Engine braking : compression or vaccuum

[frag]

See "Smoke after engine braking" thread for beginning of this discussion.

 

If you would like to start a new thread, I'm game.

 

We may be discussing a phenomonon that dosen't even exist in gasoline engines.

 

The more I think about it, the engine braking effect is only apparent when the transmission is manually shifted into a lower gear.

 

When simply decelarating, the car naturally slows due to no power being produced by the engine.

 

When shifting to a lower gear there is a significantly noticable amount of slowing due to increase in resistance due to gearing.

 

So..................if compression braking in a gasoline engine is negated, due to the "air spring" affect, and the small amount of combustion that takes place on the "power stroke". I can wrap my mind around that.

 

And if a closed throttle plate in the intake system can in fact produce enough resistance to provide engine braking.....................

 

Then if I jump in my '96 OB with manual trans., and run it down a nice long steep hill, turn off the ignition........................ coasting in gear, at sufficiant speed to maintaine engine RPM's around 3000......................with throttle closed there should be noticable engine braking, then with throttle open there should be a noticable lack of engine braking and increase of speed.

 

I'll give it a try.

 

Brilliant idea! If it were not so dark outside, I would go to MontRoyal (10 minutes ride) and try it myself. Will certainly do it tomorrow.

[CNY_Dave]

Well, a possible fly in the ointment- with the throttle open, the engine will be moving a significant amount of air through the intake and exhaust valves, and moving the air through those small orfices will take some work.

 

Also, since the intake/exhaust timing isn't symmetric, I think there may not be as much 'air-spring' effect as you might otherwise get. For example, the ex valve is going to open before BDC, wasting some of the air-spring's energy.

 

It will be interesting to see what difference you notice.

 

 

Dave

[jamal]

keep in mind that with the car off the lights and brakes and steering don't work so well.

 

Then, if you were to pop the key to acc and then back to on so that the steering didn't lock up, the car would start back up again once you touched the throttle or took it out of gear.

 

Engine braking is more noticeable in lower gears because the engine has more of a mechanical advantage over the wheels.

 

Engine braking occurs on the compression stroke so the valves are closed anyway. Because of the volume of the intake manifold I don't think throttle position would make a noticeable difference.

[frag]

See "Smoke after engine braking" for the beginning of this discussion

 

You have to remember that the PCV system is tied into the intake, on the piston side of the throttle plate.

 

All that increased vacuum in the intake is sucking on the crank case. There is vacuum in the crank case, not atmospheric pressure.

 

Maybe not full atmospheric pressure but more pressure than in the cylinder. Did you ever suck on a PCV valve (new:) ) with you mouth?

I did and it closes when the vaccuum exceeds a certain amount.

[WAWalker]

Ok, I just did another experiment with the Select Monitor on my car.

 

I'm for sure learning some things I didn't know. To me that is part of the beauty of this board. It keeps my mind working, and helps me to learn new things. I work on these cars day in and day out, but will never know all there is to know.

 

So, I took the car out on the freeway with the Monitor hooked up and selected some data peramaters to record.

 

Vehichel speed

RPM

MAF

O2 Sensor

Fuel injection

Manifold relative pressure (vacuum)

(Atmospheric pressure here is 25.5) (Vacuum is -17.3 inHg on fully warmed up engine)

 

As soon as I go closed throttle on decel. This is from crusing speed of 80 mph, 3200 rpm.

Fuel injection = .26 ms

MAF = 3.91 g/s

02 Senosr = 0.00 volts

(These reading stay fixed no change so I won't list them again)

So what little fuel is being injected is buring (or not detectable) according to the O2 sensor.

Vacuum = -20.5 inHg

 

Down shift from D to 3 @

MPH = 68

RPM = 2725

Vacuum = -20.5 inHg

 

Rpm comes up to 3050, speed drops to 66 mph, vaccum come up to -20.8 inHg.

