The Basic Difference b/w 2 Stroke Engines & 4 Stroke Engines

2 & 4 Stroke Engine Principles

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.

Please click on either the 2 or 4 stroke links below to proceed
The links below are to pages that explain the basic operating principles of 2 & 4 stroke motorcycle engines

2 Stroke Engines

4 Stroke Engines

Pros & Cons of 2 & 4 Stroke Engines

2 Stroke Engines

Pros	Cons
Simple to produce	High noise levels
Easy & cheap to manufacture	High Pollution levels (caused by burning of oil with the fuel)
Relatively high power output for CC	Low fuel economy
Ideal for smaller motorcycles & scooters up to 125cc due to low production cost & good power output	Narrow power band
	Due to the way oil is burnt in the cylinder, re-bores are required every so often

4 Stroke Engines

Pros	Cons
More even power spread across the rev range	More complicated to produce than a 2 stroke engine
Good fuel economy	High manufacturing costs due to more complex design
Lower levels of exhaust emissions
More efficient engine system

Please note: As a general rule,due to lower manufacturing costs & simplicity of design, 2 stroke engines have been commonly used in motorcycles & scooters up to 125cc, motorbikes & scooters over 125cc have tended to be mainly 4 stroke. But, as technology has advanced, most of the problems associated with either 2 or 4 stroke engines have been or are being engineered out.So, although smaller engines still tend to be 2 stroke & larger motorcycle engines tend to be 4 stroke, engines have remained this way due to manufacturing costs rather then because of limits in design or engineering technology.