Engine Performance Parameter MCQ Quiz in বাংলা - Objective Question with Answer for Engine Performance Parameter - বিনামূল্যে ডাউনলোড করুন [PDF]

Concept:

Mechanical efficiency is given by:

Indicated power = Brake power + Friction power

Calculation:

Given:

Indicated power = 12 kW, η_mech = 90% = 0.9

Mechanical efficiency is:

Now,

Friction Power = Indicated power – brake power

Friction power = 12 – 10.8 = 1.2 kW

Explanation

Dynamometer – The work output or power output from the piston of the engine is found by a brake mechanism attached to the shaft of the engine, this mechanism is known as Dynamometer.

Types of dynamometer

Heenan and Froude dynamometer is a hydraulic dynamometer invented by William froude in 1877.

Concept:

The compression ratio is the ratio of volume before compression and after compression.

Let VS = Stroke volume

VC = Clearance volume

Explanation:

Mechanical efficiency:

• Mechanical efficiency is defined as the ratio of brake power (delivered power) to the indicated power (power provided to the piston).

• Mechanical efficiency takes into account the mechanical losses in an engine.
• In general, the mechanical efficiency of the engine varies from 70 – 90 % and it depends on speed and design of the engine and can fluctuate based on:
  • Engine Design and Technology: Older engines tend to have lower efficiencies, while newer engines with advanced technologies like direct injection and variable valve timing can achieve higher efficiencies.
  • Engine Load and Speed: Engines operate at their peak efficiency at specific load and speed conditions, often around their cruising range. At lower or higher loads and speeds, efficiency typically drops.
  • Temperature: Engines operate most efficiently within their normal operating temperature range. Cold starts and overheating can significantly reduce efficiency.
  • Maintenance: Poor engine maintenance, like clogged air filters or worn-out spark plugs, can lead to decreased efficiency.

Important PointsEngine Performance Parameter:

Indicated Power:

• It is the power developed inside the engine cylinder.

Brake Power:

• This is the actual power available at the crankshaft and may be called power output of the engine. It is always less than indicated power.

Indicated thermal efficiency:

• It is the ratio of IP to fuel power.

Brake thermal efficiency :

• It is the ratio of bp to fuel power.

Concept:

In Prony Brake Dynamometer,

Torque T = Wl in N-m.

Brake Power  in kW

where l is the length of the lever

Calculation:

Given:

Speed of engine N = 1000 rpm, load on brake W = 1000 N and length L = 750 mm = 0.75 m.

T = 1000 × 0.75 = 750 N

Thus B.P = 78.54 kW

Explanation:

Peak torque appears earlier in the RPM range because of the engine's volumetric efficiency characteristic.

Volumetric efficiency =

Theoretical mass intake = Displacement Volume × Air density at that temperature

Volumetric efficiency:

It is a measure of the engine breathing capacity - varies from low (during low speeds) to a peak at about 2/3rd of the speed range and again to a low value at higher speeds.

The reason for this is the engine piston speed and valve timing.

At low speeds, the valves stay open for a longer time but the suction in the engine is less because the piston moves slowly. Because of this, the air intake into the engine is lesser than what is maximum possible.

As the engine picks up speed, the higher piston speed creates more suction which makes the engine intake more and more air with each stroke. This increases the volumetric efficiency which peaks at a speed for a given engine and thus increasing the Torque.

Beyond this speed, flow losses at the intake valve dominate due to the increase in air intake velocity and this causes lesser and lesser air to be taken in per stroke. This reduces the Volumetric efficiency.

Concept:

Compression ratio = 

Work done = Mean effective pressure × swept volume

swept volume = (v1 – v2)

Calculation:

Given Compression ratio (r) = 8, v₁ = 0.9 m³/kg, Net heat interaction for cycle = 1575 kJ/kg

For any cycle, the net heat interaction = Net work interaction

⇒  

Work done = Mean effective pressure × swept volume (v₁ – v₂)

∴ 1575 × 10³ = p_m × (0.9 – 0.1125)

p_m = 20 bar

• The Morse test is a test conducted to determine the power developed in each cylinder in a multi-cylinder IC engine
• In this test first of all the power developed by all the cylinders together is determined experimentally
• Then the power of the individual cylinders is determined by cutting off the power supply to the spark plug of the cylinder under test
• Then the power developed by the engine with the remaining cylinders is determined experimentally and this value is subtracted from the first value and this gives you the power developed in the cylinder, whose spark plug was cut off
• Similarly, this test is carried out on all the cylinders of the engine individually

Thus Morse test is used to find out the frictional power of the multi-cylinder engine.

 

The methods are used to find out the frictional power are -

(a) Willan's line method

(b) Morse test

(c) Motoring test

(d) Retardation test

First indicated power is found out by cut out one cylinder at a time simultaneously. When all cylinders are working then it is possible to find out the Break Power of the engine. From Indicated Power and Break Power we can find out the Frictional power.

But the main point is that the Morse test is only applicable to multi-cylinder engines.

Here in the options, only one option has a multi-cylinder engine and that is option (3) and hence it is correct. 

Explanation:

Mechanical efficiency is defined as the ratio of brake power (delivered power) to the indicated power (power provided to the piston).

Mechanical efficiency takes into account the mechanical losses in an engine.

Additional Information

Engine Performance Parameter :

Indicated Power:

It is the power developed inside the engine cylinder.

Brake Power:

This is the actual power available at the crankshaft and may be called power output of the engine. It is always less than indicated power.

Indicated thermal efficiency :

It is the ratio of IP to fuel power.

Brake thermal efficiency :

It is the ratio of bp to fuel power.

Concept:

Indicated thermal efficiency:

It is the ratio of indicated power (I.P.) and energy in fuel per second.

η_ith =  

where  = mass flow rate of fuel (kg/s) and C.V. = calorific value of fuel (kJ/kg)

Calculation:

Given: 

I.P. = 15 kW, C.V. = 40000 kJ/kg,  = 0.001 kg/s

Indicated thermal efficiency, 

ηith = 37.5 %