How/why exactly does engine braking work?

W

White95Max

New member
I noticed that my Protege's ability to engine-brake is different from the Maxima. Is the fact that it's a smaller engine the reason? Lower compression ratio?

And how is a car able to slow down by utilizing the compression of the engine?
 
Well, instead of using the expansion of the burning intake mixture to drive the pistons down, you're using the potential energy of the vehicle in motion to compress the intake charge...kind of like running the engine backwards...you're using the potential energy to spin the engine back up to speed...in a hybrid vehicle this energy would be captured through the electric drive motors acting as generators to recharge the battery (regenerative braking).
 
Hmm...



So why did the Maxima have a greater ability to engine-brake than the Protege?



Excuse me, I have to go wash the Protege and apply some Paste Glaz, so I'll check back later. :)
 
You also have to take into consideration the difference in drivetrain drag between the two vehicles. It may not be that much different, but they are certainly different. For instance, just different size tires on the same car can make a difference.
 
so whats a 'jake' brake...same kinda thing??...some one said a jake will hold the exhaust valves open ???
 
steck said:
so whats a 'jake' brake...same kinda thing??...some one said a jake will hold the exhaust valves open ???
Click to expand...



A jake brake is a braking system used primarily on semi-trucks or other large vehicles that modifies engine valve operation to use engine compression to slow the vehicle. They are also known as compression brakes. The name comes from Jacobs Vehicle Systems.



When the accelerator is released on a truck its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses. A compression brake uses a solenoid to open the exhaust valve at the top of the compression stroke. This releases the compressed air in the cylinder preventing it from returning its energy back to the piston.



The driver controls consist of an on/off switch and, sometimes, a multi-position switch that controls which cylinders the brake is active on. When the compression brake is turned on, it will activate when the driver releases the accelerator. There is also a switch on the clutch pedal that will deactivate the compression brake when the clutch is disengaged.



Jake brakes often make a loud chattering or machine gun noise while being used, which has led many communities to ban them. There are signs on the side of some freeway offramps, or at municipal boundaries, that will say "Engine brakes restricted", etc. They are referring to jake/compression brakes. This has led to special mufflers on trucks that also muffle the sound of the compression brake.





From http://en.wikipedia.org/wiki/Jake_brake
 
White95Max said:
A jake brake is a braking system used primarily on semi-trucks or other large vehicles that modifies engine valve operation to use engine compression to slow the vehicle. They are also known as compression brakes. The name comes from Jacobs Vehicle Systems.



When the accelerator is released on a truck its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses. A compression brake uses a solenoid to open the exhaust valve at the top of the compression stroke. This releases the compressed air in the cylinder preventing it from returning its energy back to the piston.



The driver controls consist of an on/off switch and, sometimes, a multi-position switch that controls which cylinders the brake is active on. When the compression brake is turned on, it will activate when the driver releases the accelerator. There is also a switch on the clutch pedal that will deactivate the compression brake when the clutch is disengaged.



Jake brakes often make a loud chattering or machine gun noise while being used, which has led many communities to ban them. There are signs on the side of some freeway offramps, or at municipal boundaries, that will say "Engine brakes restricted", etc. They are referring to jake/compression brakes. This has led to special mufflers on trucks that also muffle the sound of the compression brake.





From http://en.wikipedia.org/wiki/Jake_brake
Click to expand...





i should have stayed in school!!! that was alot to swallow! cheers though for the info!
 
On a 4-stroke car, the piston is pumping air in on the intake and out on the exhaust stroke. So the bigger the engine, the more of a pump it is. Gearing also affects this, and the faster the engine is turning, the more it is pumping. So engine braking works by basically giving no fuel to the engine when you lift off the gas. Of course it's also giving hardly any air since the throttle plate is closed. But anyway, this pumping is using energy, thus slowing the car down. I'm sure a lot of factors regulate how effectively it does this.



A diesel is generally a two-stroke motor. So exhaust is not actually pumped out, rather the pressure from combustion forces it out. Since there's no real combustion on engine braking, I believe this is why a jake-brake needs to be used. I'm not sure how that wikipedia thing is explaining it as it refers to an exhaust valve on the top of the combustion chamber, in which case you wouldn't need to open it on the compression stroke?



I dunno, beats me... :nixweiss
 
OK here's another related question...

