Volumetric Efficiency - EngineKnowHow

A defining limit of how much torque an engine can produce is the mass of air in the cylinder and one way to measure engine performance is to measure how effectively the engine is working as an air pump, or how well it takes in and expels the air / fuel.  Relative to the amount of air in the cylinder, the required amount of fuel is very small and therefore the major challenge for the engine is the intake and exhausting of the air.

 

A method to determine the engine’s performance as an air pump is to determine the volume of air delivered to the cylinder relative to the volume available in the cylinder per engine cycle, this is known as the volumetric efficiency.

 

The volumetric efficiency is calculated by:

 

 

 

 

The pumping performance of just the inlet ports and valves can be determined by considering the density in the intake plenum as opposed to atmospheric density.

 

An unboosted, spark ignition engine typically has a maximum volumetric efficiency between 80 – 90% whilst the volumetric efficiency is higher for a diesel engine.

 

Improving Volumetric Efficiency

In terms of increasing the volumetric efficiency and therefore the performance of a particular engine under wide open throttle conditions efforts are made to increase the mass flow of air through the engine.  This can be achieved via:

  • Boosting – A turbocharger or supercharger compresses the air, thereby increasing the density of the air and the mass of the air. Boosted engines allow an engine to achieve volumetric efficiencies above 100%.
  • Inlet Valves / Port Design – Under full throttle conditions a major restriction to air flow is the inlet valves and the inlet port, specifically the cross sectional area of the port / valve and the distance between the valve head and the cylinder head. Therefore for high mass flow a large valve diameter and lift are required in addition to fast opening and closing speeds.
  • Frictional Losses -As the air passes through the intake system its velocity is reduced by frictional losses due to surface roughness and bends in the circuit. Therefore the flow rate can be increased by smoothing the internal surfaces via machining, polishing etc, (highest potential with the inlet ports) and having an intake / exhaust system that minimises bends and the total length.  These frictional losses are also a dependent upon engine speed.
  • Flow Restrictions – Air filters, intercoolers, throttles, catalytic converters, particulate filters and mufflers are all flow restrictors which inhibit the ability of the air to flow into the cylinder and exhaust products to flow out of the cylinder.
  • Fuel Injection – Liquid fuels which vapourise in the intake system or cylinder due to fuel injection systems cool the air due to the fuel’s heat of evaporation, thereby increasing the density and therefore the mass of the air.
  • Liquid Fuels – Gaseous fuels, eg. Liquefied Petroleum Gas or Natural Gas displace the air as opposed to liquid fuels which vapourise and mix with the air charge.
  • Ramming – With the pulsating nature of the intake system due the opening and closing of the intake valves it is possible to time the pressure waves in the ports to result in maximum pressure at the intake valve when open. This phenomena can be further exploited via variable valve timing, camshaft profile and valve lift systems and inlet system design.
  • Exhaust Tuning – In the same manner as the intake system, pulses exist in the exhaust system due to the opening and closing of the exhaust valves. These pulses can be timed such that a reduced pressure occurs at the exhaust valve whilst the valve is open aiding in the expulsion of combustants from the cylinder.   This phenomena can be further exploited via variable valve timing, camshaft profile and valve lift systems and exhaust system design.
  • Intake / Exhaust Pressure Ratio – During valve overlap conditions if the exhaust system pressure is higher than the intake system pressure, exhaust products will flow into the intake system, displacing the fresh air and increasing the air’s temperature, thereby decreasing its density and mass.
  • Internal Exhaust Gas Recirculation – Due to incomplete exhausting of the cylinder, if the intake system at the time of valve opening is less than the cylinder pressure, exhaust products can flow into the intake system, displacing the fresh air and increasing the air’s temperature, decreasing its density.
  • Compression Ratio – The exhaust system will never completely remove all of the exhaust products during the exhaust stroke and these remaining exhaust products displace fresh air entering the cylinder and increase the air’s temperature, lowering its density. As the compression ratio increases the clearance volume (the volume with the piston at TDC) will decrease, reducing the volume for exhaust gases to remain in the cylinder resulting in less exhaust gases remaining in the cylinder.  This phenomena is a major reason for the increased volumetric efficiency of diesel engines.