BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is concerned with an apparatus for reducing the emission of pollutants in the exhaust gas of an internal combustion engine by modifying the fuel supplied to the engine at least during a start-up period. More specifically, the present invention is concerned with both fractionation and catalytic treatment of conventional liquid hydrocarbon fuel used for an internal combustion engine.

Background and Related Art

Apparatuses and techniques for modifying the fuel fed to an internal combustion engine are known in the art. For example, U.S. Patent 3,794,000 to Hodgkinson, dated February 26, 1974 discloses a fuel system for separating volatile fuel from gasoline. The apparatus used in this system comprises a vaporizing chamber into which gasoline is introduced to provide vapors which are removed by a pump and condensed under pressure to form a liquid which is stored in a storage container disposed within the va¬ porization chamber. The line from the pressurizing pump to the storage container is extended around the storage container to provide additional heat exchange with the gasoline (column 2, lines 54-55 and column 6, lines 30-31) thereby increasing volatilization of the gasoline. When the engine is started the condensate is used as fuel. When the engine reaches a satisfactory operating tempera¬ ture, a -valve cuts off the flow of the condensate and al- lows normal gasoline to flow to the carburetor (column 5, lines 13-20). The storage container for the condensate includes a level sensor which, when the level of conden¬ sate is low, triggers the pump-to draw additional vapors

from the vaporization chamber. When the vapor pressure in the vaporization chamber is low, additional gasoline is drawn from the gas tank and sprayed into the vaporization chamber (column 5, lines 35-44). In addition, when the storage tank is full, the switch turns the condensing pump off. After the cold-start period, gasoline from both the gas tank and the volatile-depleted gasoline from the va¬ porization tank are used to run the engine (column 5, line 65 through column 6, line 15). U.S. Patent 4,394,002 to Allen, dated September 14, 1982, discloses an apparatus and a method for fractionat¬ ing the gasoline fuel for an internal combustion engine, and combusting the different fractions in repeating se¬ quence in the engine in order to improve fuel mileage. Heat from the engine is utilized via the engine coolant in the fractionation process.

U.S. Patent 3,855,980 to Weisz et al, dated December 24, 1974 discloses a fuel system for an internal combus¬ tion engine in which conventional fuel from a fuel tank is passed through a zeolitic catalytic converter heated by engine exhaust gases before being supplied to the engine. The effect of the catalyst is said to improve the octane content of the fuel fed to the engine.

U.S. Patent 4,429,675 to Talbert, dated February 7, 1984 teaches the separation of conventional liquid fuel (gasoline) into a vapor fraction and a liquid fraction using either ultrasonic energy or the heat in the exhaust gases of the engine. The heavier fraction is used, if at all, only at start-up. Other patents teach generally the use of light fractions of a fuel. See, e.g., U.S. Patents 1,202,610; 1,384,512; 4,441,477; 3,800,533; 4,562,820; 3,963,013 and German Patent document DE 3002-544, dated July 31, 1981.

SUMMARY OF THE INVENTION

The present invention provides a method for abating pollutants discharged from an internal combustion engine fueled by a liquid hydrocarbon-fuel, at least during the

cold-start period of operation, by extracting low boiling point fraction of the liquid hydrocarbon fuel, and supply¬ ing the low boiling point fraction to the engine during start-up. Specifically, the method comprises (a) heating the liquid hydrocarbon fuel to vaporize therefrom a low boiling point fraction, condensing the low boiling point fraction to obtain therefrom a distillate fuel, and stor¬ ing the distillate fuel in a storage container; (b) sup¬ plying the distillate fuel to the engine as part of a start-up fuel/air combustion mixture for engine start-up and during an initial engine operating period; and (c) after the initial engine operating period, terminating the supply of distillate fuel to the engine and supplying the liquid hydrocarbon fuel of (a) to the engine as part of a second fuel/air combustion mixture for a subsequent engine operating period. The method further comprises (d) refin¬ ing at le s one of the distillate fuel and the liquid hy¬ drocarbon fuel by contacting it under cracking or isomer- izing conditions with a cracking catalyst prior to supply- ing the fuel to the engine, and (e) regenerating the cracking catalyst during operation of the engine by by¬ passing the supply of distillate fuel or liquid hydrocar¬ bon fuel to the engine around the cracking catalyst, and flowing air through the cracking catalyst under catalyst regeneration conditions.

