Naked Science Forum
Non Life Sciences => Technology => Topic started by: lunar11 on 19/04/2012 20:55:16
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When I am driving my car and approach traffic lights when they are red, I often change down gears and allow the engine to 'do the braking' hence preserving my brakes. However, when changing down gears and subsequently slowing down the RPM (revolutions per minute) increase, though I am not pressing the accelerator. Will this still increase my fuel consumption, eventhough I am slowing down.
Should I avoid doing this practice and just apply the brakes earlier and wear them out sooner?
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If you're wanting to look at the "big picture" and consider every part, then you probably want to add the clutch to the equation. When you shift down, you have to engage the clutch to actually slow down the car. I've never driven a manual, but I should understand the mechanics well enough to know that the RPM increases because the clutch/engine locks on to the still-moving-wheels, not because you've pressed the accelerator; so there is no greater than usual gas consumption. What there is though is a greater-than-usual clutch usage, the replacement of which is likely to cost you far more than replacing your brake pads. Not that they would wear out at the same rate.
Having said all that, if I did drive a manual, I would slow down using the engine. Not because of the above. But because it seems cool -_-
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I like using the engine to decelerate. But, it is a good point that it may put extra stress on the clutch.
The clutch itself is actually a fairly cheap part, but it is a pain to dig out of the middle of one's car. When "engine braking", one typically doesn't slip the clutch, however, the weak link I've had most problems with are the clutch springs which can cause the clutch to lock up if they fall out.
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I hardly ever touch the brakes, apart from the final stop or emergency. I allow the revs to drop to a point where the engine is close to but not laboring under low revs that way when I change down the revs don't rise appreciably and there is minimal wear on the clutch. This method just requires starting to slow down earlier. It is also calming for both myself and my passengers and it pays off in other ways. It is less stressful. Why most people have to accelerate up to a stop sign then frantically brake is beyond me.
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Hi lunar11, engine breaking will indeed decrease your fuel consumption and increase the lifetime of your brakes.
In modern vehicles (or better, vehicles with modern ECUs: 'electronic control units'), you won't even use a single drop of fuel when doing this while you are in gear, because the car cuts off all fuel supply and uses the wheels coupling through clutch to keep the engine running above idle/stationary rpm. And just before the engine reaches the idle rpm, you'll need to downshift.
In older, "dumber," cars, the engine will use the same amount of fuel as if it were stationary when doing the engine breaking manoeuvre, because that's the amount of fuel that always goes to the engine when you're not touching the accelerator
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Brakes are cheap and easy to replace and need use to keep them in good trim whereas engines often cost more that the value of the car to ovehaul.
brakes also on a modern card are computer controlled to keep the wheels turning and improve control and tyre wear.
Brakes are for stoping and engines for speeding up, keep it that way
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I switched to driving an automatic transmission diesel car some years ago and one of the things that you had to get used to was the lack of engine braking and heavier brake use unless you deliberately forced the gearbox into a lower gear. However the latest DSG gearboxes when you are going down a reasonably steep hill and apply the brakes lightly, change down one or two gears to help with engine braking and this will hold in low gear until you touch the accelerator again. very effective and minimises brake use except on the very steepest hills. Very useful as I live in a very hilly bit of the UK.
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I think this arrangment is more to maintain sufficient brake servo action than to provide engine braking
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Interesting syhprum I hadn't thought of that. Mind you the brakes are so good on this car you only have to think and it will stop very unlike my first cars before servos where you really had to stamp on them hard.
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The main argument for using engine braking (with a manual transmission) was that, should you suddenly need to accelerate, you will be in an appropriate gear. You are going to burn off the kinetic energy one way or the other, so it's more a matter of control than anything else.
However, if you are going down a long hill, it's better to use engine compression rather than the brakes to keep your speed in check. Brake rotors and pads have to dissipate an enormous amount of energy during a long descent, and they can overheat, and fail.
