Automotive Upper End Theory + Service--part 2

INTAKE AND EXHAUST VALVES

The intake and exhaust valves are commonly called poppet valves (Fgr__8). They tend to pop open and close. When they open, they allow intake air to flow into the combustion chamber or allow the exhaust to leave it. When closed, they must (along with the cylinder head gasket, piston rings, and spark plug) seal the chamber. The heads of the intake and exhaust valves have different diameters. The intake valve is the larger of the two. An exhaust valve can be smaller because exhaust gases move easier than intake air.

Valve Construction: Today, most valves are made from special hardened steel, steel alloys, or stainless steel. Other metals are often used in high-performance valves. Heat is an important factor in the design and construction of a valve. The material used to make a valve must be able to withstand high temperatures and be able to dissipate the heat quickly. Most of the heat is dissipated through the contact of the valve face and seat. The heat then moves through the cylinder head to its cool ant passages. Heat is also transferred through the valve stem to the valve guide and again to the cylinder head (Fgr__9).

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Fgr__8 A poppet valve. HEAT-- Heat dissipation to cooling system; Valve guide

Fgr__9 Valves cool by transferring heat to the liquid passages in the cylinder head.

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Intake and exhaust valves are typically made with different materials. Intake valves are typically low alloy steels or heat- and corrosion-resistant high-alloy steels. The alloy used in a typical exhaust valve is chromium for oxidation resistance with small amounts of nickel, manganese, silicon, and/or nitrogen. Heat resistance is critical for exhaust valves because they face temperatures of 1,500°F to 4,000°F (816°C to 2,204°C). Intake valves need less heat resistance because the intake air and fuel tend to cool them.

Intake valves also need less corrosion protection because they are not exposed to the corrosive action of the hot exhaust gases.

A valve can be made as a single piece or two pieces.

Two-piece valves allow the use of different metals for the valve head and stem. The pieces are spin welded together. These valves typically have a stainless steel head and a high-carbon steel stem. The stems are often chrome plated so the weld is not visible. One piece valves run cooler because the weld of a two piece valve inhibits the flow of heat up the stem.

Today's engines require higher quality valves that contain more nickel to withstand the heat. Most exhaust valves and some intake valves have 4% nickel content. The intake valves with high nickel are used in turbocharged engines. Older valves are alloyed with 2% nickel. The alloys used to make valves depend on the intended use and the design of the engine. Newer engines also tend to have lighter valves than what was used in the past. The lighter weight decreases the amount of power lost moving the valves and allows for higher engine speeds.

Stainless Steel Valves -- Stainless steel is commonly used to make valves. Stainless steel is an iron-carbon alloy with a minimum of 10.5% chromium content.

Stainless steel does not stain, corrode, or rust as easily as ordinary steel. There are different types of stainless steels used to make valves. Austenitic stainless steels contain a maximum of 0.15% carbon, a minimum of 16% chromium, and nickel and/or manganese to give it strength and improve its heat resistance. Stainless steel is nonmagnetic.

Inconel Valves -- An alloy that is being used by many manufacturers is Inconel. Inconel has a nickel base with 15% to 16% chromium and 2.4% to 3.0% titanium. This alloy is normally used in high temperature applications and has good oxidation and corrosion resistance. Inconel is difficult to machine; therefore, Inconel valves are replaced when they are deformed or damaged.

Stellite Valves -- Another alloy that is used in high temperature applications is stellite. Stellite is an alloy of nickel, chromium, and tungsten and is nonmagnetic. Stellite is a hard-facing material that is welded to valve faces and stems. It may also be used on the stem tip for added wear resistance. It comes in various grades depending on the mix of ingredients that are used in the alloy. This alloy has high resistance to wear, corrosion, erosion, abrasion, and galling. Stellite is available in many different grades, which are determined by the materials used in the alloy.

Sodium Filled--Some exhaust valves have a hollow stem. The hollow section of the stem is partially filled with sodium (Fgr__10). Sodium is a silver-white alkaline metallic substance that transfers heat much better than steel. At operating temperatures, the sodium becomes a liquid. When the valve opens, the sodium splashes down toward the head and absorbs heat. Then as the valve moves up, the sodium moves away from the head and up the stem. The heat absorbed by the sodium is then transferred to the guide where it moves to the coolant passages in the head. Sodium-filled valves should not be machined.

!WARNING! Never cut open a sodium-filled valve. Sodium will burn violently when it contacts water.

Titanium Valves -- Titanium alloys are added to valves to lighten them. Some high-performance engines have titanium valves. These valves dissipate heat well, are durable, and are very light. A titanium valve weighs less than half of a comparable steel valve.

Fgr__10 Some exhaust valves are partially filled with sodium to help cooling. Partially filled with metallic sodium; Hollow head and stem.

Ceramic Valves -- Ceramic valves are being tested for future use. Ceramic materials weigh less than half of what a comparable size steel valve weighs and can withstand extreme temperatures without weakening or becoming deformed.

