MSefi.com read only forum (Jan.15,2018)

Hiya guys,

Just a though as i was driving home today :-

If injectors meter fuel by time ie. if the injector is open 5ms then it will inject x amount of fuel. We know that if the fuel pressue is changed then the amount of fuel injected will also change.... So if the fuel is cold then it will be denser therefore its volume will be smaller, therefore wouldn't the amount injected also change....??

Just a thought please correct me if i'm wrong...

Many thanks

Dave

quattrodave wrote:Hiya guys,

Just a though as i was driving home today :-

If injectors meter fuel by time ie. if the injector is open 5ms then it will inject x amount of fuel. We know that if the fuel pressue is changed then the amount of fuel injected will also change.... So if the fuel is cold then it will be denser therefore its volume will be smaller, therefore wouldn't the amount injected also change....??

Just a thought please correct me if i'm wrong...

Many thanks

Dave

Theoretically yes, but the change would be so small that it would be negligible. Water for example only changes by a few micrograms from 10C to 40C.

C|

I wouldn't be so quick to dismiss the effect. Water changes density about 1.2% from the triple point to 50C, but dense hydrocarbons change more than twice that (this is why I see an approximate 1 qt difference in cold to hot oil readings on my Porsche, which has a 9 quart sump). Lighter fractions probably are somewhere between the two, so I would not be at all surprised to see a 2% difference in fueling due solely to realistic fuel density changes. If anyone has a CDC available, just look up the coefficient of thermal expansion of iso-octane for instance and then we'll know for sure (my CDC is packed up in a box somewhere awaiting shipping to California).

ERic

Oops, I meant CRC in the above post...

Mmm, Atlanta where it's nice and warm... 15F here in Michigan this morning, so the fuel is pretty dense.

It looks like light fractions have a higher coefficient than the heavy ones in the crankcase. I wonder how much viscosity increase at reduced temperatures counters the density increase?

efahl wrote:Mmm, Atlanta where it's nice and warm... 15F here in Michigan this morning, so the fuel is pretty dense.

Actually is was high 20s for much of Atlanta this morning. So a little warmer, but I guess it depends how you look at it.

Although the amount injected will remain, by & large, very similar the amount that reaches the combustion chamber on a large throttle transient will vary enough to notice.

With the effects being due to increased wall hang up and the evaporation of less of the light ends of the fuel with reduced temp.

MS

Also, i seam to remember from a few years ago, didn't the FIA ban cooling of fuel in F1..?!

Many thanks

Dave

Aren't injectors mass metering devices ? if they were volumetric metering devices, I could see how temp has an effect.

so no matter it's temperature, x mass of fuel is ejected for y amount of time ?

if this is wrong, can you please explain why ?

SeanS wrote:Aren't injectors mass metering devices ? if they were volumetric metering devices, I could see how temp has an effect.

so no matter it's temperature, x mass of fuel is ejected for y amount of time ?

if this is wrong, can you please explain why ?

I may be looking at it wrong, But I don't see how a fixed orifice with a timed opening would be metering a mass quantity.

To me it could only meter a volume.

but why ?

I'm not too sure about the specifics (this is why I'm asking), but I always thought that the only way to accurately meter mass is through a fixed orifice given a known pressure.

Didn't NASA do the same thing for one of their probes ? For a known pressure through a sintered element that was the only way they could accurately meter mass for precise combustion events. I know this is gas flow, but thats why I'm asking the experts. I was hoping that an ME could clear this up .. wiki (or anywhere else I look) doesn't provide me enough of the info to continue in my studies ...

If you look at true Bosch datasheets for injectors, everything is rated mass flow and NOT volumetric ... but word on the street is typically volumetric metering, and those references are typically from service manuals. I could see how someone is told to dump X amount of fuel into a beaker and see if it's ~= to y cc's . .. I'm an engineer and I'd like to know the engineering truth, not the service manual approximation.

I'm hoping someone can please bore me with the math ... or at least some solid references other than the local garages webpage.

SeanS wrote:but why ?

I'm not too sure about the specifics (this is why I'm asking), but I always thought that the only way to accurately meter mass is through a fixed orifice given a known pressure.

Didn't NASA do the same thing for one of their probes ? For a known pressure through a sintered element that was the only way they could accurately meter mass for precise combustion events. I know this is gas flow, but thats why I'm asking the experts. I was hoping that an ME could clear this up .. wiki (or anywhere else I look) doesn't provide me enough of the info to continue in my studies ...

If you look at true Bosch datasheets for injectors, everything is rated mass flow and NOT volumetric ... but word on the street is typically volumetric metering, and those references are typically from service manuals. I could see how someone is told to dump X amount of fuel into a beaker and see if it's ~= to y cc's . .. I'm an engineer and I'd like to know the engineering truth, not the service manual approximation.

I'm hoping someone can please bore me with the math ... or at least some solid references other than the local garages webpage.

I think all of the ratings that are advertised in mass quantities would be at a standard pressure and density. In the field the density would be subject to change with temperature and specific fuel blend. The 1 or 2 percent variation in density that has previously been dicussed in this thread would obviously have an effect on air fuel ratios, but since most manufacturers are using closed loop systems to trim the air fuel ratio that small amount shouldn't be significant. And before closed loop systems were used 1 to 2 percent was probably considered within tolerance.

As far as NASA, I'm sure that they have specifically programmed there systems for the exact fuel blend and use sensors to monitor the fuel temperature so that they can be more precise with the fuel delivery calculations.

Vicoor wrote:I think all of the ratings that are advertised in mass quantities would be at a standard pressure and density. In the field the density would be subject to change with temperature and specific fuel blend. The 1 or 2 percent variation in density that has previously been dicussed in this thread would obviously have an effect on air fuel ratios, but since most manufacturers are using closed loop systems to trim the air fuel ratio that small amount shouldn't be significant. And before closed loop systems were used 1 to 2 percent was probably considered within tolerance.

ah yes, this makes sense - under controlled conditions, they eject x mass .. thank you.

Lance wrote:The mass flow is determined by multiplying the volume flow rate by the specific density of the fluid. So mass flow is a function of specific density.

I thought this was the case, and even worked it all out on a spreadsheet. I took this to a coworker and he made a really good argument that injectors are mass-flow metering devices, he is very experienced with this stuff, so I took his word for it.

I think I'm going to have to make some custom code and test this out to see what happens.