Balance shafts are used in piston engines to reduce vibration by cancelling out unbalanced dynamic forces. The counter balance shafts have eccentric weights and rotate in opposite direction to each other, which generates a net vertical force.
Balance shaft in Ford Taunus V4 engine.
Lanchester's vertical force balancer. The eccentric masses are labelled "C" and "D".
Valve timing gears on a Ford Taunus V4 engine. The balance shaft runs off the small gear on the left (the large gear is for the camshaft, causing it to rotate at half the speed of the crankshaft).
A straight-four engine is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.
Diagram of a DOHC straight-four engine
1989-2006 Ford I4 DOHC engine with the cylinder head removed
2006-2009 Nissan M9R diesel engine
1908–1941 Ford Model T engine