The Free Piston Engine

Engines that run on petroleum fuel are used in vehicles and stationary plants everywhere. This is the two-stroke diesel engine. Blue air is compressed on the right outside the cylinder, fuel injection, self-ignition in the cylinder, red exhaust gases driven out on the left. Here, the reciprocating action of a piston turned into rotary movement by a crankshaft. A rather cumbersome process preventing really smooth running, but a very high compression ratio achieving a good thermal efficiency. And here in the gas turbine, the compressor absorbs much of the power developed. The fuel consumption is high, the thermal efficiency low. But the engine runs smoothly because of the simple single rotary moving part. An engine can be evolved that has a high compression ratio, simple moving parts, and is free from vibration. Using two similar two-stroke engines make an opposed-piston engine. Better balance than before, but still a complexity of crankshafts and connecting rods. Try removing them. Springs might do for returning the pistons to the centre, but air cushions work better. Having got rid of the crankshafts, we must take the power from our engine in the form of hot gas, which we can use to turn the blades of a turbine. Our engine is thus a machine producing hot, compressed gas. A gasifier, in fact. And so we can dispense with the turbine's compressor and combustion chambers, and lead the gas onto the blades of the turbine. Let us try and do without the auxiliaries. The air needed to scavenge the cylinder came from an outside blower. Put back the pistons, but reshape them. Surround the cylinder with a jacket, lose the blower, lengthen the injector, and put back the air cushion. Now let's insert simple automatic valves which open and close with differences of pressure, here and here. Air from outside can be drawn in through the suction valves by the reshaped compressor pistons. The suction valves close, the air is compressed, forced through the delivery valves and stored under pressure in the jacket surrounding the cylinder. When the pistons are moved outwards again, the suction valves open. If holes are put round the cylinder where the inlet from the blower used to be, the store of air will expand into the cylinder at the right moment to scavenge it. This time, when the pistons return, some of the air is trapped in the cylinder and compressed. Fuel is injected, combustion takes place, and a fresh supply of compressed air scavenges the cylinder. The gas leaves the gasifier in a series of pulses. To improve the scavenging of the cylinder and smooth the gas flow to the turbine, we form in the exhaust a large chamber called a gas collector. The cool air mixed with the combustion gases reduces the temperature of the turbine blades. Now we have a perfectly balanced engine with a high thermal efficiency which can run on a wide range of fuels. Now, let us look at the engine itself. This one is on test. Starting is effected by first manoeuvring the pistons to outer dead point by means of a small air-operated piston. A charge of air is released into the two cushion cylinders, bouncing them inwards and starting the cycle. The white circles are the delivery valves. The white squares on the transparent cylinders are the suction valves. This time, the valves change to blue when they open and to white when they close. On the outward journey, the suction valves open. The compressor pistons draw air into the compression chamber. Outer dead point is reached. The suction valves close. The air cushions bounce the pistons inwards. The air is compressed and forced into the jacket surrounding the inner cylinder. Here is the cycle. In this free-piston gasifier, there are six fuel injectors. A light mechanism is needed to synchronise the movement of the pistons. It can be seen more clearly if the transparent cylinders are removed first. This movement drives the fuel injection pump and the cylinder lubricator. An eccentric geared to the central spindle of this synchronising assembly operates the plunger of the pump. The moving parts come to rest at inner dead point. But injection must take place over a period that includes inner dead point, so an accumulator pump is used. Returning to one end of the engine, this is the simple procedure for withdrawing a piston. First, the compressor piston. Next, the diesel piston. Now, the layout of the suction and delivery valves can be seen. These are automatic non-return valves. In this engine, the suction valve is of the reed type and the delivery valve of the plate type. The mechanical lubricator on the control side feeds oil into the compression and diesel cylinders to lubricate the surfaces of the pistons. Thus, the working parts are few and maintenance is relatively simple. Most applications use this 1,000-horsepower unit known as the GS-34 gasifier, now seen in plan. Several gasifiers combined can develop several thousand horsepower. There are three main uses: in locomotives, ships and stationary plant. There is a 2,000-horsepower free-piston locomotive in France. The two gasifiers provide power for two turbines geared to the axles. Cost, weight and fuel consumption are reduced because no electric or hydraulic transmission is needed. In the engine room of a free-piston ship, only the turbine has to be aligned to the propeller shaft. For maintenance or repairs, one gasifier can be stopped with only a small reduction in the ship's speed. The 9,000-tonne oil carrier, the Morar, was the first ocean-going ship designed with a free-piston engine. She has four gasifiers. There are many more applications in stationary plant. The power station at Ajaccio in Corsica has six gasifiers coupled in pairs to three turbines. Above 4,000 horsepower, the engine is lighter and less bulky than the diesel. The gasifiers can be arranged in almost any formation. At Augusta in Sicily, a large power station has a separate free-piston installation that provides internal electric power for the station. This auxiliary plant uses ten gasifiers that supply gas to two turbo-alternators. Absence of vibration means that heavy foundations are not needed. Multiple gasifier sets are being installed in many countries. Fifteen gasifiers are used in a chemical process plant. This set supplies gas through a constant-pressure ring main to six turbines, each driving a compressor. At Singapore, there is a large free-piston power station. Six sets of eight gasifiers provide power to six turbo-alternators. The total output is 36 megawatts. This combination of diesel engine and gas turbine is now firmly established for propulsion and power generation.