2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello Tommy Cookers.

Any drawing / photo of the master rod / slave rods you mentioned?

Thanks
Manolis Pattakos

[manolis]

Hello Tommy Cookers

You write:
"the 4 bank approach was (in)famously used by RR in the WW2 4 stroke 24 cyl Vulture (and the 2 stroke Exe and Pennine)"


According the Internet, this is the R-R pennine sleeve-valve aero engine:




Is it a 2-stroke or a 4-stroke engine?


The master rods seem conventional master rods. This makes the stroke of the “side” cylinders substantially longer, it also makes the leaning angle of the opposite to the master-rod slave-rod, by far bigger than the leaning angle of the master rod.

I.e. each crankpin of the crankshaft drives four pistons which performed substantially different motion (substantially different piston motion profiles).


The R-R pannine 24 cylinder engine can be regarded as comprising 4 different straight-six-cylinder engines, each having its own piston-motion profile.


The width of each crankpin (the width of each master connecting rod) is some 10% bigger than the bore (as shown in the red ellipse).

The crankshaft is assembled (see the tightening nut in the yellow circle).

The mechanism driving the sleeve valves is shown in the blue ellipse.

At the top end of each cylinder there is an immovable “piston” (comprising compression rings), as shown in the green ellipse), just like in the famous 4-stroke Bristol Hercules and Bristol Centaurus Radial engines.

Thanks
Manolis Pattakos

[Tommy Cookers]

my source (the Graham White book) now seems misleading re Exe and Pennine big end design
the Exe was a split b.e. (like the Vulture), the Pennine wasn't
and apologies, yes of course, they were 4 strokes, but air-cooled, so the bore centres would be larger and so ease the b.e. design

fwiw I now think the Vulture piston motion had 3 geometries, eg for commonality of link rods and compression height ?
(I have scanned the diagram from the GW book but attempts to send it to you in a PM don't seem to work)
but surely a geometry common to all the link rod piston motions is in principle possible ?

another source says the only remaining Vulture problem was b.e. bolt failures
and that RR wanted it cancelled as they were busy and preferred their later PV24 design (presumably another liquid-cooled X-24)
in stating that the Vulture had Merlin-size bore centres maybe GW is being informed by recollections of the PV24 design
an X-24 basically as 2 short-stroke Merlins that could have been looking at 4000 - 4500 hp

[manolis]

Hello Tommy Cookers.

You write:
"I have scanned a diagram but attempts to send it to you in a PM don't seem to work"

E-mail it to me: manolis@pattakon.com


There are several design compromises in the R-R Pennine (asymmetry among cylinders, complication, friction, cost, crankshaft strength, etc, etc).
The PatAT Cross Radial 4-cylinder 2-stroke turbocharged Diesel architecture seems a good choice for aero engines in comparison to the R-R Pennine and in comparison to the conventional Opposed Piston engines (Achates Power, EcoMotors etc).

Thanks
Manolis Pattakos

[manolis]

Hello Tommy Cookers

Thanks for the drawing:



showing the split of the master rod of the Rolls-Royce Vulture.


You write:
“fwiw I now think the Vulture piston motion had 3 geometries, eg for commonality of link rods and compression height ?”

If I understand the question correctly, the answer is: No.

The Vulture piston motion (and the Pennine piston motion) had 4 geometries.

The stroke of the “master piston” in the “top” cylinder and the stroke of the piston in the “bottom” cylinder were equal.
But the leaning angle of the slave connecting rod in the “bottom” cylinder was by far bigger than the leaning angle of the master rod, resulting in substantially different piston motion geometry and in substantially heavier thrust loads on the “bottom” cylinder liner and on the bottom piston skirt.

The two side pistons had common stroke which was substantially longer than the “master piston” stroke.
But the geometry of the two side pistons was not the same: the one side piston was performing a faster compression and a slower expansion relative to the other side piston. See the animations in earlier posts.

So, in total there were 4 different “piston motion geometries” (or 4 substantially different “piston motion profiles”) in the Vulture and in the Pennine engines.


In comparison the PatAT Cross-Radial 4-cylinder:



has a unique piston motion geometry.

It results by combining four PatAT engines like:





that share the same crankshaft, giving full geometrical symmetry and “perfect” balancing. It also manages a substantially asymmetric transfer that helps the turbocharger to scavenge the cylinders and to maximize the quantity of fresh air trapped into them (turbocharged Diesel).

See also how it manages to have a 4-stroke lubrication in the crankcase and in the lower side of the cylinder liners whereon the thrust loads are taken (the oil scraper ring never passes over ports).

