Gas Engine Magazine

A Monovalve Diesel Engine

By Staff

67 Albion Road Ashburton, Victoria 3147, Australia

Ever seen a side valve Diesel engine with only one valve per cylinder? Warwick Bryce describes one that was made in America during the early 1930s, when diesels were still going through their evolutionary stage.

Mention of the preservation of a Monovalve diesel engine in the Reflections column started me looking through old engine books, and I turned up quite a bit of information on this unusual engine that uses only one valve per cylinder:

The Designer

In the mid 1930s the Monovalve 4 cylinder diesel engine was made in Oakland, California by the American Diesel Engine Company. Its designer and manager of the company, Charles A. Wins-low, claimed 30 years engine building experience had enabled him to design a unique engine that embodied the best marine engine features.


It was a 4 cylinder water cooled vertical engine suitable for stationary, marine and use in tractors and trucks. Power was rated continuously at 75 HP @1200 RPM with an intermittent rating of 125 HP @2000 RPM. Bore and stroke of 5 by 7 inches gives a displacement of 665 cubic inches.


The engine is of side valve layout but has only one valve per cylinder that handles both inlet and exhaust. A simple casting forms the cylinder head that has an injector squirting almost horizontally into the combustion chamber that is formed in the underside of the head and is half over the valve and half over the piston. Injection equipment is conventional Bosch plunger pump and pintel type injector set at relatively low 82 atmospheres. A large diameter exhaust manifold with a venturi shaped entry fits over the much smaller exhaust port. An inlet manifold communicates with the exhaust manifold at the entry to the venturi.


Exhaust gas rushing out the port at high speed produces a low pressure at the venturi, which draws fresh air into the exhaust manifold behind the exhaust gases. The valve does not close at the end of the exhaust stroke but remains open for the inlet stroke. During the inlet stroke, a suction is produced by the combined effect of the piston moving down and the exhaust continuing down the exhaust pipe at high velocity. This draws air from the aircleaner through the inlet manifold into the venturi section, where it is sucked back through the port and hence into the cylinder.


Several advantages are claimed for this system, such as simplified construction, particularly of the head, extended valve life due to the cooling the valve receives during the inlet stroke, a cooler exhaust pipe due to the air drawn into it, and improved combustion due to the heat picked up by the air from the exhaust manifold.

Photos by Ted Paproski.


Another striking feature of this engine is that it is a side valve diesel engine. I have never heard of any others; even Diesel’s first successful engine had valves in the head and an overhead camshaft. Thinking about it, the Mono-valve engine really had to be a side valve, since the valve is held at full lift as the piston passes through top dead center. If it were overhead valve, the piston would hit the valve! On the other hand, having only one valve allowed the use of a side valve layout, yet still gave a small enough combustion chamber to get the compression ratio high enough for a diesel.

Of course the designer claims all the usual virtues such as cold starting, clean exhaust over a very wide speed range, flat torque curve and economical running of his unique design. There may be something in this, as the engine in preservation is reported to start and run just fine.

The Monovalve engine appeared to be well constructed with features such as long through bolts from the head to the main bearings, and an elaborate lubrication system. Studying other engines of the same time frame showed it had good power and the fuel consumption was similar to prechamber engines, but not quite as good as direct injection diesels of the day.

One wonders why the design did not persist, particularly since the 4 cylinder version was to be followed by a range of engines from 1 to 8 cylinders, using standardized components such as valves, pistons and rods etc. A possible drawback could have been that it was intolerant to differences in air cleaners and exhausts that would arise with various installations. The photos certainly show unusually large dual exhausts and a strange inlet system that appears to get air from some sort of flywheel fan. None of the write-ups mention any kind of forced assistance being required for the intake, so this may have been a later development.

Performance Comparison


BORE ins








5.5 x 7


75 @ 1200



7 x 6


100 @ 1000



Ted Paproski, Bothell, Washington, USA, for photographs and description of his engine, and the article out of Pacific Fisherman.


Gas Engine Magazine

High Speed Diesel Engines, International Correspondence Schools

Diesel and Gas Turbine Publications

Pacific Fisherman

  • Published on Oct 1, 1994
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