Expansion Chamber Design According to Blair
Introduction
All the information presented was taken from the book ‘Design and Simulation of Two Stroke Engines’ written by Dr. Gordon P. Blair, professor at Queens University Belfast, and published by the Society of Automotive Engineers. If you find this interesting, get this book.
Speed of Sound in a Gas (Sound Pressure Wave Velocity)
One of the main parameters involved in expansion chamber design is the speed of sound, this governs the speed of the pressure pulses that we use in the chamber.

Where:
T
_{
exc
}
is Exhaust gas temperature in Celsius
273 added to T
_{
exc
}
to convert to Kelvin
R is 287 the Gas Constant of air in J/kgK
g
is 1.4 the Specific Heat Ratio of air
a
_{
0
}
is the speed of sound in m/s

Brake Mean Effective Pressure
BMEP is used in several of the expansion chamber design parameters, and is calculated as shown.

Where:
kW is engine power, kW (1bhp=746W)
SVcc is swept volume, cc
RPM is engine speed, rpm
BMEP is in Bar

Average Exhaust Temperature
Now we must determine the exhaust gas temperature in Kelvin (k = C + 273.15). This is usually a function of the engine's BMEP.
Engine

BMEP, Bar

Av. Exhaust Temp,
°
C

Grand Prix Racer

11+

650

Enduro

8

500

Motorcross

910

600

Road Bike

5

350

Tuned Length of the Expansion Chamber
Blair’s formula assumes that the tuned length of the expansion chamber is from the face of the piston to the beginning of the stinger and is given by the formula below.

Where:
L
_{
t
}
is tuned length, mm
83.3 = metric constant
A
_{
0
}
is speed of sound in m/s
Q
_{
ep
}
is exhaust duration, degrees

Effective Exhaust Diameter (EXD)
This is the diameter of a pipe whose area matches that of the exhaust
port.

Where:
EXD is effective diameter, mm
Width is port width, mm
Height is port height, mm

Constants
The values for k
_{
1
}
and k
_{
2
}
are ranges depending on the type of engine (enduro, motorcross or road racing) and if broadly tuned or high specific output. k
_{
1
}
ranges from 1.05 for a high specific output road racing engine to 1.125 for a broadly tuned enduro engine. k
_{
2
}
ranges from 2.125 for a broadly tuned enduro to 3.25 for a high specific output road racing engine.
These constants have been interpolated from the following table.
Engine

BMEP, Bar

K
_{
0
}

K
_{
1
}

K
_{
2
}

Enduro

8

0.7

1.125

2.25

Motocross

910

0.65



Grand Prix Racer

11+

0.6

1.05

3.25

Two Stage Diffuser Expansion Chamber Dimension
Calculation
A diagram of a typical twostage
diffuser expansion chamber is shown above. Note that the length of the
header pipe section LP01 includes the length of the exhaust port, i.e. LP01
is measured from the piston face.
Dimension Calculation Two Stage Diffuser
The following table gives the dimension for the twostage diffuser expansion
chamber section diameters.
D
_{
1
}
= K
_{
1
}.EXD



D
_{
3
}
= K
_{
2
}.EXD

D
_{
4
}
= K
_{
0
}.EXD


The next table gives the dimensions for the two stage diffuser expansion
chamber section lengths.
LP01 = 0.10L
_{
T
}

LP12 = 0.41L
_{
T
}

LP23 = 0.14L
_{
T
}

LP34 = 0.11L
_{
T
}

LP45 = 0.24L
_{
T
}

LP56 = LP45

Three Stage Diffuser Expansion Chamber Dimension
Calculation
A diagram of a typical threestage diffuser expansion chamber is shown above. Note that the length of the header pipe section LP01 includes the length of the exhaust port, i.e. LP01 is measured from the piston face.
Dimension Calculation Three Stage Diffuser
The following table gives the dimension for the threestage diffuser expansion
chamber section diameters.
D
_{
1
}
= K
_{
1
}.EXD



D
_{
4
}
= K
_{
2
}.EXD

D
_{
5
}
= K
_{
0
}.EXD


Notice that two extra parameters are required for diameter calculation. These are given next.
Notice also that an extra coefficient has been introduced. This coefficient
K_{h}
is called the horn coefficient, with typical values between one and two. Small values of
K_{h}
are best suited
to Grand Prix engines with narrow power bands, larger values are for wider more flexible engines.
The next table gives the dimensions for the threestage diffuser expansion
chamber section lengths.
LP01 = 0.10L
_{
T
}

LP12 = 0.275L
_{
T
}

LP23 = 0.183L
_{
T
}

LP34 = 0.092L
_{
T
}

LP45 = 0.11L
_{
T
}

LP56 = 0.24L
_{
T
}

LP67 = LP56



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