Because it is a accepted term. A CAT has rare elements in it the reacts with hot gas to convert the emissions passing thru it. These same elements are destroyed quickly by high sulpher in fuel. A thermo reactor helps burn and lower the suspended particulates that diesels are famous for and are aways present. Why do you think the EPA ordered sulpher out? It was so they could start putting true CATs on oil burners and fully address the very high NOx and true grams per mile emissons that they produce. (one diesel makes as much NOX as about 5 to 10 cars in grams per mile) The reason diesels appear clean is because they have a high volume of air flowing thru them which dilutes emissions and makes PPM low when actually grams per mile is high. Gas engines have a much lower air flow thru them and at same PPM emit far less grams per mile than a diesel. Diesels are finally going to have to comply fully with emission requirements like gas engines have been for over 30 years now. You will see diesel ratings decline in coming years and economy suffer too as regs kick in because allow with exoitic injection systems to limit NOx they will also lower CR and boost to reduce NOx generation to begin with. Thye will either have to get bigger (more displacment for same power with less boost) or see ratings fade. Diesel have had a unfair edge in emissions for many years and loop hole is finially closing and long overdue.
----------------- TheSnoMan.com