Here's a 22-page thread on the topic:
http://ecomodder.com/forum/showthrea...wer-13727.html , and I've posted a few thoughts in that thread that are relevant to this one.
I think you're crazy for trying to run a heat engine between Th=100°C and Tc=30°, especially considering your working fluid will achieve an even smaller range of temperatures.
Also, if it must run off of engine coolant, what about a working fluid other than water? Maybe alcohol?