It has to use some assumption for MPG… could it be a constant based on the original rating?
It also computes and will display 1)Ave MPG since I last reset it and 2) current MPG. It doesn’t seem like changes in the above two calculations affect the remaining miles though.
It has quite an accurate fuel flow meter, allowing it to compute MPG, also a less-accurate level sensor in the tank, allowing it to approximate the number of miles left at the current MPG - my last car couldn’t tell me the miles left if I’d just reset the startpoint for the average MPG, but my current one doesn’t care - I suspect it stores the last average MPG even if it’s reset, so that it can give a fairly meaningful ‘miles left’ value while it’s acquiring a new average.
The first car I had one of these things on was a SAAB. The owner’s manual said that it computed miles left on the average mpg for the last 20 miles, which is different than the overall average, which is pretty stationary if you haven’t reset if for a while, or the current mpg reading which wanders all over the place. I think most of them do something similar - some sort of moving average of the mpg readings so that the “miles left” is reasonably damped. I know that ever since they’ve started putting those things in, I use the “miles left” reading rather than the gas gauge. It’s far more accurate.
My SAAB (one of my very few complaints I have) has this, and I think it has a mind of its own. I’ve been getting 34-36 (I do a lot of highway driving), but when I’m in the city, it plumments to 19-21. The first SAAB I had was literally all over the place. Going down hill without the gas engaged made my mpg go up to 99 mpg. Every time I’ve seen this feature, I find it very suspect. I just trust my gas guage and calculate it the old fashioned way – by hand.
The current mpg SHOULD do that. It’s just a reading off the flow meter mentioned above, perhaps damped, and is telling you the mileage you are getting at the very moment (or at least for the last couple minutes). The average mpg, or the miles left reading are another story.
Most (if not all) of these trip computers have two reading instant and average. As has been said the engine can calculate with a very high degree of accuracy the amount of fuel being injected into the engine. The vehicle speed is also an input to the engine control module. If you know how much fuel you are burning, and your speed it does not take Einstein to figure out what your current fuel mileage is. This display will jump all over the place, as it changes with every little hill, and your inputs on the gas pedal.
As far as the 99 goes down the hill, it is actually infinity but the unit will not display above 99, so that is what it shows. This is because your car does not burn any fuel when it is decelerating. None, zero nada.
Now to get average fuel mileage, the computer looks at the instant fuel mileage over a period of time, does a little math, and you have a display of what the average is. You could do it with a pen and paper if you wished.
To get miles to zero, the computer looks at how much fuel is in the tank, and at the fuel mileage over the last 16-20 miles or so does the math. Two gallons left at 30 mpg = 60 miles to empty.
Fuel gauges are horribly inaccurate, so most of these give a pretty good approximation when the tank is full or nearly so. When the tank is very low, the calculation get off. Also how and where you are driving can lead to issues. Let’s say you have been rolling down the freeway at a steady speed and your miles to empty shows 30 miles. (1 gallon left in tank @30mpg). You get off the freeway and head up a steep hill from a stop light. You are now getting 3 mpg. There is no way in hell you are going to make 30 miles on that last gallon. To add insult to injury, most of the fuel has run to the back corner of the tank, and you have less fuel available to the pickup.
Are you talking about a hybrid, because even decelerating downhill a standard engine will burn fuel.
Also, a lot of these WILL display 3 digits for mpg. Whatever you may think of the safety issues of coasting down hills, I have a manual, and have managed to run the mpg reading quite high by kicking into neutral on straight but steep multilane highway stretches. Coasting, with the engine idling, at 65 mph or so, I’ve gotten the mpg reading way into the 100s.
(For those in the Bay Area - I used to drive down 92 into San Mateo off 280 on a daily basis. If the traffic is light, this provides an example of the sort of stretch I’m talking about.)
No it won’t. Your standard every day modern car does not burn any fuel on deceleration.
here is previous post of mine on the subject.
Poor Rick. Isn’t that about the tenth time this month you’ve explained that point?
It sure seems like it. It really is taking longer than we thought.
Rick, I just read your linked post. That is pretty damn cool and completely unexpected. It also seems to have been totally unadvertised, so I think you are going to be spending the rest of your time on the board letting people know about it.
Funny, when I was reading your post I immediately thought, “Wow, you will probably use less gas leaving a manual in gear while you slow down rather than putting it in neutral,” and sure enough, it turns out that was the point you were making. Is it safe to say that pretty much every car manufactured since XXX uses this fuel injection cut-off?
I keep hearing this all the time, but my car’s (Citroen C4) trip computer thinks otherwise.
When going downhills with the car in gear and not pressing the gas pedal at all, the instant consumption for most of the time is 0.0 but occasionally it jumps at something like 0.2 or 0.3 and then goes back to 0.0 (values are litres per 100km).
So either it is some glitch in the software or the car does indeed burn a tiny amount of fuel.
Didn’t we have a thread about this not to long ago, when one manufacturer shut off the injectors during descents and they got a lot of complaints about people loosing the heat during the winter descending the mountains? And that most cars do burn some fuel in these situations?
And your explanation in your previous post does not make sense. You would get lower emissions with a hot catalytic converter, and it gets it’s heat from fuel. If you shut off the fuel you cool down the converter and get higher CO and HC emissions. (Note that CO2 and H2O are not ‘emissions’)
Alot of great info here. To answer my specific question then, do I understand correctly that:
In addition the the Ave MPG (since reset) and Current MPG that can be displayed, the system keeps a running average over the last 15-20 miles that it uses for Miles on the Remaining Fuel?
Yup.
Fuel cut-off typically doesn’t last long enough to let your cat cool all the way down.
Even if it did, a car heats its cat up nice and quick at highway speeds.
It sounds like you’re saying that the hot engine exhaust is what makes the converter hot. Did you never do the science experiment/demonstration where you put a hot platinum wire (or platinum coated nichrome loop,) in methanol vapor and it remains glowing hot due to the surface catalysis? It produces its own heat. Low flow exhaust rich in emissions products can cause the converter to get considerably hotter than with higher flowrates… exhaust is functionally a coolant in some conditions.
Edit 2: Apparently you can melt copper by exposing it to ammonuim hydroxide vapor by a similar exothermic catalysis.
Yeah, it was the last paragraph in my linked post
As far as cooling off the converter, you are mistaking engine coolant temp with exhaust temp. They are not necessarily linked. The fact that you are compressing air heats it. The air flowing into the converter is not cold. While it is not red hot, it sure as hell isn’t cold. There is a volume issue also. With the throttle plate closed, there is very little air going though the exhaust. Grades that are long enough to allow the coolant to cool off are very rare.
Also converters need a dose of oxygen every so often to be able to work. When you convert HC -> H[sub]2[/sub]O + CO[sub]2[/sub] you need three oxygen atoms for every HC molcule you deal with. You can get some from NO[sub]X[/sub], but NO[sub]X[/sub] is only produced during steady state cruise. So the engineers talk about the oxygen storage capability of a converter. Meaning that when you shut the throttle, the converter gets flooded with oxygen. This oxygen is stored in the pebble bed of the converter to be used when needed to convert the HC and CO.