valve (thing)

Valves are vital to the internal workings of any four stroke internal combustion engine. They allow air/fuel into, and exhaust gas out of, an engine cylinder. An internal combustion engine has one or more cylinders, where a fuel/air mixture is detonated by a spark plug to create motion. The fuel/air mixture is sucked in through a valve, and exhaust gases are pushed out through another valve. Each cylinder has two or more valves.

I'll ignore how the valves are opened and closed for this writeup (partly because some of it's already done for me); elves are pushing them around or something.

Some understanding of how valves relate to the inner workings of engines would probably be useful here. I'll not go into too much detail of how a four-stroke engine works, suffice it to say that it contains one or more cylinders (most commonly there are four), each of which contains a piston. The pistons produce power by going through what's called the ottoman cycle (assuming some basic knowledge of engines here):

1. The piston starts at the top of the cylinder bore and descends. As it does, an inlet valve at the top of the cylinder (the cylinder head) opens to allow the air/fuel mixture into the cylinder. The descent of the piston creates a vaccuum effect that sucks the mixture in. This is the intake stroke.
2. The piston reaches the bottom of the bore. The inlet valve closes and the piston travels back up the bore, compressing the mixture trapped inside the cylinder. This is the compression stroke.
3. The piston reaches the highest point of its travel and the air/fuel mixture is at maximum compression. At this point a device on the roof of the cylinder creates a spark, which detonates the air/fuel mixture. The explosion pushes the piston back down the bore and ultimately drives the crankshaft (which the piston is connected to). This is the power stroke.
4. The piston reaches the bottom of the bore again. The cylinder is now full of waste gas left over from the explosion. This has to go. An exhaust valve at the top of the cylinder now opens and as the piston rises again it pushes the waste gas out through it. This is the exhaust stroke.

A single valve is a trumpet-shaped piece of solid or hollow metal which sits in a small bore in the cylinder head. The exhaust valves in some cars are filled with sodium to help them cool.

xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|__/  \__|xxxxx
xxxxx|--------|xxxxx

     CYLINDER
(combustion chamber)

When the valve opens it descends into the cylinder, allowing gas to flow past it:

xxxxx|   ||   |xxxxx
xxxxx| G || G |xxxxx
xxxxx|   ||   |xxxxx
xxxxx| A || A |xxxxx
xxxxx|   ||   |xxxxx
xxxxx| S || S |xxxxx
xxxxx|   ||   |xxxxx
xxxxx| F || F |xxxxx
      L  ||  L
  W O __/  \__ O W
      --------

If any readers have ever experienced something like a cambelt failure it might be clearer now why such damage can occur (repair bills of thousands are not unheard of). See, an open valve is directly in the path of the piston. It is only the exquisitely precise and ingeniously self-regulating timing of an engine that prevents the pistons and valves from colliding.

In older engines, the fuel/air mixture was not compressed as much (the bores were longer or the pistons were shorter), so piston travel did not overlap with the valve travel. It was impossible for the piston to hit any valves on the upstroke, even if they were open. If the timing belt failed in an engine like this, the engine would just stop running.

More recent and current engines use much greater compression. The piston travels further up the bore (which may also be shorter) and does overlap with the valve travel. So, if the valves stop operating for any reason (because of a cambelt failure, for example) and any are left protruding into the cylinder head, they're going to get smacked by the piston as it rises. This is why cambelt failure is so bad. Pistons move fast. Although very small parts, and frequently quite cheap themselves, valves are critical and expensive to reach if damaged. They can also take other parts with them like valve guides, the cylinder head, the piston and in severe cases maybe even the crankshaft, camshaft(s) and hydraulic lifters.

There is at least one valve each on inlet and exhaust duties in a four-stroke engine. The majority of modern four-cylinder engines of two litres and above have two inlet and two exhaust valves per cylinder. This allows more air/fuel into the cylinder at once and allows exhaust gases to escape more efficiently. There are also some 12-valve engines that have one inlet and two exhaust valves per cylinder, presumably in an attempt to compromise between the performance of a 16-valve engine and the economy of 8-valve engines (it is commonly small hatchback cars that have this sort of engine).

There's a bit more to how valves work - they are opened by force but have to close themselves. I won't get into how they're opened; suffice it to say they are pushed downwards from above at precise intervals. The diagram below is an exploded view of the valve assembly from my car's engine, which I imagine is similar to that of most engine designs:

xxxx|     ___     |xxxx
xxxx|    |___|<--------- F
xxxx| ___________ |xxxx
xxxx||___________|<----- E
xxxx|         ___ |xxxx
xxxx| ___-----___||xxxx
xxxx||___-----___ |xxxx
xxxx| ___-----___|<----- D
xxxx||___-----___ |xxxx
xxxx| ___-----___||xxxx
xxxx||___-----___ |xxxx
xxxx| ___-----___||xxxx
xxxx||___-----    |xxxx
xxxx|     ___     |xxxx
xxxx|    |___|<--------- C
xxxx| ___________ |xxxx
xxxx||___________|<----- B
xxxx|_____   _____|xxxx
xxxxxxxxxx| |xxxxxxxxxx
xxxxxx           xxxxxx
           _
          |-|
          |-| <--------- A
          |-|
         \/\/\

This whole assembly sits in a small bore in the cylinder head (x). There is a circular hole at the bottom of this that leads to the combustion chamber. The valve stem goes through there, and the valve seat is a short distance below.

• A: Valve stem - the thin end of the valve. It has a thread on the end of it. It slots through the rest of the components and F is screwed onto it.
• B: Valve spring washer - this sits on the bottom of the valve bore and provides something for D to bear on.
• C: Valve stem oil seal - this stops oil from the top of the engine (camshafts, tappets/hydraulic lifters etc) from getting into the combustion chamber. You don't want oil down there.
• D: Valve spring - this pulls the valve back up into the cylinder head when the pressure from above has ceased. It is very strong.
• E: Valve spring seat - this is sort of a 'bowl' that the top of the valve spring sits in. It is held in place by:
• F: Valve stem key - this screws onto the thread on the top of the valve stem. It holds the assembly below it in place.

So, when the valve assembly is pushed downwards from above, it compresses the valve spring, which is ultimately attached to the top of the valve stem. When the pressure is released, the valve spring immediately expands (they are very strong springs, remember?) and pulls the valve back into its seat in the cylinder head.

Just as an aside, the ultimately limited speed at which valve springs are able to close an open valve is one of the factors that limits the maximum revolutions per minute of overhead valve engines. Formula 1 engines use a method of electromagnetic valve actuation, allowing potentially insane engine speeds.

• My car. Bless.
• Vauxhall Motors Ltd & Bell and Howell PSC; "EPC Electronic Parts Catalogue"
• Haynes Publishing; "Vauxhall/Opel Omega"; "General engine overhaul procedures"