV IMEP).The drivability problems of vehicles start to be noticed when COV IMEP exceeds 10% for SI engines [3].[4] and [5] also stated that cyclic variability affects driving comfort, reduces the thermal efficiency and increases pollutant emissions.
COV for maximum pressure is defined with the following formula [6]: where ( 1) is the standard deviation and ( 2) is the avarage value for maximum pressure.
[7] studied cyclic variation for a SI engine fuelled with gasoline-hydrogen blends of 2.14%, 5.28% and 7.24% under lean-burn settings.An increase of air-fuel ratio also increased the COV and is more noticeable at air-fuel ratios beyond 1.1, showing that cyclic variations increase when fuel mixtures become leaner.The addition of hydrogen reduces COV, the minimum cyclic variation being observed for the largest hydrogen fraction.The COV is basically unaffected for hydrogen fractions above 5.28%, for any air-fuel mix.Using a DI gasoline engine with PFI hydrogen injection, [8] investigated the COV of IMEP.With 11.67% hydrogen addition, the maximum COV reached 1.07% compared with almost 20% for gasoline use only.
A single cylinder gasoline engine was modified to operate on hydrogen with manifold injection [9].When running only H2, the engine produced 29% less maximum indicated power, BTE increased with 2%.COV IMEP was around 0.85% for hydrogen engine at λ = 0.75 and 1.3% for gasoline engine at λ=1.3.The cyclic variations in all combustion periods for hydrogen engine were lower than for gasoline engine.The authors concluded that even at lean mixtures, hydrogen produces a more stable engine operation than gasoline at rich mixtures.Dual fuelling SI engines with gaseous renewable fuels is an efficient approach because it combines the advantages of both fuels.Hydrogen DI, compared with PFI, has the advantage that reduces abnormal combustion phenomena like preignition, backfire and knock, but in terms of emissions, a higher level of NOx and an increase in cyclic variation was noted.Using 18% EGR flow rate, showed a reduction of NOx with 77.8% [2].
[10] studied the effect of hydrogen DI on combustion stability and showed that for hydrogen fractions under 5.3%, the COV drops from 3.3% to 0.51%, with the excess air-fuel ratio decreasing from 1.8 to 1.The authors agreed that an increase of hydrogen fraction will reduce the COV for a given excess air ratio.At stoichiometric conditions, a slight COV increase was observed because of the concentration of hydrogen at the top of the combustion chamber which creates a lean mixture near the cylinder walls that affects the combustion stability.[11] directly injected 10% hydrogen in a SI under lean burn conditions with 15% throttle opening.The results showed that an increase in excess air ratio reduced the mean value of the maximum cylinder pressure and increased the COV because the mixture becomes leaner and deteriorates the flame propagation speed, reducing the maximum cylinder pressure.Notable cycleto-cycle variations occur when the engine is operating with very lean mixtures, but with hydrogen, these variations are much reduced compared to other hydrocarbon fuels [12].
According to [2], [4] and [8], the main sources of cyclic variation in a SI engine are: (a) turbulence formation in the cylinder flow field, (b) variation in fuel-air ratios, (c) the homogenous nature of fuel mixtures, (d) characteristics of spark discharge and flame development, (e) the quantity of leftover and recirculated gases in the cylinder.A high turbulence flow can affect the fuel concentration in the cylinder.A lower fuel-air ratio than desired ratio for optimum operation, slows down the flame propagation process, which is important during the initial flame development phase which directly affects the cyclic dispersion during combustion.During operation of a SI engine, only a few cycles are obtained at optimum spark timing, so faster combustion cycles need advanced spark timing and slow combustion cycles need late spark timing.This fluctuation of spark timing also add up to cyclic variation.The air-fuel mixture within the cylinder is not an homogenous mix with the recirculated exhaust gases and this fact affects the combustion process but because of the higher burning velocity of hydrogen than that of gasoline, the cyclic variation can be reduced.There are also other methods to reduce the cyclic variation such as: using a correct air/fuel ratio, controlled spark timing, using fuel with high RON.
Recent studies try to focus on creating a predictive model for a premixed hydrogen in an ICE [13].
Besides efficiency, they also predicted the NOx emission and abnormal combustion phenomena [14] used a simple first-law zero-dimensional model that predicted reasonably well the trends obtained by experiment, though the simplifications limit its predictive capability.[4] model incorporates cycle-tocycle variability and takes into consideration the effect of hydrogen addition and was validated by direct comparison with experimental results.The authors discovered that for lean mixtures, when the fraction of hydrogen increases, COV decreases to a minimum value, which depends on the fuel-air equivalence ratio.At fuel ratio near 0.9 and 70% H2 by volume are the minimum values of COV.Set side by side with pure gasoline, hydrogen addition can make the ignition more stable and accelerate combustion rate to improve the brake thermal efficiency even more under lean burn condition.Additionally, it reduces the CO and HC emissions because of more complete combustion, but generates more NOx due to the higher combustion temperature [15] [16], [17], [18], [20], [21], [22].Decreasing of spark advance can improve the NOX [19].The purpose of this paper is to determine the effects of fuelling a 4-cylinder spark ignition engine with different fractions of gaseous hydrogen on energetic performance and cyclic variability over 250 cycles.
The engine was fueled at first with gasoline to establish the reference at the engine load χ=55%, engine speed of n=2500 min-1 and different air-fuel ratios (λ).

