Engine Components : Valves & valve trains

Valves & valve trains:

• Valves
•  Valve seats
•  Valve seats in cylinder heads
• Valve rotation
•  Valve stem oil seals
•  Intake valves
• Valve trains 
• Valve-timing diagram
•  Variable valve timing

Valves

Gasoline engines must control the flow of combustible mixture they take in, and when it goes in.
In small 2-stroke gasoline engines, this is done by ports that are opened and closed by the piston skirt during the engine cycle, or by pressure-operated reed valves.
Diesel engines are different. Their power and speeds are controlled by the amount of fuel injected, so it isn’t necessary to control airflow into the intake manifold.
Almost all 4-stroke gasoline and diesel engines use valves, which are located in the cylinder head. This is also true for 2-stroke diesel engines that are used in road vehicles.
Valves experience enormous stress even in normal conditions. In a 4-cylinder car driven at around 90 kph, each valve opens and closes about 30 times a second. Exhaust valves withstand huge temperatures and they can become red-hot.

A valve must not soften at high temperatures. It needs good hot strength to stand up to being forced against the seat, and to prevent tensile failure in the stem. It needs good fatigue properties to overcome cracking.
Various surface treatments are used to help the valve resist wear, burning and corrosion.
Inlet valves are made of steels mixed with chromium or silicon to make them more resistant to corrosion, and manganese and nickel to improve their strength.
Exhaust valves are made of nickel-based alloys.
Some high performance applications use especially hard-wearing titanium alloys.

Valve seats

A poppet or mushroom valve has 2 main parts, a stem and a head. It fits into a port in the head. Its face makes a gas-tight seal against the seat.
During operation, the head near the face of the valve transfers heat to the seat. Some is conducted up into the valve stem. The stem transfers heat on to the guide, so the stem is the valve’s coolest part.
The valve seat and guide are also cooled by coolant in passages around the valve ports.
When a valve does not seat properly, there’s a smaller area where heat transfer can occur. That means the face will overheat. Local hot spots can reach such extreme temperatures that the edge of the valve can actually burn.
The width of the valve seat is important. A narrow seat is desirable because a thin circular contact with the valve face forms an efficient seal.
But a wider seat is better for transferring heat from the valve to the cylinder head.
A common compromise is for the inlet valve to have a narrower seat than the exhaust valve

Valve seats in cylinder heads

In some cast-iron cylinder heads, the seats are cut directly into the edge of the valve port. These valve seat areas are machined from the metal of the cylinder head. In some engines, the valve seat area is hardened during manufacture.
In others, hard metal valve seat inserts are pressed into the machined holes. Valve seat inserts are metal rings that match the shape of the valve. They are usually made of an iron alloy. They are used in aluminium cylinder heads to provide a sealing surface for seating the valve.
Leaded fuels leave a deposit on the valve that protects the valve seat. With unleaded gasoline, however this deposit doesn’t occur, and all cast iron heads used with unleaded gasoline have hardened valve seats.
The faces of the valve are ground at an angle of 45 degrees or 30 degrees. Some engines use 30 degrees or 45 degrees face angles for inlet valves, and 45 degrees for exhaust valves.
Valve seats are often ground to the same angle as the valve face, but they can differ. The difference is called an interference angle. An interference angle allows for a quick bedding-in of the valve face to the seat on new engines. It may also allow for slight changes in angle as a valve heats and expands.

Valve rotation

As the valve opens and closes, it has a natural tendency to rotate, very gradually, so that it keeps seating in a new place. This produces a slight wiping action which helps keep the face and seat free of carbon. It also helps prevent sticking in the valve guide and distributes heat around the valve seat.
On some diesels, the inlet valve has a shroud or mask on the back of the valve head. This is designed to cause turbulence in the incoming air. The position of this mask is critical for best operation, so the valve is pinned to prevent it rotating. If the rocker arm is slightly offset to the valve stem, it can help this rotation.
Some engines even use positive valve rotators. The valve operates in a valve guide and it is exactly concentric with the valve seat. The valve guide is the hollow cylindrical part in which the valve stem moves.
The valve guide area can be machined from the metal of the cylinder head, or holes can be drilled for pressed-in guides. Cast-iron guides are necessary in aluminium-alloy heads to provide a suitable bearing surface for the valve stem.
Many heads use replaceable valve guides that are a form of metal bush pressed into holes in the cylinder head.
Other cylinder heads have guides cast as part of the cylinder head, then bored to the size of the valve stem during manufacture.

