Green hydrogen, produced using renewable energy, is nowadays one of the most promising alternatives to fossil fuels for reducing pollutant emissions and in turn global warming. In particular, the use of hydrogen as fuel for internal combustion engines has been widely analyzed over the past few years. In this paper, the authors show the results of some experimental tests performed on a hydrogen-fueled CFR (Cooperative Fuel Research) engine, with particular reference to the combustion. Both the air/fuel (A/F) ratio and the engine compression ratio (CR) were varied in order to evaluate the influence of the two parameters on the combustion process. The combustion duration was divided in two parts: the flame front development (characterized by laminar flame speed) and the rapid combustion phase (characterized by turbulent flame speed). The results of the hydrogen-fueled engine have been compared with results obtained with gasoline in a reference operating condition. The increase in engine CR reduces the combustion duration whereas the opposite effect is observed with an increase in the A/F ratio. It is interesting to observe how the two parameters, CR and A/F ratio, have a different influence on the laminar and turbulent combustion phases. The influence of both A/F ratio and engine CR on heat transfer to the combustion chamber wall was also evaluated and compared with the gasoline operation. The heat transfer resulting from hydrogen combustion was found to be higher than the heat transfer resulting from gasoline combustion, and this is probably due to the different quenching distance of the two fuels.
Keywords: hydrogen; combustion; CFR engine; knocking; heat exchanges
Over the last decades, changes in climate have dramatically drawn the attention of the scientific community, which has pledged to find a solution. The release of anthropogenic greenhouse gas emissions is the most important issue to address in order to reduce global warming. To this purpose, the only mid-long-term solution is to replace fossil fuels with renewable energy sources. To mitigate the intrinsic randomness and periodicity of solar and wind energies, an effective storage system is needed. A feasible solution is the electric energy storage by using batteries which, however, has some disadvantages: the great amount of energy needed for both manufacturing and end-of-life disposal, and the rarity of raw materials. Both Shu et al. [[
Green hydrogen has some unquestionable advantages over the traditional fossil fuels when used in ICE. First, owing to its high laminar flame speed, as reported by Dahoe [[
There are also some issues related to the use of hydrogen as fuel for internal combustion engines: its high reactivity (low activation energy and high flame speed) often produces, for near stoichiometric mixtures, pre-ignition or auto-ignition phenomena, and in turn, knocking; and the high flame temperature and short quenching distance (Table 1) produce high heat transfer to the combustion chamber wall [[
Considering the growing interest in research on hydrogen-fueled engines, the authors decided to study the hydrogen combustion with particular reference to SI engines. In this work, some preliminary experimental tests were performed on a CFR (Cooperative Fuel Research) engine in order to explore some aspects of hydrogen combustion such as the combustion duration, the amount of NOx emitted, the engine thermal efficiency, the effects of wall heat transfer and the knock resistance. All the mentioned parameters were studied imposing a variation of engine CR and excess air ratio λ (i.e., the ratio between actual and stoichiometric A/F ratio). The CFR engine, which is the standard engine employed for fuel octane rating, was used in this research because its results are easily replicable by other laboratories and its compression ratio can be easily varied in order to simulate different SI engines geometries. The results obtained with hydrogen were compared with the ones obtained with gasoline in a reference operating condition in order to highlight the differences with respect to a conventional fuel.
The novelty of the present work lies mainly in two aspects. Firstly, it determines an experimental correlation between the engine compression ratio (CR) and the air-fuel (A/F) mixture ratio (λ) that defines a range of knock-free operating conditions. Secondly, the study establishes an experimental correlation between the engine CR and the amount of heat exchanged with the combustion chamber walls. Although other authors [[
In the subsequent section, the experimental setup and test execution are described in detail.
The engine employed in this work is a standard CFR S.I. engine [[
Because the engine set-up and the measuring equipment have been widely described in previous works by the same authors [[
By moving the cylinder head, the CR of the engine can be adjusted over a wide range. The original CFR was modified by implementing two separate electronic injection systems (as reported in Figure 1) in order to be able to control the amount of gasoline or gaseous fuel injected and to obtain mixtures with different A/F ratios. In the test performed, both air and fuel were measured with proper mass flow meters. The in-cylinder pressure was measured by a piezoelectric sensor flush mounted on the combustion chamber whereas the piston position was evaluated by using an optical encoder connected to the engine crankshaft. The in-cylinder pressure sensor was also used to detect knocking, as described in [[
Table 3 reports the accuracies of the measurement equipment used in the tests.
Figure 2 shows a scheme of the experimental layout.
