Advanced Engine Technology
Ali Momeni Movahed
University of Ottawa
Department of Mechanical Engineering
10/3/2023
• Introduction to Engine Technologies
• Engine Thermochemistry
• Fuel and Induction System Technology
• Exhaust System
• Cooling and Lubrication System Technology
• Emission regulations
• Gasoline Engines
• Diesel Engines
• Natural Gas Engines
• Electric and Hybrid Engines
• Advanced Technologies in Emission Control System
• Engine Control Unit (ECU)
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Mark Distribution:
Assignments: 10%
Project: 35%
Final exam: 55%
Project:
Students are supposed to study one of the aspects of combustion
engines in more detail.
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Chemical energy
Mechanical energy
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Internal Combustion Engine
External Combustion Engine
[Internal Combustion Engine Fundamentals, John B. Heywood, Second Edition]
[1999 Encyclopedia Britannica Inc.]
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[www.global.kawasaki.com]
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Engine Classifications
Power cycles
• Otto cycle
• Diesel cycle
Working cycle
• Four stroke
• Two stroke
Geometry
• Inline engines
• V-type engines
• Rotary engines
Valve or port design and location
• Over-head (or I-head) valves
• Under-head (or L-head) valves
• With two, three, or four valves per cylinder
• Fixed or variable valve control (timing,
opening and closing points, and lift)
Fuels
• Gasoline
• Diesel
• Compressed natural gas
• Liquefied petroleum gas
• Hydrogen
Method of cooling
• Water cooled
• Air cooled
Fuels
• Gasoline
• Diesel
• Compressed natural gas
• Liquefied petroleum gas
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Engine components
•
•
•
•
•
•
•
•
•
Cylinder block
Piston and rings
Connecting rod
Crankshaft
Camshaft
Gudgeon pin
Cylinder head
Combustion chamber
Valves and valve
assembly
• Oil pan
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Valve Springs: Keeps the valves
closed.
Cam Shaft: The shaft
that has intake and
exhaust cams for
operating the valves.
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Connecting rod
Attaches piston to the crank shaft.
Crank shaft
Converts the reciprocating motion into
rotary motion.
Flywheel
Absorbs and releases kinetic energy
of piston to have a smoother rotation
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Three rings. Top two rings are
compression rings (to seal
the compression pressure in the
cylinder) and the third one is an oil
ring (scrapes excessive oil from
the cylinder walls)
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Intake stroke:
Intake valve is open, piston goes down, there is a slightly negative pressure inside
the combustion chamber and fresh air is drawn into the engine.
Compression stroke:
The valves are closed, piston moves toward the TDC, the mixture is compressed,
the pressure and temperature increase.
Combustion stroke:
The valves are closed, piston moves toward the BDC, the chemical energy is
released, the pressure and temperature dramatically increase.
Exhaust stroke:
The exhaust valve is open, piston moves toward the TDC, the.
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Geometrical Relationships for Reciprocating Engines
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Geometrical Relationships for Reciprocating Engines
rc = 9 to 12 for spark-ignition (SI) engines
rc = 14 to 22 for compression-ignition (CI) engines
B/L = 0.8 to 1.2 for small and medium size engines
B/L = 0.5 to 0.8 for large slow speed CI engines
R = 3 to 3.5 for small and medium size engines
R = 5 to 9 for large slow speed CI engines
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Bore to stroke ratio can affect the engine friction by:
Changing crankshaft bearing friction and cylinder
friction.
• At higher bore to stroke ratios, the bearing friction is
high because of the larger piston area and
consequently larger forces to the crankshaft bearings.
• The corresponding shorter stroke results in decreased
cylinder friction.
• For high speed engines, the bore to stroke ratio is
usually higher while for high engine efficiency, the
bore to stroke ratio should be lower.
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Geometrical Relationships for Reciprocating Engines
𝑑𝑠 𝑑𝑠 𝑑𝜃
𝑆𝑝 =
=
= −𝜔(𝑎𝑠𝑖𝑛𝜃 +
𝑑𝑡 𝑑𝜃 𝑑𝑡
𝑑2 𝑠
𝑑 𝑑𝑠
𝑑
𝑎𝑝 = 𝑑𝑡 2 = 𝑑𝑡 𝑑𝑡 = 𝑑𝑡
𝑑 𝑑𝑠 𝑑𝜃
=
∙
𝑑𝜃 𝑑𝜃 𝑑𝑡
2
𝑑𝑠 𝑑𝜃
𝑑𝜃 𝑑𝑡
𝑎2 𝑠𝑖𝑛𝜃𝑐𝑜𝑠𝜃
𝑙 2 − 𝑎2 𝑠𝑖𝑛2 𝜃
𝑑 𝑑𝑠
𝑑𝜃
𝑑𝑠
1)
2
𝑑2 𝜃
= 𝑑𝑡 𝑑𝜃 ∙ 𝑑𝑡 + 𝑑𝜃 ∙ 𝑑𝑡 2
𝑑2𝑠 2
= 2.𝜔
𝑑𝜃
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Otto cycle:
It consists of four reversible processes:
• Isentropic Compression
• Isochoric (constant volume) Heating
• Isentropic Expansion
• Isochoric Cooling
In reality, the last process should be replaced by exhaust
and intake processes.
Simplifications to the real cycle include:
1) Fixed amount of air (ideal gas) for working
fluid
2) There is no combustion process
3) There is no intake and exhaust processes
4) Engine friction and heat losses not considered
5) Specific heats independent of temperature
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𝑉𝑡 = 𝑉𝑑 + 𝑉𝑐
𝑉𝑡
𝑟𝑐 =
𝑉𝑐
𝑊𝑜𝑢𝑡
𝜂𝑂𝑡𝑡𝑜 =
𝑄𝑖𝑛
𝑞𝑖𝑛 − 𝑞𝑜𝑢𝑡 = 𝑤𝑛𝑒𝑡
⇒ 𝑞𝑖𝑛 = 𝑢3 − 𝑢2 = 𝐶𝑣 𝑇3 − 𝑇2
⇒ 𝑞𝑜𝑢𝑡 = 𝑢4 − 𝑢1 = 𝐶𝑣 𝑇4 − 𝑇1
𝑤𝑛𝑒𝑡
𝑞𝑜𝑢𝑡
𝑇4 − 𝑇1
𝜂𝑡ℎ,𝑂𝑡𝑡𝑜 =
=1−
=1−
𝑞𝑖𝑛
𝑞𝑖𝑛
𝑇3 − 𝑇2
𝑇1 𝑇4 /𝑇1 − 1
𝑇1
1
=1−
= 1 − = 1 − 𝑘−1
𝑇2 𝑇3 /𝑇2 − 1
𝑇2
𝑟
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• For a given compression ratio, the
thermal efficiency of the ideal Otto
cycle would be higher at higher
specific heat ratio.
• The compression ratio can also
increase the thermal efficiency of
the Otto cycle.
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Diesel cycle
𝑞𝑖𝑛 − 𝑤𝑏,𝑜𝑢𝑡 = 𝑢3 − 𝑢2 ⇒ 𝑞𝑖𝑛 = ℎ3 − ℎ2 = 𝐶𝑝 𝑇3 − 𝑇2
𝑞𝑜𝑢𝑡 = 𝑢4 − 𝑢1 = 𝐶𝑣 𝑇4 − 𝑇1
𝑤𝑛𝑒𝑡
𝑞𝑜𝑢𝑡
𝑇4 − 𝑇1
1
𝑟𝑐𝑘 − 1
𝜂𝑡ℎ,𝐷𝑖𝑒𝑠𝑒𝑙 =
=1−
=1−
= 1 − 𝑘−1
𝑞𝑖𝑛
𝑞𝑖𝑛
𝑘 𝑇3 − 𝑇2
𝑟
𝑘 𝑟𝑐 − 1
𝑉1
𝑉3
𝑟=
𝑟𝑐 =
𝑉2
𝑉2
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• ηth,Otto > ηth, Diesel with the same
compression ratio for both
cycles.