Thanks
Manolis Pattakos

[J.A.W.]

Manolis, thanks for the interesting mechanical analysis..

R-R never got far with their single crankshaft X-configuration engines & eventually did a 'Chinese copy' of Napier's Sabre..
( see 1st post of current page of "Feliks..." thread for a Sabre H-24 graphic) R-R's Sabre copy failed due to harmonics issues..

Unlike the Sabre, the R-R H-24 fired 2 cylinders simultaneously, & coupled the crankshafts through a simple spur gear..

[manolis]

Hello J.A.W.

The famous 4-stroke Sleeve-Valve engines have several advantages and a couple of compromises like the inevitable high specific lube consumption and the passing of some compression rings over ports made on the sleeve valve (actually the passing of ports of the sleeve valve over some immovable compression rings).

Here is a modification of the 4-stroke Sleeve Valve design to 2-stroke:



It is substantially different than the R-R Crecy.

The exhaust ports are at the top of the cylinder.

The transfer ports are at the bottom of the cylinder.

The exhaust starts substantially earlier than the transfer; the exhaust ends before the transfer (asymmetric transfer).

Such an arrangement would achieve near double specific power, without adding significant compromises / side-effects to the 4-stroke Sleeve Valve engines.

As in the 4-stroke Sleeve-Valve engines, an immovable piston seals the top of the cylinder.


A bigger stroke would improve the “through scavenging efficiency” in expense of higher mean piston speed of the moving piston.


But then, why not to move the immovable piston too, and half this way the mean piston speed, and eliminate the Sleeve Valve completely, and make the engine full balanced as regards the inertia forces and moments, and eliminate the load on the main bearings of the crankshaft, like:



(more for the above Opposed Piston PatOP engine at http://www.pattakon.com/pattakonPatOP.htm )

Thanks
Manolis Pattakos

[gruntguru]

I see what you did there! You fixed the sleeve to the block and attached the junk piston to the crankshaft (very clever). The reed valves are a nice touch too.

[J.A.W.]

Well Manolis, that sleeve valve 2T does look a bit like Uniflow's made in metal unit, way back on page 5 of this thread..

The Sabre was intended to be capable of running as a 2T, but even running 1/2 time as a 4T, the best available prop's
of the day - were at their limits coping with its power outputs..

& as for wear/oil consumption, it was noted that the relative motion between sleeve & piston actually reduced these things,
by comparison to more conventional aero-engines.

I guess a frictional drag/wear/heat analysis would be a meaningful comparative for your interesting latest design.

[manolis]

Hello Gruntguru.

You are right.

You can see the transition from the 4-stroke Sleeve Valve to the 2-stroke PatOP the way you write: the sleeve is fixed to the block and the junk piston turns to a reciprocating / conventional / working piston.


This animation:



shows how simply the above PatOP turns to a twin-charge engine.

The compressed air from the turbocharger goes to the intercooler – not shown – and then it feeds the PatOP engine through the open throttle valve (green).

At high revs – heavy load, i.e. when the pressure from the turbocharger is high, the one-way valve stays closed / idle.

At lower revs / partial load, i.e. when the pressure from the turbocharger is low, the “green” throttle valve closes and the PatOP engine uses its built-in piston-type scavenge pump and the one-way valve for the scavenging.

Thanks
Manolis Pattakos

[Tommy Cookers]

looking for something relating to the benefits of such lavish asymmetrical transfer timing, I found this read
http://dragonfly75.com/motorbike/2StrokeDesign.pdf

[manolis]

Hello J.A.W.

You write:
“Well Manolis, that sleeve valve 2T does look a bit like Uniflow's made in metal unit, way back on page 5 of this thread..”

The sleeve valve of R-R Crecy:



and the sleeve valve of Uniflow:



have, both, a substantial difference as compared to the present sleeve valve:



The top end of the R-R / Uniflow sleeve valves remain permanently into the combustion chamber, making inevitable the location of the exhaust ports substantially lower in the cylinder and closer to the transfer ports.

This, in turn, makes easy the “short-circuit” of the transfer and exhaust ports (unless the piston stroke is a couple of times longer than the bore) and increases substantially the quantity of the residual gas worsening the combustion and overheating everything.

In comparison, the exhaust ports of the present sleeve valve arrangement, shown in the stereoscopic animation, are at the top end of the cylinder, away from the transfer ports. The residual gas decreases. The top end of the sleeve valve exits from the cylinder and is cooled. The shape of the combustion chamber remains the same of the 4-stroke Pennine, Vulture, Centaurus, Hercules etc.



