<P> If the camshaft is located in the crankcase, a valve train of pushrods and rocker arms will be required to operate overhead valves . Mechanically simpler are side valves, where the valve stems rested directly on the camshaft . However, this gives poor gas flows within the cylinder head as well as heat problems and fell out of favor for automobile use, see flathead engine . </P> <P> The majority of modern automobile engines place the camshaft on the cylinder head in an overhead camshaft (OHC) design . There may be one or two camshafts in the cylinder head; a single camshaft design is called single overhead camshaft (SOHC). A design with two camshafts per cylinder head is called double overhead camshaft (DOHC). Note that the camshafts are counted per cylinder head, so a V engine with one camshaft in each of its two - cylinder heads is still an SOHC design, and a V engine with two camshafts per cylinder head is DOHC, or informally a "quad cam" engine . </P> <P> With overhead camshafts, the valvetrain will be shorter and lighter, as no pushrods are required . Some overhead camshaft designs still have rocker arms; this facilitates adjustment of mechanical clearances . </P> <P> A four valves per cylinder design usually has two valves for intake and two for exhaust, which requires two camshafts per cylinder bank . If there are two camshafts in the cylinder head, the cams can sometimes bear directly on cam followers on the valve stems (tappets). The cam followers aid in noise reduction, dampened vibration, shock absorption and the carrying of axial load . This latter arrangement is the most inertia free, allows the most unimpeded gas flows in the engine and is the usual arrangement for high performance automobile engines . It also permits the spark plug to be located in the center of the cylinder head, which promotes better combustion characteristics . Beyond a certain number of valves, the effective area covered decreases, so four is the common-most number . Odd numbers of valves necessarily means the intake or exhaust side must have one valve more . In practice this is invariably the intake valves - even in even - numbered head designs, inlet valves are often larger in size than exhaust . </P>

In which one of the following four-stroke cylinder arrangements