Part VII — IC Engines and Power Plant Engineering

Chapter 7: Internal Combustion Engines and Power Generation Basics

7.1 Classification of IC Engines

Internal combustion (IC) engines are classified by ignition type into Spark-Ignition (SI) engines (petrol/gasoline engines, using a spark plug to ignite a pre-mixed air-fuel charge, typically operating on the Otto cycle) and Compression-Ignition (CI) engines (diesel engines, where air is compressed to a high temperature that spontaneously ignites injected fuel, typically operating on the Diesel cycle). Engines are also classified by stroke cycle into four-stroke engines (intake, compression, power/expansion, exhaust — one power stroke per two crankshaft revolutions) and two-stroke engines (combining these four events into two strokes/one crankshaft revolution, providing more power per cycle for a given size but generally less fuel-efficient and higher-emission than four-stroke designs).

7.2 Four-Stroke Engine Cycle

In the four-stroke cycle: the intake stroke draws the air-fuel mixture (SI) or air alone (CI) into the cylinder as the piston moves down; the compression stroke compresses this charge as the piston moves up; the power (expansion) stroke follows ignition/combustion, with expanding gases driving the piston down and delivering work; and the exhaust stroke expels burnt gases as the piston moves up again, completing the cycle.

7.3 Engine Efficiency and Performance Terms

Compression ratio is the ratio of total cylinder volume (at bottom dead centre) to clearance volume (at top dead centre) — SI engines typically have lower compression ratios (limited by knocking/pre-ignition risk) than CI engines, which rely on high compression for auto-ignition. Mean Effective Pressure (MEP) is a theoretical average pressure that, if constant throughout the power stroke, would produce the same net work output as the actual varying pressure — a useful figure of merit for comparing engines of different sizes. Volumetric efficiency compares the actual volume of air/mixture drawn into the cylinder to the theoretical swept volume, reflecting how effectively the engine "breathes."

7.4 Power Plant Engineering Basics

A thermal (steam) power plant converts fuel's chemical energy into electricity via the sequence: boiler (heat generation, converting water to high-pressure steam), turbine (steam expansion drives a generator), condenser (steam is condensed back to water), and feed pump (returns water to the boiler) — essentially a practical Rankine cycle implementation. A diesel power plant uses diesel engines directly coupled to generators, commonly used for smaller-scale or standby/backup power generation due to relatively higher fuel costs compared to large-scale thermal/hydro plants. A nuclear power plant uses controlled nuclear fission (typically of uranium-235) to generate heat, which is then used similarly to a conventional thermal plant to produce steam and drive a turbine-generator.

7.5 Practice Set — IC Engines & Power Plant Engineering (16 MCQs)

1. Spark-Ignition (SI) engines typically operate on which idealised thermodynamic cycle?
   (a) Diesel cycle (b) Otto cycle (c) Rankine cycle (d) Brayton cycle
2. Compression-Ignition (CI) engines typically operate on which idealised thermodynamic cycle?
   (a) Otto cycle (b) Diesel cycle (c) Rankine cycle (d) Carnot cycle exclusively
3. In a CI (diesel) engine, ignition occurs due to:
   (a) A spark plug igniting a pre-mixed charge (b) Air being compressed to a high temperature that spontaneously ignites injected fuel (c) An external flame source (d) Electrical resistance heating
4. A four-stroke engine completes one power stroke per:
   (a) One crankshaft revolution (b) Two crankshaft revolutions (c) Four crankshaft revolutions (d) Half a crankshaft revolution
5. A two-stroke engine, compared to a four-stroke engine, generally provides:
   (a) Less power per cycle for a given size (b) More power per cycle for a given size, but often less fuel efficiency (c) Identical efficiency in all cases (d) No practical power output
6. The four strokes of a four-stroke engine cycle, in order, are:
   (a) Intake, compression, power, exhaust (b) Power, intake, exhaust, compression (c) Exhaust, intake, power, compression (d) Compression, exhaust, intake, power
7. During the compression stroke, the piston:
   (a) Moves down, drawing in charge (b) Moves up, compressing the charge (c) Remains stationary (d) Moves down, expelling exhaust
8. Compression ratio is defined as the ratio of:
   (a) Clearance volume to total cylinder volume (b) Total cylinder volume to clearance volume (c) Bore to stroke (d) Power to torque
9. CI (diesel) engines, compared to SI (petrol) engines, typically have:
   (a) Lower compression ratios (b) Higher compression ratios, relying on high compression for auto-ignition (c) No compression at all (d) Identical compression ratios always
10. Mean Effective Pressure (MEP) is best described as:
    (a) The maximum instantaneous pressure in the cylinder (b) A theoretical average pressure producing the same net work as the actual varying pressure (c) The exhaust gas pressure only (d) The atmospheric pressure at sea level
11. Volumetric efficiency compares:
    (a) Actual air/mixture volume drawn in to the theoretical swept volume (b) Fuel consumption to distance travelled (c) Engine weight to power output (d) Compression ratio to MEP
12. In a thermal (steam) power plant, the boiler's primary function is to:
    (a) Condense steam back to water (b) Generate heat, converting water to high-pressure steam (c) Drive the generator directly (d) Store fuel
13. In a thermal power plant, the turbine's primary function is to:
    (a) Generate heat (b) Expand steam to drive a generator (c) Condense steam (d) Pump water into the boiler
14. A thermal power plant's basic cycle is essentially a practical implementation of the:
    (a) Otto cycle (b) Diesel cycle (c) Rankine cycle (d) Carnot cycle exactly
15. Diesel power plants, compared to large-scale thermal/hydro plants, are commonly used for:
    (a) The bulk of a nation's baseload power generation (b) Smaller-scale or standby/backup power generation (c) Only nuclear applications (d) Only offshore wind integration
16. A nuclear power plant generates heat primarily through:
    (a) Combustion of coal (b) Controlled nuclear fission (typically of uranium-235) (c) Solar radiation (d) Wind turbine rotation

Answer Key

1.(b)

2.(b)

3.(b)

4.(b)

5.(b)

6.(a)

7.(b)

8.(b)

9.(b)

10.(b)

11.(a)

12.(b)

13.(b)

14.(c)

15.(b) 16.(b)