If the throttle plate is closed when you let off the gas, then how does ANY combustion take place? Where does the oxygen come from?
 
Um...asking that question is like asking how the car idles when you don't have your foot on the gas. First of all, the throttle butterfly doesn't seal perfectly, and besides there is some sort of bypass (GM calls it Idle Air Control Valve, or IAC) which meters air around the throttle plate for proper idle. And, um, combustion doesn't take place that well when you lift off the gas; this is a moment when there are a lot of emissions, that almost killed manual transmission cars in the early 80's, until they got the emissions techniques worked out.
 
Note on Jake Brakes... You can't use them on wet and snowy roads!



The new Engine 1 (due to be delivered within the near future) that is replacing the present Engine 1 has a jake brake.
 
Aurora40 said:
On a 4-stroke car, the piston is pumping air in on the intake and out on the exhaust stroke. So the bigger the engine, the more of a pump it is. Gearing also affects this, and the faster the engine is turning, the more it is pumping. So engine braking works by basically giving no fuel to the engine when you lift off the gas. Of course it's also giving hardly any air since the throttle plate is closed. But anyway, this pumping is using energy, thus slowing the car down. I'm sure a lot of factors regulate how effectively it does this.



A diesel is generally a two-stroke motor. So exhaust is not actually pumped out, rather the pressure from combustion forces it out. Since there's no real combustion on engine braking, I believe this is why a jake-brake needs to be used. I'm not sure how that wikipedia thing is explaining it as it refers to an exhaust valve on the top of the combustion chamber, in which case you wouldn't need to open it on the compression stroke?



I dunno, beats me... :nixweiss
Click to expand...

Most diesel engines built today are 4 stroke. Detroit Diesel may still be making some 2 stroke engines in thier industrial division yet, but I am not sure about that. Ocasionally I can still hear an old Detroit 2 stroke screaming down the road in an old truck. As far as on road trucks today running Cat, Cummins, and the newer Detroit's are all 4 stroke and that is why it works to hold open the valves at the correct time.

You know you are a motorhead when you love it when a trucker hits the Jake Brake as they go through a tunnel. Made me smile just to think about it :)
 
Heheh, well the only diesel I'm really very familiar with was on the LAV, which was a Detroit Diesl turbo 2-stroke... :nixweiss



If regular diesels are 4-stroke, why open the exhaust valve on the compression stroke? It'll be opened anyway on the next stroke, between which time nothing will be taken in.
 
When you open the exhaust valves as the piston is at the top of the compression stroke, you relieve the pressure in the cylinder that would be used to drive the piston down on the power stroke. By doing this the engine uses energy to compress the air during the compression stroke that was taken in during the intake stroke, but not allowed to produce much energy in return during the powerstroke. Those engines have alot of heavy rotating mass that that requires a large amount of energy just to keep themselves running. When you reduce thier ability to produce energy during the powerstroke, they make for a effective brake for the truck.
 
I'm pretty sure it has a lot to do with gears. When a car has short gears it will slow down more than the same car with long gears. For example my brother had basically the same car as mine and he had 2:42s and I have 3:73s, my car will slow down more so than his just on engine, however my top speed is lower than his.
 
bluemoon said:
When you open the exhaust valves as the piston is at the top of the compression stroke, you relieve the pressure in the cylinder that would be used to drive the piston down on the power stroke. By doing this the engine uses energy to compress the air during the compression stroke that was taken in during the intake stroke, but not allowed to produce much energy in return during the powerstroke. Those engines have alot of heavy rotating mass that that requires a large amount of energy just to keep themselves running. When you reduce thier ability to produce energy during the powerstroke, they make for a effective brake for the truck.
Click to expand...



I guess this would allow a bit less conservation of energy, but even with no special feature, 1/2 the four-stroke cycle would still be spent wasting energy since no fuel would be applied. Perhaps that isn't enough in a big truck, though.



I guess I always associated them (and diesels) with 2-stroke operation. But I never thought that much about it... :)
 
Pontman43 said:
For example my brother had basically the same car as mine and he had 2:42s and I have 3:73s, my car will slow down more so than his just on engine, however my top speed is lower than his.
Click to expand...



Do you have an automatic? I'd think 3.73:s's would raise the top speed over 2.42:1's due to putting the motor more in the powerband at the drag-limited top speed.
 
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