Coasting gas mileage

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  #1  
Old 01-13-07, 09:39 AM
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Question Coasting gas mileage

I have a car with automatic transmission and fuel injectors. Can someone tell me if the injectors cut off and the engine just sucks air while coasting downhill, maintaining full speed? Would this also happen when coasting to stop sign while slowing down, with or without brakes applied?

Thanks.
 
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  #2  
Old 01-13-07, 09:44 AM
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No. If the injectors stopped completely, the engine when stall. When coasting, the amount of fuel being consumed is approximately the same as when idling.
 
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Old 01-14-07, 07:50 AM
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It depends on the make and model. For example, I know for a fact the '02 Volvo S40 completely cuts fuel off to the engine when three conditions are met:

1. transmission in drive
2. foot off the pedal
3. engine rpm above 1500

I'm not sure which makes and models have this feature, but it seems like a no-brainer to me. Considering all the complicated things a car's computer can do, you'd think EVERY car would have this design feature and cut fuel to the engine using those three inputs. Unfortunately, I don't think this is the case.
 
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Old 01-14-07, 12:33 PM
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I wouldn't think any engine would be designed to completely shut off while rolling, you need engine running for power steering and power brakes to work!

That's the main reason freewheeling [coasting with motor off] is illegal in most [maybe all] states.
 
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Old 01-14-07, 12:47 PM
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Kestas is right.

My Nissan Maxima's fuel injection will completely shuts down if those certain criteria's are met.

Marksr- engine does not shut off, fuel is just stopped. Sounds strange, I know, but thats how fuel injection works since the advent of OBDII
 
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Old 01-14-07, 02:34 PM
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Marksr:
Many years ago some cars actually had a nob on the dash for freewheeling they also had a nob for throttle(old time cruse control). Have a nice day. Geo
 
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Old 01-14-07, 02:46 PM
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Gas engines use a mix of gas and air in order to fire up so I would think that even when idling engine still uses gas , it coulnd't keep firing on air only.
Diesel engines do suck 100 % air to the cylinder where once it's compressed , fuel is added, which ignites on contact with the hot air, that's why a diesel engine is more efficient.
 
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Old 01-14-07, 02:53 PM
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Modern gas engines use fuel injection. Shutting fuel off to the engine is easy - just don't let the injectors fire. Air still goes through the engine though. Like HotinOKC said, this doesn't shut off the engine. It's still in gear and freewheels, running the alternator, ps pump, and other functions.
 
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Old 01-14-07, 05:27 PM
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Originally Posted by geogrubb View Post
Many years ago some cars actually had a nob on the dash for throttle(old time cruse control).
My 51 ford f1 has a throttle control on the dash, has a cable and works similiar to a manual choke. Of course today it is illegal to have it hooked up
 
  #10  
Old 01-15-07, 08:58 AM
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Ummm, on what automatic transmission vehicles does the engine continue to turn over if you're coasting with the engine off (no fuel delivery)??? Neither my wrecker nor my wife's Camry will do that or as far as I know, any of the AT equipped vehicles I've ever owned; or am I missing something here? If you could it would then be theoretically possible to push start it. I don't know of TOO many vehicles with AT's that you can push start.
 
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Old 01-15-07, 10:37 AM
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With the car in gear, and at about 1500 rpm, the fuel is cut off, at least in my Maxima it does this.

The engine is still operating off the transmission. I had some information about this, but I don't know where it is now.

I thought it was crazy when someone told me that the engine is running without fuel, but it does do it.
 
  #12  
Old 01-15-07, 10:48 AM
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So at what point do the injectors start putting fuel back into the cyl to keep the engine running. From reading some of the post your saying when you coast the fuel is shut down, no fuel while coasting, coasting, coasting and now you have come to a stop! What now if no fuel is being delivered? Do you have to hit the starter to get it going again?
I would think fuel is cut off to a point but to keep everything in balance a minute amount of fuel would have to enter the cyl to keep it running as mentioned above with enough to make it idle.
 
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Old 01-15-07, 11:39 AM
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I think what's confusing people is that in the past we've all experienced an engine conking out on the road. The engine shuts down, the dash lights come on, and you pull over to the side of the road.

I'm guessing there may be a sprag clutch that keeps the engine revving, which keeps all the ancillary equipment (trans fluid pump, alternator, ps pump) going, until the engine takes over again.

As a side note, if you know how to drive such a car, you can take advantage of this feature by keeping your foot off the pedal until you're ready to accelerate or resume your speed. There's a natural tendency to feather the throttle under such conditions, usually when there's heavy traffic on the highway. If you can train your foot to stay off the throttle under these conditions, you can take full advantage of this feature.
 