Valve stem oil seals

Oil seals are fitted to the valve stems or the guides on both intake and exhaust valves. They prevent too much oil passing down into the combustion chamber.
The coil spring on the outside holds the sealing edge against the valve stem. The angle at the top of the seal forms a small reservoir of oil to lubricate the stem and guide. If there is too much oil there, carbon deposits form in the port and on the valve head.
Umbrella seals shed the oil and keep it away from the end of the valve guide. Worn seals or guides or too much valve-guide clearance will let oil pass the intake valve.
The inlet valve is more likely to pass oil through its guide than the exhaust valve. This is because of the low pressure in the inlet port that draws in the oil.
The exhaust valve can still have problems because of exhaust pulsing. This creates a low pressure area behind the gases, which can cause oil to pass down the valve guide.
Some engines however don’t use oil seals on their exhaust valves.
Valves are normally held on their seats by 1 or 2 coil springs that are compressed between the cylinder head and a retainer on the valve stem.
The spring retainer is held on the end of the valve stem by conical shaped collets. Collets are also known as cotters, keepers or keys. The springs usually have their coils closer at the bottom than the top. This makes different parts of the spring vibrate at different frequencies, and prevents wasteful valve spring vibration. They can also be made of wire with an especially shaped strong section that limits valve bounce.
Unless the valve is held on its seat, it also allows leakage from the combustion chamber. Carbon builds up on the valve stem.

Intake valves

Intake valves pass only air or air-fuel mixture so they run at much lower temperatures than exhaust valves.
They are usually larger than exhaust valves because the pressure forcing the charge into the cylinder is much lower than the pressure forcing the exhaust gases out of the cylinder. Exhaust gases under pressure need much less space.
Different engines use different valve combinations.
Having more than 1 inlet valve provides better breathing. An additional inlet valve allows larger inlet passages and a freer flow into the cylinder, so the engine receives a better charge.
Similarly, two exhaust valves mean the cylinder can be designed with larger exhaust ports, which provides a freer flow of exhaust gases out of the cylinder.

alve trains

The valve train includes all of the components that are driven from the camshaft to the top of the valves. There are different types of valve trains, depending on how many camshafts there are, and where they are located.
In an overhead valve or pushrod system the valves are in the cylinder head, but the camshaft is in the block near the crankshaft. A valve lifter or tappet rides on the cam. As the cam lobe reaches the lifter, it rises, transfers the motion to the pushrod. This then moves a rocker which in turn pushes the valve open.
There are different kinds of lifters. A solid lifter is usually a hollow, cast iron cylinder mounted in a bore in the crankcase. It is free to rotate slowly, which distributes wear from the cam over the face of the lifter.
The gap between the valve tip and the valve train is called valve clearance or valve lash. This must be maintained when the cam is not applying pressure to open the valve. It can be adjusted with a screw and locknut built into the rocker arm. These adjustments are needed regularly.
Many engines now use hydraulic valve lifters. Their purpose is to make the engine quieter and eliminate the need for valve clearance adjustment. When the engine is operating, oil under pressure from the engine’s lubrication system is supplied to the lifter.
The oil is assisted by spring tension to maintain zero valve clearance but through a system of valves it is trapped in the lifter as the camshaft lifts it. Since oil is not compressible, the lifter acts like a solid lifter. When the valve is closed, any oil lost during the previous lift is replaced, and zero valve clearance is maintained.
Rocker arms transfer motion to the valves. The rocker arm rocks up and down using a pivot mechanism. Some rocker arms are made of cast steel or aluminium alloy. Others are a steel pressing.
Hydraulic valve lifters usually use stamped, or pressed, sheet metal or cast aluminium rocker arms.

Valve-timing diagram

To see how valve-timing works in a 4-stroke engine cycle, let’s show piston motion as a circle. In this simple cycle, each stroke is shown as a semi-circle.
This intake valve opens at top dead center, and closes at bottom dead center. The blue line shows that period and it matches the intake stroke.
The exhaust valve opens at bottom dead center, then closes at top dead center before the new air-fuel mixture enters the cylinder.
In practice, the intake valve usually opens earlier than top dead center, and stays open a little past bottom dead center.
The exhaust valve opens a little before bottom dead center, and stays open a little past top dead center.
When the valves actually open and close, can be measured by angles. To make these angles easier to read, let’s use a spiral instead of a circle.
This intake valve opens 12° before the piston reaches top dead center.
And it closes 40° after bottom dead center.
The exhaust valve opens 47° before bottom dead center - and stays open - until 21° past top dead center. This gives exhaust gases more time to leave.
By the time the piston is at 47° before bottom dead center on the power stroke, combustion pressures have dropped considerably and little power is lost by letting the exhaust gases have more time to exit.
When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called lead.
When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag.
On the exhaust stroke, the intake and exhaust valve are open at the same time for a few degrees around top dead center. This is called valve overlap. On this engine, it is 33°.
Different engines use different timings. Manufacturer specifications contain the exact information.

