A car engine converts the chemical energy stored in fuel into mechanical energy that turns the wheels. Most gasoline cars use a four-stroke internal combustion engine, where fuel burns inside cylinders to push pistons. Understanding this process helps explain power, efficiency, emissions, cooling, lubrication, and many common vehicle problems.
The engine is a carefully timed system, not just an explosion chamber.
Key Facts
- Four-stroke cycle: intake, compression, power, exhaust.
- Engine displacement = number of cylinders × volume swept by one piston.
- Swept volume of one cylinder = πr^2s, where r is cylinder radius and s is piston stroke.
- Power = torque × angular speed, or P = τω.
- Thermal efficiency = useful work output ÷ heat energy input.
- In a four-stroke engine, each cylinder produces one power stroke every two crankshaft rotations.
Vocabulary
- Piston
- A piston is a moving metal cylinder that slides inside the engine cylinder and transfers gas pressure to the crankshaft.
- Crankshaft
- The crankshaft is the rotating shaft that converts the piston's back-and-forth motion into rotational motion.
- Spark plug
- A spark plug is an electrical device that ignites the compressed air-fuel mixture in a gasoline engine.
- Valve
- A valve is a timed opening that controls when air enters the cylinder and when exhaust gases leave.
- Fuel injector
- A fuel injector is a controlled nozzle that sprays fuel into the intake air or directly into the cylinder.
Common Mistakes to Avoid
- Thinking the piston directly turns the wheels, which is wrong because the piston first drives the crankshaft, then power passes through the transmission and drivetrain.
- Confusing the intake and power strokes, which is wrong because intake draws in air and fuel while the power stroke happens after ignition forces the piston downward.
- Assuming more fuel always means more power, which is wrong because the engine also needs the correct air-fuel ratio, spark timing, compression, and airflow.
- Ignoring cooling and oil flow, which is wrong because overheating or lack of lubrication can damage pistons, bearings, valves, and cylinder walls.
Practice Questions
- 1 A four-cylinder engine has a cylinder radius of 4.0 cm and a piston stroke of 8.0 cm. Using swept volume = πr^2s, find the total engine displacement in cm^3.
- 2 An engine produces 180 N·m of torque at 3000 rpm. Convert 3000 rpm to rad/s using ω = 2π(rpm)/60, then calculate power using P = τω.
- 3 Explain why opening the intake valve at the wrong time would reduce engine performance, even if the spark plug and fuel injector are working correctly.