Intake and Exhaust Manifold Design and Tuning - EngineKnowHow

The development of the geometries for the intake and exhaust systems is extremely complex.  From a performance point of view, several phenomena must be considered:

 

  • The pulsed flow through the intake and exhaust systems due to intake and exhaust valves opening / closing at different times across the cylinders and variations in piston speed and accelerations.
  • The differing distances from the intake valves to the throttle or intake plenum entrance and from the exhaust valves to the turbocharger / exhaust manifold exit.
  • The changing intake and exhaust valve opening and closing times or valve lift due to variable valve timing and lift systems.
  • Fuel atomising in the intake manifold or carburettor, throttle body and port fuel injection systems.
  • The introduction of Exhaust Gas Recirculation (EGR), both internally and externally to the intake manifold or the removal of external EGR from the exhaust system.

 

Intake Manifold Design

Important design considerations for intake manifolds are:

 

  • The reduction of pumping work (further discussed in “Pumping Work in the Intake System”)
  • Good distribution of the intake charge and exhaust gas recirculation to the cylinders to ensure consistent Air-Fuel Ratios and combustion stability
  • Intake runners long enough to reduce unintended interaction between the cylinders, such as cylinders closer to the intake plenum entrance starving the cylinders further away from the plenum entrance
  • A large enough plenum volume that dampens pressure pulses in the manifold but a small enough volume to provide adequate throttle response.
  • Intake runner diameter large enough to ensure low flow resistance but small enough to ensure high charge velocities for air / fuel mixing and carrying of fuel droplets to the cylinder when not direct injected.
  • When fuel isn’t injected directly into the cylinder, the manifold is heated to ensure maximum air / fuel mixing.

 

An important note is that in engines that don’t feature fuel injection directly into the cylinder, the air flow in the intake manifold will have a large effect on the fuel flow in the intake system, however the fuel flow will have little effect on the air flow.

 

Intake Manifold Tuning

Due to the pulsing nature of the air flow in the intake manifold, pressure waves exist that travel along the length of the intake runner / port.  When the pressure waves are timed such that the maximum point of the pressure wave occurs at the intake valve as it is closing, and the piston is acting to force the intake charge back out of the cylinder, additional charge can be rammed in the cylinder.  This phenomenon is known as the Ram Effect and generally increases with engine speed due to the increasing inertia of the air.  When these pressure waves are timed to increase the volumetric efficiency of the engine at the certain engine speed and load then the intake manifold is tuned.

 

The plot below shows the pressure in an intake runner ≈150mm before the intake valve on a 2L, I4, naturally aspirated petrol engine at full load.   With this intake design, at 1200rpm the maximum pressure in the intake runner will reach the valve too early to coincide with the valve closing.  However, at 4800rpm the pressure peaks when the valve is closing resulting in the ramming of the cylinder and a benefit in volumetric efficiency.

 

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

The pressure waves in the intake runner / port travel at the speed of sound and in both directions due to backflow from the cylinder pressure, the action of the piston and / or boost pressure forcing the intake charge into the cylinder.  Additionally, when a pressure wave travelling along the runner / port enters a greater volume, eg. the intake plenum or the cylinder, a reflected pressure wave will occur travelling in the opposite direction with the opposite sign, eg. a compression wave → expansion wave.  Finally, a pressure wave that enters from a larger volume to smaller volume, eg. from the plenum into the intake runner or the cylinder into the intake port, a reflected pressure wave will occur in the opposite direction but with the same sign, eg. a compression wave → compression wave.

Generally, the amplitude of the pressure wave is defined by the diameter of the pipe whilst the length of the pressure wave is defined by the length of the pipe, engine speed and intake charge velocity.  When an intake runner / port length is fixed, a tuned system would be optimal at a single engine speed and load.  Variable Intake Manifold systems take advantage of this flow phenomena by varying the length of the inlet runners, either continuously or discretely, to alter the timing of the pressure waves to increase volumetric efficiency.  These systems can also exploit this variability to optimise the Fluid Motion for different engines speeds and loads as well.  Production examples include Opels “Twin Port”, Yamahas “YCC-I” and Ford’s “Dual Stage Intake”.

