Heat Pump on Exhaust

[t vago]

Originally Posted by pgfpro

We ran the stock ecu, map, and injectors for a couple weeks before we installed Crome. So I didn't have any data logs of running it N/A.

Hm... Perhaps a exhaust blocking experiment is in order, then. Gotta get my A-B MPGuino code working first!

[jeff88]

Originally Posted by IamIan

I do not follow why you are referencing a nozzle that is not used in the application being discussed ( Automotive Exhaust ) ?

For all intents and purposes, would a valve which creates a restriction still have the same characteristics of the flow of a gas (exhaust) as a nozzle?

I'm wondering if the de Laval nozzle is the same theory of putting your thumb over a hose to make a more powerful water stream?

Originally Posted by t vago

Hm... Perhaps a exhaust blocking experiment is in order, then. Gotta get my A-B MPGuino code working first!

Can you expand your thought process a little more. Putting a turbo in and just blocking the exhaust will have two very different results, so what are planning on doing? I do like to see experiments!

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This is merely tangential to my original thought process of this thread, but I'm wondering if somebody were to put this de Laval nozzle just before the exhaust turbine of a turbo, would the efficiency of the turbo improve. (I know this creates more back pressure on the engine, I'm only worried about the turbo at this stage.)

Also, if somebody were to put one in front of the turbine and/or on the compressor side (or anywhere on the intake side), if that would increase the PSI of the charged air. Would it increase turbo efficiency for having one or both sides of the turbo "choked"?

[Occasionally6]

Originally Posted by IamIan

#2> Chocked subsonic is not necessarily = to chocked sonic is not necessarily = to chocked supersonic ... the way you wrote it by just putting an = sign ... assumes all three are = ... that is not necessarily the case... there can be chocked flow in each of the 3 cases ... but that does not mean all three are =... or that they all have the same choked flow.

By definition, if the velocity in the venturi is sonic, the flow is choked.

By definition, if the flow through the venturi is subsonic, it is not choked.

Choked flow is just that; choked flow, with the mass flow rate constant (for a particular upstream pressure and orifice size).

Further along the duct the velocity of the gas can exceed the speed of sound (supersonic) if it is allowed to expand. The flow in the venturi is still at Mach 1 and the mass flow rate is the same whether or not the later expansion occurs i.e. choked flow means the mass flow rate is fixed for simple sonic or supersonic flow.

And what?


I do not follow why you are referencing a nozzle that is not used in the application being discussed ( Automotive Exhaust ) ?

Where there is an orifice in choked flow and an expansion further along the duct, there is the possibility of the de Laval type flow occurring.

[t vago]

Originally Posted by jeff88

Can you expand your thought process a little more. Putting a turbo in and just blocking the exhaust will have two very different results, so what are planning on doing? I do like to see experiments!

It's not so much "blocking" the exhaust; rather, it's the idea that placing a restriction in the exhaust may:

• (of course) deliberately raise backpressure, which would
• cause the EGR system to suck in more exhaust gas, which would
• (this is the key here) cause intake manifold vacuum to lower, which would
• cause an increase in fuel economy.

This would only work, of course, at part-throttle. Peak engine power would of course suffer as a result.

This is the idea that I am gaining from pgfpro's post above, where he saw a 10% increase in fuel economy at part-throttle, after installing that eBay kit on his son's Civic.

[Occasionally6]

Originally Posted by jeff88

For all intents and purposes, would a valve which creates a restriction still have the same characteristics of the flow of a gas (exhaust) as a nozzle?

It's possible.

I'm wondering if the de Laval nozzle is the same theory of putting your thumb over a hose to make a more powerful water stream?

No, because it requires compressible flow with sonic and supersonic velocities, something not occurring with water and at the flow rate from a tap.

With the thumb over the hose, the velocity of the water does increase but it's a long way from becoming sonic.

[Occasionally6]

Originally Posted by t vago

It's not so much "blocking" the exhaust; rather, it's the idea that placing a restriction in the exhaust may:

• (of course) deliberately raise backpressure, which would
• cause the EGR system to suck in more exhaust gas, which would
• (this is the key here) cause intake manifold vacuum to lower, which would
• cause an increase in fuel economy.

This would only work, of course, at part-throttle. Peak engine power would of course suffer as a result.