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Old 01-15-07, 02:35 PM
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Question

Thanks for the inputs, everyone. I sure started an interesting discussion.

I’d like to add something else to the pot. My car has RPM’s = 2200 at about 60 MPH, and gets about 35 MPG. It doesn’t downshift unless I am going up a pretty significant grade. So, while driving flat or slightly uphill/downhill I must be wasting gas. Surely, I could have another gear or two which could handle that condition, with a RPM of 1500 and maybe even 1000 for downhill. I heard of a Japanese car that had 8 speeds. Maybe an 8 speed or 4 speeds with Overdrive could really save some gas and give a much higher average highway mileage. This would be much less expensive power-train than a Hybrid, and less repair cost, thus lowering insurance cost as well. With the auto industry migrating to hybrids, I am afraid that they will never do something as straight forward as this.

Hope you folks decide to stir the pot some more. Thanks in advance for inputs.
 
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Old 01-15-07, 02:55 PM
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Coasting gas mileage

justdoit007 -

The thing you are missing when try to save money by just lowering rpms is that the engines have different efficiencies at different speeds. Lowering the rpms (or raising them) may make the engine operate less efficiently and you could use more gas. That is why they have gears - power and efficient speed.

You might be interested - Volkwagen just brought out a new version of a two door coupe from an old car line called Bugatti, that was made in the 1920's and 30's. It has an 8 speed transmission and a double clutch setup. Top speed is 255 mph, price is over $1,000,000 and they are sold out for years. It must be real economical!!! (LOL)

Dick
 
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Old 01-15-07, 02:56 PM
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my 2 cents

I dont think the engine shuts off reason being if you were driving down the road and shut car off with the iginition first thing you notice is lack of power steering.
On a side note when I was young and stupid I actually blew a muffler apart by doing this stupid stunt. Going about 70 I shut down engine and turned on key and all I heard was a loud explosion followed by the sound of a blown muffler. Car had a carb so that might have did it.
 
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Old 01-15-07, 03:23 PM
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Used to to that on motorcycle when I was young and crazy; flip off the kill switch for a few seconds and then back on. Good for a pretty good bang; never blew a muffler off, tho.
 
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Old 01-16-07, 04:55 AM
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Simple answer for automatics, torque lockup. Until the lockup is released, the engine and transmission are locked together, that keeps the engine turning without fuel until either the pedal is depressed again, or the RPM or speed drops below the point determined in the computer. In any case, once the lockup is released, the injectors are turned back on.

Works the same way for a manual, just without the torque converter. Until RPM or speed drops, fuel is shut off.

Worked that way on my 85 Pontiac, so it's nothing new for fuel injected cars.

While the savings for YOUR car may not be significant, multiply times the millions and millions of cars on the road, and it adds up.
 
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Old 01-16-07, 08:10 PM
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A different size tire will give you the same results as a gear change. If your interested in this you can calculate different size tires and how it changes your gear ratio.
 
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Old 01-17-07, 01:41 PM
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Smile Thanks for inputs.

If I may summarize:

While coasting, engine stays engaged with ‘torque lockup’ and injectors shut down as long as engine RPM is above some level around 1500 RPM, controlled by the computer. This would apply to steep downhill coasting or slowdown while coming to stop. It appears that the auto companies are on target here.
(Search ‘torque lockup’ for many references.)

Example: http://dnr.louisiana.gov/sec/execdiv/techasmt/ecep/auto/f/f.htm

Torque peaks out at some medium RPM, ~ 3500-4500, where maximum efficiency occurs. The engine is much less efficient at 1000-1500 RPM, but if little energy is needed to keep car rolling on flat or slightly downhill terrain, gas mileage could be much higher in a higher gear while not pulling on engine.

Increasing wheel size would be equivalent to increasing all gear ratios causing less low speed acceleration and highway speed hill climbing capability. Adding extra gears would give higher mileage under easy driving conditions, while not giving up anything at the lower speeds or steep grades.

I think a typical aerodynamic family sedan or compact equipped with an 8 speed transmission for high mileage during highway travel would complement the new Volt car now being touted by GM. The Volt would be perfect for mostly city driving. My 8 speed transmission car would be ideal for anyone that uses interstate, car pool lane driving or lives in the country, where immediately upon leaving the driveway it becomes highway travel. The auto manuf could install a governor to prevent driving at really excessive speeds.

Reference AT: http://auto.howstuffworks.com/automatic-transmission1.htm

For what ever it is worth, I’m emailing GM with my AT ideas.
 
