fredag 4. juli 2014

Deltic Rebirth? The Opposed Piston Engine Strikes Back?

I "stumbled upon" in my own way, the "new" breed of opposed piston engines which are allegedly being funded by the Bill Gates Foundation. They are two stroke turbo diesels which use a double cylinder per bank, with long conrods connecting the outer piston effort to a common single axis crank shaft.
Although this approach solves the "problem" of reconciling forces* at both ends of each cylinder into a single power stream, it does still have the main inherent emissions issues of a swept port,  two stroke design  ( * in the Napier Deltic this  necessitated an expensive gear box which reverses the one gear lower sump shaft with the other two while then marrying all three axis to a single power output shaft)  

The main issue is that lubrication oil from the piston side walls and the sumps or injection sprayed "dry sumps" ends up exiting with the exhaust and burns partially, or worse, builds up (especially after time on idle )around both the exhaust and the air intake vents/ports and then leads to incomplete combustion when injection and final compression to ignition happens. Incomplete ignition is one thing usually completely avoided in four stroke diesels and modern 2 stroke traditional cylinder head motors like the EMD 645 derived engines.

Deltic pistons are more complex than traditional pistons because they require oil routing in order to effect cooling of the piston. In the absence of a heat dissapating cylinder head with water cooling or oil cooling channels in such heavy casting, then much of the heat from ignitions builds up on the piston heads and in napier's development process they identified that this needed oil cooling for the piston heads:  which means even more lub' oil than  the spray injected crank ends and bearings area can gather in the legnth of the cylinder where it meets the pistons. Inevitably some of this lub oil migrates along the piston head and is swept into the combustion area of the cylinder and exhaust chambers. The air intake side is under positive pressure by super charging (roots blower in a deltic) and turbo charging, and this stops build up to some extent at the lip of the intake but that may exacerbate migration of oil into the cylinder due to this positive pressure.

This oil migration to the combsution area of the cylinder is limited by the piston rings around the top third of the piston head however they do not provide a complete scraping or sealing when they sweep over the "gas"  ports.

One way of further reducing oil ingress to combustion areas is to then add more piston rings along the head and /or an oil scavenging scraper collar which is a consumable part fitted to the piston's base and scraping a swept path at a desirable D400s /BR Class 50, one reason they were too effective in preventing oil getting to the piston walls were it is needed.  In a swept port dry sump two stroke engine with oil cooled piston heads such a collar may well work quite well, and may be something which can be fitted from the crank case inspection points in a larger engine or by at least avoiding taking the pistons out as a outward end extra ring would otherwise.

Deltic "clagg" ie smokey exhaust  was always a feature of the power units when they were first revved from idle. All the excess lub' oil suddenly gets heated and blown out, some burning and there is also a lot of water vapour formed if the exhaust collective is cold , leading to plumes of white to grey smoke steam with a rancid smell. Add a little incomplete combustion of diesel from the air- in-port side issue and you get nasty deltic clagg, not good if you ever want to hear that Napier drone in new locomotives, gun boats etc.

Under way however, as you will see in the opening credits of the 1960s classic 'Get Carter', the deltic power units produced quite little smoke in service life.  So at 1500rpm lub oil was not able to build up due to the scraping of the piston and higher positive pressure on the intake side.

General motors EMD have overcome much of the issue with swept air ports while also steadily increasing the volume and pressure of air which is aspirated by/into the cylnder. So it is not a wild goose chase to clean up a deltic.

One approach is to use a different porting and exhaust rooting design, where you may have a positive pressure at the crucial timing points when the piston sweeps. Another is altering porting design , perhaps with multiple ports, which resist ingress of lub oil.

Alternatively the most complex which is not soild state, ie it requires new moving parts, is to utilise a rotating valve mechanism on the exhaust port which allows for the port to be shut when the piston is sweeping it (or if exposed to the crank case side on an imaginery longer stroke design). This could be a fairly simple cam lobe type arrangement on a single shaft per bank of cylinders, where the valve is wheel like and has an open side cut which is designed to be timed in with necessary exhausting. Alternatively a camshaft lobe  valve actuation could be used, if then introducing a degree of complexity into otherwise an elegant design with a high degree of "solid state" solutions.

Another alternative is to reduce the amount of lub oil needed to lubricate the walls of the piston and to dissapate the heat of combustion from the piston head. Here in Napier's day there was nothing available apart from metal with the best qualities to work with oil cooling, but now ceramic piston heads, rings and cylinder liners are realisable. The deltic has modest piston sizes, modest compression and kw/cylinder and a design which lends itself to cooling circuit improvements in the cylinder linings and design of the triangular engine "block".  All these point to use of low lubrication ceramics as a possibility.

I prefer the idea of the following set of " solid state" technolgies over cam shaft valve systems:

1) ceramic piston heads and rings
2) oil blocking and scraping system on the crank side of the piston which prevents most crank case oil from entering the swept area and reaching the gas ports or combustion area of the cylinder.
3) a high tech solution for the small amount of piston wall and conrod pin which need lubrication: perhaps using low pressure diesel injection, or a complete burning lubricant or a non burning synthetic lubricant.
4) Twin scrolled air intakes which are thus always providing a positive pressure at the in-port side, even at idle from super and turbo charging
5) a design for exhaust manifold which enables a slight positive back pressure on the exhaust port when it is being swept such that it presses lub' oil away.