Automotive Lower End Theory and Service--part 2--CAMSHAFTS

CAMSHAFTS

A camshaft (Fgr__29) has a cam for each exhaust and intake valve. Each cam has a lobe that controls the opening of the valves. The height of the lobe is proportional to the amount the valve will open. The camshaft in older engines had an eccentric to operate the fuel pump and a gear to drive the distributor and oil pump. Some diesel engines have cam lobes for fuel injectors, fuel injection pumps, and/or air starting valves. The camshaft fits in a bore above the crank shaft. The bore is either in the center of the block or slightly off to one side, unless the engine has overhead camshafts. Many camshafts are cast iron or steel; others are forged steel units. To reduce weight and add strength, the camshaft in many late-model engines is a steel tube with lobes welded to it.

On OHV engines, the camshaft lobes work with lifters, pushrods, and rocker arms to open the valves.

As the cam lobe rotates, it pushes up on the lifter, which moves the pushrod and one end of the rocker arm up. Since a rocker arm is a lever, as that end goes up, the other moves down. This action pushes down on the valve to open it. As the cam continues to rotate, the valve spring closes the valve while maintaining contact between the valve and the rocker arm, thereby keeping the pushrod and the lifter in contact with the rotating cam.

OHC engines may have separate camshafts for the intake and exhaust valves. As these camshafts rotate, the lobes directly open the valves or open them indirectly with cam followers, rocker arms, or bucket-type tappets. The closing of the valves is still the responsibility of the valve springs. Service to the camshaft(s) in an OHC engine is usually part of the procedures for reconditioning the engine's cylinder head.

A camshaft is driven by the crankshaft at half its speed. This is accomplished through the use of a cam shaft drive gear or drive sprocket that is twice as large as the crankshaft sprocket. During one full rotation of the camshaft, the intake and exhaust valves open and close once. To synchronize the opening and closing of the valves with the position and movement of the pistons, the camshaft is timed to the crankshaft.

In the valve timing diagram (Fgr__30) valve action is shown in relation to crankshaft rotation. The intake valve starts to open at 21 degrees BTDC and remains open until it has traveled 51 degrees past BDC. The number of degrees between the valve's opening and closing is called intake valve duration time (252 degrees). The exhaust stroke begins at 57 degrees before BDC and continues until 15 degrees ATDC. The total exhaust valve duration is 252 degrees.

The specifications for the camshaft used for the figure show the duration of the intake valve to be the same as the exhaust. This is typical; however, some cam shafts are designed with different durations for the intake and exhaust valves. These camshafts are called dual-pattern cams. Different engine designs require different valve opening and closing times. Therefore, each engine design has a unique camshaft.

The actual design of the cams and lobes varies with the type of lifter or follower used in the engine.

There four distinct types of lifters: solid nonroller, hydraulic nonroller, solid roller, and hydraulic roller.

Camshafts designed for solid and hydraulic nonroller cams are often called "flat-tappet" cams. A camshaft must be matched with the type of lifter for which it was designed.

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Installing a camshaft with a different profile is a popular way to increase the performance of an engine. Choosing the right cam for an engine can be tricky. There are many things that must be considered, such as performance goals, type of transmissions, the weight of the vehicle, and other modifications made to the engine. Most camshaft suppliers offer guidelines for choosing the right cam.

Basically, opening the valves more and keeping them open longer allows more air and fuel into the engine but allows more mixture to escape through the exhaust. Longer duration camshafts improve high engine speed performance but weaken low-speed power. Lower duration improves low-speed torque, but it limits high-speed power. Typically if the vehicle is driven mostly on the street, a camshaft with a modest increase in duration and higher lift is best.

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Camshaft Terminology:

Many different terms are used to define the specifications of a camshaft. The actual shape of a cam lobe is called the cam profile. The profile determines the duration and lift provided by the camshaft. Valve overlap is controlled by the placement of the lobes on the camshaft.

Camshaft duration is how long the cam holds the valves open. It’s expressed in crankshaft degrees. The width of the lobe determines the cam's duration. Lift is the distance the lobe moves the lifter or follower to open the valve. Maximum lift is determined by the height of the lobe. The valve is fully open only when the lifter is at the top of the lobe. Camshaft lift does not always express how far the valve is open. Rocker arms increase the actual amount of valve opening.

Both valves are open slightly at the end of the exhaust stroke and the beginning of the intake stroke; this is called valve overlap. Overlap is critical to exhaust gas scavenging. A camshaft with a long overlap helps empty the cylinders at high engine speeds for improved efficiency. However, since both valves are open for a longer period, low-rpm cylinder pressure tends to drop. Because the amount of over lap has an effect on cylinder pressure, it affects overall engine efficiency and exhaust emissions. Valve over lap also helps get the intake mixture moving into the cylinder. As the exhaust gases move out of the cylinder, a low pressure is present in the cylinder. This low pressure causes atmospheric pressure to push the intake charge into the cylinder. Less overlap provides for more pressure in the cylinder at low speeds, resulting in more torque at lower speeds.

