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Straight-4
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The straight-4 or inline-4 engine is a four cylinder internal combustion engine with all four cylinders mounted in a straight line 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.
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The straight-4 or inline-4 engine is a four cylinder internal combustion engine with all four cylinders mounted in a straight line 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-4.
The straight-4 layout is the simplest design which is in perfect primary balance and confers a degree of mechanical simplicity which makes it popular for economy cars. However, it suffers from a secondary imbalance causing minor vibrations that rapidly get worse as engine size and power increases. As a result, larger cars usually use more complex engine designs with more cylinders.
Displacement
This straight engine configuration is the most common in cars with a displacement up to 2.4 litres. The usual "practical" limit to the displacement of straight-4 engines in a car is around 2.7 litres. However, Porsche used a 3.0 L four in its 944 S2 and 968 sports cars; Mitsubishi still employs a 3.2 litre four-cylinder turbodiesel in its Pajero (called the Shogun or Montero in certain markets) and Tata Motors employs a 3.0 litre four-cylinder diesel in its Spacio and Sumo Victa. Larger four-cylinder engines are used in industrial applications, such as in small trucks and tractors, are often found with displacements up to about 4.6 L. Classic and Antique vehicles tended to have larger displacements to develop horsepower and torque. The Model A Ford was built with a 3.3 litre straight-4 engine. Diesel engines for stationary, marine and locomotive use (which run at relatively low speeds) are made in much larger sizes.
Balance and smoothness
The straight-4 engine is much smoother than one, two, and three cylinder engines, and this has resulted in it becoming the engine of choice for most economy cars, although it can be found in some sports cars as well. However, the straight-4 is not a fully balanced configuration.
While an even-firing straight-4 engine is in primary balance because one 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° of the crankshaft rotation than the bottom 180°. Therefore, across the 4 cylinders two pistons are always accelerating faster in one direction while two others are accelerating slower in the other, which leads to a secondary dynamic imbalance—an up-and-down vibration at twice the crankshaft's speed. This imbalance is tolerable in a small, low-displacement, low-power configuration, but the vibrations get worse with increasing size and power.
The reason for the piston's higher speed during the 180° rotation from mid-stroke through top-dead-center and back to mid-stroke is that the minor contribution to the piston's up/down movement from the connecting rod's change of angle here has the same direction as the major contribution to the piston's up/down movement from the up/down movement of the crank pin. By contrast, during the 180° rotation from mid-stroke through bottom-dead-center and back to mid-stroke the minor contribution to the piston's up/down movement from the connecting rod's change of angle has the opposite direction of the major contribution to the piston's up/down movement from the up/down movement of the crank pin.
Most straight-4 engines below 2.0 L in displacement rely on the damping effect of their engine mounts to reduce the vibrations to acceptable levels. Above 2.0 L, most modern straight-4 engines now use balance shafts to eliminate the second-order harmonic vibrations. In a system invented by Dr. Frederick W. Lanchester in 1911 and popularized by Mitsubishi Motors in the 1970s, a straight-4 engine uses two balance shafts, rotating in opposite directions at twice the crankshaft's speed, to offset the differences in piston speed. However, in the past there were numerous examples of larger straight-4s without balance shafts, such as the Citroën DS 23 2347 cc engine that was a derivative of the Traction Avant engine, the 1948 Austin 2660 cc engine used in the Austin-Healey 100 and Austin Atlantic, the 3.3 L flathead engine used in the Ford Model A (1927), and the 2.5 L GM Iron Duke engine used in a number of American cars and trucks. Soviet/Russian GAZ Volga cars and UAZ SUVs, vans and light trucks used aluminium big-bore straight-4 engines (2.5 L or later 2.9 L) with no balance shafts in 1950s-1990s. These engines were generally the result of a long incremental evolution process and their power was kept relatively low compared to their capacity. However, the forces increase with the square of the engine speed—that is, doubling the speed makes the vibration four times worse—so modern high-speed straight-4s have more need to use balance shafts to offset the vibrations.
Four cylinder engines also have a smoothness problem in that the power strokes of the pistons do not overlap. With four cylinders and four cycles to complete, each piston must complete its power stroke and come to a complete stop before the next piston can start a new power stroke, resulting in a pause between each power stroke and a pulsating delivery of power. In engines with more cylinders, the power strokes overlap, which gives them a smoother delivery of power and less vibration than a four can achieve. As a result, six and eight cylinder engines (and sometimes five) are generally used in more luxurious and expensive cars.
Automobile use
Notable straight-4 engines
The smallest automobile production straight-4 engine powered the 1961 Mazda P360 Carol kei car. Displacing just 358 cc, the Mazda OHV was a conventional but tiny pushrod engine. Honda produced, from 1963 to 1967, a 356 cc straight-4 engine for the T360 truck. Straight-4 motorcycle engines are built down to 250 cc, e.g. in the Honda CBR 250.
Most straight-4 engines, however, have been over 0.7 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 first in the 944 S2 (1989-1991), but the largest modern non-diesel was the plain 3.2 L (3188 cc) 195 in the 1961 Pontiac Tempest.
Currently, one of the largest straight-4 engines is the 2.7 L Toyota 3RZ-FE engine.
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 , 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 17,000 cc motor. Its cubic capacity is over twice the size of the Cadillac's 500 CID 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 1,500 rpm maximum, and had a specific output of about 10 hp/L. The US tractor industry both farm & industrial relied on large 4 cylinder power units until the early 1960s when 6 cylinder designs came into favor. International Harvester built a large 5.7 L (350 CID) 4 cylinder for their WD-9 series tractors.
Other technologically or historically notable engines using this configuration include:
- Willys L-134 engine - Nicknamed the Go Devil engine. Powered the World War II Jeep and post-war models. Notably undersquare, with 3.125 in (79.4 mm) bore and 4.375 in (111.1 mm) stroke.
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 I, 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. The Offenhausers won five straight victories at Indianapolis from 1971 to 1976, and it was not until 1981 that they were eliminated as competitors by engines such as the Cosworth V8.
Many cars produced for the pre-WWII voiturette Grand Prix motor racing category used straight-4 engine designs. 1.5L supercharged motors found their way into cars such as the Maserati 4CL and various ERA models. These were resurrected after the war and formed the foundation of what was later to become Formula One.
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 L 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. Enlarged to 2.0 L for Formula 1 in 1958, it evolved into the large 2495 cc FPF that won the Formula One championship in Cooper's chassis in 1959 and 1960.
Motorcycle use
The smallest production motorcycle straight-4 engine was the 4-stroke engine powered the 250 cc Benelli/Moto Guzzi 254. For racing, Honda built straight-4 engines as small as a 125 cc for the Honda 125/4. This engine was replaced by a 125 cc straight-5 engine. The largest proprietary straight-4 engine in a commercially-produced motorcycle is the 1402 cc engine in the Suzuki GSX1400.
Modern straight-4 motorcycle engines first gained their popularity with Honda's SOHC CB750 in the '70s. Since then, the straight-4 has become one of the most common engine configurations in street bikes. Outside of the cruiser category, the straight-4 is simply the most common configuration because of its relatively high performance-to-cost ratio. All of the Japanese motorcycle manufacturers offer motorcycles with straight-4 engines, as does MV Agusta and BMW who employ both longitudinal and transverse-mounted engines. Even the modern Triumph company has offered straight-4-powered motorcycles, though they were discontinued in favor of a triple.
2009 Yamaha R1 has an interesting straight 4 engine that does not fire at even intervals of 180 degrees. Although it is a true straight 4 engine it uses an offset crank and fires unevenly, thus sounds like a V4.
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