In a four-stroke cycle, the minimum temperature inside the engine cylinder occurs at the
Beginning of suction stroke
Understanding Minimum Temperature in a Four-Stroke Engine Cycle
The operation of an internal combustion engine, specifically a four-stroke cycle engine, involves significant variations in temperature and pressure within the engine cylinder throughout its cycle. The four strokes are suction (intake), compression, power (expansion), and exhaust.
Let's examine how the temperature changes during each stroke:
- Suction Stroke: The piston moves downwards, and the intake valve opens. A fresh charge of air-fuel mixture (for petrol engines) or just air (for diesel engines) enters the cylinder. This incoming charge is relatively cool compared to the hot exhaust gases from the previous cycle.
- Compression Stroke: Both intake and exhaust valves are closed. The piston moves upwards, compressing the charge. Compression causes a significant increase in both pressure and temperature.
- Power Stroke: After compression, the spark plug ignites the fuel-air mixture (or fuel is injected and ignites due to high temperature in diesel engines). Combustion occurs rapidly, resulting in a very high temperature and pressure. These high-pressure gases push the piston downwards, producing power. This is where the peak temperature occurs.
- Exhaust Stroke: The piston moves upwards, and the exhaust valve opens. The hot exhaust gases are expelled from the cylinder. While gases are leaving, the temperature decreases from its peak, but the cylinder walls and remaining gases are still quite hot.
Temperature Variation During Exhaust and Suction Strokes
Considering the exhaust and suction strokes:
- At the beginning of the exhaust stroke, the temperature is near its maximum, just after combustion.
- As the exhaust stroke progresses, hot gases are pushed out, and the temperature inside the cylinder decreases.
- At the end of the exhaust stroke, most of the hot gases have left, but the cylinder walls are still hot, and some residual hot gas may remain. The temperature is lower than during the power stroke but still elevated.
- At the beginning of the suction stroke, the exhaust valve closes, and the intake valve opens. Fresh, cool air-fuel mixture or air enters the cylinder. This incoming charge is significantly cooler than the cylinder walls and any remaining exhaust gases. As the cool charge enters and mixes, it cools the cylinder contents. This influx of relatively cool charge marks the point where the cylinder temperature is lowest in the entire cycle before the compression begins to raise it again.
- By the end of the suction stroke, the cylinder is filled with the fresh charge. Although heat transfer from the walls might warm it slightly, and slight adiabatic effects might occur, the temperature is still relatively low, but it is beginning to rise slightly in preparation for the compression stroke.
Determining the Minimum Cylinder Temperature Point
Comparing the temperatures at different points:
- Beginning of exhaust stroke: High temperature (near peak).
- End of exhaust stroke: Moderately high temperature (after expulsion of most hot gases).
- Beginning of suction stroke: Lowest temperature (due to influx of cool charge).
- End of suction stroke: Low temperature (cylinder full of fresh charge), but slightly higher than the very beginning of the stroke as heat transfer from walls occurs and compression is imminent.
Therefore, the minimum temperature inside the engine cylinder during a four-stroke cycle typically occurs at the beginning of the suction stroke when the cooler intake charge starts entering the cylinder and mixing with the hot residual gases and cooling the cylinder walls.
Revision Table: Engine Cylinder Temperature
| Stroke Phase | Valve Status (Typical) | Piston Movement | Cylinder Contents | Temperature Trend | Relative Temperature |
|---|---|---|---|---|---|
| Beginning of Suction | Intake Opens, Exhaust Closes | Starts Down | Residual Gas + Incoming Fresh Charge | Decreasing / Lowest Point | Minimum |
| End of Suction | Intake Closes | Ends Down | Fresh Charge | Slightly Increasing (before compression) | Low |
| Beginning of Exhaust | Exhaust Opens | Starts Up | Hot Exhaust Gases | Decreasing from Peak | High (near peak) |
| End of Exhaust | Exhaust Open | Ends Up | Mostly Exhaust Gases (expelled) + Residual | Decreasing | Moderately High |
Additional Information on Four-Stroke Engine Cycle
The four-stroke cycle is the most common type of internal combustion engine cycle used in automobiles. Each cycle requires two complete revolutions of the crankshaft and involves the four distinct piston strokes:
- Intake (Suction): Draws fuel-air mixture or air into the cylinder.
- Compression: Compresses the charge to increase its temperature and pressure.
- Combustion (Power/Expansion): Ignition and burning of the fuel produce high pressure, pushing the piston.
- Exhaust: Expels the burnt gases from the cylinder.
Efficient operation depends on precise timing of valve openings and closings, which is controlled by the engine's camshaft.
Important Questions from IC Engine Cycles
- The indicated thermal efficiency of an internal combustion engine is defined as:
The working cycle of two stroke engine is completed in _______ revolutions of crank shaft.
The ratio between net work for one cycle in a process and displacement volume is -
The fourth stage of the compression ignition process is:
The air-fuel ratios used in a compression ignition engine lie in the range of:
