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High power performance with zero NOx emission in a hydrogen-fueled spark ignition engine by valve timing and lean boosting
Fuel 128 (2014) 381–389
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High power performance with zero NOx emission in a hydrogen-fueled spark ignition engine by valve timing and lean boosting Jongtai Lee b,⇑, Kwangju Lee a, Jonggoo Lee a, Byunghoh Anh a a b
Graduate School of Sungkyunkwan University, 300, Chunchun-dong, Jangan-gu, Suwon-si, Gyeonggi-do 440-746, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 300, Chunchun-dong, Jangan-gu, Suwon-si, Gyeonggi-do 440-746, Republic of Korea
h i g h l i g h t s Backfire could be controlled by retardation of the intake valve opening timing in a H2 engine. Combustion was stable until an ultra-lean region below the NOx generation. A BMEP equal to that of a gasoline engine was realized without backfire. A NOx emission of 0 ppm was realized with the lean boosting. Thermal efficiency reached a high level of 39.1% which is difficult to achieve with the Otto engine.
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Article history: Received 13 December 2013 Received in revised form 28 February 2014 Accepted 7 March 2014 Available online 26 March 2014 Keywords: Hydrogen-fueled engine Lean boosting Backfire control Valve timing Ultra-low NOx
a b s t r a c t For a hydrogen engine to be successfully commercialized in a short period of time, it must have power on the same level as gasoline engines, almost no NOx emissions, and high efficiency. This simultaneous achievement would be possible with the use of low temperature combustion, which supercharges an ultra-lean mixture, because NOx from a hydrogen engine is thermally produced. However, increasing the energy input for higher output produces backfire, which is a significant problem for hydrogen-fueled engines with external mixture, and why the simultaneous achievement of high output and low-exhaust is not easily accomplished. This research investigated the potential of simultaneously achieving high output, similar to the output of a gasoline engine, without backfire, by using a complex valve timing variation and lean boosting. The study achieved almost zero NOx emission and high efficiency in a single cylinder engine built for research purposes with a valve timing variation system. Experimental results revealed that retarding the intake valve opening timing could control the backfire generated when increasing the output of the hydrogen engine, and the method above is also effective with lean boosting. Stable combustion in the lean region of U = 0.2, which can reduce temperature below the NOx generation temperature, was possible due to the large ultra-lean limit of hydrogen. In addition, the exhaust was almost NOx free due to the low temperature combustion, by supercharging the ultra-lean mixture under high power operation similar to that of gasoline. The above results verified that almost pollution-free emission without backfire, and simultaneous high power and efficiency could be achieved in a hydrogen engine through a method of intake valve timing retardation and lean overcharging. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The hydrogen engine can utilize hydrogen produced from any source, and since its fuel does not include a carbon component, it is consequently one of the next generation engine systems that ⇑ Corresponding author. Tel.: +82 31 290 7442, mobile: +82 10 8920 7442; fax: +82 31 299 4754. E-mail addresses: [email protected], [email protected] (J. Lee). http://dx.doi.org/10.1016/j.fuel.2014.03.010 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
can simultaneously solve both the CO2 and fossil fuel energy depletion problems. Among the hydrogen engines being developed, the hydrogen engine with an external fuel mixture is more efficient and durable than the direct injection type, and is considered capable of commercialization in a short period of time with low cost. However, it also generates NOx in its exhaust gases, which undermines its value compared to fuel cells. Additionally, the power output is relatively lower than the power output of petroleum-based engines
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due to the decrease in the mass air flow from the low hydrogen density [1–6]. The fuel of a hydrogen engine does not contain carbon or nitrogen components, and the NOx is only produced as thermal NOx, which is dependent on the combustion gas temperature [7–9]. Therefore, in order to simultaneously resolve both the NOx and low power issues, lean boosting has been proposed. Lean boosting is one of the low temperature combustion methods, and supercharges a lean mixture to supply the energy required for high power. The resulting increase in the intake flow mass allows the max temperature of the combustion gas to be below the thermal dissociation temperature [10–12]. However, increasing the supplying energy for higher power introduces one of the biggest problems of a hydrogen engine with external mixture, namely, backfire, and it is not easy to simultaneously achieve both high power and low exhaust. Backfire is caused by the reflux of flame from the hotspot or crevice volume in the rich region, into the intake valve during the valve overlap period [13–16]. With that in mind, various backfire controlling methods have been proposed. One of the methods retards the opening timing of the intake valve involved in the valve overlap period to control the reflux timing of the source of the flame and thus the backfire generated in the rich region [17]. This valve timing retardation method is relatively easy to achieve, and the decrease in volumetric efficiency produced by retarding the valve opening period during supercharging could be compensated, and it is effective in a hydrogen engine with SI. The complex process of retarding the intake valve opening timing and lean boosting supercharging could achieve high power without backfire in a hydrogen engine with external mixture, and produce an almost pollution-free NOx emission. However, it is still not known whether it is possible to produce power similar to that from a gasoline engine, or whether, even though the lean limit of hydrogen is very large, NOx emissions can be reduced to an almost pollution-free level with a stable ultra lean combustion, below the thermal dissociation temperature, under high power operation. This research investigated the possibility of simultaneously achieving high power, almost pollution-free NOx emission, and high efficiency without the generation of backfire, through the complex applications of valve timing variation and lean boosting to a hydrogen engine with external mixture, which has the highest potential for near-term commercialization. A single cylinder engine with a varying valve timing system was built for research purposes and used to analyze backfire control, combustion stability during ultra-lean supercharging, NOx emission and experimental performances.
2. Experimental explanation 2.1. Experimental setup Fig. 1 illustrates the actual picture of the external mixture hydrogen engine built for the experiment with Mechanical Continuous Variable Valve Timing (MCVVT). The hydrogen research engine is a DOHC single cylinder engine with a compression ratio of e = 10.5, and a bore and stroke of 86 mm. Table 1 lists the main specifications. Crank, cylinder head, cam-shaft and assemblies of a conventional 4 cylinders engine were modified for the experiment of a single cylinder. MCVVT is a timing belt of a certain length that independently and continuously varies the opening and closing periods of an intake valve and exhaust valve while the ignition is operating, using the principle of the phase angle of the camshaft, connected to the crank shaft, varied with respect to the two idling gears [18]. Hydrogen
Fig. 1. Hydrogen research engine with MCVVT system.
Table 1 Specification of test engine. Valve mechanism
DOHC
Bore stroke Compression ratio Displacement volume Valve timing control Duration of IVO Duration of EVO Range of valve timing
86 86 mm 10.5 499.6 cc MCVVT system 257 °C A 224 °C A 50 °C A
gas was supplied using a low pressure solenoid type injector for a natural gas engine. The insert location of the injector was 70 mm in front of the intake valve. For cooling, coolant was independently supplied to the cylinder head and cylinder jacket. As the ignition system, a type of distributor with grounded secondary coil, which easily emits a residual charge that could cause a backfire, was used [19]. Fig. 2 is the schematics of the experimental setup. The experimental setup was composed of the previously mentioned hydrogen engine, power measurement system, hydrogen supply system, intake and exhaust system equipped with supercharger, cooling system, and data acquisition. The hydrogen fuel came from a high pressure cylinder charged to 15 MPa and went through a primary and secondary regulator to lower the pressure to 0.3 MPa. The gas with the lowered pressure was then supplied to the engine through the gas injector. An accumulator with a volume of 3.8 L was installed between the hydrogen flow meter and gas injector to decrease surging due to the fuel injection. The hydrogen injection was controlled by an injector controller (Mobiltek, MT2010) of the peak and hold method, and measured with a hydrogen flow meter (Bronchus, P-113AC -HAD-55 -V) of thermo coupling method. Supercharging to increase fresh air was produced with an external supercharging ring blower (DongBu, DBR-1002) and a piston type compressor: the external supercharging work was not considered in the experimental result. Bypass valves were
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Fig. 2. Schematic diagram of experimental setup.
