Japanese Engine Research
at the
Smithsonian National Air and Space Museum


Japanese Aircraft Engines

Book Research in Progress

Robert Mawhinney, Museum Specialist, NASM


Often, Japanese engines are summarily dismissed as “inferior." This is seldom the case. Considering the factors hampering the industry at the time, Japanese engineers produced many successful designs that rivaled any in the West. The mid-20s saw the first Japanese engine designs emerge, and within a decade, the Japanese had fielded two separate air arms capable, for the short term, of waging war with any western power.

Published information about Japanese aircraft engines of World War Two (as well as other Japanese subjects of the time period) is often confusing, misleading and occasionally just wrong. Published English-language information, based primarily on Technical Air Intelligence Center (TAIC) reports, tends only to repeat type and model identification mistakes made during the early days of war. In addition, TAIC reports were understandably focused on fighter powerplants and ignored the more mundane types used for transports and trainers. No document has yet been published that accurately and completely covers the broad range of Japanese engines.

The primary reason for the confusion surrounding Japanese engines is the type and model designations used by the Japanese military. The Imperial Japanese Army (IJA) and the Imperial Japanese Navy (IJN) never formed a cooperative relationship. These military branches operated completely independently with regard to specifications, procurement and their relationships with manufacturers. (This extended to the point that the different service branches even chose different caliber ammunition for their weapons.) The designation nomenclatures for aircraft engines were service unique as well. The IJN gave their engines a multi-letter code while in development and a name and model number when the engine reached service status. The IJA also gave a development designation and service status designation based on the Japanese calendar year it was accepted into service. In 1942, a third system of nomenclatures was introduced, the “Unified” type designation system. In addition to this, most engines had a “popular” name. This name was often used in official military and company documents.  A single type/model of engine, in service late in the war, could have as many as six unique nomenclatures. 


With this book, I intend to provide a single-point, concise reference to Japanese aircraft powerplants in the time frame leading up to and including World War Two.

The purpose for a book of this type is to eliminate the confusion surrounding Japanese powerplants. In addition, the book will provide a reference manual for engine identification and specifications for comparison purposes and will highlight the unique aspects of Japanese aircraft powerplants.

To accomplish this goal, it is necessary to include every type and model of engine that may or may not have appeared in literature on the subject to date. This includes obsolete types, experimental types and even license-built types. Fictitious types reported by U.S. Intelligence during the war will also be included. This will serve to eliminate lingering questions and uncertainty regarding an engine type.

The “big picture” view of the Japanese aircraft powerplant industry will also be covered. To understand the industry as a whole, it is necessary to understand the problems faced by the companies producing engines.

Material shortages were common. The workforce was often unskilled and minimally trained. Fuel for testing powerplants was in short supply. In fact, much of the advanced research and development being conducted focused on planning for the lack of quality metals and the lack of petroleum-based fuel. This occurred at the same time that advanced research and development in America was concerned almost exclusively with improving engine performance and increasing production—no expense spared. Bombing raids forced workers from their homes, destroyed factories and forced companies to divert resources into factory dispersion programs.  Providing the facts regarding the industry will allow the reader to draw logical conclusions about the engines.

The Basics of Identification --- Explanation of the Unified Engine Designation System

This system was created in April 1942 to minimize the confusion created by the different designations used by the Japanese Navy and Army. Under the old Navy and Army systems a single engine could have as many as six unique names.

This designation system has four basic parts:

Part 1) Ha - The abbreviation for Hatsudoki (Engine)

Part 2) Single digit, engine type identifier

                1 - Air Cooled Inlines

                2 - Air Cooled Single Row Radials

                3 - Air Cooled 14 Cylinder Two Row Radials

                4 - Air Cooled 18 Cylinder Two Row Radials

                5 - Air Cooled Over 18 Cylinder Multi Row Radials

                6 - Liquid Cooled 12 Cylinder

                7 - Liquid Cooled Over 12 Cylinders

                8 - Diesels

                9 - Special Engines

Part 3) Single Digit, Bore/Stroke identifier (Millimeters)

                1 - 140/130

                2 - 150/170

                3 - 140/150

                4 - 140/160

                5 - 130/150

                0 - 130/160

Part 4) Specific two digit type identifier with-in the above groupings

Example: Ha45-11 (Nakajima Homare 11)


Explanation of the Navy engine designation system:

The Imperial Japanese Navy used two systems to identify their aircraft engines. While an engine was in the development or experimental stage it received a 4-5 character code to identify its manufacturer, basic type, design and revisions of the basic design. Once an engine was accepted into service it was given another designation that consisted of a name plus a model number. The experimental designation was then dropped.


Experimental codes: Example- NK9B

First Character:

                M - Mitsubishi

                N - Nakajima

                A - Aichi

                K - Kawasaki

                I - Ishikawajima

                Y - Air Technical Arsenal

                G - Hitachi

Second Character;

                K - Air cooled

                E - Liquid Cooled

                D - Diesel

Third Character; Design number

Fourth Character; Revision code


The example above is a Nakajima built air cooled engine. Nakajima's 9th design and the second revision of that design. It is a Nakajima Homare 11 built to Navy specification.

Service Designation: Example- Kinsei 62

Service designations are given in series (i.e. 00,10, 20, 30, 40, etc.) When a designation is given ending in 0, it refers to the entire series. No production models end in "0". The first model of a production type ends with a "1".

Therefore a Kinsei 62 would be the second version of the 60 series Kinsei's.


Explanation of the Army engine designation system:

The Imperial Japanese Army used two designation systems to identify aircraft engines. While an engine was in the development or experimental stage it received a Ha number. The Ha numbers were assigned in consecutive order and could be given sub-model numbers or special identifiers.

Examples: Ha24, Ha115 Model 1, Ha13 "Ko"

When the engine reached production/acceptance status the Ha identifier was retained and its production Type designation was added. Army Type numbers refer to the Japanese calendar year the engine was accepted into military service.

 Japanese calendar years:  

                99 is 2599 or 1939

                0 or 100 is 2600 or 1940

                1 is 2601 or 1941

                2 is 2602 or 1942

                3 is 2603 or 1943

                4 is 2604 or 1944

                5 is 2605 or 1945


In addition to the Type number the engine was identified by its basic type and its nominal horsepower (metric) rating.

Example: Type100 1250hp Air Cooled Radial (Ha41)

In many cases the Ha number is the only specific designator with-in several engine designations that have the same Type and HP identification.