David Napier and son James founded the company in 1848, and made a wide variety of products including steam-powered printing presses and a centrifuge for sugar manufacturing. After his father's death in 1873, James Napier specialised in beautifully-crafted precision machinery for making coins.
His son Montague Napier inherited the business in 1895 and switched to making motor cars, high-performance luxury cars in particular. Napiers made the first ever six cylinder car, many winning races, and expanded into marine engines as well. Their 1905 boat Napier II set the world marine speed record over a mile at almost 30 knots.
Early in World War I, Napier were contracted to build engines from other companies designs: initially a Royal Aircraft Factory model and then Sunbeams. Both proved to be rather unreliable, and in 1916 Napier decided to design their own instead, an effort that led to the superb W-block 12-cylinder Lion. The Lion was a best-seller for the company, and they eventually abandoned all other products.
In the 1930s the introduction of much larger and more powerful engines from other companies suddenly ended sales of the Lion. Napier quickly started work on newer designs, leading to the 16-cylinder Rapier and the 24-cylinder Dagger, both air-cooled H-block designs. Neither proved very reliable, due to poor cooling of the rearmost cylinders, and even the Dagger's 1,000hp was already smaller than contemporary designs when it shipped.
Starting from scratch, the Napiers decided to use the new sleeve valve design in a much larger H-block 24-cylinder engine, soon to be known as the Sabre. The engine was very advanced and proved to be difficult to adapt to assembly line efforts, so while the engine was ready for production in 1940, it wasn't until 1944 what production versions were considered reliable. At that point efforts were made to improve it, leading eventually to the Sabre VII delivering 3,500hp, making it the most powerful engine in the world, from an engine much smaller than its competition.
Napier also worked on Diesel cycle aircraft engines. In the 1930s they licensed the Junkers Jumo 204 for production in England, which they called the Culverin. They also planned on producing a smaller version of the same basic design as the Cutlass, but work on both was cancelled at the outbreak of World War II.
During the war Napier were asked by the Royal Navy to supply a Diesel for use in their patrol boats, but the Culverin's 720hp was not nearly enough for their needs. Napier then designed the Deltic, essentially three Culverns arranged in a large triangle (deltoid). Considered one of the most complex engine designs of its day, the Deltic was nevertheless very reliable, and served for many years after the war as an engine for trains.
Last of the great Napier engines was the Nomad, a "turbo-compound" design that combined a Diesel engine with a turbine to recover energy otherwise lost in the exhaust. The advantage to this "odd" design was fuel economy, it had the best specific fuel consumption of any aircraft engine built, even to this day. However even better fuel economy could be had by flying a normal jet engine at much higher altitudes, while existing designs filled the "low end" of the market fairly well. The Nomad was largely ignored by the market, and eventually cancelled.
Today Napier is no longer in the engine business, with the ending of the Deltic sales in the 1960s they had no new modern designs to offer. They continue on today as a primary supplier of turbochargers, which can be found on many engines.