Overhead camshaft
Overhead camshaft (OHC) valvetrain configurations place the
camshaft within the
cylinder heads, above the combustion chambers, and drive the
valves or
lifters directly instead of using
pushrods. When compared directly with
pushrod (or OHV) systems with the same number of valves, the reciprocating components of the OHC system are fewer and in total will have less mass. Though the structures that support the system may become more complex, most engine manufacturers easily accept the added complexity in trade for better engine performance and greater design flexibility. The OHC system can be driven using the same methods as an OHV system, these methods may include using a
timing belt, chain, or in less common cases, gears.
Many OHC engines today employ
Variable Valve Timing and
multiple valves to improve efficiency and power. OHC also inherently allows for greater engine speeds over comparable cam-in-block designs.
There are two overhead camshaft layouts:
*
Single overhead camshaft (SOHC)
*
Double overhead camshafts (DOHC)
Single overhead camshaft is a design in which one camshaft is placed within the
cylinder head. In an inline engine this means there is one camshaft in the head, while in a
V engine there are two camshafts: one per cylinder bank.
The SOHC design is inherently mechanically more efficient than a comparable
pushrod design. This allows for higher engine speeds, which in turn will by definition increase power output for a given
torque. The cams operate the valves directly or by a short
rocker as opposed to overhead valve
pushrod engines, which have tappets and long pushrods to transfer the movement of the lobes on the camshaft in the engine block to the valves in the
cylinder head.
SOHC designs offer reduced complexity compared to
pushrod designs when used for
multivalve heads, in which each cylinder has more than two valves.
|
Overhead view of Suzuki GS550 head showing dual camshafts and drive sprockets. |
A double overhead camshaft (also called
double overhead cam,
dual overhead cam or
twincam) valvetrain layout is characterized by two camshafts being located within the cylinder head, where there are separate camshafts for inlet and exhaust valves. In engines with more than one
cylinder bank (
V engines) this designation means two camshafts per bank, for a total of four.
Double overhead camshafts are not required in order to have
multiple inlet or exhaust valves, but are necessary for more than 2 valves that are directly actuated (though still usually via tappets). Not all DOHC engines are
multivalve engines — DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared, however today DOHC is synonymous with multivalve heads, since almost all DOHC engines have between three and five valves per cylinder.
The first DOHC engines were two valve per cylinder designs from companies like
Fiat (1912),
Peugeot (1913),
Alfa Romeo (
6C- 1925,
512 - 1940),
Maserati (
Tipo 26, 1926), and
Bugatti (
Type 51, 1931). Most
Ferraris used two valve per cylinder DOHC engines as well.
When DOHC technology was introduced in mainstream vehicles, it was common for the technology to be heavily advertised. While the technology was used at first in limited production and sports cars, the Fiat group is historically credited as the first car company to use a belt driven DOHC engine across their complete product line, comprised of coupes, sedans, convertables and station wagons, in the mid-1960's.
*
Valvetrain*
Pushrod*
Cam-in-block*
Camless*
OHV, SOHC, DOHC engine animated diagrams
