Waste Heat Power Recovery?

[Docengineheat]

Nearly everybody knows that both gas and diesel engines waste lots of heat from the coolant system and exhaust system.

I'm interested in the forums opinions on why there do not appear to be any viable systems that recover this waste energy and use it to improve engine efficiency. What comes to mind is using a steam engine that is powered by the exhaust heat.

Anybody have any insight on why this hasn't been done?

 

[ivymike]

added mass, limited quality of waste heat

 

[ivymike]

then there's that whole cost issue...

 

[Slowzuki]

Quantity of waste heat is large, the quality is low. Turbos do extract some waste energy from the exhaust.

Some stationary diesels recover heat form the exhaust for district heating, in a mobile vehicle is would take a lot of weight.

Diesels cause very low temperature change in the exhaust and water jacket at low load due to the large mass flow of air running through the engine so the heat recovery is only really practical at high loads.

I have often thought about using the coolant from a vehicle to heat a shop. The woodsworkers here have quick connects on their pickups cooling system so they can warm up their skidders engines before starting in the winter. If I included a 20 L tank of coolant on my vehicle and circulated to my shop when I returned home I would get a head start on the fire that is just being lit in the stove.

Ken

 

[Docengineheat]

Granted that the quality of coolant heat is low with a temperature of 200 deg F or so, but the quality of exhaust heat is quite high.

The exhaust gas temperature of a diesel engine will be anywhere from 700 to 1300 deg F, while the exhaust gas temperature of a gas engine can be up to 1800 deg F. Considering that the temperature of superheated steam in a typical power plant is only 1000 deg F, the exhaust gas heat must be regarded as high quality.

I agree that cost and weight would be a consideration. But a steam turbine would only be about the size of a turbocharger, and a exhaust boiler could be a "flash boiler". I don't think the weight of those two devices would be all that much, and the cost should be manageable.

Any other thoughts?

 

[GraviMan]

I'm suprised in these CO2 conscious days that the adiabatic engine has not found greater favour to research grants. Ceramic liners and piston crowns are a pain to mass produce, due to their brittle nature, but this is not the only material.

Why not use machined graphite or silicon components a la space shuttle. I mean the technology is supposed to filter into every day life isn't it? Graphite liners may even go some way to reducing friction. I'm sure these materials could take the pounding of a diesel engine, although careful design would be required. It may even be possible to use these in place of sodium cooled valves.

The real advantage is that such an engine would would obviate the need for any cooling! Obviously it would need to be designed from the outset as a compound engine, ie a turbo charger would extract as much heat/pressure from the exhaust as possible. Inlet timing would take into account the higher boost pressures, but better would be to use a high speed generator/motor a la Turbo Genset. In a world moving towards hybrids, I'm sure use could be fould for the electrical power produced - maybe airconditioning or heating!

Mart

 

[GregLocock]

One problem you face is that the average power output in a car is rather low - in a 200 hp car the average is around 22 hp. So, assuming that 1/3 of the energy ends up in the exhaust, how much plant are you prepared to install in a car to retrieve 22 hp, and what would you do with it? A battery that will efficiently accept charge at 15 kW is going to be quite large, and then you have to find a use for the energy generated. Sadly it won't replace the alternator where we size the system to cope at idle, pretty much.

If electric traction assist ever gets popular then I think we will see ic engines with very large (high back pressure) turbo alternators on them - they have been around for donkey's years. I am a little surprised the Prius doesn't do this.

For stationary installations the answers are much more sensible - use a natural gas powered diesel, driving a mains generator, feed excess electricity back into the grid and use low grade waste heat for hot water and heating.



Cheers

Greg Locock

 

[Docengineheat]

"One problem you face is that the average power output in a car is rather low - in a 200 hp car the average is around 22 hp."

I'm not sure exactly what you are saying -- is it that in a 200 hp car only about 22 hp is normally produced? Or is the amount of power retrieved from waste heat only 22 hp for a 200 hp car (i.e., about 11%).

I would think that the most natural use of the recovered power would be to introduce it back to the driveshaft.

As far as how much plant you are prepared to install -- I would think that if you got a 10% increase in power, it would certainly justify at least a 10% increase in plant, and maybe even a 20% increase in plant, given that the engine will be more economical.

I'm just pulling numbers out of the air, here, but consider a large diesel truck engine in a truck that normally gets 6 mpg, and assume that the truck goes 120,000 miles a year. Thats 20,000 gallons of diesel. At $2 a gallon we're looking at a yearly fuel bill of $40,000.

