Exhaust Heat Recovery: Simulation of a Rankine Cycle System
 

Here we have a very interesting paper where they analyse using exhaust heat to power a rankine cycle system to recover power. I've long wondered how much power you can really harness from the exhaust.

http://www.evs24.org/wevajournal/php...J3-3260191.pdf

I did not read the whole paper, but I've cut up a bunch of the info to share here.

Here we have the general premise. Capture heat from the exhaust to vaporize a fluid, use the resulting pressure to spin a pump/turbine, condense the gas back to a liquid, and pump it back into the high pressure side to turn back into a gas again.

http://ecomodder.com/forum/attachmen...1&d=1409948209




Basically, they used a 2nd gen Prius as an example vehicle under two operating points. Point #1 is about 5kW engine output, point #2 is 20 kW.

http://ecomodder.com/forum/attachmen...1&d=1409948120



And, here we have the power recovery they calculated. Php is the high pressure side pressure, Plp is low side pressure.

http://ecomodder.com/forum/attachmen...1&d=1409948290

The Rankine cycle is commonly used in power plants since it's a very efficient power cycle but it wouldn't work well for the primary power source for a car due to the constant changing power needs. Using it to capture waste heat and charge a battery is a solid plan though.

I think you're going to start seeing these on production vehicles shortly. First will likely be on heavy duty trucks.

This study is a little late though. BMW has had functional prototypes (search for Turbosteamer) for over a decade, and many other manufacturers have already been studying these systems.

I heard that Formula 1 engines use 1/3 of the fuel energy for power, 1/3 goes out the exhaust as heat and 1/3 is radiated through the coolers.

That 2/3's is a lot of wasted energy.

Build the boiler around the converter. Better yet converter around the cylinder head then the boiler.:rolleyes::cool:

This type of thing is neat, but on the flip side you got vehicles with electric and fuel assisted heaters that heat the coolant to maintain operating temperature cause the vehicle cant maintain it. I see this a lot with diesels including the big trucks. :eek:

Put coolant/heater system on the outlet of exhaust. Still enough heat there. Put preburner(with limited air injection) in converter to get heat up fast(some already have that)just enough too vaporize fuel send that(exhaust from preburner) back too engine.

But if they didn't 'waste' that energy making the engine run, the engine would not be able to get that 1/3 energy out of the fuel.

A turbocharger sounds like the obvious way to recover exhaust energy and I wonder why this system is better?

Absolutely.^^^

But if the F1 guys were allowed under the rules to recover some of the spent exhaust gas heat (they recover the exhaust energy with turbos this year) and the spent radiant heat they would have even more power.

Turbo's really don't do much for economy. They allow you to retain economy (out of boost) and still have the option of having power (on boost). My F-150 gets fantastic mileage if I mostly stay out of the boost. But when I spool them up look out! Fuel gets used. It's fast though.:thumbup:

Do you know how big the boiler heater is going to be?

I put the turbo on my diesel I instantly got 2 to 2.5 MPG increase. Then adding the second turbo and intercooler got me maybe another 0.5mpg.

Turbos just don't really increase the fuel economy of gas engines. On a gas engine the turbo and throttle fight each other constantly, any gains are lost there.

That's interesting. IDK much about diesel turbo setups aside from having a bunch of F250/350's that I didn't do any work or mods on. I put fuel in them, took them in for their services and drove them to death.

Oh wait, I did have an alternator go out on one once that I pulled over and changed in a Ford dealers parking lot. So I guess I have been under the hood of one.;)

I think the % of a turbo helping mpg is in relation to the cr of the engine.

The exhaust thing is neat, but like my insight half the time the engine is off and in stop n go traffic the water temp hovers around 154 degrees. This maybe a better idea for a stationary engine used in power or pumping?

On petrol turbo engines if the turbo is big enough it has no effect on FE at light to medium load.

I have personally done this on at least 4 different Honda's.

One with a .63 A/R T3 hot side and the other three with .82 A/R hotsides. All four engines had T4 compressors.

I would run them for at least a month with the stock ecu, injectors and map etc. They all drove just like before with no decrease in power. Also all had the exhaust change to 2 1/2" and one a 3" Back pressure was less or equal to pre-turbo form at light to medium load. The engines wouldn't make boost until 4000rpm at WOT.

Turbos allow an engine to make power more efficiently. However, for economy, you aren't trying to make a lot of power, just enough to cruise along. Modern turbo engines are becoming mire common and get better fuel economy... but only compared to the larger engines they replaced.

Adding a separate cycle to capture heat from the exhaust allows you to store that energy for later. With a turbo, when you capture waste energy, you need to immediately use it to compress the incoming air. I think this is where the big gains will be seen. Although, BMW's system was connected to the driveline and the power was used to propel the car and not stored, and still saw significant gains.

I get it now! So why not a turbocharger connected to a generator?

My guess is that the turbocharger would require the engine to work harder to push the exhaust gases through it, whereas the boiler built into the exhaust would allow the exhaust to pass through with very little restriction, having a minimal effect on the performance of the engine.

I'm not sure about this. I'm pretty sure that turbo's don't take any additional energy to spin.

I remember some stuff about the Rankine cycle. Mother Earth News was holding it up in the late 70s or early 80s as the holy grail of future energy generation. If I recall correctly the OTEC plant, which pulled super-cold water up from ocean depths, used a low differential Rankine cycle.

Yup. Here's more about that: Ocean thermal energy conversion - Wikipedia, the free encyclopedia

SAAB-Scania did work with turbocompounding for their big trucks and I think they're even available currently, I guess this Rankine system would be a natural next step. I think though that due to space constrictions it would be hard to have one of these systems on anything much smaller than a medium-duty truck; Peterbilt and some others already offer hybrid models and this would free up the engine from having to twist a genny to charge the traction battery. You can see though how it would nicely coincide with the radiator, so it'd be great to put some of that energy back to use instead of just throwing it over the side.

Sterling Cycle engine.

While the study shows that there are huge gains to be made (~25%). I'm more interested in how we can use this to develop a cheap DIY solution. Is there even a way to do this? I don't know. It does seem like a fair amount of complication. And, as you cut corners you'll definitely loose efficiency. But, starting with a 25% increase is not a bad place to start.

Also, remember they started with Prius which is innately more efficient than most gas engines. Larger gains may be had from conventional engines.

That's almost the thing they are running now on Formula 1. They are running turbos, but only for obtaining a similar power output as the older engines, but the interesting thing is in the exhaust turbine, where they are not boost limited, but instead of a wastegate they run it into some kind of electric motor/generator (called ERS-H) to recharge the batteries so they can spool quickly, eliminate turbo lag and adding 160 hp when combined with the normal engine and the ERS-K (which takes energy from braking).

Because the alternator belt system in use now is about 98% efficient.
A turbine is no where near that efficient.

A turbocharger does NOT act like a heat engine on a car. It will capture some of the excess energy that is the exhaust gases being at higher than atmospheric pressure when the exhaust valve open. This is useful for race cars that are run at WOT most of the time (I read that it boosts efficiency by 7% at WOT) but not for street cars which have very little excess pressure running at 30% load.

Look up BMW Turbosteamer. They were able to generate a consistent few kW from the exhaust with a Rankine cycle generator, 10kW peak. Not a DIY project though.

I imagine in the future as high temperature thermoelectric tech advances we'll be able to get most of the benefit in a simple bolt on package. Thermoelectric generators that are half as efficient as the BMW steam turbine cycle could completely alleviate all accessory loads on the engine and keep a hybrid system's battery topped up.

Diesel engines run WOT all the time.

Touche :)

But this is more about excess pressure at blowdown, which for a diesel still corresponds to load. At lower loads a turbo-generator will not be efficient at all on a diesel engine and the restriction it introduces could negate a lot of the benefit. That said the setup that they use in Formula 1 where the turbine is very free-flowing and uses the electric motor to augment boost would probably work very well on a diesel street car, albeit very expensive.

rankine cycle
 

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