Excessive Knock

Question 1

What is excessive diesel knock?

Answer 1

A faint audible knocking sound produced when diesel fuel ignites spontaneously and burns rapidly due to high cylinder gas pressures.

Question 2

What causes diesel knock?

Answer 2

Diesel knock occurs when high cylinder pressures lead to auto-ignition (detonation) of diesel oil, causing a rumbling or clattering sound.

Question 3

What is ignition lag?

Answer 3

The time span between the injection of diesel fuel and its ignition. It is a crucial factor that influences the occurrence of diesel knock.

Question 4

How does long ignition lag affect diesel knock?

Answer 4

If ignition lag is too long, it can result in too much fuel igniting simultaneously, leading to a mild explosion and a loud knock.

Question 5

What factors can contribute to abnormally loud diesel knock?

Answer 5

Factors include:

• Low operating temperatures (thermostat stuck open)
• Early injection timing
• Low compression
• Fuel contamination
• Oil consumption

Question 6

What is the relationship between ignition lag and fuel ignition?

Answer 6

Ignition lag is critical; a too-long lag results in a delayed ignition that can cause multiple fuel injections to ignite at the same moment, resulting in excessive knocking.

Question 7

What mechanical condition can lead to increased diesel knock?

Answer 7

Conditions such as low compression can cause increased diesel knock due to reduced cylinder pressure and efficiency.