Transcript The Motor Vehicle Problem

The Motor Vehicle Problem
朱信
Hsin Chu
Professor
Dept. of Environmental Eng.
National Cheng Kung University
1
1. An Overview of the Problem of Air
Pollution from Motor Vehicles



The first gasoline-powered automobiles appeared
in 1886.
By 1900 world production was only about 20,000
vehicles per year, compared to about 30 million in
1999.
Although any one car consumes little fuel and
emits small amounts of pollutants, together the
roughly 500 million of them in the world
consume large amounts of fuel and emit large
amounts of pollutants.
2
1.1 Emissions


There are about 123 million autos in the US and
about 70 million trucks.
Next slide (Table 13.1)
Motor vehicles are the source of three-fouths of
the US emissions of CO, and 40 to 50% of the US
emissions of HC and NOX.
3


“Off-road vehicles” in Table 13.1 include aircraft,
railroads, boats, construction equipment, and farm
equipment.
From Table 13.1, autos and light trucks contribute
much more to the US emissions than do these
other sources.
5
1.2 The Regulatory History of Motor
Vehicle Air Pollution Control


Motor vehicles did not attract much attention as
air pollution sources until about 1950.
As coal combustion sources were controlled, and
as natural gas replaced coal as the principal urban
heating fuel in the US, a new type of air pollution
was discovered in Los Angeles.
6


A type of eye and nose-irritating air pollutant,
later named smog, occurred there, mostly in the
summer.
Professor A. J. Haagen-Smit demonstrated the
eye-irritating materials were largely formed from
emissions from autos.
7



California began to regulate emissions from autos
in 1963.
In the clean Air Act of 1970 US Congress began
federal regulation of autos, requiring stricter rules
for any states that already had state rules (only
California), but also requiring fairly strict rules
for the rest of the country.
The history of these regulations is shown in Table
13.2 (next slide).
8
2. The Internal Combustion (IC) Engine


External combustion engines were developed
before internal combustion engines.
James Watt’s 1776 steam engine was the first
general-purpose heat engine that converted heat
from combustion to a steady flow of power to a
rotating shaft.
10



For 100 years steam engines, with combustion in
a boiler external to the power –producing part of
the engine, were the only combustion engines.
These steam engines launched a giant
technological expansion, which, among other
things, led to the development of much better
machine tools.
The improved machine tools made it possible to
build the first IC engines.
11


The first commercially successful IC engines
(combustion inside the power-producing parts)
were those of Otto and Langen about 1876.
For a given power output these engines were
substantially smaller and lighter than external
combustion engines and had a higher thermal
efficiency (lower fuel consumption).
12


Those features made them the natural choice for
motor vehicles.
The steam engine held on in railroad locomotives
until the 1950s, when the major cost savings
brought by diesel engines led to its replacement.
13
2.1 The Four-Stroke IC Gasoline Engine


The four-stroke IC gasoline engine has been the
power source for most of the autos and small
trucks ever built.
Next slide (Fig. 13.1)
A cross-sectional view of a typical auto engine.
14


Most auto engines have four such piston and
cylinders, some have six or eight.
To begin a cycle, with the piston at the top (top
dead center, TDC) during the first stroke the
piston moves downward while the intake value is
open, so that an air-fuel mixture is sucked into the
combustion chamber (the space within the
cylinder, above the piston).
16



When the piston is at the bottom (bottom dead
center, BDC), the intake valve closes, ending the
intake stroke.
As the piston rises again to the top during the
compression stroke, both valves are closed, so
that the air-fuel mixture is compressed.
Near the top of that stroke the spark plug fires,
igniting the air-fuel mixture.
17


In its next downward travel, the power stroke, the
piston is driven by the high-pressure combustion
gases, which do the actual work of the engine.
At the bottom of the piston travel, the exhaust
valve opens, and on its next upward travel the
piston pushes the burned gases out into the
exhaust system.
18



The cycle is named for its four strokes-intake,
compression, power, and exhaust.
The spark plug fires every second upward travel
of the piston.
Power is produced only during the power stroke.
Each of the other three strokes consumes power.
19
2.2 Pollutant Formation