Variable valve timing

Valve overlap is the amount of time the intake and exhaust valves are both open at once. Less overlap produces a smooth idle and more slow speed torque, but poor high speed performance because there is not enough time for complete scavenging to occur. More valve overlap allows better engine breathing at high speeds, but poor performance at low speeds, rough idling, and higher exhaust emissions.
Engines with fixed valve timing can only operate most efficiently at one specific speed. Engines that can vary valve timing and/or valve lift can operate efficiently at a wider range of speeds, and deliver better performance at high speeds, with a flatter torque curve.
There are two types of variable valve timing, or VVT – cam phasing and cam changing.
Cam phasing VVT varies valve timing by shifting the phase angle of the camshaft. At high engine speeds, the inlet camshaft phasing can be rotated in advance to enable earlier intake, increasing the amount of valve overlap. This is controlled by the engine management system, and actuated by hydraulic valve gears.
Phasing change is either continuous or fixed. Continuous systems normally vary the phasing angle between 0 and 40 or more degrees according to engine load and speed requirements. Fixed phasing systems alter phasing by a specific angular value at a specific speed and load condition.
Single overhead camshaft engines can use cam phasing. However, double overhead camshaft engines can receive greater benefit from phasing change VVT as the intake and exhaust camshaft can be controlled separately.
Some manufacturers choose to alter phasing on both intake and exhaust camshafts, but it's also common that that only the inlet camshaft is phase controlled with the exhaust camshaft fixed.
Cam changing VVT uses different cam profiles to lift the valves depending on engine load and speed.
One common system uses two rocker arms for normal operation on its two intake valves, with a third, higher profile, rocker arm between the other two arms. At engine speeds above 5000 to 6000 rpm, the engine ECU activates an oil pressure controlled pin that locks the three rocker arms together. The center rocker arm follows a larger and more aggressive profile, transferring its movement to the intake valves which now open further and for longer.
When engine speeds fall below the threshold speed, oil pressure is removed from the pin and a spring deactivates the pin. The rocker arms are no longer locked together and the valves are controlled by the less aggressive outer lobes.
Cam changing VVT can also be used in a similar way to deactivate a second intake valve at low engine speeds, increasing the velocity and swirl of the air/fuel mixture as it enters the combustion chamber.
Throttle-less valve control engines do not use a throttle butterfly to control engine power. Instead intake valve lift is controlled between to 0 and 10 millimeters or .39 of an inch by the engine management computer.

Valve-timing diagram

To see how valve-timing works in a 4-stroke engine cycle, let’s show piston motion as a circle. In this simple cycle, each stroke is shown as a semi-circle.
This intake valve opens at top dead center, and closes at bottom dead center. The blue line shows that period and it matches the intake stroke.
The exhaust valve opens at bottom dead center, then closes at top dead center before the new air-fuel mixture enters the cylinder.
In practice, the intake valve usually opens earlier than top dead center, and stays open a little past bottom dead center.
The exhaust valve opens a little before bottom dead center, and stays open a little past top dead center.
When the valves actually open and close, can be measured by angles. To make these angles easier to read, let’s use a spiral instead of a circle.
This intake valve opens 12° before the piston reaches top dead center.
And it closes 40° after bottom dead center.
The exhaust valve opens 47° before bottom dead center - and stays open - until 21° past top dead center. This gives exhaust gases more time to leave.
By the time the piston is at 47° before bottom dead center on the power stroke, combustion pressures have dropped considerably and little power is lost by letting the exhaust gases have more time to exit.
When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called lead.
When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag.
On the exhaust stroke, the intake and exhaust valve are open at the same time for a few degrees around top dead center. This is called valve overlap. On this engine, it is 33°.
Different engines use different timings. Manufacturer specifications contain the exact information.