I studied mostly turbulent combustion and spray combustion, PDF methods and flamelet methods.
HCCI is a very hot topic right now, with some very interesting challenges. Especially with regards to controlling ignition delay and certain unwanted combustion products. In the US I only know the group in Berkeley and Stanford doing HCCI, but probably almost any combustion group is investigating it. Make sure to go to a conference and talk to other people in the field. Input from other people, especially from outside your own group, can give some nice insights.