Valve questions


#1

Just for my own education…

What is valve lash and how is it adjusted?

What does it mean to have a burnt valve and why does it happen?

Do camshafts open and close the valves? How does this mechanically differ on an OHC vs pushrod?

Just trying to learn. Thanks


#2

You can easily get your answers by searching the web. There is a ton of info there for you. Search “valve lash Wiki”, “burned valve Wiki” to get going.


#3

First, look here and here for a good explanation and images of how a poppet valve looks and works.

It’s essential that the (mushroom shaped) head of the valve rest firmly against the valve seat when it’s closed.
Two reasons for this.
First, obviously, to block the flow of gasses so pressure can build up in the cylinder. The mating surfaces have to match up to form a seal.
Second, in the case of the exhaust valve, when hot exhaust gasses are passing by when the valve is open the valve head heats up.
The valve stem carries away some of that heat, but it’s a relatively long distance between the outer rim of the valve head and the stem, so heat flow is limited.
When the exhaust valve closes heat flows across the mating surface to the valve seat, which stays relatively cool because there’s a coolant passage nearby.
The valve spring can press the valve firmly closed when the cam isn’t holding it open.
Also, when the combustion chamber is pressurized that gas pressure holds the valve closed even tighter.
That advantage is one reason poppet valves are universally used instead of other types of valves.

In order for the valve spring and gas pressure to do their job there has to be some mechanical slack when the cam isn’t pushing the valve open, that’s the valve clearance.
There has to be enough clearance to tolerate expansion/contraction with temperature, but too much clearance can make noise when the cam takes up the slack to open the valve and also create a hammering effect that can deform components.
Many modern cars have an element (hydraulic lifter) that automatically adjusts the clearance.


#4

Thanks.

So I’m assuming the advantage to 4 valves vs 2 is just the ability to move air and exhaust faster?

On a side note, does the fuel injector squirt directly into the cylinder or into a chamber where the valve will then open and release fuel into the cylinder? Im thinking its the latter bc a fuel injector would probably get all burnt up if it was exposed to the combustion chamber.


#5

New info quest - go to Wiki and look up ‘fuel injection’ and ‘direct fuel injection’. You have a good question, makers are now switching from injecting before the intake valve to injecting directly into the cylinder for better efficiency and hp. It’s been in use since WWII (Germans used it first, I think).


#6

Fuel delivery has gone through an evolution over the years, bringing the fuel delivery point closer and closer to the combustion chamber, and most recently delivering it in the chamber (direct injection).

First there were carburetors and later throttle body injection.
Fuel is introduced in the passageway common to all cylinders, about 6 inches to a foot away from the intake valve.
Under certain conditions, like when it’s cold, some of the fuel can drop out of the air stream and collect on the passage walls.
So the carb has an accelerator pump to deliver extra fuel to keep the engine from running lean.
Later that fuel can evaporate and make the engine run rich.
The layout of the intake system can also cause that evaporated fuel to be delivered un-evenly to the different cylinders
The distance and condensation makes it difficult to maintain a precise fuel/air mix, especially under changing conditions.

Later came multi-port fuel injection, which is what most engines currently use.
Each cylinder has its own fuel injector, located about an inch from an intake valve.
The fuel sprays against the back of the valve head, which is pretty hot, and the fuel quickly evaporates.
Fuel is usually sprayed while the valve is closed, to give the fuel time to fully evaporate before the valve opens.
Now the fuel quantity can be adjusted much more quickly under changing conditions.
It’s even possible to tailor delivery to each cylinder, but that’s not generally done AFAIK.

Most recently car makers are using direct injection.
Fuel is delivered directly inside the chamber, which is also how it’s done with diesels.
This brings the ability to time the injection and have the fuel un-evenly distributed within the chamber (stratified charge).
For instance there can be a rich mixture near the spark plug when it fires for easy ignition, and a lean mixture at the far end to ease the rise of temperature and pressure as the burn proceeds, which reduces the tendency to knock or create oxides of nitrogen (a pollutant).
Honda’s CVCC engines back in the '70’s had this goal.
One disadvantage of direct injection is that when the engine is cold liquid fuel can dribble down past the pistons an into the motor oil.
And yes, the fuel injectors have to be made of materials that can withstand the high combustion temperatures.