• The compression ratio is
usually higher for diesel
engines by a factor of ~ 2.
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Diesel cycle
• In diesel cycle, compression ratio must be higher (e.g. 18:1 for diesel
cycle vs. 9:1 for Otto cycle) for the auto-ignition.
• Improved efficiency, but bigger and heavier engine block required.
• Fuel mix is leaner, i.e.  < 1.0.
• Low CO and HC, but high NOx.
• Lots of particles including soot.
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Actual engine efficiency (typically 25% to 30%), is much lower than the ideal
thermal efficiency, due to the following factors:
• Deviation of actual cycle from the idealize Otto or Diesel cycle
• Air is not a perfect gas
• Friction losses
• Heat loss
• Combustion is often incomplete
 In a combustion engine approximately 30%–40% of the fuel energy is
converted to useful work. The other 60%–70% is usually lost in the form of
heat to:

Cooling fluid

Exhaust system

Radiation & friction
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Indicated vs. brake power
nR is the number of crank revolutions for each power stroke.
For four stroke cycles, nR = 2
For two stroke cycles, nR = 1
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Important Engine Characteristics
𝑚𝑒𝑝 =
𝑤𝑛𝑒𝑡
𝑣1 − 𝑣2
• MEP is a theoretical constant pressure that can produce same net work as
the actual net work in the cycle if acted on the piston during the power
stroke.
• MEP is used to compare two engines with the same engine size, i.e the
power is higher for the engine with higher MEP.
• Power is Work/Unit Time.
• Rated horsepower (rhp) is approximately 80% of the full engine power.
Engines should not be run at full load for very long time to prevent failure.
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Important Engine Characteristics
• The engine’s maximum
performance (Pmax and Tmax)
over its full operating speed
range.
• The engine’s fuel consumption
within this operating range and
the cost of the required fuel.
• The engine’s noise and air
pollutant emissions within this
operating range.
𝑠𝑓𝑐 =
𝑚𝑓
𝑚ሶ 𝑓
=
𝑤𝑛𝑒𝑡
𝑃
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Important Engine Characteristics
• Maximum rated power . The highest
power an engine is allowed to
develop for short periods of operation.
• Normal rated power . The highest
power an engine is allowed to
develop in continuous operation.
• Rated speed . The crankshaft
rotational speed at which rated power
is developed.
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Important Engine Characteristics
𝑃𝑖,𝑚𝑎𝑥
𝑣𝑑
Specific Power
𝑃𝑠𝑝 =
Specific Weight
𝑊𝑠𝑝 =
𝑒𝑛𝑔𝑖𝑛𝑒 𝑤𝑒𝑖𝑔ℎ𝑡
𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑤𝑒𝑟
Specific Volume
𝑉𝑠𝑝 =
𝑒𝑛𝑔𝑖𝑛𝑒 𝑣𝑜𝑙𝑢𝑚𝑒
𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑤𝑒𝑟
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Combustion in engines
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Compositions of dry air
Actual air contains some water vapor depending on the temperature,
and degree of saturation. Typically the proportion of water vapor by
mass is about 1%, but it can rise to about 4% under extreme conditions.
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Combustion Reactions
The maximum amount of chemical energy released from a fuel can be
released at stoichiometric condition. If sufficient oxygen is available, a
hydrocarbon fuel can be completely oxidized. The carbon in the fuel is then
converted to carbon dioxide, and the hydrogen to water.
Stoichiometric combustion reaction
Lean combustion reaction
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Where:
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The first law of thermodynamics
For a constant pressure process:
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- Heating value is the heat of reaction for one unit of fuel
- Heating value is calculated using 25°C for both the reactants and the products
- Higher heating value is used when water in the exhaust products is in the liquid
state
- Lower heating value is used when water in the products is vapor
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Adiabatic flame temperature
Adiabatic flame temperature is the maximum temperature that the
flame can reach to.
Open system
Closed system
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Chemical Equilibrium
Where:
Ni: Number of moles at equilibrium
Nt: Total number of moles at equilibrium
P: Total pressure in units of bar
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Equivalence Ratio Determination from Exhaust Gas Constituents
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Combustion in SI engines
The volume fraction curves rise more
rapidly because:
- The density of unburbed mixture is
higher than the density of burned gases.
- There is some unburned mixture behind
the flame.
The combustion process can be divided
into 4 different phases:
-
Spark ignition
Early flame development
Flame propagation
Flame termination
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- If the start of the combustion is advanced, the compression work
increases.
- If the start of the combustion is delayed, the peak cylinder pressure
occurs later.
- The optimum timing which gives maximum brake torque occurs when
magnitude of these two opposing trends offset each other.
- With optimum spark setting, max pressure occurs at about 15 degrees
CA after TDC (10 - 15), half the charge is burned at about 10 degrees
CA after TDC.
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Factors Influencing Combustion
- Engine speed
- Equivalence ratio
- Residual gas fraction
- Induction pressure
- Compression ratio
- Combustion chamber design
- Spark advance
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Self-Ignition Temperature
The temperature that the mixture can self-ignite even without a
spark plug or other external igniters.
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Cyclic Variations in Combustion
The coefficient of variation (COV) in indicated mean effective
pressure
vehicle drivability problems usually result when COVimep exceeds about
10 percent.
COV increases by leaning the mixture.
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Combustion in CI engines
Start of injection
Start of combustion
End of injection
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The combustion process in compression ignition engines includes the
following stages:
Ignition delay - The fuel vaporizes and mixes with the combustion air.
Premixed combustion phase – combustion of the fuel which is mixed with
high temperature air. This phase only lasts a few crank angles.
Mixing controlled combustion phase –the burning rate is controlled by the rate
at which mixture becomes available for burning.
Late combustion phase – the last phase of combustion where very small
amount of heat is released.
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Induction system
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Induction system
- The intake system consists of an intake manifold, a throttle, intake valves,
and either fuel injectors or a carburetor to add fuel.
- The intake manifold is a system to deliver air to the engine through pipes
to each cylinder, called runners.
- The inside diameter of the runners must be large enough to reduce the flow
resistance and increase the volumetric efficiency of the engine.
- On the other hand, the diameter must be small enough to increase the air
velocity and turbulence, which is necessary to carry fuel droplets and also to
increases evaporation and air-fuel mixing.
- The length of a runner and its diameter should be sized together to equalize
the amount of air-fuel mixture that is delivered to each separate cylinder.
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- To minimize flow resistance, runners should have no sharp bends, and the
interior wall surface should be smooth with no protrusions such as the edge of
a gasket.
- Some intake manifolds are heated to accelerate the evaporation of the fuel
droplets in the air-fuel mixture flow.
- Air flow rate through the intake manifold is controlled by a throttle plate
usually located at the upstream end.
- By adding the fuel further upstream the manifold there is more time to
evaporate the fuel droplets and have a better mixing. However, this can also
reduce engine volumetric efficiency as a results of displacing incoming air
with fuel vapor.
- Early fuel addition also makes it more difficult to get good cylinder-tocylinder AF consistency because of the asymmetry of the manifold and
different lengths of the runners.
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- Gasoline components evaporate at different temperatures and at different
rates. Therefore, the composition of vapor in the air flow might not be exactly
the same as that of the fuel droplets carried by the air
- The air-fuel mixture which is then delivered to each cylinder can be quite
different, both in composition and in air-fuel ratio.
- This effect can be reduced or eliminated by using multipoint port fuel
injection, with each cylinder receiving its own individual fuel input.