Valve Terminology -- Valves have a round head with a tapered face used to seal the intake or exhaust port. This seal is made by the valve face contacting the valve seat. The angle of the taper depends on the design and manufacturer of the engine. The distance between the valve face and the head of the valve is called the margin. The valve stem guides the valve during its up-and-down movement and serves to connect the valve to its spring through its valve spring retainers and keepers. The keepers are fit into a machined slot at the top of the stem, called the valve keeper groove. The stem moves within a valve guide that is either machined into (integral type) or pressed into the head (insert type).

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Tip or rocker arm contact area Fillet (neck) Face Valve spring retainer lock grooves Stem Margin Head

Fgr__11 The parts of a typical valve.

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Valve Stems: Little oil passes through the clearance between the stem and valve guide. Therefore, the surfaces of the guide and the stem are designed to minimize friction.

Valve stems have two common types of coating to prevent wear and reduce friction: chrome plating and black nitriding. In addition to these coatings, the tips of the stem are hardened or stellited to resist damage from the constant hammering they face as the stems are pushed open.

Chrome-plated stems help prevent valve stem scuffing and galling and provide protection against wear during initial engine starts when no oil is present on the valve stem. Chrome coating is also widely used on high-performance valves. The thickness of the chrome plating can vary from 0.0002 to 0.001 inch (0.0051 to 0.0254 mm).

Many foreign manufacturers use a black nitride coating rather than chrome plating on the valves.

Black nitride is applied to the entire valve, not just the stem. The finish of the surface is smoother than chrome; therefore, less friction is produced by the stem. The nitride coating protects the stems against scuffing and wear.

Valve Seats: The valve seat (Fgr__12) is the area of the cylinder head contacted by the face of the valve. The seat may be part of the casting and machined in the head (integral type) or it may be pressed into the head (insert type). Insert seats are always used in aluminum cylinder heads. They are also used to replace damaged integral seats.

Valve seats provide a sealing area for the valves.

They also absorb the valve's heat and transfer it to the cylinder head. Seats must be hard enough to withstand the constant closing of the valve. Due to corrosive products found in exhaust gas, seats must be highly resistant to corrosion. When the head is made of cast iron, it has integral seats because cast iron meets those requirements. Cast iron is also used to make seat inserts. Most are induction hardened. These are hardened through electromagnetism, which heats the surface of the seat.

Many late-model engines with aluminum heads have sintered powder metal (tungsten carbide) seats.

Powder metal seats are harder and more durable than cast-iron seats.

Valve Guides: Valve guides support the valves in the head and pre vent the valves from moving in any direction other than up and down. The inside diameter of a guide is machined to provide a very small clearance with the valve stem. This close clearance is important for the following reasons:

¦ It keeps oil from being drawn into the combustion chamber past the intake valve stem during the intake stroke, and it keeps oil from leaking out to the exhaust port when the pressure in the exhaust port is lower than the pressure in the crankcase.

¦ It keeps exhaust gases from leaking into the crank case area past the exhaust valve stems during the exhaust stroke.

¦ It keeps the valve face in perfect alignment with the valve seat.

Valve guides can be cast integrally with the head, or they can be removable (Fgr__13). Removable or insert guides are press-fit into the head. Aluminum heads are fitted with insert guides. Guides are made from materials that provide low friction and can transfer heat well. Cast-iron guides are mixed or coated with phosphorus and/or chrome. Bronze alloys are also used. These may contain some aluminum, silicon, nickel, and/or zinc.

Valve Spring Retainers and Oil Seals: The valve assembly is completed by the spring, retainer, and seal. An oil seal is placed over the top of the valve stem. The seal acts like an umbrella to keep oil from running down the valve stem and into the combustion chamber. The valve spring, which keeps the valve in a normally closed position, is held in place by the retainer. The retainer locks onto the valve stem with two wedge-shaped parts that are called valve keepers.

Fgr__14 shows the components that make up a valve spring assembly.

Fgr__12 Valve seats.

Fgr__13 (A) Integral and (B) removable valve guides.

Valve Rotators: Some engines are equipped with retainers that cause the exhaust valves to rotate. These rotators prevent carbon from building up between the valve face and seat. Carbon buildup can hold the valve partially open, causing it to burn.

Valve Springs: A valve spring closes the valve and maintains valve train contact during valve opening and closing. Some engines have one spring per valve. Others use two or three springs. Often the second or third spring is a flat spring called a damper spring, which is designed to control vibrations. To dampen spring vibrations and increase total spring pressure, some engine manufacturers use a reverse wound secondary spring inside the main spring.

Low spring pressure may allow the valve to float during high-speed operation. Too much pressure will cause premature valve train or camshaft lobe wear and can also lead to valve breakage.

Other Valve-Related Parts: Other parts are associated with the valves.

