Page 21The Power Curve

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You have taken a set of readings, and entered them to a spread sheet.

The type of chart you want to plot is shown on the right. Watts vs Rpm.
A “hilltop” chart.

When an engine is running slowly, under heavy load, there is a good time within each stroke
for the gas to be heated inside the cylinder. The force of the rotating shaft is called the torque.
The multiplication of the torque and the engine speed, gives the engine power, the wattage.
“Power” is the speed at which work is being delivered.

At low revs the torque is good, but with low revs, the multiplication yields a low power value.

If the load is lessened slightly, the revs rise, but with less time in each engine revolution,
there is slightly less time for gas to be heated and slightly less torque is created.

However, the revs have risen more than the torque has fallen. Multiplying, the power rises.

Further loosening of the brake lowers the friction and lets the engine run more rapidly.
Until the point is reached when the drop in torque is balanced by the gain in revs.

Hereafter, the reducing torque, or force per revolution outweighs any increase in rpm.

The power curve starts to descend.

If the brake clamp were totally loosened, there would be no friction. The revs would soar.

do not do thisover revving engines destroy themselves.

IF, the engine were running without load, then, whilst the revs would be very high,
there is no load on the engine. There is no torque being delivered.
The multiplication of high revs by zero torque, is zero. The power is zero.

So the power curve must come from zero – a stationary engine - then rise to some peak,
and then fall away again – to zero – when, with no load, no power is being delivered.

But does your chart look like this one below?

What are all those low down points? What curve?

Read on...