What Is Diesel Fuel Knock?

Introduction. The characteristic “knocking” noise associated with diesel engines is caused by high pressure-rise rates associated with fuel auto-ignition during the premixed combustion stage. Diesel knock [1] and combustion roughness [2] are common terms for this noise.

Does diesel knock cause damage?

Bad noises can signal a problem with your diesel engine in a variety of ways:

  • There is a rattling noise. When you speed your vehicle, you may hear this type of noise. It’s created by an air-fuel mixture in the cylinder that ignites prematurely due to engine compression. This is known as pre-ignition, and it can harm the engine’s pistons, valves, and connecting rods.
  • The sound of a diesel engine ticking. Typically, reciprocating components such as valves, pistons, rods, and pushrods are to blame. Low oil level, ill-adjusted valves, rod knock, or a noisy lifter are all indicators of the sound.
  • The sound of a diesel engine banging. The injectors are the source of the knocking sounds. Normally, the lubricant in the purge dampens the noise, but if the noise persists after fifteen minutes of operation, you should inspect the engine more closely.
  • Noise from the meshing of the timing chain. Because the timing chain connects the crankshaft and camshaft, it must be kept in good working order at all times. When the engine is cold, the noise created by a defective timing chain is rattling, and when the engine is fully warmed up, the noise is toned down. These diesel engine issues are fairly prevalent, and it’s critical to address them as soon as possible.

Can a diesel knock Be Fixed?

What is the best way to tell if the engine noises you’re hearing are good or bad? After years of troubleshooting diesels, I’ve discovered that performing a Diesel Purge is the best way to evaluate whether the internal noises you’re hearing are normal or not. Let me explain; with a diesel engine, the majority of the banging and pinging is caused by injector “nailing” and ignition knock. Most of these noises will go away in ten to fifteen minutes if you run diesel purge through your engine. The purge lubricant will lessen “nailing” or hammering in the injectors, while the clean fuel will reduce combustion banging. I often fantasize of being able to run my engine on diesel purging all of the time. The diesel purge is working its way through the pump and injectors, “softening out” all those harsh sounds, and the engine produces such a beautiful sound. (If you put high-quality waste vegetable oil in a diesel engine, the same thing can happen.) If the noise(s) you’ve been worried about disappear during a purge, you can relax. The source of the noises is almost certainly fixable.

In earlier Mercedes diesel engines, the fuel injectors are the source of the most noise. They make ticking, pinging, rattling, and even snapping sounds. This type of injector noise will not do any serious damage to your engine. In most circumstances, diesel purge will silence all injector noises while also softening the knocking noise. If the nailing or banging sounds from your diesel injectors returns after a purge, I propose rebuilding your fuel injectors with the Monark nozzles offered on our website. We provide everything you need, including tools and instructions, to rebuild and pressure balance diesel fuel injectors in your garage.

If, on the other hand, the noise does not go away while the purge is being run through your engine, you should be concerned. You’ll have to look for the source of the noise elsewhere (s). If the deep knock continues, it could be dangerous, and the vehicle should not be driven until the source is identified. See my whole guidebook for additional information on diesel engine noise diagnostics.

How do I stop my diesel from knocking?

Detonation can occur when an unburned fuel/air mixture beyond the flame front is exposed to a combination of heat and pressure for an extended length of time (beyond the fuel’s delay period). Detonation is defined as the explosive ignition of at least one pocket of fuel/air mixture outside of the flame front in an almost immediate manner. Around each pocket, a local shockwave is formed, and the cylinder pressure rises abruptly – possibly above its design limits – causing damage. (Detonation is more efficient than deflagration, but it is normally avoided because it damages engine components.)

Engine parts can be damaged or destroyed if detonation is permitted to continue under extreme conditions or over a long period of time. The most common negative impacts include particle wear produced by moderate banging, which can spread through the engine’s oil system and inflict wear on other components before being captured by the oil filter. Similar to the damage caused by hydraulic cavitation, such wear appears as erosion, abrasion, or a “sandblasted” appearance. Severe knocking can result in catastrophic failure, with physical holes melted and pushed through the piston or cylinder head (i.e., combustion chamber rupture), depressurizing the damaged cylinder and introducing massive metal fragments, fuel, and combustion products into the oil system. Shock waves are known to quickly break hypereutectic pistons.

  • the use of a high-octane fuel, which raises the fuel’s combustion temperature and minimizes its inclination to detonate
  • Increasing the air–fuel ratio changes the chemical reactions during combustion, lowers the combustion temperature, and raises the detonation margin.
  • lowering the manifold pressure by lowering the throttle or boosting the boost pressure

Because pressure and temperature are inextricably linked, knock can be reduced by lowering peak combustion chamber temperatures through compression ratio reduction, exhaust gas recirculation, proper ignition timing schedule calibration, and careful design of the engine’s combustion chambers and cooling system, as well as lowering the initial air intake temperature.

When certain fuels are utilized, the inclusion of specific elements like as lead and thallium will greatly reduce detonation. Tetraethyl lead (TEL), a soluble organolead chemical added to gasoline, was widely used until it was phased out due to harmful pollution concerns. With various hydrocarbon fuels, lead dust applied to the intake charge will also lessen knock. Manganese compounds are also used in gasoline to decrease knock.

What happens when diesel knock occurs?

Sharp sounds produced by early burning of a portion of the compressed air-fuel mixture in the cylinder of an internal-combustion engine. The charge burns in a fully operating engine, with the flame front moving smoothly across the combustion chamber from the moment of ignition.

Why do diesel engines knock when cold?

Many diesel engines, however, create an unpleasantly harsh banging sound during cold starts and warm-up at low outside temperatures. A significantly higher cylinder pressure gradient results in a louder and more unpleasant combustion noise under cold running circumstances.

What causes excessive diesel knock?

When the factors governing combustion result in excessively long ignition delay periods, this situation frequently arises. As a result, severe diesel knock is frequently a symptom of underlying issues such as tainted or substandard fuel, injection system issues, or inadequate alternate fuel substitution rates.

What causes fuel knock?

When fuel burns unevenly in your engine’s cylinders, knocking happens. Fuel will burn in discrete, regulated pockets rather than all at once when cylinders contain the proper balance of air and fuel. (Think of sparklers rather than fireworks.) After each pocket has burned, it produces a little shock, which ignites the next pocket and repeats the cycle. Engine knocking occurs when fuel burns unevenly and shocks fire at inopportune times. What’s the end result? An obnoxious noise and the risk of damage to the cylinder walls and pistons of your engine.

What causes engine knock in a diesel engine?

The operation of a diesel engine differs from that of a gasoline engine. Fuel is combined with air and compressed in a gasoline engine before an electric spark ignites the mixture. Only the air is compressed in a diesel engine. The gasoline is then injected into a compressed air-filled cylinder, where the heat from the compressed air ignites the fuel without the use of an electronic ignition system.