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Volumetric efficiency
𝑚𝑎
𝜂𝑣 =
𝜌𝑎,𝑖 𝑉𝑑
where 𝑚𝑎 is the mass of air per cycles, 𝜌𝑎,𝑖 is the density of air and 𝑉𝑑 is the
engine volume.
- Typical maximum volumetric efficiency for naturally aspirated SI engines is
80-90%
- The volumetric efficiency for CI engines is higher than SI engines.
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There will be a certain engine speed at which the volumetric efficiency
is maximum.
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The fuel induction systems are classified in 4 categories:
- Carburetor
- Throttle body Fuel Injection System
- Multi Point Fuel Injection System
- Direct Gasoline Injection System
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Throttle body Fuel
Injection System
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Multi Point Fuel
Injection System
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Direct Gasoline
Injection System
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There are several parameters that affect the engine volumetric
efficiency.
Fuel
In a naturally aspirated engine, volumetric efficiency in never 100% because
fuel is also being added and the volume of fuel vapor will displace some
incoming air.
If the fuel vaporize later in the intake system, the volumetric efficiency is
higher. On the other hand, the earlier that fuel vaporizes, the better are the
mixing process and cylinder-to-cylinder distribution consistency.
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Heat Transfer-High Temperature
Intake system is hotter than the surrounding air temperature and will
consequently heat the incoming air. This reduces the density of the air and
the volumetric efficiency of the engine.
At lower engine speeds, the air flow rate is slower and the air remains in the
intake system for a longer time. Therefore, the intake air gets even hotter at
low speeds, which lowers the volumetric efficiency.
Valve Overlap and Intake and exhaust valve location
When both intake and exhaust valves are open at the same time, some
exhaust gas can get pushed through the open intake valve back into the
intake system
Engine compression ratio
Compression ratio can increase the effects of valve overlap
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Fluid Friction Losses
The viscous flow friction that affects the air as it passes through the air filter,
throttle plate, intake manifold, and intake valve reduces the volumetric
efficiency of the engine intake system.
Viscous drag, which causes the pressure loss, is a function of the square of
flow velocity. This results in decreasing the efficiency on the high-speed end.
Smooth walls in the intake manifold, the avoidance of sharp corners and
bends contribute to decreasing intake pressure loss.
Choked Flow
As air flow is increased, it eventually reaches sonic velocity at some point in
the system. This choked flow condition is the maximum flow rate that can be
produced in the intake system.
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Closing Intake Valve After BDC
This potentially can increase or decrease the volumetric efficiency depending
on the pressure inside the combustion chamber.
Exhaust Residual
During the exhaust stroke, not all of the exhaust gases get pushed out of the
cylinder by the piston, a small residual being trapped in the clearance volume.
EGR
In some engines, some exhaust gas is recycled (EGR) into the intake system
to dilute the incoming air. This exhaust gas can displace some incoming air
and also heats the incoming air and lowers its density and the volumetric
efficiency of the engine
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Intake Valve
The distance which a valve opens is called valve lift and is generally on the
order of a few millimeters to more than a centimeter.
𝑙𝑚𝑎𝑥 < 𝑑𝑣 /4
where 𝑙𝑚𝑎𝑥 is valve lift when the
valve is fully open;
𝑑𝑣 is the valve diameter.
Valve discharge coefficient
CDv=Aact/Apass
where Apass is πdvl
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- Two or three smaller intake valves give more flow area and less flow
resistance than one larger valve.
- Moreover, they can be better fit into a given cylinder head size with enough
clearance to maintain the required structural strength.
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- Intake valves normally start to
open somewhere between
10° and 25° bTDC and should
be totally open by TDC to get
maximum flow during the
intake stroke.
- Intake valves normally finish
closing about 40°-50° aBDC
for engines operating on an
Otto cycle.
- At different engine speeds,
the valve needs to open and
close at different times.
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- Variable valve timing systems are being developed for automobile
engines.
- VVT allows for changing valve lift and gives much faster opening and
closing times in comparison with camshaft system.
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Superchargers and Turbochargers
- Superchargers and turbochargers are compressors mounted in the intake
system and used to raise the pressure of the incoming air.
- Superchargers are mechanically driven directly off the engine crankshaft.
- They are running at speeds about the same as engine speed.
- They increase the engine load which is disadvantage of superchargers
compared to turbochargers.
- The advantage of the superchargers is their quick response to throttle
changes.
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𝑊𝑠𝑐 = 𝑚𝑎 (ℎ𝑜𝑢𝑡 -ℎ𝑖𝑛 )=𝑚𝑎 𝑐𝑝 (𝑇𝑜𝑢𝑡 -𝑇𝑖𝑛 )
where:
𝑊𝑠𝑐 : power needed to drive the supercharger
𝑚𝑎 : mass flow rate of air into the engine
𝑐𝑝 : specific heat of air
ℎ : specific enthalpy
𝑇: Temperature
Assumptions:
compressor heat transfer,
kinetic energy, and
potential energy are negligible.
𝑊𝑠𝑐,𝑎𝑐𝑡𝑢𝑎𝑙 = 𝑊𝑠𝑐,𝑖𝑠𝑒𝑛 /𝜂𝑠
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Turbochargers
- The compressor of a turbocharger is powered by a turbine mounted in
the exhaust flow of the engine
- The advantage of this system is that only waste energy is used to drive
the compressor not the useful engine work.
- The turbine in the exhaust flow causes a more restricted flow, resulting in
a slightly higher pressure at the cylinder exhaust port and consequently
slightly lower engine power.
- The disadvantage of turbochargers is turbo lag, which occurs with a
sudden throttle change. It takes several engine revolutions to change the
exhaust flow rate and to speed up the rotor of the turbine.
- This effect can be reduced by using lightweight ceramic rotors with very
little mass inertia
- Most turbochargers and superchargers, are equipped with an aftercooler
to lower the compressed air temperature.
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𝜂𝑠,𝑐𝑜𝑚𝑝 =
𝑊𝑠𝑐,𝑖𝑠𝑒𝑛
𝑊𝑠𝑐,𝑎𝑐𝑡
𝑊
𝜂𝑠,𝑡𝑢𝑟𝑏 = 𝑊 𝑠𝑐,𝑎𝑐𝑡 =
𝑠𝑐,𝑖𝑠𝑒𝑛
𝑚𝑎 𝑐𝑝 (𝑇1 −𝑇2𝐴 )
𝑚𝑎 𝑐𝑝 (𝑇1 −𝑇2𝑠 )
𝑊
𝜂𝑚 = 𝑊𝑐,𝑎𝑐𝑡
𝑡,𝑎𝑐𝑡
𝜂𝑡𝑢𝑟𝑏𝑜 = 𝜂𝑠,𝑐𝑜𝑚𝑝 × 𝜂𝑠,𝑡𝑢𝑟𝑏 × 𝜂𝑚
The overall efficiency of the turbochargers range from 0.7-0.9
73
Stratified charge engines
- A rich mixture that ignites quickly is desired around the spark plug, while
the major volume of the combustion chamber is filled with a very lean
mixture for better fuel economy.
- Combinations of multiple valves and multiple fuel injectors, along with
flexible valve and injection timing, are used to accomplish the desired
results.
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INTAKE-GENERATED FLOWS
- Swirl flow: Organized rotation of the charge about the cylinder axis.
- Tumble flow: Charge rotation about an axis orthogonal to the cylinder axis.
PISTON-GENERATED FLOWS
-Squish
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INTAKE-GENERATED FLOWS
76
Swirl Generation during Induction
Different types of swirlgenerating inlet ports: (a)
deflector
wall; (b) directed; (c) shallow
ramp helical; (d) steep ramp
helical
77
Tumble flow
- Tumble is set up, usually in pent-roof combustion chambers with four valves (two
intake valves) with inclined valve stems, by positioning the intake ports to bring the
entering airflow into the cylinder through the upper portion of the open area
between the valve head and valve seat.
