Zero emission engines 
Vehicles and other mobile machinery used for transport (over land, sea, air, rail) and for other uses (agricultural, mobile power generation, etc.) contribute heavily to climate change and pollution, so zero emission engines are an area of active research. These technologies almost in all cases include an electric motor powered by an energy source compact enough to be installed in the vehicle. These sources include hydrogen fuel cells, batteries, supercapacitors, and flywheel energy storage devices.
In some cases, such as compressed air engines, the engine may be mechanical rather than electrical. This mechanical engine is then powered by a passive energy source like compressed air, or a combustible non-polluting gas like hydrogen.
The above engines can be used in all vehicles, from cars to boats to propeller airplanes. For boats, energy sources such as nuclear power and solar panels can also be a viable option, in addition to traditional sails and turbosails.
A concept like vegetable oil economy produces emissions; however, the only emissions are things that were first taken out of the atmosphere when the plants were growing. So there is no net emission. The same concept can also be applied to similar fuels such as nitrous oxide and syngas.
See also 
- Air engine
- Carbon neutrality
- Zero emission project
- Zero Emissions Research and Initiatives
- Zero-emissions vehicle
- Dixon, Lloyd; Isaac Porche, Jonathan Kulick (2002). Driving Emissions to Zero: Are the Benefits of California's Zero Emission Vehicle Program Worth the Costs?. RAND Corporation. ISBN 0-8330-3212-7.
- A special issue of the Journal of Cleaner Production that focuses specifically on Zero Emissions
- Description of what 'zero emission' means