much of the current japanese effort to promote the development and sale of low-pollution automobiles focuses on natural gas vehicles , which use methane -based city gas , and which are categorized according to method of fuel storage : compressed natural gas vehicles -lrb- cngvs -rrb- , liquefied natural gas vehicles -lrb- lngvs -rrb- , and absorption natural gas vehicles -lrb- angvs -rrb- .