Multi-valve

From WOI Encyclopedia Italia

In automotive engineering, an engine is referred to as multi-valve (or multivalve) when each cylinder has more than two valves.

All tappet-valve, four-stroke internal combustion engines have at least two valves per cylinder - one for intake of air and fuel, and another for exhaust of combustion products. Adding more valves improves the flow of intake and exhaust gases, potentially improving combustion efficiency, power, and performance.

Most multivalve engines use an overhead camshaft to actuate the valves, and many use double overhead camshafts (DOHC). However this is not always the case: Chevrolet recently introduced a 3-valve version of its Generation IV V8 which uses pushrods to actuate forked rockers, and Cummins makes a 4-valve pushrod straight-6 Diesel, the Cummins 600.

History

The first multivalve engine was built by Peugeot in 1912 for Grand Prix racing. The technology was also attempted by Bugatti, Bentley, and Stutz, but it was not until the 1970s that this technology became widespread. The first was Jensen in the 1972 Jensen Healey roadster. This used a Lotus developed version of a GM design which resulted in a 1973 cc (2.0 litre) DOHC engine that delivered 140 bhp. Others, including Cosworth (on the 1975 Chevrolet Vega's 2300 engine), Lotus Cars (on the 1976 Esprit - which used a 160 bhp version of the same engine first seen in the Jensen Healey), and BMW (on the 1979 M1's M88 engine). Triumph also introduced a single overhead cam 16-valve head on the Slant-4 in their Dolomite Sprint.

Ferrari followed Lotus and GM in to the multivalve designs with their Quattrovalve 308. From there, Honda and Toyota rapidly spread the technology to their mainstream models in the 1980s.

Modern multivalve engines

Today, multivalve engines are used by nearly all manufacturers. They are common among the Japanese and European makers. The US manufacturers have lagged, though improvements to their pushrod designs have caused some to question the benefits of multivalve engines.

General Motors began using 4-valve DOHC heads with their Quad-4 and Northstar engines in the 1990s. The company worked with Lotus, a subsidiary at the time, to adapt two pushrod engines for 4-valve DOHC cylinder heads: The LT5 V8 from the Corvette ZR-1 and the 3.4 L LQ1 V6. Pushrod engines are still the norm at GM, however.

Ford's DOHC success came with their (multivalve-optional) Modular V8, SHO V6, and Mazda-developed B-family of I4 engines. Their Duratec family consists entirely of multivalve engines, and is used across the product line.

DaimlerChrysler's Mercedes-Benz used 3-valve SOHC engines for many years, but recently switched to 4-valve designs. Their American Chrysler operation has developed a number of successful multivalve OHC I4 and V6 engines, but relies on pushrod V8s.

VAG companies like Volkswagen and Audi now use 5-valve engines in many of their vehicles after acquiring the technology from Bugatti who developed it for their EB110 supercar.

Motorcycles

Rudge-Whitworth introduced the 4-valve Ulster pushrod operated 500cc single cylinder machine in 1929.

Honda introduced the 4-valve XL250 SOHC single cylinder engine in 1972. In 1977 Honda introduced the 3-valve SOHC CB400 twin cylinder models. Yamaha introduced the 5-valve DOHC four cylinder FZ750 in 1984.

In 1978 Honda introduced the CX500, a V-twin with pushrod operated 4-valve cylinder heads.

In the 1979 to 1992 era, Honda produced a limited number of racing and production motorcycles with 8-valve, V4 engines in 500 and 750cc displacements, the NR series.

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