2 stroke thread (with occasional F1 relevance!)

All that has to do with the power train, gearbox, clutch, fuels and lubricants, etc. Generally the mechanical side of Formula One.
manolis
82
Joined: Tue Mar 18, 2014 9:00 am

Re: 2 stroke thread (with occasional F1 relevance!)

Post by manolis » Sat Mar 24, 2018 11:25 am

Hello Pinger


Manolis wrote: “The lean mixture is not a limitation, any longer.”

Pinger replied: “Yes it is as it will incur greater pumping losses with its greater air requirement”


No.
The greater air requirement is not causing greater pumping losses.
Think of the Diesels.
Then think of the conventional 4-stroke spark ignition engines at idling, wherein a small quantity of air flowing through the engine (say 1/6 of the engine capacity per two crankshaft rotations) maximizes the pumping loss.


Quote from “your” link (end of last page):

The KHD Dz 700 two-stroke Diesel

Image

Dr. Ing (Engineer) Adolf Schnürle, who was employed by Humboldt-Deutz, had earlier developed a new method for two-stroke cylinder porting.

Schnürle was put in charge of the new Humboldt-Deutz diesel engine project.

Preliminary tests were conducted in Cologne, Germany on small single-cylinder and two-cylinder engines.

In 1937, the Dz 700 was built. It was an eight-cylinder, two-stroke, air-cooled, diesel engine.

The engine had a 3.15 in (80 mm) bore and a 3.94 in (100 mm) stroke, giving a total displacement of 245 cu in (4.0 L).

The Dz 700 produced 158 hp (118 kW) at 2,800 rpm.

The Dz 700 had a diameter of around 38 in (1 m) and weighed only around 120 lb (55 kg).

A blower (weak supercharger) forced air through manifolds in between and then into the cylinders.
Utilizing Schnürle porting, the two intake ports were positioned slightly lower in the cylinder than the two exhaust ports, and all were covered and uncovered by the piston.

End of Quote.



As a Diesel, it is unthrottled (permanently WOT).

At partial loads the engine runs from lean to very lean to extremely lean (idling).

This means that at partial loads it passes through the engine several times more air than if it were running stoichiometric.


Here is the BSFC of a Diesel versus the BMEP versus the RPM, from Achates Power (2-stroke, Opposed Piston):

Image


Quote from Achates Power: “The Achates Power engine has an extremely flat brake-specific fuel consumption (BSFC) map, illustrating that its high efficiency also extends to low loads.”

Take the point (1600rpm, 1.0bar BMEP).

While the air that passes per unit of fuel burnt is more than 10 times more than at the same revs and full load (at 14bar, 1600rpm), the BSFC is almsot the same.

If you take into account the significant mechanical friction of the engine (which absorbs a big part of the power provided at so light load), it is obvious that the pumping loss has very little to do with the quantity of the air passing through the engine.


Another example of what the pumping loss is (and is not), is the Toyota PRIUS engine that runs on Atkinson / Miller cycle and achieves top BTE.

At partial loads the intake valves stay open for a good part of the compression stroke to allow a significant portion of the entered air-fuel mixture to return beck to the intake manifold. Even at full load the 1/3, or so, of the entered mixture returns to the intake manifold; despite this “reciprocation” of air, the BTE is above 40%.



The KHD Dz 700 two-stroke radial Diesel of Adolf Schnürle provides its peak power at only 2,800rpm.
The modern Diesels for cars run at low revs, too (peak power at, or around, 3,500rpm is quite often).
The Diesel fuel injected into the compressed hot air needs time to prepare and get burned.
The part of the Diesel fuel that burns substantially after the TDC is burned at a substantially lower efficiency (because the actual expansion ratio is small).

The SkyActiv-X and the PatBam are, just like the conventional Diesel engines, compression ignition engines.
But they are also rid of the limitations of the Diesels.
The rev limit is way higher, the efficiency is higher (for the same compression ratio), the emissions are way lower etc.


Having said that,
do take another look at the PatBam 2-strokes, say like the following Radial:

Image

or like the following double OPRE Tilting PatBam for Portable Flyers:

Image

and think of them as Diesel 2-strokes wherein the Diesel fuel (and the problems associated with the Diesel fuel, like: slow combustion extending at low expansion ratios, low maximum revs and so low specific power (bhp per lb of engine weight), NOx emissions, particulates emissions etc) is replaced by low octane gasoline that burns almost instantaneously (HCCI substantially constant volume combustion).


