Method of operating vehicle during its in-plant assembly
BACKGROUND OF THE INVENTION 
     In assembly plants or storage depots, newly assembled vehicles are usually driven under their own power between several locations all within a short distance of each other. As a result, the vehicle engine runs for very short intervals with a fuel rich mixture at temperatures well below its normal operating temperature (50-70% of normal operating temperature, i.e., 350°-400° C. at plug tip). 
     In spark ignition internal combustion engines, such operation frequently leads to fouling of the engine spark plugs by carbon and unburnt fuel deposition on electrodes as result of imperfect combustion. This is to be distinguished from gum deposits which form as fuel residue and cannot be burnt away at normal operating temperatures. The fouling provides difficulties in moving vehicles within the plant or depot. Additionally, when the vehicle is delivered to the retailer, it is often necessary to fit a new set of spark plugs to the engine before the vehicle is sold to the customer. This adds to the final cost of the vehicle. 
     This invention relates to methods of operating newly assembled motor vehicles in vehicle assembly plants or storage depots. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method of operating a newly assembled vehicle in a vehicle assembly plant or storage depot, the vehicle having a spark ignition internal combustion engine and comprising driving the vehicle under its own power with a fuel-rich mixture between a plurality of locations within the plant and switching off the engine when the vehicle is at rest in the locations, characterized in that the engine is driven from a fuel composition containing a gasoline fuel and a minor proportion of a mineral oil. 
     We have found that the addition of a minor proportion of mineral oil to the fuel composition significantly reduces the risk of fouling of spark plugs in newly assembled vehicles operated in plants or depots. 
    
    
     SUMMARY OF THE DRAWING 
     FIG. 1 is a schematic diagram of an in-plant driving cycle used to test the performance of the fuel mixture of this invention. 
    
    
     DETAILED DESCRIPTION 
     It is well known to use fuel compositions containing so-called upper cylinder lubricants when driving the vehicle under normal conditions and some fuel additives used for this purpose do contain mineral oil. However, such fuel compositions are not used in newly assembled vehicles because the benefit of upper cylinder lubrication is only obtained when the vehicle is driven for prolonged periods. 
     The benefit of the invention can be obtained using any mineral oil heavier or more viscous than gasoline. Viscosity of oil is determined by use of a viscosimeter, a device that determines the length of time required for a definite amount of oil to flow through an opening of a definite diameter. Temperature is taken into consideration during this test, since high temperature decreases viscosity and low temperature increases viscosity. The viscosity of the additive should be greater than 15 SUS (Saybolt Universal Seconds) at 100° F. (37.8° C.), which is roughly the viscosity of conventional gasoline. The oils preferably suggested to be used are standard engine lubricating oils having a viscosity of 24-1500 SUS at 100° F. (37.8° C.), or a viscosity classified by the U.S. Society of Automotive Engineers (SAE J300D). In referring to viscosity, the lower numbers refer to oils of lower viscosity (thinner). The Society of Automotive Engineers rates oil viscosity in two different ways, for winter and for other than winter. Winter-grade oils are tested at 18° C. There are three winter grades, SAE5W, SAE10W, and SAE20W, the &#34;W&#34; indicating winter grade. For other than winter use, the grades are SAE20, SAE30, SAE40 and SAE50, all without the &#34;W&#34; suffix. Some oils have multiple ratings, which means they are equivalent in viscosity to several single-rating oils. An SAE10W-30 oil, for example, is comparable to SAE10W, SAE20W and SAE30 oils. 
     
         ______________________________________                                    
SAE GRADE                                                                 
______________________________________                                    
          MAX VISCOSITY IN CENTIPOISE                                     
          at 18° C.                                                
 5W        1250                                                           
10W        2500                                                           
20W       10000                                                           
          VISCOSITY RANGE IN CENTISTOKES                                  
          at 100° C.                                               
20        5.9-9.3                                                         
30         9.3-12.5                                                       
40        12.5-16.3                                                       
50        16.3-21.9                                                       
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     For example, we have found that the incorporation of heavy fuel oils such as diesel oil in the gasoline reduces in-plant fouling. Preferably, however, the mineral oil is a petroleum based lubricating oil, and motor vehicle engine oils are especially suitable. 
     Any minor proportion of mineral oil is included in the composition, 0.2-10% by volume. Preferably, however, the mineral oil constitutes less than 5% by volume thereof to reduce the amount of pollutants emitted in the engine exhaust fumes. In this respect, the most satisfactory results have been obtained using 0.4% by volume of mineral oil. 
     Any gasoline suitable for spark ignition internal combustion engines may be used. Standard gasoline fuels having Research Octane Numbers of from 90 to 100, and preferably from 95 to 98, will normally be appropriate. 
     The following examples illustrate the invention: 
     Using the fuel composition identified in Table II below, a motor vehicle was subjected to the test procedure described below which simulates the conditions under which newly assembled motor vehicles are driven between various locations in a vehicle assembly plant or storage depot. 
     The motor vehicle was a Ford Escort (Registered Trademark) saloon car having a conventional, manualtransmission, 4-cylinder, spark ignition, internal combustion engine fed by a carburetor fitted with a manually operated choke control. The engine was fitted with standard AGR 22 spark plugs. 
     Prior to each test procedure, the engine was fitted with clean spark plugs and the fuel tank was drained and then filled with the test fuel composition. The carburetor was adjusted so that the choke control has no effect upon the position of the throttle control and the idle jet was adjusted to produce an idle speed of 800-50 revolutions per minute with the engine at ambient temperature. 
     The vehicle was then driven around the circuit illustrated in the drawing starting point 1 and passing through points 2 to 14 in numerical order. The total length of the circuit was 0.1 miles (0.15 kn). Table 1 summarizes the routines performed at points 1 to 14 in the circuit. 
     
