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What car was this in? I was always told that the brake of a modern car is much stronger than the engine so you can always come to a stop, even with a stuck acce

What car was this in? I was always told that the brake of a modern car is much stronger than the engine so you can always come to a stop, even with a stuck accelerator

mrguyorama 2y ago
While the breaks physically will be ABLE to overcome the engine in anything that isn't a dodge hellcat, if the peddle is stuck to the floor and the engine is at full throttle, you won't have much or any vacuum boosting effects! You will have to quite literally STAND on the brake peddle like the bad old days, and a lot of people driving today have never experienced unassisted braking.
deleted 2y ago
[deleted]
blake1 2y ago
In most modern cars, there is a pressure booster in the power brake system, enabling braking force that can overcome the engine’s horsepower and quickly stop the car. But they are designed to continually maintain the braking force for a limited amount of time—30 seconds or so—after which this boosting ability is depleted. Once that happens, braking must be fully supplied by muscle power via the mechanical backup. This is challenging if the engine is stuck in a wide open throttle (WOT) state, because the driver must overcome the cars weight in addition to the engine. For a small car like a Toyota Corolla, this requires a few hundred pounds of downforce on the pedal. For a large 300hp SUV, this could require a thousand pounds of downforce. As you said, the brakes can bring the car to a stop, but the car will start reaccelerating if the engine isn’t shut off. (Sorry for mixing physical units.)
topspin 2y ago
While it is the case, by design and typically by regulation, that brakes have more braking force on paper than the engine and drivetrain can produce, in the real world it is not so straightforward. ICE engines have active heat rejection and conventional brakes don't. So ultimately in any prolonged fight between an engine and brakes the engine will win. Hot brakes have a lower coefficient of friction. This is the brake "fade" experienced on long downhill runs or heavy use, such as while towing heavy loads. In extreme cases brake pad outgassing, brake pad glazing and boiling brake fluid, all a consequence of heat, will degrade brake power. Traditionally vacuum assist is used to amplify brake force in passenger cars. The vacuum reserve is, however, finite and little vacuum is available from an engine with a stuck open throttle. When the throttle is even partially open and the engine RPM is kept low (such as when fighting it with the brakes) the vacuum drops severely. When boost runs out brake force is greatly reduced. This all changes in heavier vehicles where more robust systems are employed, such as compressed air brakes with large pressure reserves. Some vehicles have enough power to overcome the brakes on driven wheels. Some vehicles have "low range" gearing that can also easily overcome the brakes. The situation described by helsinkiandrew was probably a combination of brake fade that emerged while fighting the engine and lack of brake boost due to low vacuum because the throttle was somehow stuck. But there isn't enough information to say for certain.

What car was this in? I was always told that the brake of a modern car is much stronger than the engine so you can always come to a stop, even with a stuck accelerator

mrguyorama 2y ago

While the breaks physically will be ABLE to overcome the engine in anything that isn't a dodge hellcat, if the peddle is stuck to the floor and the engine is at full throttle, you won't have much or any vacuum boosting effects! You will have to quite literally STAND on the brake peddle like the bad old days, and a lot of people driving today have never experienced unassisted braking.

deleted 2y ago

[deleted]

blake1 2y ago

In most modern cars, there is a pressure booster in the power brake system, enabling braking force that can overcome the engine’s horsepower and quickly stop the car. But they are designed to continually maintain the braking force for a limited amount of time—30 seconds or so—after which this boosting ability is depleted. Once that happens, braking must be fully supplied by muscle power via the mechanical backup. This is challenging if the engine is stuck in a wide open throttle (WOT) state, because the driver must overcome the cars weight in addition to the engine. For a small car like a Toyota Corolla, this requires a few hundred pounds of downforce on the pedal. For a large 300hp SUV, this could require a thousand pounds of downforce. As you said, the brakes can bring the car to a stop, but the car will start reaccelerating if the engine isn’t shut off. (Sorry for mixing physical units.)

topspin 2y ago

While it is the case, by design and typically by regulation, that brakes have more braking force on paper than the engine and drivetrain can produce, in the real world it is not so straightforward. ICE engines have active heat rejection and conventional brakes don't. So ultimately in any prolonged fight between an engine and brakes the engine will win. Hot brakes have a lower coefficient of friction. This is the brake "fade" experienced on long downhill runs or heavy use, such as while towing heavy loads. In extreme cases brake pad outgassing, brake pad glazing and boiling brake fluid, all a consequence of heat, will degrade brake power. Traditionally vacuum assist is used to amplify brake force in passenger cars. The vacuum reserve is, however, finite and little vacuum is available from an engine with a stuck open throttle. When the throttle is even partially open and the engine RPM is kept low (such as when fighting it with the brakes) the vacuum drops severely. When boost runs out brake force is greatly reduced. This all changes in heavier vehicles where more robust systems are employed, such as compressed air brakes with large pressure reserves. Some vehicles have enough power to overcome the brakes on driven wheels. Some vehicles have "low range" gearing that can also easily overcome the brakes. The situation described by helsinkiandrew was probably a combination of brake fade that emerged while fighting the engine and lack of brake boost due to low vacuum because the throttle was somehow stuck. But there isn't enough information to say for certain.