---
title: "Two-stroke motoring test: 5 kW motoring"
subject: "Power Plant Equipment"
chapter: 5
key: "semester_7/power_plant_equipment:motoring-test-5kw"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/motoring-test-5kw
site: BCT Engineering
---

# Two-stroke motoring test: 5 kW motoring

Power Plant Equipment · Chapter 5: Diesel Power Plant

- Asked in: 2081 Baisakh, 8 marks
- Video: [MOTORING TEST & WILLIAM'S LINE METHOD FOR MEASURING FRICTION POWER](https://www.youtube.com/watch?v=AaGAOH6N7UU) — Professor kartik
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/motoring-test-5kw
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-5

## Question

A diesel power plant operated by a two-stroke diesel engine was motored when the meter readings was 5 kW. Then the test on the engine was carried out for one hour run and the following observations were recorded: Brake Torque = 300 N.m, Speed = 1200 rpm, Fuel consumed = 6.5 kg, Calorific value of fuel = 40 MJ/kg. Determine: i) Mechanical efficiency ii) Indicated thermal efficiency iii) Brake thermal efficiency

## Short answer

Motoring test gives friction power 5 kW. BP=2π×1200×300/60=37.70 kW, IP=42.70 kW, heat supplied=72.22 kW. Thus ηm=88.29%, ηith=59.12%, and ηbth=52.20%.

## Answer

**Data:** two-stroke engine; motoring power (friction power) $FP = 5$ kW; $T = 300$ N·m; $N = 1200$ rpm; fuel used = 6.5 kg in 1 h; $CV = 40$ MJ/kg.

In a **motoring test** the engine is driven by a motor with fuel cut off; the power drawn equals the friction power, so $IP = BP + FP$.

#### Brake power

$$
BP = \frac{2\pi NT}{60} = \frac{2\pi\times1200\times300}{60} = 37{,}699\ \text{W} = 37.70\ \text{kW}
$$

#### Indicated power

$$
IP = BP + FP = 37.70 + 5 = 42.70\ \text{kW}
$$

#### Heat supplied

$$
Q = \dot m_f\times CV = \frac{6.5}{3600}\times40{,}000 = 72.22\ \text{kW}
$$

#### i) Mechanical efficiency

$$
\eta_m = \frac{BP}{IP} = \frac{37.70}{42.70} = 0.8829 = 88.29\ \%
$$

#### ii) Indicated thermal efficiency

$$
\eta_{ith} = \frac{IP}{Q} = \frac{42.70}{72.22} = 0.5912 = 59.12\ \%
$$

#### iii) Brake thermal efficiency

$$
\eta_{bth} = \frac{BP}{Q} = \frac{37.70}{72.22} = 0.5220 = 52.20\ \%
$$

(Check: $\eta_{bth} = \eta_m\times\eta_{ith} = 0.8829\times0.5912 = 0.522$.)

**Answer:** $\eta_m$ = **88.29 %**, $\eta_{ith}$ = **59.12 %**, $\eta_{bth}$ = **52.20 %**. (These efficiencies are higher than in a real diesel engine, about 30–40 %; they follow from the given data.)

## Check yourself

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

1. What does the 5 kW motoring reading represent?
   A. Friction power of the engine
   B. Brake power
   C. Heat supplied
   D. Indicated power
   Answer: A. In a motoring test the engine is driven with fuel cut off, so the power drawn equals friction power. (Key point: Motoring test meaning)

2. What is BP for T = 300 N.m at 1200 rpm?
   A. 2 pi x 1200 x 300/60 = 37.70 kW
   B. 1200 x 300/60 = 6 kW
   C. 300 x 1200 = 360 kW
   D. 2 pi x 300/60 = 31 W
   Answer: A. BP = 2 pi N T/60 = 37,699 W = 37.70 kW. (Key point: Brake power from torque)

3. Which pair is correct?
   A. IP = BP - FP = 32.7 kW; eta_m = 115%
   B. IP = BP + FP = 42.70 kW; eta_m = BP/IP = 88.29%
   C. IP = FP = 5 kW
   D. IP = BP x FP
   Answer: B. IP is brake power plus friction power; mechanical efficiency is BP/IP. (Key point: Indicated power and mechanical efficiency)

4. How is heat supplied computed from 6.5 kg in 1 hour and 40 MJ/kg?
   A. (6.5/3600) x 40,000 = 72.22 kW
   B. 6.5 x 40 = 260 kW
   C. 6.5/40 = 0.16 kW
   D. (6.5 x 3600) x 40 = 936 MW
   Answer: A. Fuel mass flow in kg/s times CV in kJ/kg gives kW: 0.001806 x 40,000 = 72.22 kW. (Key point: Heat supplied)

## Close variants set in other exams

- [Two-stroke motoring test: 4.5 kW motoring](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Amotoring-test-4-5kw) — Power Plant Equipment, chapter 5, asked 6×
- [Two-stroke motoring test with specific fuel consumption](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Amotoring-test-sfc) — Power Plant Equipment, chapter 5, asked 6×
- [Two-stroke motoring test: 2.2 kW motoring](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Amotoring-test-2-2kw) — Power Plant Equipment, chapter 5, asked 6×
- [Two-stroke motoring test: 2 kW motoring](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Amotoring-test-2kw) — Power Plant Equipment, chapter 5, asked 6×
- [Two-stroke motoring test: 1.5 kW motoring](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Amotoring-test-1-5kw) — Power Plant Equipment, chapter 5, asked 6×

## Related in Power Plant Equipment

- [Engine test: 4-cylinder, B.P. 31.4 kW](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aengine-test-bp-31-4kw) — Power Plant Equipment, chapter 5, asked 2×
- [Diesel engine 75 kW: fuel consumption and efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-75kw-fuel-consumption) — Power Plant Equipment, chapter 5