Inspired from the game "Planetary Annihilation" a Halley Delta V Engine is a engine built in the ground that is big enough to move the planet when used. This can be used for two things in my mind. War, and a savior from global warming (as it can be used to move us further from the sun like scientist are trying to use meteors for). It can be used for war by smashing planets into each other. But if used to save us from global warming, what could be the backlashes of such an engine on the environment, and would such an engine even be possible?
Edit: So basically I'm seeing this engine would not be safe for use in a planet we plan on keeping alive. In that I conclude its best use is for warfare in the destruction of planets and possibly damaging planets nearby as chunks of the two planets collide. And for those complaining about not having enough info about the engine, it would most likely be powered by some sort of electrical motion such as repulsion rather than something such as fossil fuels considered that might not be available on all planets.