WHILE DRIVING ON THE BELTWAY, THE CAR ACCELERATED THEN STARTED SLOW DOWN. I MERGED OFF OF THE BELTWAY AND ONTO A RAMP TO GET SOMEWHERE SAFE. AT THE RAMP THE CAR LIGHT OF THE AIR BAG CAME ON THEN, THE CAR CAME TO A STOP, AND ALL THE LIGHTS CAME ON. AND THE HOOD OF THE CAR WAS SMOKING. I LIFTED THE HOOD AND SAW THE FIRE UNDER THE ENGINE. WE MOVED AWAY FROM THE CAR AND LUCKILY A MAN IN A WORK TRUCK SAW US AND HAD A FIRE EXTINGUISHER AND PUT THE FIRE OUT. THE CAR IS WITH THE DEALER BUT, NOTHING HAS BEEN COMPLETED AT THIS TIME. THE CAR HAS BEEN WITH THEM SINCE 5/3/17.