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I am not sure if if this is the right forum to ask this, but even my driving instructor cannot give me a clear answer, hence the question. Learning driving on a toyota vios, automatic transmission. The interrelated questions I have are

  • Does the accelerator control and ensure gas is flowing through the engine? The harder I press, the more gas flows in and the faster is the engine RPM. Is my mental model correct? Also, if this is the case, does it mean unless I step on the accelerator, no gas flows in and the engine does not spin (unless it was spinning, because of inertia)?
  • While driving at, suppose 50kmph, suppose I take my foot off the accelerator, and neither do I press the brake. Does it mean the engine will still spin for sometime (because of inertia) and the car will move some distance (may be even a few miles) before eventually coming to a halt? Is it like when I stop pedaling when riding a bicycle?
  • What happens if I press both the accelerator and foot brake together? As one tries to move the car, and another resists, will some parts internally grind against each other, eventually damaging the parts, maybe even an accident? Do not want to try this, just want to know what happens in that case. Or is the car somehow smart enough to not let the engine spin when the foot brake is pressed?
  • Does pressing the foot brake, in automatic, also decouple the transmission (wheels) from the engine, in addition to resist the wheel with brake shoes? Is it similar to neutral in manual?
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    I want to see your driving instructor's face when you're driving 50kmph. ;-)
    – Monkey47
    Jun 6 at 23:41
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    I think you need a new driving instructor. The first thing mine did was explain the basics of how the engine/transmission works.
    – SiHa
    Jun 7 at 8:04
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    @Della Monkey47 is probably (humorously and not a little pedantically) reading your "kmph" as "thousands of miles per hour." Kilometers per hour is more usually abbreviated "kph" or "km/h" or "km/hr."
    – TypeIA
    Jun 7 at 10:19
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    Your confusion probably stems from the mechanism of (your specific flavor of) drive train. Traditionally, the engine speed is linear to car speed (and the car obviously doesn't stop immediately so neither does the engine), unless you disengage one from another (by pressing the clutch pedal in a manual). Automatic transmissions complicate things. I second the advice of others: try to get your license in a manual.
    – aross
    Jun 7 at 12:22
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    @Della consider (with an instructor or just someone who knows how to drive, if it's legal in your area) going to a large, empty parking lot somewhere and driving the car around aimlessly a bit to get a feel for this sort of stuff. I guess you didn't have that lesson. It seems like people often memorize procedures (like when to press the accelerator) without really learning how the thing actually works. A bit like the mom who cuts the ends off the roast without knowing why.
    – user253751
    Jun 8 at 9:16

6 Answers 6

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In short, find a better driving instructor and, if possible, learn to drive a car with manual transmission.

The long (still simplified) answer:

Does the accelerator control and ensure gas is flowing through the engine? The harder I press, the more gas flows in and the faster is the engine RPM. Is my mental model correct? Also, if this is the case, does it mean unless I step on the accelerator, no gas flows in and the engine does not spin (unless it was spinning, because of inertia)?

Generally, yes, but there are other factors that affect the amount of fuel allowed in the engine:

  • Idle. When the engine is turned on, some fuel is allowed in the engine even with the accelerator pedal fully released. This is why automatic cars inch forward when the transmission is in "D" position. You need to apply brakes to prevent this. This is also how e.g. steering assistance or air conditioning are powered.
  • The actual amount of fuel depends also on the engine RPM at the moment.
  • Other, less important factors also impact the engine fuel consumption/power.

While driving at, suppose 50kmph, suppose I take my foot off the accelerator, and neither do I press the brake. Does it mean the engine will still spin for sometime (because of inertia) and the car will move some distance (may be even a few miles) before eventually coming to a halt? Is it like when I stop pedaling when riding a bicycle?

Mostly. An automatic car will not, in fact, completely come to a halt on a level road - see about the "idle mode" above.

Slopes can alter this behavior.

Uphill, the idle engine power may not be enough to creep the car forward.

Downhill, the car can gradually accelerate all the way to its designed maximum speed (and even more) with no action on the accelerator pedal. It is up to the driver to apply brakes and common sense.

What happens if I press both the accelerator and foot brake together? As one tries to move the car, and another resists, will some parts internally grind against each other, eventually damaging the parts, maybe even an accident? Do not want to try this, just want to know what happens in that case. Or is the car somehow smart enough to not let the engine spin when the foot brake is pressed?

