4 ms·

It's not "nearly". It's exactly the same effect. Compression braking. Even if you're not on the throttle the engine is still rotating as it's connected to the w

It's not "nearly". It's exactly the same effect. Compression braking. Even if you're not on the throttle the engine is still rotating as it's connected to the wheels. If the engine is rotating it is compressing air. If it's compressing air but not igniting fuel (throttle closed) then it will suck energy out of the wheels to compress the air. You can do it in an automatic, you just have to force it to select a lower gear using the gear number options (1, 2, 3, 4) or using the tiptronic mode. The lower gear means the engine will displace more air in the same amount of time, increasing the rate it pulls energy from the wheels. People think you can't do it in automatics because they try very hard to keep engine RPM low where the effect is diminished.

peterbecich 1y ago
Related to this: https://en.wikipedia.org/wiki/Compression_release_engine_brake?wprov=sfla1 https://en.wikipedia.org/wiki/Compression_release_engine_bra...
agurk 1y ago
If it takes energy to compress the air in the cylinder - doesn't it also release most of that energy when that compressed air is expanded on the subsequent stroke? If you release the compressed air without pushing the cylinder down you would lose that energy, but you would need a extra device to do so (by lifting a valve at the right time). This option does exist for large vehicles like trucks as a compression release engine brake [0], but this isn't something you'd have on a family car. In a petrol engine you always want the same ratio of petrol to air in the mix that is taken into a cylinder. As you want to vary the amount of fuel, and therefore power developed, you have to be able to therefore limit the amount of air that is sucked in. Otherwise the engine would always run at full power. There is a mechanical restrictor called a throttle plate that lives inside the throttle body that restricts how much air the cylinder can pull in (and therefore how much fuel is injected to get the same fuel/air mix). This is controlled by the throttle. When you are coasting, this plate is in its most closed position. This creates significant resistance on the intake stroke, and is where the majority of energy is lost during engine braking. This is also known as a pumping loss. Diesels always intake the same amount of air, so they can compress it enough to autoignite the fuel. They vary the amount of fuel injected to the same volume of air. This means no throttle body or plate, so unless an extra exhaust restrictor has been added there is minimal engine braking on a diesel engine. [0] https://en.wikipedia.org/wiki/Compression_release_engine_brake https://en.wikipedia.org/wiki/Compression_release_engine_bra...
- MindSpunk 1y ago
  You're right. Petrol engines are air pumps so I should've realised the explanation wasn't correct. Though the ECU would be doing the AFR management on modern EFI engines as the injectors aren't vacuum operated like Carburetors were. You should be able to cut fuel injection when coasting in a modern engine, can't run lean if there's no fuel at all. Not sure if carbs could do the same.
  - agurk 1y ago
    On all the fuel injected engines I have owned there is a physical cable that controls the position of the throttle plate. There is an airflow mass sensor the other side of the plate to measure the amount of air and therefore how much fuel needs to be injected. So interestingly in these sort of engines you're really just controlling airflow to the engine rather than fuel/air mix like on one with a carburettor. More modern engines have electronically controlled throttle plates, and this is definitely somewhere you could do something clever like you suggest - cutting fuel flow but also maximising airflow when there is zero throttle input. I assume engine braking is generally considered a beneficial thing by manufacturers, but it could be fun to be able to customise the amount. Or do something like have the braking come on gently at first then harder. Maybe even try and have a linear or flat response curve vs. engine rpm.
    - yetihehe 1y ago
      > Or do something like have the braking come on gently at first then harder. You can do this by letting go of gas pedal slowly. I have "current amount of fuel used" info in my car (liters/100km), it shows pretty clearly, that when going fast and slowly letting go of gas, amount of fuels slowly goes to 0. If I let go of gas fast, the engine is intelligent enough to not close throttle as fast as possible, still probably takes 1 second. > More modern engines have electronically controlled throttle plates, and this is definitely somewhere you could do something clever like you suggest - cutting fuel flow but also maximising airflow when there is zero throttle input. They cut fuel flow and close throttle plate almost completely but still allow some small amount of air, in order to actually do engine braking. If you need to coast, you can apply clutch in manual. Don't know that much about automatic, but from what I've driven, they use "lift gas" as a "engine braking" signal, so probably they can't really coast that good.
jve 1y ago
I'v bee downshifting manually with automatic especially when riding downhill. The effect with Chrysler T&C 2009 was very noticable and effective. Toyota Sienna 2015 - the braking effect is unfortunately minor.
bigfatkitten 1y ago
Many modern automatics are smart enough to do this for you if you tap the brakes while rolling downhill.
yakkers 1y ago
An aside on your last point: In the D gear, a lot of auto transmissions will be set up to allow for the wheels to spin somewhat independently of the engine (in any gear) when you aren't accelerating, which accounts a lot for how well an automatic will coast when you release the throttle, even if you're not going fast enough for top gear (especially apparent with older four speed boxes). Using the numbered gear options will enable clutches/bands that provide more engine braking.
- MindSpunk 1y ago
  Huh, interesting. I had thought it was just from the higher gears being used. Vast majority of my driving is with manual, I've only really driven modern autos with 5+ gears (and a Toyota hybrid, which don't really have gears at all).
vladvasiliu 1y ago
It depends on the implementation. I ride an automatic motorbike which will downshift if I grab the brakes a bit harder, even if I'm only slowing to a speed to which it wouldn't otherwise shift. For example, if I brake somewhat hard from 130 km/h to 90, it will downshift from 6th to 5th. When riding normally, it would stay in 6th down to around 50.
frollogaston 1y ago
Auto trans downshift is designed for long hills, not downshifting frequently to stop signs etc like you'd do with a stick. Even with paddles, there's a delay, or it briefly goes neutral, or it doesn't rev-match well. Or you can't double/triple-downshift, which is worse when you have 8-12 gears. Allegedly wears them down faster too, which idk but would not be surprised if it were true given how unhappy it feels.