The principal pollutants emitted from simple
gasoline-powered IC engines are carbon
monoxide, hydrocarbons, and nitrogen oxides.
Auto engines produce more of them per unit of
fuel burned than other combustion processes
principally for the following reasons:
20
1)
2)
Auto engines are often oxygen deficient, which
most other combustion systems are not. → CO,
HC
Auto engines preheat their air-fuel mixtures,
which most combustion systems do not. → NOX
21
3)
4)
Auto engines have unsteady combustion, in
which each flame lasts about 0.0025s.
Almost all other combustion systems have
steady flames that stand still while the materials
burned pass through them. → CO, HC
Auto engines have flames that directly contact
cooled surfaces, which is not common in other
combustion systems. → HC
22
2.2.1 Carbon Monoxide, CO


In automobile engines we often have less than
stoichiometric air.
Let the excess air ratio E be negative values or it is much
easier to use the oxygen deficit z.
For any hydrocarbon fuel with formula CxHy:
n stoich

oxygen
 x
y
4
If there is less than stoichiometric oxygen, we can write:
n oxygen  x 
y
4
 z  n stoich oxygen  z  n stoich oxygen (1  E )
23


All gasoline are mixtures of many components,
but they can be characterized as having an
approximate average formula CxHy, where for a
typical gasoline x is about 8 and y is about 17.
Gasoline manufacturers change these values from
one location to another and with season of the
year (smaller values in winter and in cold climates
than in summer and warm climates).
24

For complete combustion (E = z = 0) of this fuel, the
equations is:
y

 y
C x H y   x   O 2  xC O 2  
4

2



 H 2O

(1)
If there is not enough oxygen to complete the reaction,
then the combustion products will contain CO, H2 and
unburned hydrocarbons.
At combustion temperatures the most common of these
products of incomplete combustion is CO.
25

If we assume that an oxygen deficit of z mols per mol of
fuel is fed, and if we assume that all of the oxygen deficit
causes CO formation, we can rewrite Eq. (1) as:
y


 y
C x H y   x   z  O 2  ( x  2 z )C O 2  
4


2


 H 2 O  (2 z ) C O

Each mol of oxygen from air brings with it
(0.79/0.21 = 3.76) mols of nitrogen, so that the total mols
of combustion products will be:
n total
out
y


 y
 3.76  x   z    x  2 z      (2 z )
4


2
(2)
26

The mol fraction of CO will be:
yCO 

2z
 
 
y
y


3.76 x 
z x


4
2
Example 1: Calculate the expected CO mol fraction for
combustion of a gasoline with x = 8, y = 17, and with the
air supplied being 90 percent of that required for
complete combustion.
27

Solution:
y
y



n oxygen  0.9 n stoich oxygen =  x+  z   0.9  x  
4
4



Therefore,
y
17 


z  0.1  x    0.1  8 
  1.225
4
4 


and
yCO 
2 1.225



3.76  8  17
 1.225   8  17
4
2



 0.042  4.2%
28
2.2.2 Air-Fuel Ratio (A/F), Equivalence
Ratio (Ø)


Example 2: Calculate A/F and Ø for Example 1.
Solution:
The A/F is always stated in weight terms in the IC
literature (normally lb/lb in the US).
In general it is written as:
 
y


x
 z  3 2  3 .7 6 2 8 


4
A

F
12 x  1 y
29

For this example we have
A

 8  17 / 4  1.225   32  3.76
28 
12 8  17
F
 13.39
 8  17 / 4   32  3.76 28 
A
and  

 14.88
12.8  17
 F  stoich

The equivalence ratio is defined as:
 

 A / F  stoich
AF

14.88
 1.11
13.39
actual
The normalized A/F ratio λ=1/Ø = 0.9.
30

Table 13.3 (next slide) shows the A/F ratios that
actually occur in IC engines.
31



From Table 13.3, an IC engine would operate
satisfactorily for any normalized A/F ratio
between 0.5 and 3.5.
However, based on experience with actual
engines the operable range is from about 0.8 to
1.3.
The smaller operable range is mostly due to the
large heat losses from the small amount of
combustible mixture in the cylinder to the
surrounding cooled cylinder walls and head.
33



