21st Century - Engines

Detail Level 2

Detail Level - 1 provides less detailed information.

Bore & stroke of a cylinder are its its diameter in inches (in.) or millimeters (mm) and length of piston travel from TDC to BDC.   When bore and stroke are of equal size, then the engine is called a square engine.   Engines with larger bore than stroke are called oversquare engines.   These engines use larger valves and longer connecting rods; thus, they run at higher speeds.   However, they are larger than undersquare engines.   Undersquare engines have shorter connecting rods that produce more power at lower engine speeds.   A square engine is a compromise between the two extremes.   Crank throw is the distance from the crankshaft's main bearing centerline to the crankshaft throw centerline.   The stroke of an engine equals twice the crank throw. ^{Erjavec 189}

Displacement is the volume of a cylinder when it is at BDC. Also, the engine when the cylinder displacement is multiplied by the number of cylinders.   An engine with more displacement produces more torque than one with less displacement.

Compression ratio is the cylinder volume at BDC divided by the cylinder volume at TDC.   It is a measure of the amount of compression of the air/fuel mixture during compression stroke.   The ratio will change with engine wear and carbon and dirt buildup in the cylinders. (The latter will increase the ratio because volume at TDC will be less.)   The higher the compression ratio, the more powerful the engine and the hotter the engine.   Low octane gasoline will burn faster and may explode rather than burn which causes preignition.   As the compression ratio increases, the octane of the gasoline should be increased to prevent abnormal combustion.   The SVC compression ratios vary between 8:1 and 14:1.

Variable compression ratios of some engines is achieved by altering the slope of the engine in relation to the engine block.   This changes the volume of the combustion chamber.   In the Saab Variable Compression (SVC), the cylinder head is pivoted at the crankshaft by a hydraulic actuator with movement up to 4°.   The Engine Management System adjusts this angle and therefore the compression angle in response to engine speed, load and fuel quality.   The cylinder head is sealed to the engine block by a rubber bellows. ^{Erjavec 190}

Engine efficiency is measured in 3 ways:

1. Thermal efficiency is the amount of heat transformed into meachanical power.   This is about 30%.   70% of the heat is lost to the engine parts, air, coolant water and oil lubricant.

2. Volumetric efficiency is the amount of cylinders' volume filled with the air/fuel mixture.   This is from 80% to 100%.

3. Mechanical efficiency is the amount of power transmitted to the crankshaft.   This is less than 30% because of the friction losses of moving parts.   The efficiency at the wheels is about 15% because of friction losses along the drive train.

Torque & horsepower are measures of engine power.   Torque is the amount of work (force x displacement) that the engine produces to turn the crankshaft.   Torque increases with engine speed until about 1700 rpm (revolutions per minute); thereafter it decreases.   Brake horsepower increases until about 3500 rpm and then decreases. Friction horsepower increases continually with engine speed.

Engine IDs: (1) Casting numbers identify major engine parts.   (2) Engine codes ....

Engine noises have various causes: ^{Erjavec 198-199}

1. Ring noise: Heard during acceleration as a high-pitched rattling or clicking.   Can be caused by worn rings or cylinders, broken piston ring lands, or insufficient ring tension against the cylinder walls.   Corrected by replacing rings, pistons, or sleeves, or reboring the cylinders.

2. Piston slap: Heard as a hollow, bell-like sound while engine is cold and often gets louder as engine accelerates.   Caused by piston slapping against cylinder wall, which can be caused by worn pistons or cylinders, collapsed piston skirts, misaligned connecting rods, excessive piston-to-cylinder wall clearance, or lack of lubrication from worn bearings.

3. Piston pin knock: Heard as a sharp, metallic rap that sounds like a rattle if all the pins are loose.   Most noticable when engine is idling and hot.   Sounds like a double knock at idle speeds.   Caused by worn piston pin, piston pin boss, piston pin bushing, or lack of lubrication from worn bearing.

4. Ridge noise: Heard as a high-pitched rapping or clicking that becomes louder upon deceleration.   Caused by piston ring striking ridge at top of cylinder.

5. Rod-bearing noise: Heard as a light tap or heavy knock or pound depending on how badly the bearings are worn.   Caused by worn or loose connecting rod bearings that can be heard at idle and speeds over 35 mph.

