How to make alternator produce 110 v? If the output on alternator is producing 220 v . without using S-down transformer?

It depends on speed of prime mover (Engine) In case of diesel generators, if we reduce speed to half or 50% then induces voltage is also reduced to 50%. Lets say 220V to 110V.

it can be easily done by controlling the exiter voltage 

The prime mover input to the alternator has to reduced by 50%. As emf induced is directly propotional to speed, voltage generated will be reduced to 50%. (220V to 110V).