Automobile Engine Knowledge

The automobile engine is the device that provides power to a vehicle, serving as its heart and determining the vehicle's power performance, fuel economy, stability, and environmental friendliness.

There are two common types of automobile engines: petrol engines and diesel engines. Both are reciprocating piston internal combustion engines that convert the chemical energy of fuel into mechanical energy.

The advantages of petrol engines include high rotational speed, light weight, low noise, easy starting, and low manufacturing costs.

Diesel engines, on the other hand, offer higher compression ratios, greater thermal efficiency, and superior fuel economy and emissions performance compared to petrol engines.

Classification of internal combustion engines:

1. According to the working method of the intake system, they can be divided into four types: naturally aspirated, turbocharged, supercharged, and twin-charged.
2. By piston motion, they are categorised as reciprocating piston internal combustion engines and rotary piston engines.
3. By cylinder arrangement, they include inline engines, V-type engines, W-type engines, and horizontally opposed engines.
4. Based on the number of cylinders, they can be single-cylinder or multi-cylinder engines. Modern vehicles commonly use three-, four-, six-, or eight-cylinder engines.
5. By cooling method, they are classified as water-cooled or air-cooled engines. Water-cooled engines, which provide even cooling, reliable operation, and effective cooling, are widely used in modern automotive engines.
6. By stroke number, they are either four-stroke or two-stroke internal combustion engines. Four-stroke engines are predominantly used in automobile engines.
7. By fuel supply method, they include carburettor engines, electronic fuel injection engines, and direct injection engines.

Working principle:

A four-stroke petrol engine mixes air and petrol in a specific ratio to form a combustible mixture, which is drawn into the cylinder during the intake stroke. The compressed mixture ignites, generating thermal energy. The high-temperature, high-pressure gas acts on the piston crown, driving the piston in a reciprocating linear motion and outputting mechanical energy via the connecting rod, crankshaft, and flywheel mechanism. The four-stroke petrol engine completes one working cycle through the intake, compression, power, and exhaust strokes.

The working principle of a four-stroke diesel engine is similar to that of a petrol engine, with each cycle comprising intake, compression, power, and exhaust strokes. However, as diesel has a lower auto-ignition temperature and higher viscosity compared to petrol, diesel engines employ compression ignition (combustion by compression), whereas petrol engines use spark plugs for ignition.

Structure:

The engine consists of two main mechanisms—the crank-connecting rod mechanism and the valve train—as well as five systems: cooling, lubrication, ignition, fuel supply, and starting. Key components include the cylinder block, cylinder head, piston, piston pin, connecting rod, crankshaft, and flywheel.

The working chamber of a reciprocating piston internal combustion engine is called the cylinder, which has a cylindrical inner surface. The piston, moving reciprocally within the cylinder, is connected via the piston pin to one end of the connecting rod, while the other end is linked to the crankshaft. The crankshaft is supported by bearings on the cylinder block and rotates within them, forming the crank-connecting rod mechanism.

As the piston moves reciprocally in the cylinder, the connecting rod drives the crankshaft to rotate. Conversely, when the crankshaft turns, the connecting rod journals move in a circular motion within the crankcase, pushing the piston up and down via the connecting rod. Each full rotation of the crankshaft causes the piston to move up and down once, continuously varying the cylinder volume from small to large and back again in a repeating cycle.

The top of the cylinder is sealed by the cylinder head, which houses the intake and exhaust valves. The opening and closing of these valves allow air intake into the cylinder and exhaust expulsion. The valve operation is driven by the camshaft, which is itself driven by the crankshaft via a timing belt or gears.

Performance indicators:

Engine performance indicators characterise the engine's capabilities and serve as benchmarks for evaluating different engine types. Key performance indicators include power output, fuel economy, environmental impact, reliability, and durability.

Power output indicators:

These metrics represent the engine's work capacity, typically evaluated using effective torque, effective power, and engine speed.