What is the effect of speed of piston on the knocking in 2 stroke and four stroke engine?

Answer 1

First of all there is no difference to knocking or pinking in a 2 or 4 stroke as it only occurs on the compression stroke in either case and in either type of cycle that is essentially the same. Nocking and pinking are essentially the same the terms used are for different types of engine as they are descriptive terms of the noise that occurs in the respective engines i.e. nocking in a diesel (compression ignition engine) and pinking in a petrol (spark ignition engine).
In either type of engine it is necessary to ignite the fuel and create the maximum pressure in the cylinder when the piston is at or past top dead centre so as to develop optimum force down on the piston during the expansion stroke. However if the fuel is ignited and attains it maximum pressure due to expansion of the gasses to early in the cycle i.e. before top dead centre, or the rise to this pressure happens too early the piston will still be trying to move upward. This creates excessinve pressure in the cylinder leading to the excessive nocking sound or pinking sound (note a compression ignition engine has an inherant knocking sound due to the higher compression ratios used anyway). So as you can see it is crucial that the expansion of the gasses attain thier maximum pressure at the correct point.
The relationship between the speed of the engine/piston becomes important when you look at the fuel. A set amount of fuel takes a set time to combust. For example a measure of fuel may take 1 second from initiation of ignition to attaing maximum burn rate and maximum pressure. If this fuel is ignited at 1 second or less from the piston reaching top dead centre all will be well, however if the engine is slowed down and the fuel is ignited one second before the piston reaches top dead centre, the piston will be at a shorter distance from TDC as with a slower rotational speed the piston is moving slower so the fuel must be ignited when the piston is closer to tdc. To counter this problem the point of injection or ignition must be varied in relation to the distance the piston has to travel to top dead centre. At higher speed the fuel must ignite earlier, i.e. at an earlier crank angle or distance of the piston to tdc, to allow sufficient time for the fuel to begin combustion. At lower speeds the timing is retarded i.e. at a later crank angle or with the piston closer to tdc.

So at the end of the day as the rpm of the engine varies the timing or point or ignition needs to be varied in the number of degrees before topdead centre that it is initiated the time periond will be the same but the distance and thus the speed of the piston will vary also. So knocking will only occur if the fuel is ignited or injected at a piont that is too early in cranke angle and period of time for the speed that the engine it doing at that time.

Answer 2

I'm not too clear about what you mean. To begin with 'knocking' relates to an oil engine. It is quite different in principle from a similar-sounding effect in a petrol engine called 'detonation' or 'pinking'. If you happen to be thinking of a petrol engine and if by 'piston speed' you mean engine speed (piston speed varies) then an increase of speed would tend to reduce liability to pinking.

Answer 3

Hi. Knocking will hammer the heck out of the piston. I think the speed stays the same (the crankshaft doesn't slow) as it would in a normal running engine. Fastest at mid-stroke, stopped at TDC and BDC.