According to one aspect of the invention, one or both of refining of step (d) and regenerating of step (e) may comprise heating the cracking catalyst, which may be ac¬ complished using heat generated by operation of the engine and/or electrical power that may be generated by the vehi¬ cle or that may come from a battery.

The method may further comprise carrying out step (a) by heating the liquid hydrocarbon fuel with heat generated by operation of the engine. The use of heat generated by the engine for this purpose or for steps (d) and (e) as stated above may be accomplished by transferring heat from either or both of the engine exhaust gas and the engine coolant fluid to the liquid hydrocarbon fuel. Alterna-

tively, the heating in step (a) may optionally be replaced by a step which separates a low boiling point fraction from the liquid hydrocarbon fuel prior to condensing the low boiling point fraction. In a further aspect of this invention, when the ex¬ haust gas is flowed through a heat exchanger in the gas tank to heat the liquid hydrocarbon fuel, the exhaust gas is preferably treated by the removal of pollutants such as unburned hydrocarbons, carbon monoxide or nitrogen oxides by contacting the exhaust gas under abatement conditions with an exhaust gas purification catalyst, before flowing the exhaust gas into the heat exchanger.

Still another aspect of the invention comprises that the step of condensing the low boiling point fraction may comprise passing the low boiling point fraction in heat exchange relation with a heat exchanger utilizing coolant from an air conditioning system powered by the vehicle= Alternatively, the condensing step may comprise increasing the pressure in the storage container, e.g., by introduc- ing pressurized air into the storage container.

According to another aspect of the application, the method may comprise measuring the temperature of the li¬ quid hydrocarbon fuel and ceasing the supply of heat thereto when the temperature reaches a predetermined lev- el. In addition, or as an alternative, the method may comprises measuring the quantity of liquid hydrocarbon fuel in the fuel tank and ceasing the supply of heat thereto when the quantity falls to a predetermined level. In another aspect, the present invention provides an improvement in an internal combustion engine system com¬ prising an internal combustion engine, a fuel tank for storing a liquid hydrocarbon fuel, and a liquid fuel transfer means including a fuel pump and a fuel line con¬ necting the fuel tank in liquid flow communication with the engine for transferring the liquid fuel from the fuel tank to the engine, and an exhaust gas discharge pipe con¬ nected to the engine for discharge of exhaust gas from the engine. The improvement comprises liquid fuel heating

means disposed within the fuel tank to vaporize a low boiling point fraction from the liquid hydrocarbon fuel, and a vapor line connecting the fuel tank in vapor flow communication with a condenser zone for condensing the low boiling point fraction into a liquid distillate fuel. The improvement further comprises a distillate fuel storage container connected in liquid flow communication with the condenser zone, and a distillate fuel line connecting the distillate fuel storage container in liquid flow communi- cation with the fuel line. A distillate fuel control valve may be disposed in the distillate fuel line to con¬ trol the flow of distillate fuel therethrough. A liquid fuel control valve is disposed in the fuel line to selec¬ tively control the flow of fuel through the fuel line from the fuel tank or the distillate fuel line. There may also be a control means responsive to start-up of the engine and operationally connected to both the liquid fuel con¬ trol valve and the distillate fuel control valve, to open the distillate fuel valve and operate the liquid fuel control valve to permit the flow of liquid distillate fuel into the fuel line during an initial engine operating pe¬ riod, and to close the distillate fuel valve and operate the liquid fuel valve to permit the flow of liquid hydro¬ carbon fuel therethrough at the end of the initial engine operating period. The apparatus may further comprise a cracking catalyst bed disposed in the fuel line for refin¬ ing fuel flowing therethrough into a more efficiently com¬ bustible composition and regeneration means for regenerat¬ ing the cracking catalyst while the engine is in opera- tion.

According to one aspect of the invention the improve¬ ment may further comprise a fuel by-pass line disposed in the fuel line, through which fuel flowing to the engine by-passes the cracking catalyst bed. In another aspect of the invention, the improvement further comprises an exhaust gas catalytic converter posi¬ tioned in the exhaust gas pipe in gas flow communication with exhaust gas discharged therethrough to catalyze the

conversion of noxious components of the exhaust gas to in¬ nocuous components. The liquid fuel heating means may be dimensioned and configured to transfer heat from the ef¬ fluent of the exhaust gas catalytic converter to the liq- uid hydrocarbon fuel in the fuel tank.

