Cam questions - getting my head round them

[obi_waynne]

Here are a few questions that I think I know the answer to but I am now starting to question this understanding.

How many revolutions does a cam make in relation to one crank revolution?

Does a SOHC and DOHC setup change the above at all?

If a cam has a duration of 200 degrees does this mean the valves are open or closed for the 200 degrees of rotation?

Does the cam lobe open or close the valves?

 

[HDi fun]

Camshafts rotate one turn for every two of the crankshaft. Single or dual per cylinder bank makes no difference to this.

If a cam has 200 degrees duration this means that the valve or valves it controls are open for 200 degrees of CRANKSHAFT rotation. Since the camshaft rotates at 1/2 of crank speed the lobe on the camshaft would only need to have 100 degrees duration to achieve this.

The lobe opens the valve. They are closed by spring pressure. Kawasaki has an interesting and, as far as I know, unique variant where the valves are actively closed by the cam as well.

 

[obi_waynne]

Thanks HDi I thought it was 720 degrees of cam per crankshaft so phew, but I thought the lobes closed the valves! Thanks for clearing that up for me. Talk about back to basics!

 

[claymore]

The lobe opens the valve. They are closed by spring pressure. Kawasaki has an interesting and, as far as I know, unique variant where the valves are actively closed by the cam as well.

Isn't that what the desmotronic does on the ducati?

 

[Loz]

I never fully grasped how cams worked either, thanks for the explantaton HDI :bigsmile: I do know that the next big thing is engines that will have cams that are electronically actuated, completely negating the need for a cam shafts. The Ducati system looks interesting, I wonder why it's never caught on?

 

[Loz]

Here's another cam question, what's valve overlap?

 

[pgarner]

valve overlap is where both the intake and exhaust valves are open at the same time.

the ducati system never caught on due to the patents on it

 

[pgarner]

copied from my instructional book for teaching the cadets proulsion

To describe the modified valve and ignition timings, the following terms are
used:

a. “Valve lead” means that the inlet valve opens before the piston has reached
TDC, and that the exhaust valve opens before BDC.

b. “Valve lag” means that the inlet valve closes after the piston has passed
BDC and that the exhaust valve closes after the piston has passed TDC.

c. “Valve overlap” means that both inlet and exhaust valves are open together.
It is a period when the inlet valve opens before TDC and the exhaust valve
does not close until after TDC (see Fig 1-7).

d. “Ignition advance” describes the fact that ignition occurs before the piston
reached TDC on the compression stroke.
(Note that timings of all these events (ie ignition, and the valves opening or closing)
are all measured in degrees of crankshaft rotation - eg “120 before TDC” etc.)


12. Increasing the efficiency of the engine by using valve lead, lag and overlap
is made possible by a natural feature of the piston engine called “ineffective crank
angle”. This means that near TDC and BDC a fairly large rotation of the crankshaft
causes only a small linear movement of the piston.


13. The valves are spring-loaded into the closed position and are opened



THE PISTON ENGINE
by a mechanism geared to the crankshaft. The one shown at Fig 1-4 is a camoperated
pushrod, but there are other methods. The mechanism operates at half the crankshaft
speed so that each valve opens and closes only once for each two revolutions of
the crankshaft. As shown earlier the valves are timed to open and close at special
positions of the crankshaft and piston, measured in degrees of crankshaft rotation.
For example, in Fig 1-7 the inlet valve leads 300 before TBC and the exhaust valve
lags 150 after TDC. The inlet valve lead ensures that the maximum charge of mixture
enters the cylinder on the induction stroke, and the exhaust valve lag ensures
maximum scavenging of the burnt gases on the exhaust stroke.

please not that this is copyrighted by Air Training Corp -RAF Cranwell

 

[HDi fun]

Isn't that what the desmotronic does on the ducati?

Thankyou for the picture and confirmation of the trademark. That's exactly what it is.

Electrically operated valve gear is being trialled by some engine makers with seemingly good results. I think the problems they're experiencing are magnetic reluctance issues with fields that rapidly change direction in close proximity and very quickly.

Perhaps piezo actuator transducers could be used?

 

[Sid447]

.............If a cam has a duration of 200 degrees

does this mean the valves are open or closed for the 200 degrees of rotation?

There's two ways of quoting cam details.
1] Advertised or Total duration.
2] Duration @ 0.050" or 1.27mm

Advertised duration is usually taken from .006" tappet lift (most manufacturers, afaik) or .004" duration.

In an attempt to have a standard (to compare apples with apples) .050" tappet lift duration has become pretty universal in N.America and Aus.

In Europe, most specs seem to be total or advertised duration figures.

e.g. An E30 BMW 325i used a 264* cam (total duration).
where-as a 7-litre GM V8 uses a 211* cam (0.050" figures quoted only).

 

[obi_waynne]

I didn't realise that different areas used different standards. Thanks for that Sid