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knocking

Knocking (also knock, detonation, spark knock, pinging or pinking) in spark ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.
Knocking should not be confused with pre-ignition—they are two separate events. However, pre-ignition can be followed by knocking.
The phenomenon of detonation was described in November 1914 in a letter from Lodge Brothers (spark plug manufacturers, and sons of Sir Oliver Lodge) settling a discussion regarding the cause of "Knocking" or "Pinging" in motorcycles. In the letter they stated that an early ignition can give rise to the gas detonating instead of the usual expansion, and the sound that is produced by the detonation is the same as if the metal parts had been tapped with a hammer. It was further investigated and described by Harry Ricardo during experiments carried out between 1916 and 1919 to discover the reason for failures in aircraft engines.

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  1. A

    Worcester Greenstar Ri Boiler Knocking

    Hi there, My boiler (Worcester Greenstar Ri) has started making a soft knocking noise when it turns on, and again a few times when it turns off. I've attached a short video of the sound. It isn't constant, it seems to happen within the first 10 mins of the boiler starting up, then you won't...
  2. S

    Worcester Oil Heatslave 20/25 Thumping / Knocking Noise

    Hi, I wonder if anyone can help. I have a Worcester Oil Heatlslave 20/25 that has started making a fairly loud knocking noise and the the pressure gauge flicks a little when the noise thumps. The hot water still works, I've turned off the heating for the time being. Any ideas on what the...
  3. T

    knocking in bathroom drain

    This morning I heard knocking coming from my bathroom sink drain! The central heat outdoor unit is just outside the wall, and it was running. I went outside, but it seemed fine. When the heat unit went off, the knocking stopped. What the hey? I have a video, but I didn't see a way to add it to...
  4. N

    Groaning sound, Knocking pipes.

    This is regarding the hot water, but I didn't have a clue where else to post it. We live in a 22 year old Barratt house, we have lived here since new. The start of last year, we had to have our hot water cylinder replaced as it started leaking (closed system). Because it was a different make...
  5. S

    Saniflo water goes back in and repumps to loud banging in pipes

    Hello, I have had a Saniflo PRO XR for three years. Still under warranty and I put cleaner/descaler every three months. It is used once or twice a day. The first year there were no problems. Then it suddenly started to go off intermittently during the night. I took out the Elbow where the non...
  6. P

    Knocking from front of house

    Hi, I have heard knocking from the front of the house pipes/radiators? for about a year now. I can hear it when sat in the living room and also the bedroom above it. All night, every two minutes a knock knock knock - three times always on the dot. Especially when its cold or the radiators...
  7. H

    Knocking noise from PVR

    Knocking sound coming from PVR. Have an in-line pressure tank set at 3.5bar, but downstream a 3 bar PVR. The PVR seems to knock at random times for hours and hours, which is driving me crazy. Is the inline pressure tank pressure set to high?
  8. T

    Mains cold water knocking out gravity hot water in bath shower mixer

    Hi all. My customer has a bath shower mixer that gets cold from the mains and hot from a vented cylinder and the cold knocks out the hot. I'm thinking about putting in a thermostatic bath shower mixer but would appreciate any other ideas! Cheers all/
  9. B

    Loud knocking noise from inside wall

    Hello, We’ve had a new boiler, hot water tank, pump, T-valve and thermostats fitted and are using the original radiators without zone valves (small house). The guy that fitted it says he’s too busy to come back and take a look at the problems. If we run just the heating, after it reaches the...
  10. king of pipes

    Corona virus knocking on the door

    The virus has struck down a few member of my wifes family the brother-in-law has it , sister-in - law and her husband so caused quite a bit of disruption their end luckily we haven't had contact with them, the people who have been in contact are now in isolation aswell they are ok fortunately...
  11. K

    Knocking pipes - resolved but mystified

    Hi all , I have had an issue for months - water supply pipes knocking randomly at all hours of day and night - I tried a couple of fixes to no avail - water arrestor , cistern flushing valve. Recently I fully opened the cold mains stopcock and voila - no more knocking. So relieved to get a...
  12. G

    Knocking on pipes when heating starts

    Every time the heating comes on there is a knocking sound from the pipes that lasts 15-20 minutes, then intermittently while the heating is on. Only started over last 4 months. System / house 5 years old. Knocking/banging pipe noise always from same place. Why and what can be done?
  13. C

    Loss of pressure and knocking noise

    Hi all, Came back from a week away and my combi boiler lost pressure completely which happen every now and then. When I open the refill valve, pressure is going back extremely slowly (slower than usual) After a bit it started doing a very hard knocking noise. Turned it off for now but unsure...
  14. C

    Grant Boiler Knocking only a year in...sound files attached

    Can anyone offer any guidance please. System installed a year ago. Oil fired grant vortex pro. Recently started knocking - have read and I see it could be anything from kettling through to pipes. I attach sound files links as someone may recognise the sound: Dropbox - Boiler 2.mp3 Dropbox...
  15. T

    Knocking in pipes - stops when stopcock off

    Hi there - I have an issue with my pipes that I am hoping someone can advise on (even if it's just to tell me to call out a plumber!). I can hear a knocking noise in my pipes, in particular in the area near the boiler and in the bathroom. It persists when the boiler is turned off (even when I...
  16. R

    Knocking sound in pipes for hours after last use

    Hi there, I've recently moved house into an old property (built 1938). Something that keeps me awake at night is an intermittent knocking coming from somewhere in the walls, that I believe must be the plumbing (doesn't sound like the house 'settling'). It's not Really loud (not deafening), but...
  17. D

    Knocking / Banging when hot water switches on

    Hi All I recently fitted a new shower and since then, everytime my hot water kicks in to get up to the set temperature, there is a knocking / banging noise that continues until it reaches the temperature. I have read a few things online about completely draining the tank and then turning the...
  18. M

    Knocking out tap hole on ceramic sink

    Never done one before is it just tap it out?
  19. C

    Just installed a new boiler, there are banging or knocking noise from the radiator.

    Just replaced a new boiler. when the heating is on I can always hear banging or knocking noise from the radiator and sounds like machine gun. never happen with the old boiler. someone said that it's the radiator valve that's causing the problem & need replacing. just incase I'd waste money if...
  20. S

    Knocking in pipes

    I have a top level flat which is experiencing intermittent knocking in the mains pipe. I have been experiencing this issue for a couple of months now and am none the wiser as to the issue. It appears to be quite intermittent in, however there seems to be an increase in the frequency during the...
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