5 ms·

> especially if you are shifting a certain way because you can see traffic or the lack of it, up ahead. Downshifting instead of using brakes is more environmen

by sebcat 10y ago

> especially if you are shifting a certain way because you can see traffic or the lack of it, up ahead. Downshifting instead of using brakes is more environmentally friendly as well. Though you can do this with an automatic too (or semi-automatic I guess). My mother has a car with paddle shifters which you can use if you want, or you can just keep it in automatic. Best of both worlds. Edit: I drive stick, though. It's what I know.

sliken 10y ago
Downshifting more environmentally friendly? Because burning more gas is better than heating up break pads? Reference?
- toomuchtodo 10y ago
  Uses engine compression to bleed off kinetic energy (no additional fuel use). Not more environmentally friendly, but saves on brake wear. Semis utilize this mode of braking heavily: https://en.wikipedia.org/wiki/Compression_release_engine_brake https://en.wikipedia.org/wiki/Compression_release_engine_bra...
  - geon 10y ago
    Actually, it uses a tiny bit less fuel, since it doesn't have to idle.
- sebcat 10y ago
  Was taught to do it in drivers ed, which features environmental driving here. The point is not to aggressively downshift, but to let go of the gas and then downshift when the RPMs are lower. You still want to keep the RPMs relatively low. This is preferred to a situation where you would keep your speed and use the brakes at a later point in time. I should probably not have called it downshifting.
- sniglom 10y ago
  Basic understanding of an injected engine and power conservation should be enough here. When you're downshifting you're braking with your engine. This means two things: 1. Less fuel is burned. The injectors are closed while the engine is braking. 2. Momentum, not fuel, is used to drive the alternator, AC etc. If you instead were to brake the car, it would need to use fuel just to keep the engine running. In the same way as it would if you were idling parked. Even more fuel is needed to combat the resistance from your AC and alternator.
- wastedhours 10y ago
  Engine braking in most petrol cars is an efficient tactic due to DFCO[0] - less efficient than coasting in neutral, but you're not as in control of the car when not in gear, so the combination of both being in control and getting a "free" slowing effect (especially when judging upcoming traffic, as opposed to coming to a complete halt) can be better than just mashing the middle pedal. [0] Deceleration fuel cut off
  - mantas 10y ago
    If you're slowing down to traffic light, it's more efficient than coasting in neutral. You've to slow down anyway and slowing down with engine braking is 0.0l/km. While coasting still burns some fuel. In addition to that, some newer cars charge battery when engine braking. Which is effectively saving fuel later.
- Gibheer 10y ago
  You can downshift and use the motor for breaking. When doing that, you don't burn any fuel at all, as the kinetic energy of the car will help keep the motor turning. You have the same effect in automatic transmissions when letting go of the gas pedal.
- TheNewAndy 10y ago
  No reference, but engine braking means you don't need to send petrol to the engine (since the engine is being driven by the car's momentum), so there should be no fuel usage. When the car is stopped, then it will use more petrol (to keep the engine from stalling). Assuming all things are equal, then when you brake with your foot brake, the energy that gets turned into heat in the brake pads could have instead been turned into energy that keeps the engine spinning (and so it doesn't need to come from petrol).
