inlinefours

An inline-four, or I4, is a piston internal-combustion engine with four cylinders arranged in a single straight row along a shared crankshaft. It is the most common engine configuration in modern cars due to its compact length, relatively simple construction, and low manufacturing cost. Many engines in daily use are inline-fours, spanning naturally aspirated and turbocharged designs.

In a four-stroke cycle, each cylinder completes a power stroke once every two crankshaft revolutions. With four cylinders firing in sequence, the engine delivers a smooth and balanced torque output for its size, though it is not perfectly free of vibration. Typical configurations include single or double overhead camshafts, port or direct fuel injection, and ascent into turbocharged variants. Valve timing and fuel delivery systems have evolved to balance efficiency, power, and emissions.

Firing order varies by design, but common patterns such as 1-3-4-2 help minimize vibrations and ensure even firing intervals. The inline-four is known for good primary balance but can exhibit secondary dynamic imbalances, which manufacturers mitigate with counterweights, balancers, or chassis isolation. Modern versions often feature variable valve timing, direct injection, and turbocharging to boost performance and efficiency.

Applications and packaging: the short length of an inline-four makes it ideal for transverse engine layouts in front-wheel-drive vehicles and for lightweight, compact cars. It dominates mass-market automotive propulsion and is also used in motorcycles and some small aircraft engines. While larger engines offer smoother operation at high power, the inline-four remains favored for its balance of cost, efficiency, and packaging.