INJECTOR FUEL FLOW

Engine output is in direct relation with fuel supplied to the engine, however installing injectors, which are too big, will not make more power. It is therefore very important to match the fuel injector flow characteristics to specific engine applications. Matching the fuel flow characteristics of fuel injectors is as important as matching the carburetor jets for a specific engine application. The fuel flow of the injectors and the carburetor has to be matched to the air flow requirements of the engine over a broad RPM operating range.

The dynamic range must encompass the minute quantities of fuel required at idle conditions and the large quantities of fuel required at maximum engine output. It must also cover the required fuel amounts during transient response. The dynamic range of the fuel injector is further stressed in turbo charged applications because of the additional fuel required due to the higher engine air mass flow rates generated by the turbocharger.

The following equation sizes fuel injectors for specific engine applications.

Injector Static Flow Rate [lb/hr] = (Engine HP* BSFC)/ (Number of  injectors * DC of Inj.)
Engine HP = Realistic HP output estimate of the engine
BSFC = Brake Specific Fuel Consumption [lb/HP*hr].
               Good approximation 0.50
Duty Cycle of Injector = Maximum opening time of injector / cycle time.
Maximum Duty Cycle = 0.90

Example:
Engine HP = 400HP
Number of Injectors = 8
Injector Static Flow Rate [lb/hr] = (400 * 0.50) / (8 *0.90) = >27.78 b/hr

Note: If the application requires a static flow rate that falls in between two available  injectors always use the next injector with the higher flow rate.

For the example above if only 25 lb/hr and 30 lb/hr injectors are available, choose 30 lb/hr injectors.

FUEL PRESSURE

In certain occasions matching of the injectors' fuel flow for a specific engine application cannot be done due to injector availability or the fuel flow step between the available injectors is too large. Since the fuel injector is a pressure/time-metering device, increasing the fuel pressure can increase the fueling level. Increasing the fuel pressure is limited mainly to four factors: burst pressure of the components in the fuel system, increase of opening time of the injector, reduced life expectancy of the fuel system components and limitations of the fuel pump. Most injectors are limited to a burst pressure of 125 psi. Reducing the fuel pressure to match the required fuel flow can be done but lower fuel pressures affect the atomization efficiency of the fuel injector nozzle. To project potential fueling levels by changing the fuel pressure, the following equation can be used:

M1/M2 = √P1 / √P2
M1 = rated mass flow rate of the injector at fuel pressure P1 in lb/hr
M2 = new mass flow rate of the injector at fuel pressure P2 in lb/hr
P1 = existing fuel pressure setting in psi
P2 = new fuel pressure setting in psi

Example:
Rated mass flow rate M1 = 30 lb/hr
Existing fuel pressure P1 = 43.5 psi
Required fuel mass flow rate M2 = 35 lb/hr

P2 = (M2/M1)2*P1
P2 = (35/30)2*43.5
P2 = 59.21psi = >60psi

To obtain a fueling level of 35 lb/hr the system pressure has to be increased to 60 psi.

After increasing the fuel pressure to obtain certain engine output, idle, off-idle and light load condition will have to be re-tested. Increasing the fueling level at the upper end, requires the fuel injector to run at smaller pulse widths at idle conditions. When running at pulse widths smaller than 1.8 ms the injector might be running in the non-n linear portion of its dynamic range. Such condition can lead to engine "hunting" during idle to hesitation during off-idle conditions.