According to another aspect of the invention, the ap¬ paratus may further comprise fuel tank temperature sensing means, and the liquid fuel heating means may be responsive to the temperature sensing means, wherein when the temper- ature of the liquid hydrocarbon fuel reaches a predeter¬ mined level, the liquid fuel heating means stops heating the fuel tank. According to yet another aspect of the in¬ vention, the apparatus may comprise a fuel tank level sensing means, and the liquid fuel heating means may be responsive to the level sensing means, wherein when the level of the liquid hydrocarbon fuel falls to a predeter¬ mined level, the liquid fuel heating means stops heating the fuel tank.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic block diagram illustrating one embodiment of an internal combustion engine system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

AND SPECIFIC PREFERRED EMBODIMENTS THEREOF

Figure 1 schematically illustrates an improved engine system 10 in accordance with an embodiment of the inven¬ tion. The system includes a fuel tank 12 encased with a layer of thermal insulating material 13 and having a fill pipe 14 fitted with a cap. Fuel tank 12 is connected, as described below, to supply fuel to an internal combustion engine 16 fitted with a carburetor (or fuel injector) 18 having an air inlet 18a. In the illustrated embodiment, engine 16 has an exhaust pipe 20, 20a fitted with a con¬ ventional catalytic converter 22 which contains a catalyst for the abatement of pollutants. For example, in the case of a gasoline-fueled engine, catalytic converter 22 may

serve to catalyze the conversion of hydrocarbons, carbon monoxide and nitrogen oxides contained in the exhaust gas to H ₂ 0, C0 ₂ and N ₂ .

Liquid fuel transfer means is provided by a fuel line 24, 24a, 24b which extends from fuel tank 12 to carburetor 18 and in which is disposed a solenoid-operated valve 26, a fuel pump 28, valve 59, a cracking catalyst bed 30 and a valve 70. Preferably, valve 59 and valve 70 are both so¬ lenoid-operated valves. Cracking catalyst bed 30 contains a cracking catalyst, for example, a solid super acid cata¬ lyst such as Zr0 ₂ :S0 ₄ which is capable of refining liquid hydrocarbon fuels such as gasoline at room temperature in order to crack heavier molecules to lighter molecules and optionally to isomerize certain molecules to form an im- proved fuel which is more readily combusted in the engine and which permits leaner operation. Depending on the type of catalyst used and the extent and rate of cracking de¬ sired, it may be desirable to heat cracking catalyst bed 30. This may be effectively accomplished by diverting at least a portion of the hot exhaust gas exiting from engine 16 via exhaust pipe 20 into line 32. The hot exhaust gases would then flow through a coil 32a disposed within cracking catalyst bed 30 to heat catalyst bed 30. The ex¬ haust gas is then withdrawn via line 32b to pipe 20 for discharge through catalytic converter 22. In other em¬ bodiments, a plate-type heat exchanger may be used in place of coil 32a to heat catalyst bed 30. Alternatively, catalyst bed 30 could be heated by an electrical resis¬ tance heater powered from the vehicle battery. A vapor line 34 connects the vapor space 12a above the gasoline (or other liquid hydrocarbon fuel contained within fuel tank 12) with a condenser 38 which is equipped with a distillate fuel level sensor 39 having associated therewith a sending unit D connected by a first control line 39a to valve 26, by a second control line 39b to valve 62, and also by a third control line 39c to valve 56. Valves 26, 62 and 56 are all preferably solenoid- operated control valves. Vapor line 34 is provided with a

check valve 36 to ensure that there is no backflow from condenser 38 to the fuel tank 12. An air pump 40 provides compressed air through line 42 through a check valve 44 and valve 46 which is preferably a solenoid-operated valve, into the vapor space 38a above condensed distillate fuel Fd contained within the distillate fuel storage con¬ tainer 38b provided, in the illustrated embodiment, by the bottom portion of condenser 38. A compressed air by-pass line 48 extends from valve 46 to fuel line 24a, for a pur- pose to be described below. Air pump 40 may also provide compressed air to carburetor 18 via air inlet 18a. Alter¬ natively, or in addition, air pump 40 may provide com¬ pressed air through line 41 to exhaust pipe 20 for entry into catalytic converter 22. Valves (not shown) control the air flow into air inlet 18a and/or exhaust pipe 20.