For steady operation at most driving speeds, the best fuel
economy occurs at a λ of about 1.2.
For acceleration or hill climbing the requirement is not
best fuel economy but maximum power output, which is
found at λ≒0.95.
Most engines idle more smoothly at values between 0.90
and 0.95. (At low speeds there is more time for heat
losses to put the flame out.)
34



Cold starting poses a special problem for IC
engines.
When the engine is cold the exhaust heat is not
available, and the temperature in the compressed
mixture is so low that much of the liquid fuel is
not vaporized.
Only the most volatile parts of the fuel will be
vaporized under this condition.
35



To make λ, based on the vaporized part of the fuel,
be low enough for the engine to start, one must
put more total fuel into the air-fuel mixture.
In carburetor autos, this excess fuel is added by a
choke. This was operated by hand on older cars
and is now operated by thermostatic or electronic
sensors.
Fuel injection engines regulate the amount of fuel
injected, taking the same variables into account.
36
2.2.3 Hydrocarbons (HC)


At all values of λ one measures unburned HC in
the exhaust gases of gasoline IC engines.
Most of these are the result of flame quenching.
37


IC engines must have some kind of lubrication
where the piston slides up and down in the
cylinder.
In auto engines this is provided by the motor oil,
which is pumped from a sump at the bottom of
the crankcase through holes drilled or cast in the
block, bearings, crankshaft, connecting rods, wrist
pins, and cylinders to holes on the side of the
piston.
38


The piston rings, which are the actual sliding
surface between piston and cylinder, ride on this
oil film.
Normal hydrocarbon lubricants cannot stand
temperatures much higher than about 250-3000F
(121-1490C) for long periods.
39


The principal purpose of the cooling system of an
auto engine is to keep the temperature of the
lubricant film between the piston rings and the
cylinder wall at or below the temperature.
Heavily loaded engines, in trucks or autos that
pull trailers, have separate radiators to cool the oil.
40


If the temperature becomes significantly higher
than that, the lubricants decompose, leaving
behind solid carbon residues that cause the engine
to seize.
An engine operated without its cooling system is
destroyed in a few minutes.
41



Research engines have been built that use solid
lubricants (MoS2) that can stand very high
temperatures.
These engines have no cooling system and
operate at temperatures comparable to the melting
point of steel.
They have excellent fuel economies but very
difficult materials-engineering problems.
42


The cooling of the cylinder walls and head makes
them cold enough that in a narrow quench zone
adjacent to them the flame goes out, and the
hydrocarbons in that part of the air-fuel mixture
are not burned up.
Example 3: Estimate the hydrocarbon
concentration to be expected in the exhaust gas
from an engine with a piston diameter of 6 cm, a
stroke of 5 cm, and a quench zone thickness of
0.2 mm at λ=1.
43

Solution:
We assume that all of the surface of the cylinder
and the head has a quench zone.
The top of the piston is not cooled and does not
apparently play a significant role in flame
quenching.
44

The ratio of the volume of the quench zone to the volume
of the combustion chamber with diameter D, piston
travel L, and quench zone thickness t (assuming a flat
head) is:
tA
V
t  4 D   D L 
2


4
2
D L
4 
1
 t 

L D
4 
 1
 0.02 cm 

  0.017  1.7%
 5 cm 6 cm 
45


Thus we would expect 1.7% of the total hydrocarbons in
the fuel to appear in the exhaust.
From Eq.(2),
Total mol of combustion products = ntotal

 3.76 x 
= 3.76  8+
y
4

17
4
x
 
y
2
  8   172 
 62.6 m ol com bustion products/m ol of fuel
46

mol fraction of unburned fuel in exhaust = yunburned

0.017 m ol unburned/m ol fuel
 272 ppm
62.6 m ol com bustion products/m ol fuel

The calculation shows that we would expect a higher
hydrocarbon concentration in the exhaust from a small
engine than a large one, which is observed.
47


