Injection Fuel Pump Sizing etc

[gerrym]

So you have decided on an injection setup for your Zetec, high output Twincam etc.

What size of injection pump should you fit, how many amps, how big should the fuel line be etc?

A few sizing calcs follow:- (these have all come from an Excel sheet to facilitate the what-ifs and customisation).

A:/ How much fuel does the engine need at maximum output.

Assume fuel is petrol, calorific value is 5.5MJ/kg, density 0.74 kg/litre = 33.3 MJ/litre.
With a peak gross engine output of 180HP (134kW),thermal efficiency of 23%, engine needs 0.58MJ/s of fuel input=0.175 litres/sec of the above petrol (1.052 litres/minute, 13.87 Imperial Gallons/hour).

But engine needs higher instantaneous fueling for acceleration, plus the fuel must recirculate fuel to carry away heat, enable the fuel rail back pressure regulator to maintain its setpoint etc, so assume pump is sized for 150% of above calculated requirement=95 litres/hour.


to continue (see part B)

[bcmc33]

The whole thing seems far too complicated for me - I simply went to my local scrap yard and bought a Mondeo pump, and it works perfectly.

[gerrym]

Part B: Fuel Line Sizing.

Assuming an in-line (out of tank) fuel pump is used for the retrofit installation. Consulting a Bosch catalogue, one of their smallest pumps is the model 254190 (see attached performance curve on hexane).

Flow will be at least 180 litres/hour (i.e about twice what was previously calculated).

The return line from the injectors, downstream of the back pressure regulator, must have a back pressure less than the set-point of the regulator (otherwise it can't hold the setpoint and injection flowrates won't be predicatable)

For the limiting case with the engine at idle, assume zero fuel consumption so all the pump flowrate is being returned back to the tank (or swirl pot).

For a 3/8" OD line, 1.2mm wt, ID would then be 7.1mm, nominal velocity around 1.3m/s. Assuming an equivalent length of 8m from regulator back to tank (including bends, restrictions etc), pressure drop is around 0.2 bar (or less than 10% of 2.5 bar setpoint for the selected bosch injectors and correct spray pattern). So this size is OK. For a 1/4" OD line, 4.5mm ID, velocity rises to > 3m/s and pressure drop is > 2 bar, which is excessive compared to setpoint and so the actual regulator setpoint will drift and be a function of the instantaneous consumption. In addition, the pressure drop will cause flashing/vapourisation of the petrol and hence excessive loss of the light ends from the tank venting system.