Heat Pump on Exhaust

[Occasionally6]

Maybe start with this one:

Internal Combustion Engines - Google Books

It's this book, from page 169:

Internal Combustion Engines (Combustion Treatise Series): Constantine Arcoumanis: 9780120597901: Amazon.com: Books

You can search for more. There's no magic in it.

[Occasionally6]

Originally Posted by jeff88

You're engine uses the heat of the exhaust to some benefit. That is what I was theoretically trying to do with the heat pump idea, albeit in a different form. I was really hoping to just put a turbo on it and instead of compressing air for combustion, it compressed it for heating & cooling. Some of that is done via the heater core, but certainly not the a/c.

Thinking some more:

Maybe intercooling the compressed air first and then allowing it to expand for the cooling? Might work. This is done in passenger aircraft where there is a convenient source of high pressure air available in the form of bleeding some air from the jet engine compressors.

[jeff88]

I think you're exactly describing what I was thinking. The only difference in this system is that the compression of the coolant comes from the turbo, rather than a belt driven compressor. You would still need an intercooler (or condenser) and an evaporator inside the cabin.

(I think I originally described it as compressing the air for cooling, when I should have said the compression of coolant)

[IamIan]

Originally Posted by Occasionally6

Maybe start with this one:

You can search for more. There's no magic in it.

Thanks for the reference.

I didn't think there was "Magic".

But ... I am still of two minds about this.

- - - - - -

#1> It does not change the results of my original claim you posted this in line of reasoning in response to.

Originally Posted by Occasionally6

It's not as bad as it's made out to be. If the flow across an exhaust valve is supersonic, the flow is choked. If that is so, there is scope to increase the pressure in the exhaust manifold without having any effect on the mass flow rate across the exhaust valve at all, at least for (the early) part of the exhaust stroke.

That increase in pressure you write about is the same back pressure I wrote about ... and on its own ... just taking that away ... still results in a decrease in ICE efficiency.

- - - - -

#2> I think your reference is not actually claiming this ( supersonic flow ) you described , is actually happening in real engines.

I think the passage from your links you are referring to is:

When the exhaust valve first starts to open, cylinder pressure is high, hence the gas can theoretically expand through a large pressure ratio, down to atmospheric pressure. Thus the Energy available to the turbine can be high. However, the exhaust valve flow passage at that instant of low valave lift is very restrictive and it is likely to be choked ( sonic flow at the valve throat).

They claim "Sonic" not Supersonic ... they aren't the same thing.

Bold Added

The author claims this is only "likely" to happen ... and even that 'likely' is only in the theoretical case described.

But we know in the real world ... some of the conditions of that theoretical case don't follow.

For example the low pressure at that point is NOT atmospheric pressure ... as he assumed in his theoretical case ... there is back pressure in the exhaust system ... with a lower pressure difference there will be a slower speed... the gasses also have momentum , etc ... the real world is not that theoretical case.

[Occasionally6]

Originally Posted by jeff88

I think you're exactly describing what I was thinking. The only difference in this system is that the compression of the coolant comes from the turbo, rather than a belt driven compressor. You would still need an intercooler (or condenser) and an evaporator inside the cabin.

(I think I originally described it as compressing the air for cooling, when I should have said the compression of coolant)

If you only compress air you don't need an evaporator, just an orifice through which the air can expand. That's a bit different to a conventional car (or building) A/C system which uses the change of state in a refrigerant to absorb and reject heat energy, and is a closed system.

A refrigerant based system can produce a cooling work (energy flow) that is a multiple (maybe 3-4x) of the compressor work. The air based system is only at 1:1 (ideally) but would be simpler to implement.

You do need the intercooler to bring the compressed air back to (near) ambient temperature and so to allow cooling below ambient when the air is expanded. If you search for "air standard refrigeration" you should find some more info.