This example assumes that the hydrocarbons in
the exhaust have the same chemical composition
as those in the fuel.
Next slide (Table 13.4)
Typical composition of hydrocarbons in untreated
auto exhaust.
48


The methane, ethane, acetylene, propylene,
formaldehyde, and other aldehydes were not
present in the fuel and must have been formed by
incomplete combustion, mostly in the quench
zone.
The benzene, toluene, and xylenes were present in
the fuel.
They are the gosoline components with the
slowest burning velocities, and hence the highest
probability of passing, unburned, into the exhaust.
50


The calculation in Example 3 suggests that the
percent hydrocarbon in the vehicle exhaust is
independent of air-fuel ratio.
Next slide (Fig. 13.2)
The typical emissions of NOX, CO and HC from
automobile engines as a function of λ .
51


From Fig. 13.2, the CO concentration does not become
zero at λ>1.0 but rather continues at some low value,
even when there is plenty of excess air in the exhaust gas.
At the high temperatures of the flame the reaction that
actually consumes CO:
CO 
OH
CO2 
H
is an equilibrium reaction that does not go to completion.
53


As the temperature is lowered toward the exhaust
temperature, the equilibrium shifts strongly to the
right, but the reaction becomes very slow below a
temperature of 2,200 K.
Therefore, some of the CO found in the exhaust is
due to the noncompletion of this reaction even
though adequate oxygen is present.
54



In conclusion, we can say that the CO consists of
“oxygen deficit” CO and “incomplete reaction”
CO, with the latter being pratically independent of
λ.
The HC consists of “oxygen deficit” HC and
“quench zone” HC, with the “quench zone”
amount being about the same at all values of λ.
At λ=0.9 we would expect about 4% CO and
about 0.1% HC.
55
2.2.4 NOX


Peak temperatures in an auto engine are of the
order of 2,700 K, whereat NO is formed from N2
and O2.
Next slide (Fig. 13.3)
The calculated temperature and NO concentration
history for a typical single combustion in an IC
engine.
56


Example 4:
For the example shown in Fig. 13.3 estimate the
temperature before the beginning of combustion,
the temperature in the burned gas just after
combustion begins, and the temperature in the
burned gas at the end of combustion.
Solution:
The figure shows that combustion begins about 15
degrees before TDC and continues until about 15
degrees after TDC.
58


At 15 degrees before TDC the piston has traveled about
98% of its travel from BDC, so we can safely ignore
further piston travel and assume that the gas in the
cylinder has been compressed by the stated compression
ratio of 7.
From standard thermodynamics texts for reversible
adiabatic compression, we find:
T2  T1
 
V1
R /  cP  R 
V2
59

Taking common values of T1 and cP, we write:
T2  528 R (7)
o

R /( 3.5 R  R )
 1154 R  694 F  641 K
o
o
The temperature rise during the first part of the buring
process, which is assumed to be adiabatic and to occur at
constant pressure, is given by:
T 
m fuel  H com bustion
m com bustion products c P , com bustion products
60

For a typical gasoline △Hcombustion is about 19,020
Btu/lbm and cP, combustion products is about 0.33 Btu/lbm/0F,
so that
T 
1 lbm fuel
15.88
19, 020 B tu / lbm
lbm com b . prods .
lbm fuel
  0.33
B tu
lbm
o
F

 3629 F  2016 K
o

This calculation overstates the temperature increase
because it assumes that the whole combustion chamber is
filled with air-fuel mixture.
61


In Fig. 13.1, we see that when the exhaust valve
closes at the top of the exhaust stroke, the
combustion chamber will contain a substantial
amount of exhaust gas.
The incoming air-fuel mixture mixes with this
residual exhaust gas.
62


Measurements indicate that the residual exhaust
gas is about 15% of the total charge to the
cylinder.
Thus the heat released per pound of total gases in
the combustion chamber will be about 85% of
that just computed, and the temperature rise 85%
of that computed above, or △T = 3,0850F =
1,714K.
63



