Valve Gear

Shape of the cam determines: 3 things

1. Point of opening and closing 2.Velocity of cam opening 3.Amount of valve lift

What do Valve Guide Inserts do?

1. Provides for easy replacement when worn 2.Allows a more suitable material with wear resistant qualities to be used

Advantages of chain drive camshafts are:

1. reliable- chains can take more abuse and not affected as much by foreign particles. 2.Accuracy-chains can be tightened to compensate for wear 3.Use of the chain drive permits camshaft to be positioned higher on the engine-reduces timing errors due to elasticity

When two or more valves on a cylinder must be opened/closed simultaneously, one rocker arm may be used to operate two valves. This is accomplished by using:

1.A forked rocker arm (TVSOM 6M601) 2.A valve Bridge

Methods of driving the camshaft include:

1.Belt drives 2.Gear drives

Valve gear includes:

1.Camshafts 2.Cams A. Intake B. Exhaust C. Fuel injection 3.Cam followers 4.Push Rods 5.Valve bridges 6.Valves A. Intake B. Exhaust

Push Rods

1.Connects the cam follower to the rocker arm 2.Weak link in valve gear

Shape of cam flanks determines velocity of opening or closing: 2 Things

1.If convex or concave, slower opening and closing 2.If straight, faster opening and closing

Valve requirements are ? (6)

1.Must be opened as quickly and widely as possible 2.Must remain open as long as possible 3.Must close as soon as possible 4.Must be made as light as possible 5.Must be made as robust as possible 6.Must be able to withstand high temperatures

If valve lash is excessive:

1.Noisy operation 2.Loss of power - due to incorrect valve timing because valve opens later and closes earlier 3.Excessive wear

Hydraulic lifters allow for zero valve lash at all load conditions so it

1.Reduces pounding of valve stem 2.Reduces pounding of valve seat 3.Reduces valve failure by approximately 50% 4.Not used in most large engines because of added cost and problems with lube oil contamination

Cam follower types

1.Roller-most common 2.Flat-Used in smaller and high-speed engines

The number of cams per cylinder is determined by: 2 things

1.The engine operating cycle 2.The method of scavenging in a 2-stroke cycle

A valve cage consists of hollow cylindrical castings containing:

1.Valve 2.Valve guide insert 3.Valve seat insert 4.Valve springs, keepers, and retainers 5.Cooling passages

If valve lash is insufficient:

1.Valve will not seat properly 2.Valve leakage and burning of the valve/valve seat will occur 3.Loss of compression and thus loss of power

Valve rotation is typically applied to exhaust valves and

1.ensures that the valve is kept at an even temperature so that it does not distort and leak by 2.Valve rotation also helps dislodge any build-up of deposits on the valve and seat

Methods of Camshaft Construction Include: (2)

1.integrally forged 2.separately fabricated with cams bolted, keyed, or hydraulically shrunk to camshaft

In rocker arms, One end in contact with valve stem, the other in contact with either

1.push rod 2.Cam in Overhead Cam design

To prevent spring surge,

2 to 3 smaller springs may also be used since small springs have a high natural frequency of vibration

What are The most common angles?

30 degrees and 45 degrees

What uses Belt Drives?

Automobiles

Larger camshafts may be What to reduce weight?

Bored Hollow

Cams are case-hardened to increase what?

Durability

Which Valve experiences the most severe operating conditions?

Exhaust Valve

What is the 45 degree angle good for?

Exhaust Valves because it prevents valve seat deposits but may increase wear due to valve pounding

Pumping Loss:2 stroke vs 4 stroke

Exhaust valves are important on 2 and intake important on 4

Valve Spring Seat:

Located at the lower end of the spring on the cylinder head

Valve Seat Width

Seat width is critical to valve life - it is determined by the manufacturer and increases in proportion to engine size

resonance will cause

Spring Surge

What uses Gear Drive?

Sulzar Diesel engine

Valve lash is defined as the clearance between:

The valve stem and the rocker arm or the push rod and the rocker arm

What are Valve Cages used for?

To allow easier removal for replacement or reconditioning in large engines

Rotocap is used in which type of engine?

Trunk type

What is stellite?

a cobalt-based alloy that resists the tendency most metals have to soften as temperatures increase

Spring surge is

a situation where the center coils of the spring oscillate due to their own inertia

Cam

an eccentric projection on a revolving disk used for opening and closing a valve through intermediate valve gear

What does a 2 stroke cycle cycle have?

at least one cam per cylinder for fuel injection and may have one or more cams for exhaust, depending the number of exhaust valves

The purpose of the valve spring is to

close the valve

Intake valves always operate in relatively cool temperatures and can therefore be constructed of

common ferritic steel

Valve cages also

eliminates the need to disturb the cylinder head and liner simply to change a valve

Arrangement of cams on camshaft determines?

engine firing order

Rocker Arms

held on its pivot by a shaft or guide which allows it to pivot when actuated by push rod or cam

What is Camshafts Typically fabricated of?

high strength alloy steel forging

What is the 30 degree good for?

intake valves

Camshaft speed in a 4-stroke cycle engine

is ½ the speed of the crankshaft

Camshaft speed in a 2-stroke cycle engine

matches the speed of the crankshaft

Rotocap is a

mechanical device which produces valve rotation by a small amount as the valve opens

A 4-stoke cycle engine typically has

one fuel injection cam, one intake cam, and one exhaust cam

Valve springs have a closing force directly

proportional to the amount they are compressed

valve is rotated as it reseats, a light grinding action takes place. This

removes deposits and ensures a good seal

if the amplitude of vibration increases with each application of force to its natural frequency it causes

resonance

Exhaust valves are typically made of

silicon chromium (Silchrome) steel or some other form of austenitic steel such as Nimonic

The spring retainer is combined with a

split keeper or "collet" to grasp the valve stem

To increase the wear resistance properties of exhaust valves,

stellite may be used to build up the face of the valve or the valve seat

What is the Valve Seat Angle?

the angle between the valve seat surface and the cylinder head surface

What is Interference Angle?

the angle of the valve face is made ½ to 1 degree less than the angle of the seat

What drives the camshaft?

the crankshaft

A disadvantage of the interference angle is that

the narrow contact area results in less heat transfer from the valve face to the valve seat

natural frequency of vibration depends on

the shape and mass of the spring

Valve Spring Retainer:

the upper spring seat and consists of a hardened washer shaped to fit the spring