installed at each outlet of the supercharger (ring blower) and the entrance of the surge tank to control the supercharging pressure. In addition, the supercharging temperature was controlled with a liquid-cooled intercooler. In order to reduce surging due to supercharging, 2 surge tanks were employed between the intake pipes of the engine and the outlet of the intercooler, and an air flow meter was installed between the surge tanks. The NOx emission was measured with an exhaust gas analyzer (Horiba, MEXA-554JKNOx). The test of changing the exhaust pressure was performed by roots blower and throttle valve in exhaust pipe. 2.2. Experimental method Depending on the opening timing of the intake valve, an arbitrary ignition source within the combustion chamber can cool or completely combust the fuel. Likewise, the closing timing of the exhaust valve can cause a backfire when high temperature exhaust gas or residual gas comes into contact with fresh air [20]. Therefore, the major experimental variables to identify whether backfire is suppressed or not were set as the intakeexhaust valve timing and supplying energy. The intake valve opening period was varied sequentially from BTDC 20 °C A to ATDC 20 °C A and the exhaust valve opening period was varied sequentially from BTDC 20°CA to ATDC 30°CA. The supplying energy was increased until backfire occurred at each of the intakeexhaust valve timings. In addition to the above, abnormal combustion and performance characteristics of the low temperature combustion produced from the lean boosting method were analyzed. The major experimental variables for the analysis were the fuel–air equivalence ratio (equivalence ratio hereafter) and boost pressure for increasing the amount of fresh air. The equivalence ratio was varied step-by-step by U = 0.1 increments, from the combustion capable lean region, to the rich region where knocking occurs. The experimental power objective was to achieve as high power with the hydrogen engine as a gasoline engine. Therefore, the boost pressure was varied by 0.01 MPa in the engine from the natural intake condition to the power output at the gasoline fuel changing (U = 1.0) and the pressure 10% higher than the boost pressure capable of ultra-low NOx emission. Assuming the power at the gasoline fuel switching (U = 1.0) was WOT, the load was varied step-by-step by 25%. For the case above, the opening timing of the intake valve was fixed at the TDC that was capable of controlling backfire under a high
load, as determined by the experiment results. The closing timing of the exhaust valve was fixed at the ATDC of 10 °C A with high exhaust efficiency. The engine rotation and ignition timing were fixed at 1600 rpm and MBT, respectively, in each experiment. 3. Results and discussions 3.1. Backfire control with valve timing retardation at high power Fig. 3 illustrates the backfire limit equivalence ratio, which is the equivalence ratio right before backfire occurs, with varying intake valve opening timing (IVO) and exhaust valve closing timing (EVC). The backfire limit equivalence ratio increased when IVO was retarded from BTDC 20 °C A, and backfire did not occur even when the experiment continued beyond U = 1.0, up to U = 1.6, after TDC. This seems to be due to a complex reason in which the slowly combusting flame from the hot spot within the combustion chamber and the piston crevice volume either cools down or burns until the intake valve opens again due to the retardation of IVO. Fresh air
Fig. 3. Exhaust valve closing timing and intake valve opening timing versus fuel–air equivalence ratio about the backfire.
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and exhaust gas coexist during the valve overlap period, which varied with the advancement and retardation of IVO and EVC. When there is an identical backfire limit equivalence ratio during the period with no valve overlap, it means that the backfire did not occur, as the exhaust gas directly ignites the fresh air. However, residual high temperature gas in the cylinder could be involved in backfire, since the gas influences the ignition condition of the mixture within the heat source of the combustion chamber wall and crevice volume. To verify this the backfire limit equivalence ratio was identified with respect to varying exhaust pressure, as illustrated in Fig. 4, The change in exhaust pressure can be understood as the change in residual gas because the amount of residual gas decreases as the exhaust gas decreases. As illustrated in the figure, the backfire limit equivalence ratio showed a slightly reducing trend as the exhaust pressure, or the residual gas, was reduced. However, the difference was very small, small enough to assume it was constant, and that the residual gas did not have a significant influence on the backfire generation. The residual gas is heavily influenced by the EVC, and the result above verifies that the generation of backfire is not highly dependent on EVC, as noted in Fig. 3. This also means that the IVO is more important than EVC in controlling the backfire. Therefore, the EVC was fixed to ATDC 10 °C A, which results in a high exhaust efficiency. It is a well-known fact that the temperature of the wall of a combustion chamber increases as the engine rotation increases due to the increase in the supplying energy per unit time, and the timing of backfire controlling IVO may be changed. When IVO was retarded to TDC to determine this, the backfire limit equivalence ratio with respect to the change in the engine rotation was compared to IVO BTDC 10 °C A, the design valve timing of the base engine, and Fig. 5 illustrates the comparison. Backfire was generated around U = 1.3, which is beyond the equivalence ratio of U = 1.0, with the intake valve timing of IVO BTDC 10 °C A and at 1000 rpm. The backfire limit equivalence ratio decreased as the engine rotation increased, so that it became U = 0.7 around 1600 rpm and remained almost constant. The generation of a certain source of ignition is facilitated by the increase in the supplying energy per unit time due to the increase in the engine rotation, and the backfire limit equivalence ratio decreases due to the decrease in the period per unit crank, and the factor shortening the combustion or cooling period. The similarities in the backfire limit equivalence ratio starting from 1600 rpm could be explained by the factors above cancelling each other, due to
Fig. 4. Backfire limit equivalence ratio as a function of exhaust pressure.
Fig. 5. Fuel–air equivalence ratios versus engine speed at each case.
the increase in burning rate from the increase in turbulence within the cylinder. Backfire was generated with the equivalence ratio of U = 1.3 at 1000 rpm due to a rich mixture, even with low heating value. This was due to various factors, such as the decrease in burning rate from the use of a rich mixture and the lengthened combustion of flame from the crevice volume. With IVO TDC, however, backfire was not generated, regardless of the engine rotation, when the measurement was made from the lean region beyond an equivalence ratio of U = 1.4. The result above shows that backfire can be controlled regardless of the engine rotation within the operational range of this experiment, when IVO is fixed at TDC. However, when IVO is retarded, backfire could be controlled with a decrease in the supplying energy, by decreasing the air intake. Fig. 6 illustrates whether backfire is generated or not with respect to the increase in supplying energy when IVO is retarded. The supplying energy here is increased until right before knocking generation, by lean boosting and ignition timing retardation. When IVO is retarded to TDC under the natural intake condition, the supplying energy can be increased by a maximum of 15.6% compared to the supplying energy from IVO BTDC 30 °C A, due to the increase in the equivalence ratio from the backfire
Fig. 6. Supplying energy versus intake pressure.
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control. In addition, a lean boosting was conducted with the increase in the boost pressure at IVO TDC to eliminate the influence of decreased charging efficiency due to the IVO retardation. The result showed a trend of limited increase in the supplying energy due to the knocking, but no backfire generation. When the boost pressure was 0.21 MPa, the supplying energy was increased by approximately 60.05%. This is in contrast to the supplying energy under the natural intake condition, when backfire was still not generated under IVO TDC. Such a result shows that when IVO is retarded, the backfire is not controlled by decreasing the supplying energy by decreasing the air intake, and that backfire could be controlled by retarding the IVO, even with the application of lean boosting. As it as mentioned earlier, intake valve opening timing retardation was verified to be effective in suppressing the backfire generation while achieving the gasoline engine level power output with a hydrogen-fueled engine. The verification was made from the suppressed backfire generation at IVO TDC under the conditions of variation in exhaust valve closing timing, increase in engine revolutions, and increase in supplying energy. To identify the conditions in which a hydrogen engine achieves the power of a gasoline engine, the brake mean effective pressure, BMEP, was investigated with respect to the changes in the equivalence ratio and boost pressure, as Fig. 7 illustrates. Gasoline engine power was measured for the experimental engine by modifying the hydrogen engine fuel supply system to take gasoline. Due to the use of a single cylinder, an increase in mechanical loss from the utilization of MCVVT, and the decrease in the volumetric efficiency at the intake, the BMEP of a gasoline engine was approximately 0.72 MPa below the conditions of WOT and equivalence ratio U = 1. That value is lower than the BMEP of a conventional engine. However, since a goal of this research was to identify the potential of achieving such power when a gasoline engine is converted to a hydrogen engine, the value was used as the representative value of gasoline engine power for this study (hereafter gasoline-level power). Under the conditions of IVO BTDC 20 °C A and 10 °C A in a natural intake condition, normal operation was limited to the vicinity of the equivalence ratio range of U = 0.7–0.8 due to backfire generation, and the maximum value of BMEP remained at approximately 85% of a gasoline engine. Even if IVO is retarded to TDC and the equivalence ratio capable of normal operation is expanded to U = 1.0, the maximum value of BMEP is similar to the previous one due to the decrease in the amount of air intake caused by the IVO retardation. Gasoline-level power or higher was achieved
Fig. 7. BMEP versus fuel–air equivalence ratios at each case.
