OVERHEAD CAMSHAFT ENGINE
* INTRODUCTION :-
OHV means OverHead Valve –
An engine design where the camshaft is installed inside the engine block and valves are operated through lifters, pushrods and rocker arms (an OHV engine also known as "Pushrod" engine).
Although an OHV design is a bit outdated, it has been successfully used for decades. An OHV engine is very simple, has more compact size and proven to be durable.
* Classification :-
Single OverHead Cam
Double OverHead Cam
Pushrod Engines
* Single OverHead Cam
Single overhead camshaft (SOHC) is a design in which one camshaft is placed within the cylinder head.
In the SOHC engine the camshaft is installed in the cylinder head and valves are operated either by the rocker arms or directly through the lifters (as in the picture).
The cam actuates rocker arms that press down on the valves, opening them. Springs return the valves to their closed position.
These springs have to be very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms.
Advantage:-
Valves are operated almost directly by the camshaft - easy to achieve the perfect timing at high rpm. Also it's possible to install three or four valves per cylinder.
Disadvantage -
An OHC engine requires a timing belt or chain with related components - more complex and more expensive design.
* Double OverHead Cam
A double overhead camshaft valve train layout is characterized by two camshafts located within the cylinder head, one operating the intake valves and one operating the exhaust valves.
Some engines have more than one bank of cylinder heads and these have more than two camshafts in total, but they remain DOHC.
The term "twin cam" is imprecise, but will normally refer to a DOHC engine.
Also not all DOHC engines are multivalve engines—DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared.
Today, however, DOHC is synonymous with multi-valve heads since almost all DOHC engines have between three and five valves per cylinder.
DOHC or Double OverHead Cam –Today the setup is used in many cars.
Since it's possible to install multiple valves per cylinder and place intake valves on the opposite side from exhaust vales.
DOHC engine can "breath" better meaning that it can produce more horsepowers with smaller engine volume.
The main reason to use double overhead cams is to allow for more intake and exhaust valves.
More valves means that intake and exhaust gases can flow more freely because there are more openings for them to flow through. This increases the power of the engine
Compare: The 3.5-liter V6 DOHC engine of 2003 Nissan Pathfinder has 240 hp, similar to 245 hp of the 5.9-liter V8 OHV engine of 2003 Dodge Durango. Pros: High efficiency, possible to install multiple valves per cylinder and adopt variable timing.Cons: More complex and more expensive design.
* Pushrod Engines :-
Like SOHC and DOHC engines, the valves in a pushrod engine are located in the head, above the cylinder. The key difference is that the camshaft on a pushrod engine is inside the engine block, rather than in the head.
Working :-
The cam actuates long rods that go up through the block and into the head to move the rockers. These long rods add mass to the system, which increases the load on the valve springs. This can limit the speed of pushrod engines; the overhead camshaft, which eliminates the pushrod from the system, is one of the engine technologies that made higher engine speeds possible.