- jfitbsidbfb 10y ago
  Downshifting to slow down uses zero fuel. Modern cars cut fuel completely when coasting is detected based on throttle position and whether gear is engaged among other sensors. This is only used a lot in manuals since the torque converter causes freewheeling at lower rpms. It's known as deceleration fuel cut and it's standard in OEM and aftermarket engine controllers. Typically the engine "restarts" when rpm drops to a bit above idle speed but as mentioned it doesn't happen as much in automatics because of the freewheeling. In automatics it only occurs when deceleration starts at high rpm or the clutch within the torque converter is locked during steady highway driving. I assume shifting into "low" may enable deceleration fuel cut in more situations assuming the driver may be using it to slow down, but I have no evidence of that usage
- deleted 10y ago
  [deleted]
- 24gttghh 10y ago
  I had assumed engine-braking will heat up the engine more and use more oil and increase general wear on the engine and reduce it's service life. New brake pads seem cheaper and more environmentally sound than having the engine serviced/replaced and burning more oil...
manmal 10y ago
Downshifting doesn't really slow you down if you have one of those new 3 cylinder gas engines, for their lack of a throttle valve.
ollie87 10y ago
It's funny, I just got a new car (manual, of course) and it has "rev-matching" so when you shift down a gear it automatically heel-toes (basically a throttle blip) which means there's not much engine braking going on during downshifts. Granted when driving quickly and dropping a cog or two to overtake a slow moving vehicle it's great. Slowing down for traffic? Not so good. Took some getting used to changing down gear and not feeling the car slow. EDIT: To clarify the folks downvoting me here's and example of how it works: 1) You're in 6th gear doing 60mph. 2) You come up to a tractor doing 25mph. 3) You brake and slow, as you do you compound change from 6th gear down to 3rd. 4) As you place the car into 3rd the ECU gives the engine some extra throttle and match the speed of the gearbox. You can physically see the revs rise BEFORE you release the clutch. 5) You release the clutch and don't feel any transmission shunt or immediate engine braking. This reduces wear and makes it smoother. For more detail watch this YouTube video: https://www.youtube.com/watch?v=FnXLZg_O1rk https://www.youtube.com/watch?v=FnXLZg_O1rk
- exDM69 10y ago
  Are you sure it actually blips the throttle and it's not just the synchromesh gear in the gearbox? There's no need to blip with a synchronized gearbox, which has been the standard in road vehicles since the 1930s or so. Even if it does blip, you can still utilize engine breaking but you have to shift early and sometimes skip gears.
  - ollie87 10y ago
    Not sure why I got downvoted here. It actually blips the throttle. It's not the syncho, trust me. http://www.minif56.com/forum/353-mini-cooper-f56-forum/7010-rev-match-function.html http://www.minif56.com/forum/353-mini-cooper-f56-forum/7010-... It's not the first car to have it either, and there is actually PLENTY of reasons why you'd need to blip or heel-toe when driving even in a car with a synco'd gearbox. https://www.youtube.com/watch?v=FnXLZg_O1rk https://www.youtube.com/watch?v=FnXLZg_O1rk https://www.youtube.com/watch?v=p4n9tMdISBg https://www.youtube.com/watch?v=p4n9tMdISBg
    - 10y ago