without backfire generation when IVO was fixed to TDC and supercharged, however, a knock occurred due to the increase in the supplying energy in the rich region. The minimum boost pressure and equivalence ratio able to generate gasoline-level power under the above conditions are in the vicinity of 0.13 MPa and U = 0.6, respectively. When the boost pressure is increased even more, gasoline-level power could be achieved in an even leaner mixing ratio, and the maximum BMEP of 0.912 MPa was achieved when the lean boost pressure was 0.21 MPa. 3.2. The possibility of pollution-free NOx emission at high power As it is shown in Eq. (1), the gas temperature (Tg) in the combustion chamber is proportional to the supplying energy (Q), and it is inversely proportional to the mass of fresh air (mi) and specific heat at constant volume (Cv). The maximum temperature within the engine decreases even with identical supplying energy when a large quantity of lean mixture is supplied.
Tg ¼ T 1
n1
þ Q mi
n X mk k¼1
mi
!1 C mk
ð1Þ
T1, e, and n represent initial compression temperature, compression ratio, and polytropic index, respectively, and the subscript x corresponds to air, fuel and residual gas. The decreases in the maximum gas temperature for hydrogen and hydrocarbon fuels are significantly different from each other, even if identical energy was supplied, because hydrogen fuel and hydrocarbon fuels have significantly different material properties. The lean boost pressure is the boost pressure that has the lean equivalence ratio when boosted under identical supplying energy. Fig. 8 illustrates the reduction rate of the reduction rate of the maximum temperature and the specific heat at constant volume
Fig. 8. Decrease rate of temperature and constant-volume specific heat of intake mixture mass versus boost pressure about each fuel with same supply energy at U = 1.0 in natural aspirated condition.
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of fresh air with respect to the variation in the lean boost pressure to compare the low-temperature combustion effect with those of gasoline and natural gas. The supplying energy of each fuel was fixed to the measurement from the natural intake condition and equivalence ratio of U = 1.0. The material properties of C8H18 and CH4 were used as the properties of gasoline and natural gas. The maximum gas temperature reduction rate is the percentage temperature reduction resulting from an increase in lean boost pressure under natural intake condition. The maximum gas temperature reduction rate represents the low temperature combustion effect, and tended to increase for all three types of fuels due to the increase in the mass of fresh air resulting from the increase in lean boost pressure. However, while the values of gasoline and natural gas fuels were similar to each other, the hydrogen values were relatively lower than the others. As mentioned earlier, the supplying energy was constant. There were differences in fresh air masses with respect to the mass of each fuel, depending on the change in lean boost pressure, however, the mass of fresh air is generally proportional to the boost pressure. The specific heat at constant volume of hydrogen is approximately 10.183 kJ/kg K at 300 K and it is approximately 6.2 times larger than gasoline (1.639 kJ/kg K) or natural gas (1.736 kJ/kg K). Therefore, the specific heat at constant volume of a hydrogen–oxygen mixture is significantly larger than other fuels and the difference decreases as the lean boost pressure increases, due to the increase in mass ratio of oxygen to hydrogen from the supercharging of the lean mixture. Due to the factors above, the maximum gas temperature reduction rate of hydrogen with lean boosting is lower than for other fuels. For example, the maximum gas temperature reduction rate, which represents the low temperature combustion effect of hydrogen, at a boost pressure of 0.15 MPa is approximately 23.1% lower than that of gasoline fuel. Consequently, to lower the maximum gas temperature of hydrogen to below the thermal dissociation temperature of NOx, hydrogen requires a more lean mixture than hydrocarbon fuels, due to its small gas temperature reduction rate and high adiabatic flame temperature. Fig. 9 illustrates the maximum gas temperature in the combustion chamber of the hydrogen engine with respect to the changes in boost pressure and equivalence ratio. The maximum gas temperature was computed from the gas pressure within the cylinder and the ideal gas equation. The maximum gas temperature value for an identical boost pressure decreases rapidly as the equivalence ratio decreases. However, the maximum gas temperature value for an identical
Fig. 9. In-cylinder maximum temperature versus fuel–air equivalence ratios and boost pressure.
equivalence ratio is almost constant, even with the increase in boost pressure, due to the simultaneous increase in supplying energy and mass of the mixture. This trend is shown in the regions that yield gasoline-level power as well. The generation of thermal NOx theoretically takes place due to the Zeldovich NO reaction when the flame temperature is above 1850 K. It has been reported that NOx is generated from the flame temperature of 2200 K or above from experimental results [21– 23]. As expected, the condition that yields a temperature below NOx generation temperature in a hydrogen engine occurs in the super lean region, regardless of the change in boost pressure, and the region is the region below the equivalence ratio of U = 0.35. From the experiment, the boost pressure that satisfied the region above was approximately 0.17 MPa. Fig. 10 illustrates the cycle coefficient of variation, COVimep, of the hydrogen engine with respect to the change in the equivalence ratio at each boost pressure in the region capable of producing gasoline-level power. COVimep is the Root Mean Square (RMS) of the indicated mean effective pressure, and it is the ensemble average of the data of 100 cycles. Conventionally, the lean limit of a gasoline engine is approximately the equivalence ratio of U = 0.7. Even under natural intake conditions CO or HC increases, even if NOx decreases, when a leaner low temperature combustion is employed, and this, again, causes the problem of exhaust emission. On the other hand, the hydrogen engine showed stable combustion with COVimep less than 5% until the equivalence ratio of U = 0.3, at which the thermal dissociation temperature of NOx is generated. In addition, the cycle variation showed an even larger decreasing trend when the boost pressure was increased in an identical equivalence ratio. When the boost pressure was increased above 0.16 MPa, combustion was facilitated by the increase in the initial combustion temperature from the increased supplying energy and the increase in the flow rate from supercharging, and this resulted in a stable low temperature combustion even with the super lean mixture around the equivalence ratio of U = 0.2. Fig. 11 illustrates the NOx emission according to changes in equivalence ratio and boost pressure, and Fig. 12 illustrates the regions capable of NOx emissions of less than 10 ppm. The maximum NOx emission, 2120 ppm, was observed when the mass of fresh air was increased right before knocking occurred, at the boost pressure capable of producing gasoline-level power. However, when the equivalence ratio became approximately
Fig. 10. COVimep versus fuel–air equivalence ratios at each boost pressure.
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Fig. 11. NOx versus fuel–air equivalence ratios above bmep of gasoline level. Fig. 13. Boost pressure versus fuel–air equivalence ratio in each load for zero emission and increasing NOx regions.
every load. The case of 100% load of gasoline-level power in the NOx pollution-free region is mentioned above. 75% load achieves 0 ppm from respective boost pressure and equivalence ratio of 0.15 MPa and U = 0.3. NOx emission at 50% load was verified to be below 10 ppm at its maximum, and 0 ppm was achieved from a boost pressure and equivalence ratio of 0.12 MPa and U = 0.315, respectively. 0 ppm of NOx was achieved even with a natural intake condition (U = 0.27) with 25% load. 3.3. Simultaneous achievement of high performance due to lean boosting
Fig. 12. NOx versus fuel–air equivalence ratios above bmep of gasoline level under 10 ppm of NOx.
