Four-stroke cycle

Definition

The four-stroke cycle is a thermodynamic process used in internal combustion engines where the engine completes four distinct strokes: intake, compression, power, and exhaust. Each stroke corresponds to a specific movement of the piston within the cylinder and plays a critical role in converting fuel into mechanical energy. This cycle is fundamental to understanding how many modern vehicles operate efficiently while minimizing emissions.

5 Must Know Facts For Your Next Test

1. The four-stroke cycle includes four main strokes: intake, where the air-fuel mixture enters the cylinder; compression, where this mixture is compressed to increase efficiency; power, where combustion occurs and pushes the piston down; and exhaust, where burnt gases are expelled.
2. Each complete cycle of the four-stroke engine requires two revolutions of the crankshaft, leading to a power stroke once every two revolutions.
3. The efficiency of the four-stroke cycle comes from its ability to use separate strokes for intake and exhaust, allowing for better control over combustion conditions.
4. The design of a four-stroke engine often includes components like valves and camshafts to control the timing of each stroke, significantly impacting performance and efficiency.
5. Four-stroke engines are commonly found in cars and motorcycles due to their ability to produce more power with lower emissions compared to two-stroke engines.

Review Questions

• Compare the four-stroke cycle with the two-stroke cycle in terms of efficiency and emissions.
  • The four-stroke cycle is generally more efficient than the two-stroke cycle because it uses separate strokes for intake and exhaust, allowing for better combustion control. This separation minimizes the loss of unburnt fuel, leading to lower emissions. In contrast, two-stroke engines complete their cycles in just one crankshaft revolution but tend to mix intake and exhaust phases, often resulting in higher emissions and lower fuel efficiency.
• Evaluate how the design features of a four-stroke engine contribute to its performance during each stroke of the cycle.
  • Design features such as valves and camshafts play a crucial role in controlling when air-fuel mixtures enter the cylinder during the intake stroke and when exhaust gases are expelled during the exhaust stroke. This precise timing ensures that the compression stroke can maximize pressure before combustion, leading to a powerful expansion during the power stroke. These engineering choices directly influence performance metrics such as horsepower, torque, and fuel economy.
• Assess the impact of advancements in four-stroke engine technology on environmental concerns related to vehicular emissions.
  • Advancements in four-stroke engine technology have significantly reduced harmful emissions from vehicles. Innovations such as fuel injection systems, turbocharging, and variable valve timing improve combustion efficiency and allow for cleaner burning of fuel. These developments help meet increasingly stringent environmental regulations by lowering carbon monoxide, nitrogen oxides, and hydrocarbon emissions. As manufacturers continue to refine these technologies, they contribute positively to global efforts aimed at reducing air pollution and combating climate change.

Related terms

Internal Combustion Engine: A type of engine that generates power by burning fuel and air within a combustion chamber, utilizing the energy produced to drive pistons.

Otto Cycle:

A thermodynamic cycle that describes the functioning of a spark-ignition internal combustion engine, which is a specific implementation of the four-stroke cycle.

Piston:

A cylindrical component that moves up and down within the cylinder of an engine, compressing fuel-air mixtures and converting combustion energy into mechanical work.