HB Engine
Where Yanmar’s diesel story began
A bold idea, ahead of its time
Founded in 1912, Yanmar spent its early years perfecting engine technology. But in the early 1930s, founder Magokichi Yamaoka set a bold challenge: build a small, affordable diesel engine that everyday farmers could use one that ran on low-cost heavy oil and dramatically reduced physical labour.
At a time when diesel engines were large, heavy, and expensive, this idea was revolutionary.
17 months of persistence
Development of the HB engine began in July 1932. Under Yamaoka’s direct leadership, a dedicated team experimented relentlessly with combustion chambers, fuel injection, and atomisation. The goal was straightforward yet demanding: stable combustion, minimal smoke, and reliable performance in a compact format.
After 17 months of trial and error, success came. On December 23, 1933, the prototype ran smoothly. Designed for 3 hp, it surprised everyone by delivering around 5 hp under load. The HB was born and with it, what later became known as Diesel Commemoration Day.
Small engine, smart design
The HB was a horizontal, water-cooled diesel engine, affectionately nicknamed “yokosui” (from yokogata suirei horizontal and water-cooled). This clever layout made it easy to transport, simple to operate, and perfectly suited for field work.
Despite its compact size, the HB proved something groundbreaking:
a small diesel engine could be fuel-efficient, durable, and reliable, while remaining affordable for small-scale users.
Powering progress
The impact was immediate. The HB engine helped mechanise agriculture and small industries, powering:
- Farm machinery
- Generators
- Welding equipment
- Concrete mixers
Its success led directly to a family of horizontal water-cooled diesel engines for both land and marine applications. In Yanmar’s history, the HB is recognised as the true starting point of its compact diesel legacy and it is officially registered among Japan’s Essential Historical Materials for Science and Technology.
One small engine. A lasting legacy.
From a humble 5–6 horsepower engine in 1933 to millions of engines powering the world today, the HB reminds us of what Yanmar has always stood for: practical innovation, real-world impact, and engineering that serves people.
That spirit still drives Yanmar every engine, every day.
