Dynamometers and engine testing

Measurement of brake power

Brake power is obtained from the shaft torque $T$ and speed $N$ by a dynamometer, which absorbs the engine power and measures the torque:

$$BP = \frac{2\pi N T}{60\,000}\ \text{kW}$$

($T$ in N m, $N$ in rpm.) Dynamometers are of two kinds: absorption types (Prony, rope, hydraulic, eddy current), which convert the power to heat, and transmission types (torsion) for power transmitted without absorbing.

Rope brake dynamometer

        flywheel with water-cooled rim
       ______
      /  O   \   rope turns around, ends carry:
      \______/   S (spring balance)   W (dead load)

Several turns of rope are wound around the flywheel drum of the engine. One end carries a dead weight $W$ and the other a spring balance reading $S$. Wood blocks keep the rope in place. Net load $= W - S$ and torque is $T = (W - S)(R + r_{rope})$, where $R$ is the drum radius. Cooling water is circulated in the drum. It is simple and cheap and suits small engines, but the drum heats up and the friction is not steady.

Hydraulic (water brake) dynamometer

A rotor with vanes (on the engine shaft) turns in a casing full of water. The vanes churn the water against fixed casing pockets, and the casing, which is free to swing, tends to turn with the rotor. The torque is balanced by a weight or load cell at an arm, and the water supply is adjusted to vary the absorbing load. Heat is taken away by the flowing water. It is compact and good for high-power engines, easy to control and stable.

Quantities measured in a performance test

• Speed (tachometer) and torque (dynamometer) for the brake power.
• Fuel consumption (burette and stopwatch or flow meter) for sfc.
• Air consumption (air box with orifice, or hot-wire meter) for A/F and volumetric efficiency.
• Temperatures of the cooling water inlet/outlet, exhaust gas, oil and ambient; and the cooling water flow, for the heat balance.
• Exhaust gas composition (gas analyser), and indicated diagram if IP is needed.

A test may be a constant-speed test (Willans line, load variation), a variable-speed test at full throttle (performance curves) or a heat balance test.