In a sane car, brakes outpower the powertrain by a great margin. If you press on brakes strong enough, no pressure on the accelerator pedal will make the car move.

Don't really try this, a wrong order of releasing the pedals can make you car vigorously jump forward, up to and including crashing into some obstacle.

Yes, if you insist on pressing and holding the accelerator and the brakes together, some parts wear intensively and can overheat in a few minutes. Repairs will be expensive.

This (among other things) is why automatic cars are controlled by a single foot and this is one of the first things you learn in the driving lessons.

Then again, pressing both pedals (to an extent) is sometimes used when driving off-road.

Does pressing the foot brake, in automatic, also decouple the transmission (wheels) from the engine, in addition to resist the wheel with brake shoes? Is it similar to neutral in manual?

Not really. An automatic transmission always applies some torque to the wheels. If the accelerator is not pressed, the internal friction that applies this basic torque is considered safe. You are not expected to manually disengage the transmission when waiting for a green light.

If you really want to disconnect the transmission, there is "N" position on most automatic transmissions. "P" is the same, but it also applies an internal brake mechanism that blocks the wheels.

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Note, most of these answers are very specific to an automatic transmission and also a gasoline (petrol) engine -

The position of the throttle generally controls the volume of air allowed to enter the engine. The air volume will depend on the throttle position, current RPM of the engine and the current speed of turbo if one is fitted. The engine ECU will add the correct amount of fuel to match the air intake. More fuel can mean higher revs, but it could also just be to maintain the revs when going up an incline. The ECU may cut fuel to the engine if the momentum of the car is driving the engine, but usually there will be fuel getting used even with the engine idling.

If you take your foot off the throttle, the momentum of the vehicle may well keep the engine rotating even without throttle. However automatic transmissions often disengage the engine in these circumstances, so the engine revs will drop to idle while the vehicle still moves at speed. The car will keep moving until frictional forced slow it.

If you press the brake and throttle at the same time, you will likely just overheat the transmission fluid in the torque converter. You could also cause wear to the clutches in the transmission. No metal parts are likely to be harmed. I don't know of any cars that stop you pressing both pedals at the same time.

When you press the brake, the engine will not be disconnected from the drive train, however the act of releasing the throttle may make it happen. Some autos will provide engine braking in certain circumstances, or it may be possible to select a low gear when going down a hill for example which will keep the engine connected to the drive train to add extra engine braking.

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    "I don't know of any cars that stop you pressing both pedals at the same time." In many "modern" cars (roughly 2000-ish and newer), the ECU will cut the throttle if you apply brake & throttle at the same time. The assumption being that some people will left-foot brake and that the crossover isn't intentional. Of course, that makes doing burn outs harder... Technically, this doesn't stop you from pressing both, but the computer will only respond to the brake and not both, so effectively it does.
    – FreeMan
    Jun 6 at 19:09
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    @FreeMan Thanks for that, I will give it a try in my wife’s car :) Maybe I just haven’t tried this in a modern enough car. I know I have accidentally caught the throttle when pressing the brake on my 2002 Land Rover Discovery while wearing big boots and the engine was definitely trying to drive the car.
    – HandyHowie
    Jun 6 at 20:24
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    Actually, I think some started doing that after the mid- late-80s "unintended acceleration" debacle that Audi went through in the US where morons where hitting the throttle and the brake at the same time, then blaming the car for moving when they were trying to stop. Unless history has been edited, you should find details about this on Wikipedia or other sites.
    – FreeMan
    Jun 7 at 10:58
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Does the accelerator control and ensure gas is flowing through the engine? The harder I press, the more gas flows in and the faster is the engine RPM. Is my mental model correct? Also, if this is the case, does it mean unless I step on the accelerator, no gas flows in and the engine does not spin (unless it was spinning, because of inertia)?

Traditionally, on completely mechanical cars (no computer controlling fuel injection) accelerator controls the air intake, and the air intake controls the fuel intake. But yes, indirectly it controls fuel intake. RPM does not necessarily goes faster, it depends on the load, and the engine might simply not have enough power to increase the rotation (maybe on a too steep inclination, maybe wind is too fast relatively to the car, maybe the manual car is in the wrong gear, etc). Lastly, not pressed does not mean no air/fuel intake, but just barely, to keep the engine running without load, so the engine should work without the accelerator input.

While driving at, suppose 50kmph, suppose I take my foot off the accelerator, and neither do I press the brake. Does it mean the engine will still spin for sometime (because of inertia) and the car will move some distance (may be even a few miles) before eventually coming to a halt? Is it like when I stop pedaling when riding a bicycle?