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Squish flow
- Squish is the name given to the radial or transverse gas motion that occurs toward
the end of the compression stroke (and early part of the expansion stroke) when a
portion of the piston face and cylinder head approach (or separate from) each other
closely.
79
PRECHAMBER ENGINE FLOWS
- Historically, small high-speed diesel engines used an auxiliary combustion
chamber, or pre-chamber, to achieve fast fuel-air mixing rates.
- The two most common designs of auxiliary chamber are: the swirl
chamber, where the flow through the passageway enters the chamber
tangentially producing rapid rotation within the chamber, and the prechamber
with one or more connecting orifices designed to produce a highly turbulent
flow but no ordered motion within the chamber
80
CREVICE FLOWS
- The engine combustion chamber is connected to several small volumes
usually called crevices because of their narrow entrances. Gas flows into
and out of these volumes during the engine operating cycle as the cylinder
pressure and volume change.
81
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Engine cooling system
- About 35% percent of the total chemical energy is converted to useful
crankshaft work.
- About 30% of the fuel energy is carried away from the engine in the
exhaust flow in the form of enthalpy and chemical energy.
- Temperatures within the combustion chamber of an engine reach values
on the order of 2700 K and up.
- Removing the heat from the engine is very important to keep the engine
away from thermal failure.
- On the other hand, it is desirable to operate an engine as hot as possible
to maximize thermal efficiency.
Two types of cooling systems:
- Water-cooled systems
- Air-cooled systems
84
The amount of energy available for use in an engine is:
𝑊ሶ 𝑓 = 𝑚ሶ 𝑓 𝑄𝐻𝑉
Where:
𝑚ሶ 𝑓
fuel flow rate
𝑄𝐻𝑉
heating value of the fuel
85
About 35% percent of
the total chemical energy
is converted to useful
crankshaft work. The rest
of the energy can be
divided into heat losses,
parasitic loads, and what
is lost in the exhaust
flow.
86
For any engine:
𝑃𝑜𝑤𝑒𝑟 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 = 𝑊ሶ 𝑠ℎ𝑎𝑓𝑡 + 𝑄ሶ 𝑒𝑥ℎ𝑎𝑢𝑠𝑡 + 𝑄ሶ 𝑙𝑜𝑠𝑠 + 𝑊ሶ 𝑎𝑐𝑐
Where:
𝑊ሶ 𝑠ℎ𝑎𝑓𝑡 =brake output power (25-40%)
𝑄ሶ 𝑒𝑥ℎ𝑎𝑢𝑠𝑡 =energy lost in the exhaust flow (20-45%)
𝑄ሶ 𝑙𝑜𝑠𝑠 =all other energy lost to the surroundings by heat transfer
(10-35%)
𝑊ሶ 𝑎𝑐𝑐 =power to run engine accessories
87
For many engines, 𝑄ሶ 𝑙𝑜𝑠𝑠 can be divided to:
𝑄ሶ 𝑙𝑜𝑠𝑠 = 𝑄ሶ 𝑐𝑜𝑜𝑙𝑎𝑛𝑡 + 𝑄ሶ 𝑜𝑖𝑙 + 𝑄ሶ 𝑎𝑚𝑏𝑖𝑒𝑛𝑡
Where:
𝑄ሶ 𝑐𝑜𝑜𝑙𝑎𝑛𝑡 = (10-30%)
𝑄ሶ 𝑜𝑖𝑙 = (5-15%)
𝑄ሶ 𝑎𝑚𝑏𝑖𝑒𝑛𝑡 = (2-10%)
𝑊ሶ 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 ≈10%
88
Temperature distribution for
an IC engine at steady state
operating condition.
89
- The exhaust valve and port operate hot because they are located
in the steady flow of hot exhaust gases and create a difficulty in
cooling. The valve mechanism and connecting exhaust manifold
make it very difficult to route coolant or allow a finned surface to
give effective cooling.
- The piston face is difficult to cool because it is separated from
the water jacket or outer finned cooling surfaces.
90
Engine Warmup
- As a cold engine heats up to steady-state temperature, thermal
expansion occurs in all components.
- In cold weather, the startup time to reach steady-state
conditions can be as high as 20-30 minutes.
- Fairly normal operating conditions may be experienced within a
few minutes, but it can take as long as an hour to reach
optimum fuel consumption rates.
- Driving before total engine warmup causes some loss of power
and fuel economy.
91
92
Heat transfer in intake system
The walls of the intake manifold are hotter than the intake air, therefore:
𝑄ሶ 𝑐𝑜𝑛𝑣 = ℎ𝐴 𝑇𝑤𝑎𝑙𝑙 − 𝑇𝑔𝑎𝑠
where:
𝑇= temperature
ℎ= convection heat transfer coefficient
𝐴= inside surface area of intake manifold
Some systems have special localized hot surfaces, called hot spots, in
optimum locations, such as immediately after fuel addition or at a place
where maximum convection occurs.
93
Heat transfer in intake system
- Convective heating in the intake manifold helps to evaporate the fuel
to get better homogenous mixture.
- As fuel vaporizes in the intake manifold, it cools the surrounding flow
by evaporative cooling.
- This helps to cool the cylinder walls and to keep them from
overheating.
- The heating of inlet air in SI engines should be limited to keep the
maximum temperature of the mixture at the end of the compression
stroke lower than the self-ignition temperature.
94
Heat transfer in intake system
- The convective heat transfer for multipoint port fuel injector engines
is less critical as a result of finer fuel droplets, and higher
temperature around the intake valve to assure necessary fuel
evaporation.
- Multipoint fuel injectors often spray the fuel directly onto the back of
the intake valve face. This speeds evaporation as well as cools the
intake valve.
- The intake air temperature is higher in the engines equipped with
superchargers or turbochargers so many of these systems are
equipped with aftercooling, which lowers the temperature.
95
Heat transfer in combustion chambers
- All three primary modes of heat transfer (conduction, convection,
and radiation) play important roles for heat transfer inside
combustion chamber.
- The air-fuel mixture entering a cylinder during the intake stroke may
be hotter or cooler than the cylinder walls, so the heat transfer is
possible in either direction.
- During the compression stroke, the temperature of the gas
increases, and there is a convective heat transfer to the cylinder
walls at the end of the compression stroke.
96
Heat transfer in combustion chambers
- During combustion peak gas temperatures on the order of 3000 K
occur within the cylinders, and effective heat transfer is needed to
keep the cylinder walls from overheating.
- Convection and conduction are the main heat transfer modes to
remove energy from the combustion chamber and keep the cylinder
walls from melting in the compression stroke.