The above-described CFR engine, fueled with hydrogen, was used to perform a series of experimental test: the engine CR and A/F ratio was varied in a wide range and the corresponding pressure curves were acquired. The cylinder pressure was used to evaluate, by means of the Rassweiler and Withrow method [[
The cylinder pressure was used also to evaluate the engine indicated thermal efficiency (ITE):
(
being P
(
The IMEP, in turn, was evaluated integrating the experimental pressure as a function of the in-cylinder volume:
(
where N
As is known, ITE may be considered the product of two different efficiencies that take into account different energy loss phenomena. The first is the ideal thermodynamic cycle efficiency that, in the case of SI engine, is the Otto cycle whose efficiency is:
(
where k is the isentropic coefficient equal to 1.4 for air.
Besides incomplete combustion, there are two other significant phenomena that reduce the thermodynamic efficiency: heat loss to the combustion chamber wall and non-isochoric combustion. These effects are represented by the internal efficiency η
(
In this way, a qualitative comparison between the different operating conditions in terms of heat losses can be performed because these are inversely proportional to the internal efficiency.
As most of the heat losses occur during combustion and are proportional to the burning mixture temperature, in the aforementioned comparison it is useful to evaluate the average combustion temperature T′
(
where T
The aforementioned approximation assumes that all the mass within the cylinder is involved in the combustion process at the same temperature T
A test performed with stoichiometric gasoline–air mixture and engine CR equal to 6.5 was used as a reference operating condition to compare all the results obtained with hydrogen and to highlight the main differences (pro and cons) between a conventional and an innovative fuel. In all of the aforementioned experiments, the SA was adjusted to achieve a location of peak pressure (LPP) approximately 15 CAD after top dead center (ATDC), which is a commonly used indicator of optimal combustion phase, as reported by Heywood [[
Table 4 resumes all the operating parameters used in the mentioned tests; in this table the spark advance (SA) is expressed in CAD before top dead center (BTDC).
During all the mentioned tests the engine pollutant emissions were measured.
A preliminary test was performed by varying the engine CR and finding the minimum λ value that allows a knock-free operation. The CR was varied between 4.75 and 13.5 and the corresponding λ value between 1.30 and 1.94. This test enables the measurement of knock resistance, which is indicated by the engine CR, for various lean hydrogen mixtures. This provides valuable initial guidance to engine designers about the maximum engine CR or minimum λ that can be used for knock-free operation. Obviously, to apply this method, a proper match between CFR and actual engine compression ratios must be defined. To this purpose, the authors experimentally determined that a CFR CR equal to 6.5 may correspond to a CR equal to 10 in a smaller displacement cylinder (310.5 cm
The results, shown in Figure 3, show a linear dependence between the two parameters. This means that, unlike gasoline, the leaner hydrogen–air mixtures exhibit an increased knock resistance.
This behavior can be explained with the different processes that lead the two fuels to knock. In gasoline–air mixtures, the knocking is mainly caused by the expiration of the auto-ignition time delay whose duration depends on the pressure and temperature. This enriches the mixture and increases the combustion speed and, in turn, allows the flame front to burn all the gasoline–air mixtures before the auto-ignition time expires. On the other hand, the stoichiometric hydrogen–air mixture is very reactive, and its minimum ignition energy is very small [[
Through extrapolating the diagram in Figure 3, it can be concluded that a CFR engine having a CR of approximately 14 can operate with a hydrogen-air mixture of λ = 2. This seems a perfectly acceptable hypothesis given that the difference between a CR of 14 and 13.5 is less than 4%. The substantial increase in the CFR CR from 4.75 to 14, which corresponds to an increase in λ from 1.3 to 2, indicates that an automotive SI engine fueled with hydrogen and operated at λ = 2 could potentially employ a significantly higher CR than current engines without experiencing knock, resulting in a significant improvement in thermodynamic efficiency.
In order to highlight the effects of λ on combustion duration, a second set of tests was performed with a fixed CR of 6.5 and a variation in the hydrogen–air mixture λ from 1.5 to 2.5. To this purpose, the experimental pressure curves were manipulated to obtain the experimental MFB curves. The combustion was divided in three parts. First is the flame front development phase, which is mainly characterized by laminar flame speed propagation and ranges from spark timing to the CA corresponding to 5% of MFB. Second is the rapid combustion phase, delimited between 5% and 90% of MFB, which is often referred to as the actual combustion duration because the heat released during this phase produces the main effects on the thermodynamic engine cycle. The final 10% of the combustion duration, from 90% to 100% of MFB, is marked by flame quenching and is disregarded due to its negligible thermodynamic impacts and potential to introduce uncertainty regarding the actual cessation of combustion. Both flame front development phase (0–5% of MFB) and rapid combustion phase (5–90% of MFB) were evaluated as function of λ at constant CR (6.5).
In addition, by merging the first and second dataset, it was possible to determine the dependency between combustion duration and CR at constant λ.