A fuel line by-pass conduit 50 is connected to valve 59 which is disposed in fuel line 24a at a point upstream of cracking catalyst bed 30 and runs to fuel line 24b at a point downstream of cracking catalyst bed 30, providing a by-pass in the fuel line around catalyst bed 30.

A distillate fuel line 54 leads from the distillate fuel storage container 38b to a solenoid-operated valve 56, from whence distillate fuel line 54a runs to solenoid- operated valve 26. Distillate fuel from storage container 38b can thus be flowed into the liquid fuel transfer means through valve 26, for transfer to carburetor 18 via fuel line 24, fuel pump 28 and fuel line 24a, valve 59 and ei¬ ther through catalyst bed 30 and valve 70 or by-pass con¬ duit 50, and thence via fuel line 24b into carburetor 18. A timer 68 is connected by a control line 68a to valve 26. Fuel tank 12 is equipped with a heating fluid inlet 60, heating coil 60a and heating fluid outlet 60b. Any suitable heating fluid, e.g., the coolant liquid used to cool internal combustion engine 16, may be used. Thus, in a typical automobile engine, a side stream of heated en¬ gine coolant would be passed from the radiator into heat¬ ing fluid inlet 60 and after passing through coil 60a would be returned to the engine radiator via outlet 60b.

However, a plate-type heat exchanger would be preferred over coil 60a. Alternatively, or in addition, a heating fluid other than the engine coolant fluid could be used. The separate heating fluid could be heated by heat ex- change with hot exhaust gas discharged from engine 16 or from catalytic converter 22. In still another embodiment, hot and preferably catalytically treated exhaust gas from the engine may be flowed through the heat exchanger to heat the liquid fuel. Heating fluid inlet 60 is equipped with a control valve 62 to control the flow of heating fluid through coil 60a. A temperature probe 64 and fuel level sensor 66 are mounted within fuel tank 12. Tempera¬ ture probe 64 is equipped with a sending unit T and fuel level sensor 66 is equipped with a sending unit L which emits a signal indicating the quantity of liquid hydro¬ carbon fuel in the fuel tank. Sending units T and L are connected via control lines 64a and 66a, respectively, to provide control signals to control valve 62.

In operation, distillate fuel level sensor 39, sens- ing a low level or entire lack of distillate fuel in the distillate fuel storage container 38b, prompts sending unit D to send a signal via control line 39b to cause con¬ trol valve 62 to open, thereby permitting the flow of a heating fluid through heating fluid inlet 60, heating coil 60a and thence out of fuel tank 12 through heating fluid outlet 60b. Insulation material 13 serves to reduce heat losses from fuel tank 12. As a consequence of heating the gasoline or other liquid hydrocarbon fuel stored in fuel tank 12, a low boiling fraction of hydrocarbons from the liquid hydrocarbon fuel contained within fuel tank 12 is vaporized and passes through vapor line 34, through check valve 36 and heat exchanger 33, and into condenser 38. If the temperature of the gasoline within fuel tank 12 be¬ comes too high, sending unit T of temperature probe 64 sends a signal through control line 64a to close valve 62 to reduce or stop altogether the in-flow of heating fluid. Should the level, i.e., quantity, of fuel in fuel tank 12 become exceedingly low, fuel level sensor 66 emits a sig-

nal from its sending unit L through control line 66a to move valve 62 in a closing direction to reduce or elim¬ inate the flow of heating fluid.

Heat exchanger 33 withdraws heat from the vapor flow- ing in vapor line 34 to promote condensation of the dis¬ tillate fuel in condenser 38, which then collects in dis¬ tillate fuel storage container 38b. Heat exchanger 33 may be a plate-type heat exchanger which may utilize coolant from the vehicle air conditioning system. Compressed air may be introduced into condenser 38 by compressed air pump 40 via line 42 through check valve 44 and valve 46 to fa¬ cilitate condensation within condenser 38 by increasing the pressure therein. Any other suitable expedient may be employed to facilitate condensation of the vapor within condenser 38, as by equipping the exterior of condenser 38 with cooling fins (not shown), using a condensation coil driven b ^v the vehicle air conditioning s ^v stem* si ^ a fan to provide air-cooling, etc.

