Naphtha Launch - Naphtha Engine

Naphtha Engine

The naphtha engine is an external combustion engine, generally similar to the type of small steam engine already in use for steam launches. The working fluid is naphtha, which unusually is also used as a liquid fuel to power the boiler. Appearance is similar to a steam launch, having a small vertical boiler and vertical cylinders.

The burner for a naphtha engine uses naphtha itself, and is similar to that used for steam cars. Although such convenient and self-regulating liquid fuel burners were also used on steam launches, most launches at the time of the naphtha engine's heyday were still using solid fuel.

General arrangement

To avoid problems of premature condensation, the engine and boiler units were mounted together. Engines were three-cylinder single-acting vertical simple expansion engines, with 120° crankshaft spacing, so as to be self-starting without dead centre problems.

Boiler

The boilers were of spiral monotube form. These were single-pass water-tube_(sic) steam generators, where liquid at one end of the tube boiled by the time it reached the end, without any looped circulation. Such boilers, although with conventional water and steam, were also used for steam cars such as the White. The boiler was mounted directly atop the engine, and encased in a polished brass or copper casing with a short funnel above. The purpose of the funnel was merely to vent fumes above the heads of passengers, rather than to act as a draught to draw the fire.

Burner

Unlike coal or wood, the jet burners used did not require additional draught. The boiler was fired with a pressure jet burner, a small quantity of pressurised naphtha being tapped off from the boiler. A prominent tube and funnel on the front of the boiler casing (sometimes mistaken for a whistle) is the air inlet. The fuel jet mixes with the downwards air flow in this vertical tube, then the mixed air and fuel is fed to a ring-shaped burner inside the boiler casing. For starting, a hand air pump was used to pressurise the fuel tank and encourage fuel flow, together with pre-heating of the burner coil by lighting a little of the fuel in a tray below it.

There was no throttle valve or regulator to control the engine speed. As was commonly used on single-pass spiral monotube boilers, the burner output was regulated instead with the fuel valve and the boiler responded quickly enough for this to be sufficient.

Engine

Lubrication of the engine was performed by the fuel itself, collected in a sealed wet sump arrangement beneath the crankcase. The naphtha fuel was an acceptable lubricant and this avoided the problem of separating lubricating oil from condensed fuel vapour leaking past the piston ring seals. It did however require careful sealing of the crankcase.

The engine valvegear was simple, compared to that of contemporary steam engines. A single rotating overhead camshaft ran above the cylinders, parallel to the crankshaft. Eccentrics on this shaft actuated a transverse slide valve for each cylinder. The valve camshaft was driven at engine speed by gears from the crankshaft. A prominent handwheel on the camshaft allowed the engine to be reversed, in a similar manner to slip-eccentric valve gear by shifting the phase of the camshaft relative to the crankshaft.

Condensers

External water-cooled condensers were used, simple pipes run under the hull, along the length of the keel. Despite the obvious risks of damage from grounding, these seem to have given little trouble in service. The fuel tank was in the fo'csle of the boat, well away from the engine and risk of fire. This fuel tank was pressurised by the returning naphtha, avoiding the need for a fuel pump.

Low boiling point

One of the attractive features that first led inventors to consider naphtha as a working fluid was its low boiling point, compared to water. This allows a relatively simple engine to be constructed, although it actually represents a limitation on the efficiency of such an engine. At the time, thermodynamics was already an advanced science and the limitations of thermodynamic cycles were well understood by physicists, if not by all engineers. By limiting the range of temperatures between which the engine worked, this actually reduces the potential efficiency of the engine, rather than increasing it.

An earlier use of low boiling point fluids had been Du Tremblay's ether engine of the 1850s. This was used for a bottoming cycle as an addition to a steam engine. Waste heat rejected through the condenser of the steam engine was used to boil ether and drive a second cylinder.