AME 436 Energy and Propulsion Lecture 10 Unsteady-flow
(reciprocating) engines 5: Combustion in engines
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2 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Outline Combustion in engines Knock Misfire / flammability limits
Incomplete combustion Pollutant formation
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3 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Knock - what is it? Homogeneous reaction occurring in unburned end
gas ahead of expanding flame front Occurs when the combination of
piston compression + flame compression increases T & P of the
end gas until the homogeneous reaction rate is fast enough that a
very rapid explosion occurs before the usual turbulent flame front
reaches & consumes the end gas - horse race between flame
propagation and homogeneous reaction in end gas What does rapid
mean? Faster than acoustic waves can relax the pressure gradients -
prevents even push on the piston How fast is that? Typical engine:
Bore = 10 cm = 0.1 m, sound speed = 500 m/s, time scale 0.1m / 500
m/s = 2 x 10 -4 s = 200 s at most
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4 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Knock - movies No knock Videos courtesy Prof. Yuji Ikeda, Kobe
University Knock
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5 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Knock - why is it bad? Pressures generated by knock are not
substantially higher than those of ordinary (deflagration)
combustion but the pressure gradients are huge, causing enormous
local stresses on the piston; also causes large torque on piston,
thus tilting & rubbing against cylinder wall As the shocks
propagate into the narrow region between the piston and cylinder
wall (the crevice volume), the shock strength increases, causing
locally even more severe damage - usually the crevice volume is
where the most knocking damage occurs
http://www.llnl.gov/str/Westbrook.html
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6 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Knock Shock formation causes ringing of pressure waves back &
forth across cylinder - sounds like you're hitting piston with a
hammer, which isn't too far from the truth Tpfer et al., SAE Paper
2000-01-0252 (2000) Start of knocking Post-knock ringing Normal
combustion (no knocking)
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7 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Knock - how do different fuels compare? Different fuels are
compared by means of an octane number Determine knock-limited
compression ratio for your test fuel, using a variable compression
ratio engine called a CFR engine (used only for research, not in
any vehicle) at specified test conditions (RPM, spark timing,
intake T & P, etc.) Define n-heptane as having octane number =
zero, 2-2-4 trimethylpentane = 100 Blend these two reference fuels
to get the same knock limited compression ratio as the fuel under
test The % 2,2,4 trimethylpentane in this blend is the knock rating
Two slightly different sets of test conditions used, called
Research and Motor methods; what's displayed at the gas pump is the
average of the two methods (R+M)/2 CFR engine n-heptane 2,2,4
trimethyl pentane (iso-octane)
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8 For non-premixed-charge engines (i.e. Diesels), you want
rapid ignition once the fuel is injected, so fuel requirements are
opposite premixed-charge engines The relevant scale is called the
cetane scale, with cetane (16 carbon atoms in a straight line) =
100, heptamethylnonane (9 carbon atoms in a row, with the inner 7
each having a methyl group attached) = 15 (not zero!) AME 436 -
Spring 2015 - Lecture 10 - Combustion in Engines Diesel fuel
characterization n-cetane 2, 2, 4, 4, 6, 8, 8 heptamethyl nonane
(iso-cetane)
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9 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
What does octane number measure? Heating value? Nope - all
hydrocarbon fuels have very similar heating values as previous
discussed (lecture 2) Laminar burning velocity (S L )? Nope; S L is
practically the same for all stoichiometric hydrocarbon-air
mixtures ( 40 cm/s) since As discussed in Lecture 4, S L ~ ( ) 1/2
All such mixtures are mostly air, so thermal diffusivity ( ) same
for all Overall reaction rate ( ) is controlled by its value at
adiabatic flame temperatures (T ad ) All such mixtures have nearly
equal T ad At these temperatures, controlling reactions are H + O 2
OH + O (breaking of O=O double bond, accelerates reaction) H + O 2
+ M HO 2 + M (competing for H atoms, decelerates reaction) CO + OH
CO 2 + H(only way to oxidize CO; regenerates H) and none of these
reactions depend on the fuel molecule; fuel decomposition is very
fast by comparison at these temperatures Tendency for mixture to
exhibit homogeneous reaction at the typical T & P of end gases
(typically 30 atm, 900K)? YES!! To understand this, need to look at
what reactions occur at these T's and P's
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10 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
What reactions occur during knock? Start with fuel molecule RH,
