Honda G100K2 Applications Manual page 79

INERTIA
The single cylinder engine has unique characteristics that must be considered in the power output, power
transmission and vibration.
The illustration shows the typical cyclical speed variation of a single cylinder engine.
The horizontal line marked as " Crankshaft rpm" is the average engine speed, such as you would read on a
typical tachometer with normal damped response.
At the time of ignition, just before TDC, the instantaneous rotational speed is at its lowest point.
As combustion occurs, the piston is very rapidly accelerated to its maximum speed during the power stroke and
then the engine "coasts" and gradually looses speed until the next power stroke occurs.
As shown on the illustration, the speed change is affected by the inertia of the system, with the lightest inertia
having the highest acceleration and speed change.
During the power stroke acceleration, the instantaneous torque is very high and typically 7 or 8 times the
average torque of the engine. The momentary speed change with attached rotational inertia equivalent to a
rotary mower blade is 300 – 400 rpm.
With very light rotational inertia, it has been experimentally found that the torque is as high as 14 times the
average torque and the momentary speed change is around 1000 – 1200 rpm.
This speed characteristic can explain why it is necessary to use proper size belts and pulleys to avoid power
loss and why it is necessary to have very tight connections to the crankshaft PTO to avoid rocking, fretting or
damage.
In general, the greater the inertia the more important is the connection, as the largest inertia takes most of the
load on acceleration, stores most of the energy and supplies the most energy as its "coasts" to the next power
stroke.
Low inertia
High inertia
Crankshaft
rpm
Ignition
Ignition
Single cylinder – cyclical speed variation
25° BTDC
25° BTDC
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