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No one has mentioned stopping distance here...... Using engine breaking effectively in combination with breaking considerably reduces the distance required to stop the vehicle. I have not done the math but I always use this technique and feel much more in control of the vehicle under breaking.
Most Europeans drive manual cars, just more fun..... ;D
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Using engine breaking effectively in combination with breaking considerably reduces the distance required to stop the vehicle. I have not done the math but I always use this technique and feel much more in control of the vehicle under breaking.
If stopping distance is determined by speed and skid, I can't see how having the engine engaged to the driveshaft would make any difference.
Perhaps there would be a slight benefit of having an extra flywheel to keep the wheels from locking, but isn't that what the ABS is for?
Obviously one must pop it out of gear before coming to a complete stop!!
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I think you are right Clifford. Maybe Airtumbs needs to put his car in for a brake job!
I actually had the brakes go out on a long and twisty descent near Grasse in France many years ago. I think the brake fluid started to boil. We were four-up and fully loaded with camping gear in my '65 VW Variant. I managed to get it under control with some engine braking, pulled over, and announced that this would be a good place to get out and look at the view.
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Are you pair ganging up on me or what? ;)
Lets say your travelling at 30km/hr. You use engine breaking to stop the car and lets say you stop in 50mt. This is without brakes.
So lets say you now employ the same technique but you also employ the brakes.
How can you say that this would not make any difference?
I got this from wiki so maybe it's true but it should allow both the Geezer and the CliffordK something to get their teeth into.
"Another use for engine braking is if you need to come to a stop rapidly. That is, if you have a vehicle suddenly stop before you, or you have a pedestrian jump in front, and you must absolutely stop the vehicle. Here you can switch to the lower gear rapidly (say from the 5th to the 2nd), and use the engine for braking along with the brake pads. Thus, if you have mastered rapid downshifting, you can bring the car to a stop using this method by using all the gears from 5th down to 2nd, instead of skipping gears. This will definitely cause engine damage, but is preferable to loss of life (either to you or the other people on the road). The usual method to use engine braking is to switch to a lower gear for this purpose." http://en.wikibooks.org/wiki/Basic_Automobiles
I constantly engine break so I guess my engine is pretty damaged according to this article. However I would say going from 5th gear to 2nd is a bit extreme! I normally go from 5th to 3rd or from 4th to 2nd.
I also left foot brake sometimes but that is another story :)
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Which only goes to prove wiki's are not necessarily reliable sources of information :)
The rate of deceleration is limited by friction between the tires and the road surface. Unless you have retro-rockets, air brakes (like a plane), or a parachute, you can't do any better than that. I believe maximum deceleration is achieved when the wheels are still turning, but at a much slower speed than the road surface wants them too. If the wheels lock, the tire to road friction tends to reduce, so you really don't want to lock them. Also, you can't steer when the front wheels are locked.
So, if your brakes are crappy and they can't get the wheels to almost lock, you might be able to reduce your stopping distance with some additional engine braking, but if the brakes are up to snuff, you should be able to achieve maximum deceleration without any help from the engine.
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[joke]How about full throttle in reverse gear for reducing stopping distance in emergency cases? [::)][/joke]
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[joke]How about full throttle in reverse gear for reducing stopping distance in emergency cases? [::)][/joke]
Don't forget the other tools you have at your disposal like the handbrake :)
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The advice from multiple organisations employed globally to traind drivers in corporations is to use ones breaks to slow down, not to change down gears. That old, outmoded method was based upon the poor reliability of breaks half a cneutry ago. Of course you can choose to ignore the advice of experts and revel in the range of basic physics equations it gives you a chance to exercise. :)
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[joke]How about full throttle in reverse gear for reducing stopping distance in emergency cases? ::) [/joke]
Actually, that would reduce your stopping distance if you could ever get the tires to spin backwards - it would work like a dragster in reverse.
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So to sum this all up, Engine breaking is good for fuel economy and will help your breaks last longer?
