Why do diesel engines need straight chain hydrocarbons?

I can't figure out why diesel engines need straight chain hydrocarbons and gasoline engines need highly branched hydrocarbons. I know the optimal form of C8H18 for a gasoline engine is 2,2,4 trimethylpentane and the optimal form of cetane for a diesel engine is an unbranched C16H34. I can understand how octane would detonate better as a branched molecule since the carbocations would be more stable, but for diesel i don't see how a straight chain would be at all beneficial. If anyone has an answer I would greatly appreciate it.