Pumping loss test

[roflwaffle]

Pumping losses are only a significant issue when the engine is making power AFAIK. I've asked, and been told there is no significant difference when coasting in gear/engine off between throttle open and closed FWIW.

Originally Posted by tjts1

The simple solution is to apply manifold vacuum to the crankcase.

Wouldn't that do funky things to the oil pressure?

[tjts1]

Not really. The oil pump functions are several atmospheres are and can more than the vacuum in the crank case. A lot of race car use this trick. But you will have to make sure that all the engine seals can hold up to the abuse.
http://www.dragstuff.com/techarticles/vacuum-pumps.html

[dcb]

Originally Posted by JohnnyGrey

I wanted to test it out but the roads are a bit busy around here.

You are telling us that you cannot perform your own experiment because there is NO place to coast to a stop from 30 anywhere around you? with engine braking even? That is remarkable in itself


P.S. PLEASE don't EOC in gear if you have a carburetor, yall.

[JohnnyGrey]

You are telling us that you cannot perform your own experiment because there is NO place to coast to a stop from 30 anywhere around you? with engine braking even? That is remarkable in itself


P.S. PLEASE don't EOC in gear if you have a carburetor, yall.

Oh, I did, but the streets weren't empty enough to really take any meaningful measurements.

Pumping losses are only a significant issue when the engine is making power AFAIK. I've asked, and been told there is no significant difference when coasting in gear/engine off between throttle open and closed FWIW.

So for engine-off, closed throttle:

1. - The intake stroke works against the vacuum
2. + The vacuum returns the power on the compression stroke.
3. - Vacuum works against the the combustion stroke.
4. 0 Exhaust valve opens when chamber contents are about equal to what the manifold pressure was, air rushes in through the exhaust valve and gets pumped back out on the exhaust stroke.

Net effect: -1

For engine-off WOT:

1. 0 Intake stroke freely sucks in air.
2. - Compression stroke compresses the air.
3. + Combustion stroke, pushed down by compressed air.
4. 0 Exhaust stroke, atmospheric air is evacuated.

Net effect: 0

[JohnnyGrey]

Racers use vacuum on their crankcases not to decrease pumping losses, but to cut down on the amount of air that must be moved between the backs of the pistons. For a big-block V8 at 6K rpm, this is significant. For us, it's not.

Pumping losses come from the pressure differential between the intake and exhaust. With a shut throttle, your engine is basically an air compressor, compressing the ~2psia intake charge to the 14.7psia atmosphere. With an open throttle, it's simply moving air. There's no net compression going on.

[LostCause]

By comparing the two scenarios you posted, you would only be able to determine pumping losses caused by the throttle body...still useful, but I'm not sure what your motives are. You already know that the engine braking you are describing can be removed via the clutch, so what is to be gained beyond a numerical value?

How are you going to determine the actual pumping loss force? The only thing I can come up with is a force diagram that plots and compares all the forces acting on the vehicle between the different scenarios. Whatever your method, feel free to post how you determined your value. I wouldn't mind brushing up on my physics/logic.

Originally Posted by JohnnyGrey

Racers use vacuum on their crankcases not to decrease pumping losses, but to cut down on the amount of air that must be moved between the backs of the pistons. For a big-block V8 at 6K rpm, this is significant. For us, it's not.

Interesting. So the vacuum was used to lower the mass of air that needed to be accelerated/deccelerated during a stroke? I'm assuming this is the equivalent of lowering the reciprocating mass of the engine...

Was vacuum maintained constantly by a pump or did it hold for a significant period of time? The energy to drive a pump would almost certainly kill the benefit in a fuel economy application, but substituting helium (or another light, inert gas) for the vacuum seems doable. I wonder how quickly the gas would be contaminated or lost, though...

- LostCause

[roflwaffle]

Originally Posted by JohnnyGrey

Oh, I did, but the streets weren't empty enough to really take any meaningful measurements.



So for engine-off, closed throttle:

1. - The intake stroke works against the vacuum
2. + The vacuum returns the power on the compression stroke.
3. - Vacuum works against the the combustion stroke.
4. 0 Exhaust valve opens when chamber contents are about equal to what the manifold pressure was, air rushes in through the exhaust valve and gets pumped back out on the exhaust stroke.

