Why are heat addition processes in a petrol and diesel engine modelled as constant volume and constant pressure processes respectively?

The combustion process in a petrol engine is instantaneous. Hence approximated in the Otto cycle as constant volume heat addition. 

While the combustion process in a diesel engine takes comparitively longer time and hence approximated as constant pressure process in idealised Diesel cycle.

The often made assumptions that the SI or Otto engine is best approximated by the constant-volume cycle, and the CI or diesel engine is best approximated by the constant-pressure cyle are not necessarily correct.