According to TechNet article at SharePoint 2013 automatically generated crawled and managed properties given that the site column will have information stored and a full crawling has been completed successfully.

However, I encountered a case in which the Search service application failed to managed properties and create a invalid crawled properties. In my custom site columns definition elements.xml file I have a field called HTMLComments and the field schema was as below :

I notice that only crawled property has been created with a name HTMLCommentsOWS which not following the naming convention mentioned in the above link. as the field is HTML Field I expected to have automatically created crawled property named "ows_r_HTML_HTMLComments". I was clueless for a while and then I end up viewing the Schema for an OTB Field PublishingPageContent and it was as below

I notice that the SourceID  attribute is missing from my Field definition,I added the source Id attribute as below:

and run full crawling and I notice that the  crawling properties and managed properties has been created successfully.