I've mentioned it in another thread but if you've ever watched an F1 qualifying session you will have seen the drivers being cooled in their cars with air ejectors. The compressed air is released through a nozzle in a duct, which both allows the expansion cooling and entrains (extra) ambient air to provide the air flow.

To play around with the idea, a portable tank for holding compressed air is pretty inexpensive, as is a very basic 12V air pump (not wasted if it doesn't work as you can still use it to maintain tire pressures).

[Occasionally6]

Originally Posted by IamIan

That increase in pressure you write about is the same back pressure I wrote about ... and on its own ... just taking that away ... still results in a decrease in ICE efficiency.

It does cover it. There is reference to more equal pressures across the exhaust valve reducing the time spent in, and the effect of, the choked flow on turbine and ICE efficiency.

Where back pressure (or the lack of it) is important is in the part on the exhaust stroke during which choked flow is not occurring.

#2> I think your reference is not actually claiming this ( supersonic flow ) you described , is actually happening in real engines.

It is. It is a theoretical discussion that is non-specific because whether or not there is choked flow, and for how much of the engine cycle, depends on the particular engine and on the load it is under i.e. the actual mass flow which is occurring, the gas conditions, and the size of the hole(s) the gas is flowing through.

They claim "Sonic" not Supersonic ... they aren't the same thing.

Yeah, same thing. If you want to be pedantic, sonic is where the flow velocity exactly matches the local speed of sound, supersonic is where the flow velocity exceeds that speed of sound.

Whether it matches or exceeds the speed of sound doesn't alter the mass flow through the valve (at a particular cross sectional area). That is fixed for any given upstream pressure while sonic/supersonic flow is occurring.

[IamIan]

Originally Posted by Occasionally6

It does cover it. There is reference to more equal pressures across the exhaust valve reducing the time spent in, and the effect of, the choked flow on turbine and ICE efficiency.

Where back pressure (or the lack of it) is important is in the part on the exhaust stroke during which choked flow is not occurring.

I think your making it harder/more complicated than it needs to be.

The Back Pressure caused by a turbine in the exhaust WILL itself reduce the ICE efficiency ... which is the point I made.

The Turbine and back pressure it produces , does not magically come and go from the exhaust millisecond by millisecond ... looking at tiny fractions of a second events during the whole process is not going to change the overall negative effect on ICE efficiency that back pressure causes... the ICE efficiency is still reduced by the inclusion of the Turbine in the Exhaust.

Originally Posted by Occasionally6

It is. It is a theoretical discussion that is non-specific because whether or not there is choked flow, and for how much of the engine cycle, depends on the particular engine and on the load it is under i.e. the actual mass flow which is occurring, the gas conditions, and the size of the hole(s) the gas is flowing through.

Bold Added.

Thanks for agreeing with the point I made.

Like having a theoretical discussion about FTL or warp drives... or other fictional devices and or situations that are not the real world.

And the reference did not claim supersonic ... so it is not making that claim.

Originally Posted by Occasionally6

Yeah, same thing. If you want to be pedantic, sonic is where the flow velocity exactly matches the local speed of sound, supersonic is where the flow velocity exceeds that speed of sound.

I'm glad you seem to now agree ... that it is NOT supersonic.

Can we also agree that the conditions of that theoretical case of 'sonic' flow are very unlikely to ever actually happen in a real world vehicle ?

Meaning for one , that immediately after the combustion chamber it will NOT be atmospheric pressure ... as the theoretical assumes ... that a real world exhaust system during operation will have pressure in it higher than atmospheric pressure.

And even if some experimental , non-road going , ICE did see this instant drop to atmospheric pressure through the entire exhaust system ... by itself is also still not be enough to = sonic flow.

Originally Posted by Occasionally6

Whether it matches or exceeds the speed of sound doesn't alter the mass flow through the valve (at a particular cross sectional area). That is fixed for any given upstream pressure while sonic/supersonic flow is occurring.

Supersonic won't happen... no references to-date showing supersonic.

