OPTIMIZED FUEL MANAGEMENT SYSTEM FOR DIRECT INJECTION
`
`ETHANOL ENHANCEMENTOF GASOLINE ENGINES
`
`Backgroundof the Invention
`
`This application is a continuation of United States Patent Application Serial No.
`
`11/100,026 filed April 6, 2005, which is a continuation-in-part of United States Patent
`
`Application Serial No. 10/991,774 filed November 18, 2004, the contents of both of which are
`
`incorporated herein by reference.
`
`This invention relates to an optimized fuel management system for use with spark
`
`10
`
`ignition gasoline engines in which an anti-knock agent whichis a fuelis directly injected into a
`
`cylinder of the engine.
`
`There are a number of important additional approaches for optimizing direct injection
`
`ethanol enhanced knock suppression so as to maximize the increase in engine efficiency and to
`
`minimize emissionsofair pollutants beyond the technology disclosed in parent applicationserial
`
`15
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`number 10/991,774 set out above. There are also additional approaches to protect the engine and
`
`exhaust system during high load operation by ethanolrich operation; and to minimizecost,
`
`ethanol fuel use and ethanol fuel storage requirements. This disclosure describes these
`
`approaches.
`
`These approachesare based in part on morerefined calculations of the effects of variable
`
`20
`
`ethanol octane enhancement using a new computer model that we have developed. The model
`
`determinesthe effect of direct injection of ethanol on the occurrence of knock for different times
`
`of injection and mixtures with port fuel injected gasoline. It determines the beneficial effect of
`
`evaporative cooling of the direct cthanol injection upon knock suppression.
`
`Summaryof the Invention
`
`25
`
`In one aspect, the invention is a fuel management system for operation of a spark ignition
`
`gasoline engine including a gasoline engine and a source of an anti-knock agent whichis a fuel.
`
`The use of the anti-knock agent provides gasoline savings both by facilitating increased engine
`
`efficiency over a drive cycle and by substitution for gasoline as a fuel. An injector is provided
`
`for direct injection of the anti-knock agent into a cylinder of the engine and a fuel management
`
`30
`
`control system controls injection of the anti-knock agent into the cylinder to control knock. The
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`injection of the antiknock agent can be initiated by a signal from a knock sensor. It can also be
`
`initiated when the engine torque is above a selected value or fraction of the maximum torque
`
`where the value or fraction of the maximum torque is a function of the engine speed. In a
`
`preferred embodiment, the injector injects the anti-knock agent after inlet valve/valves are
`
`closed. It is preferred that the anti-knock agent have a heat of vaporizationthat is at lcast twice
`
`that of gasoline or a heat of vaporization per unit of combustion energy that is at least three times
`
`that of gasoline. A preferred anti-knock agentis ethanol. In a preferred embodimentofthis
`
`aspect of the invention, part of the fuel is port injected and the port injected fuel is gasoline. The
`
`directly injected cthanol can be mixed with gasoline or with mcthanol.
`
`It is also preferred that
`
`10
`
`the engine be capable of operating at a manifold pressure at least twice that pressure at which
`
`knock would occurif the engine were to be operated with naturally aspirated gasoline. A
`
`suitable maximum ethanolfraction during a drive cycle when knock suppression is desired is
`
`between 30% and 100% by energy. It is also preferred that the compression ratio be at Icast 10.
`
`With the higher manifold pressure, the engine can be downsized by a factor of two and the
`
`15
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`efficiency under driving conditions increased by 30%.
`
`20
`
`25
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`It is preferred that the engine is operated at a substantially stoichiometric air/fuel ratio
`
`during part or all of the time that the anti-knock agent such as cthanol is injected. In this case, a
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`three-way catalyst can be used to reduce the exhaust emissions from the engine. Thefuel
`
`management system may operate in open or closed loop modes.
`
`In some embodiments, non-uniform ethanol injection is employed. Ethanol injection
`
`may be delayed relative to bottom dead center when non-uniform ethanol distribution is desired.
`
`Manyother embodiments of the invention are set forth in detail in the remainderofthis
`
`application.
`
`Brief Description of the Drawings
`
`Fig. 1 is a graph of ethanol fraction (by energy) required to avoid knock as a function of
`
`inlet manifold pressure. The ethanol fraction is shown for various values of 8, the ratio of the
`
`change in temperature in the air cylinder charge due to turbocharging (and aftercooling if used)
`
`to the adiabatic temperature increase ofthe air due to the turbocharger.