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So to sum this all up, Engine breaking is good for fuel economy and will help your breaks last longer?
Yes. In fact, you could save a lot of money by only using the engine as a brake (as long as you only want to go down hills).
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[joke]How about full throttle in reverse gear for reducing stopping distance in emergency cases? ::) [/joke]
Actually, that would reduce your stopping distance if you could ever get the tires to spin backwards - it would work like a dragster in reverse.
I know it works in theory, but don't try to shift in reverse when doing anything above 20 Km/h, unless you have a somewhat decent amount of money you were otherwise going to throw away anyway :)
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The current advice in the UK is to slow down when approaching junctions using the brakes, rather than relying on engine braking.
This is not for any physical/ mechanical reasons, but because your brake lights let other road users know that you are decelerating...
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The current advice in the UK is to slow down when approaching junctions using the brakes, rather than relying on engine braking.
This is not for any physical/ mechanical reasons, but because your brake lights let other road users know that you are decelerating...
tap pedal & light worx, but how if 4 fluid circulations in tranny & engine?
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"Actually, that would reduce your stopping distance if you could ever get the tires to spin backwards - it would work like a dragster in reverse."
No it would not spinning the wheels would lead to reduced adhesion the best braking effect is acheived when the wheels are are still rotating at a rate appropriate for the speed of the vehicle.
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"Actually, that would reduce your stopping distance if you could ever get the tires to spin backwards - it would work like a dragster in reverse."
No it would not spinning the wheels would lead to reduced adhesion the best braking effect is acheived when the wheels are are still rotating at a rate appropriate for the speed of the vehicle.
That's true when only a braking force is being applied, but when the wheels are spinning in the opposite direction, you are applying a thrust in a direction opposite to the direction of travel. It works like a reverse thruster of sorts. (It probably would make the vehicle incredibly unstable too!)
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does the engine/tranny fluids experience a beneficial overflush due to downgearing?
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"Actually, that would reduce your stopping distance if you could ever get the tires to spin backwards - it would work like a dragster in reverse."
No it would not spinning the wheels would lead to reduced adhesion the best braking effect is acheived when the wheels are are still rotating at a rate appropriate for the speed of the vehicle.
Actually, the best traction is achieved with some wheel slippage. Racers use this knowledge all the time. The problem is, after you reach this maximum traction:wheel slippage ratio, the traction falls off, which puts you into a slide (in a turn) or just smokes your tires (in straight line acceleration) or puts you into a skid (when braking). The trick is to ride as close as possible to the ideal ratio, without going over it.
http://physics.stackexchange.com/questions/5838/why-does-a-tire-produce-more-traction-when-sliding-slightly (http://physics.stackexchange.com/questions/5838/why-does-a-tire-produce-more-traction-when-sliding-slightly)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi.stack.imgur.com%2Fo0kIQ.png&hash=ed4f0743dbb39bae0ee72c53e7d9e3c0)
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When I am driving my car and approach traffic lights when they are red, I often change down gears and allow the engine to 'do the braking' hence preserving my brakes. However, when changing down gears and subsequently slowing down the RPM (revolutions per minute) increase, though I am not pressing the accelerator. Will this still increase my fuel consumption, eventhough I am slowing down.
Should I avoid doing this practice and just apply the brakes earlier and wear them out sooner?
On virtually all modern cars, revving the engine up like this uses NO fuel at all; there's a fuel cut off that kicks in when you have your foot off the accelerator and when the revs are just above idle.
If you're slowing down, what you should do is keep the highest gear you can so as to keep the revs above about 1000 rpm and start slowing as early as possible.
It's surprising how much fuel you will save doing this-it can improve your mpg by maybe 3 or 4.
Using the brakes usually means your foot isn't on the accelerator, so the same thing applies, however the fuel cut-off will be happening for a shorter time.
You can even do this with automatics, if you use the gear selector, but I wouldn't recommend you try it if there's someone behind you or on a bend; braking can be a bit abrupt!