At 63 mph, rpm's start to rise, vacuum constant @ -20.8 inHg. As rpm's rise to a peak of 3575, speed stays constent @ 63 mph.

Next drop in speed and rpm's is @ 61 mph / 3375 rpm, vacuum -20.8.

 

Down shift from 3 to 2 @

MPH = 58

RPM = 3200

Vacuum = -20.8 inHg

 

Rpm comes up to 4400, speed drops to 56 mph, vaccum fixed @ -20.8 inHg.

At 52 mph, rpm's start to rise, vacuum still constant @ -20.8 inHg. As rpm's rise to a peak of 4525, speed again stays constent @ 52 mph.

Next drop in speed and rpm's is @ 48 mph / 4075 rpm, vacuum -20.8.

 

 

So...........................

At idle with throttle plate closed = -17.3 inHg

At 4400 RPM with throttle closed = -20.8 inHg (And this is the max vacuum that can be achieved without the car ever moving)

 

From 700 RPM to 4400 RPM it is only drawing another 3.5 inHg.

And from the start of inital decel to the start of "engine braking" (shifting down), the differance is .2 inHg.

 

Initally, when a lower gear is selected, there is a reduction in speed.

As rpm's rise there is no loss of speed. So is there any engine braking taking place at all?

 

I'm thinking more and more it is all gear reduction and normal friction of the rotating components that is providing the "braking".

There is combustion taking place and no more resistance due to higher vacuum then when you rev an engine up and let off the gas quickly.

 

Also taking into account that when decending a long step hill in a lower gear, the car will initally slow down, then start to build speed and rmp's again.

I read somewere that in this case the engine will be "driven" at the same speed and rpm's as it will "run" under acceleration in any given gear.

 

Is "Engine Braking" just and old wifes tail?

[porcupine73]

Sorry, I wasn't completely sure if we are talking exclusively about MT's here, or if AT's are included as well.

 

If AT's are included, depending somewhat on whether you have 4EAT phase I or phase II (not sure about 5EAT's), various one-way clutches are employed to give/not give engine braking in certain gears.

 

Maybe jake brakes could be fitted for serious engine braking!

[frag]

I did a little experiment last night along the lines WaWalker suggested.

 

1) First experiment (in a vacant parking lot): took the car to 30 Kmph in second gear and turned the ignition off just passing a selected lamp post, first with throttle closed and after that with gas pedal to the floor. The pass with throttle wide open gave me around a fifth to a quarter more distance than with the closed throttle. Not very conclusive...

2) I stopped the car at the top of a medium incline (street with no traffic near the parking lot), shut the ignition and let go of the brake. After initial acceleration the car got to a "cruising" speed of around 25 kmph. When I pushed the gas pedal to the floor (throttle wide open), I felt no clearly perceptible acceleration.

 

?????

[WAWalker]

While the site was down I had some time to do a little home work and a lot of thinking.

 

I was wrong. But I have a better understanding of the internal cumbustion engine!

 

Newton's Third Law of Motion

For every action there is an equal and opposite reaction.

 

So................................The engine can not slow the car down from any given speed any more efficently than it can accelerate the car to that same given speed.

 

So engine braking does not happen without the addition of another system to make it happen. Enter the "jake brake" on vehicles that have a real need for engine braking.

 

Have I lost my mind?

 

I'm thinking we could spend the next month performing running engine compression tests, crank case vacuum tests, try to find someone to do dyno tests...............................And we will never be sure what all is taking place inside these engines on decel. But if we did I think we would find that it all comes back to....................

 

The one thing that the engine has to over come to get the car up to speed. Friction.

[WAWalker]

With all that said.

 

I totally agree that there is energy lost due to the restriction of a closed throttle plate, but the energy losted due to the increased vacuum will be mostly regained somewere eles. Just as compression if any would be negated buy combustion.

 

If in fact. Due to the increase in vacuum and reduced air volume in the intake. The cylinder is not filling with enough air to produce compression.