Adding this temperature increase to the initial
temperature, we would find a temperature of
694 + 3,085 = 3,7990F = 4,2390R = 2,355K,
which is practically the value shown at 150 before
TDC in Fig. 13.3.
In addition, at peak temperature the combustion
reactions are not driven completely to the right.
We would expect some unreacted oxygen and
hydrocarbons to be present at equilibrium, thus
reducing the peak temperature slightly.
64



We computed the initial temperature rise by assuming the
first part of the combustion occurred at constant pressure.
This assumption is plausible because, as the first part
(near the spark plug) burns, it expands and pushes away
the remaining gas.
But later, when the remaining gas burns, it also expands
and hence compresses the gas that burned first so that it
undergoes combustion at practically constant pressure,
followed by an adiabatic compression.
65


At the end of the combustion process the gas temperature
in the combustion chamber will not be at a uniform
temperature; the first part to burn will be hottest.
But if we wish to compute the average temperature we
can proceed by assuming that the whole combustion
process occurs at constant volume, for which we can
write:
T 
m fuel  U com bustion
m com bustion
c
products V , com bustion products
66

 U combustion   H combustion for most cases

cV, combustion products for gasoline combustion is typically
about 0.26 Btu/lbm/0F, so that for the overall combustion
at constant volume we would compute
T 
1 lbm fuel
15.88
19, 020 B tu / lbm
lbm com b . prods .
lbm fuel
  0.26
B tu
lbm
o
F

 4, 607 F  2, 559 K
o
67

This shows that
 T constant
 T constant

volum e
cP
cV
pressure

0.33
 1.27
0.26
The values from Fig. 13.3 are
 T peak
 Tinitial


com bustion

2, 640  641
2, 350  641
 1.17
The differences between these values are due to heat
losses from the combustion gases and to the work of
expansion done by the gases on the piston.
68


The equilibrium NO curve in Fig. 13.3.
corresponds to the thermal NO values calculated
by the zeldovich mechanism.
The actual curve suggests that the rate of NO
formation is negligible until the temperature
reaches about 2,400 K.
69



The NO concentration rises rapidly toward the
equilibrium value, crossing it at about 22 degrees
after TDC.
From then on the concentration is higher than the
equilibrium concentration in the rapidly cooling
gas, so the concentration falls.
When the gas temperature reaches about 2,300 K
the reaction rate becomes negligible, and the NO
concentration is “frozen” at a value above the
equilibrium value.
70


The combustion period shown corresponds to
about 30o of crack angle, or 1/12 of a revolution,
and the time of combustion to about 2.5 ms.
There is very little nitrogen in gasoline, so the
amount of fuel NO is generally negligible.
71
3. Tailpipe Emissions




Table 13.2 shows that all the CO and NOX emissions and
about half the HC emissions are tailpipe emissions.
The possibilities for dealing with these are discussed next.
The cheapest and simplest approach is to change what
goes on in the combustion process, to minimize the
formation of pollutants.
The first four alternatives discussed below do that.
The remaining four require some change outside the
combustion process.
72
3.1 Lean Operation


From Fig. 13.2 it is clear that lean combustion
greatly reduces CO and HC emissions compared
to rich combustion.
Before the 1970s mechanics regularly changed
the factory carburetor settings to make the
combustion richer.
Their customers liked the car’s smooth
performance, but these changes greatly increased
emissions and lost fuel economy.
73
3.2 Exhaust Gas Recirculation (EGR)


Starting in the 1970s and continuing in many
current engines that production of NOX was
reduced by EGR, in which the incoming
combustion air is diluted with up to 20% exhaust
gas.
EGR reduces the peak flame temperatures and the
O2 content of the burned gas; both of these effects
lower the NOX formation.
74



EGR reduces the power output of the engine.
At extremely lean operation EGR can lead to
misfire and thus increased HC emissions.
Next slide (Fig. 13.5)
The effect of EGR
75
3.3 Reduce Flame Quenching



Most of the HC and CO are formed by wall
quenching of the flame.
Various techniques have been used to minimize
this.
The most obvious is to make the combustion
chamber more nearly spherical, thus reducing the
surface per unit volume.
Reducing the sizes of the crevices associated with
the head gasket, spark plug gasket, and piston
rings also contributes to reduce flame quenching.
77



