That was my idea too, that you require a transit visa of some sort. But there are several posts on the internet telling that you don't and passengers who have successfully transited DEL without having any sort of visa when their onward flight is within 24 hours. Take a look a this post, though, be careful that it specifically states that this rule is applicable to bookings under a single ticket.
http://travel.stackexchange.com/ques...without-a-visa

All the best with your search.