And say that there is still a low pressure if not equal to the intake vacuum, once the valves are closed after the intake stroke. And this low pressure/vacuum is actually "pulling" the piston up on the compression stroke. That means that the increase of vacuum due to the throttle plate restriction, is in turn producing power on the compression stroke.

 

So is it a wash? Or am I still missing something.

[frag]

Wawalker,

A good discussion is one where all participants learn something they did'nt know before and it's also been my case. But like you I end up with more questions than answers.

The first part of my little experiment shows vaccuum plays a role in the engine braking effect but much less than I expected. The second one also is disappointing in that respect.

I aggree with you that friction probably plays a greater role than I initialy tought possible, and particularly in an AWD car (4 tires, 4 bearings, 1 engine, 1 trans, 1 transfer case, 4 half shafts, 2 differentials and let's not forget alt, ps, etc).

 

If I ever find or think of something new, I'll report back.

[WAWalker]

See "Smoke after engine braking" for the beginning of this discussion

 

 

 

Maybe not full atmospheric pressure but more pressure than in the cylinder.

True.

Did you ever suck on a PCV valve (new:) ) with you mouth?

I did and it closes when the vaccuum exceeds a certain amount.

 

Many.

 

PCV valve is a one-way valve, and will allow air to flow one way but not the other. Properly working they should always flow air from the crank case to the intake as long as the intake pressure is lower than the crank case pressure.

In the case of a back fire on a NA engine (Intake pressure becomes higher than crank case pressure) the valve will close to act a a "flame arrestor" to prevent ignition of the HC's inside the crank case. Or on a forced induction engine, the PCV will close under boost to prevent pressureizeing of the crank case.

 

Back to the oil burning.

 

Oil burning on prolonged decel or imediatly after. Is mostly leaking valve guide seals. The increase in vacuum that we all agree is present, is acting more on the intake value guides as they are directly in the area of lowest pressure (Intake system). They are exposed to the increased vacuum weather the valves are closed or open.

Were as the pistion rings are only exposed to this vacuum increase when the intake valves are open.

 

The pressure on the low side (crank case side) is the same for the valve guide seals as for the pistion rings.

 

Because the intake valves are exposed to the highest amount of vacuum when closed (under decel) this is when worn valve guides are going to leak the most oil. The oil is collecting behind a closed valve. When the valve opens only a small amount of this oil will make it into the cylinder due to the low volume and speed of the air/fuel entering the cylinder.

When acceleration is resumed, air/fuel volume and volocity increases, fuel injectors are spraying more fuel at the back of the valve washing the oil into the cylinder to the point that the oil burning becomes obvious from the smoke out the tail pipe.

 

Yes, some oil can be drawn past the rings under these conditions.

So because the rings are only exposed to the increased vacuum ~1/4 the length of time as the valve guides...................and once oil is sucked past the rings it will actually creat a better seal ( think wet compression test). Worn valve guides and seals are the most likly cause of excess oil buring on or right after prolonged decel, on an engine that has compression readings that are in spec.

 

Excessive oil burning all the time, or only under acceleration will point to poor oil control due to worn rings (rings not "scraping" oil from the cylinder walls. Or blow by (compression is probably not going to be good).

[WAWalker]

Wawalker,

A good discussion is one where all participants learn something they did'nt know before and it's also been my case. But like you I end up with more questions than answers.

The first part of my little experiment shows vaccuum plays a role in the engine braking effect but much less than I expected. The second one also is disappointing in that respect.

I aggree with you that friction probably plays a greater role than I initialy tought possible, and particularly in an AWD car (4 tires, 4 bearings, 1 engine, 1 trans, 1 transfer case, 4 half shafts, 2 differentials and let's not forget alt, ps, etc).

 

If I ever find or think of something new, I'll report back.

 

I agree. It takes engergy to create vaccuum. Aparrently just not a lot of energy. And in the contexts of the disscusion, and when compaired to all the other loads you mentioned, it becomes a smaller part of the equation that one might think.