As the OPRE and PatOP ( at www.pattakon.com ) are highly unconventional 2-strokes (among others, at 6,000rpm they provide to the combustion of the Diesel fuel the time provided in the conventional engines at 4,500rpm; differently: at the same revs with the conventional Diesels, the combustion in the OPRE and PatOP is more than 30% more constant volume),
similarly the design of the PatBam 2-strokes is highly unconventional, and as such you can’t apply what you know from the conventional 2-strokes.


In an internal combustion engine the most important thing is the “combustion” itself.

Even a small improvement of the combustion is more important than a big improvement on any secondary process of the engine (say, like a kinematic mechanism enabling a lower frictional loss).

And the controllable HCCI is anything but a small improvement over the combustion: according the Mazda, their SkyActiv-X HCCI is 20% more fuel efficient than their current high-tech SkyActiv-G engines.
And I think Mazda knows better than anyone else the problems of the combustion, because they had in procuction for half a centrury the Wankel Rotary engine.

I recomend this excellent video:



Thanks
Manolis Pattakos

Pinger
8
Joined: Thu Apr 13, 2017 4:28 pm

Re: 2 stroke thread (with occasional F1 relevance!)

Post by Pinger » Sat Mar 24, 2018 11:30 am

Tommy Cookers wrote:
Fri Mar 23, 2018 9:42 pm
fwiw my guess would be that thousands of DKW motorcycles got there first
DKW cars also
Possibly.
Wikipedia says Schnurle invented loop scavenging in 1926, the article says the Deutz diesel began trials in 1937 - so more than a decade apart. I thought though, that it was widely acknowledged that Schnurle developed the porting for a diesel. Not that it matters!

Pinger
8
Joined: Thu Apr 13, 2017 4:28 pm

Re: 2 stroke thread (with occasional F1 relevance!)

Post by Pinger » Sat Mar 24, 2018 1:49 pm

manolis wrote:
Sat Mar 24, 2018 11:25 am

No.
The greater air requirement is not causing greater pumping losses.
So an air pump delivering 10litres/s and another delivering 1000litres/s can both be driven by the same engine/motor?

The larger air requirement of the diesel is countered by a number of other factors which negate that inherent disadvantage, and without turbocharging to enable that air delivery diesels are gutless and useless.

The Miller/Atkinson cycle negates its double shuffling of air by enjoying the benefit of increased expansion ratio (which the turbocharged diesel also does across its turbine - but with significant back pressure which robs energy from the crank).

If air is so easily moved - why did WW2 fighter planes use engines delivering several thousand hp to drive their props?

johnny comelately
7
Joined: Thu Apr 09, 2015 11:55 pm
Location: Australia

Re: 2 stroke thread (with occasional F1 relevance!)

Post by johnny comelately » Sat Mar 24, 2018 1:57 pm

Pinger wrote:
Sat Mar 24, 2018 1:49 pm
manolis wrote:
Sat Mar 24, 2018 11:25 am

No.
The greater air requirement is not causing greater pumping losses.
So an air pump delivering 10litres/s and another delivering 1000litres/s can both be driven by the same engine/motor?

The larger air requirement of the diesel is countered by a number of other factors which negate that inherent disadvantage, and without turbocharging to enable that air delivery diesels are gutless and useless.

The Miller/Atkinson cycle negates its double shuffling of air by enjoying the benefit of increased expansion ratio (which the turbocharged diesel also does across its turbine - but with significant back pressure which robs energy from the crank).

If air is so easily moved - why did WW2 fighter planes use engines delivering several thousand hp to drive their props?
its a different pumping loss manolis was referring to and is not directly related to the air requirement (or delivery), more the parasitic losses

Pinger
8
Joined: Thu Apr 13, 2017 4:28 pm

Re: 2 stroke thread (with occasional F1 relevance!)

Post by Pinger » Sat Mar 24, 2018 2:03 pm

johnny comelately wrote:
Sat Mar 24, 2018 1:57 pm

its a different pumping loss manolis was referring to and is not directly related to the air requirement (or delivery), more the parasitic losses
Yes, but an engine isn't designed to solely idle. Well matched to its load, even a 4T SI engine shouldn't suffer overly from throttling losses. The point that a 2T behaves inversely to a 4T regarding throttling losses seems lost. But the on-the-road advantage of lower gearing is attributed only to Mazda's HCCI. Odd.

johnny comelately
7
Joined: Thu Apr 09, 2015 11:55 pm
Location: Australia

Re: 2 stroke thread (with occasional F1 relevance!)