                       TABLE I                                                     
______________________________________                                    
Point No.                                                                 
         Routine                                                          
______________________________________                                    
1        (i)    Measure shunt resistance of spark plugs                   
                while installed in the engine                             
         (ii)   Pull choke control to ensure that choke                   
                butterfly is fully shut                                   
         (iii)  Switch on ignition                                        
         (iv)   Fully depress throttle pedal twice                        
         (v)    Start engine                                              
         (vi)   Allow time for oil light to go out then                   
                engage bottome (1st) gear and immediately                 
                drive gently away.                                        
2        (i)    Engage second gear                                        
         (ii)   Accelerate gently                                         
3        (i)    Engage third gear                                         
         (ii)   Fully depress throttle pedal immediately                  
                clutch is released                                        
4        (i)    Release throttle pedal                                    
         (ii)   Apply brakes in preparation for taking                    
                corner                                                    
         (iii)  Engage second gear                                        
5               Drive round corner in second gear with                    
                trailing throttle                                         
6               Drive gently in second gear                               
7        (i)    Engage third gear                                         
         (ii)   Fully depress throttle pedal immediately                  
                clutch is released.                                       
8        (i)    Release throttle pedal                                    
         (ii)   Apply brakes in preparation for                           
                taking corner                                             
         (iii)  Engage second gear                                        
9               Drive round corner in second gear with                    
                trailing throttle                                         
10              Drive gently in second gear                               
11       (i)    Engage third gear                                         
         (ii)   Fully depress pedal immediately                           
                clutch is released                                        
12       (i)    Release throttle pedal                                    
         (ii)   Apply brakes and bring car to a                           
                standstill                                                
13       (i)    Engage reverse gear                                       
         (ii)   Gently reverse into bay                                   
14       (i)    With gearbox in neutral and clutch                        
                engaged, note engine idle speed, if                       
                (a) less than 1200 rev/min, blip                          
                throttle to obtain 1200 rev/min and                       
                immediately switch off ignition, if (b)                   
                greater than 1200 rev/min, switch off                     
                ignition immediately.                                     
         (ii)   Measure shunt resistances of spark                        
                plugs while installed in the engine.                      
         (iii)  Allow engine to cool for at least                         
                one hour.                                                 
______________________________________                                    
 
    
     The vehicle was repeatedly subjected to the test drive until one or more of the spark plugs stopped firing and failed to fire agin. This normally occurred either when attempting to start the engine at the beginning of a test drive or when third gear had been engaged and the vehicle was being driven with the carburetor throttle wide open, when the engine misfired. The shunt resistance of failed spark plugs was usually less than 0.5 megohms. 
     Table II summarizes the number of circuits performed by the vehicle until one or more of the spark plugs failed. Examples A to G are without additive oil for comparison purposes. 
     
