Innumeracy!

I would think the innumerate poster was trying to say that the injector flowed 8.8 gms/sec not gallons. Auto specs are notoriously mixed between metric and American standard values. Some manufacturers list air flow in lbs/min or lbs/hour while listing fuel pressure in Bar. The answer is to adopt metric measurement once and for all.

Strangely enough, he'd first quoted the figure in standard cubic feet per second, which he then converted to gallons. The scfs figure was also insanely high for a fuel injector. He then went through quite a few calculations (some ok, although based on the incorrect data) but never stopped to check that his answers might be even remotely close to reasonable.

Gets back to gut feel ball parking, if a result seems way off track, it most likely is. It is no good having big fingers and a calculator if you can't do mental arithmetic.

Well put!

Sounds like apples and oranges. A nozzle will pass many cfm of air if under high enough preassure due to compression but the liquid will compress little or none. Your poster may have done the math but have his physics all wrong. You can not just interchange gases and liquids when dealing with high preassures unless you take into account compression ratios.

You're right, he was comparing apples and oranges, and his grasp of physics was no better than his grasp of math. In fact, he hadn't given any consideration to the relative mass or viscosity of the fluids involved, or whether flow rates would change linearly (or not) with changes in pressure. Nor had he considered that if it is already hard to get enough air into an engine through holes almost 2 inches across, why try to get it in through holes a few thousandths across?

But my concern was not so much that he was weak in physics, or that he did a particular calculation incorrectly, but that he would base his proposal on figures that were so far outside the realm of reasonableness. Just one injector (of 8 on the engine) draining an entire gas tank in two or three seconds??? And my REAL concern is that more than half of the people who commented (on halfbakery) didn't appear to have checked for reasonablness either.

Although I agree that it seems to excessive, in your extrapolation you must also consider that the injector is only open for a fraction of a second. I have a circuit that pulses an LED at 2.5 amps and will operate off a small battery for four years. At first it may sound crazy but the pulse is only a few microseconds out of many seconds and when you do the duty cycle calculation the it averages down to micro-amps.

it would need an opening as big as a bucket for this injector to pump that out

Depending on the pressure, could be smaller or larger but I don't disagree, it's just a matter of degree...

I agree the figure seems insanely high. A 747 can consume about 200,000 liter of fuel in about 12 hours and that works out at about 53,00 gallons over 43,200 seconds for all four engines or about 0.307 gallons per second per engine and there are a mass of injectors in each engine and these injectors run continuously. 8.8 gallons per second seems to be a couple of orders of magnitude out. Like I suspect your were I was taught that before you make any calculation take a guess at the final result then when you make your calculations if your guess was way off then your calculations need closer attention.

Whoaa! You've just confirmed the innumeracy worry. To work out 8.8 Gallons per second flow through a small orifice such as an injector, (x 8 don't forget) what sort of fuel pressure are we talking about? Think about filling your fuel tank on your car – how long does 10 gallons take? The injector could be an open conduit and not pulse at all for easier calculations. My concern is that there seems no grasp of real world practical issue's; the numbers don't paint a realistic picture. Consider the latest generation of V8 engines in F1 cars. These engines are designed for maximum fuel efficiency (not economy), running at engine speeds of up to 20,000 RPM. (333 revs per second, each piston coming to a stop twice each revolution, don't forget) It takes up to 7 seconds to put enough fuel in the car to do approximately 60 miles, through an orifice about 2 inches in diameter (I think!), at pressure. I'm not entirely sure of the amount used each time, but I believe it to be about 40 litres (10.56 US gallons). This should give you a good mental sign post of the flow rates involved, and the practicalities to be considered, such as why would you need that much fuel?

However, when motor heads talk about maximum flow from an injector they are talking about 100 percent duty cycle. Typically, this is measured by supplying the injector with the specified fuel at the specified pressure, and then energizing the injector to open it for ten seconds, or so, with the injector spraying into a small calibrated beaker.

My comment above (7350) is misplaced. It actually should be under rcapper's comment on duty cycle.

One of the hazards of commenting on an area outside of your own area of expertise. Although, then through feedback, we learn more about that area. One of the benifits of interaction on this site.

I agree with the response on the 8.8 grams/second. I have yet to see an injector rated in anything understandable to the average American mechanic. Ten years ago, when I was racing fuelie Mustangs, I had to know the density of gasoline to figure how much my injector choices would flow.

Being realistic, even if the injector would jom in 8.8 gps what would happen? It would never combust. Remember flooding? There would be locked cylinders on those with all valves closed, and fuel running out the tailpipe from those with open exhaust valves. So, no MPH at all.

I just love the term!

