PZEV engines include 2 modes of exhaust valve operation. Intake and exhaust valve lift is significantly increased to . Another reason is the large tappet clearance on the exhaust valve. The exhaust valve opens at point 1. The exhaust valve linkage required frequent adjustment. The exhaust valve opened for the piston upstroke. Dry suits are equipped with an inflation valve and at least one exhaust valve. A simple retrofit mechanism could transfer motion to open the exhaust valve. The piston rises, expelling exhaust gases through the exhaust valve (the exhaust stroke). The gasses exit the cylinder through an exhaust valve at the top of the cylinder. Each cylinder has one intake valve and one exhaust valve, pushrod-activated. Gear driven camshafts operate a single inlet and exhaust valve in each cylinder via pushrods. An exhaust valve or several like that of 4-stroke engines is used. This has a single large exhaust valve and two smaller intake valves. This vacuum only occurs if the exhaust valve is closed during the piston's down-stroke.