Engine Performance Parameter MCQ Quiz in తెలుగు - Objective Question with Answer for Engine Performance Parameter - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Explanation:

Mechanical efficiency:

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

\({\eta _m} = \frac{{bp}}{{ip}} = \frac{{bp}}{{bp + fp}}\)

• 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.

\(ip = \frac{{{p_m}LAN.n}}{{60}}\)

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.

\(BP = \frac{{2\pi NT}}{{60}}\)

Indicated thermal efficiency:

• It is the ratio of IP to fuel power.

\({\eta _i} = \frac{{ip}}{{{{\dot m}_f} \times CV}}\)

Brake thermal efficiency :

• It is the ratio of bp to fuel power.

\({\eta _b} = \frac{{bp}}{{{{\dot m}_f} \times CV}}\)

Concept:

Mechanical efficiency is given by:

\({\eta _{mech}} = \frac{{Brake\;power}}{{Indicated\;power}}\)

Indicated power = Brake power + Friction power

Calculation:

Given:

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

Mechanical efficiency is:

\({\eta _{mech}} = \frac{{Brake\;power}}{{Indicated\;power}}\)

\(0.90 = \frac{{Brake\;power}}{{12}} \Rightarrow Brake\;power = 0.90 \times 12 = 10.8\;kW\)

Now,

Friction Power = Indicated power – brake power

Friction power = 12 – 10.8 = 1.2 kW

Concept:

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

Let VS = Stroke volume

VC = Clearance volume

\(Compression\ ratio\left( {{r_c}} \right) = \frac{{Effective\ Swept\ Volume}}{{Clearance\ Volume}}\)

\({r_c} = \frac{{{V_S} + {V_C}}}{{{V_C}}}\)

\({r_c} = \frac{{{V_S} + {V_C}}}{{{V_C}}}=1+\frac{{{V_S} }}{{{V_C}}}\)

\({r_c} = 1 + \frac{{300}}{{20}} = 16\)

Concept:

In Prony Brake Dynamometer,

Torque T = Wl in N-m.

Brake Power \(B.P = \frac{{2\pi NT}}{{60}}\) 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

\(B.P = \frac{{2 \times \pi \times 1000 \times 750}}{{60}} = 78539.81\;W\)

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 = \(\frac{Actual\;mass\;of\;air\;intake\;per\;stroke}{Theoretical\;mass\;of\;air\;intake\;at\;that\;temperature}\)

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.

Concept:

Compression ratio = \(r = \frac{{volume\;before\;compression\;}}{{volume\;after\;compression}}\)

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

\(r = \frac{{{v_1}}}{{{v_2}}}\)

⇒  \({v_2} = \frac{{0.9}}{8} = 0.1125\frac{{{m^3}}}{{kg}}\)

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

∴ 1575 × 10³ = p_m × (0.9 – 0.1125)

p_m = 20 bar

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.

• 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:

The most efficient method of increasing the power of an engine is by increasing the flow of air into the engine to enable more fuel to be burnt, which can be done by two devices supercharger and turbocharger. Some of the key points are mentioned for both the devices in the tabular form below.

Explanation:

• Let’s start with the piston at the TDC.
• 1st stroke the piston moves down, and the intake valve opens.
• 2nd stroke, the intake valve closes, and the piston moves up.
• The crankshaft has rotated once, and one valve has opened.
• 3rd stroke, the spark plug fires the mixture; the piston moves down.
• 4th stroke, the exhaust valve opens, and the piston moves up.
• The engine has completed 2 revolutions and each valve has opened once.
• The camshaft travels at half the crankshaft speed because each valve opens only once on every two revolutions of the crankshaft.
• Therefore, In four-stroke engines, the camshaft gear has twice the number of teeth as on crankshaft gear.

Additional Information

• A two-stroke engine requires only two strokes of the piston to complete one full cycle.
• Therefore, it requires only one rotation of the crankshaft to complete a cycle.
• This means several events must occur during each stroke for all four events to be completed in two strokes.
• In a two-stroke engine, the camshaft is geared so that it rotates at the same speed as the crankshaft (1: 1).