`
`Fig. 2a is a graph of cylinder pressure as a function of crank angle for a three bar
`
`30
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`manifold pressure.
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`Fig. 2b is a graph of charge temperature as a function of crank angle for a three bar
`
`manifold pressure.
`
`Fig. 3 is a schematic diagram of an embodimentofthe fuel management system disclosed
`
`herein for maintaining stoichiometric conditions with metering/control of ethanol, gasoline, and
`
`air flows into an cngine.
`
`Figs. 4a and 4b are schematic illustrations relating to the separation of ethanol from
`
`ethanol/gasoline blends.
`
`Fig. 5 is a cross-sectional view of a flexible fuel tank for a vehicle using ethanol boosting
`
`of a gasoline cnginc.
`
`Description of the Preferred Embodiment
`
`Ethanolhas a heat of vaporization that is more than twice that of gasoline, a heat of
`
`combustion per kg which is about 60% ofthat of gasoline, and a heat of vaporization per unit of
`
`combustion cnergy that is close to four times that of gasolinc. Thus the evaporative cooling of
`
`the cylinder air/fuel charge can be very large with appropriate direct injection of this antiknock
`
`15
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`agent. The computer model referenced below shows that evaporative cooling can have a very
`
`beneficial effect on knock suppression.It indicates that the beneficial effect can be maximized
`
`by injection of the cthanolafter the inlct valve that admits the air and gasoline into the cylinderis
`
`closed. This late injection of the ethanol enables significantly higher pressure operation without
`
`knock and thus higher efficiency engine operation than would be the case with early injection. It
`
`20
`
`is thus preferred to the conventional approach of early injection which is used becauseit
`
`provides good mixing. The modelalso provides information that can be used for open loop (Ze.,
`
`a control system that uses predetermined information rather than feedback) fuel management
`
`control algorithms.
`
`The increase in gasoline engine efficiency that can be obtained from direct injection of
`
`25
`
`ethanol is maximized by having the capability for highest possible knock suppression
`
`enhancement. This capability allows the highest possible amount of torque when needed and
`
`thereby facilitates the largest engine downsizing for a given compressionratio.
`
`Maximum knock suppressionis obtained with 100% or close to 100% use of direct
`
`injection of ethanol. A small amountof port injection of gasoline may be useful in order to
`
`30
`
`obtain combustion stability by providing a more homogeneous mixture. Port fuel injection of
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`gasoline also removesthe need for a second direct fuel system or a more complicated system
`
`which uses oneset of injectors for both fuels. This can be useful in minimizingcosts.
`
`The maximum fraction of ethanol used during a drive cycle will depend upon the engine
`
`system design and the desired level of maximum torqueat different engine speeds. A
`
`representative range for the maximum cthanol fraction by energy is between 20% and 100%.
`
`In order to obtain the highest possible octane enhancement while still maintaining
`
`combustion stability, it may be useful for 100% of the fuel to come from ethanol with a fraction
`
`being port injected, as an alternative to a small fraction of the port-fueled gasoline.
`
`Theinitial determination of the knock suppression by dircct injcction of cthanol into a
`
`10
`
`gasoline engine has been refined by the development of a computer model for the onset of knock
`
`under various conditions. The computer modeling provides more accurate information for use in
`
`fuel management control. It also shows the potential for larger octane enhancements than our
`
`carlicr projections. Larger octane cnhanccments can increase the efficiency gain through greater
`
`downsizing and higher compressionratio operation. They can also reduce the amountof ethanol
`
`15
`
`use for a given efficiency increase.
`
`The computer model combines physical models of the ethanol vaporization effects and
`
`the effects of piston motion of the cthanol/gasolinc/air mixtures with a state of the art
`
`calculational code for combustion kinetics. The calculational code for combustion kinetics was
`
`the engine module in the CHEMKIN 4.0 code [R. J. Kee, F. M. Rupley, J. A. Miller, M. E.
`
`20
`
`Coltrin, J. F. Grcar, E. Meeks, H. K. Moffat, A. E. Lutz, G. Dixon-Lewis, M. D. Smooke,J.
`
`Warnatz, G. H. Evans, R. S. Larson, R. E. Mitchell, L. R. Petzold, W. C.Reynolds, M.