97
98
Heat transfer in combustion chambers
Heat transfer through a cylinder wall per unit surface area will be:
𝑄ሶ
1
∆𝑥
1
𝑞ሶ = = 𝑇𝑔 − 𝑇𝑐 /[
+
+
]
𝐴
ℎ𝑔
𝑘
ℎ𝑐
where:
𝑇𝑔 = gas temperature in the combustion chamber
𝑇𝑐 = coolant temperature
ℎ𝑔 = convection heat transfer coefficient on the gas side
ℎ𝑐 = convection heat transfer coefficient on the coolant side
∆𝑥= thickness of the combustion chamber wall
𝑘= thermal conductivity of the cylinder wall
99
Heat transfer in combustion chambers
The Reynolds number for the gas inside the combustion chamber can
be defined as:
𝑅𝑒 = [ 𝑚ሶ 𝑎 + 𝑚ሶ 𝑓 𝐵/(𝐴𝑝 𝜇𝑔 )
where
𝑚ሶ 𝑎 = mass flow rate of air into the cylinder
𝑚ሶ 𝑓 = mass flow rate of fuel into the cylinder
𝐴𝑝 = area of piston face
𝜇𝑔 = dynamic viscosity of gas in the cylinder
𝑁𝑢 = ℎ𝑔 𝐵/𝑘𝑔 = 𝐶1 (𝑅𝑒)𝐶2
100
Heat transfer in combustion chambers
The Reynolds number for the gas inside the combustion chamber can
be defined as:
𝑁𝑢 = ℎ𝑔 𝐵/𝑘𝑔 = 𝐶1 (𝑅𝑒)𝐶2
where
𝐶1 and 𝐶2 = constants
𝑘𝑔 = thermal conductivity of cylinder gas
ℎ𝑔 = average value of the convection heat transfer coefficient
101
Heat transfer in combustion chambers
Radiation heat transfer between cylinder gas and combustion chamber
walls is:
1 − 𝜖𝑔
𝑄ሶ
1
1 − 𝜖𝑤
4
4
𝑞ሶ = = 𝜎 𝑇𝑔 − 𝑇𝑤 /{
+
+
}
𝐴
𝜖𝑔
𝐹1−2
𝜖𝑤
where
𝑇𝑔 = gas temperature
𝑇𝑤 = wall temperature
𝜎= Stefan-Boltzmann constant
𝜖𝑔 = emissivity of gas
𝜖𝑔 = emissivity of wall
𝐹1−2= view factor between gas and wall
102
Heat transfer in combustion chambers
- Radiation heat transfer is about 10% of the total heat transfer in
combustion engines.
- N2 and O2 radiate very little and CO2 and H2O do contribute more to
radiation heat transfer.
- The solid carbon particles that are generated in the combustion
products of a CI engine are good radiators, and radiation heat
transfer to the walls in these engines is in the range of 20-35% of the
total.
103
Local heat flux
variation at one
location in a
single cylinder
engine.
104
Local heat flux
variation at
three different
locations in a
single cylinder
engine.
105
- To avoid thermal breakdown of the lubricating oil, it is necessary to
keep the cylinder wall temperatures from exceeding 180°-200°C.
- As an engine ages, deposits slowly build up on the walls of the
cylinders. These are due to impurities in the air and fuel, imperfect
combustion, and lubricating oil in the combustion chamber.
- These deposits create a thermal resistance and cause higher wall
temperatures. Excessive wall deposits also slightly decrease the
clearance volume of the cylinder and cause a rise in the
compression ratio.
106
Heat transfer in exhaust system
- To calculate heat losses in an exhaust pipe, normal internal
convection flow models can be used with one major modification:
Due to the pulsing cyclic flow, the Nusselt number is about 50%
higher than would be predicted for the same mass flow in the same
pipe at steady flow conditions.
- Nu=0.023Re0.8Pr0.3
- Pseudo-steady-state exhaust temperatures of SI engines are
generally in the range of 400°-600°C, with extremes of 300°-900°C.
Exhaust temperatures of CI engines are lower due to their greater
expansion ratio and are generally in the range of 200°-500°C.
107
Heat transfer in exhaust system
To calculate heat
losses in an exhaust
pipe, normal internal
convection flow models
can be used with one
major modification:
Due to the pulsing
cyclic flow, the Nusselt
number is about 50%
higher than would be
predicted for the same
mass flow in the same
pipe at steady flow
conditions
108
EFFECT OF ENGINE OPERATING VARIABLES ON HEAT TRANSFER
Engine size:
- If two geometrically similar engines of different size (displacement)
are run at the same speed, and all other variables (temperature, AF,
fuel, etc.) are kept as close to the same as possible, the larger
engine will have a greater absolute heat loss but will be more
thermal efficient.
Engine Speed:
- As engine speed is increased, gas flow velocity into and out of the
engine goes up, with a resulting rise in turbulence and convection heat
transfer coefficients. This increases heat transfer occurring during the
intake and exhaust strokes and even during the early part of the
compression stroke.
109
EFFECT OF ENGINE OPERATING VARIABLES ON HEAT TRANSFER
110
EFFECT OF ENGINE OPERATING VARIABLES ON HEAT TRANSFER
Engine load:
- As the load on an engine is increased (going uphill, pulling a trailer),
the throttle must be further opened to keep the engine speed
constant. This causes less pressure drop across the throttle and
higher pressure and density in the intake system. Therefore, mass
flow rate of air and fuel goes up with load at a given engine speed.
Heat transfer within the engine also goes up.
Spark Timing
- More power and higher temperatures are generated when the spark
setting is set to give maximum pressure and temperature at about
10° aTDC. With spark timing set either too early or too late,
combustion efficiency and average temperatures will be lower. These
lower temperatures will give less peak heat loss.
111
EFFECT OF ENGINE OPERATING VARIABLES ON HEAT TRANSFER
Fuel Equivalence Ratio:
- In an SI engine, maximum power is obtained with an equivalence
ratio of about Φ = 1.1. This is also when the greatest heat losses will
occur, with less losses when the engine runs either leaner or richer.
Evaporative Cooling
- As fuel is vaporized during intake and start of compression,
evaporative cooling lowers the intake temperature and raises intake
density. This increases the volumetric efficiency of the engine. As a
result, the heat transfer increase or decrease.
112
EFFECT OF ENGINE OPERATING VARIABLES ON HEAT TRANSFER
Inlet Air Temperature:
- Increasing inlet air temperature to an engine results in a temperature
increase over the entire cycle, with a resulting increase in heat
losses. 100°C increase in inlet temperature will give a 10-15%
increase in heat losses.
Coolant Temperature
- Increasing the coolant temperature of an engine (hotter thermostat)
results in higher temperatures of all cooled parts.
Engine Materials
- Different materials in the manufacture of cylinder and piston
components result in different operating temperatures.
Compression Ratio
- Changing the compression ratio of an engine changes the heat
transfer to the coolant very little.
113
Engine lubrication system
Lubricating oil adheres to the solid surfaces, and when one surface moves
relative to the other, oil is dragged along with the surface. The oil holds the
surfaces apart and one surface hydraulically floats on the other surface. The
only resistance to relative motion is the shearing of fluid layers between the
surfaces, which is orders of magnitude less than that of dry surface motion.
114
Three important characteristics are needed in a lubricating fluid:
1. It must adhere to the solid surfaces.
2. It must resist being squeezed out from between the surfaces, even under
the extreme forces experienced in an engine between some components.
3. It should not require excessive force to shear adjacent liquid layers. The
property that determines this is called viscosity.
- When an engine is not in operation, gravity pulls the shaft in any bearing
(crankshaft, connecting rod, etc.) down and squeezes out the oil film
between the two surfaces.
- In operation, the combination of a rotating shaft, viscous effects, and
dynamic forces from various directions results in hydraulic floating of the
shaft offset slightly from centre.
- The position and the thickness of the minimum oil film in the bearing will
depend on the tolerances, load, speed and oil viscosity.
115
The thickness of the minimum oil film in the bearing is on the order of 2 μm for
the main bearings in an engine
116
Engine Friction
Friction can be classified as a loss using power terms:
117
Engine Friction
Using frictional work or frictional power and rearranging these become:
Friction mep can quite accurately be related to engine speed by the
empirical equation.
118
- The magnitude of friction mean effective pressure (or friction power, or
friction work) is on the order of 10% of net indicated mean effective
pressure at WOT.
- This increases to 100% at idle, when no brake power is taken off the
crankshaft.
- A turbocharged engine will generally have a lower percent friction loss.
This is due to the greater brake output realized, while absolute friction
remains about the same.
- Most power lost to friction ends up heating the engine oil and coolant.