If we are able to increase fuel efficiency 10%, then you're looking at a fuel savings of $4,000/year. If the average size engine costs $20,000 (just a guess, folks) then a savings of $4,000 a year would probably justify a signficant investment. I'll leave it up to the accountants to say just how much, but I would think that the device could add $10,000 to the cost of the engine and it would still be a reasonable investment.

 

[patprimmer]

I know that Greg works in passanger car design, so his figures will apply to a car

I would expect that 10 to 20 HP would be the cruise hp requirement of a car. Greg works on large cars, so 22 sounds to me like cruise hp requirtement, so 1/3 recovery would be 7 hp

If you have to install extra plant to recover this waste, you need extra power to drive the heavier vehicle, which uses at least part of the saved fuel, or more than all of it if you are not carefull about your design and sums.

Re trucks inparticular, the extra plant detracts from payload, and therefore earning capacity

Regards
pat

 

[GregLocock]

Yes, I meant that a 200 hp (peak) car actually uses about 22 hp on average.

Roughly the same amount goes down the exhaust.

I must admit I like this idea - say a 150 hp engine with a 50 hp turbo alternator, driving a 50 hp motor direct coupled to the crank. The 150 hp engine would be a little larger than 150/200 of the 200 hp engine due to high back pressure, but roughly speaking engine capacity is a poor indicator of cost of manufacture so this is not a big deal.

If we assume the turbo is 70% efficient, and the electrics are 95% and the base engine is 30% then I crudely get a maximum possible fuel save of

(3/4*.3+1/4*.7*.95)/0.3-1, wow, 30% or so.

Incidentally the % improvement goes up as the engine's efficiency drops, and 30% is a pretty aggressive number. If you were running a diesel at 40% base efficiency the economy would only improve by 16%

It is not quite that easy in practice, the efficiency of the ic would drop due to the higher back pressure, but that is not a bad place to start.

Now the downside is cost. The engine+turboalternator+motor I have described would cost at least twice as much as the base engine (if it was a 200 hp gasoline engine), would weigh more, and would be more unreliable.

The sad thing is, people in general won't spend much on their car to improve the fuel economy. For the same price as the above I could give them a 350 hp turboed engine whose baseline efficiency would probably be 27%. Which would they buy?

Admittedly the truck market is more rational.

Cheers

Greg Locock

 

[Docengineheat]

I just checked some figures derived from a diesel engine simulator, set up for a turbocharged Cummins 5.9 liter diesel with the intake manifold pressure at 1.6 atms absolute pressure, and a turbocharger with an 80% efficient turbine and compressor. An exhaust turbine follows the turbocharger turbine, and also has an efficiency of 80%. The base engine output at full power is 155 hp, and is 43.8% efficient without a exhaust turbine. It is assumed that the exhaust turbine is coupled mechanically to the drive shaft without any power loss (!!!)

Here's what some of my figures look like:

Exhaust manifold pressure: 2 atms.
Power from exhaust turbine: 4.4 hp
Engine efficiency: 43.6%
Total hp: 159.4
increased efficiency: 2.8%

Exhaust manifold pressure: 3 atms
Power from exhaust turbine: 18.4 hp
Engine efficiency: 40.9%
Total hp: 163.9
Increased efficiency: 5.7%

Exhaust manifold pressue: 4 atms
Power from exhaust turbine: 27.7 hp
Engine efficiency: 38.1%
Total hp: 163.1
Increased efficiency: 5.2%

These figures are pretty much in line with Caterpillar's experience with exhaust turbines. I don't have a gas engine simulator, so I'm not sure of the magnitude of the efficiency decrease with increasing exhaust manifold pressure in that case.

 

[GregLocock]

There's something horribly wrong with the maths in my example - it's the old average speed mistake ie I need to average the inverses.


150hp of ic eff=30%
50 hp of turbo eff=.95*.7=.665

overall eta=35%

ie a 16% increase, not 30%.

I wonder if your engine efficiency sensitivity to back pressure would be less if it did not have its own turbo ahead of the 'power' turbo?


Cheers

Greg Locock

 

[Docengineheat]

The answer about having the turbocharger followed by the exhaust turbine is that the turbocharger actually increases the efficiency of the engine. So while the turbocharger does increase the exhaust manifold pressure, the corresponding decrease in efficiency from this is more than offset by the increase in efficiency from having the intake manifold pressurized by the turbocharger.

This effect, of course, is limited to unthrottled engines -- diesels -- and would not be seen on throttled engines.