You also write:
“The Sabre was intended to be capable of running as a 2T, but even running 1/2 time as a 4T, the best available prop's of the day - were at their limits coping with its power outputs.”

With the Sambre running as a 2-stroke and the power almost doubled at the same revs (note: the valve time area doubles in comparison to the 4-stroke sleeve valve version because all the top periphery of the sleeve is covered with exhaust ports, and all the bottom periphery of the sleeve valve is covered with transfer ports), the problem of the propellers of those days was an easy one.
All it takes is to eliminate / to throw away half of the cylinders of the engine (say the top 12-cylinder boxer of the "H") and so almost half of the engine weight, resulting in more lightweight airplanes and cheaper / easier to make airplanes.



You also write:
“& as for wear/oil consumption, it was noted that the relative motion between sleeve & piston actually reduced these things, by comparison to more conventional aero-engines.”

The wear yes (3,000 hours TBO in the Bristol Sleeve Valve Radials), but the specific lube consumption?

The sleeve valve architecture provides several ways to the lubricant to get burned or be lost.
If you had to design a modern sleeve valve engine, how could you control the lubricant? (and the emissions caused by the increased quantity of lubricant into the combustion chamber)

By the way, less wear does not mean less friction.



You also write:
“I guess a frictional drag/wear/heat analysis would be a meaningful comparative for your interesting latest design.”

Correct.

The Sleeve Valve engines (maybe the best 4-stroke engines in their time) were used as a reference point in order to explain the several advantages of the PatOP Opposed Piston engine.
If you follow the progressive transition from the Sleeve Valve 4-stroke engine to the PatOP engine, you understand that without introducing significant side effects, several things are substantially improved.

Worth to mention:

Besides the previously mentioned advantages, the PatOP architecture transfers the thrust loads away from the hot region of the cylinder liner, away from the cylinder ports / openings, which is a significant change allowing 4-stroke-like lubrication.

With the PatOP / OPRE pulling-rod-architecture it is provided some 30% longer piston dwell at the combustion TDC: the piston remains at the top 10% of its stroke for some 30% additional absolute time, allowing more efficient combustion at the same revs, allowing also Diesel / compression-ignition engines providing substantially more power.
The revs of the peak power go from the, say, 4,500rpm of the present conventional push-rod automotive Diesel engines to 6,000rpm, increasing a lot the peak power.

Thanks
Manolis Pattakos

[manolis]

Hello Tommy Cookers and thanks for the link.

Achates Power has an analysis of the efficiency of the various scavenging methods used in the 2-stroke engines.

Hugo Junkers was advancing, in his famous aero Opposed Piston Diesel engines, the exhaust crankshaft by 11 crank degrees relative to the intake crankshaft.

In the Napier Deltic Opposed piston, the phase difference increased from the optimum 11 to 20 degrees to comply with the specific architecture.

The scavenging of an extremely oversquare engine like the OPRE Tilting, is quite different:



With 30mm piston stroke and 84mm bore, each cylinder appears extremely oversquare (fitting with "cross uniflow(?) scavenging); but the set of the two cylinders (with the common and compact combustion chamber in the middle) has a total stroke of 60mm for an 84mm bore, which is quite normal (the total stroke to bore ratio equals to that of the BMW's boxers and is by far longer than Ducati's Panigalle 1299 wherein the "same" stroke (60.8 mm) is combined with a 40 % bigger (116mm) bore).

Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis, here is a couple of period Bristol adverts, ( inc' this one - repeating the claim of lower S-V oil consumption),

http://www.flightglobal.com/pdfarchive/ ... 01244.html

The Bristol Centaurus even allowed an effective cruise speed good enough to take back the London-to-Rome
flight speed record from the then new turbo jet, not bad for an air-cooled radial.

http://www.flightglobal.com/pdfarchive/ ... 00969.html

The Sabre power increases were wanted, for increased performance, but the prop's themselves were then at their limits.
Shedding a prop blade in flight/combat/high power settings - wasn't too welcome,
(& the rated take-off hp had increased - with added boost - from 2,180hp @ +9 lb to 3,500hp @ +20lb , within a few years).


Here is a period NACA research paper examining some of the flow efficacy aspects of S-V porting.

http://naca.central.cranfield.ac.uk/rep ... rt-717.pdf

[Tommy Cookers]

for my benefit at this point ....

http://pigeonsnest.co.uk/stuff/deltic/chapter4.pdf
http://pigeonsnest.co.uk/stuff/deltic/deltic.html