Fuel Saving Idea? 3

[XR250]

I have an idea for a way to save fuel.
I am sometimes a hyper-miler. My car is a 2023 Mazda3 6MT. I notice when I coast on long downhill highway stretches, my indicated MPG definitely increases versus driving the same stretch in gear. Same for long downhill section of backroads. I assume this is due to the lower pumping and friction losses in the engine running 2,200 RPM versus 700 RPM idle.
I also look for stop signs and red lights on my GPS and try to coast accordingly to minimize the use of my brakes (without holding up traffic of course). These strategies gain me at least 4 MPG.
This gave me the idea that software could be written to coast the vehicle based on GPS terrain, traffic and traffic sign data. This might have to be implemented in some sort self-driving software to be effective. I would assume the car would require a transmission that can coast. I know the new Mazda CX90 doen't use a torque converter in the automatic but a wet clutch so that could be held open- same with a DCT.
Is this just a crazy idea?

 

[XR250]

My Mazda3 is part-time Atkinson.

So what is the "throttle position during acceleration" consensus for best fuel economy?

 

[BrianPetersen]

I would suggest moderate acceleration. Enough to get the tach to around half-ish of redline give or take.

At higher revs and high load, that's when you get into enrichment, and higher FMEP, and so forth. At light load, that's where it's in a bad part of the BSFC map, and probably torque converter unlocked and slipping. I doubt if it's all that sensitive as long as you are somewhere in the middle third of the rev range.

If you're one of the rare folks nowadays with a row-your-own, maybe half throttle, but keep the revs in the lower half. Half accelerator-pedal position at lower revs translates to much more than half of rated torque, and that's what you want for best BSFC, but you want to stay out of floored-throttle enrichment.

N.B. my automotive experience has been with 3 and 4 cylinder underpowered econoboxes, and the van, which is the only vehicle that I have ever owned with more than 4 cylinders but is hardly a performance beast. With my motorcycles, half throttle position in first gear will send the front wheel skyward, so there's that.

 

[XR250]

If you're one of the rare folks nowadays with a row-your-own, maybe half throttle, but keep the revs in the lower half. Half accelerator-pedal position at lower revs translates to much more than half of rated torque, and that's what you want for best BSFC, but you want to stay out of floored-throttle enrichment.

That is me and basically how I drive.
#savethemanuals

 

[LionelHutz]

Cruise controls with automatic transmissions have slipped neutral going downhill for years. Part of the reason "tow mode" was added to modern pickups was not only to adjust shift points for heavier loading, but also to ensure the trans stays locked into gear to provide engine braking downhill.

That doesn't sound right. I drove a Ford not too long ago and it would downshift to engine brake with cruise on. I drove a GM too it would also downshift to engine brake with the cruise on. Neither of them were in tow mode.

 

[LionelHutz]

At WOT the engine operates in closed loop mode so the O2 sensor is ignored.

Given that the O2 sensor is the fueling feedback signal, it's not very likely the engine could be both in closed loop and ignore the O2 sensor.

 

[lucky-guesser]

Given that the O2 sensor is the fueling feedback signal, it's not very likely the engine could be both in closed loop and ignore the O2 sensor.

Newer cars with wide band O2 sensors will stay in closed loop at WOT but older cars would go into open loop (not sure when that change was made). New and old cars alike are both open loop for the first couple minutes after starting to get everything warmed up.

 

[GregLocock]

Open loop, not closed "At WOT the engine operates in closed loop mode so the O2 sensor is ignored. " And i think they tend to do this a or near full power rather than WOT in general.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Lou Scannon]

I would take the x-axis values as indicative. Highly boosted engines, when aggressively tuned for max power, may benefit from enrichment all the way up to 0.10 fuel air ratio.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[LionelHutz]

Newer cars with wide band O2 sensors will stay in closed loop at WOT but older cars would go into open loop (not sure when that change was made).

Correct, some newer cars are using wideband O2 sensors and staying closed loop most of the time. When in closed loop at WOT the O2 sensor still isn't being ignored, as I posted.

Heck, newer cars with narrowband O2 sensors stay in closed loop most of the time.

 

[romke]

The idea is employed with some more recent models in heavy duty dieselengined vehicles in Europe. When put in cruise control the vehicle is "seeing" the road in front of it based on GPS data. When noticing a hill clime the gearbox automatically shifts down just before the start of the hill and some extra power is applied to be able to climb the hill; right before the top the power is somewhat reduced and when going downhill the fuel supply is reduced in such a way that the the chosen speed will not be surpassed - when necessary the vehicle automatically chooses a lower gear or applies the engine brake or a retarder. You need some quite sophisticated software to secure this way of operating based on a lot of parameters (load, terrain, speed limits,traffic) but it is quite possible to reduce the amount of fuel needed compared to pure manual operation. At the same time the average speed can be somewhat faster.

