Hydrogen Boosted Gasoline Engines Hype, Truth, or a Mix?
Christopher P. Horvath May 16, 2011
“Get 25 – 50% improvement in gas mileage! All by purchasing and installing our hydrogen boosting system! It produces hydrogen onboard with an electrolyzer, and injects it into the engine alongside the gasoline fuel and air. By doing this, you burn the fuel more completely, resulting in better fuel efficiency, power output, mileage, and also reduced emissions!” We all want to believe hype like this, but don’t claims like these sound too good to be true? That is exactly what I have been wondering, so I decided to take a look at the science behind it to see how logically and scientifically sound the argument is. Thus, I decided to take advantage of our University’s research database and see what experimental data I could find to confirm such claims.
Figure1: Electrolyzer offered by Hydrogen-boost.com [1] It seemed that I could find little literature on the matter while searching through various databases, including ScienceDirect, Compendex, Conference Papers, Mechanical Engineering, JSTOR, Web of Science, Engineered Materials, and EI Village. However, there were a number of recent papers, in particular ones published by the International Journal of Hydrogen Energy, with experimental research coming from the College for Environmental and Energy Engineering in Beijing. All of these described various effects of the hydrogen enriched gasoline combustion, and the effects on power output, emissions, and efficiency, as influenced by various parameters of hydrogen addition (% by volume), air-fuel ratios, and spark timing. In particular, spark-ignition, gasoline engines were investigated, but advantages for diesel engines were also demonstrated. Overall, they seemed to suggest that these claims of savings and emissions control are quite possible, so we will take a closer look at some of the data and support for these arguments.
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For the various papers, the general setup took an existing 1.6L, four-cylinder, spark-ignition engine, and added a hybrid electronic control unit (HECU) which controlled the injection duration and timing of gasoline and hydrogen into the engine. This allowed for control of the percent hydrogen injected, as well as the effective excess air ratios by reduction of the amount of gasoline injection. The engine was run at a constant RPM for tests and constant spark timing, with the intake manifolds absolute pressure (MAP) representative of engine load, and measured brake mean effective pressure (BMEP) as an indicator of the power output. Now, let us take a closer look at the results of these experimental proceedings. The first paper published by Ji et al in the International Journal of Hydrogen Energy, investigated low load and lean conditions at a constant RPM of 1400 to represent city-driving conditions in heavy traffic [2]. One of the most beneficial trends seen here was that the efficiency increased with all variations in load, as shown in Figure 2. For each excess air ratio, the hydrogen addition resulted in a higher efficiency. Additionally, the higher air ratio results in better efficiency at higher loads, whereas the lower air ratio was found to be better suited for lower loads at a given hydrogen addition percentage. Volume fractions of hydrogen at 0 and 3% were tested, with excess air ratios of 1.2 and 1.4. While the BMEP increased with H2 addition at low loads, at higher loads it decreased meaning that the torque output followed accordingly, as seen in Figure 3. Regarding emissions, it was found that unburned hydrocarbons, CO2, and CO (Figures 4, 5, 6) amounts decreased for all cases with hydrogen addition, and especially so with higher air fuel ratio. However, the production of NOx was seen to increase with hydrogen addition for a given air ratio, as demonstrated in Figure 7.
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Figure 2: Variations of brake thermal efficiency with MAP at 1400 rpm and two excess air ratios
Figure 5: Variations of CO2 emissions with MAP at 1300 rpm and two excess air ratios [2]
Figure 3: Variations of BMEP with MAP at 1400 RPM and two excess air ratios [2]
Figure 6: Variations of CO emissions with MAP at 1300 rpm and two excess air ratios [2]
Fig 4: Variations of HC emissions with MAP at 1300 rpm and two excess air ratios [2]
Figure 7: Variations of NOx emissions with MAP at 1300 rpm and two excess air ratios [2]
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The results are further corroborated by another experiment with the same test facility by Ji
et al. [3]. In this case, greater excess air ratios and hydrogen volumes were investigated for a fixed speed of 800 RPM. Namely, hydrogen addition of 0, 3, 5, and 8%, with excess air ratios of 1.0, 1.18, 1.43, and 1.67 were tested. Figure 8 demonstrates some of the main findings of this experiment. From this it can be seen that a higher efficiency can be maintained at leaner conditions with greater H2 addition. On the downside, at these higher excess ratios the engine is producing less torque, but still more than the gasoline alone for the given air ratio. Greater amounts of hydrogen results in better torque output at these higher air fuel ratios. It is likely, however, that the additional torque is not required at all driving conditions, so this could potentially be an acceptable loss in power output, for a given increase in fuel economy. Additionally, it is interesting to see that the addition of hydrogen results in more stable performance at leaner condition. At points where combustion of gasoline alone would result in misfires and unpredictability, the hydrogen-gasoline mixture results in minimal variations of BMEP, even at lean conditions. Figure 9 depicts this trend, allowing for the engine to be run leaner overall with greater additions of hydrogen. As far as emissions go, unburned HC and CO decrease with hydrogen addition as seen in Figures 10 and 11. However, From Figure 12, the NOx appears to increase for a given air ratio with the addition of hydrogen. This brings us to the next point, of how to combat the formation of NOx with hydrogen addition.
