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Inline piston engine with four cylinders

A straight four engine (also called an inline four) is a four cylinder piston engine where cylinders are arranged in a line along a common crankshaft. The majority of automotive four cylinder engines use a straight four layout and the layout is also very common in motorcycles and other machinery. Therefore the term "four cylinder engine" is usually synonymous with straight four engines. When a straight four engine is installed at an inclined angle (instead of with the cylinders oriented vertically), it is sometimes called a slant four. Between 2005 and 2008, the proportion of new vehicles sold in the United States with four cylinder engines rose from 30% to 47%. By the 2020 model year, the share for light duty vehicles had risen to 59%.

A four stroke straight four engine always has a cylinder on its power stroke, unlike engines with fewer cylinders where there is no power stroke occurring at certain times. Compared with a V4 engine or a flat four engine, a straight four engine only has one cylinder head, which reduces complexity and production cost.

Petrol straight four engines used in modern production cars typically have a displacement of , but larger engines have been used in the past, for example the 1927–1931 Bentley 4½ Litre. Diesel engines have been produced in larger displacements, such as a 3.2 L turbocharged Mitsubishi engine (used the Pajero/Shogun/Montero SUV) and a 3.0 L Toyota engine. European and Asian trucks with a gross vehicle weight rating between 7.5 and 18 tonnes typically use inline four cylinder diesel engines with displacements around 5 litres. Larger displacements are found in locomotive, marine and stationary engines. Displacement can also be very small, as found in kei cars sold in Japan. Several of these engines had four cylinders at a time when regulations dictated a maximum displacement of 550 cc; the maximum size is currently at 660 cc.

Straight four engines with the preferred crankshaft configuration have perfect primary balance. This is because the pistons are moving in pairs, and one pair of pistons is always moving up at the same time as the other pair is moving down. However, straight four engines have a secondary imbalance. This is caused by the acceleration/deceleration of the pistons during the top half of the crankshaft rotation being greater than that of the pistons in the bottom half of the crankshaft rotation (because the connecting rods are not infinitely long). As a result, two pistons are always accelerating faster in one direction, while the other two are accelerating more slowly in the other direction, which leads to a secondary dynamic imbalance that causes an up and down vibration at twice crankshaft speed. This imbalance is common among all piston engines, but the effect is particularly strong on four stroke inline four because of the two pistons always moving together. The strength of this imbalance is determined by the reciprocating mass, the ratio of connecting rod length to stroke, and the peak piston velocity. Therefore, small displacement engines with light pistons show little effect, and racing engines use long connecting rods. However, the effect grows quadratically with engine speed (rpm).

Four stroke engines with five or more cylinders are able to have at least one cylinder performing its power stroke at any given point in time. However, four cylinder engines have gaps in the power delivery, since each cylinder completes its power stroke before the next piston starts a new power stroke. This pulsating delivery of power results in more vibrations than engines with more than four cylinders.

Many early racing cars used straight four engines, however the Peugeot engine which won the 1913 Indianapolis 500 was a highly influential engine. Designed by Ernest Henry, this engine had double overhead camshafts (DOHC) with four valves per cylinder, a layout that would become the standard until today for racing inline four engines. Amongst the engines inspired by the Peugeot design was the Miller engine, which was a successful racing engine through the 1920s and early 1930s. The Miller engine evolved into the Offenhauser engine which had a highly successful spanning from the 1933 until 1981, including five straight victories at the Indianapolis 500 from 1971 to 1976.

Belgian arms manufacturer FN Herstal, which had been making motorcycles since 1901, began producing the first motorcycles with inline fours in 1905. The FN Four had its engine mounted upright with the crankshaft longitudinal. Other manufacturers that used this layout included Pierce, Henderson, Ace, Cleveland, and Indian in the United States, Nimbus in Denmark, Windhoff in Germany, and Wilkinson in the United Kingdom. The first across the frame 4 cylinder motorcycle was the 1939 racer Gilera 500 Rondine, it also had double over head camshafts, forced inducting supercharger and was liquid cooled. Modern inline four motorcycle engines first became popular with Honda's SOHC CB750 introduced in 1969, and others followed in the 1970s. Since then, the inline four has become one of the most common engine configurations in street bikes. Outside of the cruiser category, the inline four is the most common configuration because of its relatively high performance to cost ratio. All major Japanese motorcycle manufacturers offer motorcycles with inline four engines, as do MV Agusta and BMW. BMW's earlier inline four motorcycles were mounted horizontally along the frame, but all current four cylinder BMW motorcycles have transverse engines. The modern Triumph company has offered inline four powered motorcycles, though they were discontinued in favour of triples. The 2009 Yamaha R1 has an inline four engine that does not fire at even intervals of 180°. Instead, it uses a crossplane crankshaft that prevents the pistons from simultaneously reaching top dead centre. This results in better secondary balance, which is particularly beneficial in the higher rpm range, and "big bang firing order" theory says the irregular delivery of torque to the rear tire makes sliding in the corners at racing speeds easier to control. Inline four engines are also used in MotoGP by the Suzuki (since 2015) and Yamaha (since 2002) teams. In 2010, when the four stroke Moto2 class was introduced, the engines for the class were a inline four engine made by Honda based on the CBR600RR with a maximum power output of Starting in 2019, the engines were replaced by a Triumph triple engine.

Inline four engines are also used in light duty commercial vehicles such as Karsan Jest and Mercedes Benz Sprinter.