Inline-four engine

Ford inline-four engine with cylinder head removed
A cutaway Renault-Nissan M9R 2.0 L Straight-4 DOHC Common rail diesel engine

The inline-four engine or straight-four engine is a type of inline internal combustion four-cylinder engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft. Where it is inclined, it is sometimes called a slant-four. In a specification chart or when an abbreviation is used, an inline-four engine is listed either as I4 or L4 (for longitudinal, to avoid confusion between the digit 1 and the letter I).

The inline-four layout is in perfect primary balance and confers a degree of mechanical simplicity which makes it popular for economy cars.[1] However, despite its simplicity, it suffers from a secondary imbalance which causes minor vibrations in smaller engines. These vibrations become more powerful as engine size and power increase, so the more powerful engines used in larger cars generally are more complex designs with more than four cylinders.

Today almost all manufacturers of four-cylinder engines for automobiles produce the inline-four layout, with Subaru and Porsche 718[2] flat-four engines being notable exceptions, and so four-cylinder is usually synonymous with and a more widely used term than inline-four. The inline-four is the most common engine configuration in modern cars, while the V6 engine is the second most popular.[3] In the late 2000s (decade), due to stringent government regulations mandating reduced vehicle emissions and increased fuel efficiency, the proportion of new vehicles sold in the U.S. with four-cylinder engines (largely of the inline-four type) rose from 30 percent to 47 percent between 2005 and 2008, particularly in mid-size vehicles where a decreasing number of buyers have chosen the V6 performance option.[4][5]


This configuration is most commonly used for petrol engine displacements up to 3.0 L. Porsche, for instance, used a 3.0 L four in its 944 S2 and 968 sports cars. Production cars with inline-fours larger than 3.0 L have included the 1927 Model A Ford (3.3 L (201 cu in)), the 1965-1980 International Harvester Scout (3.2 L (195 cu in)), and the 1961-1963 Pontiac Tempest (3.2 L (195 cu in) Pontiac inline-4 engine). A few very early vehicles had inline-four engines with much larger displacements, including the 1910 Blitzen Benz (21.5 L (1,312 cu in)) and the 1911 Fiat S76 Record (28.4 L (1,733 cu in)).

Inline-four diesel engines, which are lower revving than gasoline engines, often exceed 3.0 L. Mitsubishi still employs a 3.2 L inline-four turbodiesel in its Pajero (called the Shogun or Montero in certain markets), and several manufacturers of light commercial vehicles and large four-wheel drive vehicles, such as Fiat Powertrain Technologies, Isuzu, Nissan, Tata Motors, and Toyota employ a 3.0 L inline-four diesel.

Larger inline-four engines are used in industrial applications, such as in small trucks and tractors, are often found with displacements up to about 5 L. Diesel engines for stationary, marine and locomotive use (which run at low speeds) are made in much larger sizes.

Generally, European and Asian manufacturers of trucks with a gross vehicle weight rating between 7.5 and 18 tonnes use inline four-cylinder diesel engines with displacements around 5 L. The MAN D0834 engine is a 4.6 L inline-4 with 220 hp (164 kW) and 627 lb⋅ft (850 N⋅m), which is available for the MAN TGL light-duty truck and VARIOmobil motorhomes.[6][7][8] The Isuzu Forward is a medium-duty truck which is available with a 5.2 L inline-four engine that delivers 210 hp (157 kW) and 470 lb⋅ft (640 N⋅m).[9][10] The Hino Ranger is a medium-duty truck which is available with a 5.1 L inline-four engine that delivers 175 hp (130 kW) and 465 lb⋅ft (630 N⋅m).[11][12] The earlier Hino Ranger even had a 5.3 L inline-four engine.[13][14]

The Kubota M135X is a tractor with a 6.1 L inline-four. This turbo-diesel engine has a bore of 118 mm (4.6 in) and a relatively long stroke of 140 mm (5.5 in).[15][16][17]

One of the strongest Powerboat-4-cylinders is the Volvo Penta D4-300 turbodiesel. This is a 3.7 L-inline-4 with 300 hp (224 kW) and 516 lb⋅ft (700 N⋅m).[18][19][20]

Brunswick Marine built a 127 kW (170 bhp) 3.7 L 4-cylinder gasoline engine (designated as the "470") for their Mercruiser Inboard/outboard line. The block was formed from one half of a Ford 460 cubic inch V8 engine. This engine was produced in the 1970s and 1980s.[citation needed]

One of the largest inline-four engines is the MAN B&W 4K90 marine engine. This two-stroke turbo-diesel has a giant displacement of 6,489 L. This results from a massive 0.9 meter bore and 2.5 meter stroke. The 4K90 engine develops 18,280 kW (24,854 PS; 24,514 hp) at 94 rpm and weighs 787 tons.[21]

The largest on-road inline-4 cylinder turbo-diesel engine is the Detroit Diesel Series 50, with a displacement of 8.5 L. It is widely used in various applications such as buses, trucks, and more. Power ratings varied from 250 hp to 350 hp. It was manufactured from 1994, until 2005. The Series 50 was also marketed as the Series 50G, for its CNG and LNG versions.

Displacement can also be very small, as found in kei cars sold in Japan, such as the Subaru EN series; engines that started out at 550 cc and are currently at 660 cc, with variable valve timing, DOHC and superchargers resulting in engines that often claim the legal maximum of 64 PS (47 kW; 63 bhp). The 1.2 L, turbocharged, direct-injected, Toyota 8NR-FTS engine has a maximum power output of 114 hp (85 kW) and a maximum torque of 190 Nm (140 lbft) at a low rpm (1500 rpm).