# FREE EETC Four-Stroke Engines Questions and Answers

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#### What is the primary cause of the permanent breakdown of oil film strength in a four-stroke engine?

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Explanation:
The permanent breakdown of oil film strength in a four-stroke engine is primarily caused by the shearing of the oil layers. Shearing occurs when the oil experiences mechanical stress or turbulence, leading to the disruption and thinning of the lubricating film. This phenomenon reduces the oil's ability to maintain a protective barrier between moving engine components, resulting in increased friction, wear, and potential damage. Regular oil changes and using high-quality lubricants can help mitigate the effects of oil shearing in four-stroke engines.

#### In a two-cylinder, four-stroke engine, how many power strokes will occur in two rotations of the crankshaft?

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Explanation:
In a two-cylinder, four-stroke engine, each cylinder completes one power stroke in every two rotations of the crankshaft. Since there are two cylinders, there will be two power strokes in total during two rotations of the crankshaft. This occurs because each cylinder completes its power stroke once every two revolutions of the crankshaft in a four-stroke engine cycle.

#### How many camshaft revolutions are equivalent to one revolution of the crankshaft in a four-stroke engine?

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Explanation:
In a four-stroke engine, the camshaft revolves at half the speed of the crankshaft. This means that for every one revolution of the crankshaft, the camshaft completes half a revolution. This timing relationship ensures that the intake and exhaust valves are actuated at the appropriate times during the engine cycle.

#### During the intake stroke in a four-stroke engine, what are the positions of the valves?

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Explanation:
During the intake stroke of a four-stroke engine, the intake valve is open to allow the entry of air-fuel mixture into the cylinder from the intake manifold. Simultaneously, the exhaust valve remains closed to prevent any exhaust gases from escaping. This configuration ensures that a fresh air-fuel mixture is drawn into the cylinder for combustion during the subsequent compression stroke.

#### Which stroke follows the exhaust stroke in a four-stroke engine?

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Explanation:
In a four-stroke engine, the intake stroke follows the exhaust stroke. After the exhaust stroke, the intake valve opens, allowing the piston to move downwards and draw in a fresh air-fuel mixture from the intake manifold into the cylinder during the intake stroke. This cycle ensures the continuous operation of the engine.

#### Which of the following factors may affect the operating temperature of an air-cooled engine?

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Explanation:
All of the listed factors—engine oil quality/quantity, spark plug heat range, and carburetor jetting—can influence the operating temperature of an air-cooled engine. Engine oil quality and quantity affect lubrication and cooling, while the spark plug heat range determines the temperature at which the spark plug operates, impacting combustion and engine heat. Similarly, carburetor jetting affects fuel mixture and combustion, which can influence engine temperature. Therefore, all these factors collectively contribute to the overall operating temperature of an air-cooled engine.

#### What is used to ignite the air/fuel mixture inside the cylinder in a four-stroke engine?

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Explanation:
In a four-stroke engine, the ignition of the air/fuel mixture inside the cylinder is achieved by a spark plug. The spark plug generates an electric spark that ignites the compressed air/fuel mixture, initiating the combustion process. This controlled ignition leads to the expansion of gases, driving the piston downwards during the power stroke.

#### How many revolutions of the crankshaft are required to complete one power cycle in a four-stroke engine?

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Explanation:
In a four-stroke engine, one power cycle—consisting of intake, compression, power, and exhaust strokes—requires two revolutions of the crankshaft. This means that each piston completes its four-stroke cycle (intake, compression, power, and exhaust) once every two revolutions of the crankshaft.

#### What is the most likely cause of a broken compression ring in a four-stroke engine?

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Explanation:
Detonation, also known as engine knock, is the most likely cause of a broken compression ring in a four-stroke engine. Detonation occurs when the air/fuel mixture ignites prematurely or unevenly in the combustion chamber, leading to a rapid increase in pressure that can cause mechanical damage, including broken piston rings. Addressing detonation issues through proper fuel quality, ignition timing, and engine tuning is essential to prevent such damage in four-stroke engines.

#### What does a four-stroke engine primarily convert into mechanical energy?

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Explanation:
In a four-stroke engine, the primary conversion that occurs is from chemical energy stored in the fuel into mechanical energy. This process involves the combustion of the air/fuel mixture inside the cylinder, which generates heat and pressure, leading to the movement of the piston and ultimately producing mechanical work.

#### What causes pre-ignition in a four-stroke engine?

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Explanation:
Pre-ignition in a four-stroke engine occurs when the air/fuel mixture ignites before the spark plug fires, often due to a hot spot in the combustion chamber. This premature ignition can lead to knocking, engine damage, and reduced performance. Identifying and addressing hot spots is crucial in preventing pre-ignition issues in four-stroke engines.