Two-Stroke FRV Timing Calculator

Created: August 12, 2007


   Calculator
   How to Use this Form
   Parameter Discriptions


      
Working units:    Imperial      Metric
      
Name:
Bore:
Stroke:
Piston Height:
Conrod Length:
Wrist Pin Offset:
Top of exhaust port from crankshaft axis:
Top of port transfer from crankshaft axis:
Venturi diameter:
Shaft inlet port diameter/width:
Crankshaft diameter:
Angle between inlet port axis and TDC:
Piston deflector step height:
Exhaust port height:

How to Use this Form

This form calculates the displacement and timing events for a Front Rotary Valve (FRV) two-stroke model engine. You need to have an accurate set of measurements for the subject engine. The script called by this form does a reasonable amount of error checking on the parameters supplied, but is probably not fool-proof. If the numbers look valid, the program will calculate the opening and closing angles for the exhaust, transfer, and inlet relative to top and bottom dead center (TDC and BDC). Note that no provision is made (yet) for engines with a "DeSaxe" configuration where the cylinder axis and crankshaft axis (or wrist pin axis) are not all aligned in the same vertical plain.

Start by selecting the units of your measurements (inches, or millimetres) using the Radio Buttons at the top of the form. Then enter as many values as required. At a minimum, you need to enter values for the bore and stroke, although this will only calculate the engine displacement. You can click on the label of any of the form fields for an explanation of that parameter. Omitting the heights for a port lip, or the inlet valve data will cause that port's timing to be skipped. The others will still be calculated as all are independent. When entry is complete, press the "Calculate" button at the top right of the form. We recommend that you don't use your browser's "Back" button to return to this form as that may loose your previously entered values. Instead, use the "Return to Input" button on the results page to retain the values for modification and re-use. You can clear the values with the "Clear All Values" button at the top left of the page.

The calculations are performed using a PHP script, so this page requires a working web server for operation. It cannot be used statically in a mere web browser on a local PC (unless you have installed a local web server as detailed in the Model Engine News DVD set-up instructions). For simplicity, angular values are rounded to one decimal place.

Bore and Stroke

These are mandatory values. You can enter them as either metric or imperial units by selecting the unit type Radio Button at the top of the page. Uniform units of millimetres (mm) or inches are assumed for all measurements provided in the form. Note that we want the actual stroke, not the crankshaft throw (which will be half the stroke).

Piston Height

Mandatory parameter if performing exhaust or transfer timing calculation. This is the height of the piston from the lower edge to the perimeter of the crown. This means that if the piston crown is domed or conical, or has a deflector, you must omit this part from the measurement. This is because it is the edge of the crown (ie, the perimeter) that will uncover the cylinder port(s) and govern the timing.

Conrod Length

Mandatory parameter if performing exhaust or transfer timing calculation. This is the distance between the centers of the crankpin (big end) and wrist-pin (little end) holes.

Wrist Pin Offset

Mandatory parameter if performing exhaust or transfer timing calculation. The vertical distance from the perimeter edge of the piston crown to the center of the wrist pin hole (or gudgeon pin if you prefer).

Piston Deflector Step Height

This parameter is applicable only to
Transfer timing calculations on engines which have a step cut into the top side of the piston, forming a deflector to the transfer charge. The value required is the maximum vertical distance from the perimeter of the piston crown to the bottom of the deflector step. Omit the parameter, or set it to zero for pistons with no step—including pistons fitted with a deflector baffle on the piston crown.

Top of Exhaust Port Location

Entering a value for this parameter will produce Exhaust timing figures. It requires values for
stroke, piston height, conrod length, and wrist pin offset. The measurement required is the vertical distance from the crankshaft axis to the top edge of the exhaust port. We do not need to know the height to the port.

Top of Transfer Port Location

Entering a value for this parameter will produce Transfer timing figures. It requires values for
stroke, piston height, conrod length, wrist pin offset, and optionally, deflector step height. The measurement required is the vertical distance from the crankshaft axis to the top edge of the transfer port. We do not need to know the height to the port. Transfer is sometimes called "Bypass" as this port enables the fresh fuel/air mix to pass by the piston and enter the upper cylinder.

Intake Width

This parameter is required when calculating the inlet timing. Values are also required for
inlet port width, crankshaft diameter, and inlet port angle. The value is the width of the venturi inlet at the intersection with the crankshaft. In most cases, this will just be the diameter of the venturi hole. Note that it is assumed that the inlet is located on the engine centerline. This program does not (yet) account for offset inlets (which effectively increase the inlet duration).

Inlet Port Width

This parameter is required when calculating the inlet timing. Values are also required for
intake width, crankshaft diameter, and inlet port angle. This is the effective width of the port cut in the crankshaft. In simple cases, this will just be the diameter of a hole drilled in the crankshaft. Some engines increase this width by milling a flat across the shaft. In this case, the value would be the length of the chord formed by this flat.

Crankshaft Diameter

This parameter is required when calculating the inlet timing. Values are also required for
inlet port width, and inlet port angle. The value required is the shaft diameter measured at the inlet port location.

Angle of the Inlet Port relative to the Crankpin

This parameter is required when calculating the inlet timing. Values are also required for
intake width, inlet port width, and crankshaft diameter. The value required is the angle between the crankpin and the axis of the crankshaft inlet port. Note that the the calculations performed assume a "down-draft" inlet configuration. For engines with an updraft inlet, you can adjust the angle by subtracting it from 180 and using the result—effectively transforming the engine into a down-draft inlet.

Exhaust port height

This parameter is optional. It is not strictly required in order to calculate the timing events, but does allow the program to check for Sub-piston Induction (SPI). This is a design feature that opens the crankcase to the atmosphere at TDC through the exhaust in an attempt take in more air.

Engine Name

Enter any string you like, or leave the field blank. This value is used for the header on the timing diagram page and is intended to assist identifying the timing later, should you print the page.

 

 

Back to Design Center

 

Please submit all questions and comments to enquiries@modelenginenews.org>