The Flyers (28-14-4) remained four points behind the Eastern Conference-leading New York Rangers. The Devils (26-19-2) lost for the second straight time in the middle of a six-game homestand.
Hartnell scored in the second and third periods. His first goal came off a deflection, and his second - the 22nd of the season - came off a one-timer that eluded goalie Martin Brodeur.
Simmonds assisted on Hartnell's first goal and on Matt Read's second-period. Read leads NHL rookies with 15 goals. Simmonds scored an empty-netter with two minutes left.
Alexei Ponikarovsky, in his Devils debut, scored New Jersey's lone goal. Ponikarovsky was acquired Friday in a trade with Carolina.
Ilya Bryzgalov made 30 saves to earn his 18th win of the season. Brodeur finished with 14 saves.
The Devils had a chance to break through on the first power play of the afternoon, but the advantage was cut short after 35 seconds later when David Clarkson was whistled for interference.
The Flyers finally broke through midway through the second period. Simmonds dug the puck out from the boards and stayed with the play behind the net. Simmonds Read in front, and Read slapped the puck past Brodeur at 9:30.
Philadelphia doubled its lead on Hartnell's power-play goal. Just 10 seconds after Devils rookie defenseman Adam Larsson was sent off for interference, Kimmo Timonen sent a shot from the point that Hartnell deflected in for his 21st goal.
Just 32 seconds later, the Devils finally got to Bryzgalov. Ponikarovsky got his stick on Matt Taormina's shot and tipped it past Bryzgalov to get New Jersey within 2-1.
Brodeur denied Simmonds with a great save early in the third period, but the Flyers scored twice to put the game away.
Hartnell made it 3-1 at 8:07 with his second power-play goal, the first time this season the Devils allowed two man-advantage goals in a home game.
Devils coach Peter DeBoer pulled Brodeur for an extra skater with 2:30 left, but Simmonds foiled the plan with an empty-net goal with 2:22 remaining.
