2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello all.

In the following animation they are shown more details of the PatBam PatAT (animations in previous posts):



Among its characteristics:

"four-stroke-like" lubrication,

turbocharged (not shown; the turbocharger - assistant at cranking - serves as the scavenging pump, too),

asymmetric transfer ending after the closing of the exhaust port,

main scavenging with clean air,

early indirect fuel injection in the space underside the piston crown (the formed air-fuel mixture is the last to enter into the cylinder, eliminating the loss of unburned fuel towards the exhaust),

full balanced,

even firing,

no need for high voltage electric system,

better when running on cheap low-octane gasoline.


The strange piston structure divides both, the combustion and the scavenging.

Near the TDC the HCCI combustion is divided in two stages: after the auto-ignition and the combustion in the auxiliary chamber, the burnt gas passes into the main chamber triggering the ignition of the air-fuel mixture therein.

After the opening of the exhaust port, as the piston is further approaching the BDC, the transfer ports open and pure air from the turbocharger pushes the remaining burnt gas out of the cylinder; later the well prepared air-fuel mixture from the space underside the piston crown overfills the cylinder after the closing of the exhaust port. The scavenging completes with a gust of clean air from the turbocharger; this air pushes any fuel in the passageways of the “asymmetric’ ports into the cylinder.

Its architecture minimizes the number of bearings and the frictional losses.
It also minimizes the weight.
Even when it runs extra lean (say, AFR=2) it can still provide top specific power.

Thanks
Manolis Pattakos

[Pinger]

When the auto industry was interested in the Wankel there is little doubt that the far fewer mechanical parts utilised compared to a 4T was a big draw. That they had to pay for licensing didn't help when the fuel crisis of the time arrived which was what killed it off then. Subsequent versions from Mazda were successful certainly, but its latest Renesis as fitted to RX8s is an unmitigated disaster with them all requiring rebuilds early in their life.

That the Wankel was the last time OEMs dabbled with an unfamiliar technology and burned their fingers may be why they are hell bent on continuing with 4T but if ever there was an engine format that defined 'the law of diminishing returns' the 4T is it.

So, the 2T is still undeveloped and suffers but one problem (thus far unsurmounted) namely high levels of UBHC in the exhaust stream. A typical value for a simple 2T appears to be in the region of 4-5000 ppm and, is due to fuel loss more than the one-pass oiling utilised.

Researching what improvement has been made re the above figure for UBHC I found an article discussing EPA Phase 2 regs applied in the USA which called for an 80% reduction in UBHC for engines used in hand-held tools (eg, strimmer, leaf blower, etc). It appears that this prompted a shift to Hybrid engines (4T engines lubricated entirely by oil mixed in the fuel as per 2T) which, we can reasonably deduce emitted UBHC in the region of 800-1000pm.
Thus far in the UK I have never encountered such an engine and those I know who use 4T in hand-held tools do so not because of emissions but for reasons of transmitted to hand vibration.

I'm guessing that the European requirement was less stringent than the EPA's which allowed continued use of 2T albeit with stratified charge. Does anyone have any data on UBHC of such stratified charge engines and can anyone confirm the UBHC level from Hybrid engines (as my deduced 800-1000ppm)?
Given the article re EPA Phase 2 was from circa 2002 and possibly predates stratified charge (as adopted by Stihl) does anyone know if a current stratified charge engine could meet those EPA regs?

Notably here though, the one-pass oiling isn't necessarily the biggest problem here - though plenty will say it is. Were it so, then the Hybrids also on one-pass oiling wouldn't have got off the blocks and Mazda's Renesis engine which also has an element of one-pass oiling would surely have failed to pass its emission regs. Might the use of fully synthetic (with zero hydrocarbon content) oils be playing a part here?

[Tommy Cookers]

presumably the synthetic lubricant is dumped to atmosphere at a much lower rate, that is within limits

[manolis]

Hello Pinger.

You write:
“Subsequent versions from Mazda were successful certainly, but its latest Renesis as fitted to RX8s is an unmitigated disaster with them all requiring rebuilds early in their life.”

Take a look at https://www.rx8club.com/australia-new-z ... ld-113724/ ; the discussion starts with:

“To the wise ones.
I'm just at 190,000km's on the RX8 with the lease just about up.
Looking at purchasing the car for future project and wondering what sort of K's the 8 would cover (freeway driving) before major rebuild??”

Provided the post and the discussion are not fake,
it appears that Mazda did a great job with the rotary engine.

Worth to mention: a Wankel Rotary is so good and smooth at higher revs that it operates regularly near red (which is bad for the long term reliability), becoming an “addiction” for the driver.

Nevertheless, the operational principle of the Wankel rotary is so wrong for emissions and fuel consumption that it was phased out by Mazda some years ago.




