S'I' Engine Mixture Preparation

1
S.I. Engine Mixture Preparation

• Carburetion
• Perhaps soon to be obsolete?

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Mixture Requirements

• Engine induction and fuel system must prepare a
  fuel-air mixture that satisfies the requirements
  of the engine over its entire operating regime.
• Optimum air-fuel ratio for an SI engine is that
  which gives
• required power output
• with lowest fuel consumption
• consistent with smooth and reliable operation

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Mixture Requirements (Continued)

• The constraints of emissions may
• dictate a different air-fuel ratio and
• also require recycling some exhaust gas.
• (EGR)
• Relative proportions of fuel and air that give
  the above requirements depend on engine speed and
  load.
• Mixture strength is given in terms of air-fuel or
  fuel-air ratio or equivalence ratio.

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Mixture Requirements (Continued)

• Mixture requirements are different for full load
  (wide-open throttle or WOT) and for part-load
  operation.
• At full load, complete utilization of inducted
  air to obtain maximum power for a given displaced
  volume is the critical issue.
• At part-load at a given speed, efficient
  utilization of fuel is the critical issue.
• As seen in the next slide, at WOT, maximum power
  for a given volumetric efficiency is obtained at
  a mixture slightly richer than stoichiometric,
  F1.1

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Mixture Requirements (Continued)

• At part-load (or part-throttle) it is
  advantageous to dilute the fuel-air mixture with
  excess air or with recycled exhaust gas. This
  dilution improves fuel conversion efficiency for
  three reasons
• The expansion stroke work is increased for a
  given expansion ratio due to the change in
  thermodynamic properties,
• For a given mean effective pressure, the intake
  pressure increases with increasing dilution, so
  pumping work decreases,
• Heat losses to the walls are reduced because the
  burned gas temperatures are lower.
• In the absence of strict NOx emission control,
  excess air is the obvious diluent at part load
  and the engine runs lean

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Requirements with emission control

• For control of NO, HC and CO, operating the
  engine with stoichiometric mixture is
  advantageous so that a three-way catalyst can be
  used for emission control. In such a case, for
  further decrease in NO the diluent used is EGR.
• Amount used will depend on the EGR tolerance of
  the engine at a given speed and load based on the
  details of the engine combustion process.
• Increasing excess air or EGR will slow down the
  combustion process and increase combustion
  variability so as load decreases, less dilution
  must be provided and at idle, no EGR may be used
  and mixture will have to be made rich.

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What is carburetion?

• The process of formation of a combustible
  fuel-air mixture by mixing the proper amount of
  fuel with air before it is admitted into the
  engine cylinder.
• Comes from the words car and burette because
  the carburetor meters the appropriate quantity
  of liquid fuel (like a burette) and mixed it with
  air before sending the mixture into the engine
  cylinder.

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Factors affecting Carburetion

• Engine speed. In a 4-stroke engine running at
  3000 rev/min, the intake will take about 10 ms
  during which the fuel has to evaporate, mix with
  air and be inducted into the engine.
• Vaporization characteristics of the fuel. Will
  require a volatile fuel for quick evaporation and
  mixing with air.
• The temperature of the in coming air. Must be
  high enough to be able to evaporate the fuel and
  yet not too high as to reduce mass of fresh
  charge.
• Design of the carburetor. This will help in
  proper introduction of fuel into the air stream
  and provide proper distribution of the mixture to
  the various cylinders.

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Calculation of Air-fuel Ratio

• Applying the steady flow energy equation to
  sections A-A and B-B per unit mass flow of air
• Here, q and w are the heat and work transfers
  from the entrance to the throat and h and C stand
  for enthalpy and velocity respectively.
• If we assume reversible adiabatic conditions, and
  there is no work transfer, q0, w0, and if
  approach velocity C10 we get

