Valve Timing and Duration - EngineKnowHow

Poppet valves are the predominant type of valve used and are mechanically linked to the camshaft lobes on the camshafts.  These lobes dictate the valve profiles and timings through the:

  • Opening and closing times
  • Valve lift
  • Length of duration
  • The opening and closing times
  • The opening and closing speeds

 

The plot below shows the valve profiles of intake and exhaust valves opening and closing over the rotation of the crankshaft.

When both the intake and exhaust valve are open at the same time then valve overlap is said to occur.

 

Optimal valve timing is dependent upon engine speed / load and engine design and can therefore be exploited through variable valve timing and lift systems.

 

Exhaust Valve Opening (EVO)

Early EVO occurs during the expansion stroke and will result in a loss of work generated, or the GMEP due to the loss of expansion work. This reduction in positive work isn’t linear relative to the Crank Angle (˚CA) due to the geometric arrangement of the connecting rod and the crankshaft, with the perpendicular distance from the crankpin and crankshaft centre defining the torque generated by the engine.

 

One benefit of an early EVO is that the exhaust valves will open under higher cylinder pressures relative to the exhaust manifold, aiding in the removal of combustants due to the pressure differential between the cylinder and the exhaust manifold.  This process in known as Blow-down.  This form of exhausting reduces the work required by the piston to pump the combustants out of the cylinder, or PMEP whilst more effectively emptying the cylinder.

 

A benefit of a late EVO is the increased time for complete combustion to occur and the lower cylinder pressures due to further expansion which sucks fuel out of cylinder crevices (eg. between piston and cylinder wall)  reducing the amount of unburnt fuel exiting the cylinder.

 

Ultimately a late EVO can better exploit the expansion stroke to increase the work generated and is best suited to low speed / load conditions where there’s a greater amount of time to exhaust the cylinder.  Typically an early EVO reduces pumping losses and better empties the cylinder and is therefore more suited to higher speed / loads to reduce the amount of exhaust remaining in the cylinder to increase the mass of air inducted into the cylinder.

 

Exhaust Valve Closing (EVC)

An early EVC which results in no valve overlap (intake and exhaust valves open) and with the valve closing during the exhaust stroke and therefore prior to TDC  results in an increase in pumping losses due to the work required by the piston to compress the trapped residual gases.

 

EVC which occurs prior to TDC whilst the intake valve is open and when pressure in the intake plenum is below the pressure in the exhaust manifold (non-boosted operation or volumetric efficiency below 100%), the remaining exhaust in the cylinder will flow into the intake manifold which is then drawn back into the cylinder during the intake stoke.  The contents which weren’t exhausted into the exhaust manifold and remain in the cylinder during the next combustion event are known as the residual fraction or internal exhaust gas recirculation.  The exhaust which remains in the cylinder during combustion or the intake stroke:

  • Under part load conditions in petrol engines reduces fuel consumption as the intake valve must be further opened (reducing pumping work) to compensate for exhaust trying to flow into the intake manifold.
  • Under full load conditions, reduces the maximum torque / power as the exhaust in the cylinder restricts the amount of fresh air entering the cylinder.
  • Reduces combustion temperatures which reduces NOx emissions.
  • Increases combustion instability due to slower and more erratic combustion. This combustion instability decreases as load increases due to the increasing mass of air in the cylinder.
  • Can decreases unburnt hydrocarbon emissions following their exposure to an additional combustion cycle

 

Under boosted operation or when volumetric efficiency is greater than 100%, the pressure in the intake manifold can exceed the cylinder and exhaust manifold pressure.  This pressure differential generally eliminates back flow from the cylinder into the intake manifold and is beneficial in increasing maximum torque / power.

 

Late EVC after TDC allows exhaust gases to be drawn back into the cylinder from the exhaust manifold during the intake stroke, increasing the cylinder’s residual fraction and reducing the volumetric efficiency.    A late EVC also increases the duration of valve overlap which allows exhaust gases to enter the intake manifold which further increases the cylinder’s residual fraction when the pressure of the intake manifold is below that of the exhaust manifold.

 

Scavenging is a phenomenon which can occur during valve overlap following late EVC.  During scavenging fuel which is injected into the intake manifold (port fuel injection) or fuel which is mixed with the air prior to the intake manifold (carburettor) passes directly into the exhaust manifold via the cylinder. This phenomenon is most prevalent under the low speed, high load area of the engine’s map.

