Valve Design - EngineKnowHow

The roles of the intake and exhaust valves are to seal the cylinder during combustion whilst allowing the admittance of a fresh fuel / air and the removal of combustion products.  The valves are also the most important restriction to flow in an engine with their movement typically controlled by mechanical linkage to camshafts.   Poppet valves are the predominant type of valve used in engines and the closer the camshafts are to the valve stems, the higher the mechanical efficiency.

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For intake valves the most important design consideration is:

  • Flow through the valves

Whilst for the exhaust valves the most important design considerations are:

  • Flow through the valves
  • Cooling

 

Typically valves are designed to be as large as possible to limit their impact on flow into and out of the cylinder.

 

Flow restriction through the Valves

The restriction of flow through the intake and exhaust valves occurs in two stages and is illustrated below:

  1. Valve starts to open / nearly closed – Flow is restricted, or choked by the area between the valve head and its seat in the cylinder head
  2. Valve is close to fully open – Flow is restricted, or choked by the area between the valve stem and the port.

 

Stage 1: Intake flow choked between valve head and valve seat

Stage 1: Exhaust flow choked between valve head and valve seat

 

 

 

 

 

 

 

 

 

 

 

 

 

Stage 2: Intake flow choked between valve stem and intake port

Stage 2: Exhaust flow choked between valve stem and exhaust port

 

 

 

 

 

 

 

 

 

 

 

 

 

The plots below illustrate the actual flow area through an intake valve as it opens and closes during the intake stroke.

Source: Internal Combustion Engine Fundamentals (John B. Heywood)

As flow becomes limited by the area between the valve stem and port, increasing valve lift won’t necessarily result in increased flow.  Therefore to increase the mass of air delivered to the cylinder to achieve a higher torque or peak power, the duration of the valve being open must be increased.

 

Choking of the flow occurs as a particle cannot exceed its speed of sound, also known as its sonic velocity and this will occur at the smallest flow area.  As the temperature increases, so too the sonic velocity, therefore the valve head area and lift of the exhaust valves can be smaller than that of the intake valves due to the temperature differences between the intake and exhaust flows.  The valve diameter differences between the intake and exhaust valves is visible in the images below.

 

Valve Configuration

The number of valves installed in the cylinder head is determined through:

  • Cost – increasing the number of valves increases complexity and therefore cost
  • Flow – increasing the number of valves maximises the valve head area for maximum flow and reduced fluid resistance
  • Space and Flexibility- the valves must allow space for spark plugs and / or injectors and the flexibility to install them for optimal fuel injection, ignition and combustion
  • Fluid Motion – Valve orientation will define the fluid motion into the cylinder
  • Heat – Ensuring heat can be dissipated into the cylinder head to avoid valve failure, knock, pre-ignition or poor combustion
  • Structural Strength – Enough clearance between valves in the cylinder head to withstand combustion temperatures and pressures

 

Typically engines run with four valves, two for intake, two for exhaust.

2 Valve Cylinder Head

3 Valve Cylinder Head (x2 Intake, x1 Exhaust)

4 Valve Cylinder Head

5 Valve Cylinder Head (x3 Intake, x2 Exhaust)

Valve Acceleration and Deceleration

In a perfect engine the valve opening and closing would be instantaneous, however this isn’t possible in current production engines due to:

  • shock loading, a dramatic increase in load in a short amount of time resulting in valve and / cylinder head damage (the hammer effect)
  • valve bounce or float, valves which contact the cylinder head with too high a velocity can bounce, stopping the cylinder from sealing itself from the respective port

Camshaft design is therefore optimised to ensure the largest possible acceleration and deceleration times with the plots below taken from a current production engine.

Valve Cooling

Valves which experience high combustion and exhausting temperatures such as high performance engines utilise valves with a hollow core in the valve stem containing a metal, such as sodium that liquefies below peak exhaust / combustion temperatures.  This liquefied metal flows up and down the stem increasing the transfer of heat from the valve head through the valve stem and into the cylinder head, thereby:

  • Increasing the maximum temperature limit of the valve, allowing:
    • increased boosting
    • higher compression ratios
    • leaner combustion
  • Increasing the valve head area for increased flow and efficiency
  • Reducing the temperature of the valve head which will reduce the propensity for knock and pre-ignition