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Figure 8: Brake mean efficiency and torque against Figure 9: Variance of the indicated mean excess air ratios at 800 RPM and H2 addition levels [3] effective pressure with excess air ratio [3]
Figure 11: Variations of CO emissions with with excess air ratios and H2 addition levels [3]
Figure 10: Unburned HC emissions with excess air ratio and Hydrogen addition levels[3]
Figure 12: Variations of NOx emissions with excess air ratios and H2 addition levels [2]
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Superficially, it would seem that the production of NOx is a downside of the hydrogen addition, but a closer look reveals that it is easy to rectify. This is accomplished by operating at the leaner condition which the gasoline engine cannot do in a practical sense. Then, by looking at
Figure 10, it becomes obvious that as you move right with the air ratio, the NOx production is actually substantially decreased from the stoichiometric condition at which the gasoline SI engine would traditionally be required to operate. Overall, there are enormous benefits of operating at higher excess air ratios with the addition of hydrogen as compared with a typical engine. There is increased stability, less fuel being added, better brake thermal efficiency, and reduced exhaust emissions of HC, CO2, NOx, and CO. The only downside seems to be that there would be reduced power output at these conditions. One way to rectify this, for situations in which high output is desired such as maximal acceleration, the hydrogen could be cutoff to the cylinders, and allowed to operate with just gasoline combustion. Another way is to achieve better power output is to vary the spark timing, detonating closer to top dead center (TDC), since the mixture with hydrogen has faster flame propagation speeds and requires less time for combustion to take place. As discussed in Li et al. [4] there are optimal spark ignition points for maximum IMEP at each percent addition of hydrogen. This is seen in
Figure 13, such that the spark ignition can be adjusted for maximal power output for the designed hydrogen addition.
Figure 13: IMEP versus spark timing for two H2 addition levels and excess air ratios [4]
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So now let us take a moment to further investigate the reasons why the addition of hydrogen allows for this better combustion process to take place. Hydrogen has a low ignition energy and high burning velocity, which means that in combination with the gasoline, it allows the mixture to be easily ignited and combusted more quickly [4]. This also means that there is less post expansion combustion taking place, with more of the combustion having an impact on the cylinder head and going directly toward the torque output. There are additional benefits that hydrogen addition has to offer, the first of which is better cold start performance in a SI engine, as discussed in Wang et al [5]. The engine was started with ambient, coolant, and oil temperatures of 17°C. For greater additions of H2, it was found that mean effective pressure, and engine speed were increased for the first 20 cycles. Also, HC and CO emissions decreased due to the enhanced combustion process as facilitated by the low ignition energy and high flame speed of hydrogen. There was however increases in NOx emissions for the first 5 seconds, but then were reduced. There are also benefits of hydrogen addition for CI diesel engines as well. As discussed by Shin et al. [6] H2 addition to a diesel engine with heavy exhaust gas recirculation (EGR), greater amounts of H2 addition resulted in an increase in brake thermal efficiency, and reduced NOx emissions for each given EGR ratio. Interestingly, the H2 addition also results in a greater power output, which contrasts with the impact of hydrogen-gasoline in a SI engine. This is likely the case because the amount of diesel injected was not being reduced as in the case of the SI engine. Overall, it was seen that the addition of hydrogen resulted in more complete combustion, as demonstrated by the decrease in O2 and increase of H2O at the tailpipe. So, with all of this in mind, we will analyze some of the claims at one of the kit supplier websites. Hydrogen-boost.com provides kits, and they claim a typical improvement of 15-25% increase in mileage from their systems [1]. The on-board electolyzer (Figure 1) splits water into what they call Brown’s gas, which is then introduced into the intake manifold, accelerating the flame spread, combusting more of the vaporized fuel. Unfortunately, there seem to be a few things missing here. For one, they do not seem to do anything to reduce the amount of gasoline fuel injected, which is where the majority of the fuel savings should come from. They mention an Electronic Control Unit (ECU), but fail to describe what exactly it controls. If they are not reducing the amount of gasoline, it is also dangerous for the engine as we could see from the graphs previously shown, Figure 3 shows that the BMEP will increase, which may put the pressure above the design point of the engine. It does however result in a better thermal efficiency, as seen in