Post by johnny comelately » Sat Mar 24, 2018 2:13 pm

Pinger wrote:
Sat Mar 24, 2018 2:03 pm
johnny comelately wrote:
Sat Mar 24, 2018 1:57 pm

its a different pumping loss manolis was referring to and is not directly related to the air requirement (or delivery), more the parasitic losses
Yes, but an engine isn't designed to solely idle. Well matched to its load, even a 4T SI engine shouldn't suffer overly from throttling losses. The point that a 2T behaves inversely to a 4T regarding throttling losses seems lost. But the on-the-road advantage of lower gearing is attributed only to Mazda's HCCI. Odd.
ITs late here but I dont think he is referring to throttling losses either, it is the parastiic losses of the piston not getting a fair go.
and re HCCI I have my doubts about that at the moment , whilst it provides for a very complete combustion burn , it is very quick maybe too quick in relation to the torque pulse effect. interesting to hear if its a noisy or rough engine (whats the term for that NH something?)

J.A.W.
85
Joined: Mon Sep 01, 2014 4:10 am
Location: Altair IV.

Re: 2 stroke thread (with occasional F1 relevance!)

Post by J.A.W. » Sat Mar 24, 2018 10:44 pm

manolis wrote:
Sat Mar 24, 2018 11:11 am
Hello all.

Are the best engines in the world the giant two stroke marine engines?

With the cheapest fuel (the residual of the distillation) they achieve more than 50% BTE (BSFC less than 160gr/kWh).

Typical compression ratio: only 12:1.

And they have a big exhaust poppet valve on the cylinder head.

http://www.pattakon.com/patmar/MANs35.gif


Adding an intake-transfer poppet valve on the piston crown, the lubrication problem of the 2-strokes has been solved (portless cylinder liner, 4-stroke lubrication):

http://www.pattakon.com/patmar/PatMar.gif

Thanks
Manolis Pattakos

Hi Manolis, surely - the term "best" is ah, best applied - via technical qualifiers..

The mammoth marine 2T's are very slow-revving so the single poppet valve is a convenience,
whereas in smaller scale, the faster operating Detroit Diesel required 4 poppets, & when Toyota
tried running one of their high rpm DOHC 4V SI engines in 2T mode, even 4 poppets proved inadequate.

I'd also remind you, that lubrication systems are neither 2T, or 4T in themselves, being independent
of whether every piston down-stroke is a power-stroke, or not. Many smaller 4Ts to this day, run on
a 'splash' of lube, & some even use a fuel/oil mix

Of course, neither of these issues directly relates to Mudflap's contention..
Dr Moreau sez..
"Who breaks the law... goes back to the House of Pain!"

manolis
82
Joined: Tue Mar 18, 2014 9:00 am

Re: 2 stroke thread (with occasional F1 relevance!)

Post by manolis » Sun Mar 25, 2018 6:35 am

Hello Pinger.

You write:
“So an air pump delivering 10litres/s and another delivering 1000litres/s can both be driven by the same engine/motor?”


Yes.

It depents on the pressure difference between the intake and the exhaust of the pump.

For instance, a propeller is a kind of air pump that even when runnig at extremely low pressure differences, it can deliver huge amounts of air.



To get what the pumping loss is, do open and study the PatAir at http://www.pattakon.com/pattakonHydro.htm from where the following quote is:


“The "Ingoing Air Control" (MultiAir / TwinAir) has some inbuilt disadvantages.

After the intake valve closing, the piston continues to move towards the BDC. The charge (air or mixture) inside the cylinder undergoes an expansion. The expansion causes the charge temperature to drop increasing the heat absorption from the hotter walls (cylinder, piston crown, cylinder head, intake and exhaust valves). After the BDC the piston compresses a hotter charge and restores less mechanical energy than the mechanical energy consumed to expand the charge.

Image

That is, pure mechanical energy (yellow) from the crankshaft-flywheel is consumed inside the cylinder, with only result the increase of the charge temperature. The lighter the load, the bigger this "mechanical energy loss" and the higher the temperature of the cycle. The lighter the load, the more "expensive" the mechanical energy consumed, because it was generated at high BSFC.

The early closing of the intake valve leaves more time to the charge turbulence and swirl to fade before the combustion. The slower the combustion, the less efficient and the less clean the operation of the engine. “


When you get how it works, you will be surprised how much better than the mechanical Valvetronic (used in almost all gasoline engines of the BMW) is.