Rocker Arms: Rocker arms change the direction of the cam's lifting force. As the lifter and pushrod move upward, the rocker arm pivots at its center point. This causes a change in direction on the valve side and pushes the valve down. Rocker arms also allow the valve to open farther than the actual lift of the cam lobe. This is done by having different distances from the pivot point to the ends of the rocker arm. The difference in length from the valve end of the rocker arm and the center of the pivot point (shaft or stud) compared to the pushrod or cam end of the rocker arm and the pivot point (shaft or stud) is expressed as a ratio. Usually, rocker arm ratios range from 1:1 to 1:1.75. A ratio larger than 1:1 results in the valve opening farther than the actual lift of the cam lobe.

>> Section 8 for an explanation of rocker arm ratios. <<

The camshaft in some OHC engines rides directly on the rocker arm. One end of the rocker arm fits over a cam follower or lifter and the other end is over the valve stem (Fgr__15). Often OHC cylinder heads have a complex arrangement of rocker arms (Fgr__16). Other OHC engines have no rocker arms and the camshaft rides directly on top of the valves.

Rocker arms are made of stamped steel, cast aluminum, or cast iron. Cast adjustable rocker arms are attached to a rocker arm shaft that is mounted to the head by rocker arm brackets (Fgr__17). Cast iron rockers are used in large, low-speed engines.

They almost always pivot on a common shaft. Aluminum rockers, on the other hand, are generally used on high-performance applications and pivot on needle bearings to reduce friction.

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Spring retainer Spring O-ring Keepers (valve locks)

Fgr__14 Valve assembly with spring, retainer, seals, and keepers.

Hydraulic

lash adjuster

Exhaust camshaft

Intake camshaft

Rocker arm

Fgr__15 The camshaft in this setup rides on a rocker arm that has a hydraulic lash adjuster.

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Some domestic engines are equipped with a stamped steel rocker arm for each valve. The rocker arm is mounted to a stud that is either pressed or threaded into the cylinder head and must be replaced if worn, bent, broken, or loose. On some engines, the studs are drilled for an oil passage to the rocker arms.

Pushrods--Pushrods are the connecting link between the rocker arm and the valve lifter. Pushrods transmit cam action to the valves. Often the pushrods have a hole in the center to allow oil to pass from the hydraulic lifter to the rocker arm assembly (Fgr__18). Some engines use pushrod guide plates to limit the side movement of the pushrods.

Cam Followers--Found on some OHC engines, cam followers are cups that sit on top of the valve assembly. They provide a larger surface for the cam lobes to

move the valves. Some followers have a hydraulic unit that fits under the cup to maintain proper valve clearance. Others require periodic adjustment. Most use metal shims between the cup and cam lobe. To adjust valve clearance, a shim with a different thickness is inserted.

Camshaft Bearings--The camshaft is part of the cylinder head assembly in all OHC-type engines. The unit that holds the camshaft(s) may be a separate unit bolted to the cylinder head or the camshaft's bore is machined into the upper part of the head. In the most common design, the cylinder heads are machined to accept one or two camshafts and have caps that secure the camshaft (Fgr__19).

Multivalve Engines:

Many newer engines use multivalve arrangements.

One of the first cars to use four valves per cylinder as a way to enhance gas flow and increase horsepower was the 1918 dual-valve Pierce Arrow.

The basic idea behind using more than one intake and/or exhaust valve is simple-better efficiency. To improve efficiency, engineers need to improve the flow in and out of the combustion chamber. In the past, this was attempted by making the valves larger and by changing valve timing. Larger valves allowed more air in and more exhaust out, but the bigger valves weighed more and therefore required stronger springs to close them. The stronger springs held the valves closed tighter but required more engine power to open them. Also, when an engine is running at low speeds, the air moving past a large valve has a lower velocity than it would have if it flowed past a small valve. This reduces engine torque at low engine speeds.

Although two small valves weigh as much or more than one valve, each valve weighs less and, therefore, the spring tension on each is less. This means less power is required to open them. Also, the velocity of the air in and out at low engine speeds is quicker than it would be with large valves.

Today, multivalve engines can have three (Fgr__20), four (Fgr__21), or five valves per cylinder (Fgr__22). The most common arrangement is four valves per cylinder with two intake and two exhaust valves. All multivalve engines have cross-flow heads.

Using two intake and one or two exhaust valves increases the volume of the intake and exhaust ports.

Therefore, more air can move in or out of the cylinder. This results in a more complete combustion, which reduces the chances of misfire and detonation.

It also results in better fuel efficiency, cleaner exhaust, and increased power output. Two smaller valves have less mass than one big one, so mechanical inertia is reduced, making a higher engine speed possible before valve float occurs.

[[ Fgr__20 An engine with two spark plugs, two intake valves, and one exhaust valve for each cylinder. Daimler AG

Fgr__21 Typical layout for a cylinder with four valves.

Fgr__22 This five-valve per cylinder arrangement has three intake valves and two exhaust valves. Chrysler LLC ]]

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Next: part 3

Prev.: Combustion Chamber