Experimental test bench
The test bench is equipped with a 1.5l, 4 cylinder turbocharged Daewoo A15MF engine fueled only with gasoline at the engine load χ=55% and speed of n=2500 min-1.Parameters such as: ambient temperature, intake air temperature, exhaust gases temperature, water temperature, air consumption, boost pressure and pollutant emissions.The in cylinder pressure was measured across 250 engine cycles and the mean values of these measurements were used to determine maximum pressure (pmax), maximum pressure rise rate (dp/dα)max, heat release rate (dQ/dα) and total heat released (Q).To determine the cycle variation during combustion process, the coefficient of variability (COV) for maximum pressure (COV)pmax, maximum pressure rise rate (COV)dp/dα, angles of 10, 50 and 90% heat release as (COV)10%, (COV)50% and (COV)90% were calculated and compared with the admissible limit of 10%.

Figure 1.
Test bench schema of the SI engine fueled with gasoline and hydrogen The main components of the test bench: 1gas analyser, 2inlet supercharger pressure gauge, 3 -A15MF engine, 4gasoline injection system, 5gasoline flowmeter, 6gasoline fuel tank, 7hydrogen bottle with pressure reducer, 8second pressure reducer for hydrogen, 9flowmeter for hydrogen, 10flame arrestor, 11hydrogen injection system, 12 -electric charge amplifier, 13 -data acquisition system, 14 -PC equipped with data acquisition board and software, 15remote for fine tune of fuel injectors, 16 -PC, 17dyno control cabinet, 18differential pressure gauge for intake air, 19intake air tank, 20eddy current dynamometer, 21open electronic control unit.

Results
The energetic performances of the engine are presented in the next graphs at fuelling only with gasoline (in blue) and at fueling with 5.82% hydrogen-gasoline blend (in red), at air-fuel ratio λ=0.89.The pressure diagrams for both fuels are presented in fig. 2. Because of the small fraction of hydrogen used, the engine adjustments were kept the same.For the blended fuel, the initial combustion phase was shorter and move to the left of the TDC because of the high flame speed of hydrogen.The maximum pressure is higher for the blended fuel 36.65 bar vs gasoline use 35.16 bar.

Figure 3. Maximum dispersion over 250 consecutive cycles
In figure 3, the maximum pressure values for 250 consecutive cycles are presented.The graphic shows an improvement of dispersion in values at fueling with gasoline-hydrogen blend vs only gasoline.
The COV for maximum pressure (figure 4) is determined with formula (1).