`
`Caracotsios, W. E. Stewart, P. Glarborg, C. Wang, O. Adigun, W. G. Houf, C. P. Chou,S. F.
`
`Miller, P. Ho, and D. J. Young, CHEMKIN Release 4.0, Reaction Design, Inc., San Diego, CA
`
`(2004)]. The CHEMKINcodeis a software tool for solving complex chemical kinetics
`
`25
`
`problems. This new model uses chemical rates information based upon the Primary Reference
`
`gasoline Fuel (PRF) mechanism from Curran et al. [Curran, H. J., Gaffuri, P., Pitz, W. J., and
`
`Westbrook, C. K. "A Comprehensive Modeling Study of iso-Octane Oxidation," Combustion
`
`and Flame 129:253-280 (2002) to represent onset of autoignition.
`
`The compression on the fuel/air mixture end-gas was modeled using the artifact of an
`
`30
`
`engine compression ratio of 21 to represent the conditions of the end gas in an engine with an
`
`actual compression ratio of 10. The end gasis defined as the un-combustedair/fuel mixture
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`remaining after 75% (by mass)of the fuel has combusted. It is the end gas that is most prone to
`
`autoignition (knock). The larger compression ratio includes the effect of the increase in pressure
`
`in the cylinder due to the energy released in the combustion of 75% ofthe fuel that is not in the
`
`end gas region. The effect of direct ethanol vaporization on temperature was modeled by
`
`considcration of the cffects of the latent heat of vaporization on tempcrature depending upon the
`
`time of the injection.
`
`The effect of temperature increase due to turbocharging wasalso included. The increase
`
`in temperature with turbocharging was calculated using an adiabatic compression modelofair. It
`
`is assumedthat thermaltransfer in the piping or in an intercooler results in a smaller temperature
`
`10
`
`increase. The effect is modeled by assuming that the increase in temperature of the air charge
`
`into the cylinder ATcharge 18 ATcharge = B ATrubo Were ATturbo 18 the temperature increase after the
`
`compressor due to boosting and beta is a constant. Values of B of 0.3, 0.4 and 0.6 have been used
`
`in the modeling.It is assumed that the temperature of the charge would be 380 K for a naturally
`
`aspirated engine with port fuel injection gasoline.
`
`15
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`Fig. | showsthe predictions of the above-referenced computer model for the minimum
`
`ethanol fraction required to prevent knock as a function of the pressure in the inlet manifold, for
`
`various values of 6. In Fig. 1 it is assumed that the direct injection of the ethanolis late (1.e. after
`
`the inlet valve that admits air and gasoline to the cylinder is closed) and a 87 octane PRF
`
`(Primary Reference Fuel) to represent regular gasoline. The corresponding calculations for the
`
`20
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`manifold temperature are shown in Table | for the case of a pressure in the inlet manifold of up
`
`to 3 bar for an engine with a conventional compression ratio of 10. The temperature of the charge
`
`varies with the amount of ethanol directly injected andis self-consistently calculated in Table 1
`
`and Fig. 1. The engine speed used in these calculations is 1000 rpm.
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`Table 1
`
`Computer model calculations of temperature and ethanolfraction required for knock
`
`prevention for an inlet manifold pressure of 3 bar for an engine with a compressionratio of 10,
`
`for various values of 6 (ratio of change of the cylinder air charge temperature due to
`
`turbocharging to the adiabatic temperature increase duc to turbocharging ATcharge = B ATtubo).
`
`The engine speed is 1000 rpm.
`
`B
`
`T_charge init
`Delta T turbo
`Delta T after intercooler
`
`Delta T due to DI ethanol and gasoline
`T_init equivalent charge
`Gasoline octane
`Ethanol fraction (by energy) needed
`to prevent knock
`
`K
`K
`K
`
`K
`K
`
`0.3
`
`0.4
`
`0.6
`
`380
`180
`54
`
`-103
`331
`87
`
`380
`180
`72
`
`-111
`341
`87
`
`380
`180
`108
`
`-132
`356
`87
`
`74%
`
`82%
`
`97%
`
`Direct fuel injection is normally performed early, before the inlet valve is closed in order
`
`to obtain good mixing of the fuel and air. However, our computer calculations indicate a
`
`10
`
`substantial benefit from injection after the inlet valve is closed.
