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Fueling questions

2K views 7 replies 4 participants last post by  Haw1es 
#1 ·
So I was reading about MPG's, and came across a discussion of coasting in neutral vs coasting in gear with no throttle (engine braking). I always thought coasting in neutral was the most efficient way since you're not on the gas and using the lowest RPM you can. I had no idea the engine cuts off fuel supply when you lift off the throttle. I always thought the sound of your engine came from combustion in the chambers, and then the sound waves make their way mainly out of the exhaust. In my mind, 1000RPM used an X amount of fuel no matter how much throttle was used, 2000RPM used 2X amount of fuel, and so forth. So needless to say, I was very surprised to learn I wasn't even close in what I thought happens.

So my question is this- is fuel delivery linear through our RPM range? Someone posted that cars lean out towards the upper range, so if thats true, you actually use less fuel per RPM at the upper ranges, and more at the lower ranges?
If we take a car and do a 3rd gear pull, at WOT and 2000RPM we use an X amount of fuel. At WOT and 4000RPM we should be using 2X fuel if its linear, but in real life we would be using a little less than 2X if I understand correctly.

And if we don't use 2X fuel but a little less and the fuel delivery isn't linear, what happens to air? Is the amount of air linear since we lean out towards the top of our range?
If anybody could shed a little more light for me on the subject, I would appreciate it :)

Edit- This is mainly for N/A cars? I'm not sure if what I'm talking about is the same for turbo cars, since we stuff a lot more air in there under boost?
 
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#2 ·
Back in the days of carbureted engines fuel delivery was relatively linear with high throttle secondary linear trims based on throttle position. In old engines rolling at speed with not throttle input ( engine braking ) the throttle body plates would be sucked open by the movement of the piston and the fuel delivery was based ( using springs and connecting rods )on the position of the throttle plates.

On modern ECU controlled platforms the fuel trim is not linear, in fact is is based on a whole host of other data that the ECU is collecting. There are tuners who will spend hours pouring over fuel tables to get everything working at its best. The fact that we drive turbocharged cars makes this equation more complicated still because you need to add boost pressure to your thinking.

In your example a WOT 3rd gear pull where the engine is at 2000 RPM would use considerably less fuel than the same WOT pull at 4000RPM because the systems relative boost would be lower. As you pin the throttle you build boost up to about 4500RPM where your boost should taper off, Its not so much about throttle position or RPM as it is about the amount of air in the system that the ECU needs to mix with fuel for a proper burn. With more boost there will be more air and that means that you need more fuel to achieve a proper burn. when engine braking your engine is still sucking in air and the MAF will transmit the amount of air incoming so the ECU can send the appropriate amount of fuel for a clean burn.

The most efficient way to travel would be to keep the RPMs low and roll in neutral where available. That would allow the ECU to run the engine at idle speeds ( 1200RPM with the AC on) I live in canyon country CA just north of Los Angeles and when I get a chance I take the hills on the 14 for miles in neutral, its not uncommon for the ECU to spit out MPG numbers up in the 85-80 range. As soon as I get back in gear the figures adjust accordingly.

Rolling in gear with engine braking is less efficient on fuel economy but it is also safer than rolling in neutral because you have the transmission prepared to make emergency maneuvers if needed.
 
#3 ·
when engine braking your engine is still sucking in air and the MAF will transmit the amount of air incoming so the ECU can send the appropriate amount of fuel for a clean burn
I think the opposite is true. When your RPMs are above idle, and you are coasting in gear, the injector duty cycle should be at or near zero. The engine will actually use the road to keep turning and won't need to add any gas to stay running. As it gets to the point where your engine would normally stall (right around idle), the injectors will kick back on to keep that from happening.

I will be working on getting a boost leak tracked down this weekend, so I can pull a log or two to verify, but if someone beats me to it, so be it :laugh:
 
#4 ·
I might just need to go test that, Ive got just the road in mind.

10 seconds coasting in gear and then coasting in neutral.

Anecdotally Ive always got better coasting mileage with the car in neutral than I do with the car in gear. Highway speeds with the car in 5th are about 2800RPM.

Ill see if I have some free time tonight to test that otherwise I eagerly await your results.
 
#5 ·
Let the science begin! Haha. It could easily be different for turbocharged/not-stock-tune cars, but from what I was taught that's how it usually goes. Maybe you get better mileage because you are taking it a bit easier and not fully enjoying those roads ;) (when coasting in neutral)
 
#6 ·
OK so quick and dirty data this morning, 2500ft above sea level 48 degrees F and 65% humidity.

I took a log for 15 seconds in 5th gear rolling downhill on the I5 truck route about 62 miles per hour Injector duty cycle was in fact 0 for the length of the time, then I used some throttle for 5 seconds to get back to 62 to test again in neutral. I put the car in neutral and rolled for another 15 seconds. average injector duty cycle was 1.35 so there is an increase in fuel consumption when rolling in neutral.

The biggest thing I noticed was a marked decrease in speed when in gear, I was effectively engine braking even though i was at about 2500RPM in 5th gear. Over 15 seconds my velocity decreased a total of 4 miles per hour. When i was in neutral my velocity increased to 68 miles per hour.

That begs the question is the vehicle more fuel efficient moving downhill and accelerating due to gravity with the injectors at 1.35% or under engine braking conditions in gear with the vehicle slowing down. When I look at that I also think about the amount of fuel that will need to be added to get the vehicle back up to highway speeds, in my case 11% injector duty cycle for 4 seconds was enough to get back up to highway speeds after slowing down to 58.

Just to get some control data I also put the car in cruise control going 65 on a slight decline and the injector duty cycle increased to an average of 4.94 for the remainder of my log time.

Im not sure if thats really an answer to OPs question but thateres at least some additional data to think about. :nerd:
 
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