peterbecich 1y ago

Related to this: https://en.wikipedia.org/wiki/Compression_release_engine_brake?wprov=sfla1 https://en.wikipedia.org/wiki/Compression_release_engine_bra...

agurk 1y ago

If it takes energy to compress the air in the cylinder - doesn't it also release most of that energy when that compressed air is expanded on the subsequent stroke? If you release the compressed air without pushing the cylinder down you would lose that energy, but you would need a extra device to do so (by lifting a valve at the right time). This option does exist for large vehicles like trucks as a compression release engine brake [0], but this isn't something you'd have on a family car. In a petrol engine you always want the same ratio of petrol to air in the mix that is taken into a cylinder. As you want to vary the amount of fuel, and therefore power developed, you have to be able to therefore limit the amount of air that is sucked in. Otherwise the engine would always run at full power. There is a mechanical restrictor called a throttle plate that lives inside the throttle body that restricts how much air the cylinder can pull in (and therefore how much fuel is injected to get the same fuel/air mix). This is controlled by the throttle. When you are coasting, this plate is in its most closed position. This creates significant resistance on the intake stroke, and is where the majority of energy is lost during engine braking. This is also known as a pumping loss. Diesels always intake the same amount of air, so they can compress it enough to autoignite the fuel. They vary the amount of fuel injected to the same volume of air. This means no throttle body or plate, so unless an extra exhaust restrictor has been added there is minimal engine braking on a diesel engine. [0] https://en.wikipedia.org/wiki/Compression_release_engine_brake https://en.wikipedia.org/wiki/Compression_release_engine_bra...

MindSpunk 1y ago
You're right. Petrol engines are air pumps so I should've realised the explanation wasn't correct. Though the ECU would be doing the AFR management on modern EFI engines as the injectors aren't vacuum operated like Carburetors were. You should be able to cut fuel injection when coasting in a modern engine, can't run lean if there's no fuel at all. Not sure if carbs could do the same.
- agurk 1y ago
  On all the fuel injected engines I have owned there is a physical cable that controls the position of the throttle plate. There is an airflow mass sensor the other side of the plate to measure the amount of air and therefore how much fuel needs to be injected. So interestingly in these sort of engines you're really just controlling airflow to the engine rather than fuel/air mix like on one with a carburettor. More modern engines have electronically controlled throttle plates, and this is definitely somewhere you could do something clever like you suggest - cutting fuel flow but also maximising airflow when there is zero throttle input. I assume engine braking is generally considered a beneficial thing by manufacturers, but it could be fun to be able to customise the amount. Or do something like have the braking come on gently at first then harder. Maybe even try and have a linear or flat response curve vs. engine rpm.
  - yetihehe 1y ago
    > Or do something like have the braking come on gently at first then harder. You can do this by letting go of gas pedal slowly. I have "current amount of fuel used" info in my car (liters/100km), it shows pretty clearly, that when going fast and slowly letting go of gas, amount of fuels slowly goes to 0. If I let go of gas fast, the engine is intelligent enough to not close throttle as fast as possible, still probably takes 1 second. > More modern engines have electronically controlled throttle plates, and this is definitely somewhere you could do something clever like you suggest - cutting fuel flow but also maximising airflow when there is zero throttle input. They cut fuel flow and close throttle plate almost completely but still allow some small amount of air, in order to actually do engine braking. If you need to coast, you can apply clutch in manual. Don't know that much about automatic, but from what I've driven, they use "lift gas" as a "engine braking" signal, so probably they can't really coast that good.

jve 1y ago

I'v bee downshifting manually with automatic especially when riding downhill. The effect with Chrysler T&C 2009 was very noticable and effective. Toyota Sienna 2015 - the braking effect is unfortunately minor.

bigfatkitten 1y ago

Many modern automatics are smart enough to do this for you if you tap the brakes while rolling downhill.

yakkers 1y ago

An aside on your last point: In the D gear, a lot of auto transmissions will be set up to allow for the wheels to spin somewhat independently of the engine (in any gear) when you aren't accelerating, which accounts a lot for how well an automatic will coast when you release the throttle, even if you're not going fast enough for top gear (especially apparent with older four speed boxes). Using the numbered gear options will enable clutches/bands that provide more engine braking.

MindSpunk 1y ago
Huh, interesting. I had thought it was just from the higher gears being used. Vast majority of my driving is with manual, I've only really driven modern autos with 5+ gears (and a Toyota hybrid, which don't really have gears at all).

vladvasiliu 1y ago

It depends on the implementation. I ride an automatic motorbike which will downshift if I grab the brakes a bit harder, even if I'm only slowing to a speed to which it wouldn't otherwise shift. For example, if I brake somewhat hard from 130 km/h to 90, it will downshift from 6th to 5th. When riding normally, it would stay in 6th down to around 50.

frollogaston 1y ago

Auto trans downshift is designed for long hills, not downshifting frequently to stop signs etc like you'd do with a stick. Even with paddles, there's a delay, or it briefly goes neutral, or it doesn't rev-match well. Or you can't double/triple-downshift, which is worse when you have 8-12 gears. Allegedly wears them down faster too, which idk but would not be surprised if it were true given how unhappy it feels.