fig1 illustrates a fuel detection system for detecting the presence and amount of fuel in the cylinders of a fuel injection engine . a fuel / air mixture from the fuel inlet line 1 to the cylinders 3 of a fuel injection engine 2 passes into a sample tube 4 . sample tube 4 extends from the fuel inlet line when the fuel injection engine is in operation and fuel is being combusted . alternatively , when there is no combustion taking place in the engine , sample tube 4 can extend from the gloplug ports of engine 2 , if no plugs are utilized . sample tube 4 can also extend from the exhaust manifold of engine 2 when no ignition in the cylinders is occurring . in each event , a sample of the fuel mixture to the cylinders is to be provided . the fuel / air mixture forms a fuel vapor laden stream which moves through tube 4 to a diverter 6 at a high velocity in excess of 100 feet / second . most of the stream flows past the diverter 6 , which is a reduced tee fitting , and out through vent tube 32 to a standard fume collection system , such as a negative pressure vent system containing a duct leading to an exhaust fan with vapor filters positioned in the vent line . in this manner , the fuel vapor can be recaptured and recycled when the engine 2 is on a production test stand . a small portion of the stream is diverted by diverter 6 into a short sample tube 8 . this small portion of the stream flows through sample tube 8 at a low velocity of approximately 5 feet / second . the low velocity is attained because diverter 6 directs a sufficiently small volume of air and vapor into sample tube 8 to allow a reduction in velocity . the vapor laden stream is then slowed to a still lower velocity in the last inch of the sample tube 8 , before entry into a sensor chamber 12 which houses a sensor 10 . the air stream attains the still lower velocity due to the stream passing from sample tube 8 into the sensor chamber 12 , which has a greater cross - sectional dimension than does sample tube 8 . this final deceleration is necessary , since the output and sensitivity of sensor 10 decreases as the velocity of the air stream increases . therefore , it is desirable to minimize variances in output by having the air stream flow into sensor 10 with a minimum velocity . this avoids the use of circuitry or logic to compensate for variances in output . a portion of the vapor laden stream then flows into sensor 10 , passes through the sensor 10 and into a rear portion 16 of the sensor chamber 12 , where it exits via vent tube 18 to a fume collection system as previously described herein . the sensor 10 is supported or mounted in the sensor chamber 12 by a mounting plate 14 . the sensor 10 is connected by wiring 20 to external electronics , which are described in greater detail hereinafter , and to a monitor 22 , which may be an amplifier , a computer , such as programmable controller or any other type of voltage monitoring device . the sensor 10 has fast reponse and recovery times since no particulate hydrocarbons contained in the fuel vapor are permitted to reach the sensor . this is quite important as particulate matter entering sensor 10 can prevent the sensor 10 from functioning efficiently . the particulate hydrocarbons contained in the fuel vapor do not reach the sensor 10 since the fuel - containing stream , passing through sample tube 8 , is at a sufficiently low velocity such that the particulate hydrocarbons , which are heavier than the vapor , cannot be carried along in the air stream . in addition , the low velocity air stream prevents any significant alteration of the temperature equilibrium of the sensor 10 . thus , hydrocarbon particules or droplets are first eliminated at the diverter 6 , where the sample destined for the sensor 10 undergoes a 90 ° change in the direction of its flow due to its passing through the diverter 6 , which has a ♭ tee &# 34 ; configuration , and , therefore , simultaneously has its velocity reduced . the diverter 6 is mounted so that the sample air stream destined for the sensor 10 leaves vertically and travels upwards about one foot before entering the sensor 10 , which is mounted horizontally . in a preferred embodiment of the invention , the sample tube 4 through the diverter 6 has approximately ten times the cross sectional diameter of the sample tube 8 from the diverter 6 to the sensor 10 , and there is very little back pressure on the bypass line 34 from the diverter 6 to the vent 32 . the particles or droplets which enter sample tube 8 fall , due to the low velocity of the air stream , and are picked up by the high velocity air stream and are exhausted through vent 32 to a suitable exhaust collection system . any remaining particles or droplets are removed in the sensor housing 12 , where the sample air stream undergoes its final deceleration , due to the stream passing into the housing 12 , which has a greater cross - sectional diameter than does sample tube 8 . this causes any particles or droplets to bypass the sensor 10 . with reference to fig2 the sensing system for detecting the presence of fuel vapors is illustrated . the temperature of the sensor 10 is maintained by a heater 24 , which is connected to a power supply 26 . when voltage is applied to the sensor 10 by power supply 30 , a voltage v 1 is developed across load resistor 28 , and a voltage v 2 is also developed across the sensor 10 . when fuel vapors are present in the air stream entering the sensor 10 , this causes the resistance of sensor 10 to decrease and v 1 to increase . at the same time , v 2 decreases . the voltage monitor or amplifier 22 is connected to the circuit to monitor voltages v 1 and v 2 for control purposes . a change in either voltage , v 1 or v 2 , typically , of at least two volts , as monitored by amplifier 22 , determines the presence of the fuel vapors flowing into the cylinders of engine 2 . fig3 illustrates a graphical representation of the response of the sensor 10 when used in the method of the present invention . tests were run using a typical v - 8 diesel engine at two different speeds , 500 rpm and 1500 rpm . in fig3 the ordinate depicts the voltage , in volts , across sensor 10 as monitored by amplifier 22 . the left side of the abscissa depicts time , in seconds , after fuel begins to flow into the cylinders of engine 2 . the right side of the abscissa depicts time , in seconds , after the fuel flow to the cylinders is shut off . fig3 shows that the voltage change across sensor 10 is approximately 2 . 5 - 3 . 0 volts over a period of 3 seconds after fuel flow into the cylinders of engine 2 begins . as can be seen with reference to fig3 the response was good at both speeds . it must be noted that the amount of voltage change across sensor 10 is relatively independent of the velocity of the fuel vapor - laden air stream as it flows into sensor 10 , and the type of hydrocarbonaceous fuel being used . it is presumed that one skilled in the art can generate such graphs for fuels and flow rates other than those used to generate the data shown in fig3 . a sensor that has been found satisfactory for use in the present invention is the tgs - 812 gas sensor of figaro engineering inc . this is a fuel vapor sensitive semiconductor of the type disclosed in taguchi , u . s . pat . no . 3 , 900 , 815 . in a gas sensing device of the type described in taguchi , a powdered metal oxide semiconductor material is mixed with a material , such as stearic acid , which evaporates , sublimates or burns away when heated and produces a number of holes therein . the mixture is applied to a ceramic supporting material and is then heated at an elevated temperature . the resulting metal oxide semiconductor material is disposed between two electrodes to form the gas sensing element . the gas sensing element is used as an alarm device to detect gas leaks for safety purposes . the sensor has a signal output of several volts without amplification . there has been provided by the present invention a method and apparatus for detecting the presence of fuel vapors in the cylinders of a fuel injection engine chemically rather than detecting a physical property of the fuel . the novel method and apparatus of this invention can be utilized with an engine production test stand and facilitates fast response and recovery times in the sensing system . it should be understood that the foregoing disclosure emphasizes certain specific embodiments of the invention and that all modifications or alternatives equivalent thereto are within the spirit or scope of the invention as set forth in the appended claims .