# Automobile Engineering Questions and Answers – Performance Parameters and Characteristics

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This set of Automobile Engineering Questions and Answers for Aptitude test focuses on “Performance Parameters and Characteristics”.

1. Which of the following is correct?
a) Thermal efficiency varies directly as specific fuel consumption
b) Thermal efficiency varies inversely as specific fuel consumption
c) Thermal efficiency varies as root as specific fuel consumption
d) Thermal efficiency varies as square as specific fuel consumption

Explanation: Specific fuel consumption is the ratio of fuel consumption rate to the power produced. Thermal efficiency is the ratio of the power produced to the fuel consumption rate. Hence both are inversely proportional to each other.

2. What is mechanical efficiency?
a) The ratio of friction power to brake power
b) The ratio of the brake power to indicated power
c) The ratio of indicated power to friction power
d) The ratio of friction power to indicated power

Explanation: Mechanical efficiency is the ratio of friction power to indicated power. Mechanical efficiency takes into account the mechanical losses. The mechanical losses are divided into friction losses; power absorbed, ventilating action, and work of charging the cylinder.

3. What is volumetric efficiency?
a) A measure of the power of the engine
b) A measure of the speed of the engine
c) A measure of pressure rise in the cylinder
d) A measure of breathing capacity of the engine

Explanation: Volumetric efficiency is the ratio of the displaced volume of the fluid by the piston to the swept volume. It is the ratio of actual mass of air drawn by the engine to the theoretical mass which should be drawn by the engine.

4. What does the Sankey diagram represent?
a) Torque vs speed
b) Heat balance of the engine
c) Air consumption vs speed
d) ηbth vs brake power

Explanation: Sankey diagram represents the heat balance sheet of the engine. In this diagram, the width of the stream represents the heat quantity being considered.

5. The Otto engine consumes 9 liters of petrol per hour and develops 30 kW. The specific gravity of petrol is 0.8 and its calorific value is 45000 kJ/kg. What is the indicated thermal efficiency of the engine?
a) 33.33%
b) 25%
c) 30%
d) 32%

Explanation: ηith = heat equivalent of indicated power / heat input = (30 * 60 * 60 * 100) / (9*10-3*800)*45000 = 33.33%.

6. Which of the following relation between indicated mean effective pressure (imep), brake mean effective pressure (bmep) and friction mean effective power (fmep) is correct?
a) imep = fmep + bmep
b) imep = fmep * bmep
c) imep = fmep – bmep
d) imep = fmep / bmep

Explanation: Indicated mean effective pressure (imep) considered to be consists of the brake mean effective pressure (bmep) and the friction mean effective pressure (fmep).

7. The indicated power of the single-cylinder engine is 1.9 kW. It develops torque of 10 Nm at 1600 rpm. What is the friction power as the percentage of brake power?
a) 12.43%
b) 13.43%
c) 14.43%
d) 15.43%

Explanation: Brake power = bp, Indicated power = ip = 1.9kW, friction power = fp, speed = N = 1600 rpm, and torque = T = 10 Nm. Brake power = bp = 2πNT / 60000 = 2π * 1600 * 10 / 60000 = 1.675 kW. Friction power = fp = ip – bp = 1.9 – 1.675 = 0.225 kW. Percentage loss = (fp / bp)*100 = (0.225 / 1.675)*100 = 13.43%.

8. The bore and stroke of the single-cylinder four-stroke engine are 90 mm and 120 mm respectively. The torque is 24 Nm. What is the brake mean effective pressure of the engine?
a) 3.9 bar
b) 4.9 bar
c) 5.9 bar
d) 6.9 bar

Explanation: Brake mean effective pressure = pbm, torque = T = 24 Nm, stroke = L = 120 mm, bore = D = 90 mm. pbm = 16T / D2L = (16*24) / (0.09*0.09*0.12) = 3.9 * 105 Pa = 3.9 bar.

9. What is brake specific fuel consumption of engine whose fuel consumption if 6 grams per second and power output is 90 kW?
a) 0.24 kg/kWh
b) 0.0667 kg/kWh
c) 0.54 kg/kWh
d) 0.44 kg/kWh

Explanation: Brake specific fuel consumption = ṁf / bp where bp = brake power and ṁf = brake specific fuel consumption. Brake specific fuel consumption = 6 / 90 = 0.0667 g/kW-s = (0.0667 * 3600) / 1000 = 0.24 kg/kWh.

10. If the ip =2.62 kW and bp = 2.4 kW than fp = 5.04 kW
a) True
b) False

Explanation: Friction power = fp, brake power = bp = 2.4 kW, and indicated power = ip = 2.62 kW. ip = bp + fp ⇒ fp = ip – bp = 2.62 – 2.4 = 0.22 kW.

Sanfoundry Global Education & Learning Series – Automobile Engineering.

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