In 1872 George Brayton patented a constant pressure internal combustion engine, initially using vaporized gas but later using liquid fuels such as kerosene and oil, known as Brayton's Ready Motor,[1][2] The engine used one cylinder for compression, a receiver reservoir, and a separate power/expander cylinder in which the products of combustion expanded for the power stroke.

In the original version a gas/air mixture was created by a vapor carburetor,[3] then compressed and stored in a reservoir where it was ignited and then introduced into an expansion cylinder.

Brayton not only achieved success in making the constant pressure cycle work, but he also made and marketed a commercial product.

In 1881 John Philip Holland used a Brayton engine to power the world's first successful self-propelled submarine, the Fenian Ram.

The indicator diagrams for this engine are also reported by Dugald Clerk and show that the addition of the water has little merit in terms of power production, the cooling of the gases and expansion of the steam compensating for each other.

The reservoir on the Brayton engine allowed it to be readily started if it remained pressurized, though Clerk states that "leakage and loss were so frequent that the apparatus was of little use.

Brayton states: “I have discovered that heavy oils can be mechanically converted into a finely divided condition within a firing portion of the cylinder, or in a communicating firing chamber.” Another part reads, “I have for the first time, so far as my knowledge extends, regulated speed by variably controlling the direct discharge of liquid fuel into the combustion chamber or cylinder into a finely divided condition highly favorable to immediate combustion.” This was likely the first engine to use a lean-burn system to regulate engine speed and output.

In this manner, the engine fired on every power stroke and speed and output were controlled solely by the quantity of fuel injected.