ORIGINAL: iskandar taib
ORIGINAL: TimBle
how does a compression ignition engine ....knock?
I do understand the concept of knock, its just that in the absence of a spark to control the ignition point, in every power cycle there will always be knock.
Are you saying that its excessive?
A compression ignition engine will, um, ignite the charge at some point of the compression stroke. With spark ignition, you control this by setting the point where the spark goes off. With glow engines, you have to do it some other way. It's a combination of compression ratio, fuel (mainly, how much nitro is in it), prop, and glowplug (and probably one or two other things I've forgotten). All other things being equal, a ''hot'' glowplug will set off the charge at lower compression than does a ''cold'' one, effectively ''advancing the spark''. When the charge goes off too early, it's the same as setting the spark timing too early - you get pre-ignition, or ''knock''.
Iskandar
Exactly, in fact glow engines are compression-ignition, but we have flame propagation just like in spark ignition engines, because ignition happens at one point (on the glow plug). So, because there is ONE flame front, you must reassure that in no other place there is ignition (i.e. other flame fronts), otherwise you get "knock".