In Figure 4, the duration of flame front development (a) and rapid combustion (b) are presented for two main datasets: one with a variable CR and the other with a constant CR of 6.5. The durations are plotted as a function of λ and are compared with the reference condition of gasoline stoichiometric. The flame front development duration, characterized by laminar flame propagation speed, increases almost linearly with λ, but the dataset with variable CR shows a lower slope. This can be explained considering that higher CR produces higher pressure and temperature at the ignition time and, in turn, higher laminar flame speed [[
The rapid combustion duration, characterized by turbulent flame propagation speed, increases almost linearly with λ for the dataset with constant CR and more than linearly for the dataset with variable CR. This suggests that an increase in CR hampers turbulent combustion and further lengthens it. Moreso, in this case, the gasoline–stoichiometric mixture exhibits a longer rapid combustion phase compared with hydrogen except for λ = 2.5 and CR = 6.5.
In order to better highlight the influence of CR on the combustion phases duration, three couples of operating conditions were selected; each couple being identified by the same λ (and different CR). In Figure 5, the duration of both the flame front development phase (a) and the rapid combustion phase (b) are shown, for the three couples of operating conditions considered.
Figure 5a shows a decreasing trend of flame front development duration when increasing CR and this effect, as already stated, is due to the increment of pressure and temperature at the end of compression stroke that increases laminar flame speed; on the other hand, Figure 5b shows the trend of rapid combustion with CR and here a slight increase can be noted, in particular at higher λ values. This result contrasts with the findings in the literature [[
It can be concluded that CR has a strong reducing effect on flame front development and a weak increasing effect on rapid combustion whereas λ has an increasing effect on both combustion phases.
To give an overall view, the duration of the main part of the combustion, i.e., from spark timing to 90% of MFB, was plotted in Figure 6 as a function of λ (a) and as a function of CR (b).
Figure 6a shows an almost linear increasing trend of the main combustion duration as function of λ regardless of the dataset, variable or constant CR, and this means that the effect of λ on the duration of flame front development (Figure 4a) is roughly balanced by the effect on rapid combustion phase (Figure 4b). Figure 6b instead shows a slightly decreasing trend of the main combustion duration as function of CR; this means that the effect of CR on flame front development (Figure 5a) is greater than that on rapid combustion (Figure 5b). Compared with gasoline, hydrogen produces a shorter combustion duration, closer to the isochoric process, even with very lean mixtures, which improves the engine's thermodynamic efficiency.
As stated in previous section, the CFR raw emissions were measured in all the operating conditions of Table 4. The only relevant pollutant emitted by the hydrogen-fueled CFR engine was NOx, except for a very small amount of unburned hydrocarbons due to the engine lubricant, so it is very important to find the low NOx operating conditions.
Figure 7 clearly shows that for λ > 1.4 the NOx levels are lower than the gasoline ones and around λ = 1.9 the measured NOx are near zero (30–40 ppm) confirming that to obtain a "zero emissions" hydrogen engine it is required to adopt A/F ratios double than stoichiometric, as widely described in the literature [[
Finally, Figure 8a shows the effects of CR and λ on ITE and Figure 8b the effects on internal efficiency, which has an inversely proportional trend with respect to the heat transfer with the chamber wall and to the combustion duration, as already stated earlier.
Figure 8a highlights that, except for the operating condition with λ = 1.35 and CR = 5.5, hydrogen-fueled CFR always exhibits a higher ITE with respect to the gasoline counterpart and this is the combined result of the faster combustion (Figure 6a), the higher CR (in the case of dataset labelled "variable CR") and the yet-be-confirmed effects of the heat transferred to the chamber walls. To actually determine the effects of heat loss, the η
To further investigate this aspect, the average combustion temperature T′
In order to highlight the effects of λ on the heat loss of hydrogen combustion, the dataset labeled "CR = 6.5" reported Figure 8b must be considered. The diagram shows an almost constant trend indicating that the negative effect of combustion duration increase (Figure 6a) is balanced by the positive effect of the reduced heat loss due to the decreasing T′
Figure 9 shows the great reducing effect of CR on η
In this paper, the combustion of hydrogen in a CFR engine was analyzed in order to assess the effects of engine CR and mixture λ on both combustion speed and heat loss to the chamber wall. The combustion was divided in two parts: the first (flame front development phase) characterized by laminar flame speed and the second (rapid combustion phase) characterized by turbulent flame speed. The tests show the following results:
- Increasing λ produces an increasing effect on the duration of both combustion parts;
- Increasing CR produces a decrease of the laminar combustion duration and a slight increase in the turbulent one;
- The whole combustion duration undergoes an increasing effect by λ and a decreasing effect by CR;
- The heat loss to the combustion chamber wall undergoes a slight reduction by increasing λ (due to the combustion temperature reduction) and a great increase by increasing CR (due to the increase of both combustion temperature and convective heat transfer);
- An experimental correlation between CR and λ was determined, in order to obtain knock-free operating conditions, and a linear trend between the two parameters was found.
