An internal-combustion engine that has one power stroke per revolution.
Animation of a two-stroke glow plug engine.
The sequence of events is:
- Fuel air explosion
- Piston is driven down - power stroke
- As the piston moves towards the bottom of itís stroke, the exhaust port is uncovered
- Exhaust gases are driven out
- When the piston has bottomed out, the intake port is uncovered
- New fuel and air enters and is ready for compression and combustion.
- As the fuel mixture is being compressed a vacuum is created in the crankcase.
- The vacuum opens a reed valve and sucks air/fuel/oil from the carburetor into the crankcase.
- Simplified construction
- Great power to weight ratio - fire once every revolution for a significant power boost
- The work required to overcome the friction of the exhaust and suction strokes is saved.
- A firing stroke every revolution means a more uniform turning moment is obtained on the crankshaft and therefore, a lighter flywheel is required.
- The engines do not last as long due to poor lubrication.
- Part of the piston stroke is lost with the provision of the ports thus the effective compression is less in case of two-stroke engines.
- Have to mix lubrication oil with the fuel
- Increases the emissions
- Increases running costs
- The efficiency of a two-stroke engine depends to a great degree on the effectiveness of the scavenging process. Bad scavenging gives a low mean indicated pressure.
The two-stroke Compression Ignition engine does not suffer from the disadvantages of the spark ignition engine which are fuel loss and idling difficulty and hence CI engine is more suitable for two-stroke operation.
The two-stroke spark ignition engine runs irregularly at idle and may even stop at low speeds when mean effect pressure is reduced to about 2bar.
The inlet ports are placed on both sides of the exhaust ports so that the incoming air enters in two streams uniting on the cylinder wall opposite the exhaust ports, flows upwards, turns under the cylinder head, then flows downwards the other side to the exhaust ports. Such a system of air deflection reduces the possibilities of short-circuiting to minimum.
The two-stroke SI engine suffers from two big disadvantages-fuel loss and idling difficulty.
"Theory and Design of Automotive Engines", B Dinesh Prabhu, Assistant Professor, P E S College of Engineering, Mandya, KARNATAKA