 

[tbuelna]

Here's the reality of turbo-compounding:

Scania's turbo-compounded 12 liter DI diesel engine only gets a fuel consumption decrease of about 2% for all of the added complexity and cost associated with the compounding hardware.

http://www.scania.com/products/truc..._work.asp?ComponentID=8010&SourcePageID=715#2

Here's an analysis from Caterpillar, for a turbo-electric compounding arrangement. It predicts 5-10% decrease in fuel consumption. I find those predictions overly optimistic.

http://www.orau.gov/deer/DEER2002/Session8/Hopmann.pdf

Regards,
Terry

 

[Docengineheat]

tbuelna:

I'm not doubting you, but where is the reference on the Scania turbo-compound engine improving fuel efficiency 2%? I didn't see it in the website you referenced. Is there some place else that this figure comes from?

In the DEER 2003 (

http://www.orau.gov,

following the links to "presentation&quot conference Caterpillar put their fuel efficiency increase figure at 5%. I don't know or not if they can reach that figure, but it sure looks like the device they came up with (60 KW turbochargergenerator and 60 KW drive shaft motor) along with the electronics will sure cost a pretty penny.

I seem to recall that in the DEER 2000 conference that CAT claimed that a turbocompound unit they had similar to Scania's did get about a 5% increase in fuel efficiency ("at 62 mph across Iowa&quot.

 

[tbuelna]

Docengineheat:

You point out the drawback with turbocompounding in a roadgoing vehicle application. The Scania engine does reduce fuel consumption about 5% under optimum steady speed, steady load conditions (ie:"at 62 mph across Iowa&quot. But under a varying load/speed urban type driving cycle, it does not achieve anywhere near those numbers.

Regards,
Terry

 

[wizlish]

Let me add a few notes to this.

The Cat turboelectric compound is using the electric motor to spin the turbo up quickly to avoid turbo lag or provide more combustion air for better transient power or less smoke. They explicitly point out this only works for about 8 sec. or so. They get a bit of 'efficiency' back by letting the turbo spin the motor as a generator -- but I wouldn't look to this as supplying lots of power for use in a hybrid transmission! It's good for battery charging, but its output is going to vary with engine output -- and much attempt to draw power from it is going to slow or perhaps stall the turbocharger during light load.

Double sequential turbos and high-heat-rejection coatings have been tried. Ford did one in the 1970s that was particularly interesting -- apparently the problem was that thermal cycling caused the ceramics available for the coatings to flake as the engine aged, causing exactly the kind of turbo problem you'd expect it would.

It is not too difficult to adjust the sequential turbos to get the effective EGT very low -- a good indication that thermal energy is being recovered by expansion through the turbines. You then begin to hit problems with the water in the exhaust condensing in the subsequent components of the exhaust system. If that exhaust can rust, you'll start to get problems. If you're running a catalytic device that requires light-off, you'll have problems too...

Something that hasn't been discussed so far is the use of bottoming cycles using something other than water. The French used ether (!) as a cycle enhancement to steam engines on ships (!!) as early as the 1850s. There are a number of materials that qualify as transfer agents for organic Rankine cycle (ORC) engines -- anybody remember Learium? -- that can be selected for the appropriate closed-cycle heat extraction systems for vehicle exhaust. See the above discussion for caveats on 'how low to go' on EGTs for practical reasons; you can also get soot and other exhaust buildups on the relatively cold elements in the exhaust. On the other hand, sufficiently volatile transfer agents can produce good levels of mechanical torque from low absolute afterheat temperatures. You can also 'pipe' the transfer agents around with comparative ease either to produce mechanical power or heat exchange at locations of your choosing.

Ah, if only air preheating worked as well for IC engines as it does for steam locomotives... ;-}

 

[GraviMan]

"Something that hasn't been discussed so far is the use of bottoming cycles using something other than water. The French used ether (!) as a cycle enhancement to steam engines on ships (!!) as early as the 1850s."

Now that is sound reasoning. Brings home the point that we should all really be talking about thermodynamic fluid engines, rather than trying to distinguish between air or water cycles. Good post.

Mart

 

[gbent]

Such a combined cycle is available, it just takes a very large stationary installation to work. GE makes a combined cycle power plant they claim 60% efficiency on. They fire a gas turbine with natural gas, and use the waste heat to make steam to run a steam turbine. I don't know the size range for these units, but a wag would start at 40,000hp.
Steam is such a dangerous substance to work with, I don't think I would want something with the haphazard maintenance of the average car using steam.

 

[roadwarrrior77]

One reason only...Greed. There are some sites that research alternative fueling that use exhaust heat and coolant heat to improve efficiency in the combustion process. Check out keelynet.com or "encyclopedia of free energy".

Watts