Almost all pen and paper role playing games are battle engines. If you've ever played any wargames that use a CRT (combat results table), that's a battle engine. Battle engines are just a collection of rules and statistics (movement rate, hit points, number of attacks, etc.) applied to simulate and resolve conflict. Scissors, rock, paper is a battle engine (and I've even see wargames designed around that principle.)
What kind of game you want to design pretty much determines the kind of battle engine that you can use. You can write ones as simple as a coin toss or one that is a 5,000 line space fleet combat system. More info is required.
--
TAZ
"Do you think it is wise to provoke him?" "It's what I do." -- Stargate SG-1