You also right:
“If air is so easily moved - why did WW2 fighter planes use engines delivering several thousand hp to drive their props?”


Because they are required several thousands hp to push the airplane forwards at the required speed.

The power required increases with speed cube.


Do you know the Buatti Veiron?

Image

In order to achieve 400Km/h (111m/sec) speed, it consumes 1,000bhp (736kW).

(what this means in pushing forwards force?
736kW / (111 m/sec) = 736 kN*m/sec / (111 m/sec) = 6630N = 663 Kp (1460lb), while the weight of the car is 2,000Kp (4,400lb).
I.e. at the maximum speed the force from the road to the tires of the Bugatti Veiron equals to 1/3 of its weight).

Let’s calculate the power the same car needs at 100Km/h (62mph), i.e. at four times lower speed than its maximum:

1000bhp / (4^3) = 16bhp.

Yes, the same car that requires 1,000bhp to run at 400Km/h, the same car needs only 16 bhp to go at 100Km/h.


The airplanes had to be way faster than Veiron (otherwise they were the losers in the air fights).

Suppose Bugatti Veiron was capable for an 800Km/h speed.
What is the required power?
1,000bhp * ((800Km/h) / (400Km/h) )^3 = 1,000*2^3=8,000bhp.

So, don’t accuse the propellers for low efficiency; their actual peak efficiency is near 90%:.

Image


The air is quite easy to be displaced, provided there are not restrictions (like a throttle valve) in its flow path.

The air is so easy to escape, that the sealing of the air is a big problem.
A little bigger gap between the two ends of the compression ring of the piston of a Diesel, and (due to the leakage of air) the compression drops, the temperature of the compressed air drops and the injected fuel is not ignited any longer.


So, think of the pumping loss again.

And do look again at the plot (BMEP vs BSFC vs RPM) of Achates Power:

Image

wherein with ten times more air per unit of diesel fuel burnt (i.e. with an air fuel ratio more than 10 (1 is for the stoichiometric)) "your" pumping loss would be a catastrophe for the BSFC (Brake Specific Fuel Consumption).
But as you see in the diagram, the opposite is the true.

Thanks
Manolis Pattakos

manolis
82
Joined: Tue Mar 18, 2014 9:00 am

Re: 2 stroke thread (with occasional F1 relevance!)

Post by manolis » Sun Mar 25, 2018 6:57 am

Hello J.A.W.

The post you refer to says two things:


The most fuel efficient engines of the world are 2-stroke compression ignition Diesels having a big (diameter: about 50% of the bore) red-hot exhaust poppet valve at the top of the combustion chamber. I.e. a well designed efficient 2-stoke combustion chamber is not necessarily rid of valves (poppet, rotary etc).


The lubrication of a reciprocating piston engine having ports on its cylinder liner and piston rings passing over the ports, is way more problematic than the lubrication of the conventional 4-stroke engines and of the Port-Less 2-stroke engines (like the Patmar, or like the 4-strokes converted to 2-strokes (Renault, Toyota etc)).


Quote from http://www.pattakon.com/pattakonPatMar.htm:

"The problem as defined in Wartsila's(*) Technical Journal, Feb 2010 ( http://www.pattakon.com/tempman/Wartsil ... 2_2010.pdf ) :

"A slightly more ambitious idea is to apply the four-stroke trunk piston engine cylinder lubrication concept to the two-stroke crosshead engine, i.e. to "over-lubricate" the cylinder liner, apply an oil scraper ring, and then collect the surplus oil, clean it, and recycle it. This will of course be a radical change of concept, and whether or not it is viable remains to be demonstrated, but an outline exists and a patent is pending. The aim is to increase scuffing resistance and to achieve the same low specific oil consumption level as on the four-stroke trunk piston engines."
When the rings pass over a port, some oil is lost into the port.
On the hot exhaust ports things are worse: with tuned exhaust, and due to the inertia of the lubricant / fuel droplets, the loss of lubricant increases (the lube droplets cannot reverse direction to get back into the cylinder). And it is required a lot of lubricant around the hot exhaust port to prevent the start of scuffing."

End of Quote.


In previous posts it was presented the PatATE (more at http://www.pattakon.com/pattakonPatATE.htm ) :

Image

wherein the same ports are used for both, the intake and the exhaust, so they run at moderate temperature preventing the scuffing and decreasing the required lube specific consumption.

By the way: the recently received Search and Examination report for the PatATE is clean.


