The PatMar is a two-stroke port-less through-scavenged crosshead engine having four-stroke lubrication, four-stroke specific lube consumption and four-stroke scuffing resistance.
Click on the above gif animations to dowload the full-size controllable windows exe animation.
The problem as defined in Wartsila's(*) Technical Journal, Feb 2010 (click here for the article):
"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."
(*) Wartsila is a global leader in complete lifecycle power solutions
for the marine and energy markets
The solution: The PatMar engine applies the four-stroke trunk piston engine cylinder lubrication concept to the two-stroke crosshead engine, i.e. it "over-lubricates" the cylinder liner, applies an oil scraper ring, and then collects the surplus oil, cleans it, and recycles it.
The PatMar not only increases the scuffing resistance of the two-stroke engines, but it achieves the same scuffing resistance as on the four-stroke trunk piston engines.
The PatMar achieves the same low specific oil consumption level as on the four-stroke trunk piston engines.
The cylinder lubrication of the state-of-the-art low-speed two-stroke engines is a "total loss" or "once through" system. Once the cylinder oil has left the lubricating device it is virtually "lost", which means the dosage of the cylinder oil is crucial. The cylinder oil is partly lost to the combustion space where it is burned, and partly to the piston underside space as sludge.
In comparison, in the four-stroke trunk piston engine the cylinder liner is virtually over-lubricated with an oil scraper ring on the piston scraping the surplus oil back to the oil pan. The cylinder lubricating oil of the four-stroke trunk piston engine is identical to the engine system oil used for bearing lubrication and cooling purposes. A small amount of the cylinder lubricating oil bypasses the piston rings and ends up in the combustion space where it is consumed, however the oil scraper ring of the piston scrapes most of the oil supplied to the cylinder liner back to the engine's oil pan, from where it is drained, cleaned and recycled.
The pants (red) of the PatMar engine comprise ports (the pant-legs) for the communication of the space underside the piston crown with the air tank (brown). The crosshead (blue), the wrist pin (yellow) and the connecting rod (green) move between the pant-legs. The crosshead slippers (cyan) thrust onto the crosshead guides (not shown).
An oil scraper ring on the piston, scraps the surplus oil from the cylinder liner (brown) back to the oil pan, as in the four-stroke engines.
An oil scraper ring on the "waist" of the pants scraps the oil from the inner surface of the piston skirt back to the oil pan.
The empty space between the cylinder liner and the piston skirt is for the scraped oil.
The friction of the rings between the inner surface of the piston and the "pants waist" is more or less the friction of the sealing rings between the piston rod and the casing of the conventional low speed 2-stroke crosshead engine.
The space underside the piston crown is sealed from the crankcase.
The cylinder liner is rid of ports.
The piston crown (cyan) comprises an intake poppet valve (gray) that controls the communication of the combustion chamber with the space underside the piston crown. A restoring valve spring and the combustion chamber pressure keep the intake valve closed. During the compression, the combustion and the expansion, the closed intake valve follows the motion of the piston.
When the intake valve opens,
for instance by landing onto a shock absorber at the upper center of the pants (another shock absorber between the valve guide and the valve head smoothes the landing of the valve back onto the piston crown during the valve closing),
or, for instance, automatically by the air pressure difference between the combustion chamber and the air tank (the more the intake valves on the piston crown, the better),
or, for instance, by the hydraulic HyDesmo variable actuation mechanism controlling a pin, at the upper center of the pants, wherein the valve lands and opens; the piping for the actuation of the pin is passing through the pant-legs. For a smooth, quiet and reliable opening and closing of the intake valve, the moment the valve lands onto the pin (valve opening), or onto the piston crown (valve closing), the motion of the pin needs to match with the motion of the piston (as in the PatPortLess),
it allows air from the air tank to scavenge the cylinder. The air from the air tank arrives at the space underside the piston crown through the pant-legs / ports; the pant-legs provide adequate port-area, as shows the (red) cut-view at right:
The pressure of the scavenging air and the supporting of the intake valve during the valve opening, are the only loads the pants bear.
At the BDC the piston skirt extends completely outside the cylinder liner, enabling over-lubrication and cooling.
The air entering into the combustion chamber through the piston, cools the piston crown, the intake valve and the piston skirt from "inside".
Besides the "four-stroke lubrication" and the better cooling of the piston, the replacement of the cylinder-liner-intake-ports by the piston-crown-intake-valve enables the asymmmetrical inlet-opening and inlet-closing: the intake valve can, for instance, close at, or even before, the BDC.
With a system like the HyDesmo actuating the intake valve, the control over the engine operation / efficiency / emissions is better; for instance, at "slow steaming" the closing of the intake valve near (or at) the BDC increases the effective compression ratio (not possible in the current designs with the intake ports at the lower end of the cylinder liner).
With the "pants", the state-of-the-art low speed two-stoke marine and power station engines (like the Wartsila - Sulzer, the MAN B&W etc):
which are the by far most efficient stand alone working machines today, become:
- even more reliable (increased scuffing resistance, longer time between overhauls),
- cheaper and simpler (no special equipment and control for the cylinder liner lubrication),
- more economical (lower lubricant specific consumption),
- greener (less lubricant into the combustion chamber, better combustion, reduced emissions).
With the "pants" the low-speed two-stroke engines combine their own advantages with those of the medium-speed four-stroke engines.
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:
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.
The PatMar fits to smaller strokes, too.
Click on the above gif animations to dowload the controllable windows exe animation.
Two reed valves (one per pant-leg) form a built-in zero-cost supercharger.
Smooth intake valve landing mechanism details.
Click on the above image to enlarge.
Above: the piston is at the BDC and the intake valve is widely open.
The small yelow "air piston" (which is secured on the green/cyan valve stem and is slidably fitted inside the small blue cylinder) provides the neccesary damping action and is air cooled.
Below: the piston is at middle stroke and the intake valve is closed.
Bellow: another version of the PatMar wherein all the rings are stationary.
Click on the above image to enlarge.
Compression and oil control rings are mounted into grooves / ring-lands at the bottom of the cylinder; these rings slide over the external surface of the piston skirt (red), they seal the combustion chamber from the crankcase and they run "cold".
Other rings mounted into grooves / ring-lands made on the "waist" of the pants-shaped shell (cyan) slide along the inner surface of the piston skirt (red) and seal the space underside the piston crown from the crankcase.
Without rings sliding over the inner surface of the cylinder (blue), the cylinder can be made of various materials and needs not special treatment (like honing).
From a conventional 4-stroke to a 2-stroke PatMar.
The crosshead architecture releases the cylinder liner from the thrust loads, allowing way shorter connecting rods.
The con-rod to stroke ratio in the giant 2-stroke crosshead marine engines (those with the over 50% brake thermal efficiency) is between 1.0 and 1.25, while the typicall con-rod to stroke ratio of the 4-stroke marine engines is over 2.0
The way shorter connecting rod of the crosshead PatMar allows a total engine height not bigger than the 4-stroke of same piston stroke.
If Wartsila wants to double the 2MW per cylinder power output of the Wartsila 64 (currently the top 4-stroke medium-speed marine engine) Wartsila can easily modify it to turn into a PatMar 2-stroke crosshead. All four valves of the cylinder head become exhaust valves. The piston stroke, the piston bore and the total engine height remain the original.