Stepped piston two stroke engines

A two-stroke engine comprising a set of two cylinders each cylinder having a working part of smaller diameter and a pumping part of larger diameter;

• a piston slidably reciprocable in each cylinder and having a working part which slides in the working part of the cylinder and a pumping part which slides in the pumping part of the cylinder;
• and transfer passage means between the pumping part of each cylinder of the set and the working part of the other cylinder of the set so that at least the air for each charge required by the working part of each cylinder of the set is pumped, during operation of the engine, to such working part along transfer passage means from the pumping part of the other cylinder of the set, the piston motion within the cylinders of the set being such that the pistons are 180.degree. out of phase, the improvement wherein:
• I. the working part of each cylinder is provided with exhaust port means, main inlet ports and auxiliary inlet port means,
• Ii. the main inlet ports are arranged symmetrically about, and spaced from, a plane which contains the longitudinal axis of the cylinder and which passes through the center of the exhaust port means,
• Iii. the auxiliary inlet port means comprises at least one port in the cylinder wall opposite to the exhaust port means,
• Iv. the transfer passage means comprises main transfer passage means connected to the main inlet port means and auxiliary transfer passage means connected to the auxiliary inlet port means, and
• V. the exhaust port means of both cylinders of the set lies to one side of an axial plane containing the axes of the two cylinders of the set and the auxiliary inlet port means of both cylinders of the set and auxiliary transfer passage means of the set lies wholly on the other side of said axial plane;
• and
• Vi. means for supplying fuel to air which, when the engine is in use, passes through the auxiliary transfer passage means and the auxiliary inlet port means;
• Vii. means for sensing variations in the mass flow of air into the pumping parts of the cylinders of the set;
• and
• Viii. means for metering the fuel supply in dependence on variations in said mass flow.