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 the main parts helps explain power, efficiency, maintenance, and common problems.
A cutaway view makes the hidden motion inside the engine easier to see.
Key Facts
- A four-stroke engine cycle is intake, compression, power, and exhaust.
- Engine displacement = number of cylinders × volume swept by one piston.
- Power = torque × angular speed, or P = τω.
- For a four-stroke engine, each cylinder has one power stroke every 2 crankshaft rotations.
- Compression ratio = maximum cylinder volume ÷ minimum cylinder volume.
- Air, fuel, spark, compression, cooling, and lubrication must all work together for smooth engine operation.
Vocabulary
- Cylinder
- A cylinder is the chamber where the piston moves and the air-fuel mixture is compressed and burned.
- Piston
- A piston is a sliding metal part that is pushed by expanding gases and transfers force to the crankshaft.
- Crankshaft
- The crankshaft converts the piston's back-and-forth motion into rotating motion.
- Camshaft
- The camshaft opens and closes the intake and exhaust valves at the correct times.
- Spark plug
- A spark plug produces an electric spark that ignites the compressed air-fuel mixture in a gasoline engine.
Common Mistakes to Avoid
- Thinking the spark plug pushes the piston down. The spark only ignites the fuel-air mixture, and the expanding hot gases create the force on the piston.
- Confusing horsepower and torque. Torque is a twisting force, while power depends on both torque and how fast the engine is rotating.
- Assuming all cylinders fire at the same time. Cylinders fire in a timed sequence to keep the crankshaft turning smoothly and reduce vibration.
- Ignoring cooling and lubrication systems. Without coolant and oil, friction and heat can damage pistons, bearings, valves, and cylinder walls.
Practice Questions
- 1 A 4-cylinder engine has a displacement of 2.0 L. What is the swept volume of one cylinder in liters and in cubic centimeters?
- 2 An engine produces 180 N m of torque at 3000 rpm. Using P = τω and ω = 2π rpm ÷ 60, calculate the power in watts and kilowatts.
- 3 During the compression stroke, both intake and exhaust valves are closed. Explain why this is necessary for efficient combustion.