The Daihatsu A-series engine is a range of compact two-cylinder, internal combustion piston engines, designed by Daihatsu with the aid of their owner Toyota. The petrol-driven series has cast iron engine blocks and aluminum cylinder heads, and are of an SOHC lean burn design, with belt driven heads. The head design was called "TGP lean-burn", for "Turbulence Generating Pot". The engine also had twin-balancing shaft, which provided smoothness equivalent to that of a traditional four-cylinder engine - although it also cost nearly as much to build.
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The engine was developed with some haste in order to replace the dirty two-stroke "ZM" engines used in Daihatsu's earlier Kei cars, and was the first unit to take full advantage of the new 550 cc displacement limit in effect from 1 January 1976. It was first presented in May 1976, as the AB10. Eventually, even a turbocharged version was produced. The engine was replaced by the three-cylinder EB-series in 1985.
AB-series (547 cc)
The AB-series is a 547 cc (0.55 L) version originally installed into the 1976 Fellow Max 550 but soon found its way into Daihatsu's entire Kei car lineup. Bore is 71.6 mm and stroke is 68 mm. Unlike the export versions, the Japanese market engines have a small catalyzer fitted. The engine (AB20) was also installed in the tiny Portuguese Entreposto Sado 550 citycar, of which about 500 were built in the early eighties. Surprisingly, Daihatsu's close competitor Suzuki fitted AB10 engines into their Suzuki Fronte 7-S for a short period, as their own two-stroke engines had a hard time meeting ever stricter emissions requirements.
AB10 is the code used for engines meant to be installed in an upright position, while those intended for a horizontal position (for underfloor usage) are called AB20. The AB55 is a horizontal version equipped with a turbocharger. A downtuned version with 9 PS (6.6 kW) at 2900 rpm and 2.8 kg·m (27 N·m; 20 lb·ft) at 2000 rpm was installed in Toyota's 500 kg (1,100 lb) FG5 forklift from the late seventies.
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AD-series (617 cc)
The AD-series is a 617 cc (0.62 L) version of the AB-series engine. This somewhat larger engine does not meet Kei car regulations and was intended only for export markets. Of Daihatsu products, only the Daihatsu Cuore was ever fitted with this engine, although they were often sold as the "Domino" in export markets. Bore is 76 mm and stroke is 68 mm. The only version was an SOHC, two-valve, single carburetted unit. Innocenti also built this engine into their Mini 650, built from November 1984 until November 1987.