`
`The amount of air is constant in the case ofinjection after the inlet valve has closed.
`
`Therefore the temperature changeis calculated using the heat capacity of air at constant volume
`
`(cy). The case of early injection where the valve that admits air and fuel to the cylinderis still
`
`open is modeled with a constant-pressure heat capacity (cp). The constant volumecase results in
`
`15
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`a larger evaporation induced decrease in charge temperature than in the case for constant
`
`pressure, by approximately 30%. The better evaporative cooling can allow operation at higher
`
`manifold pressure (corresponding to a greater octane enhancement) without knock that would be
`
`the case of early injection by a difference of more than | bar. The increase in the evaporative
`
`cooling effect at constant volumerelative to that at constant pressure is substantially higher for
`
`20
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`the case of direct injection of fuels such as ethanol and methanolthan is the case for direct
`
`injection of gasoline.
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`Typical results from the calculations are shown in Fig. 2. The figure shows the pressure
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`(a) and the temperature (b) of the cylinder charge as a function of crank angle, for a manifold
`
`pressure of 3 bar and a value of B = 0.4 Two values of the ethanol fraction are chosen, one that
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`results in autoignition, and produces engine knock (0.82 ethanol fraction by fuel energy), and the
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`other one without autoignition, i.e., no knock (0.83 ethanol fraction). Autoignition is a threshold
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`phenomenon,and in this case occurs between ethanolfractions of 0.82 and 0.83. For an ethanol
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`energy fraction of 0.83, the pressure and temperature rise at 360° (top dead center) is due largely
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`to the compression ofthe air fuel mixture by the piston. When the ethanol energy fraction is
`
`reduced to 0.82, the temperature and pressure spikes as a result of autoignition. Although the
`
`10
`
`autoignition in Figure 2 occurs substantially after 360 degrees, the autoignition timing is very
`
`sensitive to the autoignition temperature (5 crank angle degrees change in autoignition timing for
`
`a changein the initial temperature of 1 K, or a change in the ethanol energy fraction of 1%).
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`The effect of evaporative cooling from the antiknock agent(in this case, ethanol) is
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`shown in Table 2, where three cases arc compared. Thefirst one is with port fucl injection of
`
`15
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`ethanol. In this case the vaporization of the ethanol on the walls of the manifold has a negligible
`
`impact on the temperature of the charge to the cylinder because the walls of the manifold are
`
`cooled rather than the air charge. The second case assumesdirect injection, but with the inlet
`
`valve open, with cvaporation at constant pressurc, where the cooling of the charge admits
`
`additional air to the cylinder. The third case assumes,as in the previousdiscussions, late
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`20
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`injection after the inlet valve has closed. It is assumed stoichiometric operation, that the baseline
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`temperature is 380 K, andthat there is cooling in the manifold after the turbocharger with 6 =
`
`0.4.
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`Table2
`
`Knock-free operation of ethanol port fuel injection (assuming no charge cooling), and of
`
`direct injection before and after the inlet valve is closed. Compression ratio of 10, baseline
`
`charge temperature of 380 K, intercooler/cooling post turbo with 6 = 0.4,stoichiometric
`
`operation, gasoline with 87 RON. Engine speed is 1000 rpm.
`
`No Evaporative Cooling
`
`Ethanol fraction
`(by energy)
`
`Max manifold pressure (bar)
`
`Cylinder pressure after
`cooling (bar)
`
`Cylinder charge temperature
`after cooling (K)
`
`0.95
`
`1.05
`
`1.05
`
`383
`
`Evaporative cooling
`Before
`After
`
`Valve Closing
`
`Valve Closing
`
`0.95
`
`0.95
`
`2.4
`
`2.4
`
`360
`
`4.0
`
`3.0
`
`355
`
`The results indicate the strong effect of the cooling. The maximum manifold pressure
`
`that prevents knock (without spark retard), with 0.95 ethanol fraction by energy in the case of
`
`port fuel injection is 1.05 bar. With direct injection of the ethanol, the maximum knock-free
`
`10
`
`manifold and cylinder pressures are 2.4 bar, with a temperature decercase of the charge of ~75K.
