The new combustion buses use the same engine technology as diesel buses, modified to burn ethanol, methanol, propane, biodiesel, or natural gas. The cleanest versions, which burn natural gas, are gaining interest among transit agencies nationwide because they are substantially cleaner than diesel buses and are often less expensive to run. Their emissions of soot and the pollutants that cause smog are 40 percent below those of the diesel buses they replace. However, they offer less impressive gains in alleviating global warming, since they reduce emissions of heat-trapping gases by only 5 to 10 percent. The next stop is hybrid and electric buses.
Hybrid buses combine a combustion engine with electric motors and batteries.
These are still in the demonstration stage, but they promise lower emissions and higher fuel economy than traditional combustion buses. Electric buses, powered by batteries, are also in the early market stage and produce zero emissions. The most promising stop in the evolution of bus technology is fuel cell buses powered by hydrogen. However, heat-trapping gases may be emitted indirectly as a result of the process that produces hydrogen for the fuel cells. When the hydrogen is harvested from natural gas, the indirect emissions associated with fuel cell buses are roughly half those of diesel buses.