where R is an organic radical, e.g. propane without an H Remove an
H atom from RH RH + O 2 R + HOO Add an O 2 to R R + O 2 ROO Produce
peroxides with O-O single bond (half as strong as O=O double bond
(120 kcal/mole vs. 60 kcal/mole), much easier to break) ROO + RH R
+ ROOH or HOO + RH R + HOOH Break O-O single bond, create chain
branching process ROOH + M RO + OH or HOOH + M HO + OH Newly
created radicals generate more organic radicals RH + OH R + HOH or
RH + RO R + ROH Note that rate of knocking reactions will be
sensitive to rates of H atom removal from fuel molecule RH
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11 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How does fuel structure affect knock? Rate of H atom removal
depends on strength of C-H bond, which in turn depends on how many
other carbons are bonded to that C - stronger bond, slower
reaction, less knock Examples: n-heptane: 6 primary, 12 secondary
C-H bonds 2, 2, 4 trimethy pentane: 15 primary, 2 secondary, 1
tertiary
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12 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How does fuel structure affect knock? Does this small difference in
bond strength matter? YES because activation energy is high If we
use bond strength to estimate activation energy thus reaction rate
(dangerous in general, but ok here) then at a typical 900K methane
: primary : secondary : tertiary exp(-E methane / T) : exp(-E
primary / T) : exp(-E secondary / T) : exp(-E tertiary / T)
exp(-105,000 cal/mole/(1.987 cal/mole-K)(900K)) :
exp(-98000/1.987*900) : exp(-95000/1.987*900) :
exp(-93000/1.987*900) 1 : 50 : 268 : 820 Comparison of fuels
Methane (RON=120, MON=120) - highest of any common fuel Larger
n-alkanes lower, e.g. propane RON=112, MON=97 Alkenes (C=C double
bonds, e.g. ethylene, C 2 H 4 ) lower Benzene - very high knock
resistance - 113 kcal/mole C-H bond strength - MON=115 Alcohols
good too - methanol: RON=106, MON=92; ethanol RON=106, MON=89 See
chart on page 472 of Heywood
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13 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How does fuel structure affect knock? Ghosh et al., Ind. Eng. Chem.
Res., Vol. 45, p. 337 (2006)
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14 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Anti-knock additives Any successful additive must inhibit knock
reactions without inhibiting flame propagation reactions! Midgley,
Boyd, Kettering (GM research laboratories, Dec. 9, 1921) after
trying 1000's of different compounds stumbled onto tetraethyl lead
- (C 2 H 5 ) 4 Pb Introduced into gasoline in 1923 - immediately
doubled knock-limited compression ratio from 4 to 8, power &
efficiency increased (by 40%, from 20.8% to 29.2% according to
AirCycles4recips.xls) Last nail in the coffin for steam and
electric vehicles in early 1900's Mechanism not understood until
1970's - production of fine PbO particle fog that acts as a
scavenger of HOO and HOOH - takes them out of system before they
lead to chain branching but PbO poisons catalytic converters for
exhaust emissions introduced in 1975 - needed to switch to unleaded
gasolines (also Pb not exactly environmentally friendly, so not a
bad idea to stop using it anyway) - trend was to increase
concentration of aromatics (benzene-like molecules) but leads to
more soot formation, also carcinogenic
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15 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Other anti-knock additives Methylcyclopentadienyl manganese
tricarbonyl (C 9 H 7 MnO 3, MMT) Methyl tert-butyl ether (MTBE)
(looked good for a while, but now prohibited - water soluble, can
be smelled and tasted in water at the parts per billion level Now:
ethanol from corn - very desirable to powerful senators from farm
states MTBE Ethanol
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16 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How do operating conditions affect knock? Key points in all this
knock stuff Whether knock occurs is a horse race between flame
propagation and homogeneous reaction of the end gas The chemical
reactions that affect flame propagation are different from those
that affect knock Homogeneous reaction rates depend primarily on
the initial T & P, whereas flame propagation depends primarily
on the final T & P Thus, operating conditions that affect knock
and flame propagation are different!!! Since knock occurs in end
gas, need to see how operating conditions affect end gas T &
P
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17 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How do operating conditions affect knock? Simple estimate of
maximum T & P of end gas in terms of intake conditions T 2, P 2
and compression ratio r - assume adiabatic compression, constant-v
combustion, reversible adiabatic compression of end gas to
(maximum) pressure P 4
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18 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How do operating conditions affect knock? AirCycles4recips.xls end
gas calculation Assumes a small portion of the total mixture has no
heat added to it but undergoes reversible compression/expansion
