Could anyone explain if there is any real advantage of DOHC compared to OHC - i.e. directly operating the valves or via a rocker? Most cars don't rev that high (unlike motorcycles or racing cars) so is there really and advantage on a family type car such as my Primera 2.0SLX?
Both DOHC and OHC can act directly on the valves.
OHC = Over Head Cam DOHC = Double Over Head Cams
Basically it's two cams instead of one, usually found in 16 valve engines, although there are a few exceptions such as the 2.0 twin cam engine used in the Ford Sierra.
...you can also get OHC engines with 16 valves, such as some Honda engines and the Chrysler engine found in the Mini One, Cooper and Cooper S.
The cams are driven by a belt or a chain from the crank, and directly operate the valves by striking followers with cam lobes (fingers that protrude from the cam shaft).
Not sure of any benefits really other than a possible lighter valve train, putting less strain on the belt/chain and associated pulleys and tensioners. It also looks better B-)
The theoretical benefit of DOHC is that you can use a hemispheric chamber with less surface heat losses. The valves can be angled into the domed roof from each side.
So there ya go :)
Erm, wasn't the CVH lump named after it's hemispheric combustion chamber? - that only had SOHC and 8 valve.
It was a one off...
The message from snipped-for-privacy@italiancar.co.uk (SteveH) contains these words:
Yes, but they way they did it was a complete bastard. Very awkward lever angles, which is why the cam shafts didn't last long.
Not on a 4V per cylinder engine . Well unless you've done something very peverse with oval cylinders
Hence it needing rockers.
Commonplace with many engines from years ago using pushrods. Riley and Daimler. Plus countless motorbike engines.
DOHC makes it easier to have a wider angle between inlet and exhaust valves which allows a more optimal combustion chamber shape - HEMIspherical for instance - and bigger valve area.
The relative costs of complex valve trains with one cam and simpler valve trains with two cams are going to be fairly close in 4-valve designs, so why not...
A
Wasn't the Riley twin (high in block) cam?
The classic Chevy Hemi was cam in V pushrod...
...trouble is you cant put ports where the pushrods live which somewhat strangles the induction side.
Once you've gone to the bother and expense of putting one cam in the head there's little extra effort required to put another one in and lots of potential gain, not just in efficiency and performance, but engineering simplicity too - no rockers, straighter, easier to machine ports, simplified manifolding, etc...
A
Still a hemispherical chamber though, and the proper "Hemi" motors were 2 valve per cylinder pushrod motors.
Just to throw another oddity into the mix, the Triumph Dollopofshite Sprint was a 16v SOHC.
Was also a Dodge... well Mopar.
You can have a hemi chamber, or any other type of chamber, with angled valves without going to DOHC by the simple expedient of using rockers. The actual advantage of DOHC is a more direct acting valve train with stiffer and lighter components, less flex and lost valve motion and lighter valve springs which generate lower parasitic frictional losses.
However, the normal direct acting cam and bucket follower may still not be the absolute optimum choice although it is the most usual. Many high revving
4v motorbike engines use twin cams with forked pivoting followers whereby one cam lobe operates both valves on each side of the chamber. This results in an even lighter valve train with the further advantage of a rocker ratio greater than 1 to multiply the cam lift up to a higher valve lift. There is also significantly less weight and machining involved in producing the cylinder head because no cam follower bore structures need to be cast into the head and finish machined. A simple rocker shaft with support between each cylinder suffices to retain the pivoting followers. The disadvantage is that pivoting followers (as with the Ford Pinto) have a linear rather than rotary wear pattern and can suffer galling and wear at relatively low mileages if the metallurgy and lubrication isn't perfectly executed.For bike engines which usually rack up much lower mileages than car engines this isn't always a major problem.
-- Dave Baker
Sandy Nuts ( snipped-for-privacy@home.com) gurgled happily, sounding much like they were saying :
No, it wasn't.
Alistair J Murray ( snipped-for-privacy@fluffy.f.co.uk) gurgled happily, sounding much like they were saying :
There's been *umpteen* pushrod lumps (and OHC lumps) with hemispherical combustion chambers. Hell, even the 2cv flat twin.
But the "classic Hemi" was a Chrysler lump.
Plonk.
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