 

I did find some equations to determine energy used to compress air but haven't run across any for vaccuum yet.

 

I guess there are worse things we could be thinking about. It's been fun.

 

Watch the EGR thread for more on the energy used to produce vaccuum, and it's effect on fuel milage. Might be a while, I will need time to do some more home work.

[Virrdog]

Interesting discussion.

 

One thing to keep in mind is that there is not one correct answer. All the things mentioned would add up to the car slowing down.

 

The car slows down in neutral, so there are already forces slowing it down there. But it slows down faster when downshifting. So this intensifies or possibly adds some other forces.

 

Friction - AWD system and engine bearings, etc. Higher engine revs would increase the engines internal friction. Energy is converted from spinning the engine.

 

Rotational inertia - Once again, only experienced in the engine with higher revs. AWD system components are going the same speed or slower through all of this. Energy is converted from accelerating the flywheel, crankshaft, pistons, accessories, oilpump, etc.

 

Increased Vacuum - Energy is definitely converted creating the vacuum. If any of that is released anywhere, I'll leave that to the smarter people.

 

So the drivetrain losses are constant and the only additional forces can come from inside the engine. So I'm pretty sure its correctly called 'engine braking'.

 

 

P.S. - I know all my terminology might not be textbook correct, but hopefully its clear.

[WAWalker]

Interesting discussion.

 

One thing to keep in mind is that there is not one correct answer. All the things mentioned would add up to the car slowing down.

 

Exactly

 

The car slows down in neutral, so there are already forces slowing it down there.

 

Yes. All the afeormentioned items that cause friction (energy loss due to heat). Power loss due to lack of cumbustion. And yes, throttle plate restriction.

It all resaults in normal engine deceleration. No one factor necessarily being significantly greater than the other to be able to define what is happening as compression or vaccuum braking. IMO.

 

But it slows down faster when downshifting. So this intensifies or possibly adds some other forces.

 

Friction - AWD system and engine bearings, etc. Higher engine revs would increase the engines internal friction. Energy is converted from spinning the engine.

 

Rotational inertia - Once again, only experienced in the engine with higher revs. AWD system components are going the same speed or slower through all of this. Energy is converted from accelerating the flywheel, crankshaft, pistons, accessories, oilpump, etc.

 

Increased Vacuum - Energy is definitely converted creating the vacuum. If any of that is released anywhere, I'll leave that to the smarter people.

 

So the drivetrain losses are constant and the only additional forces can come from inside the engine. So I'm pretty sure its correctly called 'engine braking'.

 

As there are rotating parts in the entire drive train, from the tires all the way through the transmission. There is rotational inertia in the drivetrain also created in the drivtrain also.

 

There will be some energy loss in the drive train (heat loss). Especially in the clutch on a MT, and in the clutches and torque converter in a AT.

So the higher the RPM the more energy lost in the drive train.

 

Ok, you got me again. If it wasn't for the engine is would take for ever to slow a car from 60 to X mph. It is the last link in the chain. So as far as defining the process goes............."Engine braking" works.

[WAWalker]

Oh, and just to clear the air, so we can all still be friendly.

 

None of this is personal.

 

What pushed my button and got me into this discussion was a referance (actually two) from Wikipedia. Both defining "engine braking" but one said one thing the other said something else.

 

Basicly the way I read it was...........

 

They were saying that "compression braking/engine braking" in desiel trucks was grossly inefficient, and that gasoline engines had an advantage due to the throttle plate restriction (increased vaccuum).

 

I still don't know what is really happening inside of the cylinder when "engine braking".

 

But, I do know that when towing my camp trailer behind my (throttled) gasoline powered pickup, "engine braking" what ever it may be, is grossly inefficient when decending a long step hill. Untill you get into a very low gear at a very low speed.

It is no different in a loaded 18 wheeler, when not using the "jake brake".

