by "Orange Exige" (OrangeExige)
Published 04/06/2017 at 20:59
by "Orange Exige" (OrangeExige)
Published 04/06/2017 at 20:59
Tags: fuel economy
; mpg
; gas mileage
; random thoughts
STARS: 0
Two things:
1. This one is probably pretty obvious but all other things equal, bigger fuel injectors would probably under most/all comparable circumstances decrease fuel economy. (Right?) What my thoughts are though are not necessarily fuel economy differences, but rather how the ECU sees the differences (or not...).
Will
the ECU know the difference?
When I use my OBDII scanner, my Cooper S reports impressively high fuel economy. Today for instance, it reported out 54 mpg on a ~5 mi stretch of mostly highway at a steady 55 mph. (EPA rating is probably 30 mpg highway, max)
My car’s got 380cc injectors instead of the stock 330cc ones. That’s 13% larger. Pretending that that’s the only non-stock mod (it’s not), would it be safe to assume that my car was actually getting (.87*54=) 47 mpg at that time??
2. Also today, I launched (well, rolling launch) from 4th gear. Obviously it’s slow but the question is the fuel economy comparability of this. Again, let’s pretend all other things equal:
On one hand you have a fourth gear launch which can get you up to speed (~40 mph) with say 75% throttle but only 1000-1500 rpm range. On the other, you have a normal first through third gear launch, let’s say 25% throttle but a higher 2500-3000 rpm.
What is technically more (fuel) efficient? Are they equal?
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Post-replies edit: (hopefully some of you read this) Thanks so much everybody! You all made really good points and I’m so glad to learn everyday from Oppo!
"bhtooefr" (bhtooefr)
04/06/2017 at 21:07, STARS: 1
Sometimes, if you can’t deliver the fuel quickly enough, the fuel at the end of the injection cycle will burn incompletely, so at that point, bigger injectors can actually improve efficiency. Otherwise, atomization is worsened by bigger injectors, worsening burn completion and efficiency.
To fully answer that question, I’d need to see a BSFC graph for that engine. However, on naturally aspirated or diesel engines, a decent rule of thumb is, high throttle at low to moderate RPM is most efficient, and keep it at the most efficient RPM by going through the gears. On forced induction gasoline engines, try to stay out of boost, and also low to moderate RPM.
"e36Jeff now drives a ZHP" (e36jeff)
04/06/2017 at 21:11, STARS: 1
As long as the flow rate of the injector is not so great that it is dumping in too much fuel in the minimum cycle rate it will not significantly change fuel economy(assuming you are comparing new injectors to new injectors, old vs new, the new ones will probably do better). The car measures the amount of fuel injected by looking at the exhaust with the O2 sensor to see if its lean or rich, then adjusts the injector signals to accommodate. The MAF tells it the flow rate of air into the engine, so by knowing exactly how lean or rich the exhaust gas is, it can determine the fuel flow rate. The car knows pretty accurately what the fuel flow rate is.
In theory a fully open throttle in a high gear is more efficient, as your engine is achieving maximum volumetric efficiency when operate at WOT. In reality, I think it varies car to car depending on various factors.
"bob and john" (bobandjohn)
04/06/2017 at 21:19, STARS: 1
ut rather how the ECU sees the differences (or not...). Will the ECU know the difference?.............. reported out 54 mpg on a ~5 mi stretch of mostly highway at a steady 55 mph. (EPA rating is probably 30 mpg highway, max)My car’s got 380cc injectors instead of the stock 330cc ones. That’s 13% larger. Pretending that that’s the only non-stock mod (it’s not), would it be safe to assume that my car was actually getting (.87*54=) 47 mpg at that time??
okay, 2 parts to this one.
1: the car will always need a similar amount for fuel for the HP. so regardless of the size of the injectors. the o2 sensor will tell the ECU if it is lean and then the ECU will adjust the opening of the injectors (oh, 20% isnt enough gas, i’ll leave it 30% open the next time.)
though, it wouldn’t surprise me if newer cars/injectors have a way of ID’ing the injectors (certain sizes have certain resistances, etc etc)
2: your car probably works in the first fastion I mentioned, so most likely you are correct. the Fuel gauge hasnt been re configured to know: oi dumb-ass, you have bigger injectors now) and think you are running that smaller opening time with the 330CC injectors)
"Die-Trying" (die-trying)
04/06/2017 at 21:45, STARS: 1
just kinda poking around at internet numbers, but best fuel economy OUGHT to be around the 2500 rpm range.
anytime you just kind of dump the throttle, youre just pouring a ton of fuel down the engine.
if you keep the engine in its power band, (that spot where it pulls real nice on the tachometer) you will get spectacular fuel milage, but if you lug the engine, down below its power band, or wind it up real high in the rpm range you lose economy pretty quick. if you want to find the power band(looks to start in near 2k on the graph) while going fairly slow down the road, 25 or 30 mph, put it in high gear, and mash the throttle. it wont accelerate real quick, but if you watch the tach, there will be a spot, where the rpms increase real quick, and that is the start of the power band. keep your rpms just above that, and you keep the engine pulling in its best economy.
i have an old truck with a massive cam in it. it doesnt make a lot of power below 2300, and doesnt fully clean up til around 2700 rpm. under 2700 rpm i get terrible gas milage 8-ish, but out on the highway, above 2800, i can pull in 12 or 15 on long trips, just because the engine is “happier” there.