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This is commonly called power braking, usually called up in an emergency. It's a good technique that preserves handling. If you have RWD, your front suspension

This is commonly called power braking, usually called up in an emergency. It's a good technique that preserves handling. If you have RWD, your front suspension will remain more unsprung, in any case you can quickly apply the throttle to maneuver. 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.

deleted 5y ago
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olyjohn 5y ago
Your front end still takes the weight under any deceleration event. That's why FWD cars still squat in the rear on acceleration.
- cartoonworld 5y ago
  yes, it will always rock fore and aft to some extent, but consider the difference between applying a drag to the front of the vehicle or the rear. Try this on a bicycle if you have the time. 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.
  - exporectomy 5y ago
    > If you apply only rear brake, it will cause the rear wheels to pull their suspension downwards tipping the car rearward somewhat. 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.
shkkmo 5y ago
> This is commonly called power braking, usually called up in an emergency. 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.
- adonovan 5y ago
  Also known as "jake braking". It's useful if you're a dump-truck driver who thinks midtown Manhattan is too quiet.
  - cartoonworld 5y ago
    Jake Brake is engine braking, its really similar but not the same. Theres a difference between the kind of power braking/engine braking that a car does and a commercial truck. 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_brake https://en.wikipedia.org/wiki/Compression_release_engine_bra...
  - deleted 5y ago
    [deleted]
  - MisterTea 5y ago
    A real "Jake brake" (Invented by Clessie Cummins) slows a truck down by turning the engine into an air compressor. It does this by modifying the exhaust valve timing to open at top dead center while cutting fuel. Once turned on all you do to engage it is to take your foot off the accelerator. That staccato popping sound is the compressed air blasting out of the exhaust valves. 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.
xattt 5y ago
I had a VW Golf with a DSG (DQ250) which I regularly engine braked on hills. However, I noticed that the instantaneous fuel economy meter would show some consumption on downshifting versus no consumption on coasting in drive. 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.
- cartoonworld 5y ago
  Ouch, lots of the VW fans I've known were die hard due in part to the ergonomics and feel of the manual transmission. Something about a cable I think. Too bad VW mechatronics doesn't read the throttle position for a clue here. It all just seems so obvious.
  - rand49an 5y ago
    VW transmissions haven't used a cable in many years, 30 plus.
  - xattt 5y ago
    I think the rev matching to minimize the strain on the clutch pack. The car also had standard hill hold assist to minimize the risk of wearing out the clutch over a long period of time while living in a hilly area. 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.
    - cartoonworld 5y ago
      Dang, I might take one for a spin if I get the chance. 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.
hunter2_ 5y ago
> It's a good technique that preserves handling. 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.
deleted 5y ago
[deleted]
garaetjjte 5y ago
>it is the compression of the cylinders in the engine draining momentum from the wheels 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.

deleted 5y ago

[deleted]

olyjohn 5y ago

Your front end still takes the weight under any deceleration event. That's why FWD cars still squat in the rear on acceleration.

cartoonworld 5y ago
yes, it will always rock fore and aft to some extent, but consider the difference between applying a drag to the front of the vehicle or the rear. Try this on a bicycle if you have the time. 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.
- exporectomy 5y ago
  > If you apply only rear brake, it will cause the rear wheels to pull their suspension downwards tipping the car rearward somewhat. 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.

shkkmo 5y ago

> This is commonly called power braking, usually called up in an emergency. 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.

adonovan 5y ago
Also known as "jake braking". It's useful if you're a dump-truck driver who thinks midtown Manhattan is too quiet.
- cartoonworld 5y ago
  Jake Brake is engine braking, its really similar but not the same. Theres a difference between the kind of power braking/engine braking that a car does and a commercial truck. 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_brake https://en.wikipedia.org/wiki/Compression_release_engine_bra...
- deleted 5y ago
  [deleted]
- MisterTea 5y ago
  A real "Jake brake" (Invented by Clessie Cummins) slows a truck down by turning the engine into an air compressor. It does this by modifying the exhaust valve timing to open at top dead center while cutting fuel. Once turned on all you do to engage it is to take your foot off the accelerator. That staccato popping sound is the compressed air blasting out of the exhaust valves. 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.

xattt 5y ago

I had a VW Golf with a DSG (DQ250) which I regularly engine braked on hills. However, I noticed that the instantaneous fuel economy meter would show some consumption on downshifting versus no consumption on coasting in drive. 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.

cartoonworld 5y ago
Ouch, lots of the VW fans I've known were die hard due in part to the ergonomics and feel of the manual transmission. Something about a cable I think. Too bad VW mechatronics doesn't read the throttle position for a clue here. It all just seems so obvious.
- rand49an 5y ago
  VW transmissions haven't used a cable in many years, 30 plus.
- xattt 5y ago
  I think the rev matching to minimize the strain on the clutch pack. The car also had standard hill hold assist to minimize the risk of wearing out the clutch over a long period of time while living in a hilly area. 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.
  - cartoonworld 5y ago
    Dang, I might take one for a spin if I get the chance. 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.

hunter2_ 5y ago

> It's a good technique that preserves handling. 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.

deleted 5y ago

[deleted]

garaetjjte 5y ago

>it is the compression of the cylinders in the engine draining momentum from the wheels 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.