                       TABLE II                                                    
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                                  % By  No. of                            
Ex-  Spark                        Vol.  Test                              
am-  Plug                         Nu-   Cycles                            
ple  Gap     Gasoline    Numeral  meral Before                            
No.  (mm)    Type        Oil Type Oil   Fouling                           
______________________________________                                    
A    0.8     .sup.(1)                                                     
                   Texaco  --       --     9                              
                   BS4040                                                 
                   &#34;4-star&#34;                                               
                   98 R.O.N.                                              
B    &#34;       .sup.(1)                                                     
                   Texaco  --       --     4                              
                   BS4040                                                 
                   &#34;4-star&#34;                                               
                   98 R.O.N.                                              
C    &#34;       .sup.(2)                                                     
                   Esso    --       --    10                              
                   BS4040                                                 
                   &#34;4-star&#34;                                               
                   98 R.O.N.                                              
D    &#34;       .sup.(2)                                                     
                   Esso    --       --     9                              
                   BS4040                                                 
                   &#34;4-star&#34;                                               
                   98 R.O.N.                                              
E    &#34;       .sup.(3)                                                     
                   Esso    --       --    16                              
                   BS4040                                                 
                   &#34;3-star&#34;                                               
                   95 R.O.N.                                              
F    &#34;       .sup.(3)                                                     
                   Esso    --       --     8                              
                   BS4040                                                 
                   &#34;3-star&#34;                                               
                   95 R.O.N.                                              
G    &#34;       .sup.(3)                                                     
                   Esso    --       --     6                              
                   BS4040                                                 
                   &#34;3-star&#34;                                               
                   95 R.O.N.                                              
H    &#34;             Esso    --       --     6                              
                   BS4040                                                 
                   &#34;4-star&#34;                                               
I    &#34;             Esso    --       --     9                              
                   BS4040                                                 
                   &#34;4-star&#34;                                               
1    &#34;             Texaco  Texaco   5     20                              
                   BS4040  Based                                          
                   &#34;3-star&#34;                                               
                           Oil.sup.(4)                                    
                   95 R.O.N.                                              
                           1050 SUS                                       
                           at 37/8° C.                             
                           (100° F.)                               
2    &#34;             Esso    Esso     5     19                              
                   BS4040  Bright                                         
                   &#34;3-star&#34;                                               
                           Stock.sup.(5)                                  
                   95 R.O.N.                                              
                           530mm.sup.2 /sec.                              
                           at 37.8° C.                             
3    &#34;             Esso    Esso     5     18                              
                   BS4040  Bright                                         
                   &#34;3-star&#34;                                               
                           Stock.sup.(5)                                  
                   95 R.O.N.                                              
                           530mm.sup.2 /sec.                              
                           at 37.8° C.                             
4    &#34;             Esso    Esso     5     20                              
                   BS4040  Bright                                         
                   &#34;3-star&#34;                                               
                           Stock.sup.(5)                                  
                   95 R.O.N.                                              
                           530mm.sup.2 /sec.                              
                           at 37.8° C.                             
5    &#34;             Texaco  Texaco   25    None                            
                   BS4040  20/30          after                           
                   &#34;4-star&#34;                                               
                           engine         18.                             
                   98 R.N. lub. oil                                       
6    &#34;             Texaco  Texaco   10    None                            
                   BS4040  20/30          after                           
                   &#34;4-star&#34;                                               
                           engine         18.                             
                   98 R.N. lub. oil                                       
7    &#34;             Texaco  Texaco   5     11                              
                   BS4040  20/30                                          
                   &#34;4-star&#34;                                               
                           engine                                         
                   98 R.N. lub. oil                                       
8    &#34;             Texaco  Texaco   4     None                            
                   BS4040  based oil      after                           
                   &#34;4-star&#34;                                               
                           w/Texaco       19.                             
                   98 R.N. viscosity                                      
                           improvement                                    
9    &#34;             Texaco  Esso Stanco-                                   
                                    4     27                              
                   BS4040  600 grade                                      
                   &#34;4-star&#34;                                               
                           engine oil                                     
                   98 R.N. 130mm.sup.2 /sec.                              
                           at 37.8° C.                             
10   &#34;             Texaco  Esso Stanco-                                   
                                    10    20                              
                   BS4040  130 grade                                      
                   &#34;4-star&#34;                                               
                           engine oil                                     
                   98 R.N. 25mm.sup.2 /sec.                               
11   &#34;             Texaco  Esso Stanco                                    
                                    0.1   14                              
                   BS4040  2500 grade                                     
                   &#34;4-star&#34;                                               
                           eng. oil                                       
                           550mm.sup.2 /sec.                              
                           at 37.8° C.                             
12   &#34;             Texaco  Esso Stanco                                    
                                    0.2   20                              
                   BS4040  2500 grade                                     
                   &#34;4-star&#34;                                               
                           eng. oil                                       
                           550mm.sup.2 /sec.                              
                           at 37.8° C.                             
13   &#34;             Texaco  Esso Stanco                                    
                                    0.4   27                              
                   BS4040  2500 grade                                     
                   &#34;4-star&#34;                                               
                           eng. oil                                       
                           550mm.sup.2 /sec.                              
                           at 37.8° C.                             
14   &#34;             Texaco  Esso Stanco                                    
                                    0.8   22                              
                   BS4040  2500 grade                                     
                   &#34;4-star&#34;                                               
                           eng. oil                                       
                           550mm.sup.2 /sec.                              
                           at 37.8° C.                             
15   &#34;             Texaco  Esso Stanco                                    
                                    2.0   17                              
                   BS4040  2500 grade                                     
                   &#34;4-star&#34;                                               
                           eng. oil                                       
                           550mm.sup.2 /sec.                              
                           at 37.8° C.                             
______________________________________                                    
 Footnotes to Table II                                                    
 .sup.(1), (2) &#34;Esso and &#34;Texaco&#34; are registered trademarks.              
 .sup.(3) &#34;R.O.N.&#34; means Research Octane Number.                          
 .sup.(4), (5) Texaco Base Oil and Esso Bright Stock base oils used for   
 commercial grades of engine lubricating oil.                             
 
    
     On comparing the test results obtained in Examples 1 to 16 with those of the comparative Examples A to H, it can clearly be seen that the addition of an adequate quantity of mineral oil to the gasoline based fuel composition reduced the frequency of spark plug fouling. A proportion of 0.2-10% by volume of mineral oil showed the best results and mineral oils with a viscosity of 25-550 mm 2  per second at 37.8° C. (100° F.) worked particularly well.