Yet another split-cycle engine -- comment! 2

[TDIMeister]

http://www3.signonsandiego.com/stor...ne18345-tour-de-force/?features&zIndex=112578

http://www.signonsandiego.com/news/metro/images/090608pistons.pdf

"In computer simulations, Tour said his engine achieves energy efficiencies as high as 62 percent – “that's a quantum leap improvement” – while cutting the emission of pollutants like carbon dioxide by half and nitric oxide by 80 percent."

 

[ivymike]

"The cylinders aren’t side-by-side (unlike other split-cycle designs) so there is less heat exchange."


okay, so trade dead volume for an extra crankshaft... doesn't sound like a good deal to me.

 

[TDIMeister]

"In computer simulations, Tour said his engine achieves energy efficiencies as high as 62 percent – "that's a quantum leap improvement"

Are those ideal-cycle indicated efficiency values or brake? My bet is that it's the former. Big deal. Conventional engines also do that and more. Nothing ground-breaking there. I'd like to see a loss-split diagram, as well as P-V and T-S diagrams of the proposed cycle.


– while cutting the emission of pollutants like carbon dioxide by half and nitric oxide by 80 percent."

Elsewhere it is claimed that the layout permits higher working temperatures in the hot cylinder. This tends to have the effect of INCREASING NOx emissions.

Also, the realisation of the connection between the cold and hot cylinder is not at all detailed, just an arrow pointing to an open channel. Any solution (poppet valves, etc.) will invariably result in flow losses.

Lastly, mention is made that "The hot cylinder is larger than the cool one to harness more power during combustion." Bull malarkey. Either this is faulty reporting or someone who doesn't have a knowledge in thermodynamics. The best you can do is to expand the burned charge out to atmospheric pressure. Here, it's the volumetric expansion ratio, not swept volume, that is of importance.

I will only add that there are certainly some advantages to such an engine. One is that realisation of the Miller/Atkinson cycle with a larger expansion ratio than compression, plus full expansion, is possible. With all the extra hardware though, the BMEP will not be remarkable, and FMEP and PMEP will be bad.

 

[CCycle]

This is a Brayton cycle first proposed and patented by George Brayton, Philadelphia in 1875.
The major difference is the end on pistons and the use of a poppet valve for exhaust.
The original Brayton engines used a port in the bottom of the high temperature cylinder for exhaust.
The Brayton engines were quite popular 100 years ago.
They were eventually replaced by auto cycle engines because of low efficiency and low specific power.
The end on pistons and poppet valve probably will improve operation but not to the extent predicted.
This engine is a two cycle which helps specific power even though it needs two cranks.
The valve in between the two cylinders was simple check valve.

 

[ivymike]

The Brayton cycle is quite popular today, and achieves very high specific power, just not in a piston engine configuration.

http://en.wikipedia.org/wiki/Brayton_cycle

 

[dcasto]

This looks like a 2 cycle engine with scavenging air cylinders like on old Cooper Bessemer GMV's

 

[CCycle]

"Elsewhere it is claimed that the layout permits higher working temperatures in the hot cylinder. This tends to have the effect of INCREASING NOx emissions."

Probably what the author meant to say was that compression is done in a cooler cylinder hence the compression stroke requires less power. This feature was advertised as being the main advantage for the Brayton. In actuality the energy needed to compress the charge is mostly recovered hence this is not an issue.


"... the realisation of the connection between the cold and hot cylinder is not at all detailed, just an arrow pointing to an open channel. Any solution (poppet valves, etc.) will invariably result in flow losses."

The lack of detail here is most likely because of the several options. The original Brayton used a flame arrester or "wire gauze" as detailed in the Wikipedia link. Most later engines used a check valve. Some used both. This was the Achilles heel of the engine. The compression cylinder was often of light construction. If they had a flash back it could ignite the mixture in the compression cylinder and explode the engine. It was common to see these engines retrofitted with a frangible disk which would blow out in case of flash back.

"This looks like a 2 cycle engine with scavenging air cylinders like on old Cooper Bessemer GMV's"

Right. These engines were commonly found in steel plants and oil fields. They were very rugged and would run on just about anything including producer gas and CO which was in abundance around the old Bessemer converters. They had little or no octane requirement. They usually had hot finger ignition which was better understood by the operators of that day.

 

[tbuelna]

The Tour engine has the same fundamental drawback (and with Tour's larger diameter power-side piston it's actually worse) as any opposed-piston engine: the exhaust side piston must deal with huge heat transfer conditions, which always cause scuffing failure of the piston rings.

Besides heat transfer problems, there also appears to be dynamic balance issues with the asymmetrical piston motions, there will likely be significant pumping and thermal losses at the transfer port, and the operating speed will be limited by the high operating frequencies required by the valve train.

Worst of all, the engine will be unacceptable to the automotive OEM's due to it's large physical size and cost. The engine will be very expensive to produce since it requires multiple quantities of the most expensive engine components (ie. cranks, cams, etc.)

Sadly, what most inventors fail to realize, is that "different" is not necessarily better. The reason that 99% of the world's automobiles use 4-stroke, OHV poppet-valve, recip-piston engines, is that this configuration provides the best compromise of performance and cost.

 

[j2bprometheus]

tbuelna is correct about the piston rings getting hot. For the four stroke engine, the cooling effect of the intake stroke is quite significant.


The Tour engine is a modified Cayley / Bucket / Roper engine, which was invented by Sir George Cayley in the early 1800's.

The big difference between the Cayley engine and the Tour engine is this: For the Cayley, combustion took place outside of the hot cylinder in a combustor.

Some similarities:

A. compression takes place in a cold cylinder, expansion takes place in a separate hot cylinder

B. different displacement of the cold cylinder and the hot cylinder.


Variants of the Cayley engine would shut the inlet valve to the hot cylinder part way through the expansion stroke and thus had an almost isentropic expansion for that part of the stroke. This improved efficiency but hurt power density.

 

[GregLocock]

Slightly off topic, TDI Meister mentioned a loss split diagram. I assume this effectively identifies the difference between imep and bmep, for real engine. Is this just a bar chart or something more complex?

It is obviously essential in reality, as, for example, a hot air engine will have superb thermo efficiency according to its gas cycle diagram, yet in practice its brake thermal efficiency is usually less than that of a good turbocharged diesel.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TDIMeister]

Hello Greg, nice to hear from you!

That's right, the split-loss diagrams I refer to are stacked bar graphs in usually two forms: one shows 100% fuel energy being divvied up by the useful work and different forms of losses -- deviation from air-standard cycle fuel conversion efficiency; non-ideal combustion and dissociation; heat transfer; friction, which can be further divided into valvetrain, piston/ring pack, cranktrain, etc. components; and so on. The other form only looks at the difference between indicated and brake efficiencies. Heywood has examples of these and these split-loss analyses are also widely practised in industry.

There are also energy flow diagrams that are a little different but can also be used to identify sources of losses.