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Carl Fredrik Ahl

How Engine Braking Works

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Hi,

I know that when your car roll down a hill and you switch to a lower gear, the engine will break and the car slows down. I have heard that it slows down because higher rev in the motor and therefore there will be higher friction. I don't understand how this works, can you please explain?

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The quick answer is that normally the engine 'drives' the wheels, but in your case, the wheels are driving the engine, and effectively doing work by compressing air in the cylinders ( along with gearing, friction, heat, and other losses ).

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2 minutes ago, MigL said:

The quick answer is that normally the engine 'drives' the wheels, but in your case, the wheels are driving the engine, and effectively doing work by compressing air in the cylinders ( along with gearing, friction, heat, and other losses ).

Thx for the answer, can you plz explain in more detail how this works, maybe using an illustration.

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No illustration.
But, if you are going uphill, the throttle is open and the engine has to produce power to turn the wheels.
When going downhill, the throttle is closed and the engine is producing little power, but since it is still connected to the wheels ( through the clutch/transmission ) it is forced to turn ( reciprocate ) and compress air. An internal combustion engine is basically an air pump.
this work done on the air, in turn, slows down the wheels

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The USA heavy truck industry, (perhaps world wide) for years had "engine brakes" that shut off the diesel fuel and slightly changed the timing of the exhaust(s) valves.  This effectively turned the truck diesel engine into a large compressor, absorbing considerable power.  While going down hill or slowing with the wheels driving the engine, this changed things greatly.  The better more efficient engine brake designs absorbed almost as much HP braking as the engine produced pulling.  If memory serves about 30% of the absorbed energy went out the exhaust pipe as noise and about 70% was absorbed by the cooling system.

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On 12/8/2018 at 1:17 PM, HB of CJ said:

The USA heavy truck industry, (perhaps world wide) for years had "engine brakes" that shut off the diesel fuel and slightly changed the timing of the exhaust(s) valves.  This effectively turned the truck diesel engine into a large compressor, absorbing considerable power.  While going down hill or slowing with the wheels driving the engine, this changed things greatly.  The better more efficient engine brake designs absorbed almost as much HP braking as the engine produced pulling.  If memory serves about 30% of the absorbed energy went out the exhaust pipe as noise and about 70% was absorbed by the cooling system.

The device is known colloquially as a "Jake Brake". I heard valve timing was involved, but never delved into exactly how that was accomplished. Truck diesel engines in general have quite husky valve train parts, and I do know that in some designs, "engine runaway" upon attempting shut-down was thwarted by locking the exhaust valves open.

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On 29.11.2018 at 8:02 AM, Carl Fredrik Ahl said:

Hi,

I know that when your car roll down a hill and you switch to a lower gear, the engine will break and the car slows down. I have heard that it slows down because higher rev in the motor and therefore there will be higher friction. I don't understand how this works, can you please explain?

It might be useful to use a bicycle analogy to understand this; When you ride a bike and you go down a gear, your feet suddenly start pedaling faster. When you go up a gear, you start pedaling slower. The larger/smaller gears on your bike let you control the gear ratios responsible for this. Your feet when they suddenly start pedaling faster on a bike cause little resistance compared to a car engine. Essentially what happens is you go down a gear in your car, your revs go up (your feet start pedaling faster) but the engine causes a lot more loss of energy due to resistance than your feet would when riding a bike. Obviously there are differences between a car gear box and gears on your bike and engine/feet but the principle is exactly the same.

Edited by koti

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4 hours ago, koti said:

Essentially what happens is you go down a gear in your car, your revs go up (your feet start pedaling faster) but the engine causes a lot more loss of energy due to resistance

Thx for the answer. Why is the resistance higher when the rpm gets higher and how does it slow down the car. Can you explain in more detail plz.

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11 minutes ago, Carl Fredrik Ahl said:

Why is the resistance higher when the rpm gets higher

it isn't. the resistance is higher when the rpm gets lower

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6 hours ago, Carl Fredrik Ahl said:

Thx for the answer. Why is the resistance higher when the rpm gets higher and how does it slow down the car. Can you explain in more detail plz.

Like Dimreepr said, its the other way around.

Basically, the transmission has a certain resistance which causes the car to slow down when you downshift. There are many gears connected to each other in the car’s transmission, when power is applied there is a certain loss of power due to the transmission resistance. That  loss of power due to resistance is the main factor which slows your car down when you downshift (there are other smaller factors, this is the main one) A good example would be a rally car or race car transmission which is very rigid and has a very high resistance to withstand high power. If you downshift in a car with a transmission like that, you essentially feel like you would hit the brakes really hard. 

Edited by koti

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21 hours ago, koti said:

Like Dimreepr said, its the other way around.

Basically, the transmission has a certain resistance which causes the car to slow down when you downshift. There are many gears connected to each other in the car’s transmission, when power is applied there is a certain loss of power due to the transmission resistance. That  loss of power due to resistance is the main factor which slows your car down when you downshift (there are other smaller factors, this is the main one) A good example would be a rally car or race car transmission which is very rigid and has a very high resistance to withstand high power. If you downshift in a car with a transmission like that, you essentially feel like you would hit the brakes really hard. 

Thx for the answer. Do you know an animation or video showing this?

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24 minutes ago, Carl Fredrik Ahl said:

Thx for the answer. Do you know an animation or video showing this?

 

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16 hours ago, koti said:

 

Thx, but I meant an animation or video showing how the engine braking works, when the car copresses air and shows how the car slows down.

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39 minutes ago, Carl Fredrik Ahl said:

Thx, but I meant an animation or video showing how the engine braking works, when the car copresses air and shows how the car slows down.

2

the engine turns fuel into motion, in reverse, the motion is the fuel

 

 
Edited by dimreepr

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6 hours ago, dimreepr said:

the engine turns fuel into motion, in reverse, the motion is the fuel

 

 

Yeah I know this, but I wonder if you know an animation or video where they are engine braking so that I can see directly how it works without trying to imagine the process.

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