Quote:
Originally Posted by Ecky
What I mean is, H2 and O2 (as I understand it) will react with each other at any non-absolute zero temperature. The rate with which they react varies (hugely) with temperature. What is the half-life of hydrogen gas in the conditions you're likely to find under an automobile's hood? Is the reaction rate small enough that a vast majority of the hydrogen will still be present by the time it's sucked into the cylinder? Does it survive the entire compression stroke so it can actually be used for mechanical work?
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Arrhenius rate equations do show that the relationship between the reaction rate and temperature doubles for every 10 degrees C near room temperature. This, however, assumes there is sufficient kinetic energy available to have molecular collisions that exceed the activation energy (Ea). The Ea of the oxygen/hydrogen collision is on the order of 6 kj/mole. This is quite high. At room temperature, oxygen and hydrogen can exist almost indefinitely. However, hydrogen in oxygen has a very low energy of ignition due to the large exothermic output of the reaction - an output that easily provides the Ea needed for reactive collisions. I have seen pure hydrogen run into diesel engines intake tracts in combustible mixtures (4-75%). Even with compression ratios of 17:1, the hydrogen would not ignite until extra intake heat was added and the compression heating approached 500 deg C. In gasoline engines with 13:1 compression ratios, I assure you, the hydrogen makes it into the combustion chamber available to participate in the reaction. If even a small amount of hydrogen started oxidizing in the intake tract, the combustion rate is so high it would cause untimely backfire and possible damage. This is not to be confused with backfire caused by hydrogen slip past the intake valves where the small quench distance of hydrogen causes problems with improperly timed and seated valves.