The document discusses internal combustion engines, including the workings of 4-stroke spark ignition and diesel engines. It defines external and internal combustion engines, explaining the 4 strokes of a gasoline engine: intake, compression, combustion/power, and exhaust. It also covers the 4 strokes of a diesel engine and compares the 2 engine types. The document lists assumptions for the Otto and diesel cycles and derives the efficiency expressions for both cycles. It differentiates between 2-stroke and 4-stroke engines and compares petrol and diesel engines.

1. Mechanical Technology-I
Internal Combustion Engines
1. Define external and internal combustion engines
and provide relevant examples.
Ans.
External Combustion Engines: The combustion of fuel in presence of air occurs outside
the cylinder, only the heat produced is transported inside the engine, having its own
working fluid(steam). Example: Steam Engines, Steam Turbines.
Internal Combustion Engines: In these engines, either the combustion of fuel takes place
inside the cylinder, or the products of combustion works as the working fluid. Example:
Petrol Engine, Diesel engine etc.
2. Explain the working principle of a 4-stroke
spark ignition engine with necessary illustrations. What are the advantages of
using a 4-cylinder engine over a single-cylinder engine?
Ans. The four strokes refer to intake, compression, combustion and exhaust strokes that
occur during two crankshaft rotations per working cycle of the Gasoline and Diesel engine.
Here’s a brief explanation of its working principle:
Intake Stroke: The piston travels from TDC to BDC with the intake valve open and exhaust
valve closed. This creates an increasing volume in the combustion chamber and the
pressure differential causes air and fuel to be pushed into the cylinder.
Compression Stroke: When the piston reaches BDC, the intake valve closes and piston
travels back to TDC. This compresses the air-fuel mixture and raises both the temperature
and pressure in the cylinder. Near the end of the stroke, the spark plug is fired and
combustion is initiated.
Combustion or Power Stroke: Combustion of the air fuel mixture occurs in a very short
length of time. Combustion changes the composition of the gas mixture and increases the
temperature and pressure. High pressure created by the combustion process pushes the
piston away from TDC. This stroke produces the work output of the engine cycle. As the
piston travels from TDC to BDC, cylinder volume is increased and that causes pressure and
temperature to drop.
Exhaust Stroke: Pressure and temperature are still high relative to the surroundings and this
differential causes much of the hot gases to be pushed out of the cylinder. With the exhaust
valve remaining open, piston travels from BDC to TDC in the exhaust stroke. This pushes
most of the remaining exhaust gases out of the cylinder into the exhaust system. At the end,
the intake valve starts to open, so that it is fully open by TDC when the new intake strokes
starts the next cycle.
Advantage of using 4-stroke cycle:
Power and Balance: 4-stroke cylinder can produce more power and torque than a singe -
cylinder engine. It also runs smoothly due to better balance.

2. Fuel Efficiency: They are more fuel efficient because they can distribute workloads among
the cylinders, allowing for efficient combustion.
Reduced Vibration: The balanced configuration results in fewer vibrations compared to a
single-cycle engine, leading to a smoother and more comfortable ride.
Compact Design: 4-strokes engines a relatively compact and lightweight, making them
usable for a wide range of vehicles.
Better Performance: In terms of acceleration and top speed, 4-cylinder engines tend to
perform better.
3. Describe the working principle of a 4-stroke
cycle compression ignition engine with necessary illustrations.
Ans: The Diesel engine is also known as Compression Ignition engine (CIE) since ignition
of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical
compression instead of incitation by external source (spark plug).
Here is the working principle of 4-stroke cycle:
Intake Stroke - The same as the intake stroke in an SIE with one major difference: no fuel
is added to the incoming air.
Compression Stroke - The same as in an SIE except that only air is compressed, and
compression is to higher pressures and temperature. - Late in the stroke fuel is injected
directly into the combustion chamber, where it mixes with the very hot air. This causes the
fuel to evaporate and self-ignite, causing combustion to start .
Combustion and Power Stroke - Combustion is fully developed by TDC and continues at
about constant pressure until fuel injection is complete and the piston has started towards
BDC. - The power stroke continues as combustion ends and the piston travels towards
BDC.
Exhaust Stroke: Same as with an SI engine.
4. List the air standard assumptions for the Otto
cycle and diesel cycle.
Ans. Otto cycle ( An air-standard cycle):
a. Air alone is the working fluid.
b. Heat loss is considered zero while the heat capacity of the air is assumed to be
constant.
c. The combustion process is replaced with a heat addition term Qin and Exhaust
process is changed into a heat rejection process Qout.
d. Intake and exhaust strokes occur at constant pressure.
e. Compression and expansion processes are isentropic.
f. The heat addition processes are constant-volume for Otto Cycle and constant-
pressure for Diesel Cycle.
g. Heat rejection process for all cycles is a constant-volume process.
h. All the above-mentioned processes are considered reversible.

3. Diesel Cycle(Air-standard cycle):
a. The major difference is heat addition process occurs at constant pressure.
b. For the same cylinder input and compression ratio, Diesel cycle is thermally
efficient than Otto cycle.
5. Create T-S diagrams for the Otto cycle and
Diesel cycle, and explain the different processes. Derive the expressions for
the efficiency of these cycles.
Ans.
Process 0–1: a mass of air is drawn into piston/cylinder arrangement at constant
pressure.
Process 1-2: an adiabatic (isentropic) compression of the charge as the piston moves
from bottom dead center (BDC) to top dead center (TDC).
Process 2-3: a constant-volume heat transfer to the working gas from an external source
while the piston is at top dead center. This process is intended to represent the ignition
of the fuel-air mixture and the subsequent rapid burning.
Process 3-4: an adiabatic (isentropic) expansion (power stroke).
Process 4-1: completes the cycle by a constant-volume process in which heat is rejected
from the air while the piston is at bottom dead center.
Process 1-0: the mass of air is released to the atmosphere in a constant pressure process.

