Table of Contents
Abstract 3
Introduction & Background Research 3
What Does PHI Do? 3
How Is Hydrogen Made? 4
The Water Electrolyzer 4
Electrolyzer Operation & Efficiency 5
Health, Pollution, & Compatibility Concerns 5
Design 6
Wet Cell vs Dry Cell 6
Current Leakage 6
Design Criteria 6
Design 6
Test Procedure 9
Appendix A (Design Matrices) 14
Appendix B (Electrolyzer Blueprints) 18
Bibliography 19
Contact 19
Definitions, Acronyms, Abbreviations 19
Abstract
The process of partial hydrogen injections was researched to gather an understanding of the current technology being used and how supplementing hydrogen in the fuel injection process has an effect on increasing fuel efficiency. Research revealed that the quenching distance is decreased significantly when hydrogen is introduced to the mixture resulting in more of the available fuel in the cylinder being burned.
The differences between wet cells and dry cells were also investigated. It was found that dry cells may be a better solution to on demand hydrogen production because architecture of the stack makes is simple to rebuild and it eliminates current leakage from the edges of the electrodes.
Materials were selected according to strength and ability to resist corrosion in the electrolyte solution. A structural analysis of many of the components was completed to specify component dimensions.[1]
Electrolyte research proved potassium hydroxide (KOH) to be the most viable approach to increasing the conductivity of the solution. In contrast to other electrolytes, KOH does not break down during cell operation and release other harmful chemical compounds. Periodic reservoir refilling will not require mixed solution, only distilled water.
The project was expected to go all the way to testing an ICE with and without the electrolyzer system to evaluate the change in performance. Time constraints and other complications resulted in partial completion. Although the system was not built and tested, procedures for doing so are outlined in Appendix C (Test Procedures).
Introduction & Background Research
What Does PHI Do?
Partial hydrogen injection (PHI) is the practice of feeding a small amount of hydrogen into an internal combustion engine (ICE) through the air intake. By doing so, the combined quenching distance[2] is less than that of pure gasoline. This results in a more complete combustion of the injected fuel. This process can virtually eliminate the need for a catalytic converter, as its purpose is to burn off fuel that doesn’t ignite in the cylinders. The idea here is that PHI enables an ICE to produce the same amount of power and use less fuel to do so. Table 1 shows a list of various properties, including quenching distances, for gasoline and hydrogen.
How Is Hydrogen Made?
Water electrolysis is one of the many ways to produce hydrogen and is done so by placing two electrically conductive solids (electrodes or plates) with opposing charges in a hydrogen rich, electrically conductive medium (electrolyte solution) and passing a current from the positive to the negative electrode. By passing current through the electrolyte solution, the chemical bond between water and oxygen is broken. Oxygen will appear at anode and hydrogen at the cathode, the positive and negative electrodes, respectively.
The main components of the electrolyzer are the electrodes, the electrolyte (conductive medium), the containment vessel, and the power supply that applies the electric current. It is important to note that the housing, or containment vessel, must be an electric insulator and that the system is only grounded through the negative electrode. Hydrogen production can be controlled in two ways, one of which is simply increasing the current applied to the electrodes while the other is to increase the active area of the electrodes[3].
In industrial applications, hydrogen is produced through steam reformation of hydrocarbons such as natural gas or petroleum. In this process, high temperature steam is mixed with hydrocarbons and passed through a nickel catalyst. Atomic bonds are broken and hydrogen is filtered out. This approach is very complicated as it involves careful temperature control. As a result, it has been deemed unfit for automotive applications.
Electrolytes
An electrolyte is a liquid substance which acts as a medium to conduct electricity. An electrolyte is full of ions, which are atoms that have some sort of net electric charge, either positive or negative. A dilute electrolyte has a relatively small amount of ions for its volume, while a concentrated electrolyte has a high amount of ions. Electrolytes can be divided into two groups; acids and bases. And within these groups you can have either strong acids and weak acids or strong bases and weak bases. Acids are defined as H+proton donors and Bases are hydroxide donors. These bases are a combination of a metal ion and the hydroxide ion, OH1-. Pure H2O itself is not conductive whatsoever. However, if a water-soluble electrolyte is added, the conductivity of the water rises considerably. The electrolyte disassociates intocationsand anions; the anions rush towards the anode and neutralize the buildup of positively charged H+there; similarly, the cations rush towards the cathode and neutralize the buildup of negatively charged OH−there. This allows the continued flow of electricity.
