The quantity is a lot different. Engine do it briefly at the start, boilers do it continuously

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Engine condensate is also generally only produced for the first short period of operation; once the stack warms up, the condensate is boiled off to steam, which is carried from the stack along with the products of combustion. I haven't seen everything, but I can't say I've ever seen provision made for draining off engine condensate from a stack.

Addition via edit:

Regulations I don't know, but beyond being acidic the primary difference between exhaust streams would be whether they are oily or not: natural gas fired condensing boilers, probably not; oil fired condensing boilers, possibly; IC engine exhaust gas condensing heat recovery boilers, almost certainly. How receptive the AHJ will be to accepting oily condensate is of course the question...

We decided to recommend collecting the condensate and either piping it to foul drain (large engines) or collecting it in vessels (plastic cans - small engines) and pouring into foul drain.

We cited condensing boiler condensate piping to foul drain as a precedent.

It was the equivalent of our AHJ who asked us for an indication as to best practice!

Just cannot find any other code of practice on this.

Design practice on stack temperature (think of case where there is an exhaust gas heat exchanger, like on a cogeneration set) is to go for 120C final stack temperature with natural gas fuelled set and 180C for biogas set (sulphur in fuel).

Thanks to those who troubled to help.