The majority of buses in the 20th century were powered by diesel engines. These buses would be replaced.
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 This legislation was later repealed in 1896.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.
Tech Level: 8-10
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.