Exhaust Heat Recovery: Simulation of a Rankine Cycle System

[sqidd]

Originally Posted by oil pan 4

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.

[Cobb]

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?

[pgfpro]

Originally Posted by oil pan 4

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.

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.

[darcane]

Originally Posted by markweatherill

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

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.

[markweatherill]

Originally Posted by darcane

Adding a separate cycle to capture heat from the exhaust allows you to store that energy for later.

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

[darcane]

Originally Posted by markweatherill

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.

[sqidd]

Originally Posted by darcane

My guess is that the turbocharger would require the engine to work harder to push the exhaust gases through it

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

[elhigh]

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.

[sqidd]

Sterling Cycle engine.

[Daox]

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.