Raising the temperature of the cylinder wall and
head lowers the thickness of the quench zone.
This was one of the reasons for switching the auto
coolant from water in summer and ethylene
glycol in winter to a year-round mixture of water
and ethylene glycol.
Diesel engines place the fuel in the middle of the
cylinder and they have much lower CO and HC
emissions than conventional auto engines.
78
3.4 Speed the Warmup


Much if not most of the CO and HC emissions of
a typical driving cycle occur in the first minute or
two while the engine is cold.
Most engines measure the temperature of both the
underhood air and the engine coolant.
79


When these are cold the engine uses valves to
draw air through a shroud over the exhaust
manifold into the engine air intake to speed the
warmup.
Some engines use electric heaters located between
the carburetor and the engine to warm the air-fuel
mix during this cold start period.
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3.5 Catalytically Treat the Combustion Products


Most auto manufacturers have concluded that
they cannot meet current and future emission
standards by engine modifications alone.
Instead, they have concluded that the most
satisfactory solution is to modify the engine so
that it produces the right mix of pollutants and
then treat that mix catalytically to meet the
emission standards.
81

The first attempts used two catalysts, but since then auto
manufacturers have developed the “three-way catalyst”
that promotes the following reaction:
Platinum  rhodium catalyst
NO  CO  HC        
 N 2  CO 2  H 2 O

This reaction requires very close control of the ratio of
oxidizing agent (NO) to reducing agents (CO + HC)
(Fig. 13.6, next slide).
82


The key to doing this successfully was the
development of the doped zirconium dioxide
oxygen sensor, shown in Fig. 13.7 (next slide).
Using the measured value of the exhaust gas
oxygen content, the engine computer can control
the A/F ratio to stay within the ±0.05 tolerance
(λ tolerance of ±0.003) needed to stay at the top of
the curves in Fig. 13.6.
84



The typical automobile exhaust gas catalyst is a
(5:1) mix of platinum or palladium with rhodium,
supported on an Al2O3 layer that is deposited on a
cheaper ceramic base.
The typical “light off” temperature is about 350℃.
The catalyst should cause the small amount of S
in the gasoline to exhaust as SO2 rather than the
much smellier H2S, but not oxidize the SO2 to the
more toxic SO3.
86




A typical modern auto catalyst has a volume of
about one liter and a noble metal content of about
1.5 g.
Next slide (Fig. 13.8)
The most common arrangement uses a single
structure called a honeycomb monolith.
Each channel is about 1/20 of an inch wide (4001,200 cells per in2) and about a foot long.
The channel can be hexagonal, square, or
triangular; most are square.
87
3.6 Change the Fuel

The requirements for the fuel for an IC spark
ignition engine are as follows:
(1) high heating value so that the vehicle will
have adequate range between refuelings,
without carrying an excessive fuel weight;
(2) a high fuel density so that the fuel storage
container will be of acceptable size;
89
(3) easily handled, normally as a liquid at ambient
temperatures;
(4) good antiknock properties;
(5) ability to vaporize in air-fuel system (adequate
volatility);
(6) other miscellaneous properties, like good
storage stability, limit toxicity, etc.
90



Antiknock properties are a very important part of
this list.
Returning to Fig. 13.1, we see that the
combustible mixture is ignited by the spark plug,
which is normally located at or near the middle of
the top of the combustion chamber.
The flame starts there and spreads out through the
chamber.
91


The burned gases have a much higher volume
than they had as unburned gases, so they expand
and compress the unburned gas.
As a result the temperature of the unburned gases
rises before the flame reaches them (The
propagation speed of the pressure increase is
much faster than the speed of the flame front.).
92


If the unburned gas is heated to its autoignition
temperature before the flame front reaches it, it
will spontaneously ignite, producing a loud knock.
It was apparent early in the history of the
automobile that raising the compression ratio-that
is, (the volume contained at BDC)/(the volume
contained at TDC)- increased the efficiency of the
engines but also increased their tendency to knock.
93