Condenser 38 is equipped with a pressure regulator P and a vent valve 37. When the pressure in condenser 38 rises to a point at which the flow of vapor from tank 12 through vapor line 34 is inhibited, regulator P opens vent valve 37 to reduce the pressure in condenser 38. A vent line (not shown) runs from valve 37 to fuel tank 12 to re- turn vented distillate vapors back into fuel tank 12. The pressure regulator P also controls valve 46 so that the pressure in the condenser 38 can be maintained at a preset minimal level, e.g. 5 psi, by the addition of air from air pump 40. It will be apparent that distillate fuel Fd will thus accumulate within the storage container 38b of condenser 38 when heated fluid passes through heating coil 60a, or that the system may be "primed" by providing an appropri¬ ate quantity of a light distillate fuel Fd in the storage container 38b. In any case, upon the starting of the en¬ gine, if distillate fuel level sensor 39 indicates the presence of an adequate supply of liquid distillate fuel, sending unit D emits a signal -via signal control line 39a

to valve 26 to close off the flow of liquid fuel from fuel tank 12 to fuel pump 28 and emits a signal via signal control line 39c to open valve 56 to the flow of distil¬ late fuel from storage container 38b via line 54 and 54a to valve 26 as long as there is an adequate supply of li¬ quid distillate fuel in storage container 38b. The dis¬ tillate fuel then flows to the engine through the liquid fuel transfer means, i.e., through fuel line 24, pump 28, fuel line 24a, valve 59, and through catalyst bed 30 or by-pass 50 to fuel line 24b, into carburetor 18, wherein it is mixed with air introduced via air inlet 18a to pro¬ vide a start-up fuel/air combustion mixture for engine start-up. If there is not an adequate supply of liquid distillate fuel in storage container 38b, fuel is drawn from tank 12 as described below. Heating means may be provided to heat catalyst bed 30 to improve its conversion performance, e.g., a side stream of hot exhaust gases may be drawn directly from the engine exhaust gas manifold and passed through line 32, coil 32a and line 32b, to heat catalyst bed 30, or a heating coil powered through the vehicle's electrical system may be used to heat catalyst bed 30.

The period of time during which engine 16 is supplied with the distillate fuel Fd from storage container 38b may be determined by timer 68 which sends a signal via timer signal line 68a to solenoid-operated valve 26. The timer signal may allow valve 26 to open for the flow of liquid distillate fuel according to signals received from sending unit D for a predetermined period of time after engine start-up, for example, two minutes, after which time the engine can be expected to combust the liquid hydrocarbon fuel more efficiently. Alternatively, a temperature sens¬ or (not shown) may be used to hold valves 26 and 56 in position to supply distillate fuel Fd to engine 16 (sub- ject to the availability of such fuel as indicated by sending unit D) until the engine attains a predetermined operating temperature which may be indicated by the tem¬ perature of the engine exhaust-gas manifold. In yet an-

other alternative, or in addition, temperature sensors may sense the temperature of the exhaust gas fed to the cata¬ lytic converter 22 or the temperature of the catalytic converter 22 itself, and use such temperature to determine the period for which valve 26 allows distillate fuel Fd to be fed to engine 16. During this initial period of opera¬ tion, e.g., about the first two minutes of operation, the engine is thus supplied with a light distillate fuel Fd which has been cracked by treatment in catalyst bed 30 to provide a fuel which is rapidly ignitable and which may be efficiently combusted in a lean fuel/air mixture. These factors greatly reduce the amount of hydrocarbon pollu¬ tants emitted in the exhaust gas during the initial period of engine operation. This is of great benefit because during this initial period the catalytic converter 22 is cold, i.e., it has not yet attained its normal operating temperature range, and is therefore much less efficient in catalyzing the oxidation of hydrocarbons and carbon monox¬ ide than it is when it has been heated to a temperature within its normal operating range, e.g., from about 300° to 800°C. Thus, hydrocarbon emissions are significantly reduced during the critical initial cold-start period de¬ spite the inactivity of the catalytic converter.

After the predetermined time has lapsed, or the pre- determined engine exhaust gas or pollution abatement cata¬ lyst temperature has been attained, timer 68 or another appropriate control mechanism switches valve 26 to stop the flow of distillate fuel Fd from storage container 38b and to begin the flow of the liquid hydrocarbon fuel, e.g., gasoline, from fuel tank 12 to engine 16 via the liquid fuel transfer means described above. Engine 16 is thus supplied with the gasoline or other liquid hydrocar¬ bon fuel stored in fuel tank 12 for a subsequent engine operating period following the initial engine operating period. The gasoline or other liquid fuel remaining in fuel tank 12 may be considered enriched in octane depend¬ ing on the amount of fuel of low boiling point fraction which has been removed therefrem. Preferably, the subse-

quent engine operating period begins after the catalytic converter has been heated to within its normal operating temperature range.

