Just read something interesting in the Owner's Manual. On pg 90 of the R56 manual there is a subheading "Coasting". The last paragraph (sentence really) of that sections says " Fuel supply is shut off automatically when the vehicle is coasting." Earlier in the section it defines coasting as letting off the accelerator with the car in gear.
Is this real! I have never heard of anything like that (but the last car I really understood was my 1972 VW Type III). This would indicate that there was no combustion taking place in the cylinders and only the torque from the wheels is turning the crankshaft. Somehow that doesn't make sense to me. How would the car know if I was "coasting to a stop" or meerly decelerating a bit. Does this mean that there is only fuel going into the cylinders when the car is accelerating? I don't think so.
Would someone with some knowledge on this please chime in. I tend to kick the car out of gear and coast at idle - old habit from my depression era father who taught me to drive.

Greg

 

It seems reasonable to me. When you're coasting (by that definition), the car is driving the engine, if you want it to fire up again you just start injecting gas again.

What's the difference between that and coasting but the ECU is still pumping a trickle of gas in, enough to make it idle, except in that case its just wasting the gas. Why does it have to know if its decelerating, or decelerating to a stop? Its still decelerating and the engine is helping the process. It says nothing about not supplying gas when the car is maintaining speed (not accelerating or decelerating), it has to supply gas to keep the constant speed.

Coasting is when the instantaneous MPG reading pins at 99.9, which is as close to infinity (some miles/zero gallons) as it can display. If you kick it out of gear, it starts to show some MPG at slower speeds, as its got to keep the engine alive at idle, you're not allowing it to save that gas by keeping the engine turning.

I really don't understand why you're having a problem with this.

 

the electronic engine system can shut off the injectors while engine breaking. But this ISN"T coasting! Bad manual, bad manual! While under engine braking, there are lots of parasitic losses (friction) that act as a drag on the car.

What you learned from your depression era father applies to carborated motors, where the engine keeps sucking a lot of air, even with the throttle closed, and this uses much more gas than just letting the engine idle while in neutral.

For what it's worth, in the hills I live in, I actually get better gas mileage when I put the car in neutral because of the time that I have to use the gas pedal when engine breaking that I don't have to do when I just coast. The longer the slope when you can engine brake without using the gas pedal the more it tilts towards leaving it in gear. What's best? Depends on the specific location you drive in.

Matt

 

I would recommend you simply take your foot off the gas. When you do that the engine cuts the fuel just as if you were in neutral. The instant MPG display will read 99.9 MPG as well. Taking the car out of gear while moving is illegal in some states as well dangerous since you will not have power in an emergency.

 

but if you do that, you won't be able to coast as far. While it's true that there are laws in some states against coasting, they owe thier roots to a very different problem than not having power in an emergency. They have to do with the tendancy of cars to stall (this is from way back when cars had carbs and manual chokes). If this happened with power steering and brakes, you were hosed.

For example, while it's illegal (in some places) to be in neutral, it's not illegal to be in the wrong gear! So you can STILL have no power when it's needed, but be compliant with the law. Also, it's a very grey area when you are coming up to a light or stop sign. What's coasting? You HAVE to go into neutral sometime, otherwise the engine lugs! You're coasting when you change gears, is this illegal too? Don't think so...

Whatever, this is an area where there is no right answer, and you should do whatever makes you happy.

Matt

 

Most cars nowadays have some sort of decel fuel shutoff. When you aren't on the gas, you really aren't using any gas, basically. At idle you'll be using just enough to keep the engine turning, but when you lift off the pedal coast down, it cuts off fuel. It's all about finding that next % fuel economy gain.

 

You know Dr., that has to be the one of the silliest laws on the books. How are they gonna enforce that?
I am not coasting. I am just timing my shifts with a sun dial.

 

that the cut off is there, but it comes at a price. The price is called engine braking. That means you slow down faster if you just lift off the gas than if you coast. It depends on the exact situation to tell what actually gets you more gas saved.

And FWIW, the gas comes back on at about 1200 RPM as a transition to idle as you slow and push the clutch in. Also, the system doesn't work when the car is cold, as the car keeps some fuel flowing to get it warmed up to normal operating temp. Cold engines have higher emissions.

Like all things, it's not so simple....

Matt

 

Engine braking with fuel cutoff and extra drag versus engine running at idle with less drag. Which has better economy? They both have positives...and negatives. We need a fuel-cut neutral coast mode!

 

Since the throttle is electronic, the car knows when you are "off throttle"... This may be obvious, but I did not see it mentioned above...

 

In California (according to the "driver's handbook" it is illegal to coast in nuetral) ...yet I can't imagine how they'd enforce this. I do admit that I've coasted in neutral many times on long downhill streches of highway.

when in gear, your car won't coast as freely, as there is friction from the gears, etc...

I am surprised they recommend this in our manuals though, I always figured this sort of thing was frowned upon.

 

I think what they are recommending is that you leave the car in gear when coming up to a stop or turn instead of putting the car in nuetral.

 

Yea, I can't see how coasting in neutral while idling could be much more economical than coasting in 6th gear.

The manual, for some previous car I had, warned not to coast to a stop in neutral because it could leave some gears in the transmission spinning at high speed and be hard on the syncros when trying to put in gear. I wonder if this would be an issue with the MINI.

The way the California law might be enforced is after an accident, if a driver explains their loss of control because they lost power steering and power brakes when the engine died while coasting in neutral. This was much more likely to happen with out of tune pre-computer cars.