This is why you pass so many semi's comeing down mountain passes just creaping along with the hazard lights flashing.

 

So I thought that to totally dismiss the idea of "compression braking" as it is so inefficient, and say that "vaccuum braking" was better was streching it.

 

I still feel that way to a point. But will always be curious as to what is really going on inside the cylinders.

 

"Engine braking" either way is grossly inefficient in slowing the weight of a vehicle going down a long step hill. Untill you reach a very low speed in a very low gear. Even in a car.

[Virrdog]

As there are rotating parts in the entire drive train, from the tires all the way through the transmission. There is rotational inertia in the drivetrain also created in the drivtrain also.

 

There will be some energy loss in the drive train (heat loss). Especially in the clutch on a MT, and in the clutches and torque converter in a AT.

So the higher the RPM the more energy lost in the drive train.

 

Ok, I did forget a couple of things... everything in the transmission up to but not including the output shaft does spin faster... more losses there. But everything from the output shaft through the tires only has frictional losses. Their momentum alone (without the friction) tries to keep the car going the same speed. So those pieces only hinder the work of the engine braking. Case in point: a carbon fiber one-piece driveshaft makes engine braking more effective. Why? Less rotating mass for the engine through inputshaft pieces to slow down.

 

 

Ok, you got me again. If it wasn't for the engine is would take for ever to slow a car from 60 to X mph. It is the last link in the chain. So as far as defining the process goes............."Engine braking" works.

Yeah, as is mentioned in the post after yours - it's not a huge force, but does help bring speeds down where friction and the heat capacitance of brakes can keep things under control. It's not proportional to the weight of the vehicle (its more or less fixed in a range created by engine design and gearing) so the more mass the vehicle possesses, then the less effective the engine braking.

[electryc_monk]

I really liked reading this thread. Stimulating dialogue!

 

I have one tiny tidbit to suggest to your last post about diesel engine brakes being ineffcient.

 

I will simply disagree on the innefficentcy word useage. Reasoning? I used to be an OTR driver-trainer and the "Jake brake" was actually scarily efficent in some cases.

 

I have an exhaust brake on the F350 7.3L PSD out in the driveway and let me tell you that it works -granted in a smaller scale- nearly as well as the setups in the KW's, Freightshakers, Peterbilts, "Nationals, Volvo's, and MAcks - etal.

 

The big truck jake's (tangent) that I've worked with have adjustable settings:

on a 6 cyl. engine you can have 3 settings; 2 cyl, 4 cyl, and all cyl braking.

the owners manuals warn against loss of vehicle control if using the jake's without a trailer.

jake's are even a backup sleep warning for a few drivers out there.... LOL put the jake on max and if their foot falls off the throttle pedal the jakes go off and wakes them up to pull over. LOL

 

the unit on the F350 is actually a vacuum actuated flapper valve that closes off the exhaust at the rear of the turbo.

 

so this is what .... a nickels worth of free advice? (tilts head to the side and smirks)

[WAWalker]

I was saying................"Engine braking" weather gas or diesel, is inefficient. Thus the need for an additional system to provide efficient "engine braking", enter the "Jake Brake", exhaust brakes etc.

[keltik]

I think this discussion is very intriguing. Ill agree that the major part of engine braking is due to the mechanical advantage in lower gears giving the engines rotating parts more of a drag on the car. I can also say that whether the throttle is open or closed makes little difference.

 

From what ive learned on my daily commute (long straight downhill), my Holden and all of my Subarus will shut off the injectors (or make some sort of adjustment) after a couple seconds of 'engine braking'. If the idle switch is activated and the engine rpm is above 1800 - there will be a change in exhaust note and an increase in braking until the rpm drops below 1800.

 

When descending long grades such as the Kaimai mountain pass - ill knock the a/c on. It makes one helluva difference. Even switching the main beam lights on slows me down quicker.

 

IMHO its the engines accessories and cams that give the greatest braking effect, rather than compression or vacuum effects.