petrol engine

A petrol engine converts the chemical energy in gasoline into mechanical work through controlled explosions inside metal cylinders. A mixture of air and vaporized fuel enters each cylinder, is compressed by a rising piston, then ignited by a spark plug at high pressure and temperature. The expanding gases drive the piston down, which rotates a crankshaft, and the cycle repeats. This differs fundamentally from diesel engines, which ignite fuel through compression heat alone, without spark plugs. Petrol engines dominate light vehicles because they start easily in cold conditions, rev quickly, and run cleanly at modest power outputs.

Most petrol engines are four-stroke designs, completing one full combustion cycle in four piston strokes: intake, compression, power, and exhaust. Two-stroke variants, common in chainsaws and small motorcycles, combine these strokes into two, offering higher power density but producing more emissions and requiring oil mixed into fuel for lubrication. The displacement, measured in cubic centimeters (cc) or liters, sets the working volume: a 2.0-liter engine has four cylinders of 500 cc each. A 1.5-liter petrol engine in a hatchback typically produces 100 to 120 brake horsepower; larger versions in trucks or performance cars can exceed 400 horsepower.

Fuel injection has largely replaced carburetors since the 1990s. Modern systems spray pressurized petrol directly into each cylinder or the intake port, controlled electronically to optimize the fuel-air mixture. This improves efficiency, reduces emissions, and allows engines to meet increasingly strict standards. The fuel must have sufficient octane rating, typically 95 or 98 RON in Europe, to prevent knocking, a metallic pinging caused by uncontrolled detonation that damages pistons and reduces power. High-compression engines, around 11:1 or 12:1, demand premium fuel; lower compression engines operate on regular fuel at ratios near 9:1.

Failure modes include carbon buildup on intake valves and injectors, reducing airflow and causing rough idle or hesitation. Spark plugs wear out after 20,000 to 30,000 kilometers, causing misfires and fouling. Timing belt failure, which occurs without warning after 100,000 kilometers or more depending on the engine design, can cause catastrophic valve damage if not replaced on schedule. Overheating, often from coolant leaks or thermostat failure, warps cylinder heads and gaskets, leading to loss of compression and oil contamination.

The term "petrol" derives from the British shortening of "petroleum spirit." Americans and others use "gasoline" or "gas," though the latter is misleading since the fuel is liquid. Petrol engines remain the baseline technology against which alternatives are measured: diesel engines offer better torque and fuel economy for heavy duty work; hybrid systems add electric motors for urban driving; and battery electric vehicles eliminate combustion entirely. For cost, simplicity, and refueling convenience, the petrol engine remains the dominant power source in personal transport globally.