It employed a hemispherical
cylinder head,
chain-driven
single overhead camshaft (SOHC) and eight
valves (two per cylinder). United States passenger car versions had a small secondary intake valve referred to as the "Jet Valve". This valve induced swirl in the intake charge, enabling the use of leaner fuel/air mixtures for lower emissions. It was designed as a cartridge containing the valve spring and seat which simply screwed into a threaded hole in the head, similar to a
spark plug but inside the cam cover. The
rocker arms for the intake valve were widened on the valve end to accommodate the cartridge, which was equipped with a very soft valve spring in order to avoid wear on the camshaft intake lobe. Modifications to the head were thereby reduced as the Jet Valve negated the necessity for a three-valve-per-cylinder design. In 1975, the
Astron 80 introduced a system dubbed "Silent Shaft": the first use of twin
balance shafts in a modern engine. It followed the designs of
Frederick Lanchester, whose original patents Mitsubishi had obtained, and proved influential as
Fiat/
Lancia,
Saab and
Porsche all licensed this technology. The
4D5 engine is a range of four-cylinder
belt-driven overhead camshaft
diesel engines which were part of the "Astron" family, and introduced in 1980 in the then new fifth generation
Galant. As the first turbodiesel to be offered in a Japanese passenger car, it proved popular in the emerging
SUV and
minivan markets where Mitsubishi was highly successful, until superseded by the
4M4 range in 1993. However, production of the 4D5 (4D56) continued throughout the 1990s as a lower-cost option than the more modern powerplants. Until now it is still in production, but made into a modern powerplant by putting a common rail direct injection fuel system into the engine. == 4G51 (1.85 liters)==