Help Me Understand - Fuel Injector UOM

[Ewheels]

I have a super random and technical/nerdy question for those of you who care...

Fuel injectors are sized or quantified by either "lbs" or "cc" or more specifically lbs/hour or cc/min whether using Standard or Metric units. My confusion comes from these UOM not matching.
cc/min is volume over time or flow rate of liquid out of the injector but lbs/hour is force over time which would be....average force over an hour?? Does that make sense? I would think the Standard UOM should be cubic inches per min so it's still volumetric flow rate.

Someone learn me, please

 

[67GTA]

I have a super random and technical/nerdy question for those of you who care...

Fuel injectors are sized or quantified by either "lbs" or "cc" or more specifically lbs/hour or cc/min whether using Standard or Metric units. My confusion comes from these UOM not matching.
cc/min is volume over time or flow rate of liquid out of the injector but lbs/hour is force over time which would be....average force over an hour?? Does that make sense? I would think the Standard UOM should be cubic inches per min so it's still volumetric flow rate.

Someone learn me, please

Lbs/hr is a flow rate as well - it’s how many pounds of fuel will flow in one hour. As different fuels have different weights, I assume they use gasoline as the standard measure?

There’s a simple conversion formula that’s used to go back and forth:

cc/min = lb/hr × 10.5
Or
lb/hr = cc/min ÷ 10.5

Using this formula, it works out to 6 lbs per gallon. Gasoline is approx. 6.0 - 6.3 lbs/gal, so perhaps the formula uses an even 6 lbs/gal for simplicity.

 

[JAJ]

I have a super random and technical/nerdy question for those of you who care...

Fuel injectors are sized or quantified by either "lbs" or "cc" or more specifically lbs/hour or cc/min whether using Standard or Metric units. My confusion comes from these UOM not matching.
cc/min is volume over time or flow rate of liquid out of the injector but lbs/hour is force over time which would be....average force over an hour?? Does that make sense? I would think the Standard UOM should be cubic inches per min so it's still volumetric flow rate.

Someone learn me, please

Fuel energy depends on the mass of fuel, not the volume, so that's why lbs/hr or other measures of mass are used in the ECU. Devices like injectors, fuel lines and pumps don't know anything about mass, they're volume limited, so you have to convert from mass (which is what the ECU cares about) to volume (the limiting factor for injectors, lines and pumps) in order to make sure that your fuel system can deliver enough volume of fuel to supply the mass that the engine requires. Make sense?

 

[Ewheels]

Lbs/hr is a flow rate as well - it’s how many pounds of fuel will flow in one hour. As different fuels have different weights, I assume they use gasoline as the standard measure?

This is what I thought but still seems odd. Weight per time would definitely be a flow rate but it would vary whether you use gasoline, ethanol, methanol, ect. Volume per time would be globally more consistent regardless of what liquid you choose to flow through them.

@JAJ the conversion makes sense and the ECU necessity for mass of fuel makes sense but still seems odd to put a UOM of mass/hr when talking about flow rates.
Does this mean a 47 lb injector will deliver greater volume of a less dense liquid in the same time??

 

[Scootsmcgreggor]

Does this mean a 47 lb injector will deliver greater volume of a less dense liquid in the same time??

No. If you look at E85 setups they require massive injectors. So lbs/hr rates must be standardized at a certain fuel density/mass.

 

[Ewheels]

No. If you look at E85 setups they require massive injectors. So lbs/hr rates must be standardized at a certain fuel density/mass.

Gotcha. So @67GTA you nailed this one

 

[JAJ]

This is what I thought but still seems odd. Weight per time would definitely be a flow rate but it would vary whether you use gasoline, ethanol, methanol, ect. Volume per time would be globally more consistent regardless of what liquid you choose to flow through them.

@JAJ the conversion makes sense and the ECU necessity for mass of fuel makes sense but still seems odd to put a UOM of mass/hr when talking about flow rates.
Does this mean a 47 lb injector will deliver greater volume of a less dense liquid in the same time??

I suspect the concept of a "47 lb" injector is some kind of convention, sort of a shorthand way to tell one injector from another. Presumably the 47 lbs/hr applies at a specific rail pressure, temperature and fuel type (E0 AKI 97 gasoline most likely).

E0 gasoline weighs about 6.1 pounds per gallon, so a 47 lb injector will flow about 7.7 gallons in an hour at a rail pressure (most likely) of around 40 psi. If you pump a fluid with a different density, either higher or lower, so long as the viscosity is the same as E0 gasoline, it'll still flow 7.7 gallons an hour. So, for instance E85 is actually denser than E0 (6.5 lbs/gallon vs 6.1 lbs/gallon), so the "47 lb" E0 injector becomes a "50 lb" injector with E85.

The other thing to keep in mind is that AFR is the ratio of pounds of air to pounds of fuel. Stochiometric AFR for gasoline E0 is around 14.7:1 versus E85 at around 9.8:1. If a hypothetical single cylinder (one injector) engine can use 47 lbs/hr of E0 running flat out, then the same airflow will require about 70 lbs/hr of E85, which translates to 10.8 gallons an hour. It'll need a bigger injector and possibly bigger lines and pump as well.

 

[Grant 302]

^ That’s the most complete answer here.