No, far from it (and I believe this is where my answer differ most from others). The engine can output power, or it can take power. If you release the accelerator at speed, the engine will not produce enough power to sustain the RPM, so it will take the load of the car inertia backwards (from wheels -> transmission -> engine), and will actively drag the car, sometimes strongly (in a manual car you can control it to an extent). This is what is called engine braking, and usually recommended when going downhill for an extended period of time.

What happens if I press both the accelerator and foot brake together? As one tries to move the car, and another resists, will some parts internally grind against each other, eventually damaging the parts, maybe even an accident? Do not want to try this, just want to know what happens in that case. Or is the car somehow smart enough to not let the engine spin when the foot brake is pressed?

As said in the comments, the car's computer will (probably) try to prevent you from damaging your car. But every car has a coupling mechanism between the engine and the transmission. It is either a clutch (in manual cars), or either a computer controlled clutch and/or a torque converter (in automatic cars). So, if there is only a clutch, and it is engaged, something in the system (engine <-> clutch <-> transmission <-> brakes) will have to give way. Often the engine will shutdown with a bump (and a slight chance of breaking something). If the clutch is partially engaged, it will take the hit and burn (and you will smell it). If completely disengaged, there is no connection between the engine and the transmission, so the engine can run freely and will accelerate very quickly without load (maybe the engine is damaged if the computer fails to limit the RPM for too long).

In automatic cars, the torque converter is a kind of always partially engaged hydrodynamic clutch: the metal parts do not touch, and the power is transmitted through oil. In this case, the propeller side will turn with heavy load, the turbine side will be tightly locked by the brakes, and you will heat a lot of oil in the process (and possibly damage the torque converter).

Whatever the case, don't do that.

Does pressing the foot brake, in automatic, also decouple the transmission (wheels) from the engine, in addition to resist the wheel with brake shoes? Is it similar to neutral in manual?

Generally no. The engine needs a minimal RPM in order to function. The transmission will shift back to lower gears in order to maintain the engine working as the speed decreases. It will only decouple (in case of an automatic clutch) if the car is about to stop and it would bring the RPM to below the minimum. In case of only torque converter, nothing is decoupled, and it will simply absorb the engine's low RPM into the oil as heat, as it is designed to do.

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  • +1 for mentioning engine braking. However, that is basically non-existent in an automatic transmission. The torque converter will absorb almost all of the wheel's drive and not transmit it to the engine for braking effect.
    – FreeMan
    Jun 7 at 11:15
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    @FreeMan I had an automatic recently and I could feel the engine braking going downhill. This particular model had both a clutch and a torque converter. The clutch would engage at around 20 km/h and higher, so that the engine would be strongly coupled to the transmission most of the time. I thought that was the norm on modern automatic cars.
    – lvella
    Jun 7 at 13:09
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    @FreeMan My experience with automatic transmissions has been that engine braking works fine. There are several hills that I routinely descend; I control vehicle speed simply by pushing the auto transmission down to 2nd or 3rd gear. Engine RPM jumps up accordingly, and I seldom have to touch the brake while others around me have their brake lights lit solid for the duration of the descent. This is with vehicles in the "light truck" (pickup/van) category though; maybe other classes of passenger car would behave differently.
    – Greg Hill
    Jun 8 at 23:00
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Excellent questions from a learner driver! To address them in turn, assuming a modern fuel-injected engine with an automatic drivetrain:

  • Yes, the accelerator pedal does control the flow of air and fuel into the engine. This raises the "load" condition of the engine, and generally also raises the engine speed (RPM). In newer cars, the throttle (the valve plate that regulates airflow into the engine) is controlled electronically and can move somewhat independently of the accelerator pedal. The exact amount of fuel injected is estimated by the engine control system based on air flow and/or pressure and temperature, and corrected using oxygen sensors in the exhaust. The accelerator pedal position will also influence the choice of gear ratio in an automatic transmission. I find it useful to think of the accelerator pedal as controlling the overall power demand on the powertrain (engine plus transmission).

