Heat Pump on Exhaust

[euromodder]

Heat pump systems are complex too.

I kinda like VW's approach:
Run coolant through the exhaust manifold to quickly heat up the coolant and get the engine up to temp.


I've never had a car that warms up this quickly.
I'll have to check in winter, but it certainly looks promising !
Coolant is now up to normal temp in about 1 km.
When hypermiling Hägar the diesel, it wouldn't get up to normal temp before hitting the motorway, after 9 km.
Being a CNG engine, the coolant is also used to heat the gas evaporator.

Then again, a CNG evaporator could be the cold side of a heat pump

[jeff88]

I actually like the idea of running coolant around the exhaust to heat up the engine. I'll have to look into this, I might try it one day, once all my other plans actually come to fruition. Is there a thermostat so that it doesn't continue to heat the engine after it gets to operating temperature?

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.

VW does some of what I was thinking, but in a much simpler form. Thanks for the input and yet another idea to add to the list!

[aerohead]

In Israel,they have used solar ponds to super-heat a low boiling point refrigerant to power a gas turbo-generator to produce electricity.
The exhaust heat might provide for such a process.Coolant too.

[IamIan]

Originally Posted by jeff88

Maybe this is another one of those give power back here, but take power from there and thus it's a net unchange or worse net loss. These are just a few ideas, I haven't completely refined my thoughts, but I'm willing to bet there is a diamond in the rough here.

Nail on the head.
possible , but No Free Lunch.

A Turbo in the Exhaust creates back pressure which reduces the efficiency of the ICE... normally a Turbo gets some of this back by also compressing the intact air ... but if you only take away then the ICE sees a reduction in efficiency... and that air pressure in the exhaust is also 'thrust' slowing it down also is a net negative.

Of the energy you take away you won't have 100% conversion to something else... be it Heat Pump or Peltier style... always losses... which means you have to take more than you get.

I think the only way for such a exhaust system to be a net benefit ... is if what it is taking is already waste , with virtually no negative impact for taking it.

The coolant to heat up the ICE faster I think would fall into that category.

Just a few other ideas along this line I've run into in the past.
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Tigers pdf ... use switched on and off ~80% efficient generator from exhaust instead of traditional alternator... gives less parasitic loss than a traditional alternator... but it isn't a net benefit by itself ... it is just less parasitic losses than a traditional alternator.
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Link ... NREL researched the potential for a Peltier style device to improve net vehicle efficiency ... after the + and - all balance out... they were left with seeing a potential for only ~1.5% increase in net vehicle efficiency with current devices ... and only up to ~2.5% with future devices ( that they hoped would exist soon ) ... given the cost of such a system ... ~1.5% improvement is rather tiny.
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Link ... these People built one to test out of off the shelf Peltier modules ... Their Best case was about ~1kw of harvested electricity when the Engine was under about ~225 HP load. ... ie about ~0.6% of the Engine's load ... a lot of money and several additional pounds ... just for 0.6%... if you were able to get the same results ... maybe ~500Watts when the ICE was under a ~112HP load ... or ~225W @ ~56HP Load ... etc.

[serialk11r]

Running AC off the exhaust is not a good idea, you'd be subject to variable amounts of heat available and variable temperatures, aka your AC isn't really going to work most of the time. Belt driven wastes power by overrunning the compressor but at least the compressor has adequate power all the time and functions.

Exhaust driven turbines I think are a good idea if the idea is to act like a muffler that returns some energy back to the system, but they also don't generate much power and cost too much money for what they accomplish. Better to just save the energy upstream at the engine using Atkinson cycle. At maximum power like F1 most of the time, it's useful because then the turbine produces a significant amount of power and raises the efficiency of the engine appreciably.

I think exhaust heat regeneration devices that actually come on cars in the near future will be Seebeck effect (thermoelectric) devices, once they figure out some efficiency improvements, or similar very high power to weight ratio heat engines that don't rely on a gas turbine. To condense a low boiling point fluid and drive a turbine would require a low temperature condenser which is going to be difficult. If they figure out some nice 800C working temperature thermoelectric devices that are cheap (not even more efficient), you can run 80C engine coolant through the cold side and end up with enough power for the car's basic electrical demands under cruising conditions, with pretty little effort.

[Occasionally6]

Originally Posted by euromodder

I kinda like VW's approach:
Run coolant through the exhaust manifold to quickly heat up the coolant and get the engine up to temp.

Later model Prius also use the exhaust heat directly.

[Occasionally6]

Originally Posted by IamIan

A Turbo in the Exhaust creates back pressure which reduces the efficiency of the ICE... normally a Turbo gets some of this back by also compressing the intact air ... but if you only take away then the ICE sees a reduction in efficiency... and that air pressure in the exhaust is also 'thrust' slowing it down also is a net negative etc.

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.

Originally Posted by serialk11r

Running AC off the exhaust is not a good idea, you'd be subject to variable amounts of heat available and variable temperatures, aka your AC isn't really going to work most of the time.

That would be OK though if you could store the energy in a cold sink. i.e. run the A/C compressor hard when the exhaust gas is available and use latent heat storage around the evaporator to cover the intermittency.

The latent heat storage may already be used in some cars (BMW again?) with engine stop-start.

The A/C system would have to be larger though, because the peak refrigeration work would be higher.

[IamIan]

Originally Posted by Occasionally6

If the flow across an exhaust valve is supersonic, the flow is choked.

SuperSonic Air Flow seems like a mighty big IF to me ... especially in the exhaust of ground vehicles.

[Occasionally6]

It always is (supersonic) in the initial part of the exhaust stroke/end of power stroke.

The pressure in the exhaust manifold/runner isn't constant either. It pulses high around the exhaust valve opening event and is lower through the remainder of the exhaust stroke.

[IamIan]

Originally Posted by Occasionally6

It always is (supersonic) in the initial part of the exhaust stroke/end of power stroke.

Do you have a reference?