by sebcat 10y ago

sliken 10y ago

Downshifting more environmentally friendly? Because burning more gas is better than heating up break pads? Reference?

toomuchtodo 10y ago
Uses engine compression to bleed off kinetic energy (no additional fuel use). Not more environmentally friendly, but saves on brake wear. Semis utilize this mode of braking heavily: https://en.wikipedia.org/wiki/Compression_release_engine_brake https://en.wikipedia.org/wiki/Compression_release_engine_bra...
- geon 10y ago
  Actually, it uses a tiny bit less fuel, since it doesn't have to idle.
sebcat 10y ago
Was taught to do it in drivers ed, which features environmental driving here. The point is not to aggressively downshift, but to let go of the gas and then downshift when the RPMs are lower. You still want to keep the RPMs relatively low. This is preferred to a situation where you would keep your speed and use the brakes at a later point in time. I should probably not have called it downshifting.
sniglom 10y ago
Basic understanding of an injected engine and power conservation should be enough here. When you're downshifting you're braking with your engine. This means two things: 1. Less fuel is burned. The injectors are closed while the engine is braking. 2. Momentum, not fuel, is used to drive the alternator, AC etc. If you instead were to brake the car, it would need to use fuel just to keep the engine running. In the same way as it would if you were idling parked. Even more fuel is needed to combat the resistance from your AC and alternator.
wastedhours 10y ago
Engine braking in most petrol cars is an efficient tactic due to DFCO[0] - less efficient than coasting in neutral, but you're not as in control of the car when not in gear, so the combination of both being in control and getting a "free" slowing effect (especially when judging upcoming traffic, as opposed to coming to a complete halt) can be better than just mashing the middle pedal. [0] Deceleration fuel cut off
- mantas 10y ago
  If you're slowing down to traffic light, it's more efficient than coasting in neutral. You've to slow down anyway and slowing down with engine braking is 0.0l/km. While coasting still burns some fuel. In addition to that, some newer cars charge battery when engine braking. Which is effectively saving fuel later.
Gibheer 10y ago
You can downshift and use the motor for breaking. When doing that, you don't burn any fuel at all, as the kinetic energy of the car will help keep the motor turning. You have the same effect in automatic transmissions when letting go of the gas pedal.
TheNewAndy 10y ago
No reference, but engine braking means you don't need to send petrol to the engine (since the engine is being driven by the car's momentum), so there should be no fuel usage. When the car is stopped, then it will use more petrol (to keep the engine from stalling). Assuming all things are equal, then when you brake with your foot brake, the energy that gets turned into heat in the brake pads could have instead been turned into energy that keeps the engine spinning (and so it doesn't need to come from petrol).
jfitbsidbfb 10y ago
Downshifting to slow down uses zero fuel. Modern cars cut fuel completely when coasting is detected based on throttle position and whether gear is engaged among other sensors. This is only used a lot in manuals since the torque converter causes freewheeling at lower rpms. It's known as deceleration fuel cut and it's standard in OEM and aftermarket engine controllers. Typically the engine "restarts" when rpm drops to a bit above idle speed but as mentioned it doesn't happen as much in automatics because of the freewheeling. In automatics it only occurs when deceleration starts at high rpm or the clutch within the torque converter is locked during steady highway driving. I assume shifting into "low" may enable deceleration fuel cut in more situations assuming the driver may be using it to slow down, but I have no evidence of that usage
deleted 10y ago
[deleted]
24gttghh 10y ago
I had assumed engine-braking will heat up the engine more and use more oil and increase general wear on the engine and reduce it's service life. New brake pads seem cheaper and more environmentally sound than having the engine serviced/replaced and burning more oil...

manmal 10y ago

Downshifting doesn't really slow you down if you have one of those new 3 cylinder gas engines, for their lack of a throttle valve.

ollie87 10y ago

It's funny, I just got a new car (manual, of course) and it has "rev-matching" so when you shift down a gear it automatically heel-toes (basically a throttle blip) which means there's not much engine braking going on during downshifts. Granted when driving quickly and dropping a cog or two to overtake a slow moving vehicle it's great. Slowing down for traffic? Not so good. Took some getting used to changing down gear and not feeling the car slow. EDIT: To clarify the folks downvoting me here's and example of how it works: 1) You're in 6th gear doing 60mph. 2) You come up to a tractor doing 25mph. 3) You brake and slow, as you do you compound change from 6th gear down to 3rd. 4) As you place the car into 3rd the ECU gives the engine some extra throttle and match the speed of the gearbox. You can physically see the revs rise BEFORE you release the clutch. 5) You release the clutch and don't feel any transmission shunt or immediate engine braking. This reduces wear and makes it smoother. For more detail watch this YouTube video: https://www.youtube.com/watch?v=FnXLZg_O1rk https://www.youtube.com/watch?v=FnXLZg_O1rk

exDM69 10y ago
Are you sure it actually blips the throttle and it's not just the synchromesh gear in the gearbox? There's no need to blip with a synchronized gearbox, which has been the standard in road vehicles since the 1930s or so. Even if it does blip, you can still utilize engine breaking but you have to shift early and sometimes skip gears.
- ollie87 10y ago
  Not sure why I got downvoted here. It actually blips the throttle. It's not the syncho, trust me. http://www.minif56.com/forum/353-mini-cooper-f56-forum/7010-rev-match-function.html http://www.minif56.com/forum/353-mini-cooper-f56-forum/7010-... It's not the first car to have it either, and there is actually PLENTY of reasons why you'd need to blip or heel-toe when driving even in a car with a synco'd gearbox. https://www.youtube.com/watch?v=FnXLZg_O1rk https://www.youtube.com/watch?v=FnXLZg_O1rk https://www.youtube.com/watch?v=p4n9tMdISBg https://www.youtube.com/watch?v=p4n9tMdISBg
  - 10y ago