#7

Very interesting indeed!

Now, why havent they invented an injector that shoots both air and fuel directly in (computer controlled) allowing them to eliminate the intake valve all together.

Then eliminate the exhaust valve some how, then the cams. Less moving parts and problems.

I’ll call them AF injectors (air/fuel). They can be unbolted and swapped out at their inevitable failure just like an individual coil.

BAM! i’m gonna be a millionaire!


#8

I had to remove my fuel injectors while changing my cracked intake on my 98 ford mustang gt. Theyre connected to fuel rails and are right next to the spark plugs in the head. Im assuming they were using DFI by 1998 then.

Interestingly enough, one injector was cracked (factory original from 98). I swapped in a single new one to the tune of about $70


#9

Nope, those injectors went into the area before the valve. Ford is only now switching to direct injection.


#10

Oh ok. So mine would be considered multiport?


#11

The new Toyota/Subaru sports car even uses a combination of direct injection and injection just before the valve (called “port injection” because it fire into the intake port). It allows the engine to take advantage of the attributes of each.

But for every squirt of fuel you need the equivalent of about 14 squirts of air to create combustion. The process of combustion is the hydrogen and carbon atoms in the hydrocarbon (gasoline) tearing themselves apart and bonding to oxygen atoms. For that you need the oxygen, which comprises about 22% of the air. You need the air. Thus, you need a system to get it in the cylinder, and the best that’s yet been developed is intake valves.

For the record, that need for the hydrocarbon molecules to be in contact with oxygen to burn is the very reason fuel is vaporized. If you light gas on the ground, only the vapors and the very surface molecules burn. The gas under the surface actually doesn’t burn until the layer above it is gone, exposing it to oxygen.

And then you need to get the burned fuel (now gasses) out of the cylinder. Again, the best idea so far has been valves.

Although, there was a small design firm some years back that came up with a constantly rotating shaft with holes in line with intake and exhaust ports, like a string of ball valves, that replaced the traditional valvetrain and all it’s reciprocating masses. He developed it to a working prototype. I always wondered what happened to that design. I assume it wasn’t deemed durable enough long-term to put on the market.

By the way, I like the way you’re trying to figure out how things actually work. That’s my way of thinking as well.


#12

“Now, why havent they invented an injector that shoots both air and fuel directly in”

Hate to burst your bubble, but:
Fuel injectors ARE valves!
Such an air/fuel injector would have to move a large volume of air, and would need a passage as big as the current setup.
You’d also need a pump to inject the air, but why bother when the piston is already there to do a fine job of pumping air.
There have been attempts to eliminate the cam and operate the valves with electrical solenoids, much the way current fuel injectors work.


#13

It’s possible in theory at least for the valve operation to be replaced by electrically controlled solenoids. It’s a pretty tough environment to make a system like that reliable is one of the problems engineers would have to address. But there is merit to your idea. The engine looses some of its power to the force needed to overcome the valve springs. Might could improve mpg with solenoid actuated valves. Interesting idea.

A valve clearance measurement is an indirect way of determining if the valve is opening and closing the right amount. The reason valve clearance is measured as part of routine maintenance is that if the valve doesn’t open enough, the engine will lose power. On the other hand, if it doesn’t close tightly against the valve seat – the seat operates also as a heat sink for the valve – the heat from the exhaust gasses will not be dissipated away from the valve and the valve will overheat and may start to disintegrate.

A valve can fail to open and close correctly for two common reasons on overhead cam designs. The cam or cam follower can wear, which will cause the valve not to open all the way. This would measure as too much clearance. Or the valve and valve seat can wear away which prevents the valve from completely closing. This would measure as too little clearance. The second one is more problematic than the first as I understand from what the experts here say.