Two-Stroke Engines
Summary:
The two-stroke internal combustion engine operates through a simple cycle comprising a compression and power stroke, utilizing ports instead of valves. Two-stroke engines are lighter, noisier, and more cost-effective than four-stroke engines, producing half the power and generating higher torque. They require more lubricating oil and have lower thermal efficiency. Depending on the scavenging process, two-stroke engines are classified as crankcase scavenged or separately scavenged. Scavenging, trapping, and charging efficiency are crucial factors affecting engine performance. By optimizing these parameters and considering factors like bore diameter, stroke length, compression ratio, engine speed, and air density, one can determine the scavenging ratio, scavenging efficiency, and trapping efficiency of a two-stroke compression ignition engine.
Excerpt:
Two-Stroke Engines
INTERNAL COMBUSTION ENGINE
VI. TWO-STROKE ENGINE
A. WHAT WE KNOW ABOUT THE TWO-STROKE CYCLE SO FAR:
The two-stroke cycle only comprises two processes:
1. Compression Stroke – where two-stroke engines use ports instead of valves. The gas enters through the inlet ports while the piston moves towards the TDC that compresses the mixture (air for CI engine, air-fuel for SI engine). The mixture will ignite Depending on the ignition type (either a spark plug for SI or cylinder temperature for CI).
2. Power Stroke – the ignited gas pushes the piston back to BDC, producing work. The exhaust port expels the flue gas while the inlet port sucks in another batch of the mixture. These events happen simultaneously.
| Two-Stroke | Four-Stroke | |
|---|---|---|
| Power Cycle | One revolution | Two revolutions |
| Noise | Lighter and noisy | Heavier and less noisy |
| Cost | Cheaper, easier | Expensive, harder |
| Power | Develops full | Develops half the power |
| Torque | Generates high | Generates less |
| Specific | Develops high | Develops less |
| Efficiency | More smoke, less | Less smoke, more |
| Lubrication | Requires more | Requires less |
| Wear & Tear | Poor lubrication | Good lubrication |
| Valves | Uses ports | Uses valves |
| Thermal | Lesser | Higher |
| Efficiency |
In this engine, the pumping function is not done in the working cylinders, but due to a mechanism called the scavenging pump, the event where the fresh batch of the mixture fills in the space left by the exhaust gases from the cylinder is called scavenging.
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