Question

Which ONE or MORE of the following can be caused by scavenging in a two-stroke Diesel engine?

• A. Dilution of incoming fuel-air mixture
• B. Loss of incoming air
• C. Pushing out exhaust gases by incoming air
• D. Autoignition

Hint:

Remember scavenging in a Diesel engine moves only air, not a fuel-air mixture, and some of that air always escapes with the exhaust.

Answer 1

The Correct Option is B, C

Scavenging in a two-stroke engine is the process of clearing burnt exhaust gases from the cylinder using a fresh charge of scavenge air before the next power stroke starts. The question asks what effects actually follow from this process.

1. Dilution of incoming fuel-air mixture: wrong. In a Diesel engine the cylinder takes in only air during scavenging, fuel is injected separately much later near the top of the compression stroke, so there is no premixed fuel-air charge to dilute.
2. Loss of incoming air: correct. Some of the fresh scavenge air always escapes straight through the exhaust port before it can be trapped, this short-circuiting is a well known scavenging loss.
3. Pushing out exhaust gases by incoming air: correct. This is the basic working definition of scavenging itself, the incoming air physically displaces and pushes the burnt gases out through the exhaust port.
4. Autoignition: wrong. Autoignition is linked to compression and injection timing of the fuel charge, it is not something scavenging with air causes.

So scavenging causes some loss of fresh air along with the exhaust, and it is the mechanism that pushes exhaust gases out, matching options B and C.