Superchargers - Two-stroke Engines

Two-stroke Engines

A two-stroke engine does not have an induction stroke where low pressure can draw in air. In addition a supply of air at higher than ambient pressure is needed to blow out the burnt gases from the previous combustion cycle.

In small trunk engines this is commonly achieved by using the crankcase as a supercharger. As the piston descends during the power stroke the underside of the pistons compresses the air in the crankcase. As it nears the bottom of its stroke a valve or port will open and allow the compressed air charge to escape into the cylinder.

In larger engines other forms of supercharging are needed. These engines are likely to be using crossheads and so have limited under-piston volume. They are also likely to have a crankcase shared by several cylinders. In these cases other means of supercharging are necessary and most, if not all, of the methods listed above have been employed.

Some engines, such as large marine diesels, will use a combination of superchargers. These will use turbocharging, for its efficiency gains, at medium and high speeds. For starting and running at low speeds, when the turbocharger may be unable to supply adequate air, an electrically driven blower will be used. On these engines mechanically driven superchargers are unlikely to be employed due to fuel efficiency being a major design criterion of this engine type.