valve system

1  valve Train

To coordinate the four-stroke cycle, a group of parts called the valve train opens and closes the valves(moves them down and up,respectively).these valve movements must take place at exactly the right moments.the opening of each valve is controlled by a camshaft.there are different types of valve trains depending on how many camshafts are and where they are lacated.

1)  Overhead camshaft (OHC)valve train

The  cam  is  an  egg-shaped  piece  of  metal  on  a  shaft  that  rotates  in  coordination  with  the crankshaft. The metal shaft, called the camshaft, typically has individual cams for each valve in the engine. As the camshaft rotates, the lobe, or high spot of the cam, pushes against parts connected to the stem of the valve. This action forces the valve to move downward. This action could open an inlet valve for an intake stroke, or open an exhaust valve for an exhaust stroke.

Valve in modern car engines are located in the cylinder head at top of the engine.this is know as an overhead valve(OHV) configuration.in addition,when the camshaft is located over the cylinder head,the arrangement is known  as  an overhead  camshaft(OHC)design.some  high-performance engines have two separate camshaft,one foreach set of inlet and exhaust valves.These engine are known as dual overhead-canshaft (DHOC)engine.

2) push-rod valve train

The  camshaft  also  can  located  in  the  lower  part  of  the  engine,within  the  engine  block to transfer  the  motion  of  the  cam  upward  to  valve,additional  parts  are  needed. 

In this arrangement, the cam lobes push against round metal cylinders called cam follower.  As the lobe of the cam comes up under the cam follower, it pushes the cam follower upward  (away from the camshaft). The cam follower rides against a push rod, which pushes against a  rocker arm. The rocker  arm  pivots on a shaft through its center. As one side of the rocker  arm  moves up, the other side moves down, just like a seesaw. The downward-moving side of the  rocker arm pushes on the valve stem to open the valve.

Because   a   push-rod   valve   train   has   additional   parts,it   is   more   difficult   to   run   at   high speeds.push-rod    engine    typically    run    at    slower    speeds    and,consequently,produce    less horse-power than overhead-camshaft designs of equal size.(remember,power is the rate at which work is done.)

2  Valve clearance

When the engine runs in compression stroke and power stroke, the valves must close tightly on their seats to produce a gas-tight seal and thus prevent the gases escaping from the combustion chamber. If the valves do not close fully the engine will not develop full power.Also the valve heads will be liable to be burnt by the passing hot gases, and there is the likelihood of the piston crown touching an open valve, which can seriously damage the engine.

So  that the  valves  can  close  fully  some  clearance  is  needed in  the operating  mechanism.this means that the operating mechanism must be able to move sufficiently far enough away from the valve to allow the valves to be fully closed against its seat by the valve spring.however,if the clearance is set too great this will cause a light metallic tapping noise.

3   valve timing

The time at which valves open and close ( valve timing ) and the duration of the valve opening is stated in degrees of crankshaft rotation . For example , the intake valve normally begins to open just before the piston has reached the top dead center . The valve remains open as the piston trave ls down to BDC and even past BDC . This is intake valve duration .An example of this could be stated as follows : IO at 17°BTDC , IC at 51°ABDC ( or , intake opens 17°before top dead center , intake closes 51°after bottom dead center ) . Intake valve duration in this case is 248° of crankshaft rotation .

This leaves 129° duration for the compression stroke since compression ends when the piston reaches TDC . At this point the power stroke begins . The power stroke ends when the exhaust valve begins to open approximately at 51° before bottom dead center . The duration of the power strokein this case is also 129°.

Since the exhaust valve is opening at 51°BBDC , this begins the exhaust stroke . The exhaust stroke continues as the piston passes BDC and moves upward to past TDC . With the exhaust valve closing at 17°TTDC , the duration of the exhaust stroke is 248°.

It is apparent from this description that the exhaust valve stays open for a short period of time during which the intake valve is also open . In other words , the end of the exhaust stroke and the beginning of the intake stroke overlap for a short period of time . This is called valve overlap . Valve timing and valve overlap vary on different engines.

Opening the intake valve before TDC and closing it after BDC increase the fill of air-fuel mixture in the cylinder . Opening the intake valve early helps overcome the static inertia of the air-fuel mixture at the beginning of the intake stroke , while leaving the intake valve open after BDC takes advantage of the kinetic of the moving air-fuel mixture . This increase volumetric efficiency.

As the piston moves down on the power stroke past the 90°ATDC position , pressure in the cylinder has dropped , and the leverage to the crankshaft has decreased due to connecting rod angle and crankshaft position . This ends the effective length of the power stroke , and the exhaust valve can now be opened to begin expelling the burned gases . The exhaust valve remains open until the piston has moved up past the TDC position . This helps to remove as much of the burned gases as is possible and increase volumetric efficiency .