`
`Thefinal case, with injection after inlet valve closing, allows a manifold pressure of 4 bar, a
`
`cylinder pressure (after cooling) of 3 bar, and a charge temperature decrease of ~120 K.It should
`
`be noted that the torque ofthe late injection case after the valve has closed is actually higher than
`
`that of the carly injection case, even thoughthe carly injection case allows for additional air (at
`
`15
`
`constant pressure). For comparison, the model is also used to calculate the manifold pressure at
`
`which knock would occur for port fuel injection of 87 octane gasoline alone. This pressure is ~
`
`0.8 bar assuming spark timing at MBT (Maximum Brake Torque). Conventional gasoline
`
`engines opcrate at | bar by retarding the timing at high torque regions where knock would
`
`otherwise occur. Thus the model indicates that evaporative cooling effect of direct injection of
`
`20
`
`ethanolafter the inlet valve has closed can be significantly greater than that of the higher octane
`
`numberrating of ethanol relative to gasoline.
`
`A manifold pressure of 4 bar is very aggressive. Table 2 is indicative of the dramatically
`
`improved performance of the system with direct injection after the inlet valve has closed. The
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`improved performancein this case can be traded for increased compressionratio or reduced use
`
`of the anti-knock agent.
`
`It should be noted that, as mentioned above, the calculations of autoignition (knock) are
`
`conservative, as autoignition for the case shownin Fig. 2 occurs relatively late in the cycle, and it
`
`is possible that the fucl has been combusted before it autoignitcs. Also it should be notedthat
`
`the calculations in Fig. 2 break down after autoignition, as the pressure trace would be different
`
`from that assumed. Figures similar to Fig. 2 are used to determine conditions where autoignition
`
`would not occur, and those conditions are then used to provide the information for Fig. 1. The
`
`initial tempcraturcs of the cases shown in Fig. 2 arc 341 K for 0.82 cthanolfraction, and 340 K
`
`10
`
`for 0.83 ethanolfraction, a difference of 1 K (the difference due to the cooling effect of the
`
`ethanol).
`
`Becauseofthe large heat of vaporization, there could be enough charge cooling with
`
`carly injection so that the rate of vaporization of cthanol is substantially decreased. By instcad
`
`injecting into the hot gases, which is the case with injection after the inlet valve has closed, the
`
`15
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`temperature at the end of full vaporization of the ethanolis substantially increased with respect
`
`to early injection, increasing the evaporation rate and minimizing wall wetting.
`
`The optimum timing of the injection for best mixing and a ncar homogencouscharge is
`
`soon after the inlet valve closes, provided that the charge is sufficiently warm for antiknock
`
`agent vaporization. If, on the other hand, a non-uniform mixture is desired in order to minimize
`
`20
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`ethanol requirements and improveignition stability, then the injection should occur later than in
`
`the case where the best achievable mixingis the goal.
`
`Late injection of the ethanol after the inlet valve has closed can be optimized through the
`
`use of diesel-like injection schemes, such as injectors with multiple sprays. It is important to
`
`inject the fuel relatively quickly, and at velocities which minimize any cylinder wall wetting,
`
`25
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`which as described below could result in the removal of the lubrication oils from the cylinder
`
`liner. Multiple sprays from a nozzle that has multiple holes results in a distributed pattern of
`
`sprays, with relatively low injection velocities. This is particularly important for ethanol, because
`
`of the higher volumethroughputs (as compared with gasoline) of ethanol for equal energy
`content.
`
`30
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`Injection after the valve has closed may require that a modestfraction of the fuel (e.g.
`
`25%) be port injected in order to achieve the desired combustion stability. A tumble-like or swirl
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`motion can be introducedto achieve the desired combustion stability. The port injected fuel can
`
`be either gasoline or ethanol.
`
`Use of the computer model for operation with gasoline alone givesresults that are
`
`consistent with the observed occurrence of knockin gasoline engine vehicles, thereby buttressing
`
`the credibility of the projections for cthanol. The computer model indicates that for knock-free
`
`gasoline operation alone with a compression ratio of 10, knock imposes a severe constraint upon
`
`the allowed manifold pressure for a naturally aspirated gasoline engine and very limited (7.¢., less
`
`than 1.2 bar) manifold pressure can be achieved even with direct injection of gasoline unless
`
`spark retard and/or rich operation is used. These changes, however, can reduce efficiency and
`
`10
`
`increase emissions.
