Truck & Car Guides / Car guides
How car engines work
The four-stroke cycle in plain English, the main parts of a gasoline engine, cylinder layouts, turbochargers and the difference between horsepower and torque.
The big idea
A car engine turns the chemical energy in fuel into motion. It does this by burning a small amount of fuel mixed with air inside a sealed space called a cylinder. The burning gas expands and pushes a piston down. The pistons are connected to a crankshaft, which turns their up-and-down motion into rotation, and that rotation eventually reaches the wheels through the transmission.
Most gasoline car engines use the four-stroke cycle, also called the Otto cycle after the German engineer Nikolaus Otto, who built a successful four-stroke engine in 1876.
The four strokes
| Stroke | Piston moves | What happens |
|---|---|---|
| 1. Intake | Down | The intake valve opens and the cylinder fills with air (and fuel, in some designs) |
| 2. Compression | Up | Both valves close and the mixture is squeezed into a small space |
| 3. Power | Down | The spark plug fires, the mixture burns, and expanding gas forces the piston down |
| 4. Exhaust | Up | The exhaust valve opens and the piston pushes the burned gas out |
Each cylinder makes power on only one stroke out of four, so engines use several cylinders firing at different times to run smoothly. Many people remember the cycle as "suck, squeeze, bang, blow."
The main parts
- Engine block: the main metal casting that holds the cylinders.
- Cylinder head: sits on top of the block and contains the valves and spark plugs.
- Camshaft: opens and closes the valves in time with the pistons, driven by a timing belt or chain.
- Fuel injectors: spray fuel into the intake or directly into the cylinder.
- Oil system: pumps oil to lubricate and cool moving parts.
- Cooling system: circulates coolant through the engine and radiator to remove heat.
Cylinder layouts
| Layout | Description | Common in |
|---|---|---|
| Inline (straight) | Cylinders in one row | Most 3- and 4-cylinder cars |
| V | Two rows at an angle, like a V | V6 and V8 engines in larger cars and trucks |
| Flat (boxer) | Two rows lying flat, opposite each other | Some Subaru and Porsche models |
Displacement is the total volume swept by all the pistons, given in liters or cubic inches. More displacement generally means more power, but modern technology lets small engines produce a lot.
Turbochargers and superchargers
A turbocharger uses exhaust gas to spin a turbine connected to a compressor, which forces extra air into the engine. More air means more fuel can be burned, so a small turbo engine can make the power of a larger one. A supercharger does the same job but is driven directly by the engine. Turbos are now very common because they help engines meet fuel economy targets.
Horsepower vs torque
Torque is twisting force: how hard the engine turns the crankshaft. Horsepower is the rate of doing work: how quickly that force is delivered. In US units, horsepower equals torque (in pound-feet) multiplied by engine speed (rpm), divided by 5,252. That is why the torque and horsepower curves on a graph always cross at 5,252 rpm.
Diesel trucks have huge torque at low rpm, ideal for moving heavy loads. Sports cars often make peak power at high rpm. See diesel engine basics for how diesels differ.
The best thing you can do for any engine is follow the oil change and maintenance schedule in your owner's manual. See maintenance and warning lights.