The friction isn't what's working here, it is the compression of the cylinders in the engine draining momentum from the wheels, think of it as negative torque.
As the forward momentum of the vehicle causes the rotation of the drivetrain to begin to overcomes the speed of the engine output shaft at the clutch or torque converter, torque is effectively being reversed. Instead of engine output pushing the rotation of the transmission, the forces invert causing the transmission to drive the engine speed upwards. Most ECMs will recognize the zero throttle position and cut off fuel, but older vehicles may not have this capability and especially carburetor fed motors are mostly incapable of reducing fuel delivery in this manner.
If you need to stop quickly and have the time, you can downshift even in an automatic, just throw it into 3 or maybe 2 while braking. If you have an electronic dual-clutch transmission with a "manual" mode, the electronic management controls will prevent the engine from over revving and causing damage.
Its important to keep the RPMs low if you do this a lot, if you redline, you redline no matter what your intentions are. I'd wager that running your ICE at 80-90% redline at low speeds for extended periods is also going to affect longevity.
There are other effects you might notice, such as additional bump and over/under steering as your suspension geometry responds to the unusual load. Also, other components will become loaded in usual ways during this maneuver causing some possibly unexpected handling characteristics to present themselves. Its good to practice this in a safe location, before employing it in the field.
If I may venture to say, it gets even more interesting; some vehicles are constructed in a manner where this happens all the time, the old school Porsches were notorious for an effect known as "Throttle Steer", a description of which exceeds the scope of this post, but a curious person will find several references to this in automotive literature.
It turns out that the logic will try to rev match to be in a lower gear before cutting off the fuel again.
Rev matching wasn't perfect either, as going down more than one gear would cause the engine to lazily rev up until it got close enough in speed before the clutches fully engaged.
The DSG offered a Sport mode (obviously to be used when late for work) that would downshift on deceleration, offering a mild form of automated engine braking. I could never quite tell whether would wearout the clutch packs or the brakes faster as it seemed to be a real push-pull scenario between the two.
Too bad VW mechatronics doesn't read the throttle position for a clue here. It all just seems so obvious.
The design of the VW Golf Mark VII was absolutely brilliant. The EU model got a few why-didn’t-I-think-of-that extras that the NA market didn’t get, particularly around towing. There was a hidden tow hitch that popped out when a button on in the trunk was pressed, as well as reversing assist that used the electric mirror control knob (!) as steering control.
I talked to a mkVII GTI owner today at work, I asked him about his DSG. He said he liked it a lot, but mentioned that he found the shifting too fast and got a DSG tune and either paid or on his own was able to modify the shifting, but I didn't have time to ask him what he meant or for more details about the tune.
He's a chemistry PhD, and is into German stuff. Hes also got a w124 e320 that he's un-rusting (!) so I'd say he has pretty good taste.
DSG tunes are also a thing but then it becomes a rabbit hole and a money pit…
When traction is plentiful, yes. When approaching the limits of traction (in snow, or while turning, etc.), regular braking (or for curves, you could say trail braking) is usually best because the jerk of engine braking right when the clutch is engaged can upset the balance just enough to break loose.
That said, it does have an anti-lock effect in snow which is handy for cars that lack ABS, so as long as the jerk is accounted for, it can be a net win.
I've never heard it called power braking, always engine braking or j-braking.
It isn't just useful in emergencies but also in steep descents with heavy loads to make sure your brakes stay cool enough to be useful. It may be obvious, but the other technique that really helps keep your brakes cool is descending more slowly.
On old rigs like my '61 Mack the jake brake is switched manually. You have to be sure to turn it off once you get close to engine idle rpms otherwise the engine will stall.
A quieter version called a bleeder brake which does the same thing though it leaves the exhaust valve slightly open during the entire compression stroke eliminating the popping sound. There is also the exhaust brake which puts a butterfly valve on the exhaust line creating back pressure when closed. Though the exhaust brake is known to cause problems with some valve train setups.
I typed a lot about this elsewhere, but essentially trucks have crappy brakes because of reasons (thermal ones, in large part) and as a result, in order to gather enough brake performance to not be extremely hazardous, they often must apply their engine brakes/jake brake/engine compression release brake in order to safely navigate with their loads in normal traffic or an emergency braking scenario (such as being cut off in midtown by some a-hole).
https://en.wikipedia.org/wiki/Compression_release_engine_bra...
And you do have to be careful not to grab too low a gear and/or don't let the clutch out too fast, or you can mechanically overrev the engine.
IMO compression braking (jake brake) would be a better analog.
A.k.a. the "money shift" as it results in spending lots of money, typically from grabbing 2nd instead of 4th (each being relatively down and to the left).
The "squat" you see on accelerating FWD vehicles is something different called wheel climb I think and is related to the effects of braking and center of gravity that we are discussing. When you gun the throttle, the wheel torque applies an opposite force as the wheels are turning in relation to the car. This causes a lever-like interaction forcing the front up and the rear down. During the engine brake maneuver the situation is reversed, as the torque is reversed in this scenario.
Imagine you had a car with independently activated front and rear brakes: Its trivial to understand that in a front-brake it will cause the front of the vehicle to tip forwards, and the rear will respond by tipping upwards. This unloads the rear suspension causing excessive camber, reducing tire contact, and if you were applying the brakes here, braking performance.
If you apply only rear brake, it will cause the rear wheels to pull their suspension downwards tipping the car rearward somewhat. As the stopping vehicle weight then pulls on the rear axle load, it applies a weak lever-like corresponding downward force on the front axle, reducing your center of gravity over the front-brake scenario. This will improve your handling performance, like I said.
Additionally, highway vehicles are designed with proportional braking that applies more of the braking force to the front rather than the rear. This, among other reasons, is why you can fishtail when you slam on the brakes. If you want to try it out, pull your ABS fuse the next time it rains, snows or there is some other slippery conditions and tool around in an empty parking lot.
This is why power braking is more suitable for RWD vehicles. It alleviates some of the effects of proportional braking and maintains a more favorable center of balance and suspension geometry.
It isn't that it doesn't work on FWD vehicles (It works well, and I did not mean to imply that it did not) only that RWD vehicles preserve more of their handling with this maneuver because of the facts regarding how a vehicle's suspension and center of gravity respond to braking and suspension load.
Are you sure? I'm trying to draw a free body diagram of that, and I get a net moment tipping the car forward. Can you explain it using physics principles?
> Try this on a bicycle if you have the time.
You can't easily compare because the rear wheel easily skids with a fraction of the braking force the front wheel can take.
Yes.
It's pretty confusing, I did the same thing and came up with the same answer as you. Then I typed a sentence about "even on fully independent multi-link rear suspensions" before going off into the weeds. If the suspension were rigid, it seems like it should do the opposite, climbing upwards, but there is more going on in the car than the assumptions we typically make with a model.
A rear suspension is of course not rigid. It is, in short, a vertical damped spring mounted on a lever attached to the frame in front of it's axis. The axle/wheel hub is attached to the bottom of this damped spring at the end of the lever.
During braking, the rear will deflect rearwards and also be drawn towards the centerline of the vehicle following the arc of a circle drawn on the ground. The suspension will control the forces through the trailing arm, directing them towards the rear along a second arc drawn on the side of the vehicle, centered on the rotational vertex where the trailing arm is bolted to the car in front of its axle. The sway bar will control both sides to the same level which will be the apogee of the circle, closest to the rear bumper.
You can test this by pulling your parking brake below 5mph. If this doesn't work, call your mechanic :)
>You can't easily compare because the rear wheel easily skids with a fraction of the braking force the front wheel can take.
That is because braking the front bike wheel causes the lever to apply an upward force on the rear axle, pulling the front wheel rearwards, causing absurd instability and poor handling. This also happens to the rear wheel, it wants to hop upwards in response to the brake forces. That's why the proportional brakes exist in highway vehicles, to stabilize the braking forces into the most efficient braking scenario. If the front wheel were mounted in a reverse mounted trailing arm suspension (This doesn't exist, so I don't know what else to call it.) it would drop the front of the bike as happens to a car instead of flipping you off the bike. The side effects of this are atrocious understeer and a nonsensical center of gravity.
The reason the wheels move like this is to control for a characteristic called caster. Bikes have negative caster, if you let go of the handlebars at low speed, they will flop to one side or another at random, causing you to crash. Cars have positive caster which is the most stable configuration for the steering to operate. This is also why your steering wheel returns to center when you let go of the wheel while moving forward, and it is why the braking forces can be stabilized in this manner.
Some vehicles don't have swaybars, or instead they might have a "solid rear" rear subframe to which both knuckles are attached, and moves as one large unit. They always have shock absorbers though. I'll just keep considering only the rear axle here.
I can tell you are away of, but do note, that even without a trailing arm configuration but something else, the wheels will still always want to go back and towards the centerline of the vehicle during braking. The suspension geometry will determine how this is controlled, usually the move is to go down. I've never seen one that goes up unless something is broken, usually the shocks. People normally call this excessive movement "nose dive". It happens because a functional shock absorber momentarily control the climb, allowing the suspension to control the wheel characteristics. Without it, the forces will yank the wheel out of configuration.
In most vehicles, without the sway bar controlling both sides of the suspension, or the shock absorber damping, you will experience many varieties of reduced braking and handling performance unless both wheels on the axle are reacting in exactly the same way, which is very unlikely.
If the spring is undamped, aside from the "nose dive", it will start to oscillate in an uncontrolled manner during braking, causing hopping. This will cause the wheels to both not brake well and also lock up. This will still affect the solid rear, causing wonky camber.
If one wheel locks up, the ABS will kick in. This is normally fine, and increases performance in all categories, but only if the wheels are in contact with the ground and exhibiting correct camber. If they aren't and are instead hopping because of the undamped spring, the ABS won't be able to control much, it will in fact suck.
If the sway bar is absent/broken/ the linkage or bushings damaged, and one wheel has different handling or braking performance than the other wheel on the axle, (such as in all the time or during hopping or ABS engagement) the ride height will change as the wheel reacts along the second circle, causing body roll which will alter camber and other factors on every other wheel, especially the opposing wheel, which will experience much worse performance in every category.
All this stuff will show up on the tire. You can often tell whats wrong just by glancing at the tire tread.
Does it really? After all, while it will take energy when cylinder goes up and air is compressed, immediately after that the compressed air will be instead pushing cylinder down, returning that energy to crankshaft. There will be loss from compressed air heat migrating into cylinder walls, but it doesn't seem to be large enough to be responsible for majority of engine braking. It would also mean that diesel engines should engine brake much stronger due to higher compression ratio, but this isn't the case.
It seems to me that engine braking is just various friction losses from all parts of the engine, without single main source.
EVs on the other hand get a partial refund for the energy they spent going uphill in the first place.
Don't over do it.
However, unlike regenerative braking, you won't end up with more fuel in the tank afterwards.
Truckers will sometimes install a valve to bypass the muffler, so they can let someone know that they just cut them off, and that they should try moving out of the way.
The bypass valve on semi's you're referring to is not the exhaust. The huge BRAAAAAAAP that you hear coming from trucks when someone cuts them off aren't an exhaust bypass valve, its them slamming on their brakes to avoid a tragic 20-car pileup.
It is a release of the compressed air-fuel mixture momentarily before combustion called a Jake Brake or Compression Release Engine Brake[0], that Semis and large commercial vehicles are equipped with that provide for additional braking in an emergency.
What they're doing is releasing the compressed air-fuel into their exhaust, just prior to combustion. This causes the engine speed to drop instantly, slowing the truck down. It will surprise you to learn that semi trucks have pretty terrible braking performance, loaded or unloaded. They all use drum brakes.
It is strictly prohibited to bypass a muffler or emissions control device in the US. While cars get away with it all the time, a commercial truck carries additional registration, inspection, and license restrictions which makes this not only dangerous, annoying, needless and harmful, but extremely expensive.
Commercial Driver's face much steeper fines for equipment that is out of specification. Many commercial drivers are required to inspect their vehicles before they depart for even so much as a top indicator light which is burnt out, or they may face harsh penalties and fines that would make your skin crawl. $10,000 citations are levied daily.
[0] - https://en.wikipedia.org/wiki/Compression_release_engine_bra...
Another fact that might seem counterintuitive at first is that a heavily loaded truck can stop faster than an empty one. This is because it has more weight on the wheels, which means more force before they start to skid, and the brakes themselves are not the limiting factor.
Coming from manual transmission, I was super excited when it happened the first time.
No. Climbing the hill the car uses extra energy (compared to a flat road) because its fighting gravity. On the way back down the hill regen will recover some of that gravity-fighting energy but nowhere near all of it.
https://phys.org/news/2017-09-e-dumper-world-largest-electri...
https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...
It’s actually very interesting to me that while driving that car, I developed a sort of feel for how trading potential and kinetic energy affected my available range, and I had an imaginary boundary drawn in my head that defined all the places where I’d be able to make it home “for free” that was kind of like a topo map. In particular, if I managed to crest the hill where Canlis sits on highway 99, I knew that despite all the ups and downs in between me and home, I’d be able to make it there without having to fire up the gas engine. :D
I would imagine that at best, you could maybe match the range. If there's any scenario where going up and down a hill would yield more range than driving flat, I'd be very interested in how/why.
Cruising on level terrain is a mediocre load on the traction system to offset the rolling and aerodynamic friction. Climbing a steep grade at the same speed will increase the load significantly, but that extra kinetic output energy is being stored as gravitational potential energy rather than lost like the baseline cruising load. Then, when you descend the mountain on the other side, you recover that gravitational energy to offset the rolling and aerodynamic friction. You want to be "falling" down the mountain grade at your terminal velocity where no braking and no traction force is required to maintain your cruising speed. It will not work well for winding descents where you need to brake for turns.
I've seen this work with turbocharged ICE cars with elevation gains of 4k foot or more and hundreds of miles distance. I've repeated on many trips to prove to me that it isn't simply a fluke (such as extreme headwind or tailwind). Going over a pass yields me a better trip MPG than covering the same several hundred miles on relatively flat ground.
I could see this benefit for a hybrid car too, since I believe they mostly handle highway cruise on ICE power. However, it seems unlikely for a BEV car unless there is something about battery and power electronics that I do not understand, that would give them a significant efficiency boost at high power.
Suppose you're going to make a 100 mile (160 km) trip. You have two possible routes. One is a straight shot on level ground. The other is straight, too, but it takes you up a 10% grade for the first 10 miles (gaining 1 mile in altitude), then you continue on level ground for 80 miles, and then you descend a matching 10% grade at the end.
It seems like the 80 miles of cruising on level ground at high altitude would use less battery than 80 miles of cruising at sea level (if the speeds are the same).
There will be some losses due to climbing and descending. Maybe climbing and descending is a little less efficient. Also, you're definitely traveling a very slightly longer distance. But those losses might be made up for spending the bulk of your trip in thinner air.
I hope the prosecutor has enough leverage with the court! Justice truly is blind!
New EV users often forget that unlike 'gallons of gasoline' or 'kWh', 'estimated miles of travel' are not a true unit of energy.
Why not design them to be aerodynamic for headwinds, and maximize wind-resistance for tailwinds? I wonder how much energy would be gained.