Toyota Motor Co., Ltd. had been conducting research on diesel engines from early on, and around 1939 began efforts to develop a diesel engine, studying the diesel engine made by Junkers. However, this engine did not lead to commercialization before the war because of various factors in the industry and Japanese government agencies.

Around the spring of 1948, Toyota established a diesel engine research laboratory inside the Kamata Plant and began R&D on diesel engines anew, inviting such experts as Rihei Nagano1 and Takeshi Oka.2 As a result, five prototypes each of the Models Y and Z diesel engines were produced in 1950 (Table 1-36).

During this period, due to a labor dispute that had begun in April 1950, the Kamata Plant was closed in June of that year, and research on diesel engines was moved to the Head Office in Koromo. In the same year, Toyota began testing the Model Y diesel engine using the revamped Model BM truck, and also conducted testing using the revamped Model BX truck in March 1951. Neither of these efforts, however, led to commercialization in the end and research on diesel engines was suspended once again.

Subsequently, diesel engine development was resumed in October 1954 and a prototype of the D diesel engine was completed in August 1955. A modified prototype truck was manufactured by installing this engine in the Model BA truck and, following various types of testing and improvements, was launched as the Model DA60 truck in March 1957.

Manufacturing of the D diesel engine was accomplished through cooperation among various Toyota Group companies. Casting and machining of the cylinder block and the cylinder head was done by Toyoda Automatic Loom Works; machining of the crankshaft was performed by Toyoda Machine Works, Ltd.; machining of gears was done by Aichi Kogyo Co., Ltd.; the injection pump and electrical components for the diesel engine were made by Nippondenso Co., Ltd.; and vehicle assembly was performed by Toyota Motor Co., Ltd. Table 1-37 shows the specifications of the D diesel engine.

The performance of a diesel engine greatly depends on the performance of its fuel injection pump, which can be likened to the 'heart' of the engine. The D diesel engine used a fuel injection pump made by Nippondenso utilizing a technology introduced by Robert Bosch GmbH of West Germany.

The idea of a technology introduction from Robert Bosch was brought to Kiichiro Toyoda, who had become a counselor in the autumn of 1951, by Dr. Tokushichi Mishima. Dr. Mishima was a classmate of Kiichiro from the Engineering Faculty at the Tokyo Imperial University and was a metallurgist known globally for his invention of MK magnetic steel. When the patent on MK magnetic steel was licensed to Robert Bosch, the company asked Dr. Mishima whether any Japanese corporations would be interested in forming a technical partnership. Because Dr. Mishima had served as a research advisor to Toyota Motor Co., Ltd. during its founding period, he brought the issue of technical partnership to Nippondenso via Kiichiro.

Nippondenso and Robert Bosch concluded a technical assistance agreement related to electrical parts in November 1953 and later, in February 1955, added clauses related to fuel injection pumps and spark plugs. Based on this agreement, Nippondenso completed the first prototype of an injection pump in March 1956 under designing and manufacturing guidance from Robert Bosch.

When the D diesel engine was first being developed, testing was done using fuel injection pumps and injection nozzles made by Robert Bosch. However, these components were replaced one after another with components made by Nippondenso as its manufacturing technologies advanced. Subsequently, by the time the 5-ton Model DA60 truck equipped with the D diesel engine was launched on March 14, 1957, the performance level of the fuel injection pumps made by Nippondenso equaled that of Robert Bosch.