according due to the pressure increase/decrease of the rest of the
gas in the cylinder that DOES have heat input due to combustion End
gas undergoes heat loss according to the usual formula The
following plot shows effects of compression ratio, spark advance,
mixture strength, T intake and P intake on the P-T history of end
gas
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19 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
P-T trajectory of end gas (combustion process only) Baseline: r =
8, = 1.3, f = 0.068, Q R = 4.5 x 10 7 J/kg, T in = 300K, P in = P
exh = 1 atm BurnStart = 0.045, BurnDuration= 0.105, BurnRateProfile
= 0, h = 0.01, T wall = 400K comp = exp = 0.9 For lean mixture
case: f = 0.051, BurnDuration = 0.3, BurnStart = 0.084 (best
efficiency timing)
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20 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How do operating conditions affect knock? Compression ratio As r ,
T end gas and P end gas , thus knock tendency Recall d[fuel]/dt ~ P
n exp(-E/ T), (n = order of reaction, E = activation energy) thus
both P and T affect knock P increases more than T, but reaction
rate is more sensitive to T than P T intake - increasing T intake
increases T end gas, thus knock tendency (that's why your car
knocks more on a hot day) (though P end gas isn't affected much by
increasing T intake ) P intake - increasing P intake increases P
end gas, thus knock tendency (that's why your car knocks when you
put your foot to the floor) (though T end gas isn't affected)
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21 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How do operating conditions affect knock? Spark timing As mentioned
in Lecture 9, page 24, as spark timing is advanced, there is more
burn then compress compared to compress then burn which leads to
higher T This also leads to higher P, thus higher T end gas Also,
igniting earlier means more time for knock to occur Fundamental
tradeoff between increasing spark advance to obtain best thermal
efficiency, and decreasing spark advance to minimize knock and NO x
Spark advance up to 0.05 increases thermal efficiency (see next
page), but there's a penalty to pay in terms of knock (T end gas
increasing) and NO x (T max increasing) T max and T end gas don't
increase much as advance increases, but both knock and NO x are
high activation energy processes so a little increase in T matters
Note knock and NO x are controlled by very different reactions
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22 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Spark advance
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23 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How do operating conditions affect knock? Mixture strength Many
experiments show less knock with leaner mixtures Leaner mixtures
burn slower, so need more spark advance, but even when spark timing
adjusted for best efficiency, still less knock that stoichiometric
mixtures Less heat release, thus lower peak P, thus lower T end gas
Ronney, et al., J. Auto. Eng., (Proc. Instit. Mech. Eng., Part D),
Vol. 208, pp. 13-24 (1994).
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24 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
How do operating conditions affect knock? Engine RPM - generally
most important effect is less time for knock to occur - at higher
RPM, more turbulence, S T increases, time needed for flame
propagation to occur decreases, but turbulence has no effect on
time for homogeneous reaction of end gas
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25 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
HCCI engines Actually, burning rapidly at minimum volume yields the
best possible thermal efficiency, but damage due to knocking means
we want to burn fast but not too fast HCCI - Homogeneous Charge
Compression Ignition engines take advantages of this - controlled
knocking Video courtesy Prof. Yuji Ikeda, Kobe University
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26 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
HCCI engines By using homogeneous reaction instead of flame
propagation, conventional flammability/misfire limits don't exist -
recall for homogeneous reaction, burn time is nearly independent of
equivalence ratio ( ) As a result, one can burn very lean mixtures,
low T ad, low peak temperature, low NO x formation Lean mixtures -
can obtain part-load operation without throttling and its losses
Since we're asking for knock, use high compression ratios, thus
high th Much more difficult to control the rate and timing of
homogeneous reaction than a propagating spark-ignited flame;
various control schemes being studied Variable intake temperature
Variable exhaust gas recirculation Variable compression ratio and
valve timing Cycle-to-cycle control probably needed As a result,
not in commercial use yet
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27 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Comparison of gasoline, diesel & HCCI
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28 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Comparison of gasoline, diesel & HCCI J. Dec., Proc. Combust.
