Two days ago my friend and I were driving a rented car and we've noticed that when coasting, the computer said the fuel consumption is 0, but when the friend disengaged the clutch, the fuel consumption actually went up (!) to some value. We've been wondering what's going on, and we actually suspected a software bug in the fuel consumption indicator. Thanks!
In my head, using the engine/transmission to "brake" when in gear going down hills or slowing down seems like a bad idea. Something is getting stressed in order to slow the car down.
Having to replace my brakes makes sense, they get worn out, and its relatively inexpensive (especially if I do it myself). I have always been wary of staying in gear to brake. And, as it so happens, my manual has 130k miles on it, stock engine, transmission, and clutch. Never had any problems.
I would love to understand the science behind why using the engine/transmission/clutch to brake is better for the car than just using the brakes.
As for wear on the engine: the engine is actually designed to mesh with the transmission. Engine braking is not much different than accelerating.
When I was learning how to drive a manual, my Dad was a huge fan of them and all of our family cars were manual. I was taught that it makes a huge difference when you're driving in the mountains to use engine braking instead of your brakes. He also explained the difference in braking in traffic and using engine braking to do so and how it supposedly saves on gas as well - something about idling the engine versus continuing to have the engine running?
I'd be interested to hear from people who regularly drive in the mountains and the advantages/disadvantages of engine braking versus using your brakes and if that was some myth or actually real.
While you're accelerating, you're giving the engine gas to keep it moving and prevent a stall. While braking you aren't accelerating, but you still need the engine to keep going at speed.
If you depress the clutch, it disconnects the engine from the wheels. The engine will then get a trickle of gas (even if you don't accelerate) to keep it moving while the car breaks.
If you don't depress the clutch, the engine stays in connection with the wheels. Their motion drives the engine - the reverse of normal - keeping it moving without burning any gasoline at all.
It's not generally a big difference, but if you're in the hills of Colorado you might care more.
https://en.wikipedia.org/wiki/Compression_release_engine_bra...
Engine braking on a gasoline engine is actually quieter than normal operation.
http://www.jacobsvehiclesystems.com/about-us/environmental-h...
That said, I drive a manual and use the engine for braking...
Growing up in the mountains, in Colorado, it was drilled incessantly into me how important it was to use engine/transmission braking on long hills, etc.
I know it, I understand it ... and it always irked me.
I never liked the idea that I would save wear and tear on a consumable that is meant to be regularly worn and repaired and instead transfer that wear and tear to an integral part of the car that is non-trivial to replace or repair.
I would think very carefully about this if I were towing a boat or a horse trailer, but in a normal car it just seems backwards.
It's the reason that sports cars generally come with bigger brakes. Sure, a Toyota Corolla might be able to stop in a similar distance, but things change when you have to do it 5 times in a row.
For most people, it’s not really an issue though.
That condition is known as brake fade/fading and occurs mostly with drum brakes and can occur on bicycles, motorbikes and automobiles.
Braking heats the brake drums, which then expand. The drum may expand in diameter to the extent that the brake pads no longer contact the drum sufficiently enough to slow the vehicle. Your brakes "fade" away.
Many vehicles have disc brakes in front and drums on the rear, an arrangement usually sufficient even in hilly country. Of course when pulling a boat or trailer having all disk brakes would be a safer bet.
Brake drum fade is by far the most common manifestation, but there are other types of "fade", e.g., brake fluid can get heated up enought to boil and reduce braking system pressure, brake pads can "slip" more at extremely high temperatures:
http://bicycles.stackexchange.com/questions/30449/do-mechani...
Last ditch efforts: shift into a lower gear, use any emergency brake (which is mechanical and bypasses the braking fluid system and so won't fade due to boiling fluid), and finally, use the inside of a hill/mountain as a giant brake pad by sliding your car's body into it as gently(!) as possible.
Finally I feel compelled to warn anyone who ever pulls a trailer, boat or RV about a potentially fatal phenomenon they may encounter on even gently sloped roads: undamped driven harmonic oscillation between towed and towing vehicle. My first experience:
VW Beetle towing a U-Haul trailer on a Pennsylvania turnpike mountainside. Traffic moderate in both directions.
On a long downhill segment the trailer hitch began to move to-and-fro left and right, gently at first but, as I attempted to correct with steering, rapidly growing in amplitude. My steering reaction time and corrections were unfortunately timed precisely so as to _increase_ the amplitude of the oscillation. In a flash the rear end of the Beetle was hopping right and left! Insight - I need to dampen the oscillation. I gripped the steering wheel, braced both forearms against my legs and reduced all steering corrections to a minimum (I just kept the car on the road and in the proper lane). The car's front end skidded left and right as I kept the wheels as straight forward as possible. Then I slowly applied the brakes. This brought the oscillations under control and the speed down. I continued the trip at a much slower speed despite the honking of frustrated drivers behind me.
