Is Bourke Engine a hiden Atkinson cycle?

[MKimagin]

I just wonder if Bourke Engine a hidden Atkinson cycle?

According to the people that make temperature measurement of exhaust on existing Bourke prototype, the temperature is around 200F (see the thread71-98313).
That may suggest that it might be Atkinson cycle with shorter compression and longer expansion cycle.
Does any body make some dynamic analysis if that assumption might be true?

 

[EdDanzer]

After reviewing the Bourke thread, it cannot be anything but a 2 stroke Otto cycle. Atkinson cycle engines have a longer power stoke than intake and compression.

I doubt any 2 stroke will ever meet the emissions for 2010 cars and trucks.

 

[globi5]

Can you provide a concept drawing that goes through all the cycles of the Bourke engine?
It appears to be comparable to a 2 cycle engine, but the piston spends more time at TDC which reduces the temperature at BDC. Depending on how the inlet port and outlet ports are arranged or timed the compression stroke could be shorter than the expansion stroke.

Regarding 2 cycle engines:
The problem with conventional 2 cycle gasoline engines is that the outlet port is open for a longer period of time than the inlet port. So the air/fuel mixture has lots of time to leave through the exhaust port. By using fuel direct injection, there are ways to prevent this:

http://www.rotax.com/en/Engine/2004/Snowmobile/Engine.Models.htm

This is also not an issue with 2 cycle diesel engines. So it is not entirely clear to me, why emissions couldn't theoretically be just as good as with 4 cycle diesel engines. Maybe they just don't work that well on load changes and thus are mainly used on big ships where load is more or less constant.

 

[globi5]

After looking at the drawings and animations more closely, I noticed there's nothing but the piston regulating the gas exchange this means that the inlet and exhaust ports always open and close symmetrically. Therefore it is definitely not a hidden 'Atkinson Cycle' - even if this nomenclature was valid for a 2 cycle engine.

 

[malbeare]

Is the Beare Head a hiden Atkinson cycle?
Here is my blerb on this .
Please tell me if I am on the right track

SWEPT VOLUME

Imagine a four stroke engine single cylinder capacity one litre
Bore 115mm stroke 96.3mm swept volume of 1000cc.
Imagine a Beare Head with a bore of 75mm and a stroke of 56.5mm a swept volume of 250cc.
Imagine that the combustion chamber volume is 100cc. the trapped volume when both upper and lower main piston are at their closest proximity with the top piston down as far as possible and the main piston at TDC.
INTAKE.
The main large piston is at TDC, the upper smaller piston is at the top of its bore or BDC, so the cylinder volume is 250cc plus 100cc equals 350cc.
As the main piston descends it is increasing volume. At the same time the upper piston is descending reducing volume.
At main piston BDC the main piston has swept 1000cc and the upper piston has descended half its bore as it is synchronized at half the main piston rotational speed. It has swept 125cc and reduced the swept volume by 125cc
Therefore the cylinder volume at main piston BDC is 1000cc add the combustion chamber, add the volume left in the upper piston of 125cc so the total volume is 1225cc
So the swept volume of the intake stroke is 1225cc minus the volume at the start of the intake stroke of 350cc
875cc
COMPRESSION.
The cylinder volume is 1225cc
The main piston ascends while the upper piston continues to descend, both pistons are reducing volume.
At TDC main piston has swept 1000cc while the upper piston has swept a further 125cc
Cylinder volume is now 100cc
So the swept volume is 1225 minus 100cc
1125cc.

EXPANSION
Cylinder volume is 100cc
The main piston descends while the upper piston ascends.
Both pistons are increasing volume.
At BDC the cylinder volume is main piston 1000cc and upper piston is 125cc.
Total cylinder volume is 1225cc
So the swept volume is 1225 minus 100cc
1125cc

EXHAUST
The cylinder volume is 1225
From BDC the main piston ascends reducing volume while the upper piston continues to ascend increasing volume
At TDC the main piston has swept 1000cc, the upper piston has increased volume by 125cc
The total cylinder volume is combustion chamber 100cc and upper piston volume 250cc
350cc
So the swept volume is 1225 minus 350cc.
875cc


The total swept volume over the four strokes is 4000cc
Intake 875cc add compression 1125cc add expansion 1125cc add exhaust 875cc
Therefore the nominal average capacity of the Beare cycle engine is
1000cc
Similar arguments and dissertations could apply to the miller cycle.

The waters could be muddied somewhat more by considering only the trapped volumes after all the ports have been closed. The Japanese used to apply this principle to two-strokes with corrected compression ratios.
There is a further complication if the upper piston crank is delayed or advanced in its rotational relationship with the main crank, or if it is a conventional crank and con rod or a scotch yoke drive. All have effects on the swept volume in regards to crank angle position.
But the net results are that the Beare cycle has advantages in gaining efficiency, pumping losses are reduced as less energy is expended to suck intake and pump out exhaust. And more energy is extracted during the expansion stroke
If the upper piston is delayed in its rotational relationship by about 20 degrees the maximum volume no longer occurs at BDC but is at173 main crank degrees on intake and maximum volume occurs at 548 for expansion and minimum volume occurs at 361 and the rate of change in volume during combustion is less than the conventional four stroke maintaining a closer relationship to the theoretical ideal of constant volume combustion.. and therefore higher maximum cylinder pressures are achieved even though the compression ratio and open throttle cranking cylinder pressure may be the same as the conventional four stroke.







A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[globi5]

I'm not sure whether there's an official definition of what an Atkinson cycle is, but if there was one it probably be something like this:

compression volume < expansion volume
intake volume = compression volume
expansion volume = exhaust volume
(volume can be substituted with stroke)

So, if one simply applied this definition it is not the same.

But, regarding the merits of the beare engine (if I may and if I understood everything correctly, which may not be the case):
* The inlet and exhaust port valving mechanism (2nd piston) appear to be less restrictive than the poppet valve mechanism.
* The 2nd piston (aluminum) might have less hot-spots than a traditional exhaust poppet valve.
* Pumping losses are reduced at partial throttle simply because less volume will be expelled and presumably increase temperature of fresh air/fuel mixture, which should improve and speed up combustion at partial throttle: Honda actually was applying a similar principle on a 2 cycle engine:

http://www.motorcycle.com/mo/mchonda/exp2_tech.html

(One can claim though, that the same can be achieved with an EGR system).
* If the rate of change in volume during combustion is indeed prolonged, this would certainly be an advantage.
* It is imminent though, that the intake and exhaust is tuned in a way that the remaining exhaust gases will be expelled at full throttle by the gases itself otherwise power will suffer, because the remaining exhaust gases will harm VE. (Unless you mount a blower to your engine similar to the opposed piston engines from Junkers).

However, sucking and expelling exhaust gases in an Ottocycle should not require any significant work apart from overcoming friction. So, I believe it is not valid to call this pumping losses of the Ottocycle, unless, you think of pumping losses due to partial throttle operation. (Which, by the way an Otto 2-cycle does not have - as opposed to the 4 cycle engine - because the back of the piston does not need to work against atmospheric pressure at partial throttle.)

 

[EHudson]

The Bourke is a two stroke with a wet sump. Not an Atkinson cycle.

Best reading on the Bourke: Contact the Experimental Aircraft Association Library, Oshkosh, Wisconsin, and ask for a copy of J. David Kirk's article on said engine that appeared close to 30 years ago in the 'Sport Aviation' magazine.

A short synopsis: There is no magic.