Lobe Terminology:

Other terms are used to describe a camshaft's profile:

¦ Base Circle -The base circle is the cam without its lobe. It’s also the part of the cam where valve adjustments are made.

¦ Nose -The nose is the highest portion of the cam lobe measured from the base circle. This point provides for the maximum amount of lift.

¦ Ramp (Flank) -The ramps are the sides of a cam lobe that lie between the nose and base circle. The ramp on one side is for valve opening and the other for valve closing. How quickly a valve will open and close depends on the steepness of the ramp.

¦ Clearance Ramps - Clearance ramps are at the very beginning or end of the opening and closing ramps. The opening clearance ramp removes any slack in the valve train before applying pressure against the valve spring to open the valve. The closing clearance ramp restores that clearance.

These clearance ramps are important to the durability of the valve train.

¦ Lobe Separation--Lobe separation is the angle formed by the centerlines of the intake and exhaust lobes. This angle is measured at the camshaft and the degree rating reflects the angle at the camshaft and has nothing to do with crank shaft rotation. If the maximum lift points of the intake and exhaust lobes are 105 degrees apart, the camshaft has a 105-degree lobe separation angle. Lobe separation angle has a direct relation ship to amount of overlap. The larger or wider the separation, the less overlap there is.

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Outer tube Inner shaft Locking pin Exhaust lobe Inner shaft Outer tube Intake lobe assembly Exhaust lobe Fgr__31 The basic construction of the camshaft within a camshaft used for variable valve timing on an OHV engine.

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Timing Mechanisms

The camshaft and crankshaft must always remain in the same relative position to each other. They must also be aligned to each other. The alignment is initially set by matching marks on the gears for both shafts; these are called timing marks. The following are the basic configurations for driving the camshaft.

Belt Drive--Sprockets on the crankshaft and the cam shaft are linked by a continuous neoprene belt. The belt has square-shaped internal teeth that mesh with teeth on the sprockets. The timing belt is reinforced with nylon or fiberglass to give it strength and prevent stretching. This drive configuration is limited to OHC engines. Most technicians replace the belt any time it has been removed.

Chain Drive--Sprockets on the camshaft and the crankshaft are linked by a continuous chain. The sprocket on the crankshaft is steel. The sprocket on the camshaft is steel on heavy-duty applications.

When quiet operation is a goal, an aluminum sprocket with nylon teeth is used. Nearly all OHV engines use a chain drive system. Chain drives are also used on many OHC engines, especially DOHCs. Often multiple chains are used and arranged in an elaborate fashion. Most chain drives have a chain tensioner to maintain proper tightness and different silencing pads to reduce chain noise.

Gear Drive A gear on the crankshaft meshes directly with a gear on the camshaft. The crankshaft gear is usually iron or steel. The camshaft gear is steel on heavy-duty applications, or aluminum or pressed fiber when quiet operation is a major consideration.

The gears are helical. Helical gears are strong and tend to push the camshaft backward to help prevent it from walking out of the block.

Tensioners--There is a tensioner on long chain and belt drives. The tensioner may be spring loaded and/ or hydraulically operated. Its purpose is to keep the belt or chain under the correct tension as it wears and stretches. All belts and chains have a drive or tension side and a slack side. The tension side is the side that is always being pulled. The tensioner is positioned on the slack side and presses in on the belt or chain.

Variable Valve Timing--Previously, variable intake and exhaust timing was only possible with overhead cams. DaimlerChrysler became the first manufacturer to produce a cam-in-block engine with independent control of exhaust camshaft timing. This system was introduced in the 2008 Dodge Viper SRT10's 8.4-liter engine. This is the first production pushrod equipped engine with true variable valve timing (VVT). The VVT system electronically adjusts valve overlap by changing exhaust valve opening times in response to engine speed and load. The system provides an increase in horsepower and torque. It also reduces fuel consumption and exhaust emissions.

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Point of contact Cam lobe Valve lifter Taper 0.001 inch

Fgr__32 The camshaft's lobe is tapered to cause the lifters to rotate.

Pushrod Guide surface Lifter body Cam lobe Roller

Fgr__33 A typical roller lifter.

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This VVT system uses a special camshaft and phaser. The phaser is attached to the end of the cam shaft. Inside the phaser are vanes that move within a fixed cavity inside a sealed hub. The movement of the vanes is controlled by oil pressure. The applied oil pressure is controlled by the powertrain control module (PCM). The PCM transmits a signal to a solenoid to move a valve spool that regulates the flow of oil to the phaser cavity. As the applied oil pressure increases, the vanes move against spring pressure. Each vane can rotate a total of 22.5 degrees inside its chamber.