Figure 2. Secondly, this results in a greater NOx output as seen in Figure 12, if kept at around the May 16, 2011
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same air ratio. They also have various types of electrolyzers available, with the cheaper ones only raising the temperature to boil the water, and the resulting water vapor is then injected into the engine. They claim that this isn’t as effective, but still helps gas mileage. However, this is not the same science, and could possibly damage the engine. Also, they do not provide the safest method of injecting the H2, since it is provided in a mixed form with the oxidizer as a premixed fuel, making it a more flammable substance than H2 alone. An alternative, though perhaps not as efficient, would be to run a fuel cell in reverse to split the water, capturing the H2 and O2 separately. The same could theoretically be done with the electrolyzer, capturing the different gases at the cathode and anode. Nonetheless, we can do a quick calculation to see if enough H2 is being produced. Information provided says that a 20 cell system operates at 20-30 amps, and produces 96-144 liters of gas per hour. Assuming that 2/3 of the gas is H2, as per the electrolysis reaction, H2O = H2 + 0.5O2, production is 64-96 liters of hydrogen per hour, or 0.018- 0.027 liters of H2 per second. It is specified for an engine of 4.5 liters, meaning that 4.5 liters of volume is swept by all of the cylinders per stroke. For a four cylinder, four-stroke engine, there are two cylinders per revolution that inject fuel and air into the cylinders, equating to 2.25 liters of volume input per revolution. So for an engine operating at 800 RPM, or 13.3 revolutions per second, there is 30 liters/second of volume input to the engine. Then, we need to calculate the amount of gasoline fuel input, both in liquid and gaseous form. The density for an average form of gasoline (C5H12) is 0.626 kg/liter, 0.00228 kg/liter for air, and for a stoichiometric ratio we have 1 kg of gasoline for every 14.6 kg of air. Using the densities, we find that liquid gasoline occupies 0.00115% of the volume input to the cylinder, resulting in 0.0034 liters of liquid gasoline input/second. Since our H2 addition values were provided in terms of volume percent addition, it was also necessary to find the equivalent amount of gasoline in vapor form per second. The density of the gas form of C5H12 was found to be 3.02E-3 kg/liter, as per an online database [7]. Therefore, the volume of gas vapor was found to be 0.7817 liters gasoline/second, by the same method. Comparing the hydrogen addition amounts to the gasoline input, we can achieve 2.3 to 3.4% H2 addition with the 20 Cell Electrolyzer Unit. It falls short of the maximum that we have seen in the literature of 8%, but would still allow us to reach a reasonable excess air ratio of 1.5 as seen in Figure 9. The next step is to see what savings that could be achieved with one of these units at a leaner combustion configuration, and how much the parasitic load would be to achieve the H2 production. This was done assuming a 1.5 excess air ratio, with 3% hydrogen addition, and
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associated efficiencies from the literature for the matching 800 RPM engine. It was found as detailed in the steps below, that for a one hour duration with constant rates, the extra gasoline input would be 0.4 liters, while saving 4.14 liters at the leaner operating condition, equating to a net savings of 3.74 liters. The stated efficiency of the electrolyzer was also investigated, and confirmed to be a good approximation using the density and Lower Heating Value (LHV) of H2. Givens: Alternator efficiency, Ρ: 50% LHV of H2: 121,000 KJ/kg Density of H2: 8.99E-5 kg/liter Engine Mean Brake Thermal Efficiency: 20% (Figure 8) Gasoline Energy Density: 34.2 MJ/Liter Electrolyzer: draws 30 amps, producing 144 Liters of gas per hr. 75% rated efficiency. Procedure:
