What is the difference between ping and knock

Has the customer used a different fuel? Fuel octane rating has a lot to do with its ability to resist early or preignition. Check for TSB's in case of any inherent issues with the make and model you are diagnosing. Study Tips Feedback. Engine Detonation and Preignition Detonation is a problem caused by colliding flame fronts in an internal combustion engine. Engine Preignition There is a difference between detonation and preignition.

Another could be a cylinder head that has been, resurfaced to restore flatness. These changes will increase engine power but; also the risk of engine spark knock on regular 87 octane fuel. Engines that are supercharged or turbocharged, are also at a much higher risk. This is because, the forced air induction system increases compression. High back pressure is a common exhaust system problem.

This may happen because, of a clogged exhaust pipe; catalytic converter or muffler. Consequently, a clogged converter, is the most common cause of exhaust back pressure buildup.

It will restrict engine airflow, causing the engine to run hotter and lose power. Consequently, leading to pinging or knocking. Most of the time, a catalytic converter will clog because of, old age overheating or fuel contamination. Regular grade gasoline is supposed to have, an octane rating of 87 but, is not always the case.

The fix for this is to try a tank of mid-range or premium gasoline. Premium costs more but, may be required to reduce the knocking. Or, if you always buy gas at the same gas station, try a different gas station. This produces multiple flame fronts, within the combustion chamber, instead of a single flame front.

When these multiple flames collide; they do so with explosive force that produces, a sudden rise in cylinder pressure. This article defines two types of engine failures, detonation and pre-ignition, that are as insidious in nature to users as they are hard to recognize and detect. This discussion is intended only as a primer about these combustion processes since whole books have been devoted to the subject. First, let us review normal combustion. It is the burning of a fuel and air mixture charge in the combustion chamber.

It should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber in a three dimensional fashion.

Similar to a pebble in a glass smooth pond with the ripples spreading out, the flame front should progress in an orderly fashion. The burn moves all the way across the chamber and , quenches cools against the walls and the piston crown. The burn should be complete with no remaining fuel-air mixture. There is another factor that engineers look for to quantify combustion. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing 20 degrees BTDC, for example the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center.

LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. Confusion and a lot of questions exist as to detonation and pre-ignition.

Detonation is one phenomenon that is abnormal combustion. Pre-ignition is another phenomenon that is abnormal combustion. The two, as we will talk about, are somewhat related but are two distinctly different phenomenon and can induce distinctly different failure modes. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts.

The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug. Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon. Unburned end gas, under increasing pressure and heat from the normal progressive burning process and hot combustion chamber metals spontaneously combusts, ignited solely by the intense heat and pressure.

The remaining fuel in the end gas simply lacks sufficient octane rating to withstand this combination of heat and pressure. Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred.

That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes.

This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to hertz and they will pick up that spark knock. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.

One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. It can run that way for thousands and thousands of miles.

Detonation is not necessarily destructive. An engine that is making 0. Overheating scuffed piston skirts due to excess heat input or high coolant temperatures. The high impact nature of the spike can cause fractures; it can break the spark plug electrodes, the porcelain around the plug, cause a clean fracture of the ring land and can actually cause fracture of valves-intake or exhaust. The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures.

If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.

Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted.

In that case the end gas was heated to detonation by the residual heat in the valve. In a four valve engine with a pent roof chamber with a spark plug in the center, the chamber is fairly uniform in distance around the spark plug. But one may still may see detonation by the exhaust valves because that area is usually the hottest part of the chamber.

Where the end gas is going to be hottest is where the damage, if any, will occur. Because this pressure spike is very severe and of very short duration, it can actually shock the boundary layer of gas that surrounds the piston.

Combustion temperatures exceed degrees. If you subjected an aluminum piston to that temperature, it would just melt. This thin layer isolates the flame and causes it to be quenched as the flame approaches this relatively cold material. That combination of actions normally protects the piston and chamber from absorbing that much heat. However, under extreme conditions the shock wave from the detonation spike can cause that boundary layer to breakdown which then lets a lot of heat transfer into those surfaces.

Engines that are detonating will tend to overheat, because the boundary layer of gas gets interrupted against the cylinder head and heat gets transferred from the combustion chamber into the cylinder head and into the coolant.

So it starts to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat. If you look at the bottom side of a piston you see the piston pin boss. It expands directly into the cylinder wall. However, the skirt of a piston is relatively flexible. If it gets hot, it can deflect. So if the piston soaks up a lot of heat, because of detonation for instance, the piston expands and drives the piston structure into the cylinder wall causing it to scuff in four places directly across each boss.

Many times detonation damage is just limited to this. Some engines, such as liquid cooled 2-stroke engines found in snowmobiles, watercraft and motorcycles, have a very common detonation failure mode. What typically happens is that when detonation occurs the piston expands excessively, scurfs in the bore along those four spots and wipes material into the ring grooves.

Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits. Detonation caused the piston to scuff and this snowballed into loss of compression and hot gas escaping by the rings that caused the melting. Once again, detonation is a source of confusion and it is very difficult, sometimes, to pin down what happened, but in terms of damage caused by detonation, this is another typical sign.

A scuffed piston may be an indicator of a much more serious problem which may manifest itself the next time with more serious results.

Throttling back to part throttle the mixture may be leaner and detonation may now occur. I want to reinforce the fact that the detonation pressure spike is very brief and that it occurs after the spark plug normally fires.

In most cases that will be well after ATDC, when the piston is moving down. You have high pressure in the chamber anyway with the burn. Detonation is influenced by chamber design shape, size, geometry, plug location , compression ratio, engine timing, mixture temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the end gas spontaneously combusts.

Backing off the spark timing will stop the detonation. The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. A fuel can have a variety of additives or have higher octane quality.

For instance, alcohol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used. Production engines are optimized for the type or grade of fuel that the marketplace desires or offers.

At that operating point with the engine on the dynamometer and using non-knocking fuel, we adjust the spark advance. There is going to be a point where the power is the greatest.

Now our engine was initially designed for premium fuel and was calibrated for 20 degrees of spark advance. Suppose we put regular fuel in the engine and it spark knocks at 20 degrees? We back off the timing down to 10 degrees to get the detonation to stop.

The engine now suffers about a percent loss in torque output.