`
`Fig. 1 showsthat knock can be prevented at manifold pressures greater than 2 bar with
`
`direct injection of an ethanol fraction of between 40 and 80% in an engine with a compression
`
`ratio of 10. The manifold pressure can beat least 2.5 bar without engine knock. A pressure of 3
`
`bar would allow the engine to be downsized to ~1/3 of the naturally aspirated gasoline engine,
`
`15
`
`while still producing the same maximum torque and power. The large boosting indicated by the
`
`calculations above may require a multiple-stage turbocharger. In addition to a multiple stage
`
`turbocharger, the turbocharger may be of the twin-scroll turbo type to optimize the turbocharging
`
`and decrease the pressure fluctuations in the inlet manifold generated by a small number of
`
`cylinders.
`
`20
`
`With an increase in allowed manifold pressure in an engine by more than a factor of 2,
`
`the engine could be downsized by a factor of 2 (that is, the cylinder volumeis decreased by a
`
`factor of 2 or more) and the compression ratio could be held constant or raised. For example, the
`
`performanceofan eight cylinder engine is achieved by a four cylinder engine.
`
`The occurrence of knockat a given value of torque depends upon engine speed. In
`
`25
`
`addition to providing substantially more maximum torque and power, direct injection of ethanol
`
`can be used to provide a significant improvementin torque at low engine speeds(less than 1500
`
`rpm) by decreasing or eliminating the spark retard. Spark retard is generally used with gasoline
`
`engines to prevent knock at low engine speeds where autoignition occurs at lower values of
`
`torque than is the case at high engine speeds.
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`Fig. 1 can also be used to determine the ethanolfraction required to prevent knockat
`
`different levels of torque and horsepower, which scale with manifold pressure in a given size
`
`engine. This information can be used in an open loop control system.
`
`The efficiency of a gasoline engine under driving conditions using direct ethanol
`
`injection cnhancementcan beat least 20% and preferably at lcast 30 % greater thanthat of a
`
`naturally aspirated gasoline engine with a compression ratio of 10. This increase results from the
`
`substantial engine boosting and downsizing to give the same power, and also the high
`
`compression ratio operation (compression ratio of 11 or greater) that is enabled by a large octane
`
`enhancement. With more aggressive downsizing of more than 50% (where the same engine
`
`10
`
`performanceis obtained with less than one-half the displacement), the increase in efficiency
`
`could exceed 30%.
`
`Greater downsizing and higher efficiency may also be obtained by decreasing the octane
`
`requirement of the engine by using variable valve timing (VVT). Thus, at conditions of high
`
`torque, variable valve timing can be used to decrease the compression ratio by appropriately
`
`15
`
`changing the opening/closing of the inlet and exhaust valves. Theloss in efficiency at high
`
`torque has a small impact on the overall fuel economy because the engine seldom operates in
`
`these conditions.
`
`VVTcan also be used to better scavenge the exhaust gases [B. Lecointe and G. Monnier,
`
`“Downsizing a Gasoline Engine Using Turbocharging with Direct Injection” SAE paper 2003-
`
`20
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`01-0542]. Decreasing the exhaust gas decreases theair/fuel temperature. Keeping both theinlet
`
`and exhaust valves open, while the pressure in the inlet manifold is higher than in the exhaust,
`
`can be used to remove the exhaust gases from the combustion chamber. This effect, coupled
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`with slightly rich operation in-cylinder, can result in increased knock avoidance while the
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`exhaustis still stoichiometric. Cooled EGR and spark timing adjustment can also be used to
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`25
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`increase knock avoidance.
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`Any delay in delivering high engine torque at low engine speeds can decrease drivability
`
`of the vehicle. Under these conditions, because of the substantial engine downsizing, the vehicle
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`would have insufficient acceleration at low engine speeds until the turbo produces high
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`pressures. This delay can be removed through the use of direct injection of ethanol by reduction
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`30
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`of the spark retard or ethanol/gasoline with rich operation and also with the use of variable valve
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`timing.
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`Another approach would be to use an electrically assisted turbo charger. Units that can
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`generate the required boosting for short periods of time are available. The devices offer very fast
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`responsetime, although they have substantial power requirements.
`
`A multiple scroll turbocharger can be used to decrease the pressure fluctuations in the
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`manifold that could result from the decreased numberof cylinders in a downsized engine.