You also write:
“So, the 2T is still undeveloped and suffers but one problem (thus far unsurmounted) namely high levels of UBHC in the exhaust stream. A typical value for a simple 2T appears to be in the region of 4-5000 ppm and, is due to fuel loss more than the one-pass oiling utilised.”


Here are the old, the current and next “euro” emission regulations:



With their TPI (Transfer Port Injection) the 2-stroke KTM EXC250/300 of 2018 complies with the euro4 emission regulations.

For comparison,
the high-tech 4-stroke Ducati Panigale 1299 (desmodromic) is phased out because it is not euro-4 compliant (while the expensive (US80,000$) Panigale 1299SL (SuperLeggera), is euro-4 compliant).




The lubrication of the abovementioned KTM EXC of 2018 is of the “total loss” type.

Quote from http://thedirtbikerider.com/ktm-exc-tpi ... injection/ :

The amount of 2t of oil constantly adjusts the computer’s ECU, depending on the engine rev , and the scale itself is constantly changing.
But the average fuel usage is ,oil is 80: 1
Capacity of the tank for 2 Stroke oil: 700ml, enough for approximately 5 fuel tanks.

End of quote.


I.e. today, you can go and buy and drive a euro-4 compliant 2-stroke motorcycle which is cleaner / greener than many 4-strokes.



Now compare the total-loss lubrication of the euro-4 compliant KTM250/300EXC of 2018, with the “4-stroke-like” lubrication of the PatBam PatAT HCCI of the animation at the top of this page.

The space underside the piston crown is sealed from the crankcase.

The oil scraper ring (at the middle of the piston) never passes over ports.

The oil scraper ring keeps the lubricant into the crankcase (as in the 4-stroke engines), allowing plain bearings to be used and splash lubrication of the cylinder liner (whereon the thrust loads are taken).

At the BDC the lower compression ring abuts on an area of the cylinder liner whereon, 180 crankshaft degrees earlier, was abutting the oil scraper ring.

I.e. when the piston passes from its BDC, the lower compression ring abuts onto an oiled area of the cylinder liner, making unnecessary the conventional "total loss" lubrication of the ported 2-strokes.


In case the crankcase is to be used as the scavenging pump, the design can be modified as the following PatATeco 2-stroke:



More at http://www.pattakon.com/pattakonPatATeco.htm

Thanks
Manolis Pattakos

[J.A.W.]

Thanks Manolis, for the most interesting posts.

Herein: http://www.coxmarine.com/en/the-engine/the-technology - is another ( but much less revealing ) current 2T design,
which claims "F1" inputs.

[manolis]

Hello J.A.W.

Quote from “your” link:

“The secret behind the CXO300 outboard engine is our patented Opposed Piston Diesel (OPD) technology.”

End of Quote


They seem as they try to hide the “secret” of their engine.


Quote from https://newatlas.com/cox-diesel-powertain/26032/

“The Cox Powertrain concept is being developed in collaboration with the engineering R&D firm Ricardo. The project has already attracted £6.7 million (US$10.1 million) in private investment, in addition to previous MoD funding. The concept is based on an invention of Cox Powertain’s founder, former F1 designer David Cox, and is specifically designed for outboard use.



The concept uses opposed pistons, which means that the super-charged. two-stroke 3.6-liter motor has eight pistons housed in four cylinders. There’s no cylinder head, but rather each cylinder uses a centrally-placed injector, and the piston pair compresses the fuel/air mixture in the middle. Meanwhile, a Scotch Yoke crankshaft converts the reciprocating movement of the pistons into rotary motion.

End of Quote


Here is the US Patent of David Cox :



Here is an animation made with the drawings of Cox’s US Patent:



It is an Opposed-Piston Opposed-Cylinder engine (say, like the OPOC of the EcoMotors).

The cylinder liner seems as being of the “sleeve” type (it performs a short reciprocation).

The scavenge pump capacity is half than the combustion chamber capacity. Reasonably they use external turbo and/or super charger.

The big diameter pillar / pile / column / post, is not the best thing to put in the center of the combustion chamber.

But the big issue is the Scotch Yokes.

Here is the Scotch-Yoke of CMC:



which is not for an Opposed Piston engine.


In the COX engine, each combustion chamber is served by two Scotch Yokes (t
he one “double” at the sides of the central one), with each Scotch-Yoke receiving the full load from the combustion pressure.

It is supposed that the thrust loads between the cylinder liner and the piston skirt is a main cause of mechanical friction in a reciprocating piston engine.
And, depending on the con-rod to stroke ratio, these thrust loads are several times smaller than the full pressure load on the piston top.

The big question is how much of the mechanical energy is burnt in the Scotch Yoke mechanisms at the various operational conditions.



