How do decompression levers work?

[OffBalance]

Due to recent unfortunate events, I realized that my Grandpa's tractors and dozers have decompression levers, but my tractor does not. That got me to wondering, and since I am not near my Grandpa's place right now to investigate first-hand, I thought I would ask here:

Can someone explain to me the normal way decompression levers work? In other words, do they have additional valves in the head, shift the camshaft, or do something else funky with the injectors or otherwise?

Also, what is the main purpose of having them? Simply for easy starting? Somewhere I also heard that you can use them in case of a runaway engine but that seems fishy.

Enquiring minds want to know...

Thanks!

 

[mark777]

In simple basic terms: When the decompression handle is pulled back (on many models) it mechanically prevents the exhaust valves from seating, resulting in zero compression. And yes for ease of starting by faster engine rotation. It also provides an opportunity, on cold starts, for oil to lubricate entire engine before releasing decompression handle to fire under compression. Models without this decompression setup are usually equipped with glow plugs.

Hope this helps.

Mark

 

[OffBalance]

Perfect, thanks for the info!

 

[Soundguy]

To go along with marks good advice.. the part you heard about a decomp lever stopping a 'runaway' would 'defacto' have to work. Diesels are compression engines. if the exhaust valve stays open.. = no compression.. = no fire=no motive force.... engine has to stall out under load.

Situations like working near leaking propane or natural gas make for runaway situations on diesels. normally, when you throttle back, you cut fuel.. but with gas present in the air, the diesel can suck a burnable 'charge' mix righ via the breather.

Lots of mechanics when they test start an engine will have a choke plate handy near the intake for these reasons.. IE stuck open fuel.. or other problems.. etc.

Soundguy

 

[Paddy]

On my KAMA 454. there is a shaft inside the valve cover. When you pull on the knob it rotates the staft. On the shaft levers press on the exhaust valves over-riding the rocker arms to control the valve position.

When starting, I pull the De-copression lever the get her to roll over a few times. When I release the decompression knob, she fires evertime. This will save your starter as well!

Patrick