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`The temperature of the air downstream from the turbochargeris increased by the
`
`compression process. Use of an intercooler can preventthis temperature increase from increasing
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`the engine’s octane requirement. In addition, in order to maximize the poweravailable from the
`
`engine for a given turbocharging, cooling of the air charge results in increased massofair into
`
`10
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`the cylinder, and thus higher power.
`
`In order to minimize emissions, the engine should be operated substantially all of the
`
`time, or most of the time, with a stoichiometric air/fuel ratio in order that a 3-way exhaust
`
`catalyst treatment can be used. Fig. 3 shows a 3-way cxhaust treatment catalyst 10 andair,
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`gasoline and ethanol control needed to maintain the substantially stoichiometric ratio of fuel to
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`15
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`air that is needed for its effective operation. The system uses an oxygen sensor 12 as an input to
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`an electronic control unit (ECU) 14. The ECU 14 controls the amountof air into a turbocharger
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`16, the amount of gasoline and the amount of cthanol so as to insure stoichiomctric opcration.
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`During transients, open-loop algorithms from a stored engine map (not shown)are used to
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`determineair, gasoline and ethanol flows for keeping substantially stoichiometric combustion in
`
`20
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`a cylinder of the engine 18.
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`Thus when variable ethanol octane enhancement is employed, the fuel management
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`system needs to adjust the amounts ofair, gasoline and ethanol such that the fuel/air ratio is
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`substantially equal to 1. The additional control is needed because, if the air/gasoline ratio
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`determined by the fuel management were not be corrected during the injection of ethanol, the
`
`25
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`mixture would no longer be stoichiometric. In contrast to the lean boost approach of Stokes et a/
`
`.[ J. Stokes, T. H. Lake and R. J. Osborne, “A Gasoline Engine Concept for Improved Fuel
`
`Economy —The Lean Boost System,” SAE paper 2000-01-2902] stoichiometric operation with a
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`3-way catalyst results in very low tailpipe emissions.
`
`There are certain regions in the engine operating map where the ECU 14 may operate
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`30
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`open loop,that is, the control is determined by comparison to an engine map lookup table rather
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`than by feedback from a sensed parameter which in this case is engine knock (closed loop). As
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`mentioned previously, open loop operation during transients may be advantageous.
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`Another situation where open loop control can be advantageous would be under high
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`load, where fuel rich conditions (where the fuel/air ratio is greater than stoichiometric) may be
`
`required to decrease the temperature of the combustion and thus protect the engine and the
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`exhaust system (especially during prolonged operation). The conventional approach in gasoline
`
`engine vehicles is to use increased fuel/air ratio, that is, operating at rich conditions. The
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`presence of ethanol on-board allows for two alternatives. Thefirst is the use of ethanol fuel
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`fractions beyond what is required to control knock, thus reducing the combustion temperature by
`
`10
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`a greater amount than could be obtained by gasoline alone due to the higher cooling effect of
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`evaporation in direct ethanol injection, even while at stoichiometric conditions. The second one
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`is, aS In conventional applications, the use of increased fueling in rich operation (which could
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`result in relative air/fucl mass ratios as lowas 0.75 where a stoichiometric mixture has a relative
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`air/fuel ratio of 1). The control system can choose between two fuels, ethanol and gasoline.
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`15
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`Increased use of ethanol may be better than use of gasoline, with emissionsthat are less
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`damaging to the environment than gasoline and decreased amountof rich operation to achieve
`
`the temperature control needed. Open loop opcration with both gasoline and cthanol may require
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`substantial modification of the engine’s “lookup table.”
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`Thus, a method of operating an engineis, under conditions of partial load, to operate
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`20
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`closed loop with the use of only gasoline. As the engine load increases, the engine control system
`
`may changeto open loop operation, using a lookuptable.
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`The closed loop control of the engine can be such that a knock sensor (not shown)
`
`determinesthe fraction required of ethanol, while the oxygen sensor 12 determinesthetotal
`
`amount of fuel. A variation of this schemeis to operate the knock control open loop, using a
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`25
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`lookup table to determine the ethanol to gasoline ratio, but a closed loop to determinethe total
`
`amountoffuel.
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`In order to minimize evaporative emission of the ethanol (which hasa relatively low
`
`boiling point), solvents can be added to the ethanol to minimizethe effect. An alternative means
`
`is to place an absorptive canister between the ethanol tank and the atmosphere that captures the
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`30
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`ethanol an