Early engines has compression ratios of about 4,
most current auto engines have compression ratio
of 8 to 10, and diesel engines have compression
ratios of about 16 to 20.
In diesel engines the fuel and air are not premixed;
fuel is sprayed into the chamber near TDC.
In that case the hot gases ignite the fuel without a
spark plug.
94


Diesel engines would have a terrible knock
problem if the fuel and air were premixed, but
they have no problem if the fuel is added slowly
and if it burns as it is added.
Different families of hydrocarbons have different
resistances to knock.
95



Straight-chain hydrocarbons are the worst, highly
branched paraffins the best, olefins better than
corresponding paraffins, and aromatics almost as
good as highly branched paraffins.
The order of quality is just the reverse for diesel
fuels.
Fuels are rated by their octane number, which is
equivalent to the percent isooctane (2,2,3trimethylpentane) in a blend of isooctane and
n-heptane that has the same antiknock properties.
96


This is equivalent to assigning an octane number
of 100 to isooctane and of zero to n-heptane, and
assuming linearity of octane number on blending
of these two.
Typical motor fuels in Taiwan have octane
numbers in the range of 92-98.
97



Fuels for piston air craft engines have octane
number over 100; these are expensive, but they
allow a higher compression ratio and thus better
fuel economy and longer range.
One can improve the octane number with
additives.
The most successful has been tetraethyl lead
(TEL).
98



When it is added to gasoline at the ratio of up to
0.1% by weight, it can increase the octane number
by up to 6.
Unfortunately, although lead leaves the
combustion chamber as a gas it forms fine
particles on cooling.
These deactivate automotive emission catalysts
and increase the lead content of the atmosphere.
99


Natural gas meets all the requirements just listed
except that of easy storage on the vehicle
(2,500 psia).
Other gaseous fuels, e.g. CO or H2, can be used in
slightly modified automobile engines if a suitable
storage system can be worked out.
100



Commercial propane, sometimes called LPG
(liquefied petroleum gas), is a mixture that is
typically 90+ percent propane.
Its vapor pressure at 1000F is about 200 psig.
It is a perfectly satisfactory fuel for gasoline
engines.
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

The two lowest molecular weight alcoholsmethanol, CH3OH, and ethanol, C2H5OH - can be
used in slightly modified gasoline engines (Brazil:
sugarcane)
All of the fuels discussed in this section produce
lower air pollutant emissions for the following
reasons:
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
(1) All of these fuels have lower boiling points
than the highest molecular weight components
of gasoline, so that they are more easily
converted to the all-gas state.
(2) All of these fuels have simpler molecules than
gasoline and thus take fewer chemical steps to
be totally combusted to CO2 and H2O.
Combustion is more likely to be complete
with them than with gasoline.
103

(3) The alcohols have some of the oxygen they
need for combustion within their molecule.
This also promote more complete combustion.
The observation that oxygen-bearing fuels lead to
lower HC and CO emissions has led to the 1990
Clean Air Acd Amendments in the US requiring
regions with severe winter CO problems to use
only gasoline that contains at least 2.7 weight
percent oxygen during the winter months.
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

It appears that this requirement will be mostly met
by blending into the gasoline methanol, ethanol,
or methyl tert-butyl ether (MTBE), which is made
from isobutene and methanol.
All three of these improve the octane number of
the fuel they are blended into.
105
3.7 Computer Control


Before about 1980 all automobiles controlled A/F,
spark timing, and EGR rate with mechanical or
pneumatic devices that sensed engine speed,
throttle setting, manifold vacuum, and various
temperatures.
Starting in the 1980s, auto manufacturers
switched to computer control.
With them, the controls can react to changes
faster than the mechanical or pneumatic devices
could.
106



For example, the spark normally does not fire at
the TDC but rather some time before that (spark
advance).
However, increasing the spark advance, which
improves performance, also increases the
probability of knock.
Now some computer-controlled cars include a
knock-meter (a microphone) allowing them to
operate very close to the knock limit of spark
advance and thus to improve fuel economy.
107
3.8 Lean Burn


