How to calculate engine exhaust flow rate [with Pdf] | Exhaust mass flow rate calculation

Hey guys, Are you in search of how to calculate engine exhaust flow rate? here we will discuss about exhaust gases, exhaust flow rate measurement in four-stroke and two-stroke engines, how to measure mass flow rate, and the equipment used to measure mass flow rate.

To calculate the engine exhaust flow rate it is necessary that you should know the exhaust temperature, volumetric efficiency, engine size and RPM of flywheel. The method is quite different in the case of two-stroke and four-stroke engines.

How to calculate engine exhaust flow rate

What is exhaust gas?

It is a product of the combustion process performed in the engine cylinder. the air and fuel mixture is ignited using a spark plug in the SI engine or by auto-ignition in the CI engine which gets converted into combustion products like carbon dioxide, nitrogen products, water vapors, Sulphur dioxide, etc.

How to measure exhaust gas flow rate in 4 stroke engine?

The equation which gives the volume flow rate of exhaust in 4 stroke engine is,

`\dot{V}_{Exhaust}= \frac{T_{exhaust}}{300}\times \dot{V}_{INTAKE}`

Here,

  • `T_{exhaust}` = Temperature of exhaust gas in Kelvin
  • `\dot{V}_{INTAKE}` = Intake volume flow rate
  • `\dot{V}_{Exhaust}` = Exhaust volume flow rate

The actual Intake volume flow rate of 4 stroke engine:

`\dot{V}_{INTAKE}= (V_{stroke}\times \frac{N}{2})\times \eta _{volumetric}`

Where,

  • `V_{Stroke}` = Swept volume of the engine
  • `\eta _{volumetric}` = volumetric efficiency of engine
  • N = RPM of the crankshaft

V stroke is also known as stroke volume of the engine and it is generally provided by the manufacturer in the form of cc of the engine. But we can also calculate it by using the following formula,

`V_{stroke}=(\frac{\pi }{4})\times BORE^{2}` × stroke length

N is the RPM of the crankshaft or flywheel of an engine. in the four-stroke engine, two crank revolutions are completed within one power cycle. It means that a single stroke volume of air charge is processed for two complete the revolution of the crankshaft.

Hence for N number of revolutions of crankshaft N/2 times of stroke volume of intake charge is ignited.

The volumetric efficiency shows that the actual volume flow rate of air is always less than the theoretical volume flow rate.

Hence to get the actual flow rate of air it is necessary to multiply the theoretical volume flow rate by volumetric efficiency.

How to measure exhaust gas flow rate in 2 stroke engine?

The exhaust volume flow rate for the two-stroke engine is given by,

`\dot{V}_{Exhaust}= \frac{T_{exhaust}}{300}\times \dot{V}_{INTAKE}`

Here,

  • `T_{exhaust}` = Temperature of exhaust gas in Kelvin
  • `\dot{V}_{INTAKE}` = Intake volume flow rate
  • `\dot{V}_{Exhaust}` = Exhaust volume flow rate

The actual Intake volume flow rate of two-stroke:

`\dot{V}_{INTAKE}= (V_{stroke}\times \frac{N}{2})\times \eta _{volumetric}`

Where,

  • `V_{stroke}` = swept volume of the engine
  • `\eta _{volumetric}` = volumetric efficiency of engine.
  • N = RPM of the crankshaft

In a two-stroke engine, the Intake stroke is done in each revolution of the crankshaft.
Hence for the N number of revolutions of crankshaft N times of stroke volume of air enters into the cylinder.

How to measure mass flow rate of engine?

By applying the principle of conservation of mass we can say,

`\dot{m}_{exhaust}=\dot{m}_{INTAKE}` –(1)

But the intake mass flow rate is given by,

`\dot{m}_{INTAKE}=\dot{m}_{air}+\dot{m}_{fuel}`

`\dot{m}_{INTAKE}=\dot{m}_{air}\times (1+\frac{1}{A/F})` –(2)

Where,
A/F = air-fuel ratio of the engine

From the equations 1 & 2 we can conclude that,

`\dot{m}_{exhaust}=\dot{m}_{air}\times (1+\frac{1}{A/F})`

Which equipments are used to measure exhaust flow rate?

Raw exhaust gas flow meter is the equipment used to measure the flow rate of exhaust gas of the engine.

It uses a variable throat venturi meter which has a wide range for flowrate measurement.
This type of Venturi meter has a moving core inside of the converging section which is adjusted by a mechanism to adjust the throat area of the venturi meter.

The equipment also has the provision for temperature measurement mounted on a variable throat venturi meter.

Last words

Hence using the information provided above you can easily calculate the exhaust volume flow rate of four-stroke as well as for a two-stroke engine. It also contains information to calculate the intake flow rate of two-stroke and four-stroke engines.

Leave a Comment