turbo, expansion rate, compression, cam timing, etc

[whitegoat]

say I raised my static comp to about 20:1. I only let the intake open 1/2 the stroke time, close about 50* BBDC so I would only have half the cyl peak pressure. and more of an expansion rate.

so with this setup will this only work with a N/A engine?

Its a given that you would need a small, fast burn chamber.

Up the compression, reduce peak pressure, and increase the expansion rate.

have the exaust open about 10* BBDC.

by creating a more complete-faster burn, and more expansion, I will have recouped any power loss from having less air.? hopefully.

also the piston wont have to use so much energy to compress the charge with the higher comp ratio.

with the greater expansion rate the egt's will lower.

(sounds great except for any kind of lean burn you will have to increase the intake temp and or use a turbo.)

so how do incorporate a turbo with this?

 

[globi5]

It would probably work better with a compressor instead of a turbo (classic miller cycle).
In order to get the same power out as with a NA engine, the turbo needs to do a lot more of the compression work (since the piston is only pumping half of its displacement and doing half of the compression). At the same time the turbine has a lot less enthalpy on hand (due to the larger expansion ratio of the piston) and possibly not enough to deliver sufficient power to the compressor wheel. It might work in a static engine rpm set up where spool time of the turbo is irrelevant.
Since turbo's are getting more efficient and the environmental laws more strict, some engines running them are actually getting 'miller cycled' just not to this extend.

Last but not least, although higher compression ratio is appealing, if one looks at the thermodynamic effciency of the ideal Otto-cycle the significant compression ratio increase from 10 to 20 will actually lead to a relatively small effciency increase of 15% (relative not absolute).

 

[globi5]

PS: efficiency increase =~ power increase. (15% higher efficiency and 15% more power).

 

[patprimmer]

15% increase in thermodynamic efficiency while cutting volumetric efficiency 50% will give an increase in fuel efficiency, BUT a significant (like about 35%) decrease in power output.

The engine rpm range will be shortened considerably, maybe making current gear ratios inappropriate.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

Heywood "Internal Combustion Engine Fundamentals" has a nice section on 'over expanded' (or 'under charged'?) engines, including estimates of the efficiency gain and reduction in power.

Cheers

Greg Locock

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

 

[globi5]

I wonder, whether similar concepts (small 'miller cycled' piston engine with large turbo, CR of 10 though) have been considered in small aircrafts, where rpms can be kept relatively constant (spool time is not relevant) and where small gas-turbines would be too costly? Sort of a turbo-prop, which uses a piston engine instead of a combustion chamber between compressor and turbine and also uses the piston engine to drive the propeller (instead of a seperate turbine). This concept might not only be less expensive than a turbo-prop engine, but could also be more efficient since the maximum temperatures in the piston engine can be higher than in a combustion chamber, since it is not constantly exposed to high temperatures.
(Obviously it would still have a lower power density than a turbo-prop engine but produce a higher power density than a conventional piston engine.)