How/why exactly does engine braking work?

White95Max · 07-30-05, 01:32

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?

Setec Astronomy · 07-30-05, 01:45

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).

White95Max · 07-30-05, 01:49

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.

Setec Astronomy · 07-30-05, 01:53

I think you nailed it with the higher compression/bigger engine.

JDookie · 07-30-05, 02:32

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.

steck · 07-30-05, 04:05

so whats a 'jake' brake...same kinda thing??...some one said a jake will hold the exhaust valves open ???

White95Max · 07-30-05, 04:53

Quote:

Originally Posted by steck

so whats a 'jake' brake...same kinda thing??...some one said a jake will hold the exhaust valves open ???

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

steck · 07-30-05, 05:47

Quote:

Originally Posted by White95Max

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

i should have stayed in school!!! that was alot to swallow! cheers though for the info!

Lowejackson · 07-31-05, 06:04

The engine braking on a Formula One engine is over 1G, which is higher than most road cars maximum braking capacity

Aurora40 · 07-31-05, 07:02

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...

White95Max · 07-31-05, 09:57

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?

Setec Astronomy · 07-31-05, 10:31

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.