Pumping Work in the Intake System - EngineKnowHow

All engines must provide work to draw the fresh charge into the cylinders and to pump the exhaust from the cylinders, the only exception being boosted engines under some engine speed and load conditions.   This negative work required by the engine is known as the pumping work, or PMEP.

 

In all engines pressure losses occur in the intake system and these pressure losses increase the pumping work of the engine.  Pumping work needs to be compensated for by increased positive work,  GMEP which increases fuel consumption, particularly important during part load operation, or under full load conditions as it:

 

  • Limits the maximum torque capabilities, particularly important in naturally aspirated engines.
  • Requires increased boosting in boosted engines when not at maximum boost or the engine is knock or pre-ignition limited.

 

Pressure losses in the intake system are largely dependent upon:

 

  • The cross sectional of the pipework, as when the area increases the perimeter increases less proportionally, reducing the area of which frictional losses can occur.
  • Restrictors in the pipework which restrict the flow of air such as the air filter or the throttle in a spark ignition engine.
  • Engine speed and load, as when they increase it increases the flowrate of the air which increases the friction of the air against the pipe walls.
  • Density, as the density increases the mass will also increase, increasing friction.
  • Curves in the pipework as they impede the flow of the air.
  • Surface roughness, reduces the ability of the air near the walls to flow.

 

From the Atmosphere to the Intake Manifold – Full Load

For example, in a boosted engine the fresh air must travel through the airbox, compressor, intercooler and throttle to reach the intake manifold.  In each of these components, ignoring the pressure addition from the compressor, pressure losses will occur.

 

The plot below shows the pressure losses in the intake system between the atmosphere and the intake manifolds, ignoring the pressure addition from the compressor of a V8, twin-turbocharged, spark ignition engine under full load conditions.  Also shown is the relationship between pumping work and these pressure losses.

 

Reviewing the plot:

  • The engine, despite the help from the compressor still had to provide work to pull the air from the atmosphere and intake manifold into the cylinders (as PMEP is negative).
  • The maximum pressure losses occured at 5500rpm, the speed at which peak power occurs and the engine is consuming its maximum amount of air.
  • The pressure losses between the two banks aren’t equal. This occurred due to:
    • Different intake lengths between the two banks owing to the shared airbox location in the vehicle.
    • The two banks used the same type of turbocharger with the air from the compressor exiting from the right hand side. One bank therefore required additional bends in the intake pipework to meet the intercooler.
    • Due to the differences in pressure losses, the turbocharger in Bank #2 must provide additional boost to ensure each bank receives the same mass of air.

 

 

From the Atmosphere to the Intake Manifold – Part Load

In a spark ignition engine the load is typically controlled by restricting the flow of air into the intake manifold with a throttle.  As a result, under conditions where the throttle is partially closed the engine must provide work to draw the air into the cylinders due to the pressure loss across the throttle.  Under normal driving conditions the pressure loss across the throttle can exceed 90% of pressure losses in the intake system.

 

In order to achieve a certain load, this pumping work must be compensated through the additional combustion of fuel, increasing an engine’s fuel consumption.   As a result, in a spark ignition engine during part load conditions the greatest source of pumping work occurs at the throttle.  Conversely, a Diesel engine controls the load via the injection of fuel and runs without a throttle for load control, accounting for the increased fuel efficiency of diesel engine under part load conditions.  (Throttles can still be found in the intake systems of Diesel engines to manage external exhaust gas recirculation for reduced emissions.)

 

The plot below shows the fuel consumption of an engine targeting a fixed torque (BMEP) whilst the timing of the intake valve opening was moved due to a variable valve timing system.

Reviewing the plot:

  • At -15° Crank Angle (CA) valve overlap was occurring as the intake valve opened before the exhaust valve had closed. Due to a partially closed throttle to limit load, the intake manifold had a pressure lower than the approximately atmospheric pressure of the exhaust.  To stop the exhaust flowing into the intake manifold (due to the pressure difference) during the valve overlap, the throttle had to open more to raise the intake plenum pressure to compensate.
  • As the intake valve opening was delayed from -15 to 20°CA, the amount of valve overlap decreased, less exhaust was trying to flow into the intake system, a higher intake plenum pressure wasn’t required to compensate and therefore the throttle closed more and more.
  • As the throttle closed the intake plenum pressure decreased, increasing the pressure loss across the throttle (atmospheric pressure to intake plenum pressure), increasing the engine’s pumping work, PMEP.
  • To attain the same load, or BMEP, more fuel / air had to be combusted to extract work, GMEP to compensate for the additional pumping work.
  • As GMEP increased, the engine’s fuel consumption increased. (Despite the same work being extracted at the crankshaft, BMEP.)

 

From the Intake Manifold to the Cylinder

Under full load conditions in both spark igniton (throttle 100% open) and compression ignition (no throttle) engines, maximum torque and power is limited by the ability to move air through the intake and exhaust ports.   A measure of how effectively the engine is capable of moving the air though the inlet ports is the manifold volumetric efficiency.

 

 

 

 

 

 

Important parameters affecting the flow of air into the cylinder is the charge motion, further discussed in Fluid Motion, in addition to the timing of the air entering the cylinder, discussed in Valve Timing and Duration.

 

The flow of air (and fuel) into the cylinder is affected by friction along the surfaces of the port due to roughness of the ports surfaces, especially in the intake ports.  Typically the smoothness of the inlet ports in production engines are limited by cost and manufacturing capabilities, therefore by smoothing these surfaces, through machining or polishing, the cylinders are capable of receiving a higher mass of air through the reduction of pumping work.

 

The image below shows the difference in the surface roughness of a cast intake port and where machining of the port has been started.

Cheap engines will have a high level of surface roughness whilst high performance engines will have smoother inlet ports, achieved through machining, sanding or CNC machines or a combination of any.

Another important factor regarding the flow of the intake charge into the cylinder is the orientation of the intake runners and ports.   As discussed in “Fluid Motion into and in the Cylinder” bends in the pipework can be beneficial in increasing the turbulence of the intake flow, increasing fuel / air mixing and combustion efficiency at the expense of volumetric efficiency.  As a result, performance engines which target full load performance to the detriment of part load fuel consumption typically have intake ports designed with the minimal amounts of bends in the pipe work and funnel down into the cylinder to minimise pumping work, thereby increasing maximum torque and power.

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