---
title: "Dynamometers and engine testing"
subject: "Internal Combustion Engines"
chapter: 8
key: "semester_6/internal_combustion_engines:dynamometers-engine-testing-notes"
source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/dynamometers-engine-testing-notes
site: BCT Engineering
---

# Dynamometers and engine testing

Internal Combustion Engines · Chapter 8: Engine performance and testing of engines

- Asked in: year not given, 6 marks
- Video: [How Power & Torque Measured?🤔 Dynamometer Working | Engine Performance Series.](https://www.youtube.com/watch?v=WTJVHbGIVBY) — Engine Doctor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/dynamometers-engine-testing-notes
- Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-8

## Question

Write short notes on the measurement of brake power. Describe the rope brake dynamometer and the hydraulic (water-brake) dynamometer. State the quantities measured in a performance test of an engine.

## Short answer

Brake power from torque and speed: BP =2πNT/60000 kW. Rope brake: rope around drum, net load (W-S) gives torque, simple but friction varies. Hydraulic dynamometer: rotor churns water, reaction torque measured; compact for high power. Performance tests measure speed, torque, fuel/air flow, temperatures, exhaust composition.

## Answer

#### 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
```text
        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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is brake power obtained from a dynamometer?
   A. BP = 2 pi N T/60000 kW with T in N m and N in rpm
   B. BP = N/T
   C. BP = T/N
   D. BP = N + T
   Answer: A. Brake power follows from shaft torque and speed. (Key point: Brake power formula)

2. How is torque found in a rope brake?
   A. T = (W - S)(R + r_rope)
   B. T = (W + S) R
   C. T = W x S
   D. T = W/S
   Answer: A. The net load is dead weight minus spring balance reading, acting at drum plus rope radius. (Key point: Rope brake torque)

3. How does a hydraulic dynamometer absorb power?
   A. Rotor vanes churn water against casing pockets and heat is carried away by flowing water
   B. Friction of rope on drum
   C. Electric currents in a disc
   D. A spring twisting
   Answer: A. The vanes churn water in the casing, the casing torque is balanced at an arm, and flowing water removes the heat; load is varied by water supply. (Key point: Hydraulic dynamometer)

4. Which quantity is needed for the heat balance?
   A. Cooling water flow and inlet/outlet temperatures, exhaust gas temperature
   B. Only speed
   C. Only fuel colour
   D. Only oil level
   Answer: A. A heat balance needs water flow and temperatures, exhaust temperature, fuel and air consumption in addition to brake power. (Key point: Quantities measured in a test)

## Related in Internal Combustion Engines

- [Engine performance parameters and efficiencies](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aengine-performance-parameters-definitions) — Internal Combustion Engines, chapter 8
- [Morse test for friction power](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Amorse-test-four-cylinder-numerical) — Internal Combustion Engines, chapter 8
- [Heat balance sheet of a diesel engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Adiesel-engine-heat-balance-numerical) — Internal Combustion Engines, chapter 8
- [IP, BP and efficiencies of a petrol engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aindicated-brake-power-efficiency-numerical) — Internal Combustion Engines, chapter 8
- [Volumetric efficiency from orifice air measurement](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Avolumetric-efficiency-orifice-air-consumption) — Internal Combustion Engines, chapter 8
- [Cooling water and radiator air flow calculation](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aengine-coolant-flow-radiator-numerical) — Internal Combustion Engines, chapter 7

## Related in other subjects

- [Dynamometers and rope brake power](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Adynamometer-types-rope-brake-numerical) — Instrumentation and Measurement, chapter 7
- [Rope dynamometer trial: brake and indicated power](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Arope-dynamometer-trial) — Power Plant Equipment, chapter 5
- [Engine test: four-cylinder with rope radius](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aengine-test-rope-radius) — Power Plant Equipment, chapter 5, asked 5×
- [Engine test: fuel measured in litres](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aengine-test-fuel-litres) — Power Plant Equipment, chapter 5, asked 2×