Fig. 14 illustrates the brake thermal efficiency with respect to the change in equivalence ratio at each boost pressure. It is a well-known fact that brake thermal efficiency shows an increasing trend with increasing lean boost pressure, with the decrease in cooling loss due to the noticeable decrease in the combustion temperature. This occurs even when the combustion time increases with a lean mixture. This trend was observed to
U = 0.5 from lean boosting, NOx emissions decreased to approximately 330 ppm due to the low temperature combustion effect. As illustrated in Fig. 12, the NOx emission stays below 10 ppm, even with the high supplying energy required by gasoline-level power, when the boost pressure and equivalence ratio reach 0.16 MPa and U = 0.4, respectively. In addition, the NOx emission around the equivalence ratio U = 0.35 was pollution-free, with 0 ppm, when the maximum gas temperature was confirmed to be below the thermal dissociation, at a boost pressure of 0.17 MPa. Such a result shows that NOx can be controlled to the pollution-free level, even under high power, with the applications of valve timing retardation and lean boosting to a hydrogen engine with external mixture. Also, when the boost pressure is increased even more, stable combustion becomes possible with an even leaner mixture, and achieves pollution-free NOx emission without backfire generation even above the gasoline-level power measured in this experiment. Fig. 13 illustrates the boost pressure and equivalence ratio needed to achieve 0 ppm NOx emission for each operation under load. The maximum gas temperature increases according to the load reduction, which verifies that NOx can be controlled with
Fig. 14. Brake thermal efficiency versus fuel–air equivalence ratios at each boost pressure.
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be the same in the regions that yield gasoline-level power. The equivalence ratio that yields the maximum thermal efficiency was around U = 0.6 in a natural intake condition, and became a leaner region as the boost pressure increased. When the boost pressure reached 0.21 MPa, which was the maximum boost pressure under the conditions of this experiment, the equivalence ratio was around U = 0.35. The thermal efficiency in the region that yielded gasoline-level power was generally above 35%, and under the boost pressure of 0.21 MPa, it reached the maximum of 39.05%. For conventional hydrogen with SI in the natural intake condition, the increase in thermal efficiency was approximately 30% without considering boost contribution. Fig. 15 illustrates ratios in the following categories: ratios of total combustion to flame development, rapid burning, and final burning when the hydrogen engine was lean boosted, with the supplying energy fixed to the level required to achieve gasoline-level power. Here, the ratio of each combustion period is with respect to the total combustion period. Flame development, rapid burning, and final burning are defined to be the period required to reach the mass burning rates of 0–10%, 10–90%, and 90–100%, respectively. With the increase in lean boost pressure, the total combustion period increased proportionally due to the decrease in combustion velocity, due to the use of lean mixture, even with the same supplying energy. The flame development period ratio, which represents the difference with the increasing rate of the total combustion period, is almost the same, and the rapid burning period and final burning period ratios showed a slight increase and decrease. The gas temperature of the initial combustion and cylinder wall temperature decreased due to the use of the lean mixture, and the low temperature combustion effect with the lean boosting. In addition, the heat transfer coefficient and flame area of the mixture increased with the increase of fluid due to supercharging. Each of the combustion periods seemed to change as above, due to such complex influences. However, the increase and decrease of the rapid burning period ratio and final burning period ratio were not so significantly large, and the ratios of each combustion time were generally the same regardless of the lean boosting condition used to obtain the gasoline-level power. To determine the possibility of simultaneously achieving super low NOx, high power and high efficiency operational conditions in a hydrogen engine, COVimp was measured with respect to lean boosting, equivalence ratio reduction rate, NOx reduction rate, and thermal efficiency increase rate, as illustrated in Fig. 16. Here,
Fig. 15. Combustion durations of lean boosting at each boosting pressure in same supplying energy of gasoline engine level.
Fig. 16. Performance evaluation at constant BMEP of gasoline engine level.
each condition was aimed at achieving gasoline-level BMEP, and rates of reduction increase were represented as a percentage with respect to 0.12 MPa, which is the minimum boost pressure capable of achieving gasoline-level power. As illustrated in the figure, COVimp is below 5%, almost able to achieve the same power the same as the stable combustion condition, even with the lean boosting. NOx reached 1477 ppm at the boost pressure of 0.12 MPa, but as mentioned above, it reached the pollution-free level of 0 ppm when the boost pressure increased to 0.17 MPa, and at that point the equivalence ratio became 50% leaner and the resulting increase in the thermal efficiency was approximately 12.9%. The minimal boost pressure operating conditions which simultaneously achieved 0 ppm NOx emission and high thermal efficiency under gasoline-level BMEP measured in the experiment were identified as IVO TDC, lean boosting equivalence ratio of U = 0.35, and boost pressure of 0.17 MPa. In addition, when the lean boost pressure was increased to 0.21 MPa, pollution-free NOx emission seemed possible even with the BMEP of 0.912 MPa. 4. Conclusion This study investigated the applications of intakeand exhaust valve timing variation and lean boosting to a hydrogen engine with SI and external mixture to identify the possibility of achieving high power, high efficiency and super low NOx emission without backfire generation. The following conclusions were reached. The backfire that inevitably occurs by increasing the power of a hydrogen engine to the level of a gasoline engine could be controlled by the retardation of IVO, and the method above was confirmed to be effective along with the application of lean boosting. In addition, it was identified that EVC is not significantly effective in backfire control. The maximum temperature reduction rate of hydrogen from lean boosting is greater than those of other fuels due to the high specific heat of hydrogen at constant volume. However, hydrogen has a high lean limit which is capable of stable combustion, up to the super lean region around U = 0.2, in which the temperature could be reduced to below NOx generation temperature. Almost pollution-free NOx emission was achieved, even when operating at gasoline-level high power, with low temperature combustion supercharging super lean mixture. The results above demonstrated the feasibility of achieving high power and high efficiency simultaneously with pollution-free NOx emission in a hydrogen engine with external mixture, through the employment of intake valve timing retardation and lean boosting. However, this research was conducted with a single cylinder engine built for research purposes, and more detailed analyses
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must be conducted with multi-cylinder engine, turbocharger and various operational conditions. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea Government (MEST) (NRF-2013R1A1A2013405). References [1] Levi J, Kittelson D. Further studies with a hydrogen engine. SAE Technical Paper 780233. [2] Das LM. Hydrogen engines: a view of the past and a look into the future. Int J Hydrogen Energy 1990;15:425–43. [3] Kim JM, Kim YT, Lee JT, Lee SY. Performance characteristics of hydrogen fueled engine with the direct injection and spark ignition system. SAE 952488 1995:162–75. [4] Eichlseder H, Wallner T, Freymann R, Ringler J. The potential of hydrogen internal combustion engines in a future mobility scenario. SAE technical paper 2003–01–2267. [5] Sierens R, Verhelst S, Verstraeten S. An overview of hydrogen fuelled internal combustion engines. In: Proceedings of international hydrogen energy congress and exhibition IHEC2005, Turkey: Istanbul p. 1–12. [6] Berckmuller M, Rottengruber H, Eder A, Brehm N, Elsasser G, Muller AG, Schwarz C. Potentials of a charged SI-hydrogen engine. SAE 2003–01-3210. [7] Shravan KV, Umit OK. Computational modeling, validation, and utilization for predicting the performance, combustion and emission characteristics of hydrogen IC engines. Energy 2011;36:647–55. [8] Subramanian V, Mallikarjuna JM, Ramesh A. Effect of water injection and spark timing on the nitric oxide emission and combustion parameters of a hydrogen fuelled spark ignition engine. Int J Hydrogen Energy 2007;32:1159–73. [9] Ma Fanhua, He Yituan, Deng Jiao, Jiang Long, Naeve Nashay, Wang Mingyue, et al. Idle characteristics of a hydrogen fueled SI engine. Int J Hydrogen Energy 2011;36:4454–60.
