Combustion during the expansion stroke in an internal combustion engine (IC engine) can occur due to a weak mixture without ignition advance. This phenomenon is known as knock or detonation.

1. Combustion in an IC engine:
In an IC engine, combustion occurs in a series of four strokes: intake, compression, power, and exhaust. During the power stroke, the combustion of the air-fuel mixture generates high pressure and temperature, converting chemical energy into mechanical work.

2. Knock or detonation:
Knock or detonation refers to the uncontrolled combustion of the fuel-air mixture, which leads to a sudden increase in pressure and temperature. This can cause severe damage to the engine components, reduce efficiency, and result in a loss of power.

3. Weak mixture:
A weak mixture refers to a fuel-air mixture that has a lower ratio of fuel to air. In such a mixture, there is a limited amount of fuel available for combustion. This can lead to incomplete combustion, reduced power output, and increased emissions.

4. Ignition advance:
Ignition advance refers to the timing of the spark plug firing in relation to the position of the piston. Advancing the ignition timing means firing the spark plug earlier, allowing more time for complete combustion before the expansion stroke.

5. Weak mixture without ignition advance:
When a weak mixture is present in the combustion chamber without ignition advance, the combustion process may not be able to complete during the power stroke. This incomplete combustion can result in unburned fuel entering the expansion stroke.

6. Combustion during the expansion stroke:
If the unburned fuel-air mixture ignites during the expansion stroke, it causes an additional pressure rise in the cylinder. This can lead to knock or detonation, which is undesirable and can cause engine damage.

7. Reason for combustion during the expansion stroke:
The reason for combustion during the expansion stroke in this scenario is a weak mixture without ignition advance. The limited amount of fuel and the lack of proper timing for ignition result in incomplete combustion during the power stroke. The unburned fuel-air mixture then ignites during the expansion stroke, leading to knock or detonation.

In conclusion, combustion during the expansion stroke in an IC engine can occur when a weak mixture without ignition advance is present. This phenomenon, known as knock or detonation, can cause engine damage and reduce efficiency. Therefore, it is important to ensure proper fuel-air mixture and ignition timing to avoid this issue.