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Increase of Knock Limit in Outboard Motor Through Employment of Dual Control Direct Cooling System

Article of Honda R&D Technical Review Vol.16 No.1


Cooling performance of the outboard engines, which pump the ambient water to cool the engine and discharge it to the ocean or rivers, has been improved in a direct-cooled outboard motor without a radiator in order to increase knock limit.
The research project leading to this improvement tested a conventional single control cooling system and a new dual control cooling system in which the cylinder head and cylinder blocks were made independent and each provided with a thermostat to enable temperature to be controlled.
Tests were conducted under full engine load. Measurements of the temperature of cooling water inside the water jacket were taken to evaluate cooling performance. Knock limit was evaluated through measurements of combustion chamber pressure taken using a gasket type combustion pressure sensor positioned at the spark plugs.
These measurements confirmed that the dual control cooling system increased knock limit by 8 deg against the conventional single control cooling system.


(1) Kobayashi, H., Yoshimura, K., Hirayama, T.: A study on dual circuit cooling for higher compression ratio, IMechE/SAE, International Conference on Fuel Efficient Power Trains and Vehicles London, C427/84, p. 151-158 (1984)
(2) Yamada, T., Furuno, S., Kanda, M., Iwashita, Y., Yokoi, Y., Katagiri, J.: Improvement of Coolant Flow for Reducing Knock, 1990 JSAE Annual Congress Proceedings Fall, 902, Vol. 1, p. 189-192 (1990) (in Japanese)
(3) Ishida, N.: System Sekkei to Engine Seinou, Kouseinou Engine to ha Nanika, Tokyo, Grand Prix, p. 210-216 (1991) (in Japanese)

Author (organization or company)

Yoshihiko FUKUDA(Asaka Higashi R&D Center)、Hiroki TAWA(Asaka Higashi R&D Center)、Atsuo MAKISE(Asaka Higashi R&D Center)

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