The electrolytes we decided to look at were Salt (NaCl), Baking Soda (NaHCO3), Potassium Hydroxide (KOH), and Bleach, also known as, Hydrochloric Acid (HCl). We chose to look at these for a few reasons but mostly because they are highly available on the market. Salt, baking soda, and Hydrochloric Acid can be found at your local grocery store while Potassium Hydroxide can be picked up at Home Depot in the form of Pequa Heavy Duty Drain Opener.
Potassium Hydroxide is the final electrolyte we chose due to its electrical conductivity, availability, ability to not change concentration during use, and its reactivity to stainless steel, PVC, and polyethylene.
KOH(s)→ K+(aq)+OH–(aq)
This is how KOH dissociates when mixed with an H2O solution. KOH dissociates into a positively charged potassium atom and a negatively charged hydroxide molecule.
We decided against NaCl because it reacted negatively with the materials we wanted to use and for the sole fact that it produces chlorine gas which is extremely poisonous. The baking soda was a similar because it produces carbon dioxide and carbon monoxide which are both detrimental to what we are trying to accomplish. We chose against HCl because it produces chlorine gas and is less conductive than Potassium Hydroxide.
As for safety hazards, it has been estimated that 15 seconds of contact to the eye with concentrated KOH caustic is enough to produce permanent blindness. Potassium hydroxide is very corrosive and hazardous to handle. Goggles or safety glasses with side protectors, and plastic or rubber gloves are absolutely necessary when handling KOH. When caustic comes into contact with the skin, the natural oils of the skin are chemically converted to a soap, which initially gives a slippery feeling. Prolonged contact will dissolve the skin and give a chemical burn similar but more severe than that given by handling lime or fresh wet concrete with bare hands.
The Water Electrolyzer
Water electrolysis breaks the chemical bond of H2O: 2H2O→2H2+O2. The resulting gas mixture is commonly referred to as HHO.
Because hydrogen production is a function of active electrode area, many systems employ placing multiple cells in series to create a ‘stack.’ In a stack, electrodes are aligned in a [+n-n+… n-] configuration where n represents an electrode with no charge (neutral plate), + is the positive electrode and – is the negative electrode. Neutral plates divide the total voltage applied to the system similar to the way resistors in a series circuit do. These neutral plates decrease the amount of heat created in the system by dividing the power dissipated between plates (power = voltage x current). The number of neutral plates is based on the overall voltage applied to the system. Independent researchers have found that one neutral plate per 2.3V is sufficient to eliminate excessive heat generation(1). A system using 2.3V would not need any neutral plates. This approach to controlling heat is favorable in most applications because it eliminates the need for voltage reducing electronics that would add to the complexity of the system and cause the stack to require more cells to increase active area for equivalent hydrogen production.
Electrolyzer Operation Efficiency
From an efficiency standpoint, it is desired that the system provide as little electrical resistance as possible. The reason for this can be illustrated using the equation I=VR, where I is current ,V is voltage and R is resistance. In automotive applications, voltage is a set constant based on the power supplied by a car battery and resistance is another constant governed by the architecture of the system. That means that current has to yield to the quotient VR. The desire for a low current may be demonstrated using the equation P=VI, where P is power and V and I are as previously noted. In short, decreasing the resistance will decrease the overall power needed to run the system. With these two equations, it is easy to see how materials, solutions and electrical connections must be selected such that resistance is minimized.