As for the combustion chamber shape,
even with poppet valves, the combustion chamber can be quite compact and efficient:

Image

With the PatBam HCCI combustion completing into, say, 15 crankshaft degrees from the TDC, I can't see a better shape (either for 2-strokes, or for 4-stroke engines).

Even if at some conditions the combustion is not true HCCI, it is still a fast combustion (much faster than the TJI of Mahle used in the F1 engines, wherein for the sake of a faster combustion, the size of the poppet valves and the breathing efficiency are heavily compromized).

Thanks
Manolis Pattakos

J.A.W.
85
Joined: Mon Sep 01, 2014 4:10 am
Location: Altair IV.

Re: 2 stroke thread (with occasional F1 relevance!)

Post by J.A.W. » Sun Mar 25, 2018 8:21 am

C'mon now Manolis..

You know full well that the conditions re: combustion chamber/fuel burn -are so far apart - between a huge slow-revving marine C.I. mill, & a sporty high rpm S.I. machine - to - in practical reality, defy useful comparison..

Edit: fixed typo.
Last edited by J.A.W. on Sun Mar 25, 2018 11:27 am, edited 1 time in total.
Dr Moreau sez..
"Who breaks the law... goes back to the House of Pain!"

johnny comelately
7
Joined: Thu Apr 09, 2015 11:55 pm
Location: Australia

Re: 2 stroke thread (with occasional F1 relevance!)

Post by johnny comelately » Sun Mar 25, 2018 9:28 am

Regarding the very low BSFC of the large two stroke engines, because BSFC (g/kw/hr) is partly a measure of power density (time = per hour) they have an inherent advantage just being a two stroke. Add in slow revving giving an advantage for good gas exchange and time for combustion (i know the arguments there but)

manolis
82
Joined: Tue Mar 18, 2014 9:00 am

Re: 2 stroke thread (with occasional F1 relevance!)

Post by manolis » Sun Mar 25, 2018 1:10 pm

Hello J.A.W.

Actually the same architecture of the giant marine diesels with an exhaust valve at the roof of each cylinder (modified with the oil control system of the PatMar to be clean) can be used in truck and boat engines.

Quote from http://www.pattakon.com/pattakonPatMar.htm

“The PatMar architecture fits to more than marine and power station applications.

A 90 mm bore, 400 mm stroke four in-line PatMar with cross-plane crankshaft (as in the animation), has:

- 10 lit "two-stroke" capacity,
- 13.5 m/sec mean piston speed at 1000 rpm (and 8 m/sec at 600 rpm),
- 1.6 m x 0.6 m x 0.5 m external dimensions.

With two counterweights secured on the crankshaft and another two counterweights secured on the counter-rotating camshaft that actuates the exhaust valves, this even firing engine has the vibration free quality of the best V-8 four-stroke engines.

With 20 bar Mean Effective Pressure, typical in the marine two stroke low speed engines, the power output at 1000 rpm is 340 KW (460 bhp).

Mounted horizontally on the floor of a truck:

Image

this medium speed Diesel propulsion unit has, in comparison to the current four-stroke truck Diesel engines:

- combustion in a space with better shape and smaller "surface to volume ratio",
- lower exhaust emissions,
- better fuel efficiency and mileage,
- lower lube specific consumption,
- lower friction: the loads on the bearings compare to those of a four cylinder 2 lit Diesel engine,
- improved reliability and longer time between overhauls. “

End of Quote.


When we say 2-stroke, typically we mean a high revving sport motorcycle engines wherein the power is the requirement.

But a PatMar 2-stroke like the above can be focused on the fuel efficiency and on the emissions..

Thanks
Manolis Pattakos

manolis
82
Joined: Tue Mar 18, 2014 9:00 am

Re: 2 stroke thread (with occasional F1 relevance!)

Post by manolis » Sun Mar 25, 2018 1:20 pm

Hello Johnny comelately.

You write:
“Regarding the very low BSFC of the large two stroke engines, because BSFC (g/kw/hr) is partly a measure of power density (time = per hour) they have an inherent advantage just being a two stroke. Add in slow revving giving an advantage for good gas exchange and time for combustion (i know the arguments there but)”


No.

The BSFC has nothing to do with power, power density, time etc.

The BSFC is the ratio of the quantity (mass) of fuel burnt to the mechanical energy delivered by the engine.

