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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Fuel flow meter
for diesel engines

Click for larger image

The easiest way to measure fuel flow to an engine (for calculating fuel economy, etc) is to monitor the average pulse width of the fuel injectors. However, this doesn’t work for diesel engines with mechanical fuel injectors.

In this case, it is necessary to use a flow sensor. This is not as straightforward as it seems because the fuel pump runs continuously, keeping the fuel supply to the engine at a constant pressure. Any fuel that is not burned in the engine is fed back to the tank by a return pipe. As a result, it is impossible to know how much fuel is being burned by measuring just the flow between the fuel pump and the engine.

The solution is to use two flow sensors, one to measure the fuel flowing to the engine (the “inlet” sensor) and one to measure the fuel flowing back (the “return” sensor). By calculating the difference, the actual amount of fuel burned can be determined.

Click for larger image

There are some challenges to implementing such a scheme. At low RPM, the difference in flow rates is small so the readings must be very accurate. To make matters worse, the data from the return sensor at low RPM can be quite erratic (see graph) and must be time-averaged in order to get an accurate result.

This circuit solves those problems. It is built around an ATMega8 microcontroller (IC1). The two flow sensors, with open collector outputs, are connected to inputs INT0 and INT1 (pins 4 and 5). Two 10kΩ pull-up resistors convert the sensor outputs to logic levels. When the output from either flow sensor changes, it triggers an interrupt routine in the IC.

It measures the period of the resulting pulses and from this computes the frequency and hence flow rate. The difference is displayed on two multiplexed, 4-digit, 7-segment LED displays.

These are common cathode displays and the cathodes are driven by NPN transistors from outputs PC0-PC5 and PB2-PB3 of IC1 (pins 23-28 and 16-17), while the anodes are driven by outputs PD0-PD1, PD4-PD7 and PB0-PB1 (pins 2, 3, 6 & 11-15). The display brightness can be changed by modifying the software to alter the LED duty cycle.

By default the upper display shows the instantaneous flow rate and the lower display shows the total volume (ie, the integral of the flow rate over time) since power has been applied. The source code can be modified to show different figures if desired. For example, one display could instead show the instantaneous flow or total volume from either the inlet or return sensor.

Jaycar Cat. ZD-1202 flow sensors can be used with this circuit but only for liquids such as water. A version of this sensor that is rated for use with fuel (gasoline, diesel etc) is available from the same manufacturer. These sensors contain paddle-wheels which turn as the liquid flows. They are well-calibrated, as is required for accurate differential measurements.

The power supply is simple, consisting of a fuse, power switch, 7805 linear regulator and a number of bypass capacitors. The supply voltage can be 7-15V but the 7805 may require a heatsink for supplies above 9V. Higher supply voltages (up to 35V) are possible with a larger heatsink.

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