The camshaft is actually two camshafts: an inner shaft and an outer hollow tube-type shaft. It’s a camshaft within a camshaft. The exhaust lobes are attached to the outer shaft and the intakes are pinned to the inner camshaft through slots in the outer tube (Fgr__31). Locking pins pass through the slots and are driven through the intake cam lobe assemblies. The exhaust lobes are pressed into position on the hollow outer shaft.

The phaser hub is fit with an external gear that is driven, via a chain, by the crankshaft. The vanes are connected to the outer tube and the hub. As the vanes rotate, the position of the exhaust lobes, in relation ship to the intake lobes, changes. The amount the exhaust lobes can move is limited by the size of the oil cavity and the slots in the outer tube.

Although it’s possible to control both the intake and exhaust valves with this VVT system, Chrysler chose to alter only the exhaust valve timing. Its goal was to increase horsepower without causing rough idle and high emissions. By controlling exhaust valve timing, Chrysler was able to use high-performance camshaft profiles without affecting low-speed operation.

Camless Engines--In the very near future, four-stroke cycle engines won’t have a camshaft. In a camless engine, the valves are opened and closed electronically or with electrohydraulic devices. The electro hydraulic system uses an electromagnetic field to control hydraulic solenoids. Camless technology will allow total control of the valves' lift, duration, and overlap. This would make engines much more efficient, and there would be less engine power loss due to friction. A conventional engine loses much power through the camshaft and its drive gears and chains or belts. It’s said that this technology will improve fuel economy about 20% and provide nearly 20% more low-speed torque.

Valve Lifters--Valve lifters, sometimes called cam followers or tappets, follow the contour or shape of the cam lobe.

Lifters are either mechanical (solid) or hydraulic.

Solid valve lifters provide a rigid connection between the camshaft and the valves. Hydraulic valve lifters provide the same connection but use oil to absorb the shock that results from the movement of the valve train.

Hydraulic lifters are designed to automatically compensate for the effects of engine temperature.

Changes in temperature cause valve train components to expand and contract. Hydraulic lifters automatically maintain a direct connection between valve train parts.

Solid lifters don’t have this built-in feature and require a clearance between the parts of the valve train. This clearance allows for component expansion as the engine gets hot. Periodic adjustment of this clearance must be made. Excessive clearance might cause a clicking noise. This clicking noise is also an indication of the hammering of valve train parts against one another, which results in reduced cam shaft and lifter life.

Nonroller lifters rotate in their bore. The contact area between the cam lobe and lifter is one of the highest stressed areas in an engine. Lifter rotation places this stress at different areas at the bottom of the lifter and prevents excessive wear. Cam lobes are ground with a slight taper (Fgr__32), approximately 0.001 inch (0.254 mm). The diameter of the front of the lobe's base circle is different from the diameter at the rear of the lobe. The speed at which the lifter rotates in its bore depends on the amount of taper. The interface of the lifter and lobe taper also prevents the camshaft from moving to the front or rear while the engine is running.

Roller Lifters--In an effort to reduce the friction of the lifter rubbing against the cam lobes-and the resulting power loss-manufacturers often use roller-type lifters. Roller lifters (Fgr__33) have a roller on the camshaft end of the lifter. The roller acts as a wheel and allows the lifter to follow the contour of the cam lobe with little friction. The lifter rolls along the surface of the lobe rather than rub against it. Roller lifters may be solid or hydraulic. The hydraulic part of a roller lifter works in the same way as a flat hydraulic tappet.

The rollers also allow the lifter to follow aggressive cam lobe designs that provide more lift at a given duration than a flat tappet (Fgr__34). The lobes on a roller camshaft have steeper ramps and a blunt nose. If the same lobe was used with flat tappets, the edge of the lifter would contact the lobe. This would cause serious damage to the lifter and cam lobe (Fgr__35). Another advantage of roller lifters is that as long as they are not damaged, they can be reused on a different roller camshaft.

Roller lifters don’t and should not rotate in their bores. Therefore, the lobes on roller camshafts are not tapered. To prevent the lifters from rotating, a pair of lifters may be connected by a bar. The bar prevents the lifters from rotating but allows each lifter to move up and down independently. Some manufacturers don’t use a tie-bar; rather, they use special fixtures mounted on the block to prevent lifter rotation.

Since the lobes of the camshaft are not tapered, a roller cam will tend to walk toward the front or rear of the engine block. Cam walk can cause a number of problems to the lifters and camshaft. To prevent this problem, thrust washers or cam buttons are fitted at the ends of the camshaft. These keep the camshaft in the correct position under the lifters.