6. Main or thrust bearing noise: Heard as a dull, steady knock when caused by a loose crankshaft main bearing or a heavy thump when caused by a loose crankshaft thrust bearing, which may be heard only on hard acceleration.   In both cases, the cause is worn beariangs or crankshaft journals.

7. Tappet noise: Heard as a light, regular clicking sound that is more noticeable when the engine is idling.   It is caused by excessive clearance in the valve train.   Problem is caused by improper valve adjustment, worn or damaged parts, dirty hydraulic lifters, or lack of lubrication.

8. Combustion noises: These are either preignition or detonation noise caused by abnormal engine combustion.   Detonationn is a knock or ping most noticeable during acceleration with the engine under load and running at normal temperature.   Cause is advanced ignition timing or carbon buildup in the combustion chambers that increases combustion pressure.   Carbon deposits get hot and glow will also call preignition the air/fuel mixture, causing detonation.   Another cause is low octane fuel.   A malfunctioning EGR valve can also cause detonation.   Abnormal combustion can combine with other engine parts to cause noise.   For example, rumble is a vibration of the crankshaft and connecting rods caused by multisurface ignition.   Several flame fronts occur simultaneously from overheated deposit particles that cause much higher pressures on the cylinder at TDC.

Cylinder block is the lower part of the engine that houses the spaces where combustion of the air/fuel mixture takes place.   The upper section of the engine is called the cylinder head.   It bolts to the top of the cylinder block at the top surface ("deck") and is also part of the combustion chamber and contains valve train components.   Most cylinder blocks are one piece castings of iron (steel?) or aluminum alloys, but some current models are made of 2 parts, an upper section that contains the cylinders and a lower section that surrounds the crankshaft.   Note that considerable precision machining of the engine block (and head) are required so that the cylinder and other fitting surfaces are smooth and true.

Oil passages are drilled through the cylinder block, crankshaft, and cylinder head for lubricating oil, which cools, seals, and cleans engine parts.

Wate passages, called "jackets" are cast holes in the cylinder block and head for the passage of coolant (water + antifreeze).   These cool the engine heated during the combustion process.   The coolant is circulated by means of the cooling system.

Core plugs (expansion plugs) are placed into the core holes that are machined in the cylinder block.   These plugs, which are replaceable, allow the expansion of ice that forms in the block, thus preventing it from cracking.

Cylinder sleeves are pressed into the cylinders of some aluminum engines for additional wear surface strength.   A dry sleeve is supported from top to bottom by the block. No coolant touches it.   The wet sleeve is supported only at the top and bottom of the block.   Coolant touches the middle section of the sleeve.

Piston rings

Cylinders are the spaces where the pistons move up and down.   It surface must be smooth enough to allow proper seating of the piston rings, but rough enough to hold the oil between the cylinder wall and the piston rings.   The wall is composed of many small crisscross grooves (diamond-shaped areas) that hold the lubricating oil.

Camshaft is a shaft with an attached cam for each exhaust and intake valve.   The cam causes reciprocal (up and down) motion of the valves.   Each cam has a high point called the lobe that controls the amount of valve opening.   (Camshafts in older engines had a lobe to operate the fuel pump and a gear to drive the distributor and oil pump.   Some current diesel engines have cam lobes for fuel injectors, fuel injection pumps, and/or air starting valves.)   The camshaft is located either in the cylinder block or in the cylinder head.   (Two heads in a V8 engine.)   The camshaft fits into a bore next to the crankshaft on most inline engine, except for the the overhead camshafts.   On V8 engines the camshaft lies in a bore above the crankshaft at the enter of the block.   When the camshaft is in the block, the valves are opened through lifters, pushrods, and rocker arms.   As the cam lobe rotes, it pushes up on the lifter, which lifts the pushrod, which moves one end of the rocker arm up while the other end pushes the valve down to open it.   As the cam rotates, the valve spring closes the valve and maintains contact between the valve and the rocker arm to keep the pushrod and the lifter in contact with the rotating cam.   Overhead camshafts are mounted above the cyulinders, either in or on the cylinder head.   Pushrods are not used.   As the camshaft rotates, the cams ride directly above the valves.   The lobes open the valves by depressing the valve or by depressing the valve through the use of a cam follower, rocker arm, or bucket-type tappet.   Again, springs close the valves. ^{Erjavec 234-235}

Timing mechanisms are required to assure that the valves open and close when need during the combustion cycle.   A camshaft drive gear or sprocket (with belts or chains) is made twice the size of the crankshaft gear or sprocket and both are connected.   Thus, for every two complete turns of the crankshaft the camshaft turns once.   This rotation opens and closes the valves at the correct time.   Timing marks on the camshaft and crankshaft are used on the camshaft and crankshaft to remain their same relative position to each other.