4  Cam design and control dynamic

The  function  of  the  cam  is  to  open  and  close  the  valves  as  far  as  possible,as  fast  as  possible  and  as  smoothly  as  possible.The  closing  force  for  the  valves  is  applied  by  the  valve  springs  which  also  maintain  contact  between  the  cam  and  the  valves.Dynamic  force  impose  limits  on  cam  and  valve  lift.

The  entire  valve-train  assembly  can  be  viewed  as  a  spring/mass  system  in  which  the  conversion  from  stored  to  free  energy  causes  force  vibration.Valve-train  assemblies  with  overhead  camshafts  can  be  represented  with  sufficient  accuracy  by  a  1-mass  system(consisting  of  the  moving  mass,the  valve-train  assembly  stiffness  and  corresponding  damping).For  system  with  valve  bottom-mounted  camshafts  and  push  rods,a  2-mass  system  is  being  increasingly  used.

The  maximum  permissible  contact  stress,usually  regarded  as  the  parameter  which  limits  cam-lobe  radius  and  the  rate  of  opening  on  the  flank,currently  lies  between  600-750Mpa depending  upon  the  material  parings  used.

5  Camshaft  Drive  Mechanism

Each  cam  must  revolve  once  during  the  four-stroke  cycle  to  open  a  valve.A  cycle,remember,corresponds  with  two  revolutions  of  the  crankshaft.Therefore,the  camshaft  must  revolve  at  exactly  half  the  speed  of  the  crankshaft.This  is  accomplished  with  a  2:1  gear  ratio.A gear  connected  to  the  camshaft  has  twice  the  number  of  teeth  as  a  gear  connected  to the  crankshaft.The  gears  are  linked  in  one  of  three  ways:

1) Belt  Drive

A  cog-type  belt  can  be  used.Such  belts  are  made  of  synthetic  rubber  and  reinforced  with  internal  steel  or  fiberglass  strands.The  belts  have  teeth,or  slotted  spaces  to  engage  and drive  teeth  on  gear  wheels.A  belt  typically  is  used  on  engines  with  overhead-cam  valve trains.

2)  Chain  Drive

On  some  engines,a  metal  chain  is  used  to  connect  the  crankshaft  and  camshaft  gears. Most  push-rod  engines  and  some  OHC  engines  have  chains.

3) Gear  Drive

The  camshaft  and  crankshaft  gears  can  be  connected  directly,or  meshed.This  type  of  operating  linkage  commonly  is  used  on  older  six-cylinder,inline  engines. 

A  camshaft  driven  by  a  chain  or  belt  turns  in  the  same  direction  as  the  crankshaft.But a  Camshaft  driven  directly  by  the  crankshaft  gear  turns  in  the  opposite  direction.Timing belts  are  used  because  they  cost  less  than  chains  and  operate  more  quietly.A  typical  timing  belt  is  made  of  neoprene(synthetic  rubber)reinforced  with  fiberglass.

6  Electronic valve control system

An  electronic  value  control  (EVC)  system  replaces  the  mechanical  camshaft,controlling  each  value  with  actuators  for  independent  value  timing.The  EVC  system  controls  the  opening and  closing  time  and  lift  amount  of  each  intake  and  exhaust  valve  with  independent  actuators  on  each  value.Changing  from  a  mechanical  camshaft  driven  value  into  independently  controlled  actuator  valves  provides  a  huge  amount  of  flexibility  in  engine  control  strategy.  Vehicles  utilizing  EVC  can  realize  several  benefits  including:

1)  increases  engine  power  and  fuel  economy;                                                 2)  allows  centralized  and  distributed  EVC  systems  to  perform  at  their  full  potential;                                                                                                      3)  adapts  to  engines  of  varied  cylinder  counts.

With  all  of  the  improved  efficiencies  and  consumer  benefits,auto  manufacturers  are  eager to  get  their  first  EVC  systems  on  the  road.  The  EVC  system  is  targeted  to  operate  in  temperatures  up  to  125℃,while  the  actuator  is  targeted  to  run  up  to  6000r/min.The  actuator  can  be  controlled  in  a  centralized  system  with  a  high-speed  multiplex  bus  (up  to  10Mbps)  or  in  a  distributed  system  with  a  nominal  speed  bus. 

EVC  systems  must  be  compact  in  size,specifically  the  actuators  that  must  be  small  enough  to  fit  in  the  engine  space.A  vehicle  that  uses  a  42V  system  is  ideal  for  EVC  because  it  requires  high  voltage  to  control  the  value  actuators,  and  EVC  is  targeted  for  V8  and  V12  engines.The  EVC  system  is  also  highly  flexible,allowing  adaptability  for  a  number  of  cylinder  engines.