 

[IFRs]

If you ride a bicycle you know that you can stop pedaling and stay in gear and the bike cruises forward with only a ratchet device adding mechanical resistance. Decades ago SAAB cars had a similar ratchet device. If you took your foot off the gas, the engine went down to idle. The (manual of course) transmission was still in gear. I think it was called freewheeling. The only trick was bringing up the engine to speed without crashing into the gears. Not hard when you got used to it. There was a pull-lever above the accelerator pedal that would lock it out. Those were the days of 3 cylinder 2 cycle and v-4 pushrod motors. SAAB dropped it because the American drivers could not deal with it and the inline 4 then overhead valve then turbo had too much power to not destroy the freewheeling device. Taking them apart was a nightmare when all the little springs went "boing" and the pawls escaped all over the garage floor. I made a tool to deal with it though...

 

[TugboatEng]

Blame Americans but it's more likely the SAAB system simply didn't work. The wheels can drive the engine and accessories at very high efficiency >90%. Meanwhile, the engine at idle is very inefficient, <30%.

 

[IFRs]

Tug - have it your way but I don't think that is a fair comparison of efficiency and the fuel consumption may increase with higher but driven RPM while the car goes the same distance.

SAAB used freewheeling to prevent engine failure if the 2 stroke with oil in the gas was pushed to higher rpms with the throttle closed and therefore no oil. It worked very well for that and we got great life from our motors. SAAB also claimed higher gas mileage but I never tested it scientifically. When they began using automatic oil injection in their 2-stokes and then used 4-strokes freewheeling became obsolete. You could also upshift without the clutch which we did all the time. When SAAB discontinued freewheeling I remember the dealer telling my Dad that it was simply not suited for the American market (wink-wink) which we took as American drivers did not deal with it well and most just wanted automatics, other than enthusiasts. SAAB was not the first to use freewheeling but it was my only experience. It also was humourous to train someone how to drive a manual shift car with it locked out and then let them experience the freewheeling! I was sorry to see it go but they were the weak point of the drivetrain.

 

[TugboatEng]

So freewheeling was done for engine lubrication and not for fuel efficiency.

Interestingly, 2-stroke engines don't engine brake. It shouldn't feel much different with our without this freewheel device.

 

[romke]

Driving a twostroke SAAB with a freewheel was a remarkable experience when not used to it. If you were used to a fourcycle engined car and then entered your first corner in a SAAB, you immediately realised that you had to apply the brakes far more forcefully then you were accustomed to. The brakes where quite good compared with other cars of that era, but initially driving a SAAB was a very scary experience.

The claim that it also saved some fuel was based on a comparison with other two stroke engines of that time (DKW and cars built in the then GDR). With other two stroke cars you needed to go downhill with the throttle not fully closed or opening the throttle a number of times to make sure the engine got lubricated - spilling some fuel while doing so.

 

[dgallup]

Generally speaking, freewheeling is considered somewhat dangerous and not recommended. You can pick up considerable speed plus your brake vacuum boost and power steering are reduced. Not to say I've never done it.

I was driving to the '79 Watkins Glen GP in my Mustang very late at night. I saw a sign that said there was a long down hill into town so I threw it into neutral. Coasting at 85-90 mph, I passed a sign that said the speed limit in the town of Watkins Glen was 35 mph. Then I went past a sign that said entering the town of Watkins Glen. My sleep deprived brain did not put 1 & 1 together. Stopped at the red light at the bottom on the hill and looked in the mirror at flashing lights. Went to jail, got the guards to perp walk me to the bank in the morning so I could bail myself out. At the track during a pause in the racing the announcer said that a man had been arrested the night before for doing 90 in a 35 zone. The guy next to me said "I wonder what that cost"? I said $150.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[XR250]

You can pick up considerable speed plus your brake vacuum boost and power steering are reduced. Not to say I've never done it.

Isn't you vacuum highest at idle? Also, power steering has become electric in most cars so the engine speed is not too relevant unless your battery is weak.

 

[3DDave]

XR250 - agree - this is most clearly visible in a car with vacuum operated windshield wipers. Nothing like tromping the gas and seeing those suckers instantly stop.

 

[TugboatEng]

Vacuum is highest while coasting in gear. That is where the engine braking comes from.

 

[BrianPetersen]

... mind you, most newer engines are using VVT shenanigans to intentionally reduce volumetric efficiency at lighter engine load as opposed to closing a throttle, and/or they're turbocharged, and they're thus using a vacuum pump to reliably ensure vacuum for accessories.

... or simply eliminating the vacuum-operated stuff. My car has an electric servo brake booster. There's nothing operated by vacuum.