The straight-4 or inline-4 is an internal combustion engine with four cylinders aligned in one row. This straight engine configuration is the most common in cars with a displacement up to 2.0 litres.

The straight-4 engine is not a balanced configuration and while this is tolerable in a small, low-displacement, low-power configuration the vibrations get worse with increasing size and power. Most straight-4 engines below 2 litres in displacement rely on the damping effect of their engine mountings. Today most engineers will make use of balance shafts above that limit. A 4-cylinder engine needs two balance shafts, rotating at twice the crankshaft frequency, to be smooth. Nonetheless, there were several samples of larger Straight-4s in production using no balance shafts like the Citroën DS 23 2,347 cc engine that was a derivative of the Traction Avant engine. These engines were generally the result of a long incremental evolution process and their power was kept relatively low regarding their capacity.

Automobile use

Notable straight-4 engines

The smallest production straight-4 engine powered the 1961 Mazda P360 Carol keicar. Displacing just 358 cc, the Mazda OHV was a conventional but tiny pushrod engine. Most straight-4 engines, however, have been over one liter. A practical upper limit could be placed in the 2.5 liter range for production cars. Larger engines (up to 4.3 L) have been seen in racing and light truck use, especially using Diesel fuel. The use of balance shafts allowed Porsche to use a 3.0 L (2990 cc) straight-4 engine on road cars like the Porsche 968, but the largest non-Diesel was the plain 3.2 L (3188 cc) 195 in the 1961 Pontiac Tempest.

Other notable engines using this configuration include:

• Ford Model T engine - One of the most-widely produced engines in the world
• Austin A-Series engine - This engine powered many of the compact vehicles of the 1950s
• Honda ED engine - First use of Honda's CVCC technology
• Triumph Slant-4 engine - The first mass-produced multivalve engine for Triumph and an early turbo engine for Saab
• GM Quad-4 engine - The first multivalve American engine
• Hyundai Alpha engine - The first automobile engine designed in Korea
• Honda F20C engine - Its 240 hp from 2.0 L was the highest specific output of its time

Racing use

1913 saw a Peugeot driven by Jules Goux winning the Indianapolis 500. This car was powered by a Straight-4 engine designed by Ernest Henry . This design was very influential for racing engines as it featured for the first time dual overhead camshafts (DOHC) and 4 valves per cylinder, a layout that would become the standard until today for racing Straight-4 engines.

This Peugeot was sold to the American driver "Wild Bob" Burman who broke the engine in 1915. As Peugeot couldn't deliver a new engine during World War 1 Burman asked Harry Miller to build a new engine. With John Edward and Fred Offenhauser, Harry Miller created a Peugeot-inspired Straight-4 engine. This was the first version of the engine that would dominate the 500 miles until 1976 under the brand Miller and later Offenhauser.

Another engine that played an important role in Racing history is the Straight-4 Ferrari engine designed by Aurelio Lampredi. This engine was originally designed as a 2 litre Formula 2 engine for the Ferrari 500 but evolved to 2.5 litres to compete in Formula 1 in the Ferrari 625. For sports car racing capacity was increased up to 3.4 L for the Ferrari 860 Monza.

Yet another very successful engine was the Coventry Climax Straight-4 originally designed by Walter Hassan as a 1.5 L Formula 2 engine. It evolved into the large 2,495 cc FPF that won the Formula One championship in Cooper's chassis.

Motorcycle use

The smallest production mortocycle straight-4 engine was the 4-stroke engine powered the Honda CB 350. For racing Honda built straight-4 engines as small as a 125cc for the Honda 125/4. This engine was replaced by a 125cc straight-5 engine.