What Is Diesel Knocking in an Engine?

Fortunately, we’re experts in engine troubleshooting at Foxwood Diesel and are here to offer guidance on this topic. In this blog post, we'll explore what a diesel knock is, the reasons behind this phenomenon, and its implications.

Diesel knocking in an engine is a sound caused by vehicle cylinders that fail to burn fuel evenly. In an ideal world, the fuel-air mixture in each cylinder would be balanced, ensuring that pockets of air and fuel burn in small amounts. However, when the fuel-air mixture ignites too early or unevenly, it triggers uncontrolled combustion, creating a knocking sound. If you don’t address diesel knocking, you may face piston damage.

Common symptoms of an engine knock in diesel engines include:

You can listen out for a few different common diesel knock sounds. Identifying these sounds and understanding their impact on your engine can help you address the problem more efficiently.

Ticking diesel knock sounds are usually high-pitched and consistent in tempo. Drivers may notice them while driving without hitting the accelerator.

A rattling diesel knock sound is fast and shaky, which may get louder with acceleration. This sound is generally one of the easiest ones to detect.

This loud diesel knock may be caused by a blend of the two noises mentioned above. As they are usually lower in pitch, drivers can more easily notice these knocks while the vehicle is stationary or accelerating. However, the tempo can differ depending on the vehicle's speed.

Learning about diesel engine knocking involves understanding the diesel combustion process. Unlike gasoline engines, diesel engines don't have spark plugs to ignite the air-fuel mixture. Instead, they depend on the heat generated by the high compression of the air within the cylinder to ignite the fuel.

1. Combustion Chamber Pressure:

Diesel engines operate under high compression ratios, which can lead to increased pressure inside the combustion chamber. When the air-fuel mixture ignites, it doesn't always burn evenly or smoothly. Localised pockets of high pressure and temperature can sometimes develop, causing rapid, uncontrolled ignition.

2. Detonation:

This rapid and uncontrolled ignition is known as detonation and is one of the primary causes of diesel engine knocking. Detonation occurs when multiple flame fronts collide within the combustion chamber, creating shock waves and vibrations. These shock waves manifest as the characteristic knocking sound.

3. Fuel Quality:

The quality of diesel fuel can also contribute to diesel knocking in an engine. Poor-quality or contaminated diesel fuel may ignite unevenly, increasing the chances of detonation and knocking. Using high-quality, clean diesel fuel is essential for preventing this issue.

4. Injection Timing:

The timing of the fuel injection plays a significant role in diesel engine knocking. If the injection timing is off, it can lead to premature ignition, causing knocking. Properly timed injections are essential for a smooth and efficient combustion process.

5. Excessive Carbon Deposits:

Carbon deposits can build up within the combustion chamber, leading to hot spots that increase the likelihood of detonation and knocking. Regular engine maintenance and cleaning can help prevent this issue.

6. Overloading and Excessive Load:

Subjecting a diesel engine to heavy loads and overloading can cause it to work harder, generating more heat and pressure. This, in turn, increases the chances of knocking.

7. Engine Overheating:

Diesel engines that run at excessively high temperatures can experience more pronounced knocking due to the heightened pressure and heat within the combustion chamber.

Diesel engine knocking isn't just an auditory nuisance; it can have several consequences. These include:

Diesel engine knocking results in decreased engine efficiency and power performance as the combustion process becomes less controlled and less energy gets extracted from the fuel.

Next, diesel knocking can cause increased fuel consumption because the engine has to compensate for the inefficient combustion process, therefore burning more fuel to maintain its performance.

If not addressed, diesel knocks can trigger serious engine damage, such as damage to cylinder heads and pistons, and even engine failure. This could lead drivers to have to fork out for costly repairs.

Knocking in diesel engines may also increase harmful pollutant emissions, including nitrogen oxides, therefore contributing to air pollution.

After identifying what is causing the diesel knocking in your engine, you can try your best when it comes to fixing them – although it’s always best to try to prevent these issues from surfacing in the first place! Here are some things you can do to fix issues and prevent them from happening:

Use the right fuel

One of the most common causes of pre-ignition is incorrect fuel type usage. Flush your fuel system and replace it with a fuel type compatible with your engine—one with a suitable cetane rating. This may help resolve any rattling sounds and solve your ignition issue!

Get an oil change

Low oil levels often cause ticking sounds. Getting an oil change may solve or prevent this issue.

Have an injection timing inspection

Our team can carry out an injection timing inspection and make any modifications to the fuel-air ratio. Moreover, a fuel additive may be used to lube up poorly timed fuel injectors. If a loud knocking sound continues after lubrication, you may want to replace the injectors.

Clean your engine

One of the best practices for keeping your diesel engine in good condition is to clean it frequently. This will ensure carbon deposits are removed from the different components. This can be done with a fuel injector cleaner, or you can leave it to us!

Have regular fuel system inspections

Ensure you conduct frequent fuel system inspections and get the different parts, like injectors, pumps, and filters, services as required. Swapping out faulty components or cleaning dirty ones is critical for delivering and injecting the fuel into the engine correctly.

Replace your engine

Ensure you’re aware of the common diesel engine problems and solutions before taking action. However, if all else fails, you can replace your faulty engine with a reconditioned diesel engine or new diesel engine parts. Take a look at our repairs and replacements to see what best suits your vehicle.

While diesel engine knocking is a common phenomenon, it should not be ignored. Understanding its causes and implications is vital for maintaining a well-functioning diesel engine.

Regular maintenance, using high-quality fuel, and monitoring engine parameters like injection timing can help minimise diesel engine knocking and ensure your engine operates smoothly and efficiently. To benefit from more engine insights, please take a look at our blog.

If you are seeking regular maintenance or have noticed diesel knocking in your engine and think you would benefit from our assistance, please get in touch; our team are happy to assist with any questions you may have!