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Difficulty: HardEngine Principles and Internal Components

A technician is analyzing the operation of a single cylinder in a four-stroke internal combustion engine through a complete 720∘720^\circ crankshaft cycle. Starting from the moment fresh air and fuel enter the cylinder, in what chronological order do the following mechanical events occur?

  1. 1The intake valve opens while the piston moves downward from top dead center (TDC) to bottom dead center (BDC), drawing the fresh air-fuel mixture into the cylinder.
  2. 2Both valves remain closed while the piston ascends from bottom dead center (BDC) toward top dead center (TDC), compressing the trapped air-fuel mixture.
  3. 3The spark plug fires near top dead center (TDC), igniting the compressed mixture and forcing the piston downward toward bottom dead center (BDC) to deliver mechanical power.
  4. 4The exhaust valve opens while the piston ascends toward top dead center (TDC), sweeping spent combustion gases out through the exhaust port.

Answer

The correct chronological sequence is: (1) Intake valve opens as the piston descends from TDC to BDC to draw in the air-fuel mixture, (2) Both valves seal as the piston ascends toward TDC to compress the mixture, (3) Spark ignition occurs near TDC driving the piston downward to produce power, and (4) Exhaust valve opens as the piston ascends toward TDC to expel burned exhaust gases.
A four-stroke internal combustion engine operates in a strict sequence across two crankshaft rotations (720∘720^\circ): intake (piston moves down, intake valve open), compression (piston moves up, both valves closed), power (spark ignites mixture, piston forced down, both valves closed), and exhaust (piston moves up, exhaust valve open).

Step-by-Step Solution

1
Identify the initial event of the four-stroke cycle starting with the induction of the air-fuel charge.
The intake stroke occurs first as the intake valve opens and the descending piston creates a low-pressure area (0∘0^\circ to 180∘180^\circ crankshaft rotation).
An internal combustion engine requires fresh air and fuel in the combustion chamber before any compression or combustion can take place.
2
Determine the mechanical event following intake as the piston changes direction at bottom dead center.
The compression stroke occurs second as both intake and exhaust valves close and the piston moves upward toward top dead center (180∘180^\circ to 360∘360^\circ crankshaft rotation).
Compressing the air-fuel charge increases its density and thermal energy, allowing for rapid, complete combustion.
3
Identify the power-producing phase triggered by ignition.
The power stroke occurs third as spark ignition near TDC expands burning gases and drives the piston downward (360∘360^\circ to 540∘540^\circ crankshaft rotation).
This is the only stroke in the four-stroke cycle that delivers positive mechanical work to rotate the crankshaft.
4
Identify the final phase that purges the cylinder of combustion byproducts.
The exhaust stroke occurs fourth as the exhaust valve opens and the piston ascends to push gases out (540∘540^\circ to 720∘720^\circ crankshaft rotation).
Spent exhaust gases must be cleared from the cylinder before the next intake stroke can draw in an uncontaminated fresh charge.

Key Concept

Four-stroke internal combustion engine operating cycle and valve/piston synchronization across 720 degrees of crankshaft rotation
Estimated Time:1m 15s
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Topics

Engine Principles and Internal Components

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