Simon,

That Wartsila-Sulzer engine is a uniflow scavenge 2-stroke design. There are ports uncovered by the piston rings, but only for inlet. The single huge valve is for the exhaust, and is uncovered at the same time as the inlet ports. MAN B&W have recently done a lot of work on hydraulic actuation of this valve, for improved efficiency by making engine run closer to Atkins cycle (exhaust expanded more than starting volume of compression stage). The efficiency comes from the large volume:surface_area possible with such a huge piston, since it reduces heat rejection to the cooling system. The turbo can then capture more of the exhaust stream power.

For a while truck diesels (eg Detroit Diesel 71 and 92 series) worked on the 2-stroke uniflow scavenge principle, but ever tightening emission standards forced them to 4-stroke. The Wilksh Airmotive is also another example of this type of engine, while the Deltahawk is a Snurl type loop scavenge 2-stroke. For some additional mechanical complexity uniflow offers a less turbulent, hence more efficient, charge scavenge. There is actually an arguement for putting the inlet valve at the top with cylinder wall ports as exhaust, because the combustion fluid is naturally moving downwards at the end of the expansion process. Mercedes and AVL have also done a lot of development on automotive diesel 2-strokes, but found that they offered little benefit over a well designed 4-stroke. This is particularly true once you depend more on the turbine for compression/expansion.

To try to answer your questions about loop scavenge 2-stroke, Nick Lappos style: :ok:

1. It is simpler than a 4 stroke turbodiesel --> less error sources, cheaper to build and overhaul (important with our low numbers of engines sold in aviation, which makes us always more expensive than the road traffic folks!), possibly easier certification

Slightly, since you do not need expensive valve and cam mechanisms. Although the castings and cylinder liners require additional complexity and machining to get the ports right. Since casting development cost is ideally a one off for the entire production run, while machining costs are per engine, there is an arguement for simpler castings and more machining for low volume designs. There is also a good "parts bin" for valve train components.

2. It (should) have a better power/weight ratio than a 4 stroke. Thus you can possibly (as in the case of delta hawk) also save the reduction gearbox (with airplanes) and run the diesel engine at rpms that are more efficient for diesels, anyways

Since the scavenging process is far from ideal the power/swept volume advantage is maybe 1.5 over a 4-stroke. In practice with turbo charging this advantage is eroded, since you can find other ways to use additional blowdown pressures - overcoming flow losses or even driving ancilliaries, with a combined generator/turbine. This means that the pressure difference between inlet and exhaust possible in a 4-stroke is advantageous.

Any engine will offer better power/mass at higher RPM. Ships use low RPM diesels for better matching the propeller speed, in the same way that aircraft do. A gearbox will always weigh less than increasing engine swept volume for low RPM, at some cost increase. This is particularly true for helicopter applications where RPM reduction is a given. Belt drive is also a reduction possibility, albeit not as reliable,

3. There are no exhaust emission standards in aviation (yet) which might be a bit tougher to fulfill with these engines
What do you think?

That is true, and was also recently true for motorbike production. Once motorbikes became subject to emission standards, then all moved from 2-stroke to 4-stroke. For the costs associated with a new engine design, there is an arguement for going with 4-stroke from the outset. The Wilksch already has a valvetrain mechanism, so the cost is already there. The real problem is oil leakage into the exhaust, from the lubricated piston skirt to the ports, which is inherently reduced in a 4-stroke - although still subject to much development. In fact Detroit Diesel carried over a clever crosshead skirt piston design to seperate out the functions of load bearing and sealing in the 4-stroke S60 engine (no, that one is not a Sikorsky design).

Hope that helps!

Mart

Rudestuff, i meant to answer this post but got distracted and forgot! :\

Ah those hanger conversations on a rainy day - distant, but fond memory...

The big problem here is reliability. Very true you could replace the engines every 500 hours, although the novelty might wear off after the 4th or 5th time. In general the increased wear is an indication that the engine is seeing more stress, so will be less reliable in difficult circumstances. If the engines were at 490 hours and you pulled collective in a VRS demo there might be an inconvenient clunk sound, with a resulting auto demo. This might not be in a demo. :oh:

In principle you could put in any engine, but i would rank heli engine requirements as:

1. Reliability
2. Power-to-weight
3. Efficiency

Not putting cost there, because the objective of any designer is to achieve the above at the minimum outlay and running costs... :8

Mart