Now that the secret of the COX Diesel Marine is not a secret any longer, can anybody compare it to the PatOP engine?





Thanks
Manolis Pattakos

[Pinger]

Here are the old, the current and next “euro” emission regulations:

http://www.pattakon.com/tempman/Emissio ... ations.png

Thanks Manolis. Still looking for data on hand-held tools - but as that tends to be presented as an 'oyster map' with UBHC superimposed on speed and load, probably unobtainable.
Any info on the level of improvement over standard achieved by stratified charge units still sought though.

[Pinger]

Just had a very quick look over the Cox engine and this seems to me to be a viable direction for a 2T auto engine.
Limited supercharge (short charging time) should contain NOx, and the absence of poppet valves (which on a 2T running at any reasonable speed require massive acceleration rates).

Your comment.....

In the COX engine, each combustion chamber is served by two Scotch Yokes (t
he one “double” at the sides of the central one), with each Scotch-Yoke receiving the full load from the combustion pressure.

It is supposed that the thrust loads between the cylinder liner and the piston skirt is a main cause of mechanical friction in a reciprocating piston engine.
And, depending on the con-rod to stroke ratio, these thrust loads are several times smaller than the full pressure load on the piston top.

The big question is how much of the mechanical energy is burnt in the Scotch Yoke mechanisms at the various operational conditions.

..... kind of reinforces something I have been saying for some time. That is, the reticence of OEM to accept unfamiliar architecture. You too appear to have your doubts.

[manolis]

Hello Pinger.

You write:
“Just had a very quick look over the Cox engine and this seems to me to be a viable direction for a 2T auto engine.”


Why the Scotch-Yoke Cox design seems to you a “viable direction” for a 2-stroke auto engine?

Which are its key advantages?

And why a conventional Scotch Yoke design (like that of CMC that manufactured a number of 4-stroke Boxer engines spending a few million dollars before bankrupting) is not good for a 4-stroke auto engine?



You also write:
“.... kind of reinforces something I have been saying for some time. That is, the reticence of OEM to accept unfamiliar architecture. You too appear to have your doubts”


It is not about reinforcing some parts.
It is about the frictional losses.

To reduce the friction of the Scotch Yoke, Bourke replaced the flat / plain bearing with a track roller bearing:



Bourke failed due to the resulting extreme local loads (linear contact).




Quote from http://www.coxmarine.com/en/the-engine/the-technology

“OPD enables the four-cylinder, eight-piston supercharged 2-stroke CXO300 to achieve 1HP/KG – double the amount of a standard diesel outboard engine.”


Is the 1HP/KG a really good power to weight ratio?


According the Wikipedia,

the Junkers Jumo 205, which, like the COX, is an opposed piston 2-stroke supercharged engine (first run 1930), has a 50% higher specific power :

Jumo 205D power-to-weight ratio: 1.09 kW/kg (0.66 hp/lb) = 1.5HP/KG


The PatOP Opposed Piston prototype engine (which is running on Diesel fuel, standing free on a desk, in the following youtube video):



has:
636cc capacity,
850cc scavenging pump capacity (1.34 scavenging ratio),
20KG (44lb) engine weight (without the flywheel),
~20% additional piston dwell at the combustion dead center as compared to the OPOC of EcoMotors and to the Cox Marine, allowing it to burn efficiently the Diesel fuel at some 20% high revs and delivering some 20% higher power.

Even with only 60mN torque (which is low even for a 4-stroke 636cc engine, not for a 2-stroke 636cc with 850cc scavenging pump capacity) at 6,000rpm, its power is 50HP and the resulting power to weight ratio is more than 2.5HP/KG, i.e. 250% of the Cox Marine.

Do I miss something?

Thanks
Manolis Pattakos

[Pinger]

Hello Pinger.

You write:
“Just had a very quick look over the Cox engine and this seems to me to be a viable direction for a 2T auto engine.”


Why the Scotch-Yoke Cox design seems to you a “viable direction” for a 2-stroke auto engine?

Which are its key advantages?

And why a conventional Scotch Yoke design (like that of CMC that manufactured a number of 4-stroke Boxer engines spending a few million dollars before bankrupting) is not good for a 4-stroke auto engine?



You also write:
“.... kind of reinforces something I have been saying for some time. That is, the reticence of OEM to accept unfamiliar architecture. You too appear to have your doubts”


It is not about reinforcing some parts.
It is about the frictional losses.

Crossed wires here Manolis.
I didn't say the Scotch Yoke was what made it a viable auto engine. I said that the engine wouldn't be turbo/super charged to an inch of its life (the porting wont permit it) as is a 4T diesel thus it will be spared the high NOx that plagues said 4Ts, and that it obviates the need for poppet valves (which are unworkable (IMO) in anything other than the slowest running of 2T).