The conditions listed for the theoretical sonic flow ... are conditions that won't happen ... for real world operating vehicle engines... it is only theoretical ... like talking about FTL or warp drive.

Actually if it exceeds it ... ie goes supersonic ... it will GREATLY effect the pressure ... you will have caused a sonic boom ... and a very different kind of non-uniform pressure front ... which will itself effect the mass flow rate ... even if the upstream pressure is the same for a non-supersonic event ... it will not behave the same.

Although this aspect still seems to me to be making it overly complicated than it needs to be ... whatever the mass flow rate ... when you add a turbine to the exhaust it will reduce the efficiency of that ICE ... no mass flow rate changes that.

[jeff88]

Originally Posted by Occasionally6

If you only compress air you don't need an evaporator, just an orifice through which the air can expand. That's a bit different to a conventional car (or building) A/C system which uses the change of state in a refrigerant to absorb and reject heat energy, and is a closed system.

A refrigerant based system can produce a cooling work (energy flow) that is a multiple (maybe 3-4x) of the compressor work. The air based system is only at 1:1 (ideally) but would be simpler to implement.

You do need the intercooler to bring the compressed air back to (near) ambient temperature and so to allow cooling below ambient when the air is expanded. If you search for "air standard refrigeration" you should find some more info.

I've mentioned it in another thread but if you've ever watched an F1 qualifying session you will have seen the drivers being cooled in their cars with air ejectors. The compressed air is released through a nozzle in a duct, which both allows the expansion cooling and entrains (extra) ambient air to provide the air flow.

To play around with the idea, a portable tank for holding compressed air is pretty inexpensive, as is a very basic 12V air pump (not wasted if it doesn't work as you can still use it to maintain tire pressures).

If it is simpler, with less items to make it work, then I would imagine it *could* be cheaper for the auto companies to use an air-to-air system.

It makes sense to keep the intercooler/radiator as the heat picked up would still need to be released (or radiated) somewhere else, but with less obstacles within the system (e.g. evaporator).

So the big question then is if somebody was to get into otherwise identical cars, save for the a/c system, would they notice a difference? In other words, can an occupant be comfortable in an air-to-air system without noticing any negative effects as compared to a refrigerant system?

Also important is the efficiency of the system. Assuming we use a turbo to compress the air, would it be any more efficient than a standard refrigerant style a/c? That may be a hypothesis that needs to be determined through trial and error experimentation. If it doesn't improve engine efficiency (in any aspect: FE, power, etc.), then what would be the point?

I wonder if there is something other than air that the system could use that would be more efficient/have better cooling capacity. What about CO2? Or to expand (no pun intended) on that thought, what about exhaust gases (CO2, NOx, etc.)? What about pure oxygen?

I have not heard of the F1 style coolers, very interesting concept though. I would imagine it is similar to the compressed air in electronics dust cleaners, except the cold is transmitted out rather than at the nozzle/can itself. The idea does remind me of the water spray bottles with a fan attached that you see at amusement parks. I wonder if the concept can be used effectively in a duct and vent system that a car utilizes?

The experiment concept is pretty interesting and I already have a 12V tire air compressor for another project that didn't pan out, but as you know this experiment is going to go way down on the totem pole!

[Occasionally6]

Originally Posted by IamIan

The Back Pressure caused by a turbine in the exhaust WILL itself reduce the ICE efficiency ... which is the point I made.

To what degree i.e. does it matter if the loss is small?

The Turbine and back pressure it produces , does not magically come and go from the exhaust millisecond by millisecond ... looking at tiny fractions of a second events during the whole process is not going to change the overall negative effect on ICE efficiency that back pressure causes... the ICE efficiency is still reduced by the inclusion of the Turbine in the Exhaust.

The pressure in the manifold does vary throughout the exhaust stroke. Later in that reference is a discussion of how the turbine efficiency varies with the exhaust pressure pulse(s).

And the reference did not claim supersonic ... so it is not making that claim.

