Necrophoresis is a behavior found in social insects – such as ants, bees and termites – in which they carry the dead bodies of members of their colony from the nest or hive area. This acts as a sanitary measure to prevent disease or infection from spreading throughout the colony. Although any member of a colony can carry the bodies, it is done by designated'undertakers'. Ant undertakers have a altered development cycle, are much more than other ants to handle corpse removal, they are not restricted to performing only this task, but they do exhibit different behavioral and movement patterns than other members of the colony, which assist them in this task. Non-undertaker ants may remove dead bodies, but do so with much less consistency. Differentiating between dead and living insects is accomplished by detecting their chemical signature. Depending on the species, this can be communicated by either the absence of chemicals that are present when they are alive, or by those released in decaying corpses.
Corpses will either be taken to a random point a certain distance away from the nest, or placed in a refuse pile closer to the nest, along with other waste. The removal of corpses carrying infectious disease is crucial to the health of a colony. Efforts to eliminate colonies of fire ants, for instance, include introducing pathogens into the population, but this has limited efficacy where the infected insects are separated from the population. However, certain infections have been shown to delay the removal of dead bodies or alter where they are placed. Although placing corpses farther away reduces the risk of infection, it requires more energy. Burial and cannibalism are other recorded methods of corpse disposal among social insects. Termites have been shown to use burial when they cannot afford to devote workers to necrophoresis when forming a new colony. Although in mature ant colonies only workers feature undertaking behaviours, the queen ants from the Lasius niger species undertake deceased co-founders
A flat-four engine known as a horizontally opposed-four engine, is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, each pair of opposed cylinders moves inwards and outwards at the same time. A boxer-four engine has perfect primary and secondary balance, however the two cylinder heads means the design is more expensive to produce than an inline-four engine. Boxer-four engines have been used in cars since 1900 by Volkswagen and Subaru, they have occasionally been used in motorcycles and in aircraft. Cessna and Piper use flat four engines from Lycoming and Continental in the most common civil aircraft in the world - the Cessna 172, Piper Cherokee. Most flat-four engines are designed so that each pair of opposing pistons moves inwards and outwards at the same time, known as a "boxer" configuration. Therefore, the terms "flat-four" and "boxer-four" are used synonymously; the advantages of the boxer-four layout are perfect secondary vibration, a low centre of gravity, a short engine length.
The layout lends itself to efficient air cooling with the airflow being evenly distributed across the four cylinders. In aircraft, this avoids the need to carry heavy water cooling systems; the downsides of boxer-four engines are their extra width, the increased costs associated with having two cylinder heads instead of one, the long exhaust manifold required to achieve evenly spaced exhaust pulses. Due to these factors, inline-four engines are more used than flat-four engines, V6 engines are used where larger displacements are required; the equal and opposing forces generated in a boxer-four engine results in perfect secondary balance. Boxer-four engines are therefore better suited to displacements above 2.0 L, since they do not require balance shafts to reduce the secondary vibration. In practice, each cylinder in a boxer engine is offset from its opposing pair due to the distance between the crankpins along the crankshaft; this offset distance means that the equal and opposite forces from each cylinder pair produces a rocking couple.
The resulting vibration is not high enough to require balance shafts. As per all four-stroke engines with four cylinder or less, the lack of overlap in the power strokes results in a pulsating delivery of torque to the flywheel, causing a torsional vibration along the crankshaft axis. If necessary, this vibration can be minimised using a harmonic damper; the typical firing order for a boxer-four engine is for the left bank of cylinders to ignite one after another, followed by the right bank of cylinders, with the firing interval evenly spaced at 180 degrees. Traditionally, the exhausts from the two cylinders on each bank were merged together, with the resulting uneven exhaust pulses causing a characteristic "flat-four burble" exhaust sound; the other common exhaust configuration is to pair the cylinders with a firing interval offset of 360 degrees, in order to optimise the exhaust pulses. This configuration requires long exhaust manifolds, in order to pair the cylinders on opposite banks, results in a less distinctive exhaust sound.
In 1900, the first flat-four engine was produced by Benz & Cie, based on Benz's 1897 "contra" flat-twin engine. This engine was used in Benz racing cars, produced 20 hp, had a displacement of 5.4 L and was designed by Georg Diehl. London company Wilson-Pilcher released its first car in 1901, powered by a flat-four engine; this engine was mounted longitudinally in the chassis, water-cooled, produced 9 hp and had a displacement of 2.4 L. Unusually for its day, the bore and stroke were equal, with each being 95 mm. In 1902 the Buffum automobile was equipped with opposed four cylinder engines that were rated at 16 horsepower. Herbert H. Buffum produced an American Automobile called the Buffum in Abington, Massachusetts from 1903 to 1907. Having produced flat-twin engines, the 1926 Tatra 30 was the Czech company's first model powered by a flat-four engine. Tatra produced various flat-four engined model through the 1930s; the 1936 Tatra T97 pioneered backbone chassis layout. In 1936, English company Jowett expanded its model range from flat-twin engines to include flat-four engines.
Production of Jowett flat-four engines continued until 1954, when the Jowett Javelin saloon and Jowett Jupiter sports models ended production. The highest production flat-four engine is the Volkswagen air-cooled engine, produced from 1938 until 2006 and was most famously used in the rear-engined 1938-2006 Volkswagen Beetle and 1950-1990 Volkswagen Transporter; this air-cooled engine was designed by Porsche and was used in the 1948-1965 Porsche 356, 1953-1956 Porsche 550, 1965–1969 Porsche 912 and 1969-1976 Porsche 914. In 1982, to comply with exhaust emissions regulations a water-cooled version called the Volkswagen Wasserboxer was introduced in the Volkswagen Transporter During the 1960s and 1970s, several manufacturers produced flat-four engines including the air-cooled Citroën flat-four engine, the water-cooled Alfa Romeo flat-four engine, the water-cooled Lancia flat-four engine and the water-cooled Subaru EA engine. By the year 2000, most manufacturers had replaced flat-four engines with inline-four engines.
A notable exception is Subaru, with the latest iteration of