4. 6. Explain the valve timing diagram for Spark
Ignition Engines (SIE) and Compression Ignition Engines (CIE) with necessary
illustrations.
Ans.
7. Elaborate on the working principle of a
two-stroke cycle engine with necessary illustrations.
Ans. Combustion - With the piston at TDC combustion occurs very quickly, raising the
temperature and pressure to peak values, almost at constant volume.
❖ Expansion Stroke or Power Stroke - Very high pressure created by the combustion
process forces the piston down in the power stroke. - The expanding volume of the
combustion chamber causes pressure and temperature to decrease as the piston travels
towards BDC.
❖ Exhaust Blowdown - At about 75° bBDC, the exhaust port opens, and blowdown occurs.
- The exhaust port is uncovered as the piston approaches BDC. - After blowdown the
cylinder remains filled with exhaust gas at lower pressure.
8. Differentiate between a two-stroke and a
four-stroke engine.
Ans.

5. Difference between 4-stroke and 2-stroke engine:
4-stroke engine 2-stroke engine
Construction Complex; Valve, CAM
shaft – mechanism
Simple; Ports instead of
valves
Cycle executes 1 cycle per 2
crankshafts revolutions.
For one complete
revolution of the
crankshaft, the engine
executes one cycle
Weight Around 50% more heavier Lighter; Easier to
manufacture
Emissions More environment friendly Produces a lot of pollution
including burnt oil, fuel
leaks in exhaust.
Power to weight ratio Relatively lower Relatively higher
Durability More durable Since designed to run at
higher RPM, they tend to
wear out faster.
Efficiency Higher; Fuel consumed
once/ 4-stroke
Lower; Once every 2-
strokes
Applications Almost all modern cars,
trucks, buses
Lawnmower, Dart bike,
Scooter (Old
9. Highlight the distinctions between a petrol
engine and a diesel engine.
Ans:
Petrol Engine Diesel Engine
Induction Mixture of air and gasoline Only air is inducted
Compression peak
pressure
Almost 10 bars at the end
of compression stroke
Almost 35 bars at the end
of the stroke
Ignition The charge is ignited with
a spark plug (spark
ignition)
The fuel is injected in the
form of fine spray. The
temperature of the
compressed air is
sufficiently high to self-
ignite the fuel
(compression ignition)
Combustion The combustion of fuel
takes place approx. at
constant volume. It works
on Otto cycle.
The combustion of fuel
takes place approximately
at constant pressure. It
works on Diesel cycle.
Compression ratio Around 8 – 11 Around 16 – 24
State of combustion Homogenous combustion Myriad combustion
Noise Less noise and vibration More noise and vibration
(higher peak pressure)
Thermal efficiency Upto about 26% Upto about 40%

6. Maintenance cost Is less Is relatively higher
Running cost The running cost of petrol
engines are high due to
excessive cost of petrol/
octane.
The running cost of diesel
engines are relatively low
due to lower cost of diesel.
Applications Petrol engines are
employed in light duty
vehicles such as scooters,
motorcycles, cars.
Diesel engines are
generally employed in
heavy duty vehicles like
buses, trucks, etc
10. Why does a Compression Ignition Engine (CIE)
experience more noise and vibration than a Spark Ignition Engine (SIE)?
Ans. Compression Ignition Engines (diesel) produce more noise and vibration than Spark
Ignition Engines (gasoline) due to factors such as spontaneous ignition, higher compression
ratios, fuel properties, engine design, and faster combustion in diesel engines. These factors
lead to more intense and rapid pressure spikes, resulting in increased mechanical stresses
and greater noise and vibration levels.
11. Explain why the compression ratio of Spark
Ignition Engines (SIE) is generally lower than that of Compression Ignition
Engines (CIE).
Ans: The compression ratio of Spark Ignition Engines (SIE) is generally lower than that of
Compression Ignition Engines (CIE) due to differences in their combustion processes. In
SIE, fuel and air are mixed before ignition, and a spark plug is used to initiate combustion.
Lower compression ratios in SIE are needed to prevent premature ignition (knocking) and
to allow for more controlled combustion. In contrast, CIE relies on spontaneous ignition
from the high temperatures generated during compression, allowing for higher compression
ratios to achieve efficient combustion without the risk of knocking.
12. What will happen if we use diesel in a petrol
engine and petrol in a diesel engine?
Ans: Using the wrong fuel in an engine designed for a different type can lead to various
issues:
Diesel in a Petrol Engine:
Combustion Issues: Diesel fuel is less volatile than petrol. If used in a petrol engine, it
may not vaporize properly, leading to incomplete combustion, reduced engine
performance, and increased emissions.
Fuel System Damage: Diesel has lubricating properties that may cause damage to the
petrol engine's fuel system components, including the fuel pump and injectors.
Starting Problems: Diesel fuel requires higher temperatures for ignition. A petrol engine
may struggle to start or not start at all when fueled with diesel.

7. Petrol in a Diesel Engine
Combustion Issues: Petrol has a lower ignition temperature compared to diesel. If used in
a diesel engine, it may ignite prematurely, causing knocking, engine damage, and reduced
efficiency.
Lack of Lubrication: Diesel fuel provides lubrication to the fuel system components.
Petrol's lower lubricating properties may lead to increased wear and potential damage.
In both cases, using the wrong fuel can result in significant damage to the engine and its
components. It's essential to use the fuel type specified by the manufacturer to ensure proper
combustion, performance, and the longevity of the engine. If a mistake is made and the wrong
fuel is introduced, it's crucial to seek professional assistance to minimize potential damage and
ensure the proper cleaning or repair of the fuel system.