Source: Ford - Challenges and Opportunities of Engine Downsizing to Reduce CO2 Emissions (Andrew D.J. Fraser)

Typically early EVC is best under low load conditions to avoid high levels of remaining exhaust in the cylinder which would result in unacceptable combustion stability.  As load is increased valve overlap is beneficial for increased internal exhaust gas recirculation to reduce fuel consumption.  Under high speed and load conditions an EVC which reduces internal exhaust gas recirculation would result in increased torque / power levels due to the higher mass of air in the cylinder.

 

Intake Valve Opening (IVO)

IVO opening typically occurs prior to TDC to avoid compression of cylinder contents and signifies the start of any potential valve overlap.  Early IVO will occur during the exhaust stroke, typically enabling exhaust products to enter the intake manifold.  As discussed this phenomena is greater in petrol engines under low load / speed operating conditions due to the exhaust manifold / cylinder / inlet manifold pressure differences.

 

A late IVO ensures no valve overlap occurs and the exhaust remaining in the cylinder for the next combustion cycle was the exhaust that couldn’t be exhausted whilst the exhaust valve was open.  A late IVO will cause lower in cylinder pressures at the time of valve opening due to the action of the piston resulting in increased pumping losses due to a subsequently lower intake plenum pressure.  This increase in pumping losses has no significant effect on the volumetric efficiency as the intake charge’s velocity increases during the intake stroke following IVO.  The higher velocity of the intake charge also increases in-cylinder turbulence which is beneficial for combustion as an increase of in-cylinder turbulence results in faster combustion, increasing combustion efficiency and reducing the propensity for knock.

 

Under low load areas a late IVO would be typically targeted to reduce valve overlap, thereby reducing internal exhaust gas recirculation ensuring acceptable combustion stability.  As load increases, increasing valve overlap by earlier IVO would result in improved fuel consumption.  Under maximum load conditions and high engine speeds, maximum torque / power would be achieved though delaying IVO to reduce internal exhaust gas recirculation so as not to limit the mass of air entering the cylinder.

 

Intake Valve Closing (IVC)

Early IVC for typical engines occurs around BDC and is suited towards low load and low speed engine operating conditions due to the lower mass of air required to generate the desired power.  The reduced mass of air in the cylinder due to early IVC reduces the compressive pressure which aids in atomisation of the fuel, increasing combustion efficiency but results in lower cylinder temperatures during compression, reducing combustion efficiency.

 

During late IVC the intake valve closes during the combustion stroke, as a result the piston can force a portion of the admitted charge back into the inlet manifold, increasing the pressure of the inlet manifold and forcing the throttle to open further to combat the higher pressure which reduces pumping losses across the throttle.   This phenomena forms the basis for the Atkinson Cycle.  A late IVC also reduces the effective compression ratio, which, when counteracted by a supercharger or turbocharger leads to the basis for the Miller Engine Cycle.

 

Under conditions when the intake manifold’s pressure is higher than the cylinder’s during the compression stroke, such as boosting or volumetric efficiency higher than 100%, the delayed IVC can result in an increase in the mass of air admitted due to the higher intake plenum pressure relative to the cylinder pressure , increasing torque / power levels.

 

Under low load conditions an early IVC is favourable to ensure much of the fresh charge doesn’t flow back into the intake manifold due to the low intake manifold pressures and to increase the effective compression ratio.  As load is increased the IVC can be delayed due to increasing intake plenum pressures.

 

Duration

The optimal duration for the intake or valves is dependent upon engine speed and load.  As engine speed increases, the amount of time for the cylinder to be exhausted or to induct the fresh charge reduces.  For example:

Engine Speed Valve Duration Time Available
1000rpm 180˚Crank Angle 0.03 seconds
6500rpm 180˚Crank Angle 0.005 seconds

 

As engine load increases, an increasing mass of air needs to be inducted into the cylinders.  The table below shows the mass flow rate of air through a 3L, V6, Spark Ignition, Turbo engine at different speed and load sites:

Engine Speed Load Air flow through the Engine
2000rpm Part Load (2bar BMEP / 46Nm) 0.014 kg/s
2000rpm Max. Torque 0.110 kg/s
6500rpm Max. Power 0.310 kg/s

 

The examples above illustrate the differing requirements of the engine and the limitations within the engine must work.  Engines which are optimised to run predominantly under low load and speed conditions would favour a lower valve duration as a lower duration would require increased opening of the throttle to admit the required mass of air (shorted amount of time available), reducing pumping work whilst also reducing valve overlap, increasing combustion stability.  An engine developed to run under high speed and load conditions, such as a race engine would operate with larger valve durations to increase the mass of air which can be admitted to the engine.