Overhead camshaft, commonly abbreviated to OHC, is a
valvetrain configuration which places the camshaft of an internal
combustion engine of the reciprocating type within the cylinder heads
("above" the pistons and combustion chambers) and drives the valves or
lifters in a more direct manner compared with overhead valves (OHV)
2 Single overhead camshaft
2.1 Alternative SOHC layouts
3 Dual overhead camshaft
4 Triple overhead camshaft
Camshaft drive systems
5.1 Timing belt
5.2 Timing chain
5.3 Bevel shaft
5.4 Gear train
5.5 Cranks and rods
6 Variable valve timing
7.1 Early use
7.2 Aircraft engines
7.3 Inter-war use
7.4 Post-war use
8 See also
Compared with OHV pushrod systems with the same number of valves, the
reciprocating components of the OHC system are fewer and have a
lower overall mass. Though the system that drives the camshafts may
be more complex, most engine manufacturers accept that added
complexity as a trade-off for better engine performance and greater
design flexibility. The fundamental reason for the OHC valvetrain is
that it offers an increase in the engine's ability to exchange
induction and exhaust gases. (This exchange is sometimes known as
"engine breathing". ) Another performance advantage is gained as a
result of the better optimised port configurations made possible with
overhead camshaft designs. With no intrusive pushrods, the overhead
camshaft cylinder head design can use straighter ports of more
advantageous cross-section and length. The OHC design allows for
higher engine speeds than comparable cam-in-block designs, as a result
of having lower valvetrain mass. The higher engine speeds thus allowed
increases power output for a given torque output.
Disadvantages of the OHC design include the complexity of the camshaft
drive, the need to re-time the drive system each time the cylinder
head is removed, and the accessibility of tappet adjustment if
necessary. In earlier OHC systems, including inter-war Morrises and
Wolseleys, oil leaks in the lubrication systems were also an issue.
Single overhead camshaft
Honda D16A3 series single overhead camshaft cylinder head from a
Honda CRX Si.
Single overhead camshaft (SOHC) 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, whilst in an engine with
more than one cylinder head, such as a
V engine or a
horizontally-opposed engine (boxer; flat engine) – there are two
camshafts, one per cylinder bank.
In the SOHC design, the camshaft operates the valves directly,
traditionally via a bucket tappet; or via an intermediary rocker
arm. SOHC cylinder heads are generally less expensive to
manufacture than double overhead camshaft (DOHC) cylinder heads.
Timing belt replacement can be easier since there are fewer camshaft
drive sprockets that need to be aligned during the replacement
SOHC designs offer reduced complexity compared with overhead valve
designs when used for multivalve cylinder heads, in which each
cylinder has more than two valves. An example of an SOHC design using
shim and bucket valve adjustment was the engine installed in the
Hillman Imp (four cylinder, eight valve), a small, early-1960s
two-door saloon car (sedan) with a rear-mounted aluminium-alloy engine
based on the
Coventry Climax FWMA race engines. Exhaust and inlet
manifolds were both on the same side of the engine block (thus not a
crossflow cylinder head design). This did, however, offer excellent
access to the spark plugs.
In the early 1980s,
Toyota and Volkswagen Group also used a
directly actuated SOHC parallel valve configuration with two valves
for each cylinder. The
Toyota system used hydraulic tappets. The
Volkswagen system used bucket tappets with shims for valve-clearance
Alternative SOHC layouts
Section of a Triumph Dolomite Sprint cylinder head, highlighting the
single cam operating both inlet (directly) and exhaust (through a
rocker arm) valves.
The multivalve Sprint version of the Triumph Slant-4 engine used a
system where the camshaft was placed directly over the inlet valves,
with the same cams that opened the intake valves also directly opening
the exhaust valves via rocker arms.
Honda later used a similar valvetrain system in their motorcycles,
using the term "Unicam" for the concept. This system uses one camshaft
for each bank of cylinder heads, with the cams operating directly onto
the inlet valve(s), and indirectly, through a short rocker arm, on the
exhaust valve(s). This allows a compact, light valvetrain to operate
valves in a flat combustion chamber. The Unicam valve train was
first used in single cylinder dirt bikes and has been used on the
Honda VFR1200 since 2010.
Dual overhead camshaft
Overhead view of Suzuki GS550 cylinder head showing double camshafts
and chain-drive sprockets.
