The Saur-Sojasun rider won the 178km Belgian stage from Poperinge to Reningelst, contested on a 20km looped circuit.
Engoulvent and Kevin Lalouette (Roubaix Lille Metropole) had been involved in a breakaway and the duo managed to keep their lead to finish one minute, 45 seconds clear of the peloton.
The pair had been part of a seven-man escape from early on in the race, and at one point their lead over the chasing pack had ballooned to eight minutes and 25 seconds approximately midway through the stage.
Four kilometres from the finish the pair managed to discard their five fellow riders from the race to the line, but though Lalouette chased Engoulvent he could not catch him.
The Frenchman has a one minute, 19 second advantage in the general classification over his nearest challenger, compatriot Benoit Jarrier (Veranda Rideau).
The five-stage race's penultimate day sees the riders travel between Gravelines and Cassel in a 183km route.