As discussed above, should the level of distillate fuel Fd contained in storage container 38b drop to a pre¬ determined level deemed to be inadequate, distillate fuel level sensor 39 sends a signal via control line 39a to valve 26 which pre-empts timer 68 (or a temperature sensor or other control device, as discussed above) and switches the valve to supply the engine 16 with fuel from fuel tank 12. However, storage container 38b will be sized to hold a quantity of distillate fuel projected to be adequate to start the engine until normal engine operation replenishes the supply by vaporizing the liquid hydrocarbon fuel in tank 12. The light distillate fuel will thus be continu¬ ously replenished by vapor evaporated from the fuel in tank 12 as described above. Therefore, an ample supply of distillate fuel Fd should be available at all times for any reasonable duration of the initial operating periods. Regeneration of cracking catalyst bed 30 will be re¬ quired from time to time. Generally, regeneration may be achieved by providing sufficient oxygen in the gas flowed through catalyst bed 30 at a sufficiently high tempera¬ ture, to oxidize carbonaceous deposits which tend to ac- cumulate on the cracking catalyst during use. Heating coil 32a supplies heat during regeneration in the same manner as it does during cracking operation, as described above. Therefore, it is preferred to regenerate catalyst bed 30 when the exhaust gases are hot, i.e., after the in- itial cold-start period of operation and while the engine is running on liquid hydrocarbon fuel from fuel tank 12. During regeneration, the flow fuel by-passes catalyst bed 30 via by-pass line 50 as described below. Regeneration is initiated by a regeneration signal from a regeneration control means (not shown) at timed intervals or when a sensor indicates that the catalyst in bed 30 is not oper¬ ating effectively. Preferably, regeneration is performed during extended periods of operation and for this reason,

the regeneration control means may comprise a mechanism controlled by the vehicle operator at appropriate times. Upon receipt of the initiation signal, valve 59 closes off the flow of fuel through catalyst bed 30 and directs the fuel through by-pass line 50, and then to the engine via line 24b as before. In addition, valve 46 admits com¬ pressed air from compressed air pump 40 and line 42 into line 48 for introduction into fuel line 24a, in order to pass combustion air through catalyst bed 30 to oxidize carbonaceous deposits accumulated on the cracking catalyst contained therein, and thereby regenerate the catalyst. During regeneration, solenoid-operated valve 70 is positioned to isolate the effluent of catalyst bed 30 from carburetor 18 and to admit the discharged regeneration products emerging from catalyst bed 30 via line 72 into exhaust pipe 20 at a point upstream of catalytic converter 22 so that the regeneration by-products are treated in catalytic converter 22.

Generally, it will be observed that the embodiment of the invention illustrated in Figure 1 provides for contin¬ uous replenishment of the distillate fuel Fd by vaporiza¬ tion of the gasoline or other hydrocarbon liquid fuel con¬ tained in fuel tank 12 and condensation of light distil¬ late fuel therefrom within condenser 38. This light dis- tillate fuel is utilized during an initial cold-start en¬ gine operating period to thereby reduce the emission of hydrocarbon pollutants at least until the catalytic con¬ verter 22 has reached its normal operating temperature and thereby its usual operating efficiency. The light distil- late fuel Fd may be further enhanced by being cracked/iso- merized by being passed through the cracking catalyst con¬ tained in catalyst bed 30. After a predetermined period of initial engine operation, controlled either by a timer or by sensing the temperature of the engine or the exhaust or the catalytic converter (22 in the illustrated embodi¬ ment) temperature, the fuel supply is switched to the con¬ ventional gasoline or other liquid hydrocarbon fuel con¬ tained in fuel tank 12. This fuel is passed through the

cracking catalyst bed (30 in the illustrated embodiment) to provide a cleaner burning fuel for the subsequent en¬ gine period of operation.

While the invention has been described in detail with respect to specific preferred embodiments thereof, it will be appreciated upon a reading and understanding of the foregoing that numerous variations may be made to those embodiments which nonetheless lie within the spirit and scope of the appended claims.