A dual overhead camshaft (DOHC) valvetrain layout is
characterised by two camshafts located within the cylinder head,
one operating the intake valves and the other one operating the
exhaust valves. This design reduces valvetrain inertia more than is
the case with an SOHC engine, since the rocker arms are reduced in
size or eliminated. A DOHC design permits a wider angle between intake
and exhaust valves than in SOHC engines. This can give a less
restricted airflow at higher engine speeds. DOHC with a multivalve
design also allows for the optimum placement of the spark plug, which
in turn improves combustion efficiency. Engines having more than
one bank of cylinders (e.g. V6, V8 – where two cylinder banks meet
to form a "V") with two camshafts in total remain SOHC unless each
cylinder bank has two camshafts; the latter are DOHC, and are often
known as "quad cam".
Although the term "twin cam" is often used to refer to DOHC engines,
it is imprecise, as it includes designs with two block-mounted
camshafts. Examples include the Harley-Davidson Twin
Cam engine, Riley
car engines from 1926 to the mid 1950s, Triumph motorcycle
parallel-twins from the 1930s to the 1980s, and Indian Chief and Scout
V-twins from 1920 to the 1950s.
The terms "multivalve" and "DOHC" do not refer to the same thing:
not all multivalve engines are DOHC and not all DOHC engines are
multivalve. Examples of DOHC engines with two valves per cylinder
Alfa Romeo Twin Cam engine
Alfa Romeo Twin Cam engine and the engine of the Jaguar
XK6. Most recent DOHC engines are multivalve, with between three and
five valves per cylinder.
Triple overhead camshaft
More than two overhead camshafts are not known to have been tried in a
production engine. However,
MotoCzysz has designed a
motorcycle engine with a triple overhead camshaft configuration, with
the intake ports descending through the cylinder head to two central
intake ports between two outside exhaust camshafts actuating one of
two exhaust valves per cylinder each.
Camshaft drive systems
All valve drive systems in four stroke engines must both power the
valves and time their opening and closing. The OHC valvetrain system
may be driven by the crankshaft using the same methods as an OHV
system, but in practice (and depending on the application), lighter
weight and maintenance-free methods are more commonly used.
Main article: Timing belt (camshaft)
Timing belt, Nissan RB30E Engine
Toothed timing belt made from rubber and kevlar are commonly used to
time overhead camshaft automobile engines. Compared with
other camshaft drive systems, timing belts have low design,
engineering and production costs, low noise levels, low mass (and
consequently low inertia), low space requirements, and no need for
lubrication. Timing belt systems are also simpler and more
versatile than other drive systems; while it is possible to power
engine components such as water pumps or alternators with bevel shaft
or gear drive systems, it is simpler to do so with belts.
A disadvantage of timing belts is the need for regular replacement of
the belt. Recommended belt life varies between designs; belts made
from neoprene for older vehicles might have a recommended life of
around 20,000 miles (32,000 km) while newer designs with lower
torque loading and made from hydrogenated nitrile butadiene rubber
(HNBR) may have a recommended life of 60,000 miles (97,000 km) or
more. By 2004, the range for timing belt life for new engine
designs was 60,000–100,000 miles (97,000–161,000 km), with
some designs having no fixed replacement schedule but with continued
use based on visual inspection.
Torque loading, vibration, oil,
heat and dirt reduce belt life. Timing belts are regularly
replaced during engine overhauls.
The first known automotive application of toothed belts to drive
overhead camshafts was with Devin-
Panhard racing specials built by
Bill Devin for the
Sports Car Club of America
Sports Car Club of America (SCCA) H-modified racing
series. Some of these specials were powered by engines built from
the crankcases of
Panhard flat-twin engines and the cylinders of
Norton motorcycles. The overhead camshafts on the Norton cylinders
were driven by toothed belts from the Gilmer Belt Company instead of
the tower shafts used in the Norton engines. Jimmy Orr won the 1956
SCCA H-modified championship in an overhead camshaft
The first production automobile engine to use a toothed belt to drive
an overhead camshaft was made by Glas for use in their 1004 motor car,
introduced in September 1961.
This section needs expansion with: examples and some history. You can
help by adding to it. (January 2015)
Duplex or single row roller chains have been used to drive overhead
camshafts in automobile and motorcycle engines. By the early
1960s most production automobile overhead camshaft designs used chains
to drive the camshaft(s). Timing chain systems are noisier and
more expensive than timing belt systems. Timing chains do not
usually require replacement at regular intervals, but there have
been exceptions, including the timing chain in the Triumph Stag, which
has a recommended life of 30,000 miles (48,000 km).