 

OK...how about this? I propose that you could conceivably get better fuel economy on the highway by "pulse-and-glide driving" and by NOT using the cruise control.

Seems to me that the cruise stays "on the gas" more often than not, which would theoretically be detrimental to fuel economy. Keeping a light foot on the right pedal and coasting in gear for longer stretches (thus earning the 99.9 mpg rating on the instant mpg computer readout) should then get better fuel economy, correct?

I routinely get 34-37mpg in mostly around-town driving (average speeds of 25mph or so). When I used the cruise on a recent road trip at mostly highway speeds, I could only get 38 mpg at best (average speed of 55mph or so and no top speeds greater than 75mph and flat terrain). I didn't try pulsing, but might do so this weekend while driving from Chicago to South Haven, MI. Terrain will be similar, and I'll try a little more coasting in gear.

I'll report back with my findings (for what they're worth....)

 

The neutral law is because in the 70's gas crisis people would turn off the engine on the highway and be surprised when the steering wheel locked on them, and the lack of power assist to the brakes & steering made the car very hard to control. If you have a manual no one can stop you from depressing the clutch, but I would not coast in neutral with an automatic. You have to be careful to be off the gas pedal when you shift back into gear. I don't understand the hypermiling mindset with one of the most fuel efficient sporty cars out there. I'll take safety over a few bucks at the pump. And if you're gonna coast make sure you do it in the slow lane.

 

I know one road you can coast on for 6 miles at about 80 mph. Highway 24 between Loa and Sigurd Utah.
That was fun.

 

Doc, my recent experience out at MITM gives some credence to the latter half of your supposition (as quoted). This is a 1st Gen report in a 2nd Gen thread, so take it for what it's worth -- (come to think of it, why is this thread topic 2nd Gen specific?)...

During my 16-hour trip out to Copper Mountain, my MPG was about what I had anticipated in my 5-spd Justacooper averaging 75-80 mph with the A/C running continuously and two passengers (33-34 MPG). Once at MITM, I spent 3 solid days screaming solo around the local mountain roads. Most of the drives involved long and often steep, spirited uphill climbs and similarly long downhill runs. While climbing, I was usually in gears 2 - 4, my tach rarely fell below 4000 rpm and spent a lot of time above 5000 rpm as I tried my best to keep up with all of the "S" MINIs that seemed to climb up the mountainsides with a whole lot less effort than me . I felt like I was driving the car fairly hard during this period, and when I finally pulled into a gas station for a fill up I expected the worst when I computed my mileage. I figured it had to be at least down in the mid 20s, maybe lower. Imagine my surprise when the number came up as 39.8 MPG! (The second best mileage I've ever obtained, BTW.)

My conclusion is that the loss of efficiency during the rigorous climbs when compared to straight and level motoring was more than made up by the fuel savings during the extended downhill runs--always in gear--after the climbing was done, when I hardly had to touch the accelerator at all.

 

Wow the cars already get 30+mpg, if you're that cheap about gas you should just sell your 20K+ Cooper/S and buy yourself a Kia or get a bicycle. I seriously can't believe we're having this conversation.

 

the OP asked about coasting and what he'd learned from someone who learned about cars a long time ago.

Just because one gets better mileage than a hummer, doesn't mean getting even better mileage is not worth considering.

What I can't believe is that someone took the time to post something as content free as what you wrote! So what if you don't care. Doesn't mean other shouldn't.

Matt

 

Isn't coasting when you have it in neutral and the car is in motion? I love coasting it also saves my clutch big. I had a VW golf about 8 years old the clutch plate was supposed to be replaced at 70000km. I had it changed at 150000km

 

Coasting isn't going to make your clutch last longer. Driving down a hill in gear the clutch is fully engaged therefore there is no slippage. Whether your clutch is fully engaged or fully disengaged the wear is the same, zero.

 

My OP really had more to due with curiosity about how the engine works. I love to coast because it is fun. Dr. Obnxs hit the nail on the head when he explained about the ECU. I have since had a chance to talk to a colleague who used to program ECU's for Ford. He said that Dr. O was correct and the fuel supply is cut and the engine becomes a big compressor, but it saves fuel. Since his challenge (from his bosses) was to maximize the EPA mileage calculations they used everything they can to do that. He did note that he had to be careful how it was done so that it wad not apparent to the driver, did not wear out any other equipment, etc. So he said they programed the ECU to cut the fuel supply 3 sec after deceleration began and kick back in as soon as the pressure was put on the gas pedal, He also said that the lower end was 25 mph (fuel restarted) to insure all aux equipment kept running.
My guy agreed with Dr. O in that the situation will dictate wheter more fuel is save coasting in gear or out of gear. (Out of grear the engine is suppled with enough fuel to keep it idling). In the end there is not much difference and I will continue to do what is fun - that's why I bought the car.

 

You can even play around with your own car to see when the ECU turns the fuel back on when decelerating in gear.

Get going about 30 MPH in third gear, and then let off the gas (keep the car in gear, though). Watch the tachometer needle as you slow down. When the engine speed drops to about 1100-1200 RPM or so, you'll feel a slight shudder/jolt through the steering wheel. That's the fuel coming back on.

 

I used to do this but it gets annoying after a while and its annoying for the people behind you too. I think its more effective using my cruise control as often as I can and i try to stay on cruise control no more than 60 to save gas.