The Atlas-Imperial diesel engine

By Tom Keffer, Board President

For her first 15 years, Western Flyer was powered by a rugged Atlas-Imperial diesel. These engines were reliable and very popular in their time. Manufactured in Oakland, CA, they were exported all over the world. Many are still in service.

The original Atlas-Imperial diesel looked something like this.

Flyer came equipped with the model 6HM763, a 6-cylinder 160 HP engine. Typical of its time, it had a very large bore (9 inches) and stroke (12 inches) and turned very slowly (typically, 325 RPM). 

While it was much easier to start than most of its contemporaries, it was not a simple turn-of-the-key affair. First, you primed the oil lubrication system, then the lifters, cams, and overhead rockers had to be lubricated by hand. The crankshaft was turned over until it was in a “start” position, then the engineer built up fuel and oil pressure by hand. Finally, the engine was turned over and started with a blast of compressed air. No glow plugs were used — it was all compression heating.

A running 4HM763 Atlas engine. This engine is very similar to Western Flyer’s, except that it is four cylinders instead of six. Video courtesy of Eric Rasmussen.

Keeping it running was also a labor-intensive job. Every couple of hours the engineer on Flyer, Tex Travis, would have to run down below to lubricate the now moving lifters and rockers, spraying oil everywhere. 

The engine had no transmission, nor clutch. It was just a direct drive, engine to propeller. If the engine was turning 300 RPM, that’s how fast the prop turned: simple and efficient, without the losses normally associated with a gearbox. However, that also meant the only way to back up the boat was to run the engine in reverse! First, the fuel supply was shut off. If the boat was moving fast, it might take 5 or 10 seconds for it to slow down enough that the engine would stop turning. Then the engineer would engage a lever to shift the camshaft to a new set of lobes, designed to run the engine backwards. Finally, a blast of compressed air would be introduced, starting the engine backwards. 

If all went well, the whole process took 8 to 10 seconds. Those seconds must have felt mighty long while you’re approaching a dock doing 3 or 4 knots!

The engine was surprisingly economical, burning about 0.06 gallons per hour per HP, only slightly more than the modern John Deere engine that is replacing it.

The John Deere engine that will replace it.

The Atlas engine ran forever, was very reliable, and was quite efficient. So, what advantages does a modern engine have over it? The answer is weight, power, and ease of operation. That old Atlas engine weighed 20,000 lbs — about the same as three or four full-size pickup trucks! 

By contrast, its replacement engine weighs about 3,000 lbs, yet produces well over twice the horsepower. It will also be a hybrid engine, allowing 6-8 hours of operation on pure electrical power. Operating it is also easier: just turn a key. 

Tex will be out of a job.

Here’s a table comparing the two engines:

Atlas-ImperialJohn Deere
Model6HM7636135 AFM85
TypeInline-6, naturally aspiratedInline-6, turbocharged
Displacement4578 in3824 in3
Weight20,000 lbs3,000 lbs
Overall size (L x H x W)149” x 75” x 38”71” x 46” x 42”
Fuel consumption0.06 gph/HP0.053 gph/HP
The actual engine that will be installed in the Western Flyer.


Many thanks to Eric Rasmussen for supplying the picture of the Atlas engine and for answering my innumerable questions.

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