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U.S Department of Transport

U.S Department of Transport



Over the next 50 years, hydrogen use is expected to grow dramatically as an automotive and electrical power source fuel. As hydrogen becomes commercially viable, the safety concerns associated with hydrogen systems, equipment, and operation are of concern to the commercial motor vehicle industry. This report is intended to provide guidelines for use of hydrogen fuel as an alternative fuel by a commercial vehicle fleet operator to ensure long-term safe operation. 

In this paper by the Federal Motor Carrier Safety Administration all aspects of Hydrogen as a fuel source are explored. Below are extracts related to Hydrogen on demand fuel systems:

Hydrogen Injection Systems

A hydrogen injection system for a diesel engine produces small amounts of hydrogen and oxygen on demand by electrolyzing water carried onboard the vehicle. The electricity required is supplied by the engine’s alternator or 12/24-volt electrical system (see Section 1.5 for a description of electrolysis). The hydrogen and oxygen are injected into the engine’s air intake manifold, where they mix with the intake air. In theory, the combustion properties of the hydrogen result in more complete combustion of diesel fuel in the engine, reducing tailpipe emissions and improving fuel economy (CHEC, n.d.). Limited laboratory testing of a hydrogen injection system installed on an older diesel truck engine operated at a series of constant speeds showed a 4 percent reduction in fuel use and a 7 percent reduction in particulate emissions with the system on (ETVC, 2005).

A hydrogen injection system for a diesel engine produces and uses significantly less hydrogen than a hydrogen fuel cell or hydrogen ICE, and does not require that compressed or liquid hydrogen be carried on the vehicle. The system is designed to produce hydrogen only when required, in response to driver throttle commands. When the system is shut-off, no hydrogen is present on the vehicle.


The most abundant source of hydrogen on earth is water—every molecule of water contains one oxygen atom and two hydrogen atoms. It is relatively simple to separate the hydrogen in water from the oxygen using electricity to run an electrolyzer. An electrolyzer is a galvanic cell composed of an anode and a cathode submerged in a water-based electrolyte.

In many ways, the operation of an electrolyzer is the opposite of operating a hydrogen fuel cell. In a fuel cell, hydrogen and oxygen are supplied to the anode and the cathode, and they combine to form water while creating an electrical current that can be put to use (see Section 1.2.1 and Appendix A). In an electrolyzer, an electrical current is applied between the anode and the cathode, which causes the water in the electrolyte to break down, releasing oxygen gas at the anode and hydrogen gas at the cathode.


 Water and an onboard electrolyzer cannot be used to power a fuel cell or hydrogen ICE vehicle because of the large amount of electricity required to operate the electrolyzer. An electrolyzer can be used at a centralized fueling station to produce hydrogen, which is then compressed for on-site storage and delivery to vehicles. For a centralized electrolyzer, the electrical energy could be supplied from the electrical grid or from a dedicated renewable source, such as a wind turbine or solar cell array.


Hydrogen gas is colorless, odorless, tasteless, and noncorrosive, and it is nontoxic to humans. It has the second widest flammability range in air of any gas, but leaking hydrogen rises and diffuses to a nonflammable mixture quickly. Hydrogen ignites very easily and burns hot, but tends to burn out quickly. A hydrogen flame burns very cleanly, producing virtually no soot, which means that it is also virtually invisible.

2.1.1 Flammability, Ignition, and Luminosity

A mixture of hydrogen and air will burn when there is as little as 4 percent hydrogen or as much as 75 percent hydrogen in the mix4 This is a very wide flammability range.

In comparison diesel fuel vapors in air will burn over a range of 0.6 percent to 5.5 percent. With less than 0.6 percent diesel in the mixture it is too lean to ignite, and with more than 5.5 percent diesel in the mixture it is too rich. Natural gas will burn over a range of 5 percent to 15 percent.

It takes very little energy to ignite a hydrogen-air mixture—a common static electric spark may be sufficient.

As shown in Table 4, it takes less than one tenth of the energy to ignite a hydrogen air mixture as it does to ignite a mixture of gasoline vapors in air. Over much of its flammable range, common static electricity would be enough to ignite a hydrogen-air mixture. In some cases, the electrostatic charges or heating created by the flow of hydrogen from a leaking vessel would be enough to ignite the leaking hydrogen (Murphy, et al., 1995; Argonne, 2003).

Hydrogen flames burn very cleanly, producing virtually no soot. It is the soot created by most fuel that makes a flame visible. In addition, much of the energy radiated by a hydrogen flame is in the ultraviolet range, rather than the infrared or visible ranges of the light spectrum. Therefore, a hydrogen flame is virtually invisible to the human eye in day light, though the energy being  At room temperature and one atmosphere pressure being released by the flame may create a visible “shimmer” in surrounding air due to changes in the air density. At night, hydrogen flames are visible to the unaided human eye, and in daylight, they can be “seen” by an ultraviolet light sensor.

Read the full report Guidelines-H2-Fuel-in-CMVs-Nov2007 FINAL_0



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