This section needs expansion with: examples, inter-war Alfa Romeo,
post-war Crosley, and several sports and racing motorcycles from the
'20s to the '50s. You can help by adding to it. (January 2015)
Norton motorcycle engine showing "tower" enclosing the shaft drive to
The use of a shaft with bevel gears to drive the camshaft was common
in overhead camshaft designs before the Second World War.[citation
needed] Examples include the Maudslay 25/30 built between 1908 and
1911, the Bentley 3 Litre, the 850cc MG Midget and various
racing motorcycles including the Velocette "K" series and the
Norton Manx. The MG engine used the armature shaft of the dynamo
as the camshaft drive, which was an economical use of parts but made
Ducati single-cylinder OHC motorcycle engines used a bevel shaft
system to drive the camshaft. Some of their V-Twins also used
bevel camshaft drive. The
Kawasaki W650 (1999-2007)
and its successor the
Kawasaki W800 (2011-2016)  use a bevel shaft
drive system for its overhead camshaft. The W800 is at this moment the
only production engine to use a bevel gear and the "Final Edition",
released at the end of 2016, will mark the end of this distribution
This section needs expansion with: more examples and history. You can
help by adding to it. (January 2015)
Trains of timing gears are commonly used in diesel overhead camshaft
engines used in heavy trucks. They are less commonly used in
overhead camshaft engines in light trucks or automobiles.
Cranks and rods
J. G. Parry-Thomas
J. G. Parry-Thomas developed a camshaft drive using
three sets of cranks and rods in parallel. Parry-Thomas used this
drive system in the Leyland Eight.
W. O. Bentley
W. O. Bentley used a similar
system, called the "three-throw drive", in his six-cylinder engines,
the 6½ Litre and the 8 Litre.
The overhead camshaft of the
NSU Prinz used a two-rod system with
counterweights at both ends of the drive system. NSU began using
the system in the early 1950s and continued to use it in the Prinz 4
and Prinz Sport in the mid-1960s, although the NSU 1000 used a
chain-driven system instead.
Variable valve timing
Main article: Variable valve timing
In conjunction with multiple valves (three, four or five) per
cylinder, many OHC engines today employ variable valve timing to
improve efficiency and power.
Among the first cars to utilize engines with single overhead camshafts
were the Maudslay designed by Alexander Craig and introduced in
1902 and the Marr Auto
Car designed by
Michigan native Walter
Lorenzo Marr in 1903. The first DOHC car was the 1912 Peugeot
which won the French Grand Prix at Dieppe that year. This car was
powered by a straight-4 engine designed by Ernest Henry under the
guidance of the technically knowledgeable racing drivers Paul
Zuccarelli and Georges Boillot. Boillot, who drove the winning car
that year, won the French Grand Prix for
Peugeot again in 1913 but was
beaten in 1914 by the SOHC Mercedes of Christian Lautenschlager.
Main article: Aircraft engine
A World War I-era Hispano-Suiza V8 aviation engine, which used fully
enclosed single overhead camshaft drivetrains for each cylinder bank.
Both the Allies and the
Central Powers quickly adapted the overhead
camshaft engine designs that had been used in racing cars just before
the First World War to their liquid-cooled aircraft engines. The 1914
Mercedes SOHC racing car engine became the starting point for both
Mercedes' six-cylinder aero-engine family and the Rolls-Royce Eagle
V12, whose cylinder heads were reverse-engineered from a Mercedes
racing car left behind in England at the beginning of the war. The
Mercedes designs culminated in the Mercedes D.III. Other SOHC designs
included the fully enclosed-drivetrain
Hispano-Suiza 8 – a V8 engine
Marc Birkigt and the late-war
Liberty L-12 on the Allied
side; and the powerful
BMW IIIa on the side of the Central Powers.
Cutaway view of a
Napier Lion showing the double overhead camshaft
The aircraft engines listed above all used bevel shaft camshaft
drives. Large aircraft engines, particularly air-cooled, saw
considerable thermal expansion over the height of the cylinder block.
This made pushrod engines difficult to arrange. As well as the other
advantages of direct valve actuation by an overhead camshaft, a bevel
shaft with a sliding spline was the easiest way to allow for this
expansion. These bevel shafts were usually in an external tube outside
the block, and were known as "tower shafts".