That first experience, enhanced by both impending ignominious death on a mountainside and the sudden realization of the utility of my mathematical physics class* [1], was exhilarating.
In the years since I have myself seen this occur several more times, which makes me think it must be a not-uncommon event that requires some warning.
It once again occurred on IH 10 between Houston and New Orleans, one of the flattest pieces of land in the USA. A heavy-duty six-wheeler pickup was hauling a trailer full of goods on an extremely gentle slope at near 70-mph when his trailer hitch began to oscillate left and right. I had been following and observing his truck and noticed that the system seemed to be periodically oscillating, so I stayed well back and did not attempt to pass even on a four-lane highway. Finally things took a turn for the worse and, within 6 seconds of back-and-forth oscillation and attempts at correction, both truck and trailer were driven off the road into the grassy median. Luckily the median was wide flat grass and no harm occurred to driver, truck or trailer. I stopped and crossed the road as the driver took off his cowboy hat, waved it at his truck and trailer as if dismissing an unruly horse, bent his back, and put his hands on his knees in amazement.
I spoke to him awhile, reassured him and gave as best an explanation to him of what I saw and what he might do to prevent further mishaps. He was quite out of sorts and I'm not certain he was fully able to absorb the lesson. He was definitely astonished to find himself on the median with his truck and trailer turned around 270 degrees from the direction he intended.
Had this happened on a strip of highway without such a wide median the truck would have driven at ~60 mph into ongoing traffic at 70 mph. Had this happened on a hillside, I would estimate a 50% chance of both vehicle and trailer plummeting downhill. For these and other reasons, I think this phenomenon must kill more than a few people each year.
More discussion of fish-tailing trailers:
https://www.google.com/search?hl=en&source=hp&biw=&bih=&q=tr...
Physics can save your life:
[1] http://hyperphysics.phy-astr.gsu.edu/hbase/oscdr2.html - Under topic "Driven Oscillator Example" the red curve describes the ever-increasing amplitude experienced.
It was a fun trip with only a minute or so of harrowing possible death-down-the-mountainside. I'd recommend it to anyone!
Also if the mountain is tall enough the brakes might not make it all the way down which has happened a few times in Norway with catastrophic results for large vehicles like busses.
Suffice to say that they do not recommend against engine braking--quite the contrary. Think about it: the engine is already spinning, so spinning it at a higher RPM for a few tens of seconds does no more wear than accelerating.
The issue is not brake wear--the issue is brake fade, and potentially boiling your brake fluid!
When you're going down a long hill, downshift and save your brakes! It could save your life!
Clutches get less wear during engine braking because it doesn't slip. When you disengage and reengage the clutch because you're using the regular brakes to slow down, the clutch slips and damages itself.
In the end, the reason for engine braking isn't to reduce the cost of brakes--it's a safety measure that keeps the brakes cool for emergencies & miscalculations, since overheated brakes don't work.
And when you have to change down, while engine braking? (I have an odd learning style. Please don't mistake this for "You're wrong because..." It's a case of "I don't understand why, so...")
A friend once told me that the UK required you to half-engage the clutch while using the brakes and the engine together. I've also read something to the effect of modern brakes are much more efficient so (at least one) UK "advanced driving school" teaches that it doesn't really matter.
I have an older car (1998 Mazda 626 wagon). It's a manual, and when I drive downhill I put it into third and let it coast, braking to reduce speed. Should I be putting it into second, and letting it run up to 3500-4000 revs? Should I leave it in third, and use the brakes? Does it consume more fuel to do this, in a car of that era?
I've often meant to ask my local mechanic about this, but never think about it when I'm there.
The way I shift is as I approach a downhill I keep driving it in gear rather than coast. If its steep downhill, you can brake ahead to lower your speed to something you are comfortable at. Shift down to the gear for that speed and then back off the gas if you need engine braking. You can use brake in combination with engine braking gear if you needs to slow down more than using engine braking alone. I shift down when speed gets lower than in current gear.
The exact same components that are stressed in order to speed your car up.
Since your engine doesn't have combustion raising cylinder pressures, the total power 'absorption?' under engine braking is necessarily less than the full power output of the engine which the drive train was designed to handle.
The exception is if you have rear wheel drive and enough weight transfer to the front to get wheel hop. But, afaik, this is a problem unique to motorcycles.