  #21  
Old 01-17-07, 07:49 PM
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Two words for you my friend : De-Caf
 
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Old 01-17-07, 08:23 PM
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Are you a rocket scientologist?
 
  #23  
Old 01-18-07, 06:51 AM
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8-speed transmission?... that's a lot of shifting! Considering the soft shifts manufacturers like to design into their transmissions, this would leave a lot of dead time for the engine durings shifts while accelerating. It reaches a point of diminishing returns. Even the 7-speed automatics now marketed are pushing it. A long time ago I read that the 6-speed transmissions was considered optimal. That's why the CVT (Continuously Variable Transmission) has become popular. Plus it has fewer moving parts.
 
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Old 01-18-07, 07:43 PM
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Originally Posted by the_tow_guy View Post
Ummm, on what automatic transmission vehicles does the engine continue to turn over if you're coasting with the engine off (no fuel delivery)??? Neither my wrecker nor my wife's Camry will do that or as far as I know, any of the AT equipped vehicles I've ever owned; or am I missing something here? If you could it would then be theoretically possible to push start it. I don't know of TOO many vehicles with AT's that you can push start.
I was in the drivers seat of '68 Firebird that was being pushed and dern thing started much to my surprise.
I don't know if that would work on a modern car today.
 
  #25  
Old 01-19-07, 04:27 AM
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dont really no of any that shuts fuel off completly while the computer knows your speed and throttle position it is able to detect coast conditions and cut fuel way back, you wouldnt want to cut fuel completly as your engine would slow the car down quickly and the vehicle would not coast as long it may just be pumping air but it takes alot of energy to compress air in each cylinder and not make any use of it.
im with tow guy on the automatics the engine can completly stop or die even at 60 mph while the torque convertor clutch locks the transmission and engine together for increased mileage it is not locked at all times and its not of much benefit in coasting conditions plus it unlocks any time the brakes are applied.
and yes some of the old double pump powerglide transmissions you could push start.
 
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Old 01-19-07, 04:39 AM
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There's a lot of things some older cars could do that the newer ones can't, LOL. My memory's a little fuzzy, but I think there's some import stuff you could jump start back in those days, too.
 
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Old 01-19-07, 05:43 AM
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If I remember correctly the ford automatics back in the early '60's would push start around 35mph or so. IMO the cars back then did a lot of things better than the cars of today - except pass a gas station - at least they were easier to work on
 
  #28  
Old 01-20-07, 08:42 AM
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Smile Put to rest

Kestas, you could be right, 8-speeds might be too much. I just wish I could experiment with two more gears in my car that gave me the RPMs I’m asking for at 60 MPH. Does anyone have a supercomputer and an auto drive-train design program? Right now I have a 3 speed AT plus one overdrive gear which has a manual button. I never take it out of overdrive. A 6-speed would satisfy me. In any case I am ready to put it to rest. GM says: “We value the opinions of our customers and we always take them into consideration as we plan for future models.”
 
  #29  
Old 01-20-07, 08:56 AM
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it is possible

I know for sure a vintage 65 dodge coronet could be push started with a auto trannie speed needed is unknown I know 70 mph did the trick. However I do not think I would attempt this stunt on my 2000 Ford Taurus because I am not sure what would come apart.
My 2004 Tacoma Manuel trannie can be push started book says dont do it but I had to try just to be sure.
I know one thing for sure I miss rear wheel drive cars of the 60s with none of that computer stuff.
 
  #30  
Old 01-25-07, 07:02 PM
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Every Chrysler product with a torqueflite tranny I have ever encountered will engage above 35 mph. This is because the main trans pump turns with the tranny in neutral. When the driveshaft(or axles) turn with the tranny in neutral, the pump will build pressure, and drop it in gear ant the clutches engage to turn the engine. This is provided the tranny is in good shape. If it is somewhat worn , you just have to go faster. In the cold climates,I have always put my tranny in neutral and set the parking brake during cold winter warm up, that way the tranny fluid is circulating. This saves many Dodge truck trannies. If I was worries about saving a few test tubes of gasoline, I wouldn't drive full size,large engine 4x4 trucks on a daily basis. It's one of the many ways I can say I'm Proud To Be American!
 
  #31  
Old 01-26-07, 07:58 AM
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coasting gas mileage

The cars that could be push started had auto trans with a rear pump,as did most all the cars of the 50s and 60s then the trannys were redisigned and the rear pump was eliminated this ended push starting and brought on the "do not tow with the rear wheels on the ground" as there was no circulation of the fluid. Tom
 
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