Modern water electrolysis for use in PHI systems employs the use of corrosion resistant electrode materials. Stainless steel is a common material used in less expensive systems, but has a drawback. Over time, the corrosion of the electrodes leads to high concentrations of hexavalent chromium in the water/electrolyte solution, an extremely toxic carcinogen(2). To avoid this, some manufacturers have moved to using titanium electrodes. However, some research shows that the hexavalent chromium only leeches out of the surface of stainless steel electrodes for a short time. Leeching stops after a period of use known as the conditioning period and may take up to a week of continuous operation to do so.
In general, it is desired to use a non-caustic electrolyte. Caustic electrolytes can end up in the engine and cause premature wearing of engine components. The byproduct of the chemical reaction is also a concern. There are many things that can increase the conductivity of water, but will produce other pollutants during the electrolysis process. Much of our research will focus on determining which solution is appropriate. We’re not just trying to make the ICE more efficient, we’re trying to do it without introducing other toxins to the atmosphere.
When provided with a source of constant voltage, an electrolyzer will draw as much current as it needs to overcome the resistance of the cell. As a cell begins to increase in temperature, so does the amount of current draws. To limit the amount of current that a cell can draw, a device known as a pulse width modulator (PWM) is used to rapidly cycle power on and off.
Health, Pollution, Compatibility Concerns
Modern water electrolysis for use in PHI systems employs the use of corrosion resistant electrode materials. Stainless steel is a common material used in less expensive systems, but has a drawback. Over time, the corrosion of the electrodes leads to high concentrations of hexavalent chromium in the water/electrolyte solution, an extremely toxic carcinogen that must be disposed of properly (2). To avoid this, some manufacturers have moved to using titanium electrodes. However, some research shows that the hexavalent chromium only leeches out of the surface of stainless steel electrodes for a short time. Leeching stops after a period of use known as the conditioning period and may take up to a week of continuous operation to do so. Designs should consider the effect that the solution will have on the integrity of components in contact with the solution (hoses, reservoirs, etc.).
Electrolyte solutions are made by mixing distilled water with electrolytes such as potassium hydroxide or sodium hydroxide. Caustic electrolytes can end up in the engine and cause premature wearing of engine components. In general, it is desired to use a non-caustic electrolyte, but because the most effective electrolytes are caustic, it has become practice to use electrolytes that don’t break down and add additional gasses to the HHO mixture. Some electrolyte solutions will produce CO and CO2. Care should be taken to choose an electrolyte that will not produce toxic fumes.
Design
Wet Cell vs Dry Cell
Wet cells have a series of plates submerged in a bath of electrolyte solution. The major drawback of this design is that the electrodes will charge the water surrounding the edge of the plate, but not produce any hydrogen. This is known as current leakage and decreases the overall efficiency of the system.
Dry cells are unique in that the edges of the electrodes are not in contact with the solution. This eliminates the issue of current leakage.
Design Criteria
Components must be made of materials that are resistant to the chemical composition of the electrolyte solution. Additional care in the selection of materials should be taken with regards to material properties at temperatures upwards of 70°C. Appendix A (Design Matrices) on page 13 shows the design matrix for various elastomers considered for use. These materials are graded based on their resistance to corrosion in a bath of potassium hydroxide (3).
Design
Soft PVC was selected to be used as a gasket material based on resistance to corrosion and the availability of the material. To determine the compressive force required to seal the gaskets, ASME Pressure Vessel Codes for m and y procedures were used (4). To begin these calculations, it was necessary to estimate the maximum pressure that the seals would experience. To calculate the max pressure P, the pressure of a column of fluid was applied to the situation in Figure 1 the density ρ of the fluid is assumed to be close to that of water and the height h is assumed to never be larger than 0.5 meters. Here we will apply a factor of safety of 2 and design the system to withstand double the expected pressure.
P=2ρgh=21000kgm39.81ms20.5m=10 kPa
Continuing to the m and y procedures, we use the following equations where D is the effective diameter of the gasket (m), b is the effective seating width of the gasket (m), 2b is the effective width of the gasket for pressure (m), P is the maximum pressure (Pa), m is the gasket factor and y is the seating load (Pa). Because this is a square gasket, D is assumed to be the width of the gasket.