Aug 23 2016 03:00 PM
-
Aug 23 2016 04:30 PM
Dr. Gautam Kalghatgi
from Saudi Aramco will deliver a special seminar on Tuesday Aug 23, 1500-1630.
The seminar is titled: Knock intensity, superknock and preignition in SI engines
Abstract
Knock
is an abnormal combustion phenomenon which limits the efficiency of
spark ignition
(SI) engines. It is caused by autoignition of the fuel/air mixture in
hot spots in the “end-gas” ahead of the advancing flame front which
depends on the evolution of pressure (P) and temperature (T) during the
cycle and on the fuel anti-knock quality. Most
studies on knock focus on the onset of knock which is determined by
chemical kinetics and also ignore the stochastic nature of knock. This
presentation focuses on knock intensity (KI) which is determined by the
evolution of the pressure wave following knock
onset and highlights the stochastic processes involved.
KI is defined in this study as the maximum peak-to-peak pressure fluctuation that follows
the onset of knock. It depends on ξ=aua where uais
the speed of the autoignition front and 𝑎 is the speed of sound, when ua
is small. KI can be related to the product of a parameter Z, which depends on, Pko,
the pressure at knock onset
and the square of (∂x/∂T), which is the inverse of the gradient of
temperature with distance in the hot spot. Both Z and (∂x/∂T) were
calculated using measured KI and Pko
for hundreds of individual knocking cycles for different fuels. The
model for ignition delay as a function of pressure P and T in the hot
spot and other data needed to calculate Z were available from a previous
study (SAE 2016-01-0702). For a given fuel and
operating condition, Z varies because Pko
varies because of cyclic variation of combustion – a stochastic
process.
(∂x/∂T) depends on the evolution of the hot spot during the engine cycle
and depends on flow and turbulence – another stochastic process. All
else being equal, Z increases and hence the probability of high KI
increases as Pko
increases, e.g., by more advanced spark timing and or faster flame development. For a given Pko, Z is lower for
a fuel with higher RON.
Normally,
the primary aim in SI engines is to avoid knock. However, in modern
turbo-charged
engines extremely high intensity knock, informally termed superknock is
observed to occur occasionally. Superknock is caused by developing
detonation (DD) which results when the value of 𝜉
decreases and the pressure wave begins to couple with the autoignition
front and gets amplified. Autoignition/knock has to be initiated at high
P and T for superknock to occur. At practical operating conditions,
chosen expressly to avoid knock, this can only
happen via another abnormal stochastic phenomenon - preignition, when a
flame is established before the spark plug fires. For preignition to
occur, the local temperature has to increase beyond a critical value.
This cannot happen because of autoignition of
the fuel/air mixture and hence does not depend on RON or MON of the
fuel. A further initiation criterion has to be satisfied for a stable
flame to be established. All else being equal, the chances of
preignition (establishing a flame) increase as laminar burning
velocity increases. All else being equal, the probability of superknock
decreases as fuel RON is increased. However, even with high RON, high KI
AND superknock could occur with the right combination of P and (∂x/∂T).