18 August 1990

Technology: Oscillating motor set to shake up engine makers?

Design for an oscillating motor

REPRESENTATIVES from a major North American manufacturer of aircraft engines had discussions this week with a small company in West London about developing an entirely new type of engine for large aircraft such as jumbo jets.

The so-called quadratic engine is an internal combustion engine with pistons and spark plugs like any car engine. But the pistons are square in cross section and curved like a section of a ring and they oscillate back and forth igniting fuel at the end of every stroke.

The engine’s inventor, Michael Rodrigues of the company Supertronics of Ealing, says the engine could power the large fan at the front of a jet engine which provides most of the engine’s thrust. Such a jet engine, he says, would be one-fifth of the weight and one-fifth of the cost of a comparable turbo jet engine.

The complete 2.1-litre prototype engine – roughly equivalent to a car engine – minus its flywheel, weighs only 22 kilograms. It is also very small – the prototype power unit has a diameter of about 34 centimetres.


Rodrigues says the engine can be used for anything from small cars to airliners, ships and electricity generating stations but engineers are sceptical of some of his claims. The technology is as yet unproven and Rodrigues is about to submit the engine for testing at the Motor Industry Research Association in Nuneaton. He is currently looking for manufacturers to license the technology and has been approached by two major European car makers and an engine producer from the Soviet Union as well as the North Americans.

The quadratic engine has two pistons mounted on a flat circular plate which is pivoted around its centre. The pistons are shaped like slices of a pie with the innermost point of the slices chopped off. They oscillate back and forth against two stationary pistons of the same shape with the whole engine enclosed inside a metal casing. The spaces between the moving and fixed pistons act as the compression chambers and as the moving pistons swing back and forth the chambers expand and contract.

The engine works by a two-stroke process: as one face of each moving piston moves away from the fixed pistons, the mixture of air and fuel is pumped in through an inlet in the fixed piston, thereby pushing the exhaust out through a duct in the moving piston. As the moving piston swings back up towards the fixed piston, it compresses the fuel which is then ignited with a spark plug and forces the piston down again.

At the bottom of the stroke the spent fuel is forced out by fresh fuel/air mixture coming in. The back-and-forth motion of the moving cylinders is converted into circular motion by a connecting arm which turns a crankshaft attached to a flywheel.

Most internal combustion engines use a four-stroke process. Valves open and close to let fuel in and exhaust out, and only every other upward stroke of the piston is used to compress fuel. The intervening strokes push the exhaust out through an open valve. Two-stroke engines were abandoned for most cars because spent fuel is not removed from the cylinder effectively after combustion, which creates noxious exhaust products.

Rodrigues says his engine will burn more cleanly than conventional two strokes, and four strokes as well. First, this is because the fuel/air mixture enters through the top of the cylinder and flushes the exhaust out through the bottom duct. Secondly, he says that there is 75 per cent less friction in the engine because there are only three moving parts: the moving pistons, the connecting arm and the flywheel. It therefore has less lubricating oil which could leak into the combustion chambers.

Michael French, professor of engineering design at the University of Lancaster, says that such orbital engines are not at all new. He thinks the new engine’s greatest problem will be balance. Since the pistons are constantly accelerating and decelerating as they oscillate, they will generate an alternating torque on the engine which will make it very unstable.

The seal around the combustion chamber could be another problem, he says, because of the square shape of the pistons. Other two-stroke engines have also adopted so-called uniflow fuel intake schemes, but they still have problems with noxious emissions, he said.

Pericles Pilidis, an engineering researcher at the Cranfield Institute of Technology, agrees that vibration would probably be the engine’s biggest problem: ‘there would be a very strong hammering effect at the end of each cycle.’ As an engine for large aircraft, the jet turbine would be hard to beat, he said. It has the advantage over piston engines of a continuous flow pro cess where air is sucked in, compressed, burnt and expelled without ever having to stop the flow.