Engines designed for converting chemical potential energy into power are found everywhere. Mostly they are internal combustion engines used for generating electricity, for generating shaft power for all sorts of machines and for the propulsion of all kinds of vehicles. The most energy efficient machines are large piston engines of the sort found in sea-going vessels, rail transport and for generating electricity, efficiencies of 40-50% being possible with fuels varying from natural gas to heavy residual oils.

Engineers always want to improve engine efficiency and reduce specific capital cost, but it is often desirable to increase the absolute power output. A higher rating is accomplished by increasing the number of cylinders per engine and by increasing the size of the cylinders but this is only possible to a limited extent because of problems with size, tolerances, cooling and alignment. Moreover the forces on certain engine parts such as the piston rod and crankshaft become limiting factors.

However, high efficiencies, low specific costs and the possibility of building engines with a very high power output can be obtained by replacing the mechanical piston by a liquid piston. The liquid piston can form the end of a liquid column that replaces the mechanical piston, the piston rod and crankshaft, thus considerably reducing the number of moving parts in the engine and the need for lubrication. The presence of the liquid means that the piston and the lining of the cylinder can be cooled by direct, rather than indirect, heat transfer. Hence piston engines can be built in which higher temperatures are allowable, resulting in higher efficiencies. In addition larger cylinders can be employed, resulting in higher power outputs and lower costs.

Employing a liquid piston implies that use is made of one or more essentially vertical cylinders in which the ignition takes place at the top, and the bottom is formed by a liquid column that moves up and down and generates – in the first instance – hydraulic power.

Engineering principles

For most applications a two-stroke engine is preferable. During the power stroke liquid is pushed out of the cylinder and produces hydraulic power. This hydraulic power can be used directly when the machine is to be employed as a pump, in for example an oil pipeline or as a jet for the propulsion of a large vessel. It is also possible to use the hydraulic power to drive a liquid expansion machine that in turn can be used to drive equipment that requires shaft power – such as an electricity generator. Although strictly speaking only the upper part of the liquid column that moves up and down in the cylinder acts as a piston, the phrase ‘liquid piston’ is used here for the whole liquid column.

As in other piston engines a cylinder is the most logical form for the combustion chamber when using a liquid piston. But in principle any rotationally symmetrical form can be applied around a vertical axis, as sealing the gap between the piston and the cylinder wall is not a problem. In contrast with mechanical pistons it is not necessary for the cross-section to be the same everywhere, nor that the cylinder has a smooth surface. Although in principle the combustion chamber may be any shape, not only cylindrical, the term ‘cylinder’ is used here for all shapes.

The number of moving parts is limited essentially to the valves of the engine. Because of the generally high engine capacity the valves for the supply of combustion air and for the removal of the exhaust gases as well as for the supply and discharge of the liquid medium may have to be built in parallel units, ie not incorporated.

Because of the effective cooling of the cylinder lining, problems with the cooling of the piston, cylinder wall and cylinder head are greatly simplified. An evaporating liquid film that is applied during the compression stroke cools the cylinder lining and cylinder head. Further, the cooling of the cylinder can be improved by covering the inner wall with a porous metal or ceramic coating that is wetted during the compression stroke.

The liquid piston can be applied in both diesel and Otto engines. It is of course true that in order to get the steady flow of liquid required for most applications it would be necessary to have more than one cylinder.


Gaseous fuels, and liquid fuels such as oil and emulsions of oil in water can be used. As fouling of moving parts is less of a problem in engines with a liquid piston it is also possible to use ash-containing and solid fuels such as coal, and suspensions of such fuels in water or in oil. By operating the machine under partial oxidation conditions it could in principle also be used as a gasifier.

The working fluid

The most plausible liquid to consider for use as a liquid piston is of course water. This water need not, though, be very pure. It is for example possible to use salt solutions such as seawater. The latter fact is important in the case where no fresh water is available, as on board sea-going vessels. It is also possible to use oil. This is of special importance where the same oil can be used as fuel. It would for instance improve the option of using a liquid piston engine in pumps in oil pipelines.

Thermal efficiency

The efficiency of engines using liquid pistons can be quite high, mainly because cylinders of greater cross section are possible. Moreover the direct cooling of the cylinder wall and the piston ensures that there are virtually no limitations on the maximum temperature. The result is an internal combustion engine with a low cylinder surface/volume ratio, low specific heat losses and high efficiency. The parasitic energy normally required for lubricating oil systems can be substantially reduced and mechanical losses in the piston rod, crankshaft and gearbox are avoided. For example, in a liquid turbine of the sort that would generally be required for electrical,generating applications, losses are almost entirely limited to the hydraulic losses in the turbine. Measures to increase the efficiency such as using a supercharger can be just as advantageously applied to a liquid piston. The overall efficiency for generating electricity using liquid piston engines is expected to be 45-60% depending on the type and the capacity of the engine.


The most obvious application is to use the engine, without mechanical pistons, piston rods and crankshaft, in a piston-type pump. (Figure 1). Liquid is forced out of the pump during the power stroke: the pressure of the liquid entering the pump at the suction stroke compresses the combustion air. Such pumps can be used to advantage in water and oil pipelines.

As a compressor it can be used to drive hydraulic machinery directly, and, by using a liquid turbine or piston expansion machine, to generate shaft power and electricity (Figure2). The hydraulic power can be used to generate a pressurised fluid used as the driving fluid in a propulsion jet that could, for example, be used to power sea going vessels.

Clearly, the use of a liquid piston engine is restricted to applications in which the axis of symmetry of the cylinders can be maintained in an essentially vertical position, It’s most likely application is in place of low speed diesels in, for example, stationary engines, power units for locomotives, boats for inland navigation and large sea going vessels.