How to use the engine geometry calculator
- Choose inches or millimeters for the lengths.
- Enter the bore, which is the cylinder diameter, the stroke and the connecting rod length from center to center.
- Enter the number of cylinders and the engine speed in rpm.
- Read the mean and peak piston speed, the rod to stroke ratio, the acceleration at the top and the bottom of the stroke and the swept volume.
Formula
Swept volume of one cylinder = pi / 4 x bore^2 x stroke
Mean piston speed = 2 x stroke x rpm / 60
Rod ratio = rod length / stroke
Acceleration at top = w^2 r (1 + r / rod) and at bottom = w^2 r (1 - r / rod)
Here r is half the stroke and w is the angular speed, 2 x pi x rpm / 60. Peak speed comes from the exact slider-crank speed w r (sin t + (r / rod) sin t cos t / sqrt(1 - (r / rod)^2 sin^2 t)).
A 4.0 inch bore, 3.48 inch stroke and 6.0 inch rod at 6,000 rpm
One cylinder sweeps 716.6 cc (43.73 cubic inches), and eight of them sweep 5,733 cc, or 349.8 cubic inches. The rod ratio is 6.0 / 3.48 = 1.724.
Mean piston speed is 2 x 3.48 in x 6,000 / 60 = 17.68 m/s, which is 3,480 feet a minute. The peak speed is higher, 28.92 m/s, and it comes about 75 degrees after top dead center. The piston is accelerated at about 2,295 g at the top and 1,263 g at the bottom of the stroke, and the difference between them comes from the angle the rod makes.
Why mean piston speed is watched
Mean piston speed is a simple way to compare engines of different sizes. A long-stroke engine has to move its pistons farther in each turn, so at the same rpm its pistons are faster than those of a short-stroke engine. High piston speeds raise friction, heat and the stress on the rod and the piston, which is why builders often track this figure when they choose a stroke or a safe rev limit.
The peak speed is always higher than the mean because the piston slows down toward both ends of its stroke. A longer rod moves the peak toward the middle of the stroke and lowers the acceleration at the top, which is one reason rod ratio is discussed so often.
What the tool leaves out
This is the motion of an ideal slider-crank with the crankshaft centered under the bore. Real engines have offsets, flexing parts and combustion forces, so these numbers are for comparison and not a limit. Use the Engine Displacement Calculator for compression ratio, the Horsepower from Torque Calculator for power, and the Gear Ratio Calculator to connect engine speed to road speed. The Torque Wrench Extension Calculator helps with the bolts when you build one.
Assumptions and limits
- The crankshaft axis passes through the center of the cylinder bore. Offset engines move the numbers.
- The rod is rigid and the speed is constant at the rpm you enter.
- Acceleration in g uses 9.80665 m/s2 for one g.
- The dimensions are examples. Use the figures for your engine.
Frequently asked questions
What is a good rod ratio?
There is no single good value. Ratios between about 1.5 and 1.9 are common, and the best choice depends on the engine and its uses.
Why is the acceleration different at the top and the bottom?
The rod is not infinitely long, so the piston speeds up and slows down differently at the two ends of the stroke. At the top the two effects add and at the bottom they subtract.
Can I use this for a two-stroke or a diesel engine?
Yes for the geometry and the piston motion. The tool does not consider the cycle, so it says nothing about power or compression.
How does the tool find the peak speed?
It scans the exact speed formula in small steps from the top to the bottom of the stroke and then refines the best point.