In recent weeks, we’ve talked about several key components of industrial engines, namely the crankshaft and camshaft. As you’ll recall, the crankshaft and the camshaft are connected by a timing belt, and it is the job of both components to convert rotary motion to linear motion and vice versa, as well as to assist the other components in helping the engine develop power.
This week’s tip focuses on the one of these “other” components – the valves.
In discussing valves, we will focus primarily on the four-cycle gasoline and diesel engines that are found in most on-highway and off-highway vehicles. So, we’re not talking about two-cycle engines. A valve is a device that allows the air and fuel mixture into a cylinder and spent exhaust gases out. There are two types of valves in a four-cycle internal combustion engine, inlet (air/fuel mixture) and exhaust (spent gases).
The anatomy of a valve is fairly simple. As shown below, it is primarily composed of a stem and head. The head is domed shaped to help deflect pressure.
Valves are usually installed at a 30- to 45-degree angle to the crankshaft. This facilitates engine breathing and combustion. They also have heads of differing diameters with the intake valve typically larger than the exhaust. Valves are typically of one piece design, made from austenitic steel and are hardened with a strip of flashing applied to the seat face to help improve seating and reduce wear. Also of importance is the seated clearance of the valves as this ensures maximum power development and allows for valve expansion in the seat to minimize wear.
During engine operation, a valve is normally closed, kept so by a spring. As the cam rotates the valve opens. For intake valves, this allows the air fuel mixture to enter the cylinder. Once the mixture is in the cylinder, the cam continues to rotate, and the intake valve closes and combustion occurs. Following combustion and during the exhaust stroke, the cam opens an exhaust valve, allowing the spent gases to escape. Once all the spent gases have left the cylinder cam rotates further and the exhaust valve closes.
The amount of time a valve is opened or closed is dependent upon the shape of the actuating lobe on the camshaft. The larger the lobe, the longer then valve is open and the more air/fuel or exhaust may be handled, increasing horsepower.
Valves may be actuated by pushrods, tappets or rocker arms and this is usually determined by the location of the camshaft. An overhead cam engine may actuate a rocker either directly or through a tappet. A cam imbedded in the block or head, may use hydraulic actuators and pushrods to move the valves.
The proper lubricant for engine valves is the oil used to lubricate the engine, typically a multi-weighted engine oil. By engine design, the oil is moved under pressure from the crankcase by the oil pump, through ports and galleys at the top of the head, an area just inside the valve cover. The lubricant then flows along the head to the valve stem(s) and once there, flows down the stem, lubricating the stem and guide.
If the valve stem and guide clearance is set correctly, a proper lubrication balance will be achieved, meaning there will be little or no oil consumption. If the guide and/or stem clearance is excessive, oil will flow into the exhaust stream, fouling the seat impacting engine performance. This tendency may be measured by oil consumption. Think “blue” smoke.
I hope this tip was helpful, and let us know if you have any questions by leaving a comment below!
Younes: In a car, valve wear has many causes. It really depends on the mode of wear identified. If it is a seating issue, failure could be the result of deposit built up or stem wear. If oil is leaking down the stem, it could be guide wear or a bad stem seal. Lack of lubrication - clogged oil ports.. If the valve won't open - it is most likely not a valve problem but a lifter or acuator issue. If you can provide a picture of the valve or head I might be able to give you better guidance.