Jdwisnie, as an engineer working in the auto industry, I have to disagree with with you on the purpose of the EGR. EGR stands for Exhaust Gas Recirculation. The purpose of it is not to recirculate unburned hydrocarbons back into the engine to be burned, but to lower the temperature of combustion.
See most of the air we breath is nitrogen. That nitrogen usually passes through the engine with no change, unless the combustion temperature goes above 2900 deg F, at which point the nitrogen can chemically react with oxygen to form oxides of nitrogen(NO, NO2, NO3, ...NOx). Nox is also known as smog as it hangs in the air and is an irritant. One way to reduce the temperature of combustion is to recirculate some exhaust gas back into the engine. Most cars do it this way, some use only computer control of spark timing and fuel to do it.
Unburned hydrocarbons are burned off in the cat. If you ever had an engine apart and looked at the EGR, you would also see that it's a very small tube which recirculates only a small portion of the exhaust gas. With the EGR open, the other 90% of the unburned HC would continue on out of the exhaust. So that right there tells you it's purpose is not to recycle unburned HC.
Al lot of misinformation in here about hydrogen. Think about this: no reaction is 100% efficient. There is always a penalty to convert energy from one form to another. If you spend X amount of energy to break down water into browns gas, you can not recover 100% or more of that energy. The amount of available energy from that gas is now maybe 70% of X(the other 30% escaped as heat). So then you combust the browns gas and release maybe 70% of that available energy. You've used more energy to make your fuel than you can ever get back from it.
Where does the energy come from in HC fuels? Well, like everything else, it came from the sun, this time millions of years ago and is stored in the bonds of the molecules. When HC fuels are burned, the bonds are broken and the energy released. We have the technology to make our own gasoline or propane. But we dont make it because the process would involve adding energy to form those bonds. When that fuel is burned, the energy released would be less than the energy used to initially make the fuel. It is no different with hydrogen.
Al