I drive down several long hills each day and I have been debating which method uses more gasoline.

1) Coasting in neutral

2) Keeping it in gear but not giving it any gas

Coasting in neutral requires gas to be burned to keep the engine turning over, but the engine is turning at a low rpm.

Keeping it in gear allows momentum and gravity to keep the engine turning over, but the engine is turning at higher rpm.

Is any gas being sent to the engine when gravity is the force moving the car?

So in which of these situations would my car actually burn the most gas?

Oh, and I also know that it is illegal to coast downhill in a car, at least in my state it is.

 

Last weekend I drove to western Maryland on I 68 - there are a good many "coastable" hills on that part of the interstate. Coming through the "Cut" I saw a sign that indicated a major incline, I immediately placed the mini (MCCS) in neutral and begin the coast. I gained alot of speed and was nearing 85 at the bottom of the hill. The RPM's remained at the idle level, and my mpg went up a couple of tenths in the mile or so of coasting. I placed the car in 6th gear as my speed decreased and as was expected rpm's went up..no problems noted - smooth transition. The next hill I did not take the car out of 6th gear, and the rpm's stayed around 2,000+. I didn't gain any tenths of mpg.
It maybe illegal but if you have a manual it is easy to do and if you don't heat the brakes..I see gains from this practice.

 

When you lift off the throttle on newer cars, inc. the MINI, with the car in gear the ECU shuts off the fuel injectors until you apply the throttle. So theoreticly you would not be using any fuel while coasting down a hill. The drawback to this is that you are fighting the engine compression so it's not as freewheeling as putting the car in neutral. Best suggestion would be to leave the car in the highest gear possible if the grade is steep
enough to maintain speed if not then put it in neutral and hope for the best ! ! !

 

i have heard the same things. also, keeping it in gear will keep oil pressure higher and more oil circulating through out the motor as well as more coolant flowing through the motor.

 

ive seen this topic over and over and the reply has been that you save more fuel by keeping the car in gear while coasting vs coasting in neutral.

i downshift each time cause i like to have the car under my control,
i dont really care about the oz of gas im saving. im probably not
saving cause i blip throttle on each down shift.

 

deleted

 

Thanks for deleting the post 483. There are numerous posts about the fact that the ECU shuts off the fuel supply when you are coasting while in gear until you hit about 1900 rpm then they kick back on.

An engine doesn't need fuel to still be able to turn over while in gear.

 

Very true. Sometimes, I wish MINI had lowered the rpm at which the fuel gets re-introduced. The jerk at 1900 rpm is quite noticeable in lower gears.

If any of you have ever been in a car when the electronics are suddenly shut off, it's an interesting experience to say the least. You don't even get a real feel of engine braking, it just keeps coasting. It feels like your just cruising along slowing down gingerly until you put your right foot down and nothing happens. Good 'ol Lucas...

 

Mini incorporated in the fuel system a small amount of fuel when so when you let off the gas it has that sports car popping. The means as long as the engine is running there is still fuel applied through the injectors. If not the engine dies. LOL
Compression breaking (coasting in gear) while it may be more economical, slamming the piston rings in the other direction can cause faster engine wear.

 

Fuel does not need to be injected to allow the engine to keep turning over. How can you turn over the engine when it is on the bench......no fuel then. It acts just like an air compressor...............

 

At times I feel the nee to let others respond to ridicules responses.


This will be one of those times.

 

You mean to say you can't understand basic engine priciples.........?

or apparently spelling

 

I'm not exactly sure what you are saying either Nitominis. ScottinBend, you are correct. If you are moving at speed in gear and fuel flow is cut off to the engine, it will turn into an air pump; gradually slowing down due to friction losses. If you closed the throttlebody, you will slow down faster than if you kept the throttle plate open due to the added vacuum that the piston is sucking against. If you capped off the exhaust, it would build pressure exactly the same as an air compressor does.

 

Grammar/spelling. This is the NET and a forum. Give me a break, where have you been all your life an english teacher? Because you do better at that then you knowledge of enternal combution engines.