119
One of the best ways to quantify the friction loss in an engine is to motor the
engine (i.e., drive an unfired engine with an external electric motor connected
to the crankshaft).
= 0 since there is no combustion occurring in a motored engine.
~ 0 if the motored engine is operated at wide open throttle (WOT).
Therefore:
Thus, by measuring the electric power input to the motor driving the engine, a
good approximation is obtained of the friction power lost in normal engine
operation.
Note that all conditions of the motored engine be kept as close as possible to
the conditions of a fired engine, especially temperature.
120
- The magnitude of the friction forces is about the same for the intake,
compression, and exhaust strokes. It is much higher during the expansion
stroke, reflecting the higher pressure and forces that occur at that time.
- The piston assemblies of most engines contribute about half of the total
friction and can contribute as much as 75% at light loads.
- The valve train of an engine contributes about 25% of total friction,
crankshaft bearings about 10% of total, and engine-driven accessories
about 15% of total.
121
Forces on a piston
where Φ= angle between the connecting
rod and centreline of the cylinder.
m= mass of the piston
dUp/dt= acceleration of the piston
Fr= force of the connecting rod
P= pressure in the combustion chamber
B= bore
Ff= friction force between the piston and
cylinder walls
122
Forces on a piston
There is no motion in the Y direction, so a force balance gives:
Combining these two equations gives the side thrust force on the piston as:
- During the power and intake strokes, the side thrust force will be on one
side of the cylinder (the left side for an engine rotating as shown in
previous slide) in the plane of the connecting rod. This is called the major
thrust side of the cylinder because of the high pressure during the power
stroke. This high pressure causes a strong reaction force in the
connecting rod, which in turn causes a large side thrust reaction force.
- During the exhaust and compression strokes, the connecting rod is on the
other side of the crankshaft and the resulting side thrust reaction force is
on the other side of the cylinder (the right side in figure shown in previous
slide). This is called the minor thrust side due to the lower pressures and
forces involved and is again in the plane of the connecting rod.
123
- To reduce friction, modern engines use pistons that have less mass. Less
mass lowers the piston inertia and reduces the acceleration term in
friction equation (
)
- In some engines, the wrist pin is offset from center by 1 or 2 mm towards
the minor thrust side of the piston. This reduces the side thrust force and
resulting wear on the major thrust side.
- The friction can be reduced by having a shorter stroke. However, for a
given displacement this requires a larger bore, which results in greater
heat losses due to the larger cylinder surface area.
124
125
Engine emission
- Gas phase emissions
- Particulate emissions
- Solid particles
- Volatile and semi-volatile particles
- Internally mixed semi-volatile particles
- Externally mixed semi-volatile particles
126
- To classify the particles
based on their size,
equivalent diameters should
be defined since, in general,
soot particles are not
spherical and there is not an
exact diameter associated
with them.
- The mobility and
aerodynamic diameters are
the most common
equivalent diameters which
are widely used in particle
measurement instruments.
127
- The mobility equivalent diameter is the diameter of a sphere with the
same mobility as the particle in question.
- The aerodynamic equivalent diameter is the diameter of a spherical
particle with a density of 1000 kg/m3 which has the same terminal velocity
as the particle of interest.
- Semi-volatile compounds are organic compounds which can vaporize at
-
temperatures above room temperature.
Three distinct modes can be present in the particle size distribution.
The nucleation mode mostly consists of semi-volatile particles.
Soot particles usually dominate the accumulation and the coarse modes.
Particles in the coarse mode are usually micron size particles which are
negligible in number but their mass can be significant since their diameter
is relatively large, and the mass is a function of particle diameter cubed.
128
129
Particle terminal velocity can be calculated from:
𝐷2 𝑝 𝜌𝑝 𝑔𝐶𝑐
𝑣𝑡 =
18𝜇
where 𝑣𝑡 is the particle terminal velocity
Dp is the particle diameter
ρp is the particle density
𝐶𝑐 is slip correction factor
μ is the gas viscosity
130
Aerosol measurement and size distribution
1. Concentration
2. Size distribution
Particle mass, surface
area or number per unit
volume
Concentration versus
particle size
131
Particles are assigned to bins according to particle diameter
132
Conversion of a discrete particle size distribution to a continuous distribution
133
134
135
Mode < Median < Mean
Arithmetic mean
da=(Σnidi)/N = (summation of all areas of the bars) / (total number
of particles)
CMD (count median
diameter): 50% of all
particles are less than
Median diameter
CMD
136
Gas-phase emissions
HYDROCARBONS (HC)
- Exhaust gases leaving the combustion chamber of an SI engine
contain up to 6000 ppm of hydrocarbon components, the
equivalent of 1-1.5 % of the fuel.
- About 40% of this is unburned gasoline fuel components.
- The other 60% consists of partially reacted components that were
not present in the original fuel.
137
Causes of HC Emissions
Nonstoichiometric Air-Fuel Ratio.
With a fuel-rich mixture there is not enough oxygen to react with all
the carbon, resulting in high levels of HC and CO in the exhaust
products. This is particularly true in engine startup, when the air-fuel
mixture is purposely made very rich. It is also true to a lesser extent
during rapid acceleration under load.
If AF is too lean poorer combustion occurs, again resulting in HC
emissions. The extreme of poor combustion for a cycle is total
misfire. This occurs more often as AF is made more lean.
138
Emissions from an SI engine as a
function of equivalence ratio. A fuel rich
air-fuel ratio does not have enough
oxygen to react with all the carbon and
hydrogen, and both HC and CO
emissions increase. HC emissions also
increase at very lean mixtures due to
poor combustion and misfires. The
generation of nitrogen oxide emissions
is a function of the combustion
temperature, being greatest near
stoichiometric
conditions
when
temperatures are the highest. Peak
NOx emissions occur at slightly lean
conditions, where the combustion
temperature is high and there is an
excess of oxygen to react with the
nitrogen.
139
Incomplete Combustion
Even when the fuel and air entering an engine are at the ideal
stoichiometric mixture, perfect combustion does not occur and some
HC ends up in the exhaust. There are several causes of this.
Incomplete mixing of the air and fuel results in some fuel particles not
finding oxygen to react with. Flame quenching at the walls leaves a
small volume of unreacted air-and-fuel mixture. The thickness of this
unburned layer is on the order of tenths of a mm. Some of this
mixture, near the wall that does not originally get burned as the flame
front passes, will burn later in the combustion process as additional
mixing occurs due to swirl and turbulence.
Another cause of flame quenching is the expansion which occurs
during combustion and power stroke. As the piston moves away from
TDC, expansion of the gases lowers both temperature and pressure
within the cylinder. This slows combustion and finally quenches the
flame somewhere late in the power stroke. This leaves some fuel
particles unreacted.
140
Residual gases
High exhaust residual causes poor combustion and a greater
likelihood of expansion quenching. This is experienced at low load
and idle conditions. High levels of EGR will also cause this.
It has been found that HC emissions can be reduced if a second
spark plug is added to an engine combustion chamber. By starting
combustion at two points, the flame travel distance and total reaction
time are both reduced, and less expansion quenching results.
Crevice Volumes
During the compression stroke and early part of the combustion
process, air and fuel are compressed into the crevice volume of the
combustion chamber at high pressure. As much as 3% of the fuel in
the chamber can be forced into this crevice volume. Later in the cycle
during the expansion stroke, pressure in the cylinder is reduced
below crevice volume pressure, and reverse blowby occurs.
141
Fuel and air flow back into the combustion chamber, where most of
the mixture is consumed in the flame reaction. However, by the time
the last elements of reverse blowby flow occur, flame reaction has
been quenched and unreacted fuel particles remain in the exhaust.