Instit., Vol. 32, p. 2727 (2009).
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29 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
HCCI experiments in a single-cylinder engine
http://www-cdr.stanford.edu/dynamic/hcci_control/MODELING_talk.pdf
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30 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
HCCI experiments in 6 cyl. engine (1 cyl. HCCI)
http://www.orau.gov/deer/DEER2002/Session9/dec.pdf
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31 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
HCCI - disadvantages (opportunities?) Difficult to control timing
and rate of combustion If misfire occurs, gas mixture during the
next cycle will be too cold for auto-ignition to occur (unless
intake air heating is used), the engine will stop Cold starting?
Operating window for HCCI operation (load and engine speed) is
small - most HCCI concepts use conventional spark-ignited operation
at higher loads (less lean mixtures) Additional components for
control system - increased cost Relatively high friction losses due
to low IMEP, thus FMEP is a higher % of BMEP
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32 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Misfire / ignition limits If an engine operates too lean or too
rich, the engine will start to run roughly, misfire and may quit
altogether The limit is usually defined by a percentage of misfires
(cycles with negative MEP) or a certain percent standard deviation
in IMEP With conventional spark ignition premixed-charge engines,
the limit is at equivalence ratio ( ) 0.65 - 0.75, whereas in a
laboratory experiment (e.g. on a bunsen burner) 0.5 will burn Why
the difference? In the laboratory, the mixture can take as long as
it wants to burn, whereas in the engine, there is only a limited
time ~ 1/N available for burning - minimum turbulent burning
velocity (S T ) requirement To avoid misfire Increase burning
velocity Increase time available for burning Decrease time needed
for burning
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33 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Misfire effects of operating conditions Inlet temperature As T with
fixed, T ad , S L , thus S T (slightly) Since S L depends more on T
ad than any other property, limit criterion is basically a minimum
T ad criterion Recall T ad = T + fQ R /C v - can get a given T ad
with a high f (thus high ) and low T or vice versa Brilliant
experiments show this - limit T ad 2000K - 2100K Ronney, et al., J.
Auto. Eng., (Proc. Instit. Mech. Eng., Part D), Vol. 208, pp. 13-24
(1994).
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34 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Misfire - effects of operating conditions Intake pressure Recall S
L ~ P (n-2)/2, with n typically slightly less than 2, not much
effect But as P intake decreases, more exhaust gas (at 1 atm)
relative to fresh mixture (at P < 1 atm), so more dilution of
mixture with exhaust gas, thus lower T ad, lower S L Thus more
problems with misfire at throttled conditions (for fixed N)
Turbulence - more turbulence, higher S T - helps limit problem, but
remember too much turbulence will extinguish the flame! Engine RPM
(N) Time available for burning ~ 1/N But u' ~ u piston ~ N and S T
~ u' So (time available for burning)/(time needed) constant But
actually S T ~ u' (1- ), so you have more problems with misfire at
higher N
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35 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Misfire - effects of operating conditions Multiple spark plugs -
reduces distance each evolving flame needs to propagate, less
chance of misfire, so less misfire problem Knock additives (e.g. (C
2 H 5 ) 4 Pb) - no effect (affects knock reactions, not flame
propagation reactions)
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36 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Lean mixtures - effect on temperatures Frequently I hear the
comment from non-thermodynamicists who know engines that engine
temperature increases as you go leaner - if you go too lean you'll
burn up the engine Thermodynamically this makes no sense - leaner
mixtures have lower T ad = T + fQ R /C v (constant volume) BUT -
leaner mixtures burn slower - more heat release after some
expansion already completed - less expansion, higher T Since
chemical reactions have high activation energy, a small decrease in
T ad (leaner mixture) causes a large decrease in S L, thus much
slower burn, higher exhaust T
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37 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Lean mixtures - effect on temperatures Slow-burn effect >