Also, on a large, heavy vehicle with a manual transmission, you can get in trouble with misjudging braking time/distance if you only rely on the brakes to slow down and stop. When I have a full load in my pickup (1982 C10) I have no choice but to engine brake as the wheel brakes lose more stopping distance the heavier the load, not to mention it keeps them from overheating and failing early over time.
* 4-wheel braking, even if it is a 2wd car.
* ABS/Traction control systems are designed around brakes, not engine braking.
* Regenerative braking (if equipped) uses the brake pedal
* There is a smooth application of braking power from 80mph to 0mph. Engine braking is indexed (I can only choose which gear), and cannot bring the car to a complete stop.
* The limiting factor with brakes is traction, which isn't improved by using engine braking.
* Using the brake pedal is a single action. Engine braking involves at least 3 (clutch in, shift, clutch out).
* Brakes are cheaper to replace than clutch/transmission.
* Engine braking alone does not activate brake lights.
The only exception is when there is a danger that the brakes will overheat: descending a large hill or hauling a heavy load. The default instinct should be the brake pedal alone.
I wasn't speaking of coming to a complete stop, only of slowing down with the flow of traffic. I should have worded it better, I'm sorry.
> Using the brake pedal is a single action. Engine braking involves at least 3 (clutch in, shift, clutch out).
But if you're already downshifting while slowing down, which you should be, it's actually not extra effort. It's completely natural.
> Brakes are cheaper to replace than clutch/transmission.
That entirely depends on the vehicle and the type of clutch. In my extensive experience (I repair cars on the side, and have done so my entire adult life) the parts themselves often cost about the same. For example, when restoring my Bronco II I bought a clutch kit for $150 and a full set of front brake rotors, pads, and new calipers for about $200.
On my Crown Victoria, which was formerly a police vehicle and therefore has the upgraded heavy duty disc brakes on all four wheels, a shop quoted me $700 for a brake job. The parts alone were $350 through them. Of course I saved a ton by doing it myself, but most people can't or won't do that. The labor for a clutch swap versus a brake job is, again, dependent on the complexity of the vehicle. Most manuals still in use in the US are older model cars, with relatively easy to replace clutch parts.
> The only exception is when...hauling a heavy load.
Which I specifically spoke of. :-)
> The default instinct should be the brake pedal alone.
In an automatic, yes. In a manual, only when needing to stop suddenly or when you've downshifted to 2nd or 1st and you're stopping altogether.
Especially with hilly driving, it gives you a lot more control when slowing down, and you can accelerate straight away since your foot is still on the accelerator pedal.
There's nothing worse than being stuck behind a driver (usually a tourist) in a hilly area who is riding the brakes all the way down.
Only if your clutch is so badly broken as to make the car undriveable.
It shouldn't slip if it's fully closed.
This shifts the engine from combustion, where gas explosions cause motion, to motion causing the engine to turn. Because the engine is still in motion, when the RPM drops below 1500, then engine can resume fuel and begin combustion again.
When you disengage the clutch, the engine is disconnected from the wheels, and requires combustion again to keep running.
(Left-foot braking is a technique where you keep the power down in a bend but dab the brake to adjust the fore-aft balance of the car. It is common in rallying - there is some awesome footage somewhere on YouTube of the driver's feet in an 80s rally car. His feet move so fast it looks like he is dancing on the pedals.)
For reference, I got pretty good at this technique while driving autox, which for those unfamiliar is basically a race course around cones in a giant stadium parking lot, one car at a time. But in my 10 years of driving on public roads, I've never felt the need to use it even once.
Edit: Downvotes? For driving safely on shitty roads? I hope people don't assume Im being a jackass driver just because I use my left foot.
One racing driver technique that can be useful in normal conditions (in a manual) is heel-and-toe braking. I use it to avoid excessive engine braking while downshifting, and sometimes for hill starts without using the handbrake.
Also left foot braking is much more useful day to day than heel-toe. Especially if you ever deal with suboptimal traction. Plus, faster reaction times.
Just because YOU don't see why a feature might be useful does not mean that it isn't useful. It just means you don't see a use for it.
Similarly, while cutting the fuel on break application might be useful or even beneficial in the case of a normal / particularly bad driver (slamming down all the peddles in an emergency) it does mean a drastic change in the human/machine interface. I would say that just like ABS is a good safety feature and airbags / restraint belts are good safety features, the vehicle should CLEARLY MARK these features. Their automation may (EG as in the cases of baby seats) be counter-intuitively less safe in circumstances.
The amount is obviously negligible but it's quite crazy to spend more fuel in order to reduce emissions :)
That is what Volkswagen are being forced to do.
Of course, engine braking is obviously more efficient than friction braking, but that's not coasting, that's engine braking.