The total (chemical) energy contained into 80gr of heavy fuel oil (HFO / marine Diesel fuel) is 1kWh (here is how this results: the specific energy of the Diesel fuel is 45MJ/Kg; the energy contained into 80gr of Diesel is: 45MJ*0.080=3.6MJ=1KJ/sec*3600sec=1kW*1hour=1kWh).

1kWh is the energy provided into one hour by an engine operating at 1kW power. 1kW=1kJ/sec=1kN*m/sec.
1kWh=1kW*3600se c=3,600sec * 1kJ/sec = 3.6MJ.


A giant marine 2-stroke having a BSFC of 160gr/kWh, has a BTE of 50%.

Why?

Because if it were working at 100% BTE it would need only 80gr of fuel to provide 1kWh of energy on its power shaft.
Since it burns 160gr of marine Diesel fuel to provide the same 1kWh, its BTE is half than 100%, i.e. its BTE is 50%.


A model / RC engine burning gasoline fuel and having a BTE of 15%, has a BSFC of 530gr/kWh (which equals to 80/0.15).


The four small jet-turbines on the Delta Wing of Yves Rossy (Jetman) run at a BTE of ~2%, which means a BSFC of 4,000gr/kWh (which is 80/0.02).


The chemical energy contained into one Kg (1,000gr, 2.2lb) of Diesel fuel and into a Kg of gasoline fuel is more or less the same.

Each fuel is characterized by its own specific energy density:

Fuel Specific Energy (MJ/kg)
Gasoline: 47.3
Kerosene: 46.2
Diesel Fuel: 44.8
Ethanol: 29.7
Coal (Anthracite): 27
Methanol: 22.7
Wood: 15
Coal (Lignite): 15


So, if someone claims a 40% BTE for an engine that burns gasoline (as Toyota does for their PRIUS), then he can equivalently claim a BSFC (of 80/0.4)gr/kWh = 200gr/kWh.

If someone else claims a 400gr/kWh BSFC (Brake Specific Fuel Consumption) for his engine, the BTE (Brake Thermal Efficiency) of his engine is 80/400=20%


If something is confusing, let me know to further explain.


So, when Mazda claims a 20% fuel reduction for their SkyActiv-X HCCI engines relative to their current SkyActiv-G high tech, all they have to say is that from X gr/kWh BSFC of the SkyActiv-G, the SkyActiv-X has 0.8*X gr/kWh BSFC.

This is what the following plot of Mazda says:

Image

Similarly for the PatBam HCCI: all you have to say in order to describe its fuel efficiency is the BSFC, for instance: 178gr/kWh (which corresponds to 45% BTE).

Thanks
Manolis Pattakos

Pinger
8
Joined: Thu Apr 13, 2017 4:28 pm

Re: 2 stroke thread (with occasional F1 relevance!)

Post by Pinger » Sun Mar 25, 2018 6:23 pm

manolis wrote:
Sun Mar 25, 2018 6:35 am
The air is quite easy to be displaced, provided there are not restrictions (like a throttle valve) in its flow path.

Why does it take 1000hp to displace the air at 400km/h then?

manolis wrote:
Sun Mar 25, 2018 6:35 am

In order to achieve 400Km/h (111m/sec) speed, it consumes 1,000bhp (736kW).

Pinger
8
Joined: Thu Apr 13, 2017 4:28 pm

Re: 2 stroke thread (with occasional F1 relevance!)

Post by Pinger » Sun Mar 25, 2018 6:28 pm

manolis wrote:
Sun Mar 25, 2018 6:35 am


“The "Ingoing Air Control" (MultiAir / TwinAir) has some inbuilt disadvantages.

After the intake valve closing, the piston continues to move towards the BDC. The charge (air or mixture) inside the cylinder undergoes an expansion. The expansion causes the charge temperature to drop increasing the heat absorption from the hotter walls (cylinder, piston crown, cylinder head, intake and exhaust valves). After the BDC the piston compresses a hotter charge and restores less mechanical energy than the mechanical energy consumed to expand the charge.

http://www.pattakon.com/hydro_files/PumpingLoss.gif

That is, pure mechanical energy (yellow) from the crankshaft-flywheel is consumed inside the cylinder, with only result the increase of the charge temperature. The lighter the load, the bigger this "mechanical energy loss" and the higher the temperature of the cycle. The lighter the load, the more "expensive" the mechanical energy consumed, because it was generated at high BSFC.
That, is recoverable energy but for the heat transfer. Throttling losses are wholly worse and completely unrecoverable.
VW's new system behaves as Fiats - to avoid patent infringement while still exploiting a Miller/Atkinson cycle?