lower-T ad effect r = 3, = 1.3, f = 0.068 or 0.0612, Q R = 4.5 x 10
7 J/kg, T in = 300K, P in = 1 atm, P exh = 1 atm, BurnStart =
0.0841, BurnDuration = 0.15, 0.3 or 0.45, ExhRes = FALSE, Const-v
comb, ComplExp = FALSE, h = 0.01, T wall = 400K, comp = exp =
0.9
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38 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Lean mixtures - effect on temperatures
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39 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Lean mixtures - effect on temperatures After expansion After
blowdown After exhaust stroke Const. v Const. P
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40 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Incomplete combustion (review from Lecture 4) Most fuel is burned
in engines - very little unburned fuel under normal operating
conditions unless mixture is too lean, too rich or otherwise
disabled Flame quenching may occur in the crevice volumes where gap
< quenching distance - main source of hydrocarbon emissions from
engines Before we get to the point that raw fuel is emitted, some
fuel reacts only partially - sequence of events: Fuel + O 2 CO + H
2 CO + H 2 CO + H 2 O CO + H 2 O CO 2 + H 2 O so CO emissions are
the first sign of trouble!
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41 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Example #1 For the Otto cycle example in Lecture 9 with r = 9, =
1.3, M = 0.029 kg/mole, f = 0.062, Q R = 4.3 x 10 7 J/kg, T 2 =
300K, P 2 = P in = 0.5 atm, P 6 = P ex = 1 atm, h = 0, comp = exp =
0.9, determine the end gas temperature and pressure If this were an
ideal cycle we could use the formula on page 16, i.e. But since
this is not an ideal cycle we compress the end gas isentropically
from its pressure (9.165 atm) and temperature (611K) after
compression to the pressure after combustion (51.02 atm), i.e.
which is not much different.
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42 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Example #2 During WWII, fighter aircraft engines were equipped with
systems to inject water into the cylinders during compression. How
would water injection affect (a)Knock tendency: The water would
cool the intake charge and increase its , thus reducing the end-gas
temperature substantially. Also, while the compression ratio could
not be changed, the intake pressure could be increased (these were
turbocharged engines) without knocking, thus allowing more air to
be ingested (b)Misfire tendency: The greater mass of un-burnable
water vapor and its higher C p (compared to air) led to lower T ad,
thus more tendency to misfire. This limited the amount of water
that could be injected. (c)Power: This was the main reason for
water injection. The liquid water requires much less Work/mass to
compress than air (Work = PdV thus Work/m = Pd(V/m) = Pd(1/), and
water >> air ), yet when the liquid water vaporizes, its
volume increases tremendously, generating more expansion work /
compression work (this is why steam cycles are more efficient that
gas cycles!) Also as mentioned above, the intake air pressure could
be increased, thus allowing more fuel to be added and burned
(d)Brake thermal efficiency: The slower burning would decrease
efficiency, but the steam cycle has inherently higher efficiency,
so it could go either way. (e)Work per unit mass of (fuel + water):
This would definitely decrease, since the water provided no
additional heat release but much additional mass. In actuality,
these aircraft used a methanol/water mixture, not just water, to
recover some energy release from the injected liquid (though note
methanol has only 40% of Q R of gasoline)
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43 AME 436 - Spring 2015 - Lecture 10 - Combustion in Engines
Summary Knock A very rapid homogeneous reaction in the end gas
ahead of the flame front Horse race between knock (bad) and flame
propagation (good) Knock tendency depends on reaction rates at
initial T, P & composition of reactants (end gas), flame
propagation depends on final (burned gas) temperature, so factors
affecting knock & flame propagation are very different Knock
tendency characterized by octane number - higher ON, more resistant
to homogeneous reaction Diesel fuels - opposite characteristics
desired Misfire / flammability limits Due to insufficient ignition
source or time to burn - roughly corresponds to fixed T ad Depends
on required propagation time (~ d/S T ) vs. available time (~1/N)
Incomplete combustion Due to insufficient time to burn - CO is
first sign of trouble