Timing diagram shows the timing between the valves and the piston stokes.   Every engine design has its own valve timing requirements.   The intake valve starts to open at 21° before the piston has reached TDC and remains open until it has traveled 51° past BDC.   The number of degrees between the valve's opening and closing is called intake valve duration time.   The exhaust stroke begins at 53° before BDC and continues until 15° after TDC for a total exhaust valve during time of 200° of crankshaft rotation.   The period of time when the exhaust and intake valves are open is called valve overlap, which is critical to exhaust gas scavenging.   A camshaft with a long overlap empties the cylinders at high engine speeds for improved efficiency.   However, low rpm cylinder pressure tends to drop, which affects engine efficiency and exhaust emissions.   Overlap 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 that causes atmospheric pressure tp push the intake charge into the cylinder. ^{Erjavec 235-236}

Timing drives are either helical gear or sprocket with chain or sprocket with belt.   A crankshaft gear meshes with a camshaft gear.   The crankshaft gear is usuall steel.   The camshaft gear is steel for heavy duty applications or else aluminum or pressed fiber for quiet operation.   Helical gears are strong and tend to push the camshaft backward that helps prevent the camshaft from waling out of the block.   Chain drive uses sprockets con the camshaft and crankshaft that are connected by a continuous chain.   The camshaft sprocket is usually steel.   The camshaft sprocket may be steel for heavy duty applications or else aluminum with a nylon covering on the teeth for a quiet operation.   Nearly all overhead valve (OHV) engines uses a chain drive.   They are also used on many overhead cam (OHC) engines, especially DOHCs.   Multiple chain may be used for additional security.   A chain tensioner maintains proper tension and silencing pads reduce chain noise.   Belt drive uses a neoprene continuous belt to connect the camshaft and crankshaft.   The belt has square-shaped internal teeth that mesh with the sprocket teeth.   The belt is reinforced with nylon or fiberglass for strength and minimal stretch.   Belt drive are limited to overhead OHC engines. ^{Erjavec 236-237}

Valve lifters (cam followers, cam tappets) follow the contour of the cam lobe.   They are mechanical (solid) or hydraulic.   Solid lifters provide a rigid connection between the camshaft and the valves.   Hydraulic valve lifters are connected in the same way, but use oil to absorb the shock resulting from the valve train movement.   Hydraulic lifters are designed to compensate for the effects of engine temperature changes, which cause valve train components to expand and contract.   They maintain a direct connection between valve train parts.   Solid lifters require a clearance between the valve train parts.   This clearance allows for expansion of the parts when the engine heats up.   Periodic adjustment of this clearance must be made.   Excessive clearance might cause a clicking sound.   This noise also indicates hammering of the partsw against one another, which reduces camshaft and lifter life.   Roller-type hydraulic lifters reduce the friction between the lifter and the cam lobes.   Roller lifters have a large roller on the camshaft end of the lifter that acts like a wheel to allow the lifter to follow the cam lobe contour better than a flat-type lifter, thus reducing friction because the lifter rolls along the surface of the cam lobe instead of rubbing against it. ^{Erjavec 237-238}

Hydraulic valve lifters contain a plunger, oil-metering valve, pushrod seat, check valve spring, and a plunger return spring housed in a hardened iron body.   When the lifter rests on the cam, 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 the lifter bore.   Oil pressure seals the oil in the lifter bore.   The oil pressure seals the oil in the lifter by forcing down the check valve inside the lifter.   The oil between the plunger and the check valve forms a rigid connection between the lifter and the pushrod.   Whenever there is clearance in the valve train, a spring between the plunger and the 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.   Then no new oil can enter the lifter and the pressure on the plunger pushes it down in the lifter, which allows a small amount of oil to leak out ("leakdown").   Once the cam rotates and the lifter returns to the base of the cam, oil can again fill the lifter.   Non-roller tpe lifters must also be able to rotate in their bore when the engine is running to prevent wear on the bottom of the lifter. ^{Erjavec 238}

Camshaft bearings

Hydraulic valve lifters contain

Hydraulic valve lifters contain

Hydraulic valve lifters contain