In the early 1970s some auto manufacturers tried
to solve their emissions problems by lean burn, an
engine design based on operating almost always
at λ = 1.05-1.1. (complete combustion)
This gave good fuel economy, but normally it put
the engines near the NOX peak on Fig. 13.2,
which ultimately drove this kind of engine off the
market.
108


With the advent of computer controls it became
possible to operate engines successfully closer to
the limits of stable combustion than with the
previous mechanical controls, so there was
renewed interest in lean burn.
The goal is to operate lean enough to get to the
right side of the NOX hump on Fig. 13.2 and thus
reduce NOX production.
109
4. Alternative Power Plants
4.1 Diesel Engines
 Next slide (Table 13.5)
The differences between conventional gasoline
and diesel engines.
110


When a gasoline engine is running at its
maximum temperature, it will sometimes continue
to run even after the ignition is turned off as a
result of the spontaneous ignition by mixing with
hot gas.
Most auto engines have “anti-dieseling” devices
in their fuel systems to prevent this.
112



In a conventional auto a small amount of air per
revolution enters the engine at idle (closed throttle
plate) and a much larger amount enters at wideopen throttle.
The ratio for a typical engine between maximum
and minimum air flow is about 80:1.
Diesel engines have no throttle; they put
practically the same amount of air per revolution
into the engine at idle and at full power.
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4.2 Gasoline-Powered Two-stroke Engines



These engines discharge exhaust gas and bring in
inlet gas simultaneously.
Their spark plugs fire every revolution.
They do not have mechanical valves, but use ports,
passages, and crankcase compression to move the
inlet gas into the cylinder.
114



They are used in some lawnmowers, chain saws,
many portable power tools, some motocycles, and
most outboard boat motors.
They are simpler, cheaper, smaller, and lighter for
a given power output than typical automobile
engines.
But they are less fuel efficient and have much
higher exhaust emissions.
115
4.3 Gas Turbine Engines


These engines do their compression and
expansion with rapidly rotating (e.g., 20,000 rpm)
compressors and turbines instead of the piston and
cylinder arrangement in most IC engines.
Their burners operate in steady flow instead of
intermittently, as in other IC engines, and they use
large amounts of excess air.
116


They are used in jet aircraft, helicopters, small
electric power installations, and a few trucks.
Turbines have less weight for a given power
output than a typical automobile engine.
117



They have satisfactory fuel economy at full load
but very poor fuel economy at part load.
They respond slowly to changes in throttle setting.
These drawbacks have defeated all efforts to build
a cost-competitive automotive gas turbine engine.
118
4.4 Other Options
(1) Electric Vehicles
 The most common suggestion is the batterypowered auto.
 It produces negligible local air pollutant
emissions.
 An electric auto is also quieter than the ICpowered auto.
119




The IC engine takes its oxidizer from the air,
typically 15 pounds of air per pound of fuel.
The battery car carries both fuel and oxidizer with
it.
The extra weight is a major handicap for cars
using traditional lead-acid batteries.
Recharging is another problem.
120
(2) Fuel Cell Vehicles
 Fuel cells react fuel electrochemically with
oxygen without burning it.
 They have a higher thermal efficiency than any
fuel-burning engine.
 To date, they are only practical for hydrogen as a
fuel, which is satisfactory for space travel, but not
for autos.
121
(3) Hybrid Vehicles
 One solution to the poor range of electric vehicles
is to charge the batteries as you drive.
 This is done by hybrid vehicles, see Fig. 13.9
(next slide).
122
123



In steady level highway driving only perhaps 10%
of the engine’s maximum power is used.
A hybrid vehicle has a much smaller gasoline
engine than the same-sized conventional auto, and
it runs steadily whenever the auto is in use.
That makes it much more efficient and makes its
emissions much less than the larger, variable
speed engine of a conventional auto.
124



The battery system absorbs power in normal
driving and provides bursts of power for passing
or hill climbing.
The size of battery required is much less than for
an all-electric vehicle.
The manufacturers claim that the hybrid vehicles
have about twice the fuel economy and one-tenth
the pollutant emissions of conventional vehicles
of comparable size and performance.
125