The 'reinforcing' I refer to - as you your further criticism of the SY demonstrate - is the reluctance to deviate from the familiar piston, con-rod, crankshaft combination which is all current OEMs have ever known.

[Zynerji]

Just a turbo for that sweet engine!

Great work!

[manolis]

Hello Pinger.

You write:
“I said that the engine wouldn't be turbo/super charged to an inch of its life (the porting wont permit it) as is a 4T diesel”


Why “the porting wont permit it”?

If you look at the Cox engine GIF animation, the exhaust ports open first and close first.

The charger (turbocharger or supercharger) pushes air into the cylinder even after the closing of the exhaust ports.

Judging from the 350bhp at 4,000rpm delivered by the 3.6l Cox 2-stroke engine, the specific torque – at the peak power revs - of the Cox is more than 170mN/lt, which is not possible for a naturally aspirated Diesel.


But even without asymmetrical timing of the ports, a turbocharger can overcharge the cylinder.
Say, by keeping the exhaust and the intake at higher than ambient pressures.

Thanks
Manolis Pattakos

[manolis]

Hello Zynerji

You write:
“Just a turbo for that sweet engine!
Great work!”


It may seem strange, however there is an easier / simpler way to increase the peak power of the PatOP, without a turbo.

The capacity of the PatOP scavenging pump is 850cc and has small dead volume.

With a throttle valve at the end of the exhaust pipe, the exhaust pressure can (during the scavenging) be kept higher than the ambient pressure. The more the throttle valve closes, the higher the pressure in the exhaust.

The piston type scavenge pump pushes harder the fresh air to get into the cylinder (because the pressure in the cylinder does not drop to the ambient pressure), spending mechanical power.



But when all the ports eventually close, the cylinder contains more fresh air, enabling more fuel to be injected and more power to be delivered.



Instead of burning 636cc of air (at atmospheric pressure) per reciprocation of the opposed pistons, with the restricting throttle valve at the end of its exhaust pipe the PatOP can burn 20 to 30% more air (though the fuel economy is better with the throttle fully open).

Thanks
Manolis Pattakos

[Pinger]

Why “the porting wont permit it”?

If you look at the Cox engine GIF animation, the exhaust ports open first and close first.

The charger (turbocharger or supercharger) pushes air into the cylinder even after the closing of the exhaust ports.

But that period is very brief.

Judging from the 350bhp at 4,000rpm delivered by the 3.6l Cox 2-stroke engine, the specific torque – at the peak power revs - of the Cox is more than 170mN/lt, which is not possible for a naturally aspirated Diesel.

It is scavenging well but 115lb.ft/litre isn't going to trouble the 4Ts. But that's ok - the 4Ts can't control their NOx.

But even without asymmetrical timing of the ports, a turbocharger can overcharge the cylinder.
Say, by keeping the exhaust and the intake at higher than ambient pressures.

Until the pistons melt.....

[manolis]

Hello Pinger.

You write:
“But that period is very brief.”


Don’t believe me. Believe Wartsila:



Their giant low-speed X92 marine 2-stroke is turbocharged (there is no conventional scavenge pump) and runs continuously at 21bar Mean Effective Pressure.


Let’s make a calculation / confirmation with the data provided in the above table.

With 21bar (21Kp/cm2 = 210N/cm2) acting on the 920m (92cm) diameter piston for a distance of 3,468mm (3.468m), the energy produced is:

(92^2*pi/4)*210N*3,468m=4,841,321Joule.

And with 76rpm the power per cylinder is: (76/60)*4,841,321 J/sec = 6132kW

See the power per cylinder at the table at the bottom left. It is 6,130kW


Is the 21 bar BMEP low or high?

It is very high.

To get an idea, the high tech naturally aspirating 4-strokes (of cars, motorcycles etc) run at ~12bar BMEP (which means a specific torque of ~100mN per liter of displacement).

By the way, the typical medium speed heavily turbocharged 4-stroke marine Diesel runs at lower than 25bar BMEP (which means the 2-stroke delivers 2*21/25=70% higher power per liter of capacity).

Worth to mention:
The BMEP in a 2-stroke “acts” per crankshaft rotation, while the BMEP in a 4-stroke “acts” per second crankshaft rotation.
For the same BMEP and the same capacity, the 2-stroke makes double power.


I hope it is clear now that a 2-stroke can be turbocharged and supercharged heavily, at a degree similar to that of the 4-strokes.



You also write:
“Until the pistons melt.....”

A Diesel runs from lean to extremely lean.

The temperature of the piston and of the exhaust gas are substantially lower than in a spark ignition / stoichiometric engines.

With the intake cycle pressure and the exhaust cycle pressure shifted by some 20%-30%, the temperatures are not for melting the pistons.


Thanks
Manolis Pattakos