There have been several ways contemplated for reducing the pollution emissions from vehicles. Some were about changing the fuels (biofuels). Others were about changing the components of the engine system to increase efficiency (turbo). You can also change the type of materials used to make the engine more efficient due to the higher operating temperatures (ceramic engine block). Another is to change the engine configuration (rotary and wankle engines) The last one I can think of is the basic technology (rocket, sail, solar sail, jet, ...). There may be others. Each has their own application, and not one is sufficient for all situations.

The original post discusses how some unforseen consequences have cropped up with biofuels.