Combustion engines are somewhat outside my wheelhouse, and it's been nearly two decades since my mechanical engineering studies so I apologize in advance for possible misunderstandings and incorrect language - I may well use the wrong English terminology here, so be forewarned!

With that caveat, I was able to track down a 1935 manual for Volvo Hesselman engine model H. A., in Finnish. Here's the general description, translated to my best knowledge (see above; tried to preserve the archaic language, and some clarifications in square brackets):

Volvo-Hesselman-engine is a heavy oil engine, but a heavy oil engine that differs from Diesel engines by having a relatively low operating pressure. This has been made possible by engineer Hesselman's ingenious construction, where well balanced air supply and oil injection combined to electric ignition enables the smokeless and economical combustion of heavier fuels, such as kerosine.

The engine is designed by the engineers of the Volvo factory and its operation can be compared to gasoline engine with few and rare exceptions. One of the most notable differences is the replacement of carburetor with fuel injection pump. Many features are similar to how they appear in our top-valve equipped 75 horsepower gasoline engine.

Operating cycle is the same as in gasoline engine, unless one considers that intake manifold draws only pure air instead of gas mixture. Fuel is injected into combustion chamber in each cylinder via special pump, tube, and injector/burner. Air and finely divided fuel from the injector are mixed in a vigorous turbulence attained by specially shaped shield below the intake valve. Relatively cool cylinder wall is shielded against direct impingement by fuel by forming the tops of the pistons into a flange shape. Fuel injection occurs close to the end of the compression cycle.

From what I gathered through other sources, it seems to me that this engine - hadn't heard of it before! - is essentially, as you already described, a direct injection four stroke gasoline engine with two fuel feeds and relatively low compression ratio; a Finnish language discussion I found suggested 7:1 "or thereabouts". Direct injection is used to enable the use of heavier distillates, which was the key reason for the engine's popularity: these distillates were cheaper and more widely available at the time. There was also a small gasoline reservoir that was used in engine start to heat the engine. Fuel oil injection would then kick in automatically.

I couldn't find a direct reference to how exactly the power increase is effected, but Figure 3 ("Kuva 3") in the PDF document linked above shows the butterfly valve that the text describes as being operated by accelerator pedal. To me it seems like normal carburetor valve, but my understanding of combustion engines is shaky at best. The text also mentions that the engine described herein automatically disconnects three of its six cylinders when idling; also described is a valve that permits extra air intake in some idling conditions so that the engine doesn't produce too much smoke :). Nevertheless, "heavy smoke" caused by incomplete combustion, in turn caused by generally too low operating temperatures, seems to have been one of the noted drawbacks.

I'm more confident in the metallurgy side of things, and it seems that the improved alloys, manufacturing methods, and experience in designing better Diesel engines permitted Volvo engineers at least to ditch the Hesselman engine in 1936, apparently mostly because of aforementioned "smoke" issues. Boggles the mind to think what a smoker the engine must have been to be unacceptably polluting by the standards of the time, but there you have it.

Hope this helps!