Squish Velocity

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Squish Velocity Programme

The Squish Velocity Programme enables the user to calculate the peak squish velocity, the crank angle location of the peak squish velocity and the total energy of the squish action for one compression cycle for a two-stroke engine.

This means that it is easy to check the "tune" of an engine and make decisions on if there is scope for further modification of the engine.



Suppose you have a two-stroke engine with a 54.0 mm bore, a 54.5 mm stroke and a rod length of 110 mm. The exhaust port opens at 83 deg atdc and the peak power speed is 12000 revs/min. The combustion chamber is centrally located and has a squish area of 40% and a squish clearance of 0.85 mm. The trapped compression ratio is 7.8.

Within the engine box enter 54 for the Bore, hit the tab key and enter 54.5 for the Stroke, hit the tab key and enter 110 for the Rod Length. Enter the rest of the data and hit the calculate button. The answers are calculated as 26.01 m/s peak squish velocity at a crank angle of 9 deg btdc and the total squish kinetic energy is 3.0mJ. The value that is of most interest to the engine tuner is the peak squish velocity.

TorqSoft - Squish Velocity Programme


Programme Validation

Shown immediately below is a scanned page from the book titled "The basic Design of Two-Stroke Engines" by Professor Gordon Blair - Queens University of Belfast. It is apparent from the next image that The Torqsoft Squish Velocity Programme gives the exact same output for maximum squish velocity (MSV) and the crank angle at which this occurs.

Our programme however can handle squish angles other than zero which the QUB programme uses as default. This is important, as nearly all squish bands in practice have a slight divergent angle, though there have been some with convergent angles.

TorqSoft - Squish Velocity Programme

TorqSoft - Squish Velocity Programme


Influence of Squish Angle

The table below shows the calculated squish velocities for a 125cc engine at 0, +5 and -5 degrees squish angle.

It is evident that even a modest squish angle of +5 degrees (divergent) lowers the MSV by a significant amount. For this engine the MSV with a 5 degree squish angle has a value of 25.8 m/s which is only 66% of the value of 39.14 m/s for a 0 degree squish angle.

A squish angle of -5 degrees (convergent) gives a MSV of 30.63 m/s which is 78% of the MSV for 0 degrees.

Squish AngleMSV (m/s)

TorqSoft - Influence of Squish Angle on MSV


Measurement of Squish Clearance

TorqSoft - Squish Measurement on AM6 Engine