Lighter valve springs for better mpg?

[rbrowning]

When I worked for Associated Spring we did a lot of work on reducing the drag that the valve train put on the the engine. But the spring was the rear end of the weight loss program, not the front end. Lightening all of the other components came first, the valves, rocker arms, then the springs. Lightening the other components meant that the spring had to do less work to turn an opening valve into a closing valve.

The "beehive" springs were more a harmonic problem solver than a weight saver. Springs can do funny things at different operating speeds, and it usually isn't pretty for the engine owner. By lowering the reciprocating mass it lowers the stress at the keeper end. The beehive shave also allows you to operate a little closer to solid height before you get into an overstress condition because the reduced diameter gives a little more clearance on the end coils, but not much.

Ovate wire can also allow you to reduce the concentrated stress levels and spread the stress levels around better and theoretically you could reduce the mass of the spring, but watch your manufacturing costs go way up. Keeping the ovate orientated can be a challenge. Performance car drives aren't too concerned but I doubt you will find this in an economy car.

[Christ]

The general consensus, from what I gather, is that most of the work generated to push the spring down is just returned when the spring pushes back against the rocker, helping to turn the cam around.

I don't necessarily subscribe to this belief, for a few reasons that I hold personally, but that's not for this thread.

Regardless, you can turn the cam by hand, it doesn't even take more than a few lb/ft of force to turn the cam against the springs, so there's not enough loss there to really worry about.

[dcb]

what isn't returned is the momentum of the valves (and lifters and pushrods and rockers as applicable and some of the springs mass) as those parts are reciprocating. There isn't much there though in a shimmed OHC, up to .05hp order of magnitude by my swag.

[gone-ot]

...ferro-magnetic fluid coupling anybody? ...being controlled by DC-current, producing a modulated magnetic field, under computer control.

[dcb]

it would be nice if it were a bolt on, needs a good position sensor though, and its own brainbox, but is doable IMHO.

I would like a system where the valves stayed closed when the power is off though, so the piston doesn't bend them accidentaly when the power/brain is off. but computer control of each valve would open up some huge efficiency potential (and ugh, yes, performance), would make a vtec look like a flathead.

but some old guy and his lawyers probably already thought of it and locked it away in a patent till their demands are met. A bit of reverse-ageism built into that system.

[gone-ot]

...nice thing about a magnetic "field" is that it can both PUSH as well as PULL, so a small light spring could hold the valves "closed" while the ferro-magnetic control could override the spring to both OPEN and CLOSE the value on demand...including its opening/closing profile.

[Christ]

Dave -

Part of the reason I don't necessarily agree that the pressure from the springs is returned to the cam is that it's quite easy to turn rotation into reciprocation, but there isn't a direct link to re-exchange the energy from reciprocation (valve closing) to the rotation of the cam. Sure, you're applying pressure from the lifter/rocker against the cam, but alot of energy pushing downward on the cam isn't efficient when you're trying to spin the cam.

I'd be more than happy to review a different opinion, though.

[dcb]

there are actually engines with cam based crankshafts, dunno how efficient aside from less than 100% , but if you manage the side loads without too much drag it works (rollers probably help too).

Internal-Combustion Cam Engines.

seeing this reminds me that "Desmodromic" valves also address the spring bias problems, but it adds weight and manufacturers prefer a lighter valvetrain (you can use lighter springs and get more energy back from them that way).

[Christ]

Originally Posted by dcb

there are actually engines with cam based crankshafts, dunno how efficient aside from less than 100% , but if you manage the side loads without too much drag it works (rollers probably help too).

Internal-Combustion Cam Engines.

seeing this reminds me that "Desmodromic" valves also address the spring bias problems, but it adds weight and manufacturers prefer a lighter valvetrain (you can use lighter springs and get more energy back from them that way).

Rotary engines work that way, too, but not like the illustration you provided.

There is an ellipsis lobe on the crankshaft (called an eccentric shaft) on which the rotors act to create rotational inertia (torque).

They make a similar type of contact as a cam does with it's followers, except that the rotor is always side-loading the ellipsis, putting the most force at it's most susceptible tangent, whereas the cam's follower is not.

[dcb]

Originally Posted by Christ

...but there isn't a direct link to re-exchange the energy from reciprocation (valve closing) to the rotation of the cam.

All I'm saying is that you can "push" a cam to some degree and get energy return from the springs. when you get towards the base circle of the cam I'm sure it becomes a bit like trying to drive a worm gear in reverse, but not a lot of reciprocating going on there. And when the rpm goes up (and/or you have big clunky valvetrains) the momentum of the reciprocating valve/follower/etc increase and you need sufficient spring tension to hold that mass to the cam under the anticipated operating conditions.

If you are never, never going to exceed 1/2 redline, then maybe you can get away with springs of 1/2 tension. If you lighten the reciprocating components, you can reduce the tension. You also have to pay attention to spring harmonics when designing valve springs FYI