I'd be interested in learning the heat generated by 8.8 GPS flowing through a standard fuel injection orifice. Can any of you fluids junkies take a wack at this? Enough heat energy to melt the engine?

First let me say that I'm relatively new to this forum... a viewer of the posts since around early May. But today I have just became a registered member and have made, this, my first response. First, I think we veered off-topic on what I agree to be a serious issue and that is what has been referred to as "Innumeracy" and what the original poster asked in how we can stop it. Instead we are discussing the design characteristics and capability of a fuel injector... which is fun too ;o) So first let me chime in on the injector design thingy. Granted, I'm not a auto mechanic but I do know that injectors are simply electromagnetic valves that move a plunger allowing pressurized fuel to pass through a nozzle. The nozzle is designed to atomize the fuel to increase combustion and the fuel is pressurized by the fuel pump. The original poster said that he saw the other poster state that a "...6 liter Chevy engine could pass 8.8 gallons/sec" inferring the complete system and not just a single injector. So, assuming a multi-port system, the injectors can all open at the same time or can be sequential. But since it takes less then a second to achieve a complete engine revolution, which of the two control methods is used does not matter. It also is irrelevant how many injectors we are talking because the limiting factor is the fuel pump psi and the supply line ID. Street legal high-performance fuel pumps have 6 valves with 3/8" NPT inlet and outlet ports and run at 6 psi which comes to a flow capacity of around 110 gallons/hour or 1.8 gpm or 0.03 gps. A 6-valve performance racing fuel pump is around 130 gallons/hour through 1/2 npt porting at 13 to 15 psi. There are some free-flow designs out there from Essex Ind. that will get you around 180 gallons/hour but that's about the highest I've seen and that coverts to 0.05 gps. So even if you remove all the injectors and just let the fuel pump run wide open, to get 8.8 gps you'll still need a fuel pump with 176 times more capacity then a current top of line performance model has today. As for the amount of heat generated... I have no clue but I'm sure stuff would break ;o) Finally, regarding innumeracy... I think it's a serious problem, especially in the upcoming generations. There needs to be some mandatory (an ethically enforced) performance criteria that each child needs to meet in order to graduate high school. We're letting almost anyone become teachers, our public schools are turning into havens of drugs, alcohol and sex and we're letting these kids pass just to get them out of the system and pass them (and their problems) off to the next group. We, as a society, have become too relaxed... too at-ease with our own freedoms. Do you know where the biggest motivations comes from to drive someone to better themselves? The desire to break free from a freedom-less oppression. I believe that your youth have it too easy and thus are less motivated and as a result, under educated. Anyway, peace and thanks for listening to my first post even though it got a little soap boxy at the end ;o) And please excuse any type-o's... I went rather fast as I'm pressed for time.

Re the fuel injector: actually, he was quoting the figure (what he thought was the figure) for just one injector, NOT the entire engine. So the situation is even more dramatic than you have painted.

And you are right, I'd brought up the example to discuss innumeracy, not the physics of fuel injection. But I suppose anything that gets people thinking and discussing (albeit off topic) is good.

And they say morality has no bearing on science.

Does this qualify as innumeracy? Years ago, I was listening to a company's annual meeting where the financial guys reported a 120% growth over the previous year (!). Looking at the numbers, the increase in sales that was being referred to, though impressive, was in reality 20%. When I asked, I basically was told it was the language of finance and marketing. My reply was that I couldn't wait to go home and tell my kids that they would be experiencing a 50% growth in their allowance. :-)

Yes! Perfect... innumeracy at work.

I've checked out the site, and I have to say the concern about innumeracy still holds, but in context with this site, forget all the rules. This site discusses a wide range of things, from attack furniture to dildo door knobs! I recommend it!

I watched a top fuel drag race on TV a few years ago (2nd time I think), and one of the guys said that the reason they were kicking everyone else's butts was because they had figured out how to get 10 times the normal amount of fuel (6.5 gpm) into the engine -- in other words, 65 gpm. At that rate, each injector (if there were only one per cylinder) in an 8-cylinder engine would only be handling a little over 8 gpm.

I haven't done any fluid flow calculations (and I ain't gonna), but my gut feeling is that the pressure needed for an 8 gpm flow rate is not particularly high, but 8.8 gps is another story.

This problem with ilnumeracy goes along with Paddy's remarks about "graphing calculators." When I went to high school, there was no such thing, and when I went to college, I used a slide rule or calculator very little.

Thirty years ago, I made a statement, "We're raising a generation of button-pushers." In the meantime, we've raised a generation of teachers who are button-pushers, and they don't know how to teach our current crop of children how to do things with their minds and a pencil and piece of paper.

I feel better now.