A Hemi engine is an internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 in which the roof of each cylinder's combustion chamber

Combustion chamber

A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...

s is of hemispherical form.
Hemispherical combustion chambers, which had been used for centuries in mortars

Mortar (weapon)

A mortar is an indirect fire weapon that fires explosive projectiles known as bombs at low velocities, short ranges, and high-arcing ballistic trajectories. It is typically muzzle-loading and has a barrel length less than 15 times its caliber....

 and cannon

Cannon

A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellents to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees,...

s, were introduced on some of the earliest automotive engines, shortly after proving the concept of internal combustion engines themselves.

Hemispherical cylinder heads have been used since at least 1901; they were used by the Belgian

Belgium

Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...

 car maker Pipe

Pipe (car)

Pipe was a Belgian automobile manufacturer founded by the brothers Alfred and Victor Goldschmidt. The company was also known as 'Compagnie Belge de Construction Automobiles'....

 in 1905, the Peugeot

Peugeot

Peugeot is a major French car brand, part of PSA Peugeot Citroën, the second largest carmaker based in Europe.The family business that precedes the current Peugeot company was founded in 1810, and manufactured coffee mills and bicycles. On 20 November 1858, Emile Peugeot applied for the lion...

 Grand Prix Car of 1912, the Alfa Romeo GP

Alfa Romeo Grand Prix

A.L.F.A. 40/60 GP or GP was a fully working early racing car prototype made by the company now called Alfa Romeo. Only one example was built in 1914, which was later modified in 1921. This was creation of Giuseppe Merosi and was first Alfa Romeo DOHC engine. It had also four valves per cylinder,...

 car of 1914, Daimler

Daimler Motor Company

The Daimler Motor Company Limited was an independent British motor vehicle manufacturer founded in London by H J Lawson in 1896, which set up its manufacturing base in Coventry. The right to the use of the name Daimler had been purchased simultaneously from Gottlieb Daimler and Daimler Motoren...

, and Riley

Riley (automobile)

Riley was a British motorcar and bicycle manufacturer from 1890. The company became part of the Nuffield Organisation in 1938 and was later merged into British Leyland: late in 1969 British Leyland announced their discontinuance of Riley production, although 1969 was a difficult year for the UK...

.