By the end of the war the DOHC
Napier Lion had entered service with
DOHC straight-8 in a 1933
Bugatti Type 59
Bugatti Type 59 Grand Prix racer
Among the early pioneers of DOHC were Isotta Fraschini's Giustino
Cattaneo, Austro-Daimler's Ferdinand Porsche, Stephen Tomczak (in the
Prinz Heinrich) (in 1919); Sunbeam built small numbers of racing
models between 1921 and 1923 and introduced one of the world's first
production DOHC auto engines in 1924 – the Sunbeam 3 litre Super
Sports, an example of which came second at Le Mans in 1925. The
first DOHC engines were either two- or four-valve per cylinder racing
car designs from companies like
Peugeot Grand Prix (1912,
Alfa Romeo Grand Prix (1914, four-valve) and 6C
Maserati Tipo 26 (1926),
Bugatti Type 51 (1931). and three
years later, Harry A. Miller and
Fred Offenhauser inaugurated the line
of 4.1 litre DOHC
Offenhauser inline-4 American auto racing engines,
which began to dominate American open-wheel auto racing through much
of the 20th century.
American luxury automaker
Duesenberg was an early proponent of
overhead camshaft engines with their SOHC straight eight Model A,
produced from 1921-1927.
Duesenberg also produced successful OHC race
car engines in the 1920s. The 1928-1937
Duesenberg Model J featured a
420 cu in (6.9 L) DOHC straight eight engine producing
Crosley COBRA engine with single overhead camshaft and two valves
per cylinder. The tower gear driving the camshaft is in the
Crosley, an American manufacturer of small, low-priced cars, used a
44 cu in (724 cc) 4-cylinder SOHC engine following
World War II. The original version of the engine was developed
during the war by Lloyd Taylor and built under license by
military applications, including as a powerplant for generators.
This design was made of steel stampings, copper-brazed together to
form a strong and light block assembly. It was known as the CoBra
engine ("COpper BRAzed") and generated 26.5 hp (19.8 kW) in
automotive form. The overhead camshaft was driven by a tower shaft and
gears.  The bi-metallic construction of the CoBra engine and
the salt-based antifreeze then in common use led to electrolysis and
corrosion problems. A more traditional design with a cast iron block
assembly (CIBA) was introduced in 1949; the components and dimensions
were otherwise the same as the CoBra. Used in Crosley's Hot Shot
sports car, this small OHC engine won the first race at Sebring in
Crosley OHC was popular in H-modified racing for its
high-RPM performance. After
Crosley ceased automotive production, the
engine was produced by several other companies for many years, its
last application being the Bearcat 55 outboard motor manufactured by
Fisher Pierce, builders of the Boston Whaler boats.
When DOHC technology was introduced in mainstream vehicles, it was
common for it to be heavily advertised. While used at first in limited
production and sports cars such as the 1925 Sunbeam 3 litre, Alfa
Romeo is one of the twin cam's greatest proponents. 6C Sport, the
Alfa Romeo road car using a DOHC engine, was introduced in 1928.
Ever since this, DOHC has been a trademark of most
Alfa Romeo engines
(some Alfa V6 engines are SOHC, not DOHC. Most Alfasud boxer engines
were also SOHC). The
Jaguar XK6 engine
Jaguar XK6 engine with chain-driven DOHC
was displayed in the
Jaguar XK120 at the
London Motor Show
London Motor Show in 1948,
and used across the entire Jaguar range through the late 1940s, 1950
Fiat was one of the first car companies to use belt-driven DOHC
engines in the mid-1960s.
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Aircraft piston engine components, systems and terminology
Capacitor discharge ignition
Electronic fuel injection
Aircraft engine starting
Mean effective pressure
Overhead valve engine
Time between overhaul
Propeller speed reduction unit
Flight data recorder
Fuel and induction
Auxiliary power unit
Ice protection system
Part of the
Cylinder head (crossflow, reverse-flow)
Starter ring gear
Pneumatic valve springs
Variable valve timing
Cold air intake
Electronic throttle control
Naturally aspirated engine
Short ram air intake
Variable-length intake manifold
Warm air intake
Gasoline direct injection
Stratified charge engine
Turbo fuel stratified injection
High tension leads
Electrics and engine
Air–fuel ratio meter
Automatic Performance Control
Car battery (lead–acid battery)
Crankshaft position sensor
Drive by wire
Electronic control unit
Engine control unit
Engine coolant temperature sensor
Idle air control actuator
Mass flow sensor
Throttle position sensor
Automobile emissions control
Diesel particulate filter
Antifreeze (ethylene glycol)
Viscous fan (fan clutch)
Cylinder head porting
Reciprocating engines and configurations
& number of cylinders
Inline / straight
V / Vee
Junkers Jumo 222
Cylinder head porting
Variable valve timing
Gasoline direct injection