Operation of Hydraulic Valve Lifters--A hydraulic lifter has a plunger, oil metering valve, pushrod seat, check valve spring, and a plunger return spring housed in a hardened iron body.

When the lifter is resting on the cam's basic circle, the valve is closed and the lifter maintains a zero clearance in the valve train. Oil is fed to the lifter through feed holes in its bore. Oil pressure forces the lifter's check valve closed to keep the oil inside the lifter. This forms a rigid connection between the lifter and pushrod. When there is some clearance in the valve train, a spring between the plunger and lifter body pushes the plunger up to eliminate the clearance. As the cam lobe turns and opens a valve, the lifter's oil feed hole moves away from the oil feed in the lifter bore. New oil cannot enter the lifter. The effort to open the valve pushes the lifter's plunger down slightly. This allows a small amount of oil to leak out; this is called leakdown. When the lifter returns to the base of the cam, oil can again fill the lifter (Fgr__36).

If a hydraulic lifter is not able to leak down or does not fill with oil, a noise will be heard from the engine.

Camshaft Bearings

The camshaft is supported by several bearings, or bushings. OHV camshaft bearings are one-piece plain bearings pressed into the camshaft bore (Fgr__37). The bearings are either aluminum or steel with a lining of babbitt. Babbitt is a soft slippery material made of mostly lead and tin. Alloys of aluminum are commonly found in late-model engines.

Aluminum bearings have a longer service life because they are harder than babbitt, but they are more susceptible to damage from dirt and poor lubrication.

OHC camshafts may be supported by split plain bearings. These split bearings are similar to main and connecting rod bearings. Camshaft bearings are normally replaced during engine rebuilding. The old bearings should be inspected for signs of unusual wear that may indicate an oiling or bore alignment problem.

Ramp--Flat lifter—Ramp--Roller lifter—Ramp--Contact problem

Fgr__35 The lobes on a roller camshaft can hold the valves open longer at higher lifts. If the same lobe was used with flat tappets, the edge of the lifter would contact the lobe.

0.350

0.300 Lobe lift in inches

Ramp becomes steeper

0.250

0.200

0.150

0.100

0.050 Duration

Flat tappet lobe

0.000

Roller tappet lobe

Fgr__34 Roller lifters are able to follow very aggressive cam lobe designs that provide more lift at a given duration than a flat tappet.

SHOP TALK -- Many OHC engines don’t use camshaft bearings.

Rather the cylinder head is machined to provide support for the camshaft and to serve as bearing surfaces. When these surfaces are damaged, the entire cylinder head is typically replaced.

Balance Shafts -- Many late-model engines have one or more balance (silence) shafts to smoothen engine operation. An engine's crankshaft is one of the main sources of engine vibration because it’s inherently out of balance. Balance shafts are designed to cancel out these vibrations.

A balance shaft has counterweights designed to mirror the throws of the crankshaft. These weights are positioned to the opposite side of the weight of the crankshaft and are rotated in the opposite direction as the crankshaft. As the engine turns, the opposing weights mutually cancel out any vibrations.

Balance shafts rotate at twice the speed of the crank shaft and are synchronized or timed to the rotation of the crankshaft. If the balance shaft(s) are not timed to the crankshaft, the engine may vibrate more than it would without a balance shaft.

Balance shafts are located in the engine block on one side of the crankshaft, in the camshaft bore, or in a separate assembly bolted to the engine block (Fgr__38).

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Valve closed--Valve lifter on cam lobe base circle--Valve open--Valve lifter on cam lobe nose--Oil flowing through check valve--Oil trapped by check valve--Plunger spring eliminating valve lash Fgr__36 Hydraulic lifter operation.

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Fgr__37 The typical camshaft bearing is a full round design.

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Drive gear Crankshaft Balance shaft housing Driven gears Balance shafts Fgr__38 Balance shaft assemblies for a four cylinder engine. By Toyota Motor Sales, U.S.A., Inc.

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Balance shafts are inspected and serviced as part of reconditioning or building a short block. The shafts' journals and bearings need to be checked for wear, damage, and proper oil clearances. Check the bolts for the housing. If they are damaged, replace them. Use a vernier caliper to measure the length of the bolts from their seat to the end. Compare the length to specifications. If the bolts are too long, replace them. The drive chain for the shaft(s) should also be checked for stretching. This is done by pulling on two ends of the chain and measuring that length.

If the length is greater than specifications, the chain should be replaced.

Fgr__39 A crankshaft.

Fgr__40 Various crankshaft configurations.-- V6 splayed crankshaft 4-cylinder crank shaft; V6 -- V8

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Next: Crankshaft

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