Location of the spark plug relative to the top compression ring gap
will affect the amount of HC in engine exhaust, the ring gap being a
large percent of crevice volume. The farther the spark plug is from
the ring gap, the greater is the HC in the exhaust. This is because
more fuel will be forced into the gap before the flame front passes.
Crevice volume around the piston rings is greatest when the engine
is cold, due to the differences in thermal expansion of the various
materials. Up to 80% of all HC emissions can come from this source.
142
Leak past the exhaust valve
As pressure increases during compression and combustion, some
air-fuel is forced into the crevice volume around the edges of the
exhaust valve and between the valve and valve seat. A small amount
even leaks past the valve into the exhaust manifold. When the
exhaust valve opens, the air-fuel which is still in this crevice volume
gets carried into the exhaust manifold, and there is a momentary
peak in HC concentration at the start of blowdown.
Valve Overlap
During valve overlap, both the exhaust and intake valves are open,
creating a path where air-fuel intake can flow directly into the
exhaust. A well-designed engine minimizes this flow, but a small
amount can occur. The worst condition for this is at idle and low
speed, when real time of overlap is greatest.
143
Deposits on Combustion Chamber Walls.
Gas particles, including those of fuel vapor, are absorbed by the
deposits on the walls of the combustion chamber.
The amount of absorption is a function of gas pressure, so the
maximum occurs during compression and combustion. Later in the
cycle, when the exhaust valve opens and cylinder pressure is
reduced, absorption capacity of the deposit material is lowered and
gas particles are desorbed back into the cylinder. These particles,
including some HC, are then expelled from the cylinder during the
exhaust stroke. This problem is greater in engines with higher
compression ratios due to the higher pressure these engines
generate. More gas absorption occurs as pressure goes up. Clean
combustion chamber walls with minimum deposits will reduce HC
emissions in the exhaust. Most gasoline blends include additives to
reduce deposit buildup in engines.
144
Oil on Combustion Chamber Walls.
A very thin layer of oil is deposited on the cylinder walls of an engine
to provide lubrication between them and the moving piston. During
the intake and compression strokes, the incoming air and fuel
comes in contact with this oil film. In much the same way as wall
deposits, this oil film absorbs and desorbs gas particles, depending
on gas pressure. During compression and combustion, when cylinder
pressure is high, gas particles, including fuel vapor, are absorbed into
the oil film. When pressure is later reduced during expansion and
blowdown, the absorption capability of the oil is reduced and fuel
particles are desorbed back into the cylinder. Some of this fuel ends
up in the exhaust.
As an engine ages, the clearance between piston rings and cylinder
walls becomes greater, and a thicker film of oil is left on the walls.
Some of this oil film is scraped off the walls during the compression
stroke and ends up being burned during combustion. Oil is a highmolecular-weight hydrocarbon compound that does not burn as
readily as gasoline. Some of it ends up as HC emissions.
145
146
CI Engines.
Because they operate with an overall fuel-lean equivalence ratio, CI
engines have only about one-fifth the HC emissions of an SI engine.
In general, a CI engine has about a 98% combustion efficiency, with only
about 2% of the HC fuel being emissions. Some local spots in the
combustion chamber will be too lean to combust properly, and other spots
will be too rich, with not enough oxygen to consume all the fuel. Less than
total combustion can be caused by undermixing or overmixing.
A small amount of liquid fuel will be trapped on the tip of the nozzle. This
very small volume of fuel is called sac volume, its size depending on the
nozzle design. This sac volume of liquid fuel evaporates very slowly
because it is surrounded by a fuel-rich environment and, once the injector
nozzle closes, there is no pressure pushing it into the cylinder. Some of
this fuel does not evaporate until combustion has stopped, and this results
in added HC particles in the exhaust. CI engines also have HC emissions
for some of the same reasons as SI engines do (i.e., wall deposit
absorption, oil film absorption, crevice volume, etc.).
147
148
OXIDES OF NITROGEN (NOx)
- Exhaust gases of an engine can have up to 2000 ppm of oxides of
nitrogen. Most of this will be nitrogen oxide (NO), with a small amount
of nitrogen dioxide (NO2), and traces of other nitrogen-oxygen
combinations. These are all grouped together as NOx , with x
representing some suitable number.
- NOx is created mostly from nitrogen in the air.
- The higher the combustion reaction temperature, the more diatomic
nitrogen, N2, will dissociate to monatomic nitrogen, N, and the more
NOx will be formed. At low temperatures very little NOx is created.
149
OXIDES OF NITROGEN (NOx)
- Although maximum flame temperature will occur at a stoichiometric airfuel ratio, maximum NOx is formed at a slightly lean equivalence ratio.
At this condition flame temperature is still very high, and in addition,
there is an excess of oxygen that can combine with the nitrogen to form
various oxides.
- The amount of NOx generated also depends on the location within the
combustion chamber. The highest concentration is formed around the
spark plug, where the highest temperatures occur.
- Because they generally have higher compression ratios and higher
temperatures and pressure, CI engines tend to generate higher levels
of NOx.
150
Generation of NOx in an engine as
a function of combustion time
Generation of NOx in an SI engine
as a function of spark timing
151
Summary of pollutant formation mechanisms in a direct injection diesel engine during
“premixed” and “mixing-controlled” combustion phases
152
Kinetics of NO Formation
Rate constants for NO formation mechanism
153
Formation of NO2
unless the NO 2 formed in the flame is quenched by mixing with cooler fluid. This
explanation is consistent with the highest NO 2/NO ratio occurring at light load in
diesels, when cooler regions which could quench the conversion back to NO are
widespread.
154
Variation of exhaust (engine out) NO
concentration with percent recycled
exhaust gas (EGR)
Variation of exhaust (engine-out)
NO concentration with spark
retard.
155
156
sources of hydrocarbon emissions
157
EXHAUST GAS TREATMENT
Oxidizing catalysts for HC and CO,
Reducing catalysts for NO x,
Three-way catalysts for all three pollutant,
Traps or filters for particulates
In order to oxidize the hydrocarbons in the gas phase without a
catalyst, a residence time of order 50 ms and exhaust temperatures in
excess of 600°C are required. To oxidize CO, temperatures in excess
of 700°C are required.
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EXHAUST GAS TREATMENT
The temperatures required for gas- phase chemistry to effect these
changes, since the activation energy (or energy barrier) for breaking
these air pollutant molecule bonds is above the thermal energy
available at average exhaust gas temperatures. So only modest
pollutant reduction in the exhaust system occurs. The catalyst (e.g., a
collection of noble metal atoms on a high surface-area substrate)
facilitates the desired chemistry by reducing this energy barrier so
reactions can occur at significantly lower (and attainable)
temperatures. For CO and HC oxidation on the catalyst surface,
sufficient oxygen must be available. With a stoichiometric mixture,
the air (oxygen) associated with the CO and unburned HC is (on
average) available. For NO reduction in spark-ignition engines,
reducing agents in the exhaust gas (CO, H2, and HC) are present.
159
Illustration of the warm-up
behavior of a spark-ignition
engine/catalyst system. Catalyst
light off takes some 10 to 30s,
depending on conditions.
160
Catalyst Fundamentals
161
Catalyst Fundamentals
Catalysts are characterized by two basic properties: their activity and
their selectivity.
Activity relates to the degree to which the catalyst increases
the rate of the relevant chemical reactions. Normally, a higher activity—
faster reaction rate—in a given context is more desirable.
A catalyst can be selective: that is, accelerate certain specific reactions
much more than others. A catalyst’s selectivity, which is usually designed
into a catalyst through the elements incorporated in its formulation, is also
usually dependent on its temperature: it enhances specific reactions within a
temperature window. Diesel NO x catalysts are “selective” in this way.