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[10] White CM, Steeper RR, Lutz AE. The hydrogen-fueled internal combustion engine: a technical review. Int J Hydrogen Energy 2006;31:1292–305. [11] Stokes J, Lake TH, Osborne RJ. A gasoline engine concept for improved fuel economy-the lean boost system. SAE 2000–01-2902. [12] Nakata K, Sasaki N, Ota A, Kawatake K. The effect of fuel properties on thermal efficiency of advanced spark-ignition engines. Int J Engine Res 2010;12:274–81. [13] Koyanagi K, Hiruma M, Furuhama S. Study on mechanism of backfire in hydrogen engines. SAE technical paper 942035. [14] Kondo T, Hiruma M, Furuhama S. A study on the mechanism of backfire in external mixture formation hydrogen engine. In: Proceedings of WHEC. vol. 3, Stuttgart, 1996. p. 1547–1556. [15] Lee JT, Kim YY, Lee CY, Caton JA. An investigation of a cause of backfire and its control due to crevice volumes in a hydrogen fueled engine. ASME J Eng Gas Turbines Power 2001;123:204–10. [16] Lee JT, Kim YY, Caton JA. The development of a dual-injection hydrogen-fueled engine with high power and high efficiency. ASME J Eng Gas Turbines Power 2006;128:203–12. [17] Lee KJ, Huynh TC, Lee JT. A study on realization of high performance without backfire in a hydrogen-fueled engine with external mixture. Int J Hydrogen Energy 2010;35:13078–87. [18] Huynh TC, Kang JK, Noh KC, Lee JT, Caton JA. Controlling backfire for a hydrogen-fueled engine using external mixture injection. ASME J Eng Gas Turbines Power 2008;130:062804. [19] Kim YY, Ryu TH, Lee JT. Backfire occurrence by abnormal electric discharge in hydrogen fueled engine. Kor. Hydrogen New Energy Soc. 2002;13:65–73. [20] Lee JW, Lee TH, Shin YG, Chun KM. A study of the measurements of the residual gas and it is effect on the combustion characteristics as function of valve timing in an SI engine. KSAE 2000–03-2019. [21] Johnson T. Diesel engine emissions and their control. Platinum Met Rev 2008;52:23–37. [22] Akihama K, Takatori Y, Inagaki K, Sasaki S, Dean AM. Mechanism of the smokeless rich diesel combustion by reducing temperature. SAE Technical Paper 2001–01-0655. [23] Kitamura T, Ito T, Kitamura Y, Ueda M, Senda J, Fujimoto H. Soot kinetic modeling and empirical validation on smokeless diesel combustion with oxygenated fuels. SAE Technical Paper 2003–01-1789.
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Fuel journal homepage: www.elsevier.com/locate/fuel
High power performance with zero NOx emission in a hydrogen-fueled spark ignition engine by valve timing and lean boosting Jongtai Lee b,⇑, Kwangju Lee a, Jonggoo Lee a, Byunghoh Anh a a b
Graduate School of Sungkyunkwan University, 300, Chunchun-dong, Jangan-gu, Suwon-si, Gyeonggi-do 440-746, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 300, Chunchun-dong, Jangan-gu, Suwon-si, Gyeonggi-do 440-746, Republic of Korea
h i g h l i g h t s Backfire could be controlled by retardation of the intake valve opening timing in a H2 engine. Combustion was stable until an ultra-lean region below the NOx generation. A BMEP equal to that of a gasoline engine was realized without backfire. A NOx emission of 0 ppm was realized with the lean boosting. Thermal efficiency reached a high level of 39.1% which is difficult to achieve with the Otto engine.
a r t i c l e
i n f o
Article history: Received 13 December 2013 Received in revised form 28 February 2014 Accepted 7 March 2014 Available online 26 March 2014 Keywords: Hydrogen-fueled engine Lean boosting Backfire control Valve timing Ultra-low NOx
a b s t r a c t For a hydrogen engine to be successfully commercialized in a short period of time, it must have power on the same level as gasoline engines, almost no NOx emissions, and high efficiency. This simultaneous achievement would be possible with the use of low temperature combustion, which supercharges an ultra-lean mixture, because NOx from a hydrogen engine is thermally produced. However, increasing the energy input for higher output produces backfire, which is a significant problem for hydrogen-fueled engines with external mixture, and why the simultaneous achievement of high output and low-exhaust is not easily accomplished. This research investigated the potential of simultaneously achieving high output, similar to the output of a gasoline engine, without backfire, by using a complex valve timing variation and lean boosting. The study achieved almost zero NOx emission and high efficiency in a single cylinder engine built for research purposes with a valve timing variation system. Experimental results revealed that retarding the intake valve opening timing could control the backfire generated when increasing the output of the hydrogen engine, and the method above is also effective with lean boosting. Stable combustion in the lean region of U = 0.2, which can reduce temperature below the NOx generation temperature, was possible due to the large ultra-lean limit of hydrogen. In addition, the exhaust was almost NOx free due to the low temperature combustion, by supercharging the ultra-lean mixture under high power operation similar to that of gasoline. The above results verified that almost pollution-free emission without backfire, and simultaneous high power and efficiency could be achieved in a hydrogen engine through a method of intake valve timing retardation and lean overcharging. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The hydrogen engine can utilize hydrogen produced from any source, and since its fuel does not include a carbon component, it is consequently one of the next generation engine systems that ⇑ Corresponding author. Tel.: +82 31 290 7442, mobile: +82 10 8920 7442; fax: +82 31 299 4754. E-mail addresses: [email protected], [email protected] (J. Lee). http://dx.doi.org/10.1016/j.fuel.2014.03.010 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
can simultaneously solve both the CO2 and fossil fuel energy depletion problems. Among the hydrogen engines being developed, the hydrogen engine with an external fuel mixture is more efficient and durable than the direct injection type, and is considered capable of commercialization in a short period of time with low cost. However, it also generates NOx in its exhaust gases, which undermines its value compared to fuel cells. Additionally, the power output is relatively lower than the power output of petroleum-based engines
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due to the decrease in the mass air flow from the low hydrogen density [1–6]. The fuel of a hydrogen engine does not contain carbon or nitrogen components, and the NOx is only produced as thermal NOx, which is dependent on the combustion gas temperature [7–9]. Therefore, in order to simultaneously resolve both the NOx and low power issues, lean boosting has been proposed. Lean boosting is one of the low temperature combustion methods, and supercharges a lean mixture to supply the energy required for high power. The resulting increase in the intake flow mass allows the max temperature of the combustion gas to be below the thermal dissociation temperature [10–12]. However, increasing the supplying energy for higher power introduces one of the biggest problems of a hydrogen engine with external mixture, namely, backfire, and it is not easy to simultaneously achieve both high power and low exhaust. Backfire is caused by the reflux of flame from the hotspot or crevice volume in the rich region, into the intake valve during the valve overlap period [13–16]. With that in mind, various backfire controlling methods have been proposed. One of the methods retards the opening timing of the intake valve involved in the valve overlap period to control the reflux timing of the source of the flame and thus the backfire generated in the rich region [17]. This valve timing retardation method is relatively easy to achieve, and the decrease in volumetric efficiency produced by retarding the valve opening period during supercharging could be compensated, and it is effective in a hydrogen engine with SI. The complex process of retarding the intake valve opening timing and lean boosting supercharging could achieve high power without backfire in a hydrogen engine with external mixture, and produce an almost pollution-free NOx emission. However, it is still not known whether it is possible to produce power similar to that from a gasoline engine, or whether, even though the lean limit of hydrogen is very large, NOx emissions can be reduced to an almost pollution-free level with a stable ultra lean combustion, below the thermal dissociation temperature, under high power operation. This research investigated the possibility of simultaneously achieving high power, almost pollution-free NOx emission, and high efficiency without the generation of backfire, through the complex applications of valve timing variation and lean boosting to a hydrogen engine with external mixture, which has the highest potential for near-term commercialization. A single cylinder engine with a varying valve timing system was built for research purposes and used to analyze backfire control, combustion stability during ultra-lean supercharging, NOx emission and experimental performances.
