Japanese aero-engines for military aircraft were given a wide variety of designations depending on the customer. This led to much confusion, particularly among the Allied forces, where a single engine type could have up to six different designations. This situation emerged because of the almost total lack of co-operation in weapons procurement between the IJAAS (大æÂ¥æ¬å¸ÂÃ¥ÂÂé¸è»Âèª空é - Dai-Nippon Teikoku Rikugun Kokutai - Imperial Japanese Army Air Service) and the IJNAS (大æÂ¥æ¬å¸ÂÃ¥ÂÂæµ·è»Âèª空é - Dai-Nippon Teikoku Kaigun Koukuu-tai - Imperial Japanese Navy Air Service).
Engines could have designations in any or all of these designation systems:
Data from:Japanese Aircraft 1910âÂÂ1941 & Japanese Aircraft of the Pacific War & Japanese Aircraft Engines
Experimental engines for Army aircraft were given ã Ha â (ã¨ã³ã¸ã³ - Hatsudoki) numbers whilst under design and testing. The Army Ha numbers had no intrinsic meaning and were only sequentially allocated. Sub types could be identified by suffixes -kai, -ko etc., or -I, -II etc.. e.g.:Nakajima Ha115
Once an engine had been accepted for service in Army aircraft it was given a long designation which denoted the year of introduction, power, cooling method and layout:
e.g.:Army Type 100 1,450hp Air Cooled Radial â (Nakajima Ha111) The two or three digit Type number denoted the that the engine was introduced, identical to the Type numbers used in Japanese aircraft long designations from 1925 (From 1927 to 1930 the Type number sometimes denoted the Shà Âwa or Taisho year ):
After the Type number the power of the engine was denoted in horsepower:
After the power designator the type of engine was denoted:
Sub-types were designated by suffixes.
The IJNAS introduced a designation system for experimental engines and those under development / test before production. Once the engine entered service this designation was replaced by the name or unified system. Formed from four character groups the IJNAS experimental designation system consisted of e.g. Nakajima NK9B
The simplest of all the systems the IJNAS allocated name characters to engines combined with Model and revision numbers, introduced to reduce confusion and also to reduce the intelligence value of the designation to enemies. e.g. Nakajima Homare 11 - ()
Some engines were never allocated a designation or there is no record of such. In which case they are usually identified by the manufacturers designation. e.g. Mitsubishi A4
From 1942 the Ministry of Munitions in Japan instituted a Unified aero-engine designation system in an attempt to reduce confusion caused by previous systems. The new system prefixed engine designations with ã Ha â (ã¨ã³ã¸ã³ - Hatsudoki) followed by code numbers identifying each engine in terms of layout, no of cylinders, cooling method and sub-series model numbers.
Thus the Mitsubishi Ha-33-62 éÂÂæ Kinsei
Each engine designation in this system started with the Hatsudoki short hand character, represented by Ha in English, followed by two numbers classifying the engine:
The first digit represented the engine classification:
The second digit represented the bore and stroke of the engine:
After the classification digits a two digit number gave the model number and revision state.
Model numbers were given as:
etc.etc.
A revision (or modification) state number replaced the second digit of the model number:
Coincidentally one engine was assigned the same numerical designation in the IJAAS and joint designation systems; Nakajima Ha-45 Homare.
Data from: