A plain-language guide to how a car works from the engine out

Most of a car serves one of three purposes: it makes motion from fuel, it delivers that motion to the road, or it keeps the driver in control while the car moves. The engine is the natural starting point because it is the source of the power. Most other systems either feed the engine, carry its output onward, protect it from heat, or help the driver steer and stop. Once you know what the engine needs and what it produces, the rest of the car becomes easier to follow.

The sections below trace how the systems connect in a typical gasoline car with a front-mounted engine. Layouts differ between models, and hybrids and electric vehicles change several of the details, but the basic relationships hold. Treat this as a map of how the car fits together rather than a repair guide, and use the owner's manual for anything specific to your vehicle.

The engine makes the power

Inside each cylinder, a mixture of air and fuel burns, and the expanding gas pushes a piston down. Each piston connects to the crankshaft through a connecting rod, which turns that up-and-down motion into rotation. A four-cylinder engine has four pistons sharing one crankshaft, and because the cylinders fire at different points in the rotation, the power arrives in overlapping pulses that keep the output fairly smooth. Engine speed is measured in revolutions per minute, and the throttle controls how much air enters, which sets how much power the engine can make at a given speed.

A control computer manages the process by reading sensors for air flow, engine speed, temperature and the oxygen content of the exhaust. It then adjusts fuel delivery and spark timing to keep combustion balanced. Other engine parts depend on the same process. A camshaft, driven by the crankshaft through a belt or chain, opens and closes the valves that admit air and release exhaust. Engine oil circulates through passages to the bearings and moving parts, and it is the main reason those metal surfaces do not wear out quickly.

The transmission carries power to the wheels

The crankshaft delivers rotation to the transmission, which changes the relationship between engine speed and wheel speed. An engine works well only within a limited range of speeds, but a car needs to pull away from a stop, climb a hill and cruise on the highway without the engine racing or straining. Gears solve that problem. A low gear multiplies the engine's turning force so the car can start moving, and a high gear lets the engine turn more slowly at cruising speed.

Automatic transmissions use a torque converter, a fluid coupling that sits between the engine and the gears, along with gear sets that clutches and bands engage under computer control. Manual transmissions use a clutch the driver operates with a pedal, which connects and disconnects the engine from the gearbox. From the transmission, a driveshaft or axle shafts pass power on to a differential. The differential lets the two drive wheels turn at different speeds when the car goes around a corner. In front-wheel-drive cars, the transmission and differential are often built into one unit at the front of the car.

Cooling keeps the engine within its range

Burning fuel releases a large amount of heat, and only part of that energy becomes motion. The rest has to be carried away, or the engine would overheat, metal would weaken, and oil would break down faster. The cooling system circulates coolant through passages in the engine block and cylinder head. A water pump moves the coolant, a thermostat controls how much of it passes through the radiator, and a fan pulls air through the radiator, especially at low speed or when the car is stopped.

The temperature gauge shows whether the system is doing its job. Normal operation holds the needle steady, usually near the middle of the scale. A needle that climbs toward the hot end is a warning to act, because continued overheating can damage the engine. The owner's manual describes how to check the coolant level, and it advises waiting until the engine is cold, since the system is pressurized when hot. A steady loss of coolant over time is worth investigating, because a leak or a failing part can lead to overheating.

Steering and brakes let the driver control the car

The steering system turns the front wheels through a rack or gearbox connected to the steering wheel. Most modern cars use electric power steering, in which a motor adds assistance to the driver's effort. Some older models and certain designs use a hydraulic pump driven by the engine. Either way, the goal is the same: a small effort at the wheel produces a larger turning force at the road, while the driver keeps enough feedback to feel what the front tires are doing.

Brakes work through a different chain of parts. When the driver presses the pedal, a brake booster multiplies the force, and the master cylinder pushes hydraulic fluid through lines to each wheel. At each wheel, a caliper squeezes brake pads against a rotor, and the friction slows the wheel while turning the car's motion into heat. Brake fluid is nearly incompressible, so the pedal should feel firm. A soft or sinking pedal can point to a leak or air in the system, and that calls for an inspection before the next trip.

The electrical system starts and feeds everything

The battery stores energy for starting the engine and for running accessories while the engine is off. The starter motor draws a large current to turn the engine until it runs under its own power. Once the engine runs, the alternator produces electricity, and a voltage regulator holds its output within a safe range. That keeps the battery charged and supplies the lights, sensors, control computer, fans and infotainment system. Most gasoline cars run a nominal 12-volt system, while hybrids and electric vehicles add high-voltage systems that trained technicians handle with special procedures.

The same wiring carries signals as well as power. Sensors report engine temperature, speed, throttle position and exhaust oxygen to the control computer, and the computer sends commands back to fuel injectors, ignition coils and other actuators. Because so many functions depend on that network, a weak battery or a charging fault can cause symptoms that look unrelated, such as flickering lights or warning messages. A useful habit is to ask which system a symptom touches. A temperature rise, a soft brake pedal, a drivetrain noise and a dim charging light point to different parts of the car. That question narrows the search, and it gives a technician a clearer description of what you noticed.