Straight-four engine

The straight-4 or inline-4 is an internal combustion engine with four cylinders aligned in one row. This inline configuration is the most common in engine displacements up to 2.5L (litres). The inline-4 engine is not a balanced configuration. While a pair of pistons is always moving up at the same time as the other pair is moving down, piston speed - as with all internal combustion engines - is higher through the top 180 degrees of the stroke than the bottom 180 degrees, which leads to a secondary harmonic imbalance. 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 2.0 L in displacement rely on the damping effect of their engine mounts. 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, such as the Citroën DS 23 2347 cc engine that was a derivative of the Traction Avant engine, and the 1948 Austin 2660 cc engine used in the Austin-Healey 100 and Austin Atlantic. These engines were generally the result of a long incremental evolution process and their power was kept relatively low regarding their capacity.

Notable straight-4 engines
The smallest automobile 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 1.0 L in displacement. A practical upper limit could be placed in the 2.5 L range for production cars. Larger engines (up to 4.5 L) have been seen in racing and light truck use, especially using diesel fuel (an example is the Mercedes-Benz MBE 904). 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 modern non-diesel was the plain 3.2 L (3188 cc) 195 in the 1961 Pontiac Tempest.

In the early 20th century, bigger engines existed, both in road cars and sports cars. Due to the absence of displacement limit regulations, manufacturers took increasing liberties with engine size. In order to achieve power over 100 hp, most engine builders simply increased displacement, which could sometimes achieve over 10.0 L. One of the biggest straight-4s of its time was De Dietrich 17000 cc motor. Its cubic capacity is over twice the size of the Cadillac's 500 in3 8.2 L V8, which was considered the largest engine of its type in the 1970s. These engines ran at very low RPM, often less than 1500 RPM in maximum, and had a specific output of about 10 hp/L.

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 Arminius Miller to build a new engine. With John Edward and Fred Offenhauser, Miller created a Peugeot-inspired straight-4 engine. This was the first version of the engine that would dominate the Indianapolis 500 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 2.5L to compete in Formula One in the Ferrari 625. For sports car racing capacity was increased up to 3.4L 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.