32.2.
Fuel Consumption
Measurement of instantaneous fuel consumption rate is not simple, especially in a SI engine, because fuel flow rate into the carburettor may not be equal to flow rate to engine. However, it is possible to measure average flow rate over a short period of time. The commonly used instrument for this purpose is of the positive displacement type. The instrument consists of two main components; a metering unit which is placed in the fuel line between the supply tank of the vehicle and the engine, and an indicator which is conveniently located where it can be seen by the test operator.
The metering unit typically consists of two small chambers each containing fuel and is connectable either to the input pipe from the fuel tank or to the outlet pipe to the engine depending upon the position of the solenoid operated side valve. The two chambers are separated by a flexible diaphragm. As the fuel is delivered to the engine from the first of the small chambers the diaphragm is displaced until it reaches the extremity of its stroke. At this point, which corresponds to a fixed and known amount of fuel, an electrical contact is made which causes the solenoid to changeover the fuel connections to the two chambers and to cause the digital recorder in the indicator unit to notch on one unit. The second chamber then delivers fuel to the engine and, when the diaphragm reaches the other extremity of its stroke, again an electrical contact is made causing the solenoid valve to changeover the connections and to record another unit on the indicator. The electric supply for the instrument is the normal DC supply of the vehicle.
The calibration of the instrument is fixed by the size of the small chambers and is arranged by the manufacturer to give a convenient number of “clicks” per liter, say 500. In use, the operator resets the indicator to zero and, a the start of the test, he switches on the instrument and a stop-watch simultaneously. After a suitable period of time the indicator and stop-watch are switched off. The average fuel flow rate during the time interval is computed. Alternately, instead of using the instrument in conjunction with a stop-watch, the odometer in the vehicle may be used to provide the fuel consumption rate in km per liter instead of liters per second.
