---
title: "Two-stroke motoring test: 2 kW motoring"
subject: "Power Plant Equipment"
chapter: 5
key: "semester_7/power_plant_equipment:motoring-test-2kw"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/motoring-test-2kw
site: BCT Engineering
---

# Two-stroke motoring test: 2 kW motoring

Power Plant Equipment · Chapter 5: Diesel Power Plant

- Asked in: 2070 Asar, 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-2kw
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-5

## Question

A two stroke diesel engine was motored when the meter reading was 2 kW. Then the test on the engine was carried out for one hour and the following observations were recorded: Brake torque = 150 Nm; Speed = 900 rpm; Fuel used = 3 kg; calorific value of fuel = 40 MJ/kg; Determine: (a) Brake power, (b) Indicated power, (c) Mechanical efficiency and (d) Indicated thermal efficiency.

## Short answer

BP = 14.14 kW, IP = 16.14 kW, mechanical efficiency = 87.61%, indicated thermal efficiency = 48.41%.

## Answer

In a **motoring test** the engine is driven by the dynamometer without fuel; the motoring power equals the **friction power**: $FP = 2$ kW.

**Data:** $T = 150$ N m, $N = 900$ rpm, fuel = 3 kg in 1 h, CV = 40 MJ/kg = 40 000 kJ/kg.

#### (a) Brake power
$$
BP = \frac{2\pi N T}{60} = \frac{2\pi \times 900 \times 150}{60} = 14\,137\ \text{W} = 14.137\ \text{kW}
$$

#### (b) Indicated power
$$
IP = BP + FP = 14.137 + 2 = 16.137\ \text{kW}
$$

#### (c) Mechanical efficiency
$$
\eta_m = \frac{BP}{IP} = \frac{14.137}{16.137} = 0.8761 = 87.61\%
$$

#### (d) Indicated thermal efficiency
$$
\begin{aligned}
\dot Q &= \frac{3}{3600}\times 40\,000 = 33.33\ \text{kW} \\
\eta_{ith} &= \frac{IP}{\dot Q} = \frac{16.137}{33.33} = 0.4841 = 48.41\%
\end{aligned}
$$

**Answer:** BP = 14.14 kW, IP = 16.14 kW, $\eta_m$ = 87.61 %, $\eta_{ith}$ = 48.41 % (brake thermal efficiency = 42.41 %).

## Check yourself

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

1. In the motoring test the 2 kW reading represents
   A. Friction power
   B. Brake power
   C. Indicated power
   D. Fuel heat input
   Answer: A. Motoring without fuel measures power to overcome friction. (Key point: Friction power)

2. BP for T=150 N m at 900 rpm is
   A. 1.41 kW
   B. 14.14 kW, from 2*pi*N*T/60
   C. 135 kW
   D. 94.2 kW
   Answer: B. 2*pi*900*150/60 = 14137 W. (Key point: Brake power)

3. Mechanical efficiency is found as
   A. IP/BP
   B. FP/IP
   C. BP/IP = 14.137/16.137
   D. BP/heat supplied
   Answer: C. IP = BP + FP = 16.137 kW, so efficiency = 87.6%. (Key point: Mechanical efficiency)

4. Why is heat supplied 33.33 kW for 3 kg in one hour?
   A. 3 x 40000 kJ/h taken as kW
   B. 3/60 x 40000
   C. 3 x 40 kW
   D. 3/3600 kg/s x 40000 kJ/kg
   Answer: D. Fuel per second times CV gives kW; eta_ith = 16.137/33.33 = 48.41%. (Key point: Indicated thermal efficiency)

## Close variants set in other exams

- [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: 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×
- [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: 5 kW motoring](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Amotoring-test-5kw) — Power Plant Equipment, chapter 5, asked 6×