2. Experimental explanation 2.1. Experimental setup Fig. 1 illustrates the actual picture of the external mixture hydrogen engine built for the experiment with Mechanical Continuous Variable Valve Timing (MCVVT). The hydrogen research engine is a DOHC single cylinder engine with a compression ratio of e = 10.5, and a bore and stroke of 86 mm. Table 1 lists the main specifications. Crank, cylinder head, cam-shaft and assemblies of a conventional 4 cylinders engine were modified for the experiment of a single cylinder. MCVVT is a timing belt of a certain length that independently and continuously varies the opening and closing periods of an intake valve and exhaust valve while the ignition is operating, using the principle of the phase angle of the camshaft, connected to the crank shaft, varied with respect to the two idling gears [18]. Hydrogen
Fig. 1. Hydrogen research engine with MCVVT system.
Table 1 Specification of test engine. Valve mechanism
DOHC
Bore stroke Compression ratio Displacement volume Valve timing control Duration of IVO Duration of EVO Range of valve timing
86 86 mm 10.5 499.6 cc MCVVT system 257 °C A 224 °C A 50 °C A
gas was supplied using a low pressure solenoid type injector for a natural gas engine. The insert location of the injector was 70 mm in front of the intake valve. For cooling, coolant was independently supplied to the cylinder head and cylinder jacket. As the ignition system, a type of distributor with grounded secondary coil, which easily emits a residual charge that could cause a backfire, was used [19]. Fig. 2 is the schematics of the experimental setup. The experimental setup was composed of the previously mentioned hydrogen engine, power measurement system, hydrogen supply system, intake and exhaust system equipped with supercharger, cooling system, and data acquisition. The hydrogen fuel came from a high pressure cylinder charged to 15 MPa and went through a primary and secondary regulator to lower the pressure to 0.3 MPa. The gas with the lowered pressure was then supplied to the engine through the gas injector. An accumulator with a volume of 3.8 L was installed between the hydrogen flow meter and gas injector to decrease surging due to the fuel injection. The hydrogen injection was controlled by an injector controller (Mobiltek, MT2010) of the peak and hold method, and measured with a hydrogen flow meter (Bronchus, P-113AC -HAD-55 -V) of thermo coupling method. Supercharging to increase fresh air was produced with an external supercharging ring blower (DongBu, DBR-1002) and a piston type compressor: the external supercharging work was not considered in the experimental result. Bypass valves were
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Fig. 2. Schematic diagram of experimental setup.
installed at each outlet of the supercharger (ring blower) and the entrance of the surge tank to control the supercharging pressure. In addition, the supercharging temperature was controlled with a liquid-cooled intercooler. In order to reduce surging due to supercharging, 2 surge tanks were employed between the intake pipes of the engine and the outlet of the intercooler, and an air flow meter was installed between the surge tanks. The NOx emission was measured with an exhaust gas analyzer (Horiba, MEXA-554JKNOx). The test of changing the exhaust pressure was performed by roots blower and throttle valve in exhaust pipe. 2.2. Experimental method Depending on the opening timing of the intake valve, an arbitrary ignition source within the combustion chamber can cool or completely combust the fuel. Likewise, the closing timing of the exhaust valve can cause a backfire when high temperature exhaust gas or residual gas comes into contact with fresh air [20]. Therefore, the major experimental variables to identify whether backfire is suppressed or not were set as the intakeexhaust valve timing and supplying energy. The intake valve opening period was varied sequentially from BTDC 20 °C A to ATDC 20 °C A and the exhaust valve opening period was varied sequentially from BTDC 20°CA to ATDC 30°CA. The supplying energy was increased until backfire occurred at each of the intakeexhaust valve timings. In addition to the above, abnormal combustion and performance characteristics of the low temperature combustion produced from the lean boosting method were analyzed. The major experimental variables for the analysis were the fuel–air equivalence ratio (equivalence ratio hereafter) and boost pressure for increasing the amount of fresh air. The equivalence ratio was varied step-by-step by U = 0.1 increments, from the combustion capable lean region, to the rich region where knocking occurs. The experimental power objective was to achieve as high power with the hydrogen engine as a gasoline engine. Therefore, the boost pressure was varied by 0.01 MPa in the engine from the natural intake condition to the power output at the gasoline fuel changing (U = 1.0) and the pressure 10% higher than the boost pressure capable of ultra-low NOx emission. Assuming the power at the gasoline fuel switching (U = 1.0) was WOT, the load was varied step-by-step by 25%. For the case above, the opening timing of the intake valve was fixed at the TDC that was capable of controlling backfire under a high
load, as determined by the experiment results. The closing timing of the exhaust valve was fixed at the ATDC of 10 °C A with high exhaust efficiency. The engine rotation and ignition timing were fixed at 1600 rpm and MBT, respectively, in each experiment. 3. Results and discussions 3.1. Backfire control with valve timing retardation at high power Fig. 3 illustrates the backfire limit equivalence ratio, which is the equivalence ratio right before backfire occurs, with varying intake valve opening timing (IVO) and exhaust valve closing timing (EVC). The backfire limit equivalence ratio increased when IVO was retarded from BTDC 20 °C A, and backfire did not occur even when the experiment continued beyond U = 1.0, up to U = 1.6, after TDC. This seems to be due to a complex reason in which the slowly combusting flame from the hot spot within the combustion chamber and the piston crevice volume either cools down or burns until the intake valve opens again due to the retardation of IVO. Fresh air
Fig. 3. Exhaust valve closing timing and intake valve opening timing versus fuel–air equivalence ratio about the backfire.
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and exhaust gas coexist during the valve overlap period, which varied with the advancement and retardation of IVO and EVC. When there is an identical backfire limit equivalence ratio during the period with no valve overlap, it means that the backfire did not occur, as the exhaust gas directly ignites the fresh air. However, residual high temperature gas in the cylinder could be involved in backfire, since the gas influences the ignition condition of the mixture within the heat source of the combustion chamber wall and crevice volume. To verify this the backfire limit equivalence ratio was identified with respect to varying exhaust pressure, as illustrated in Fig. 4, The change in exhaust pressure can be understood as the change in residual gas because the amount of residual gas decreases as the exhaust gas decreases. As illustrated in the figure, the backfire limit equivalence ratio showed a slightly reducing trend as the exhaust pressure, or the residual gas, was reduced. However, the difference was very small, small enough to assume it was constant, and that the residual gas did not have a significant influence on the backfire generation. The residual gas is heavily influenced by the EVC, and the result above verifies that the generation of backfire is not highly dependent on EVC, as noted in Fig. 3. This also means that the IVO is more important than EVC in controlling the backfire. Therefore, the EVC was fixed to ATDC 10 °C A, which results in a high exhaust efficiency. It is a well-known fact that the temperature of the wall of a combustion chamber increases as the engine rotation increases due to the increase in the supplying energy per unit time, and the timing of backfire controlling IVO may be changed. When IVO was retarded to TDC to determine this, the backfire limit equivalence ratio with respect to the change in the engine rotation was compared to IVO BTDC 10 °C A, the design valve timing of the base engine, and Fig. 5 illustrates the comparison. Backfire was generated around U = 1.3, which is beyond the equivalence ratio of U = 1.0, with the intake valve timing of IVO BTDC 10 °C A and at 1000 rpm. The backfire limit equivalence ratio decreased as the engine rotation increased, so that it became U = 0.7 around 1600 rpm and remained almost constant. The generation of a certain source of ignition is facilitated by the increase in the supplying energy per unit time due to the increase in the engine rotation, and the backfire limit equivalence ratio decreases due to the decrease in the period per unit crank, and the factor shortening the combustion or cooling period. The similarities in the backfire limit equivalence ratio starting from 1600 rpm could be explained by the factors above cancelling each other, due to
Fig. 4. Backfire limit equivalence ratio as a function of exhaust pressure.