Electrical Power = 12.6 V * 30Amps = 378 Watts*3600 s = 1360.8 kJ (Assume 12.6V battery) Energy Value of H2 = 64 Liters H2 * density of H2 *LHV H2 = 1044.05 Efficiency of Electrolyzer = energy value of H2 / Power Input = 1044.05/ 1360.8 = 76.7% Extra Engine Output required = Electrical power / eta_alternator = 1360.8 kJ / .5 = 2721.6 kJ Extra Fuel Input: Engine power output / thermal efficiency = 2721.6 / 0.2 = 9072 kJ Extra Fuel Volume: Fuel Input / Energy Density = 9072 kJ / 34.2 MJ/liter = 0.265 Liters Liquid Gasoline Savings: Excess Air Fuel ratio is increased to 1.5 = AFR / AFR_Stoich = 1.5 * 14.6 = 24.382 kg air / kg fuel As per the procedure earlier, the rate of liquid gasoline addition = 0.0023 liters/s
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Savings = 0.0034 – 0.0023 liters/s * 3600s = 4.14 liters Net Savings = 4.14 liters – 0.265 liters = 3.74 liters saved % Savings = (0.0034 – 0.0023) / 0.0034 = 32.3% The same procedure was carried out for the same theoretical 4.5 L engine, operating at a more realistic 1500 RPM, with the larger 40 cell electrolyzer capable of producing between 192 and 240 liters of gas per hour. The results showed that this could provide at most 3% H2 addition for the given conditions, amounting to an excess air ratio of 1.5, savings of 7.76 liters, with parasitic gasoline load of 0.66 liters, for a net savings of 7.1 liters over an hour at constant rates. We see that this actually requires a larger system than they advertise for best operation. However, there are still likely to be savings with the smaller systems, but appropriate configuration is required. As evidenced from the examples shown, we can see that there are great savings possible from such a hydrogen boosted gasoline engine system. However, the way that it is currently marketed and likely installed does not aim toward the same improvement as evidenced in the literature. Nonetheless, the system could still be utilized and modified for appropriate usage and savings. Also, it would be even more beneficial to have a larger electrolysis machine that would be capable of providing higher amounts of H2 to meet the higher RPM needs, and for the ideal 8% of H2 addition seen in the literature. Overall, it seems like the hydrogen boosting systems currently for sale are a bit of a shot in the dark, as they are not configured for optimal performance based on research. They have a similar idea to what has been found in the literature, but seem to be missing the key points in configuration, unless there are further details found once the system is purchased. This is likely why they have to give such a wide range of potential outcomes and savings, because it is not immediately adaptable to every car system. However, if properly applied and controlled, such systems could theoretically provide great benefits in the form of fuel economy and emissions reduction, as demonstrated by experimental research reviewed here.
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References: [1] Hydrogen boosting technology and kits. <http://www.hydrogen-boost.com/index.html> May 10, 2011 [2] C. Ji, S. Wang, and B. Zhang, “Combustion and emissions characteristics of a hybrid hydrogengasoline engine under various loads and lean conditions,” International Journal of Hydrogen
Energy, vol. 35, no. 11, pp. 5714-5722, Jun. 2010. [3] C. Ji and S. Wang, “Effect of hydrogen addition on lean burn performance of a spark-ignited gasoline engine at 800 rpm and low loads,” Fuel, vol. 90, no. 3, pp. 1301-1304, Mar. 2011. [4] C. Ji, S. Wang, and B. Zhang, “Effect of spark timing on the performance of a hybrid hydrogengasoline engine at lean conditions,” International Journal of Hydrogen Energy, vol. 35, no. 5, pp. 2203-2212, Mar. 2010. [5] S. Wang, C. Ji, and B. Zhang, “Starting a spark-ignited engine with the gasoline-hydrogen mixture,” International Journal of Hydrogen Energy, vol. 36, no. 7, pp. 4461-4468, Apr. 2011. [6] B. Shin, Y. Cho, D. Han, S. Song, and K. M. Chun, “Hydrogen effects on NOx emissions and brake thermal efficiency in a diesel engine under low-temperature and heavy-EGR conditions,”
International Journal of Hydrogen Energy, vol. 36, no. 10, pp. 6281-6291, May. 2011. [7] Gas Encyclopedia, “Properties for C5H12.” <http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=81> May 15, 2011 [8] E. Sher and Y. Hacohen, “Measurements and predictions of the fuel consumption and emission of a spark ignition engine fuelled with hydrogen-enriched gasoline.,” PROC. INST. MECH. ENG.
Vol. 203, vol. 203, no. 3, pp. 155 -162, 1989. [9] C.-W. Ji, S.-F. Wang, H. Yan, F.-S. Deng, H.-L. Diao, and Y. Liu, “Experiment on combustion and emissions characteristics of an IC engine blended with hydrogen,” Beijing Gongye Daxue
Xuebao, vol. 34, no. 12, pp. 1326 -1331, 2008. [10] Hydrogen Reforming Canister (Cover Image)<http://waterpoweredcar.com/hydrobooster.html>
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