the engine of the car

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What are the differences of the engine between HEV（hybrid electric vehicle）and ICEV（internal combusion electric vehicle）?

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The engines of fuel vehicles generally use the Otto cycle, while hybrid vehicles generally use the Atkinson cycle. Their structures are basically the same, but for the same displacement, the Atkinson cycle has less intake air.

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Otto cycle

The Otto cycle is a thermodynamic cycle that describes the operation of an internal combustion engine, particularly in gasoline-powered engines. It's named after Nikolaus Otto, who developed the concept in the 19th century.

Here’s a breakdown of how the Otto cycle works:

Intake Stroke: The intake valve opens, and the piston moves down, drawing in a mixture of air and fuel.
Compression Stroke: The intake valve closes, and the piston moves upward, compressing the air-fuel mixture. As the mixture compresses, its temperature and pressure rise.
Power Stroke: When the piston reaches the top of the compression stroke, the spark plug ignites the compressed air-fuel mixture, causing a small explosion. This rapidly increases the pressure in the cylinder, pushing the piston down.
Exhaust Stroke: The exhaust valve opens, and the piston moves up again to expel the burnt gases from the combustion chamber.
This cycle repeats, powering the engine and generating mechanical energy.

The efficiency of an Otto cycle engine depends on the compression ratio (the ratio of the maximum to minimum volume of the cylinder). The higher the compression ratio, the more efficient the engine is, but it can also lead to knocking (pre-detonation), which is why higher compression ratios are generally used in high-performance or premium fuel engines.

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Atkinson cycle

The Atkinson cycle is another thermodynamic cycle used in internal combustion engines, but it differs from the Otto cycle in how it manages the intake and exhaust strokes. It was invented by James Atkinson in 1882. The key feature of the Atkinson cycle is that it uses a longer expansion stroke compared to the compression stroke, which improves fuel efficiency.

Here’s how the Atkinson cycle works:

Intake Stroke: The intake valve opens, and the piston moves down, drawing in a mixture of air and fuel.
Compression Stroke: The piston moves up to compress the air-fuel mixture. However, the intake valve is left open for a longer period, which allows some of the compressed air-fuel mixture to flow back into the intake manifold. This shortens the effective compression stroke, reducing the amount of energy needed to compress the mixture.
Power Stroke: The intake valve closes, and the piston continues its upward motion, compressing the remaining mixture. When the piston reaches the top, the spark plug ignites the mixture. Since the compression stroke was shorter, the expansion stroke is longer, which allows the engine to extract more energy from the combustion process. The piston moves down with more force.
Exhaust Stroke: The exhaust valve opens, and the piston moves up again to expel the exhaust gases.

Key Differences and Benefits:
Longer Expansion Stroke: The main advantage of the Atkinson cycle is that the longer expansion stroke (compared to compression) allows the engine to extract more energy from the combustion process, improving its thermal efficiency.
Reduced Power Output: Because the intake valve is held open longer, there’s less air and fuel in the combustion chamber, which results in less power per cycle. To compensate for this, Atkinson-cycle engines often use electric motors or hybrid systems to assist with acceleration and power output, like in hybrid cars (e.g., Toyota Prius).
Fuel Efficiency: The Atkinson cycle is more fuel-efficient than the Otto cycle at part load because of the longer expansion stroke. This is why it's often used in hybrid vehicles, where the engine is running at a more constant, optimal efficiency.

In summary, the Atkinson cycle sacrifices some power for improved fuel efficiency, making it well-suited for applications where fuel economy is prioritized over maximum power, such as in hybrid vehicles.