I know how to make it happen, but what is it? Can anyone tell me why it happens? Please satisfy me with some detail. It just seems like a lot of energy to be able to slow a car in that way.

 

In a four stroke engine you have, well, 4 strokes. Intake, compression, power, and exhaust. When you are accelerating you are intaking air/fuel, compressing it, igniting it (power stroke), then exhausting it.

When you are engine braking you intake air, compress it, then exhaust it. The work it takes to compress the air is what is slowing the car down for the most part. You also have losses from intertia and friction of the drivetrain and engine internals, as well as the intake stroke sucking in air out of a vaccuum (throttle blades closed), but it's a much smaller amount.

So basically you are turning your engine into a huge air compressor. The higher the compression ratio, and the higher rpm you are turning (moving more volume of air, in turn doing more work) the faster you are going to slow down. Thats why at high rpm you slow down quickly but at low rpm you don't have much braking effect.

 

Seems to me the pumping losses from the intake stroke being under vacuum and exhaust backpressure would have a bigger effect than the compression stroke. The reason I say that is you should get almost all of the energy back on the power stroke that was used to compress the air on the compression stroke. Meaning the net amount of work from the beginning of the compression stroke to the end of the power stroke is close to 0 with no combustion. I could be wrong though, i guess the air could loose some of its pressure due to heat transfer, or leakage through the rings.

I'm tempted to try engine braking with the ignition off and compare how fast i slow down with the throttle closed and with the throttle open. If pumping losses have a bigger effect then i'd slow down faster with the throttle closed. If compression has a bigger effect i'd slow down faster with the throttle open.

 

That is why jake brakes and exhaust brakes are needed for engine braking on diesels. They have high compression but no pumping losses. The exhaust brake increases pumping losses, the jake brake gets rid of the compressed air before the power stroke.

 

Here is a full cycle description. Take my 402 stroker for example. For every cycle I pull in somewhere close to 50.25 cubic inches of air at WOT. Now during engine breaking the throttle blade closes most of the way so the engine is really pulling a vacuum similar to that at idle but slightly more. Lets look at the strokes starting with the intake and we'll look at the losses.

INTAKE: Taking the low pressures, or vacuum, and pull a small amount of air through the intake.
INTAKE LOSSES: We are doing some work moving the air but since their isn't much air we can almost neglect it and look at what is happening with the piston. We are moving the piston with a force through a distance which is the definition of work. The force on the piston the the pressure of the crankcase times the area of the cylinder bore. Other losses here would be caused by the friction of the rings and other moving parts.

COMPRESSION: We've still got a pretty good vacuum in the cylinder and the same force we were working against on the intake stroke comes back to help us.
COMPRESSION LOSSES: Since we do have some air in the cylinder, a little more than we would have at idle, we get to the point where we have to put work into the volume to compress the air. Now when we compress a gas the temperature goes up linearly base upon Pv=nRT. So now we are compressing the gas and it is gettng hotter and some of this thermal energy transferres itself into the piston and the cylinder head at a rate proportional to its temperature and some other factors of heat transfer. Also, here again we have losses due to friction.

POWER: The piston now turns back down being pushed by the pressure of the air charge, which is a little cooler due to the heat transfer and so it doesn't have the same pressure. Again the pressure times bore are give the force.
POWER LOSSES: Once we start getting closer to the end guess what, our exhuast valve opens and there goes the remaining pressure which was pushing our piston down. Here again we have the same frictional losses.

EXHAUST & LOSSES: We've got a little bit of air left over and we need to do work on it to move it out of the cylinder. The faster the engine is running the harder it is to move this air out Try to blow through a straw and you'll see why Don't forget you are moving this all the way through the exhaust system and that take more work. Blow into a garden hose and you'll see why.


So now you can see where all of the cars kinetic energy is going. I didn't tell you above but it all gets converted to heat, which gets dumped out in the exhaust, through the radiatior and anywhere else that there is a temperature difference. Friction is a big culprit and the other are the pumping losses.

 

There may not be as much pressure remaining when the exhaust valve opens as you think if the intake valve also closes late into the compression stroke. So probably not much loss there.

 

Good explanation mack. =)

 

It also takes more energy to create a high pressure drop, as in creating the high vacuum condition. Don't forget that.
This is something the diesel doesn't do because it's not throttled.

