Magento - how to add custom discount without using coupon code in observer

Hi may be worth try with this url http://pradeepkumarrcs.blogspot.in/2011/12/applying-custom-discount-amount-in.html

I've used this code explain in blog, it uses sales_quote_collect_totals_after so it would execute every time total is calculated and your custom discount is applied on cart, Discount will be display in order,invoice and also in credit memo.