The Basic Difference
The main difference between 2 & 4 stroke engines is
the way they use oil. As any 2 metal surfaces rub together
(such as the piston & piston rings rubbing up &
down against the cylinder barrel), friction is caused. This
friction causes the metal parts to wear as they rub against
each other. In order to minimise friction & wear on
engine components, engines use oil as lubrication which
provides a thin layer of protection between the metal surfaces.
2 Stroke Engines: In a 2 stroke engine,
the fuel & air is mixed with 2 stroke oil, either pre
mixed in the petrol tank or added automatically from a separate
oil reservoir, the oil in the fuel & air mixture lubricates
the cylinder, piston & piston rings, crankshaft &
components. The 2 stroke oil is burnt with the fuel &
air in the cylinder which produces a bluish tinge to the
exhaust gases as well as that unique 2 stroke smell.
4 Stroke Engines: 4 stroke engines use
an oil sump to lubricate the internal moving components
of a motorcycle engine, which means there isn't any need
to mix oil with the fuel & air mixture. This generally
produces better fuel economy & cleaner exhaust emissions.
4 stroke engines transport the oil around the engine from
the oil sump (located at the bottom of a 4 stroke engine,
the sump is a reservoir for the engine oil) to the moving
engine parts using a series of passages in the engine casings
& grooves, the oil is usually driven round the engine
by an oil pump. 4 stroke engines use a 'recirculating oil
system', which means the oil is driven, from the sump, by
the oil pump, through an oil filter, around the engine &
back to the sump to be used again.
