Because there is more than one of them.
When you use the word Coke, you are referring to a specific product made by one company. There is only one Coke, so it's capitalized.
When you use the word coke, you are referring to anything that could be described by the word coke, excluding the single Coke product.
With diesel, the word has multiple meanings, but it is NOT Rudolph Diesel. To reference Rudy, you would always capitalize:
"But there are other internal combustion engines as well, in particular the diesel, or compression ignition engine. This was originally patented by Rudolph Diesel".
"While I hold the position of president at my company, Obama is the President of the USA."
In that last case, it's grey area, since there is only one "president" at a company, but since I was describing the job occupation which applies to thousands of people, it's normally not capitalized.