Figure 4. COV for maximum pressure (COV)pmax
After the calculation, a small improvement from 7.56% to 7.32% was noted (fig.4) when using 5.82% hydrogen-gasoline fuel blend.The dispersion for the indicated mean effective pressure is presented in figure 5.For λ=0.89, the dispersion of IMEP for 250 engine cycles for the hydrogen-gasoline blend is much reduced comparing to the gasoline use.This effect was obtained primarily because hydrogen burned more completely than gasoline, due to the excellent diffusivity properties and short quenching distance that allowed the flame to travel near the cylinder wall, achieving complete combustion.The addition of hydrogen slightly improved the stability of combustion also as pointed out in fig.6.
The COVIMEP for gasoline-hydrogen fuel is 1.28 vs 1.80 for gasoline use.At all air-fuel ratios, higher maximum pressures are observed for gasoline-hydrogen fuel.The maximum pressure rise rate (dp/dα)max was calculated using data from the in-cylinder transducer.In figure 7, hydrogen-gasoline blend has overall higher pressure rise rates (red dots) than regular gasoline fuel even at rich air-fuel ratio like λ=0.89 used for this graphic.The mass fraction burned (MFB) for 10% heat release is represented in figure 8 where a high dispersion for both fuels can be observed with a slight improvement for the gasoline-hydrogen fuel.Calculating the coefficient of variation for angle 10% of the MFB, COVG=39.26% and COVGBH6=26,89%, where G = gasoline and GBH6 = gasoline-hydrogen 5.82%, showed that the addition of hidrogen to the blend have a drastic impact on the initial burning phase, accelerating the combustion because of the high burning speed of hydrogen.The COV for angle 50% also indicates more stability for the main combustion phase for the use of gasoline-hydrogen blend 8.23% vs 12.74%.As for the final phase of the combustion, hydrogen has little to no impact 4.81% for the gasoline-hydrogen blended fuel vs 5.54% for regular gasoline fuel.The reduction of cyclic dispersion during the main phase of combustion is influenced by the lower cyclic dispersion of the initial phase at hydrogen use.The duration of the initial phase of combustion is influenced by the initial pressure, initial temperature, dosage, but as the reaction speed depends to a greater extent on the concentration than on pressure and temperature, the dosage has a greater influence, locally, in the cylinder, the presence of hydrogen influencing this aspect.Superior combustion properties of hydrogen compared to gasoline, such as lower heating value (119600 kJ/kg versus 42690 kJ/kg), higher flammability limits (=0.13…10.8versus =0.709…1.10),higher laminar flame speed (2.37 m/s versus 0.37...0.43 m/s), low minimum ignition energy (0.018 mJ versus 0.2...0.3 mJ), high diffusivity (0.61 cm 2 /s versus 0.05 cm 2 /s), the last two favor the acceleration of specific processes of the initial phase, increasing the reaction speed, reducing the duration of the initial phase, an aspect that leads to the reduction of cyclic variability during the initial phase, which correlates with the tendency to reduce the total duration of combustion and cyclic dispersion during the main phase.From the analysis of the mediated heat release laws, it can be noted that the duration of the initial phase is reduced by 7 ⁰CA at hydrogen use, and the heat release rate reaches positive values faster per cycle, by about 1⁰CA, compared to the classic fuelling.The reduction of the duration of the initial phase correlates with the reduction of the total duration of combustion by 26 ⁰CA, an aspect that correspond with the tendency to reduce cyclic dispersion and COV values at hydrogen use comparative to gasoline.In general, the reduction of the duration of the initial phase is consistent with a reduced cyclic dispersion during it.Thus, the lower COV of the beginning of combustion, COV10%, leads to the reduction of cyclic variability during the main phase, COV50%, and of the end of combustion COV90%, in the case of hydrogen use.

Figure 2 .
Figure 2. Pressure diagrams for only gasoline use and gasoline-hydrogen blend.

Figure 7 .
Figure 7. Maximum pressure rise rate over 250 engine cycles

Figure 9 .
Figure 9. Angle of 50% MFBFor the mass fraction burned for 50% heat release, the addition of hydrogen lightly improves the combustion stability as shown in figure9.