(Note: The background section of this page uses most of the same words as Terra Futura's page on natural gas buses to save time.)
Early buses were drawn by horses. These horsebuses became commonplace in 1820s. They were first used in Berlin and then spread to other parts of the world. Then, in 1830, Walter Hancock invented the steam bus. They were in England until harsh legislation nearly drove steam buses to extinction. This legislation was later repealed in 1896. Steam buses came back into services. Meanwhile, in 1882, the electric trolleybus was invented. These trolleybuses used overhead wires just like electric trains. Then, in 1895, the first internal combustion engine bus was invented. Early buses used gasoline, but, in 1920, the first diesel buses came into existence.
Diesel buses started to be used in the 1920s because they had 50% efficiency. Diesel buses went on to become the most common buses on Earth. There were two problems to contend with. First, diesel fuel had a high sulfur content which would be eliminated. Second, diesel engines released particulate matter in the form of smoke. This was solved by a diesel particulate filter. By the end of the 20th century, there was an even bigger problem. That was global warming caused by greenhouse gas emissions. Diesel fuel would be replaced with bio-diesel fuel.
Bio-diesel buses had an advantage over conventional diesel buses because of the fuel. Bio-diesel did not need to be refined at all. After being created from plant crops, the fuel was already good to go as diesel. Therefore, bio-diesel buses quickly got onto the market. They were more environmentally friendly than conventional diesel buses and, thus were commonly used. But they were not the only replacement for conventional buses. Natural gas buses were more common.
Natural gas released less emissions into the atmosphere than petroleum. Thus, it was a cleaner fuel. One of the first transit authorities to natural gases buses was the Dallas Area Rapid Transit (DART) in Dallas, TX and its suburbs. This part of a move to reduce carbon emissions in the Metroplex. The replacements began in 2013. Soon, they were commonplace. However, as hydrocarbons ran out, buses needed a new source of power. As a result, many buses became hybrids.
Tech Level: 10
In the early 21st century, hybrid electric buses started coming onto the market. As battery weight decreased significantly, hybrid buses became cheaper. They were now more widespread than ever. Equipped with regenerative braking and a start-stop system, emissions were reduced, and power was not lost when the brakes came on. Hybrid buses were a success. Soon, plug-in hybrid buses would come onto the market.