 

Good explanation, squealing! And P Mack: You will slow down faster key off with the throttle open! Give it a try. (Bonus:with an EFI engine, you won't blow the mufflers off when you turn the key back on!)
Newer engines with cylinder shut down rely on the 'air spring effect' to minimize power absorption in the disabled cylinders, but of course they leave all the valves closed.)

 

why would it slow down faster with the throttle OPEN? I imagine the key off is because there is zero combustion process AND things like transmission isn't pumping???? I cant understand why open throttle would increase pumping losses.


The best explanation for the majority of the loss in an example (that I can think of) is imagine pulling down on the end of a needle and dropping the plunger really fast and then let go. It will "bounce" back up. There is a vacuum/negative pressure there trying to pull up on the piston (over simplified). Your engine is fighting that. The more closed the throttle is, the less air gets in the cylinder the harder its pumping.
I'm sure you can come up with better examples. But yeah, closed container and you try to increase the volume.


Another question: would the heat loss from the compression actualy do anything? Id imagine it would be negligable if not negative as the cylinder walls are already REALLY hot. Heat flows from low to high and the walls are already pretty high.
Ituition tells me most of the energy is regained durring the down stroke after the compression stroke (what would be power stroke) and a negligable amount is lost in heat due to the extremely short time and nature of the whole thing.

 

As mentioned before basically we've got frictional losses and pumping losses. When we look at pumping losses it basically boils down to friction as well but just as to not confuse anyone we'll consider them being different. TreyZ28 you are absolutly right about the direction of heat flow, assuming you mistakingly switched high and low, otherwise we would be violating the 2nd law of Thermo. When heat flows from the head to the air charge it gives the power stroke a little more push from the increased pressure due to increased temperature. In a very long engine braking period like coasting down a mountain things could reverse as the block cools but the heat generated from friction might keep it at an elevated temperature. Good Point

Also interesting point on the Displacement on Demand comment from MadBill. With the valves closed the piston just compresses the air then expands it cycling over time. Its important that those valves remained closed otherwise the cylinder and piston system would act like a shock absorber on your engine. A shock absorber pushes a large volume of fluid through an orifice and this removes the energy in the form of heat which causes the system to resist further occilations. Be carefull when you distinguish a piston full of air vs. a vacated (vacuum) cylinder.

Lets suppose you could close all of the valves on your engine with each piston full of air in one case and vacated cylinders in the second case. For each case the frictional losses will be the same however in the case with air the pistons would be getting a net amound of work out of each cycle assuming the head is hotter than the compressed gas at TDC. The second law of thermodynamics says that for any cycle there has to be a loss or at best the energy in the system is to remain the same. In case 1 we are putting energy into the system through the head so here we aren't violating any laws. To figure out the net work we would add the work done due to the heat tranfer and subtract out frictional losses.

In the case of the vacuum there would be not heat transfer from the head to the vacuum because a vacuum is nothing and thus has no ability to store energy. So in this case we would start with zero and subract frictional losses. Now when comparing the two the vacuum scenario would bleed off more energy.

The reason more energy would be bled with the throttle open is because of all the of the work required to draw the air in throught the filter and into the cylinder then push it out the tailpipe

 

Well I would have never believed it without trying it myself, but there is slightly more engine breaking with the ignition off and the throttle open. I guess the increased pumping losses on the exhaust stroke and compression more than offset the decreased pumping losses on the intake stroke.

 

On another forum, my signature reads: "Without the data, you're just another man with an opinion." So, P-M, now you're the "Man with the Data"!

 

Off topic, but you do know that a bowling ball can float

 

In mercury?

 

Water

 

How can a bowling ball float in water unaided by a buoyant material

 

an egg will NEVER break by squeezing it one hand

 

I think I'd be evaluating that assertion with a hard boiled test specimen..

 

You mean a fresh egg, and if you put it into your palm and wrap your fingers around it and squeze it's very difficult to break because you're applying a non-concentrated load.

Now try squeezing it between just your thumb and forefinger. This concentrates the load. Let us know what happens, MIKEY.

FWIW:Bowling ***** that weigh under about 9 lbs. are less dense than water so they float. Heavier bowling *****, up to the 16 lb max., sink because they are more dense than water.

MadBill, if a regulation bowling ball has a circumference between 26.7 inches and 27.0 inches, you might do the math for ***** under 9 lbs. and perhaps relook at your signature quote.

What was the automotive topic of this thread?