Fig. 5. Fuel–air equivalence ratios versus engine speed at each case.
the increase in burning rate from the increase in turbulence within the cylinder. Backfire was generated with the equivalence ratio of U = 1.3 at 1000 rpm due to a rich mixture, even with low heating value. This was due to various factors, such as the decrease in burning rate from the use of a rich mixture and the lengthened combustion of flame from the crevice volume. With IVO TDC, however, backfire was not generated, regardless of the engine rotation, when the measurement was made from the lean region beyond an equivalence ratio of U = 1.4. The result above shows that backfire can be controlled regardless of the engine rotation within the operational range of this experiment, when IVO is fixed at TDC. However, when IVO is retarded, backfire could be controlled with a decrease in the supplying energy, by decreasing the air intake. Fig. 6 illustrates whether backfire is generated or not with respect to the increase in supplying energy when IVO is retarded. The supplying energy here is increased until right before knocking generation, by lean boosting and ignition timing retardation. When IVO is retarded to TDC under the natural intake condition, the supplying energy can be increased by a maximum of 15.6% compared to the supplying energy from IVO BTDC 30 °C A, due to the increase in the equivalence ratio from the backfire
Fig. 6. Supplying energy versus intake pressure.
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control. In addition, a lean boosting was conducted with the increase in the boost pressure at IVO TDC to eliminate the influence of decreased charging efficiency due to the IVO retardation. The result showed a trend of limited increase in the supplying energy due to the knocking, but no backfire generation. When the boost pressure was 0.21 MPa, the supplying energy was increased by approximately 60.05%. This is in contrast to the supplying energy under the natural intake condition, when backfire was still not generated under IVO TDC. Such a result shows that when IVO is retarded, the backfire is not controlled by decreasing the supplying energy by decreasing the air intake, and that backfire could be controlled by retarding the IVO, even with the application of lean boosting. As it as mentioned earlier, intake valve opening timing retardation was verified to be effective in suppressing the backfire generation while achieving the gasoline engine level power output with a hydrogen-fueled engine. The verification was made from the suppressed backfire generation at IVO TDC under the conditions of variation in exhaust valve closing timing, increase in engine revolutions, and increase in supplying energy. To identify the conditions in which a hydrogen engine achieves the power of a gasoline engine, the brake mean effective pressure, BMEP, was investigated with respect to the changes in the equivalence ratio and boost pressure, as Fig. 7 illustrates. Gasoline engine power was measured for the experimental engine by modifying the hydrogen engine fuel supply system to take gasoline. Due to the use of a single cylinder, an increase in mechanical loss from the utilization of MCVVT, and the decrease in the volumetric efficiency at the intake, the BMEP of a gasoline engine was approximately 0.72 MPa below the conditions of WOT and equivalence ratio U = 1. That value is lower than the BMEP of a conventional engine. However, since a goal of this research was to identify the potential of achieving such power when a gasoline engine is converted to a hydrogen engine, the value was used as the representative value of gasoline engine power for this study (hereafter gasoline-level power). Under the conditions of IVO BTDC 20 °C A and 10 °C A in a natural intake condition, normal operation was limited to the vicinity of the equivalence ratio range of U = 0.7–0.8 due to backfire generation, and the maximum value of BMEP remained at approximately 85% of a gasoline engine. Even if IVO is retarded to TDC and the equivalence ratio capable of normal operation is expanded to U = 1.0, the maximum value of BMEP is similar to the previous one due to the decrease in the amount of air intake caused by the IVO retardation. Gasoline-level power or higher was achieved
Fig. 7. BMEP versus fuel–air equivalence ratios at each case.
without backfire generation when IVO was fixed to TDC and supercharged, however, a knock occurred due to the increase in the supplying energy in the rich region. The minimum boost pressure and equivalence ratio able to generate gasoline-level power under the above conditions are in the vicinity of 0.13 MPa and U = 0.6, respectively. When the boost pressure is increased even more, gasoline-level power could be achieved in an even leaner mixing ratio, and the maximum BMEP of 0.912 MPa was achieved when the lean boost pressure was 0.21 MPa. 3.2. The possibility of pollution-free NOx emission at high power As it is shown in Eq. (1), the gas temperature (Tg) in the combustion chamber is proportional to the supplying energy (Q), and it is inversely proportional to the mass of fresh air (mi) and specific heat at constant volume (Cv). The maximum temperature within the engine decreases even with identical supplying energy when a large quantity of lean mixture is supplied.
Tg ¼ T 1
n1
þ Q mi
n X mk k¼1
mi
!1 C mk
ð1Þ
T1, e, and n represent initial compression temperature, compression ratio, and polytropic index, respectively, and the subscript x corresponds to air, fuel and residual gas. The decreases in the maximum gas temperature for hydrogen and hydrocarbon fuels are significantly different from each other, even if identical energy was supplied, because hydrogen fuel and hydrocarbon fuels have significantly different material properties. The lean boost pressure is the boost pressure that has the lean equivalence ratio when boosted under identical supplying energy. Fig. 8 illustrates the reduction rate of the reduction rate of the maximum temperature and the specific heat at constant volume
Fig. 8. Decrease rate of temperature and constant-volume specific heat of intake mixture mass versus boost pressure about each fuel with same supply energy at U = 1.0 in natural aspirated condition.
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of fresh air with respect to the variation in the lean boost pressure to compare the low-temperature combustion effect with those of gasoline and natural gas. The supplying energy of each fuel was fixed to the measurement from the natural intake condition and equivalence ratio of U = 1.0. The material properties of C8H18 and CH4 were used as the properties of gasoline and natural gas. The maximum gas temperature reduction rate is the percentage temperature reduction resulting from an increase in lean boost pressure under natural intake condition. The maximum gas temperature reduction rate represents the low temperature combustion effect, and tended to increase for all three types of fuels due to the increase in the mass of fresh air resulting from the increase in lean boost pressure. However, while the values of gasoline and natural gas fuels were similar to each other, the hydrogen values were relatively lower than the others. As mentioned earlier, the supplying energy was constant. There were differences in fresh air masses with respect to the mass of each fuel, depending on the change in lean boost pressure, however, the mass of fresh air is generally proportional to the boost pressure. The specific heat at constant volume of hydrogen is approximately 10.183 kJ/kg K at 300 K and it is approximately 6.2 times larger than gasoline (1.639 kJ/kg K) or natural gas (1.736 kJ/kg K). Therefore, the specific heat at constant volume of a hydrogen–oxygen mixture is significantly larger than other fuels and the difference decreases as the lean boost pressure increases, due to the increase in mass ratio of oxygen to hydrogen from the supercharging of the lean mixture. Due to the factors above, the maximum gas temperature reduction rate of hydrogen with lean boosting is lower than for other fuels. For example, the maximum gas temperature reduction rate, which represents the low temperature combustion effect of hydrogen, at a boost pressure of 0.15 MPa is approximately 23.1% lower than that of gasoline fuel. Consequently, to lower the maximum gas temperature of hydrogen to below the thermal dissociation temperature of NOx, hydrogen requires a more lean mixture than hydrocarbon fuels, due to its small gas temperature reduction rate and high adiabatic flame temperature. Fig. 9 illustrates the maximum gas temperature in the combustion chamber of the hydrogen engine with respect to the changes in boost pressure and equivalence ratio. The maximum gas temperature was computed from the gas pressure within the cylinder and the ideal gas equation. The maximum gas temperature value for an identical boost pressure decreases rapidly as the equivalence ratio decreases. However, the maximum gas temperature value for an identical
Fig. 9. In-cylinder maximum temperature versus fuel–air equivalence ratios and boost pressure.
equivalence ratio is almost constant, even with the increase in boost pressure, due to the simultaneous increase in supplying energy and mass of the mixture. This trend is shown in the regions that yield gasoline-level power as well. The generation of thermal NOx theoretically takes place due to the Zeldovich NO reaction when the flame temperature is above 1850 K. It has been reported that NOx is generated from the flame temperature of 2200 K or above from experimental results [21– 23]. As expected, the condition that yields a temperature below NOx generation temperature in a hydrogen engine occurs in the super lean region, regardless of the change in boost pressure, and the region is the region below the equivalence ratio of U = 0.35. From the experiment, the boost pressure that satisfied the region above was approximately 0.17 MPa. Fig. 10 illustrates the cycle coefficient of variation, COVimep, of the hydrogen engine with respect to the change in the equivalence ratio at each boost pressure in the region capable of producing gasoline-level power. COVimep is the Root Mean Square (RMS) of the indicated mean effective pressure, and it is the ensemble average of the data of 100 cycles. Conventionally, the lean limit of a gasoline engine is approximately the equivalence ratio of U = 0.7. Even under natural intake conditions CO or HC increases, even if NOx decreases, when a leaner low temperature combustion is employed, and this, again, causes the problem of exhaust emission. On the other hand, the hydrogen engine showed stable combustion with COVimep less than 5% until the equivalence ratio of U = 0.3, at which the thermal dissociation temperature of NOx is generated. In addition, the cycle variation showed an even larger decreasing trend when the boost pressure was increased in an identical equivalence ratio. When the boost pressure was increased above 0.16 MPa, combustion was facilitated by the increase in the initial combustion temperature from the increased supplying energy and the increase in the flow rate from supercharging, and this resulted in a stable low temperature combustion even with the super lean mixture around the equivalence ratio of U = 0.2. Fig. 11 illustrates the NOx emission according to changes in equivalence ratio and boost pressure, and Fig. 12 illustrates the regions capable of NOx emissions of less than 10 ppm. The maximum NOx emission, 2120 ppm, was observed when the mass of fresh air was increased right before knocking occurred, at the boost pressure capable of producing gasoline-level power. However, when the equivalence ratio became approximately
Fig. 10. COVimep versus fuel–air equivalence ratios at each boost pressure.
