DOHC vs SOHC

musbiaggi

tak paham sangat la ttg sistem enjin ..boleh bincang2

zaujahzauji

dohc ada dua cam - double over head cam

sohc single cam - single overhead cam

cluesan

kalau tgk secara fizikal, mcm mana nak bezakan? klu ada gambar lagi bagus.

t@t

Plenty of people have been talking about how the Savvy only has an SOHC engine so it is inferior, and old technology. So I decided to post this entry so that people will be better informed on what SOHC or DOHC really is.

SOHC and DOHC are basically two different ways to configure the valvetrain. In the whole valve train setup there are camshafts, lifters, rockers, valves and springs. It is best that the valvetrain is light. Valves open at intervals of 25 times a second at 3000rpm. If the valvetrain is heavy, when the camshaft lifts it the valvetrain will just keep going up until the valve spring catches it. This is called valve float. You can overcome this with stronger (which means heavier) springs, but this extra weight increases the power requirements to open the valve.

The cams are driven by the crankshaft, using either a belt or chain called a timing belt or timing chain. If the timing belt/chain snaps and the camshaft stops spinning, the piston coming back up the combustion chamber might hit the open valves. This is very costly. That抯 why you should always change your timing belt/chain at the interval specified by your car manufacturer.

SOHC refers to Single Overhead Camshaft. In the days before DOHC, it was known as OHC, with no need to differentiate between a single or double camshaft. In SOHC, the camshaft is situated in the cylinder head, above the valves. The valves are opened and closed either directly with a shim between the cam lobe and the valve stem, or via a rocker arm. SOHC engine valve configurations typically have 2 or 3 valves per cylinder. It is also possible to have 4 valves per cylinder using SOHC but this translates into a complicated combination of rocker arms and cam lobe shapes. An example would be the 4G92P in my car which is a SOHC but has 4 valves per cylinder.

DOHC refers to Double Overhead Camshaft. This arrangement uses two camshafts in each cylinder head. Two cams per cylinder head means that a DOHC V engine has 4 camshafts because it has 2 banks of cylinder heads. This allows the manufacturer to easily implement a 4 valve per cylinder setup. Most of the time it also allows the engine to rev higher. It also allows better placement of the valves in an optimized setup that gives you maximum performance. But the disadvantage of such a setup is more weight, more cost and more complexity. It takes more stuff to drive two camshafts. The main reason to use DOHC is to drive more valves per cylinder. If a SOHC setup can allow 4 valves per cylinder, having a DOHC engine will not bring that much benefits over SOHC and the additional weight becomes a burden instead. DOHC engines also allows the spark plug to be placed right in the middle of the combustion chamber. This promotes efficient combustion. With SOHC, the camshaft is usually in the middle of the head because it has to drive both the intake and exhaust valves, robbing the sparkplug of it抯 optimal location.

In the end, a SOHC 16 valve engine would have better torque on the low end where the DOHC valvetrain抯 weight results in lower torque. But at high engine speeds, the 16 valve DOHC engine抯 peak torque and horsepower would be greater. That抯 the trade-off. With the amount of valves being equal, SOHC has better low-end torque because the valvetrain package is lighter while DOHC has better top-end power.

Other benefits of DOHC would be making it easier to implement variable valve timing technologies (which I will cover in another blog post) and also you can tweak it better with adjustable cam pulleys. If you were to put high-profile cams in your DOHC engine, the cam lobe profile can also be more optimized than a SOHC engine because you can play around with the lobe shape easier with separate camshafts for the intake and exhaust valves.

Why more valves per cylinder? Why not just make 1 huge intake valve and 1 huge exhaust valve? Bigger valves weigh more than the smaller one, so controlling the extra weight as it gets flung open and close becomes difficult. The spring has to be stiffer. A stiffer spring means more energy has to be spent overcoming the valve pressure. This partially oversets the gains which a bigger valve has to offer. Another problem with a single big valve is at lower RPMs the intake velocity will be lower. I抦 sure you guys know this

t@t

Overhead camshaft, commonly abbreviated to OHC, valvetrain configurations place the engine camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters in a more direct manner compared to overhead valves (OHV) and pushrods.

Compared to OHV pushrod (or I-Head) systems with the same number of valves the reciprocating components of the OHC system are fewer and have a lower total mass. Though the system that drives the cams may become more complex, most engine manufacturers easily accept that added complexity in trade for better engine performance and greater design flexibility. Another performance advantage is gained as a result of the better optimized 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 crossection and length.

The OHC system can be driven using the same methods as an OHV system, which include using a rubber/kevlar toothed 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 - or SOHC
    * Double overhead camshaft - or DOHC

t@t

Single overhead camshaft (SOHC)


A single overhead camshaft cylinder head from a 1987 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, while in a V engine or a horizontally-opposed engine (boxer; flat engine) there are two camshafts: one per cylinder bank.

The SOHC design has less reciprocating mass than a comparable pushrod design. This allows for higher engine speeds, which in turn will increase power output for a given torque. The cam operates the valves directly or through a rocker arm, as opposed to overhead valve pushrod engines which have tappets, long pushrods, and rocker arms 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 multi-valve 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 (4 cylinder, 8 valve); a small, early 1960s 2-door saloon car with a rear mounted 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 also used a directly actuated, SOHC parallel valve configuration with two valves for each cylinder. The Toyota system used hydraulic tappets while the Volkswagen system used bucket tappets with shims for valve lash adjustment. Of all valvetrain systems, this is the least complex configuration possible.

t@t

Double overhead camshaft (DOHC)


Overhead view of Suzuki GS550 head showing dual camshafts and drive sprockets.

A double overhead camshaft valve train layout is characterized by two camshafts located within the cylinder head, one operating the inlet valves and one operating the exhaust valves. Some engines have more than one bank of cylinder heads (V8 and flat-four being two well-known examples) 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. Although most more than 2-valve per cylinder heads employ DOHC, some manufacturers still managed to use a SOHC in 4-valve layouts. Honda for instance with the later half of the D16 family, this is usually done to reduce overall costs. Also not all DOHC engines are multivalve engines桪OHC 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.[1]



http://en.wikipedia.org/wiki/Overhead_camshaft

dainese

DOHC menang....

nik2121

uiks... ni abg mus dari MBC ke?

newzahari

Masing2 ada kelibihan masing-masing....

- SOHC = horsepower kurang dan x hi reving sgt tp tork datang lebih cepat,maksudnya enjin jenis ni tak payah tarik2 sgt la contoh: waja xcampro max tork die dtg pada putaran 1920rpm je.....

-DOHC = horsepower tinggi dan hi reving tp tork datang pada putaran yg tinggi...maksudnya enjin yg jenis ni kena tarik2 skit baru ada power contoh: saga campro 1.3 max tork die datang pada 3820rpm....

Squall-Leonhart

Ada lagi perbandingan selain tu... cylinder head untuk enjin SOHC lagi slim drp yang kat enjin DOHC sebab cuma perlu muatkan satu camshaft jer. So dier lagi seswai untuk jenis motor dengan ruang enjin yang terhad macam motor kapcai dan gak skuter. Kalau sports bike lak lebih seswai pakai DOHC, bukan jer pasal high overall power dgn high-end torque jer tapi pasal ruang yang dioptimizedkan untuk enjin DOHC. Ruang enjin sports bike ni kureng seswai nak bubuh enjin SOHC sebab ada ruang besar2 tapi tak dimanfaatkan (ikut teori haku laa)   ...