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Substituting for T1 T2 from Eq. 5 in Eq. 3, we
get
By the continuity equation we can write down the
theoretical mass flow rate of air
where A1 and A2 are the cross-sectional areas at
the air inlet (point 1) and venturi throat
(point 2).
To calculate the mass flow rate of air at the
throat, we have assumed the flow to be
isentropic till the throat so the equation
relating p and v (or ?) can be used.
16
For a perfect gas we have
Thus
and rearranging the above equation we have
17
Since the fluid flowing in the intake is air, we
can put in the approximate values of R 287
J/kgK, cp 1005 J/kgK and ? 1.4 at 300K.
where
Here, pressure p is in N/m2, area A is in m2,and
temperature T is in K. If we take the ambient
temperature T1 300Kand ambient pressure p1
105 N/m2, then
18
Equation 11 gives the theoretical mass flow rate
of air. The actual mass flow rate,
, can be obtained by multiplying the equation by
the coefficient of discharge for the venturi,
Cd,a. Thus
where
The coefficient of discharge and area are both
constant for a given venturi, thus
Since we have to determine the air-fuel ratio, we
now calculate the fuel flow rate.
The fuel is a liquid before mixing with the air,
it can be taken to be incompressible.
We can apply Bernoullis equation between the
atmospheric conditions prevailing at the top of
the fuel surface in the float bowl, which
corresponds to point 1 and the point where the
fuel will flow out, at the venturi, which
corresponds to point 2.
Fuel flow will take place because of the drop in
pressure at point 1 due to the venturi effect.
Thus
19
where ?f is the density of the fuel in kg/m3, Cf
is the velocity of the fuel at the exit of the
fuel nozzle (fuel jet), and z is the depth of the
jet exit below the level of fuel in the float
bowl. This quantity must always be above zero
otherwise fuel will flow out of the jet at all
times. The value of z is usually of the order of
10 mm.
From Eq. 16 we can obtain an expression for the
fuel velocity at the jet exit as
Applying the continuity equation for the fuel, we
can obtain the theoretical mass flow rate,
20
where Af is the exit area of the fuel jet in m2.
If Cd,f is the coefficient of discharge of the
fuel nozzle (jet) given by
then
Since
If we put
, we get the following equation for the air-fuel
ratio
21
where
For the normal carburetor operating range, where
, the effects of compressibility which reduce F
below 1.0 are small.
The equivalence ratio, f, where
22
is given by
In Eq. 22, if we take T1 300K and p1 105 N/m2
then
The coefficient of discharge defined in Eq 19
represents the effect of all deviations from the
ideal one-dimensional isentropic flow. It is
influenced by many factors of which the most
important are
1.Fluid mass flow rate, 2.Orifice
length-to-diameter ratio, 3.Orifice
area-to-approach area ratio, 4.Orifice surface
area, 5.Orifice surface roughness, 6.Orifice
inlet and exit chamfers, 7.Fluid specific
gravity, 8.Fluid viscosity, and 9.Fluid surface
tension.
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• The use of the orifice Reynolds number
• as a correlating parameter for the coefficient of
  discharge accounts for the effects of mass flow
  rate, fluid density and viscosity, and length
  scale to a good approximation. The discharge
  coefficient of a typical carburetor main
  fuel-metering system orifice increases smoothly
  with increasing orifice Reynolds number, Reo.

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Air-fuel ratio neglecting compressibility of air

• If we assume air to be incompressible, then we
  can apply Bernoullis equation to air flow also.
  Since initial velocity is assumed zero, we have

Thus
Thus
25
Applying the continuity equation for the fuel, we
can obtain the theoretical mass flow rate,
, from
where A2 is the venturi in m2. If Cd,a is the
coefficient of discharge of the venturi given by
then
Since
26
If we assume z 0, then
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Carburetor Performance

• In Eq. 26, the terms A1, A2, ?a, and ?f are all
  constant for a given carburetor, fuel, and
  ambient conditions. Also, for very low flows, ?pa
  ?fgz. However, the discharge coefficients Cd,a
  and Cd,f and F, all vary with flow rate. Hence,
  the equivalence ratio delivered by an elementary
  carburetor is not constant.
• Figure shows the performance of an elementary
  carburetor. The top graph shows the variation of
  Cd,a and Cd,f and F with the venturi pressure
  drop. For ?pa ?fgz, there is no fuel flow. Once
  fuel starts to flow, the fuel flow rate increases
  more rapidly than the air flow rate. The
  carburetor delivers a mixture of increasing
  equivalence ratio as the flow rate increases.

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Discussion of Figure

• Suppose the venturi and fuel orifice (jet) are
  sized to give a stoichiometric mixture at an air
  flow rate corresponding to 1 kN/m2 venturi
  pressure drop (middle graph of Fig). At higher
  flow rates, the carburetor will deliver a
  fuel-rich mixture. At very high flow rates the
  carburetor will deliver an essentially constant
  equivalence ratio. At lower air flow rates, the
  mixture delivered leans out rapidly.
• Thus, the elementary carburetor cannot provide
  the variation in mixture ratio which the engine
  requires over the complete load range at any
  given speed.

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Summary of the Deficiencies of the Elementary
Carburetor

• At low loads, the mixture becomes leaner the
  engine requires the mixture to be enriched at low
  loads. The mixture is richest at idle.
• At intermediate loads, the equivalence ratio
  increases slightly as the air flow rate
  increases the engine requires an almost constant
  equivalence ratio.
• As the air flow approaches the maximum (WOT)
  value, the equivalence ratio remains essentially
  constant the engine requires an equivalence
  ratio of about 1.1 at maximum engine power.
• The elementary carburetor cannot compensate for
  transient phenomena in the intake manifold. It
  also cannot provide a rich mixture during engine
  starting and warm-up.
• It cannot adjust to changes in ambient air
  density due to changes in altitude.

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Modern Carburetor Design

• The changes required in the elementary carburetor
  so that it provides the equivalence ratio
  required at various air flow rates are as
  follows.
• The main metering system must be compensated to
  provide a constant lean or stoichiometric mixture
  over 20 to 80 of the air flow range.
• An idle system must be added to meter the fuel
  flow at idle and light loads to provide a rich
  mixture.
• An enrichment system must be provided so that the
  engine can get a rich mixture as WOT conditions
  is approached and maximum power can be obtained.
• An accelerator pump must be provided so that
  additional fuel can be introduced into the engine
  only when the throttle is suddenly opened.
• A choke must be added to enrich the mixture
  during cold starting and warm-up to ensure that a
  combustible mixture is provided to each cylinder
  at the time of ignition.
• Altitude compensation is necessary to adjust the
  fuel flow which makes the mixture rich when air
  density is lowered.
• Increase in the magnitude of the pressure drop
  available for controlling the fuel flow is
  provided by introducing boost venturis (Venturis
  in series) or Multiple-barrel carburetors
  (Venturis in parallel).