  • Releasing the accelerator pedal will close the throttle plate and, if the engine speed is above idle, will stop fuel from being injected into the engine. This is known as deceleration fuel cut-off (DFCO). This condition will cause the vehicle to slow due to engine braking, as the car's kinetic energy is converted into heat via friction and pumping losses in the engine (and to a lesser extent in the transmission). The engine remains connected and spinning while you are at speed. At very low road speeds, the torque converter or automatic clutch will effectively decouple the engine, and at low engine speeds, the idle air control system will let more air and fuel into the engine to avoid stalling, so you might notice a small bump in power at that point. Thanks to idle air control, even if you release the accelerator completely, the engine will still run. This also gives you the ability to "creep" on the flat with the accelerator released.

  • Partially pressing the accelerator and foot brake simultaneously will waste energy, and cause unnecessary heating and wear of the brakes, but shouldn't cause immediate damage. Most cars will not disconnect the engine from the rest of the drivetrain in this condition, although an automatic transmission with a torque converter will permit some "slip", depending on the speed. However, avoid the habit of such "two-foot" driving; even on an automatic, you should use only your right foot to operate the accelerator and brake.

  • No, pressing the foot brake (even with the accelerator pedal released) at speed will not normally decouple the engine from the wheels. In fact, the vehicle may remain in-gear and downshift to use engine braking to assist in slowing.

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In a Tesla, there is a menu option to select what happens at idle. You can set to creep forward when neither the brake and accelerator is being pressed. You can also set it to not creep forward. You can also set the car to enable regenerative braking when the accelerator is not being pressed (this has a similar effect as if you had pressed the break pedal). There is another open that will automatically engage the parking brake (auto hold) when the car slows to stop.

I have both regenerate breaking and auto hold enable. When no body is following me, I just take my foot off the accelerator at just the right time so that by the time I get to the stop sign, I'm going 0 and the auto hold kicks in. If I do have people behind me, I will use the brakes (to let them know that I'm stopping). Then I will often take my foot off of the brake, because the auto hold will keep the car in place and therefore I don't have to sit at a stop light with my foot on the brake.

So the answer to your questions depends on what car you have and features might have been set in the cars control panel.

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    Regenerative braking is used in all electric and hybrid cars. The exact behavior of the accelerator and brake pedals is a matter of software so some diversity and possible adjustments do exist. The default behavior of all recent electric cars is to emulate a traditional automatic transmission.
    – fraxinus
    Jun 7 at 19:58
  • @fraxinus Is there a point to your comment that should be reflected in my answer other than to point out that there other electric cars have regenerative braking and that they can adjust as well? Yes, the default is to mirror cars with transmissions. Was this a drive by comment, or did you intend to add value with your comment?
    – Walter
    Jun 7 at 21:31
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if this is the case, does it mean unless I step on the accelerator, no gas flows in and the engine does not spin

A lot of great answers already explained how things work. But a bit of a thought experiment, and comparison to some newer (not traditional internal combustion engine):

When you start a traditional ICE car, the battery powers the starter (basically an electric motor) which literally starts the engine mechanically. If the engine stopped turning every time the car stopped moving, then when you pressed on the accelerator to start the car moving again, the starter would have to start the engine again. Which would be a lot of wear and tear on the starter (I just had mine replaced, not cheap...) and also a significant delay instead of the immediate acceleration that we normally get. So even if the engine didn't need to produce power (alternator -> electricity to charge the battery and run the radio, HVAC, lights, etc.; plus other things connected with various belts, etc.), there would be a good reason to keep the engine running when the car is not moving.

On the other hand, an electric car's motors are there simply to power the wheels. When the car stops moving, the motors stop rotating.

The interesting twist is hybrids. With a hybrid, as long as there is enough battery power, electric motors power the wheels with the "moving = on, not moving = off" mode of operation. When battery power is low, an ICE starts up to either produce electricity or, possibly (I don't know if any currently available hybrids work this way or not) to power the wheels directly as in a traditional ICE. That ICE will not turn on/off with every start/stop of the car - it doesn't need to and would be very inefficient if it did so.

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  • Most modern ICE cars do in fact turn off the engine when the car comes to a stop, as long as the engine is warm and battery has sufficient charge. The engine restarts when the brake is released, rather than when the accellerator is pressed. Some cars will also automatically re-start the engine when they detect that the car ahead has started moving. Jun 9 at 12:29
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    Many hybrids do work like that - for example BMW PHEVs and the Chevy Volt. They can be driven fully by the electric motor, by the ICE with the electric motor providing assist, or by the ICE directly using the electric motor as a generator to charge the traction battery. This is different from say, the i3 REx which has an ICE onboard, but it has no connection to the powertrain and can only be used as an electrical generator.
    – Chris O
    Jun 9 at 14:29

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