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Fig. 11. NOx versus fuel–air equivalence ratios above bmep of gasoline level. Fig. 13. Boost pressure versus fuel–air equivalence ratio in each load for zero emission and increasing NOx regions.
every load. The case of 100% load of gasoline-level power in the NOx pollution-free region is mentioned above. 75% load achieves 0 ppm from respective boost pressure and equivalence ratio of 0.15 MPa and U = 0.3. NOx emission at 50% load was verified to be below 10 ppm at its maximum, and 0 ppm was achieved from a boost pressure and equivalence ratio of 0.12 MPa and U = 0.315, respectively. 0 ppm of NOx was achieved even with a natural intake condition (U = 0.27) with 25% load. 3.3. Simultaneous achievement of high performance due to lean boosting
Fig. 12. NOx versus fuel–air equivalence ratios above bmep of gasoline level under 10 ppm of NOx.
Fig. 14 illustrates the brake thermal efficiency with respect to the change in equivalence ratio at each boost pressure. It is a well-known fact that brake thermal efficiency shows an increasing trend with increasing lean boost pressure, with the decrease in cooling loss due to the noticeable decrease in the combustion temperature. This occurs even when the combustion time increases with a lean mixture. This trend was observed to
U = 0.5 from lean boosting, NOx emissions decreased to approximately 330 ppm due to the low temperature combustion effect. As illustrated in Fig. 12, the NOx emission stays below 10 ppm, even with the high supplying energy required by gasoline-level power, when the boost pressure and equivalence ratio reach 0.16 MPa and U = 0.4, respectively. In addition, the NOx emission around the equivalence ratio U = 0.35 was pollution-free, with 0 ppm, when the maximum gas temperature was confirmed to be below the thermal dissociation, at a boost pressure of 0.17 MPa. Such a result shows that NOx can be controlled to the pollution-free level, even under high power, with the applications of valve timing retardation and lean boosting to a hydrogen engine with external mixture. Also, when the boost pressure is increased even more, stable combustion becomes possible with an even leaner mixture, and achieves pollution-free NOx emission without backfire generation even above the gasoline-level power measured in this experiment. Fig. 13 illustrates the boost pressure and equivalence ratio needed to achieve 0 ppm NOx emission for each operation under load. The maximum gas temperature increases according to the load reduction, which verifies that NOx can be controlled with
Fig. 14. Brake thermal efficiency versus fuel–air equivalence ratios at each boost pressure.
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be the same in the regions that yield gasoline-level power. The equivalence ratio that yields the maximum thermal efficiency was around U = 0.6 in a natural intake condition, and became a leaner region as the boost pressure increased. When the boost pressure reached 0.21 MPa, which was the maximum boost pressure under the conditions of this experiment, the equivalence ratio was around U = 0.35. The thermal efficiency in the region that yielded gasoline-level power was generally above 35%, and under the boost pressure of 0.21 MPa, it reached the maximum of 39.05%. For conventional hydrogen with SI in the natural intake condition, the increase in thermal efficiency was approximately 30% without considering boost contribution. Fig. 15 illustrates ratios in the following categories: ratios of total combustion to flame development, rapid burning, and final burning when the hydrogen engine was lean boosted, with the supplying energy fixed to the level required to achieve gasoline-level power. Here, the ratio of each combustion period is with respect to the total combustion period. Flame development, rapid burning, and final burning are defined to be the period required to reach the mass burning rates of 0–10%, 10–90%, and 90–100%, respectively. With the increase in lean boost pressure, the total combustion period increased proportionally due to the decrease in combustion velocity, due to the use of lean mixture, even with the same supplying energy. The flame development period ratio, which represents the difference with the increasing rate of the total combustion period, is almost the same, and the rapid burning period and final burning period ratios showed a slight increase and decrease. The gas temperature of the initial combustion and cylinder wall temperature decreased due to the use of the lean mixture, and the low temperature combustion effect with the lean boosting. In addition, the heat transfer coefficient and flame area of the mixture increased with the increase of fluid due to supercharging. Each of the combustion periods seemed to change as above, due to such complex influences. However, the increase and decrease of the rapid burning period ratio and final burning period ratio were not so significantly large, and the ratios of each combustion time were generally the same regardless of the lean boosting condition used to obtain the gasoline-level power. To determine the possibility of simultaneously achieving super low NOx, high power and high efficiency operational conditions in a hydrogen engine, COVimp was measured with respect to lean boosting, equivalence ratio reduction rate, NOx reduction rate, and thermal efficiency increase rate, as illustrated in Fig. 16. Here,
Fig. 15. Combustion durations of lean boosting at each boosting pressure in same supplying energy of gasoline engine level.
Fig. 16. Performance evaluation at constant BMEP of gasoline engine level.
each condition was aimed at achieving gasoline-level BMEP, and rates of reduction increase were represented as a percentage with respect to 0.12 MPa, which is the minimum boost pressure capable of achieving gasoline-level power. As illustrated in the figure, COVimp is below 5%, almost able to achieve the same power the same as the stable combustion condition, even with the lean boosting. NOx reached 1477 ppm at the boost pressure of 0.12 MPa, but as mentioned above, it reached the pollution-free level of 0 ppm when the boost pressure increased to 0.17 MPa, and at that point the equivalence ratio became 50% leaner and the resulting increase in the thermal efficiency was approximately 12.9%. The minimal boost pressure operating conditions which simultaneously achieved 0 ppm NOx emission and high thermal efficiency under gasoline-level BMEP measured in the experiment were identified as IVO TDC, lean boosting equivalence ratio of U = 0.35, and boost pressure of 0.17 MPa. In addition, when the lean boost pressure was increased to 0.21 MPa, pollution-free NOx emission seemed possible even with the BMEP of 0.912 MPa. 4. Conclusion This study investigated the applications of intakeand exhaust valve timing variation and lean boosting to a hydrogen engine with SI and external mixture to identify the possibility of achieving high power, high efficiency and super low NOx emission without backfire generation. The following conclusions were reached. The backfire that inevitably occurs by increasing the power of a hydrogen engine to the level of a gasoline engine could be controlled by the retardation of IVO, and the method above was confirmed to be effective along with the application of lean boosting. In addition, it was identified that EVC is not significantly effective in backfire control. The maximum temperature reduction rate of hydrogen from lean boosting is greater than those of other fuels due to the high specific heat of hydrogen at constant volume. However, hydrogen has a high lean limit which is capable of stable combustion, up to the super lean region around U = 0.2, in which the temperature could be reduced to below NOx generation temperature. Almost pollution-free NOx emission was achieved, even when operating at gasoline-level high power, with low temperature combustion supercharging super lean mixture. The results above demonstrated the feasibility of achieving high power and high efficiency simultaneously with pollution-free NOx emission in a hydrogen engine with external mixture, through the employment of intake valve timing retardation and lean boosting. However, this research was conducted with a single cylinder engine built for research purposes, and more detailed analyses
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