The inconvenience of being stranded on a road/street/highway when the tank runs dry is only surpassed by the embarrassment that comes with it. In any case, there is no excuse for running out of fuel in a small city like Singapore, because there’s always a petrol/diesel station within a 15-minute drive, and it’s open all day and night.
On the face of it, the fuel gauge is a simple device. As a rule of thumb, it doesn’t even indicate/calculate the quantity of fuel in litres (or gallons), since all that is needed to alert the driver is the amount of fuel relative to the tank’s capacity.
In this regard, fuel gauges have just two main markings – F for Full and E for Empty. Some have a “½” marking, too, to show half-tank. The exact quantum in litres is a piece of information that is not critical to the driving, and the info is probably meaningless to the driver anyway, even someone obsessed with his car’s FC (fuel consumption).
The basic mechanical fuel gauge consists of very few parts, and its operation relies on the same simple principle as a voltmeter (voltage-measuring analogue meter).
Essentially, the fuel gauge just needs to measure voltage across a variable resistor. But said resistor must somehow vary in direct proportion to the amount of fuel in the tank. To do this, within the fuel tank is a float at one end of a lever arm-and-slider running over a strip of resistor at the other end. Known as the Sending Unit, it is a vital part of every vehicular fuel tank and works flawlessly.
As the float moves vertically with the changing fuel level, the opposite end of the lever slides across the resistor, causing a change in voltage supply at the gauge. Finally, in direct response, the indicating needle moves to the appropriate position on the gauge.
Because of their simplicity and reliability, the fuel measurement and its display have remained pretty much unchanged over the decades. However, like almost everything in the modern motorcar, some electronic devices have been included, not as a replacement for the proven classical arrangement but to complement it.
Incidentally, the fuel gauge does not provide any input to the car’s on-board computer, except in some cases to reset the average fuel consumption after the tank has been refuelled.
Where electronics are used, the Sending Unit feeds into a tiny electronic circuit to trigger a “Fuel Low” or “Please Refuel” warning light at a preset level, usually accompanied by a fuel dispenser symbol. It provides a clear visual notification that the vehicle can travel only 30km to 60km more with the remaining fuel. In fact, most fuel gauges are notoriously pessimistic, so as to provide plenty of “allowance” for the optimistic driver running on empty.
In certain cars, digital gauges with LCD or LED readouts have replaced the mechanical instrument and its analogue needle. The function of such gauges is the same, and the good old float arrangement continues to serve as the measuring device. Only the display method is different.
An electronic processor receives the signal from the Sending Unit, converts it into digital signals and updates an electronic module that controls the display. Some of the systems employ a bar graph, while the more recent ones reproduce a graphic image of the analogue fuel gauge.
Note that whatever the display, none indicates numerical values in litres or gallons – just the all-important E and F.