I have applied for fall2018
I had completed application process in Feb 2018 and still my application is in process. You have applied in May 2018, so I am not sure when you will get decision. But, may be Mechanical is not that competitive program at carleton and you may receive their decision earlier than me and get seat.
On the other hand, the visa processing time in India is close to 2 months. Suppose, if they give offer in July or Aug then there is no way to get visa sooner and join classes, which are starting in 1st week of Sep'. In such case, deferring to Jan 2019 is one of the option which I am considering. But it all depends on the University whether they will give seat in Jan or not.
Instead of Fall 2018, if you had applied for Winter 2019 then your chances would have been much brighter. Your application would have been in the first lot of screening. People have applied in Nov/Dec 2017 for Sep 2018 intake. Many would have already got offer and there would be less seats now. Anyways, all the best and hope for the best.
My advice to you would be that instead of waiting for carleton's decision, start applying for Jan 2019 and May 2019 intake in other universities. So that your application will be processed in first lot for those intakes. In the mean time you can also try to increase your IELTS score.