Piston valve Springs in diesel engine.

[omri3012]

Hallo,

I'm looking for explanation for the fact that in many diesel engines we have two springs supporting the piston valves such that one is exterior and the other interior and they have opposite helical direction. Also if anyone have a link or any information it would be very helpful.

 

[Ranger Mike]

diesel valves weight a lot more than the Internal combustion engine valves. for racing applications we run dual and in some cases triple springs. the valve spring is one of the most abused components in racing. fatigue causes failure and when the valve hangs "open" and is smacked by the piston the minimal damage is a stuck open valve. major damage happens when the valve head pops off and gets hammered to death by the piston. sometimes it puts a hole in the piston..huge smoke follows ..awesome show. I have seen cases there the piston gets so mangled , parts of it get tangled in the connecting rods and a rod is pooped thur the engine block..
diesels are low RPM applications with over 17 to 1 compression ratios. Normal IC are 8 or 9 to 1 and race cars 12 to 14 to one. the diesel needs very heavy duty (heavy being key word) parts to handle the high compression. the diesel is low revving but can run for HOURS so the requirement for very heavy duty (heavy being key word) parts again to handle heat. to counter the weight the valve needs the extra spring. the different wind direction is to ensure that they don't get tangled up with each other if things start to break in the valve train...i.e. retainers , keepers, valve stem damages etc..

 

[omri3012]

Thanks,

But what are the microscopic reasons for that the springs are in different coil direction, does it affect resonance or moments of inertia?

 

[Ranger Mike]

You have to make sure you have dual springs. You make have a single spring and a dmaper. The damper " goes solid" after the outer vavle reached max limit. The factor which causes unpredictable valve spring behavior at high reciprocating frequencies. It's caused by the inertia effect of the individual coils of the valve spring. At certain critical engine speeds, the vibrations caused by the cam movement excite the natural frequency characteristics of the valve spring and this surge effect substantially reduces the available static spring load. In other words, these inertia forces oppose the valve spring tension at critical speeds.

In a dual spring combination where the O.D. of the inner spring and the 1.0. of the outer spring nearly approximate each other so that there is a slight press fit between the 2 springs. This produces a dampening effect on valve spring vibration and surge.

OUTER VALVE SPRING: In a dual spring combination the outer valve spring is always the larger of the 2 springs. It's usually made from a heavier wire than that of the inner spring.

VALVE SPRING DAMPER: A flat wound spring coil inside the outer valve spring, which because of its rubbing contact on the inner surface of the coils, produces a friction-dampening effect on valve spring surge (harmonics).

other tricks are outer coating, and a lot of metalurgy to tech out best material..don't forget you can rev these babies 50 times a SECOND..the worst abused componet in a race engine!

 

[Bob S]

Coil springs have a tendency to twist end-to-end as they are compressed, and counter twist as they expand. Possibly using two springs with opposite helicity will minimize this.

 

[omri3012]

Thanks,

but can u explain me what are "end to end twist" and "counter twist"?

 

[Bob S]

Suppose you have a compressed spring with a right-hand helix. As the spring extends, there will be a slight left-hand end-to-end axial torque (rotation). See the posts in the following thread and the reference therin:
http://www.eng-tips.com/viewthread.cfm?qid=238479&page=8

 

[Gnosis]

Thanks,

But what are the microscopic reasons for that the springs are in different coil direction, does it affect resonance or moments of inertia?

The reason for winding the inner and outer valve springs in different directions is to eliminate any possibility that one spring could get caught between the other spring as they are being compressed. It therefore eliminates "coil bind" that could otherwise potentially destroy the engine in an instant.