And your lame no ridiculous (note spelling) response.Well just how do you suppose the engine stays running? What do you think foot off gas ..no gas engine continues to run on its own? These are areas you should leave to the Automotive Technicians. Even at idle there are pulses to the injectors that can be measured that allow fuel to flow to keep the engine running. Same if you are driving and completely let off the gas the injectors will still receive information to open to allow fuel to flow to keep the engine running. DUH ! Basic Engine Operation 101. Gasoline engine needs gas to run

 

So maybe we have the word 'run' confused with 'spin.' You can't deny that the engine still spins when you let off the gas. And yes, when you are idling, the engine must produce it's own power. When you are slowing down though, the engine doesn't need to make any power, it needs to provide engine braking (dissipating momentum through pumping losses). Most cars completely cut fuel to the engine when decelerating in gear above a certain rpm (at least manual tranny cars do). Without seeing MINI's ecu program, we can't be sure if they do this as well. The fact that we have popping during deceleration in the 3000-3500 rpm range, it leads me to believe that they don't completely cut fuel. But you can't deny that there is a very noticeable lurch around 1900 rpm where I believe full (idle) fuel flow is reintroduced. BMW's do not have the popping as they do completely cut fuel.

 

WOW....talk about a heated debate about a question that everyone has admitted they dont actually have proof to back up as which one is better. Just enjoying the show

 

Im just curious, if there was no gas being introduced into the combustion chamber, how is there sound coming out of the exhaust? Isnt the "sound" of exhaust the result of gas being ignited?

 

Nothing personal but

Search NAM on the subject. There are many early posts. Also Dr O had a write up about it in one of the MC2 issues, Ask him. Call you local MINI dealership and talk to the service Tech .(Service Measure M 18 200 04 9/27/04)

Post your findings

 

The sound coming out the exhaust isn't simply from the combusted gasses igniting. Remember, even with no fuel injected, you are compressing air. Have you ever heard an engine running open headers? The exhaust has a cracking sound at every cylinder hit. That crack is actually the combusted gasses breaking the speed of sound at the moment the exhaust valve opens (lots of gas trying to escape a tiny opening). There are ways of tuning this pulse to gain engine power but I won't go into that. I expect this pulse would occur whether there is combustion or not.

 

ok I am outa here 483fitter & Yucca patrol good luck

 

Too bad nitro, I really enjoy enlightening the uninformed.

If you all remember, the "burble" was introduced in the face-lifted models. There was quite the discussion about it. MINI was able to introduce this burble by allowing the ECU to CONTINUE to flow fuel into the engine after lifting off of the gas pedal. This was a computer controlled function. Before it was introduced the ECU was cutting off the injectors at the moment of closing the throttle body. Thereby saving fuel while slowing down while in gear.


Here is the service measure you attribute the cause to nitro




Sorry nitro, but again your ignorance and lack of understanding engine dynamics is glaring.

 

I think some of the issues here came from miscommunication and not from lack of understanding.

 

Personally, I miss compression braking. In my previous cars I downshifted and was able to maintain reasonable speed going down this one long local hill (police frequently set up radar near bottom of hill so I can't just "let 'er rip"). I find the MINI has, for all practical purposes, no compression braking unless you choose a low gear in which case the RPMs are really up there.....so I'm riding the brakes all the way down this hill.

 

It's one thing to disagree and to discuss those disagreements in a civil manner, but it is another to throw in personal comments about each other. So lets please keep to the topic.

Mark

 

Stumbled across this thread and have no meaningful input but I will throw my 2 cents in here.

As long as the engine is turning over, it's going to make some noise. I've gotten(?) a truck to backfire before by killing the engine in gear while rolling down the road. Granted it's not near as loud while it's coasting down but it's still making noise.

Now with nothing to back up my claims whatsoever, I would think a fuel injected engine would cut fuel at some point. Do some decent engine braking with a carb and you get tons of lean backfire. I don't hear that on FI engines (other than the burble on that one engine) so I'm leaning towards fuel cutoff.

Wow, this got off topic pretty fast. Here's two more cents with no evidence to validate my claims.

I say coasting is more efficient because you will retain or gain speed on the way down the hill. When you hit the next hill, you have momentum to carry you farther up the next hill so you will be on the gas later. If you keep it in gear I think you will hit the bottom of the hill at a lower speed, meaning less momentum. Less momentum means you will be on the gas sooner to carry you up the next hill.

Even if you gain only 3 mph coming down the hill idling in neutral vs engine off in gear, I think you will burn more fuel trying to regain the lost 3 mph than you would have burned idling down the hill.