inlinefour

An inlinefour, commonly written as inline four or I4, is an internal combustion engine with four cylinders arranged in a single row along the crankshaft. This compact layout is one of the most widespread configurations for light cars, motorcycles, and small trucks, offering a balance of power, efficiency, and manufacturability.

In an I4, each cylinder has its own combustion chamber and is fed by a shared intake and exhaust system. Modern examples typically use overhead camshafts (DOHC or SOHC) with four valves per cylinder, though some smaller or older designs employ fewer valves per cylinder. The straight layout allows a relatively short crankshaft and straightforward production.

Vibration and balance are notable considerations. The inline-four has inherent dynamic behavior that can produce secondary vibrations at higher RPMs, so many engines employ counterweights or balance shafts and carefully chosen firing orders (commonly 1-3-4-2) to smooth operation.

Applications and impact. The I4 dominates passenger cars in the compact and midsize segments and is common in many motorcycles. Its packaging efficiency, lower cost, and ease of turbocharging have driven its prevalence for front-drive vehicles and small sporty cars.

Variants and evolution. Modern I4s frequently incorporate variable valve timing, direct fuel injection, turbocharging, and lightweight materials to meet efficiency and emissions standards. Notable examples include engines in the Honda K-series, Toyota A-series, and many contemporary powerplants across brands.