Net effect: -1

It isn't that simple AFAIK. Depending on how your car's IAC works (totally open or closed or somewhere in between), as well as the engine's CR, and difference in combustion temperature to temperature when the engine's off, and consequently the moles of air retained in the cylinder after the exhaust valve closes, you'll end up with some difference in pressure between vacuum and atmospheric. It isn't quite -1.

[Andyman]

I've been working on some formulas related to fuel economy in cars. One of them is for pumping loss. Here it is.

HP=CID*RPM*P/792000

CID: cubic inch displacement
HP: horsepower
RPM: engine revolutions per minute
P: pressure difference between exhaust and intake in psi

The 792000 comes from these numbers:
2 rev/cycle * 60 sec/min * 12 in/ft * 550 (ft*lb/sec)/hp

A Mercury with a 429 cubic inch engine goes 60 MPH at 2200 RPM with the exhaust pressure 9 psi higher than the intake manifold pressure. What is the pumping loss?

HP=429 * 2200 * 9 / 792000
HP=10.725 horsepower (pumping loss)

How about an Accord with a 2.0 L engine running at 2500 RPM with an intake pressure of 7 psi and an exhaust pressure of 15 psi?

HP=CID*RPM*P/792000

CID=61*2.0=122 cubic inches
RPM=2500 rev/min
P=15-7=8 psi

HP=122*2500*8/792000
HP=3.08 horsepower wasted to pump air

One of the most effective ways to reduce pumping loss is to deactivate some cylinders when the load is low. This is done in some Chevrolet, Chrysler and Honda vehicles. This reduces both the displacement and the intake vacuum. I tried that in my Accord for a while by removing half of the rocker arms. The engine braking was much less. If I could turn off all the cylinders (by closing all the valves) when decellerating, the engine would turn very easily and use no gasoline while descending hills.

I put back the rocker arms after a few hundred miles because the extra vibration was destroying my exhaust system.

[Gregte]

Originally Posted by JohnnyGrey

Oh, I did, but the streets weren't empty enough to really take any meaningful measurements.



So for engine-off, closed throttle:

1. - The intake stroke works against the vacuum
2. + The vacuum returns the power on the compression stroke.
3. - Vacuum works against the the combustion stroke.
4. 0 Exhaust valve opens when chamber contents are about equal to what the manifold pressure was, air rushes in through the exhaust valve and gets pumped back out on the exhaust stroke.

Net effect: -1

For engine-off WOT:

1. 0 Intake stroke freely sucks in air.
2. - Compression stroke compresses the air.
3. + Combustion stroke, pushed down by compressed air.
4. 0 Exhaust stroke, atmospheric air is evacuated.

Net effect: 0

Consider this: If you have a cylinder and piston with no intake or exhaust valves there would be no gain or loss in pumping the piston, other than heat generated and the dissippated away, and metal to metal friction.

It you then put a small hole in the cylinder head there would be a lot of loss as air is drawn into the cylinder and then pushed out of it. In fact, this is what is often used on some racing 2-stroke motorcycles for an engine brake. A lever on the handlebar is pushed which opens a single valve in the cylinder head causing air to rush into and then back out of the cylinder with every piston stroke.

Therefore my intuitive guess regarding your original experiment is that with engine OFF, car coasting, at WOT, the motor has to pull a maximum amount of air into the cylinder (intake stroke) and then ultimately push it back out of the cylinder (exhaust stroke), regardless of the two self canceling strokes that occured in between. This should cause engine braking.

With engine OFF, car coasting, closed throttle, the piston will pull a small amount of air into the cylinder through the exhaust valve when it opens but it wont even fill the cylinder with air because the piston will already be starting up to push it out just as quickly as it started to come in. This should cause a smaller amount of engine braking than the scenario above.

Still, I am eager to hear of yours or anyone else's experience doing the experiment. I may do it myself if I can get a good opportunity.

[metroschultz]

Me too.
I am currently trapped in Ok.
I will return home in two more weeks.
I am intrigued by this test request.
I have several long strips of road that are level.
One will probly be empty of traffic @ 2-3 am
Will post results in three weeks.
S.