162
Catalyst Fundamentals
Monolith design of
catalytic converter for
spark-ignition engine
emissions control
(a) Cross-section showing a
monolith channel and washcoat
(about 1 mm2 flow area); (b)
schematic of catalytically active
(Pt) sites dispersed over the
washcoat surface which is
deposited on the monolith
substrate walls
163
Catalyst Fundamentals
164
165
Emission regulations
Driving cycles
for emission
measurement
166
New European driving cycle
167
FTP 72 driving cycle
168
FTP 75 driving cycle
169
US06 driving cycle
170
California LA92 driving cycle
171
Japanese 10-15 Mode driving cycle
172
173
US-Federal emission standards for vehicles
174
PMP test procedure for particle number measurement
175
According to the PMP programme, a particle measurement system
including two particle number counters (PNC), two dilution stages and an
evaporation tube (ET), should be used to measure the number
concentration of solid particles. According to PMP, the sample is diluted at
two separate stages. The first stage of dilution is conducted with hot air
where the temperature of the primary dilution chamber is 150°C. Later, the
sample is heated to 350°C in an evaporation tube to evaporate all semivolatile materials. All particles exiting this stage are assumed to be solid
particles. There is another dilution stage to decrease the particle
concentration and also to reduce the sample temperature. Finally, a
particle counter with the detection range of >23 nm is used to count the
particles.
176
Emission simulators
Vehicle tractive power
The tractive power (Pt) is,
Pt= (FI+FAD+FRR)V
where FI, FAD and FRR are the inertia,
aerodynamic drag and rolling resistance forces, respectively; and V is the
vehicle speed. The forces are,
FI=m dV/dt
where m is the vehicle mass, and t is time;
FAD= ρ CdAV2/2
where ρ is the density of air, Cd is the drag coefficient and
A is the cross sectional area of the vehicle; and
FRR=CRRmg
where CRR is the rolling resistance coefficient and g is the
gravity.
The vehicle specific power (Psp) is defined as,
Psp=Pt/m
177
178
179
Emission measurement methods
Techniques of Emission Analysis
Technique
Gas
Typical Range
Response Time
Non-dispersive
infra-red (NDIR)
CO
0-3000 ppm
2-5 s
CO2
0-20%
2-5 s
Chemiluminescence
detector (CLD)
NOx
0-10,000 ppm
1.5-2 s
Flame ionization
detector (FID)
Total HC
0-10,000 ppm
1-2 s
Fourier transform
infra-red (FTIR)
NOx, some HC, etc.
various
5-15 s
Paramagnetic
analyzer
O2
0-25%
1-5 s
180
Fast response gas analyzers for total HC, NO/NO2, CO and CO2 have been
developed. The fast response analyzers are miniaturized developments of
the respective conventional instruments, capable of response times
measured in milliseconds and may thus be used for true transient testing as
well as for in-cylinder and exhaust manifold measurements.
Fast Response Gas Analyzers
Technique
Gas
Max Concentration
Response Time
Fast FID
Total HC
1,000,000 ppm
1 ms
Fast CLD
NO/NO2
20,000 ppm
2-10 ms
Fast NDIR
CO & CO2
250,000 ppm
8 ms
181
Non-Dispersive Infra-Red (NDIR) detectors are the industry standard method of
measuring the concentration of carbon oxides (CO & CO2).
Each constituent gas in a sample will absorb some infra red at a particular frequency.
By shining an infra-red beam through a sample cell (containing CO or CO2), and
measuring the amount of infra-red absorbed by the sample at the necessary
wavelength, a NDIR detector is able to measure the volumetric concentration of CO
or CO2 in the sample.
182
183
A chemi-luminescence detector (CLD) is the industry standard method of measuring
nitric oxide (NO) concentration.
The reaction between NO and O3 (ozone) emits light. This reaction is the basis for the
CLD in which the photons produced are detected by a photo multiplier tube (PMT).
The CLD output voltage is proportional to NO concentration.
The light-producing reaction is very rapid so careful sample handling is important in a
very rapid response instrument.
184
185
The flame ionization detector (FID) is the automotive emissions industry standard
method of measuring hydrocarbon (HC) concentration.
The sample gas is introduced into a hydrogen flame inside the FID. Any
hydrocarbons in the sample will produce ions when they are burnt. Ions are detected
using a metal collector which is biased with a high DC voltage. The current across
this collector is thus proportional to the rate of ionization which in turn depends upon
the concentration of HC in the sample gas.
The ionization process is very rapid, so the slow time response of conventional FIDs
is mainly due to sample handling. A typical slow analyzer might have a response time
of 1-2 seconds
186
187
Magnetic type measurement system : Paramagnetic system
This is one of the methods utilizing the paramagnetic property of oxygen.
When a sample gas contains oxygen, the oxygen is drawn into the magnetic field,
thereby decreasing the flow rate of auxiliary gas in stream B. The difference in flow
rates of the two streams, A and B, which is caused by the effect of flow restriction in
stream B, is proportional to the oxygen concentration of the sample gas. The flow
rates are determined by the thermistors and converted into electrical signals, the
difference of which is computed as an oxygen signal.
188
189
Particle emission measurement methods
Tapered element oscillating microbalance
A tapered element oscillating microbalance (TEOM) can be used to measure particle
mass. It consists of a filter cartridge that is placed on one end of an oscillating tube.
The frequency of the oscillating tube correlates to the mass collected on the filter.
TEOM is not sensitive enough to measure the relatively low particle mass
concentrations.
Quartz crystal microbalance
The quartz crystal microbalance (QCM) correlates the mass of the collected particles
on a quartz crystal plate to the resonant frequency of the oscillating plate. The
problem with this method is a poor relationship between the collected particles and
the natural frequency of the vibrating plate. This poor relationship is even worse when
the particles are bigger.
190
Laser-induced incandescence
Laser-induced incandescence (LII) is another useful method for measuring the mass
concentration of black carbon particles from flames and engines. The principle of the
method is to heat the particles up to 4000–4500 K using a high energy laser beam.
The intensity of the incandescence is a function of soot volume fraction and
consequently is a function of particle mass. LII does not measure the semi-volatile
particles since the laser beam evaporates them very quickly.
Photoacoustic soot sensor
Photoacoustic soot sensor is another useful method that is employed in some
commercial mass measurement devices such as micro soot sensor (MSS, Schindler
et al., 2004). In this method, particles are heated by absorbing light, and when the
light is pulsed, the particles produce acoustic waves which are measured by a
microphone. A photoacoustic soot sensor is only able to report the black carbon
portion of the particles.
191
Afterthreatments
•
After the combustion process stops, those components in the cylinder gas mixture
that have not fully burned continue to react during the expansion stroke, during
exhaust blowdown, and into the exhaust process.
•
The higher the exhaust temperature, the more these secondary reactions occur
and the lower the engine emissions.
•
Higher exhaust temperature can be caused by stoichiometric air-fuel combustion
and high engine speed.
192
Thermal Converters
- Thermal converters are high-temperature chambers through which
the exhaust gas flows. They promote oxidation of the CO and HC
which remain in the exhaust.
CO + 0.5 O2 → CO2
CxHy + (X+0.25y) O2 → X CO2 +0.5y H2O
- For the first reaction to occur at a useful rate, the temperature must
be held above 700°C.
- The second reaction needs a temperature above 600°C for at least
50 ms to substantially reduce HC.
- It is necessary for a thermal converter not only to operate at a high
temperature but to be large enough to provide adequate residence
time to promote the occurrence of these secondary reactions.
However in modern, low-profile, aerodynamic automobiles, space in
the engine compartment is very limited, and fitting in a large, usually
insulated thermal converter chamber is almost impossible.
- NOx emissions cannot be reduced with a thermal converter alone.
193