"...the exhaust valve flow passage at that instant of low valve lift is very restrictive and it is likely to be choked (sonic flow at the valve throat)."

"Later in the exhaust process the valve area is larger and the pressure ratio is lower, hence flow becomes subsonic..."

That's pretty clear.

Meaning for one , that immediately after the combustion chamber it will NOT be atmospheric pressure ... as the theoretical assumes ... that a real world exhaust system during operation will have pressure in it higher than atmospheric pressure.

As described in that reference, the exhaust manifold pressure will vary throughout the exhaust process. When there is high pressure in the exhaust manifold there is also high pressure within the cylinder. Flow can still be choked with (relatively) high exhaust manifold pressure.

Actually if it exceeds it ... ie goes supersonic ... it will GREATLY effect the pressure ... you will have caused a sonic boom ... and a very different kind of non-uniform pressure front ... which will itself effect the mass flow rate ... even if the upstream pressure is the same for a non-supersonic event ... it will not behave the same.

The fundamental principle is that under conditions of compressible flow/sonic (or supersonic) flow the downstream pressure has no affect on the mass flow through an orifice.

[Occasionally6]

Originally Posted by jeff88

If it is simpler, with less items to make it work, then I would imagine it *could* be cheaper for the auto companies to use an air-to-air system.

That they don't is probably a clue. If it's cheap to play though, why not?

So the big question then is if somebody was to get into otherwise identical cars, save for the a/c system, would they notice a difference? In other words, can an occupant be comfortable in an air-to-air system without noticing any negative effects as compared to a refrigerant system?

You can size the A/C system how you want. Is there enough average power available from a turbine to make it workable? Possibly. The peak power from a turbine is plenty, maybe 10's of kW even from a 1.5-2.0l engine. A refrigerant based A/C compressor might take 1-2kW to drive it.

Also important is the efficiency of the system. Assuming we use a turbo to compress the air, would it be any more efficient than a standard refrigerant style a/c? That may be a hypothesis that needs to be determined through trial and error experimentation. If it doesn't improve engine efficiency (in any aspect: FE, power, etc.), then what would be the point?

Because there are unrecoverable losses across an exhaust valve it would be better to use the Atkinson expansion to extract some of the energy left in the exhaust gas and take the compressor work off the crankshaft, if you can accept the effective loss of engine size (power) or increased weight for a nominally larger engine. Or only implement Atkinson with low power demand.

I wonder if there is something other than air that the system could use that would be more efficient/have better cooling capacity. What about CO2? Or to expand (no pun intended) on that thought, what about exhaust gases (CO2, NOx, etc.)? What about pure oxygen?

CO2 is already being used as a low GHG potential replacement for R134a where a price is being placed on GHG's. It was also used as a refrigerant prior to the development of the CFC and HCFC refrigerants.

It isn't perfect because it has a critical temperature of ~31C which means it can be difficult (or impossible) to get it cold enough to condense.

O2 has some safety issues which would preclude it's use. I am pretty sure that it has to be under quite high pressure to condense when around ambient temp. too. N2 is similar.

I have not heard of the F1 style coolers, very interesting concept though. I would imagine it is similar to the compressed air in electronics dust cleaners, except the cold is transmitted out rather than at the nozzle/can itself. The idea does remind me of the water spray bottles with a fan attached that you see at amusement parks. I wonder if the concept can be used effectively in a duct and vent system that a car utilizes?

With a supply of compressed air I can't see any reason why not. I gather there is a bit of an art to designing an air ejector though.

Actually, back in the days of steam trains, air ejectors using steam to provide the entrainment were used to provide air conditioning for the cars.

The experiment concept is pretty interesting and I already have a 12V tire air compressor for another project that didn't pan out, but as you know this experiment is going to go way down on the totem pole!

At it's most basic, maybe some PVC tube, shaped to form a venturi, and a ball valve to control the flow of compressed air into a small diameter tube within it?