This is the idea that I am gaining from pgfpro's post above, where he saw a 10% increase in fuel economy at part-throttle, after installing that eBay kit on his son's Civic.

I think that's one reasonable explanation.

There's a lot of variables to know just what is going on. Change in air:fuel mixtures? Better flowing exhaust downstream of the turbine? Being able to hold higher gears? Changes in inlet manifold pressure and/or EGR rates?

[jeff88]

Originally Posted by t vago

This would only work, of course, at part-throttle. Peak engine power would of course suffer as a result.

So what you are saying is that with a higher back pressure, the throttle plate opens up a little more, thereby lowering manifold vacuum? I honestly don't know how the EGR specifically effects the intake. Does the MAF sensor recognize a smaller amount of air (because of the increase in EGR), so it opens the throttle more to allow more air in?

I would love to see this in action and see the associated FE improvements. I wonder what the comparable numbers would be compared to a turbo. He gained 10%, I wonder if just a pure restriction like you talk about would have a higher or lower increase in FE.

[jeff88]

Originally Posted by Occasionally6

No, because it requires compressible flow with sonic and supersonic velocities, something not occurring with water and at the flow rate from a tap.

With the thumb over the hose, the velocity of the water does increase but it's a long way from becoming sonic.

I was just talking about the principle. If it came anywhere near sonic I would imagine I wouldn't have this anymore!

[Occasionally6]

Originally Posted by jeff88

So what you are saying is that with a higher back pressure, the throttle plate opens up a little more, thereby lowering manifold vacuum?

If there is some extra resistance in the exhaust stroke, then extra throttle (and fuel by the way) is required to overcome it. The extra throttle reduces the work done on the inlet stroke so there's not a one-one fuel consumption penalty to increased exhaust manifold pressure.

I doubt that's the whole reason for any fuel economy benefit for just adding a turbo to an engine but might be part of an explanation as to why there was one.

I honestly don't know how the EGR specifically effects the intake. Does the MAF sensor recognize a smaller amount of air (because of the increase in EGR), so it opens the throttle more to allow more air in?

If you hold the throttle opening constant and introduce EGR (any inert gas) into the inlet manifold, the inlet manifold pressure is raised and the pumping work is reduced. The same quantity of fuel and air is consumed but there is less energy wasted in working against the vacuum (inlet man. press.) on the inlet stroke.

For a given power requirement at the crank, less air and fuel is needed, so the throttle can actually be closed more with EGR.

[t vago]

Originally Posted by jeff88

So what you are saying is that with a higher back pressure, the throttle plate opens up a little more, thereby lowering manifold vacuum?

That may happen, but the lower intake manifold vacuum would primarily result from having a larger volume fraction of inert gas (exhaust) in the intake manifold.

For a gasoline engine, the main purpose of the intake manifold and throttle is to meter the amount of oxygen that goes into each cylinder. The mechanism by which this is accomplished is by creating a volume whose pressure is lower than ambient, so as to lower the amount of oxygen in the intake manifold that is compared to ambient. This takes engine work (therefore, fuel) that would otherwise be spent in pushing the vehicle forward. A higher intake manifold vacuum requires more work, and more fuel, to maintain, than does a lower intake manifold vacuum. Similarly, given the same intake manifold vacuum, it takes more work to maintain that vacuum in a large volume intake manifold, than in a small volume intake manifold.

If oxygen could be metered into a given engine without having to vary intake manifold vacuum, such as by increasing the percentage of inert gas (such as exhaust gas) being sucked into the engine, then that engine would have to work less to maintain an intake manifold vacuum. Such an engine would consume less fuel than a similar engine that did meter oxygen via the use of an intake manifold vacuum. Production gasoline engines do exist that do not depend on a intake manifold vacuum - they instead use variable valve lift/duration to meter the amount of oxygen into each cylinder.

Diesel engines, not needing to meter oxygen to begin with, do not need to have a power-robbing intake manifold vacuum. Placing a pure exhaust restriction on a Diesel powerplant will just lower the efficiency of that powerplant.

Originally Posted by jeff88

I would love to see this in action and see the associated FE improvements. I wonder what the comparable numbers would be compared to a turbo. He gained 10%, I wonder if just a pure restriction like you talk about would have a higher or lower increase in FE.

Heh - my EGR experiment that I did earlier this year took me into the realm of MPGuino. Soon, hopefully, I can go back into EGR.