 

The plot below shows the results of volumetric efficiency from testing varying inlet runner length on a Jaguar Racing petrol engine where the inlet runners had a diameter for 50mm.

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

The testing shows that under low engine speeds the longer inlet runner offered better volumetric efficiency as the longer pipe tuned the pressure wave to reach the inlet valve whilst closing.  Additionally, at lower engine speeds the fluid friction is lower and therefore pressure losses through the pipe are lower than they would be at higher engine speeds.  As the engine speed increased the optimal length of the inlet runner reduced due to the increasing fluid friction, the increasing frequency of the pressure waves and the interactions between the pressure waves.

 

Exhaust Manifold Design

Important design considerations for exhaust manifolds are:

 

  • Exhaust runner lengths that are tuned such that exhaust from one cylinder can’t flow back into another cylinder but rather ensure a low pressure at the exhaust valve when the valve opens to aid in the extraction of combustants.
  • Exhaust runner diameters the are small enough to ensure high flow velocity whilst large enough to reduce flow resistance
  • When supplying the exhaust to a turbocharger, the pulses don’t interfere with one another to reduce the exhaust pulse at the entrance to the turbine. For example, the exhaust from an opened valve flows to the turbocharger rather than to another cylinder exhaust port.
  • Material deformation doesn’t occur following the repeated heating and cooling of the manifold, nor thermally expand and greatly alter the geometry
  • Exhaust temperature is transferred to the catalytic converter rather then lost to the environment to ensure the catalytic converter operates at maximum efficiency and as quick as possible from engine start

 

Exhaust Manifold Tuning

The same principles in terms of the pulsing nature of the system and the generation of pressure waves which apply to intake manifold also apply to the exhaust manifold. However, for the exhaust manifold it is desirable to have the lowest possible pressure at the exhaust valve when the exhaust valve is closing to aid in the extraction of combustants from the cylinder.  When the pressure waves are timed to optimise this scenario then the exhaust manifold is tuned.

 

The plot below shows the pressure in an exhaust runner ≈200mm after the exhaust valve on a 2L, I4, naturally aspirated petrol engine at full load.   At 1200rpm the maximum pressure in the exhaust runner is measured at the exhaust valve when the valve is closing resulting in a higher potential for internal exhaust gas recirculation and combustants entering the intake system, reducing the volumetric efficiency.  However, at 4800rpm the minimum pressure is measured at the exhaust valve when the exhaust valve is closing, aiding in the extraction of combustants and increasing the volumetric efficiency. Therefore this exhaust system is tuned at ≈4800rpm.

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

 

When tuning an exhaust manifold extra consideration should be given to the interaction of the different cylinders to one another due to the higher pressures relative to the intake system.  The exhaust should be tuned such that the exhaust flows toward and through a turbocharger, catalytic converter or the exit of the exhaust system as opposed to back into the cylinder or a neighbouring cylinder.

 

The plot below shows the pressure measured in the exhaust manifold for cylinders 1, 2 and 3 prior to the entry of a twin scroll turbocharger for a I6, turbo petrol engine at 1500rpm, full load.  The geometry of the exhaust manifold collected cylinders 1-3 and 4-6 into the two entry ports for the turbine and are two separate streams until reaching the turbine.

The design intent of a twin scroll turbocharger is to separate the pressure pulses from the cylinders to reduce the interaction between cylinders and ensure a more consistent feed to the turbine.  The testing results show that the system isn’t operating perfectly at 1500rpm as exhaust from cylinders 4, 5 and 6 are reaching the turbocharger turbine and flowing back into the exhaust manifold for cylinders 1, 2 and 3.   This backpressure will increase the pumping work of the engine as the piston has to work against this backflow , decreasing maximum torque.