§ How it works

An engine needs a fixed mass of fuel for every unit of power it makes. That figure is the brake specific fuel consumption (BSFC). Multiply it by the power target to get total fuel flow, then share it between the injectors while leaving headroom so none of them run wide open.

\dot{m}_{\text{inj}} = \frac{P \times \text{BSFC}}{N \times DC}

$P$ : target crank power in hp.
$\text{BSFC}$ : in lb per hp per hour. Around 0.45 to 0.55 for naturally aspirated petrol, 0.55 to 0.68 with boost, and roughly 0.80 to 0.90 on E85.
$N$ : number of injectors, and $DC$ : the highest duty cycle you allow (0.80 for 80 %).

Injectors are sold by volume flow, so the mass flow is converted using fuel density $\rho$ :

Q_{\text{cc/min}} = \frac{\dot{m}_{\text{lb/hr}} \times 453.6}{60\, \rho} \approx 10.5 \times \dot{m}_{\text{lb/hr}} \quad (\text{petrol})

Duty cycle and pulse width

A four-stroke cylinder fires once every two revolutions, so the time available for one injection is

t_{\text{cycle}} = \frac{120\,000}{n}\ \text{ms} \qquad t_{\text{open}} = DC \times t_{\text{cycle}}

Above about 85 % duty the injector barely closes before it must open again, flow becomes non-linear and the coil runs hot. Pick the next catalogue size up from the minimum shown.

§ How it works

\dot{m}_{\text{inj}} = \frac{P \times \text{BSFC}}{N \times DC}

$P$ : target crank power in hp.
$\text{BSFC}$ : in lb per hp per hour. Around 0.45 to 0.55 for naturally aspirated petrol, 0.55 to 0.68 with boost, and roughly 0.80 to 0.90 on E85.
$N$ : number of injectors, and $DC$ : the highest duty cycle you allow (0.80 for 80 %).

Injectors are sold by volume flow, so the mass flow is converted using fuel density $\rho$ :

Q_{\text{cc/min}} = \frac{\dot{m}_{\text{lb/hr}} \times 453.6}{60\, \rho} \approx 10.5 \times \dot{m}_{\text{lb/hr}} \quad (\text{petrol})

Duty cycle and pulse width

A four-stroke cylinder fires once every two revolutions, so the time available for one injection is

t_{\text{cycle}} = \frac{120\,000}{n}\ \text{ms} \qquad t_{\text{open}} = DC \times t_{\text{cycle}}

Above about 85 % duty the injector barely closes before it must open again, flow becomes non-linear and the coil runs hot. Pick the next catalogue size up from the minimum shown.

Injector Sizing

Inputsdrag · tap · type

Fig. 05Injector signal at full load

§ How it works

Duty cycle and pulse width

Injector Sizing

Inputsdrag · tap · type

Fig. 05Injector signal at full load

§ How it works

Duty cycle and pulse width