Knock? Detonation?
Thread Starter
Trial User
Joined: Jan 2002
Posts: 1,932
Likes: 0
From: Richmond, VA
Knock? Detonation?
This is going to sound like a VERY n00b-ish question, but what are these two terms? Knock (ie: "My car needs a new knock sensor") and Detonation ("[this] plus [that] could lead to detonation") Thanks. *puts on his flame suit, just in case*
Thread Starter
Trial User
Joined: Jan 2002
Posts: 1,932
Likes: 0
From: Richmond, VA
Re: Knock? Detonation? (sslude)
idont know about the knock sensor question but detonation is when you car burns the fuel on the upstroke so while the piston is going up, the fuel ignites, causing detonation.
Joined: Oct 2002
Posts: 2,052
Likes: 0
Re: Knock? Detonation? (AccordKid85)
In laymans terms you want the air/fuel to burn evenly starting at the spark plug outwards. In real life this doesn't happen, but you get a end gases igniting due to heat and/or pressure. A knock sensor can detect this and retard the timing, inject more fuel, etc. Here is the non laymens terms:
To obtain the maximum energy from the gasoline, the compressed fuel-air
mixture inside the combustion chamber needs to burn evenly, propagating out
from the spark plug until all the fuel is consumed. This would deliver an
optimum power stroke. In real life, a series of pre-flame reactions will
occur in the unburnt "end gases" in the combustion chamber before the flame
front arrives. If these reactions form molecules or species that can
autoignite before the flame front arrives, knock will occur [21,22].
Simply put, the octane rating of the fuel reflects the ability of the
unburnt end gases to resist spontaneous autoignition under the engine test
conditions used. If autoignition occurs, it results in an extremely rapid
pressure rise, as both the desired spark-initiated flame front, and the
undesired autoignited end gas flames are expanding. The combined pressure
peak arrives slightly ahead of the normal operating pressure peak, leading
to a loss of power and eventual overheating. The end gas pressure waves are
superimposed on the main pressure wave, leading to a sawtooth pattern of
pressure oscillations that create the "knocking" sound.
The combination of intense pressure waves and overheating can induce piston
failure in a few minutes. Knock and preignition are both favoured by high
temperatures, so one may lead to the other. Under high-speed conditions
knock can lead to preignition, which then accelerates engine destruction
To obtain the maximum energy from the gasoline, the compressed fuel-air
mixture inside the combustion chamber needs to burn evenly, propagating out
from the spark plug until all the fuel is consumed. This would deliver an
optimum power stroke. In real life, a series of pre-flame reactions will
occur in the unburnt "end gases" in the combustion chamber before the flame
front arrives. If these reactions form molecules or species that can
autoignite before the flame front arrives, knock will occur [21,22].
Simply put, the octane rating of the fuel reflects the ability of the
unburnt end gases to resist spontaneous autoignition under the engine test
conditions used. If autoignition occurs, it results in an extremely rapid
pressure rise, as both the desired spark-initiated flame front, and the
undesired autoignited end gas flames are expanding. The combined pressure
peak arrives slightly ahead of the normal operating pressure peak, leading
to a loss of power and eventual overheating. The end gas pressure waves are
superimposed on the main pressure wave, leading to a sawtooth pattern of
pressure oscillations that create the "knocking" sound.
The combination of intense pressure waves and overheating can induce piston
failure in a few minutes. Knock and preignition are both favoured by high
temperatures, so one may lead to the other. Under high-speed conditions
knock can lead to preignition, which then accelerates engine destruction
Thread
Thread Starter
Forum
Replies
Last Post