Another funny note is that "conventional" automatic gearboxes have the highest gear (usually the only overdrive gear) have a coasting clutch on it, so it's not even technically possible to coast with engine engaged. You're always idling if you let go of the speeder, losing energy to the torque converter that doubles as oil pump for the transmission.
When you coast with engaged clutch, you will engine brake
Energy doesn't go to the engine for free.
Adding the fuel to counter the engine braking is less
efficient than letting the engine run idle while coasting
due to losses in the power-train
This is also why it is a bad idea to use engine braking with a 2-stroke petrol engine: 2-strokes rely on oil mixed in with fuel for lubrication. When using the engine as a brake no - or hardly any - fuel is added to the incoming air while maintaining high engine speeds, starving the engine of lubrication.
Engine braking is just that, using the engine as a brake. If no fuel is added, pumping losses will make it act as a (relatively weak) brake. If you want to coast without braking, with the clutch engaged, you will need to inject fuel to counter the pumping losses of the engine, losses which are higher if the engine is not idle. Losses in the power train add to the braking effect, which must further be counted with additional fuel.
If you're going downhill with clutch disengaged and maintaining your speed, engaging the clutch will very likely increase your fuel consumption for the same task (maintaining speed). Automatics have a coasting gear (usually the last or only overdrive gear) for this very reason.
(Sorry, I had to do that.)
Side notes: 1. Coasting is not braking. People seem to be mixing "coasting to a stop" and "coasting" in general. Engine braking, when your intention is to reduce your speed, is more efficient than friction brakes, but coasting does not mean that you intend to reduce your speed. 2. Who drives a car with a 2-stroke petrol engine? :)
This is very interesting - thank you.
I shift cars into neutral while going down hills because I strongly dislike the subtle lurching of the hill descent control ...
I never thought I was saving any appreciable fuel.
However, what does happen is that (provided you don't mind speeding) you exit the bottom of the hill with much more stored energy, allowing you to coast (in or out of gear) much further before it is necessary to apply the throttle again.
So it's interesting that the direct effect of coasting in neutral does not save fuel - however I still think the net effect (again, provided you don't mind speeding when exiting a hill) is fuel-saving ...
If that is the only reason for automatic transmission to be more efficient, then i doubt it is.
It's taught for safety reasons. You have more controls of the car with the motor attached.
I remember when I was learning to drive being taught about engine braking (with an automatic) and the tradeoff between using up the brakes or burning fuel.
Well, I own a 911 that says otherwise. It's an older fuel injection system (Bosch K-Jetronic), but it's most certainly fuel injection. Porsche wasn't the only maker to use Bosch fuel injection.
I just don't think DFCO (deceleration fuel cutoff) was a feature of most of the early electronic fuel injection systems. Perhaps it was standard by the time EFI made it to economy cars. I'm fighting a massive urge to go down this historical rabbit hole and read about it, but there's work to be done today.
EDIT: no, it was neither electric nor hybrid car, but some old petrol car with manual transmission (Saab 900 or similar).
-When the clutch is engaged whilst going down hill the wheels turn the engine. No Fuel Required. -When the clutch is disengaged the engine has to turn itself over - which needs fuel.
*Disclaimer, i'm sure carbourettas (sp?) could well be doing weird ass things so whilst conceptually the above holds they may well be different.
Ditto when coasting to a stoplight - with clutch disengaged (or in neutral), it would coast for a longer distance, which - if you're careful and know your vehicle - means you can let up on the gas a lot sooner.
Not calling you out specifically, but a lot of people arguing for manual transmissions in this thread seem to base their experiences on ~10 year old cars or older, and weirdly assuming that nothing has happened since, that software and technology in cars has been magically frozen in time, despite enormous incentives for car manufacturers to improve it.
To test, watch your rpm when you left off the gas, if it drops to ~750-1000 rpm, your torque converter is slipping (as designed) and not maintaining fuel cut.
To agree with you, I think the comments in this thread started from a manual transmission standpoint and then everybody thought it applied to automatics.
In a manual you are burning fuel when the clutch is disengaged to keep the engine turning over. With the engine engaged you steal from momentum which lowers fuel use.
The mid point of keeping the engine engaged and adding enough fuel to reach your desired stopping point once the RPM enters the same range as an idling engine is actually most efficient.
The oldest car I have that has an ECU is from 1987 (although judging the fuel efficiency on that car when it was new is a bit hard since the engine has been completely reworked since then -- including an aftermarket ECU, new injectors, new ignition etc which makes the car a tad more fuel efficient than my 2012 car)
I don't think you need a computer for this: it's called engine braking. https://en.wikipedia.org/wiki/Engine_braking