There were some interesting results. One of the research projects was at the NASA Lewis research center in Cleveland. The Experiment involved a 1969 Cadillac engine with a relatively high compression ratio of 10.5 to 1. The engine was run on a test stand at a constant speed and with constant resistance. Three experiments were done. One without any boosting, to establish some data to compare to, another with bottled Hydrogen released into the induction system at 1.4lbs per hour, and finally a methane hydrogen product at .5 lbs of hydrogen per hour. The methane product used exhaust heat to heat methane and water vapor to 600 F which results in a hydrocarbon compound, and a stream of hydrogen. The researchers considered this a more efficient way to produce the Hydrogen needed to run the test. In the experiment the researchers adjusted the fuel air mixture across a specified range using the “stiochiometric” mix as a reference point. Stiochiometric meaning the perfect mix, a smaller number from that was considered extra lean, and more than that was rich.
They tested efficiency, temperature, and emissions. For our purposes we will look more closely at the efficiency and temperature.
The results were of interest. These should be instrumental in showing the relationship hydrogen addition had on combustion.
The first chart shows flame speed. Flame speed is the speed at which the flame spreads from the ignition source to the middle of the cylinder. Due to the fact that fuel is burned and does not explode the flame in a cylinder is somewhat representative to how quickly the fuel is being consumed. Different materials have different flame speed. But, the material is not the only thing that effects the speed at which the fuel is consumed; air to fuel mixture also plays a large part. For example in a cylinder with more fuel the flame propagates easier. Its the same in a fireplace fire, wood that is close together allows the flame in a fire to propagate quicker. However if you separate out those pieces of wood and give the wood a lot of space or more air, it takes more time for the flame to jump from one log onto another. Its the same in a cylinder. A lean mixture, has molecules of fuel separated by a lot of air so the flame speed begins to slow down. As we continue to lean the mixture past peak EGT we start to run into problems, one being a rough running engine, limiting how lean we can run.
The Engine runs roughly because as the flame speed continues to slow down as we lean, the cylinders begin to burn fuel differently. This difference is do to minor imperfections in each cylinder and each distribution system, that we typically don't see because fast flame speed makes up for those differences. As we stretch the time of combustion out with decrease flame speed it allows these imperfections more time to effect the quality of combustion in each cylinder. Thus instead of having all cylinders working in unison they become more out of step with each other. Making our engine start to run rough. In a perfect world we could lean with perfect smoothness while only losing power.
As shown in the graph, flame speed is highest in all cases with higher Stiochiometric ratios (richer mixture), due to the abundance of fuel the flame propagates easily. Notice however the hydrogen mix burns much faster even at extremly lean mixtures. Figure 8 shows the corresponding ignition lag. Note the reduction in lag occurred at all measured mixtures, but the reduction in lag was reduced in the lean mixtures substantially with the hydrogen addition. A faster ignition and flame spped could result in smoother engine operation at much leaner mix's. This would naturally result in better combustion.
Finally the effeciency of Hydrogen boosting is is illustrated in Figure 11. Notice thermal efficiency gains are somewhat lost on the richer mixtures, however on the leanest mixtures the hydrogen supplemented fuel's efficiency is evident.
One option that has been explored has been the use of electrolysis to produce hydrogen on site to supplement engine operation. Some sources mention it takes about 25KW of energy to produce a pound of hydrogen with this method.
We can make a few calculations to explore the feasibility of this method. Assume a standard aircraft system using 24 Volts and up to 70 amps, we will assume 30 amps are usable for electrolysis. Power= Voltage * Current , 30amps*24volts = 720 Watts per hour. So assuming 25 kw produce a pound of hydrogen, then with out 720 watts we would be producing about .03 pounds an hour. This is a good deal less than the experimented 1.4 lb per hour flow out of the bottle, and the .51 pound per hour from the methane converter. However before we completely through this idea out the window, we should consider a few more things.
These are, that according to the graphs illustrated in the experiment, a reduction from 1.4 lbs per hour to .51 lbs per hour (by well over half) resulted in very little change in the results. Meaning, we don't know exactly what level of hydrogen introduced effects combustion flame speeds, nor when thermal efficiency would be increased.
Also, in electrolysis we produce a proportional amount of oxygen. Producing .03 pounds of Hydrogen would produce .48 pounds of Oxygen. What effect a slight enriching of the induction would do to combustion, is an interesting question. Considering it would not be adding more air but create a higher density of Oxygen for the current induction air. Would it have a similar effect on flame speed and combustion quality? Materials that are not considered very flammable, in the presence of a higher concentrations of Oxygen become very flammable. Would additional Oxygen help to supplement the lower amount of Hydrogen produced by Electrolysis, therefore increasing burn quality, I think that is a very good question.
Ultimately the jury is still out on the potential hydrogen boosting has to internal combustion operation. Hopefully this article has provided some insight into the science behind the technology.