The raw NOx emissions were also measured.
All the mentioned results were compared with a reference operating condition obtained fueling the CFR with stoichiometric air–gasoline mixture. The results of this comparison are:
- Hydrogen combustion always shows a shorter duration with respect to gasoline combustion, in both the laminar and the turbulent part;
- For λ > 1.4, the NOx levels are lower than the gasoline ones and for λ > 1.9, the measured NOx are near zero;
- The heat transfer to the chamber wall is higher when fueling with hydrogen with respect to gasoline operation in particular for higher CR.
The above results confirm the findings mentioned in the literature [[
It can be concluded that hydrogen allows, with proper λ values, a faster and cleaner combustion compared to gasoline with higher CR and consequently higher engine efficiency. These considerations identify green hydrogen as the most promising substitute for conventional fossil fuels in the modern automotive market.
Graph: Figure 1 Modified CFR injection system.
Graph: Figure 2 Experimental layout.
Graph: Figure 3 Knock-free minimum λ as function of the CR.
Graph: Figure 4 Two combustion phases, for all the tested operating conditions, as a function of λ. (a) Flame front development and (b) rapid combustion.
Graph: Figure 5 Duration of flame front development and rapid combustion as a function of CR at constant λ. (a) Flame front development and (b) rapid combustion.
Graph: Figure 6 Duration of main part of combustion as a function of both λ and CR. (a) Main combustion duration as function of λ and (b) main combustion duration as function of CR.
Graph: Figure 7 NOx emissions of the hydrogen-fueled CFR as a function of λ.
Graph: Figure 8 The indicated (a) and internal (b) efficiencies as a function of λ. (a) CFR ITE as function of λ and (b) CFR ηi as function of λ.
Graph: Figure 9 Internal efficiency as a function of CR at constant λ.
Table 1 Hydrogen properties (at 300 K and 1 atm) compared with methane and isooctane (the fuel–air mixtures are stoichiometric) [[
Specification H2 CH4 C8H18 H2-Air CH4-Air C8H18-Air laminar flame speed at 360 K [cm/s] - - - 290 48 45 lower heating value [MJ/kg] 120 50 44.3 - - - density [kg/m3] 0.08 0.65 692 - - - volumetric energy content [kJ/m3] - - - 3189 3041 3704 minimum ignition energy [mJ] - - - 0.02 0.28 0.28 adiabatic flame temperature [K] - - - 2390 2226 2276 minimum quencing distance [mm] - - - 0.64 2.03 3.5 stoichiometric air-fuel ratio [kg/kg] 34.2 17.1 15 - - - flammability limits (λ) 10–0.14 2–0.6 1.51–0.26 - - -
Table 2 CFR engine specifications.
Specification Value Manufacturer Dresser waukesha Model F1/F2 Octane Compression ratio 4.5–16 Inlet mixture temperature variable Spark advance variable Bore 82.6 [mm] Stroke 114.3 [mm] Connecting rod length 254.0 [mm] Displacement 611.2 [cm3] Speed (fixed) 900 [rpm]
Table 3 Accuracy of the instrumentation used in the tests.
Sensor Accuracy Gasoline mass flow meter ±1% of reading Hydrogen mass flow meter ±1% of reading Air mass flow meter ±1% of reading Combustion chamber pressure sensor linearity error < ±0.3% FSO Combustion chamber pressure sensor thermal sensitivity shift < ±0.5% at temperature between 200 and 300 °C NOx sensor ±4% or 25 ppm absolute
Table 4 Operating parameters.
Fuel A/F Mixture Inlet Temperature (°C) Engine CR λ SA (CAD BTDC) Average Combustion Temperature gasoline 42 6.5 1.00 25 1237 hydrogen 43 4.75 1.3 5 1053 hydrogen 43 5.5 1.35 4 1037 hydrogen 43 7.0 1.46 4 1060 hydrogen 44 8.5 1.63 5 1030 hydrogen 44 10.0 1.73 5 1022 hydrogen 44 12.3 1.89 6 982 hydrogen 45 13.5 1.94 3 951 hydrogen 44 6.5 1.55 4 1034 hydrogen 44 6.5 1.65 7 1006 hydrogen 45 6.5 1.75 9 973 hydrogen 45 6.5 1.90 12 945 hydrogen 45 6.5 2.00 15 916 hydrogen 45 6.5 2.50 21 835
Conceptualization, S.B., S.C. and E.P.; Data curation, S.B. and S.C.; Formal analysis, S.B. and S.C.; Investigation, S.B. and S.C.; Methodology, S.B. and E.P.; Supervision, E.P.; Validation, E.P.; Writing—original draft, S.B.; Writing—review & editing, E.P. All authors have read and agreed to the published version of the manuscript.
Not applicable.
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
By Stefano Beccari; Emiliano Pipitone and Salvatore Caltabellotta
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