The process by which an engine converts the chemical energy of fuel into mechanical energy is essentially a transformation of energy forms based on thermodynamic principles. This process is primarily achieved through stages such as combustion, expansion, and mechanical transmission. The specific principles differ due to variations in the ignition methods between gasoline and diesel engines.
1. Core Energy Conversion Pathway
The energy conversion in an engine follows a clear physical pathway: First, fuel (gasoline or diesel) mixes with air inside the cylinder and combusts, releasing the chemical energy stored in molecular bonds as thermal energy. Subsequently, the high-temperature, high-pressure gas generated by combustion expands, driving the piston’s movement and converting thermal energy (or internal energy) into mechanical energy.
2. Key Differences Between Gasoline and Diesel Engines
Gasoline and diesel engines differ significantly in the critical steps of achieving this conversion:
Gasoline Engine: Employs spark ignition. Near the end of the compression stroke, the spark plug generates an electrical spark to ignite the pre-mixed air-fuel mixture.
Diesel Engine: Utilizes compression ignition. During the compression stroke, air is highly compressed, causing its temperature to rise sharply above the auto-ignition point of diesel fuel. At this moment, the high-pressure injection system injects diesel into the cylinder, and the fuel ignites spontaneously.
3. Detailed Explanation of the Four-Stroke Working Cycle
Taking the most widely used four-stroke piston internal combustion engine as an example, its energy conversion is completed in a recurring cycle:
Intake Stroke: The intake valve opens, and the exhaust valve closes. The piston moves downward, drawing air (diesel engine) or an air-fuel mixture (gasoline engine) into the cylinder.
Compression Stroke: Both intake and exhaust valves are closed. The piston moves upward, compressing the gas inside the cylinder, thereby increasing its temperature and pressure to create conditions for efficient fuel combustion.
Power Stroke: The intake and exhaust valves remain closed. Near the piston’s top dead center, the fuel is ignited (via spark plug in gasoline engines or through fuel injection and compression ignition in diesel engines). The rapid combustion of high-temperature, high-pressure gas expands forcefully, driving the piston downward. This is the core step where chemical energy is converted into mechanical energy.
Exhaust Stroke: The exhaust valve opens, and the intake valve closes. The piston moves upward, expelling the combusted exhaust gases from the cylinder to prepare for the next working cycle.
4. Transmission and Output of Mechanical Energy
The reciprocating linear motion of the piston cannot directly drive the vehicle and requires further conversion and transmission through the drivetrain:
The piston is connected to the crankshaft via a connecting rod, which converts the piston’s reciprocating motion into the crankshaft’s rotational motion.
The rotational mechanical energy output from the crankshaft is ultimately transmitted to the wheels through a series of drivetrain components such as the clutch, transmission, and drive shaft, thereby propelling the vehicle.
5. Auxiliary Systems Ensuring Efficient and Stable Operation
To achieve efficient, stable, and clean energy conversion, the engine relies on key auxiliary systems:
Cooling System: Circulates coolant to carry away excess heat from the engine, maintaining it within an optimal operating temperature range.
Emission Control System: Utilizes devices such as three-way catalytic converters to transform harmful gases produced by combustion (e.g., carbon monoxide, nitrogen oxides) into harmless or less harmful substances, ensuring compliance with environmental standards.
In summary, an engine converts the chemical energy of fuel into thermal energy through internal combustion, then uses gas expansion to drive the piston and convert thermal energy into mechanical energy, and finally outputs power via the crank-connecting rod mechanism. The difference in ignition methods between gasoline and diesel engines is the primary distinction in their operational processes. The entire conversion relies on the coordinated work of precise mechanical structures and auxiliary systems.
