supercharged V12 2 stroke

Here is text about crank case compresion and supercharging in 2 stroke engine..

**LINK**

"This is clearly impossible in normal two-strokes in which exhaust and ports are open simultaneously. They can, however, be "super-scavenged" by an oversize charging pump, often with some advantage, but at the expense of economy, because wastage of fuel through the exhaust port is inevitably increased. This may be tolerated in racing engines, and is no disadvantage in diesel engines of the injection type, which are charged with air only. In my experiments, however, I have failed to obtain any substantial increase of performance in two-strokes by increasing the charging pump volume."

Wrote by E. T. Westbury

As Mr Westbury said, you can't really get any supercharge on a conventional two stroke, since the exhaust port will usually be open until after the transfer port has closed. So the pressure in the cylinder cannot be higher than the exhaust pressure. The exhaust pressure can of course be made to be high at just the right moment with a suitable tuned pipe, but that will only work over a narrow power band.

The DKW mentioned above is a split single, eg two cylinders with a common combustion space, and both worked off the same crank. This allows assymetrical timing, eg the exhaust opens before the transfer, and then closes before the transfer. That would allow the cylinder to be pressurised by the crankcase pressure, and if there is a blower driving the mixture into the crankcase, supercharge will be acheived. However I think that even with this advantage, a lot of mixture must have just gone straight out the exhaust port before it closed, going by the fuel mileage I have seen quoted.

I guess for a model as per the original post, the convenience of having a common crankcase might make it worth having a separate blower to do the scavenging, and if you don't overdo the crankcase pressure the economy should not be too bad, at least by the usual standards of small engines, which tend to be pretty thirsty. It won't actually be supercharged, unless you can provide for the transfer to close after the exhaust. Some two stroke diesels manage this by having two pistons in one cylinder, inlet controlled by one piston and exhaust by the other. Then by offsetting the cranks a little you can get assymetrical timing.

John

here's the finished block only need to do the engine mounting holes. oh and the obvious finishing of the outside surfaces which will be done when all machine work is completed.

will keep you posted

dean

p.s. i hear what your saying about the open exhaust port but there is also a massive valve overlap in all high performance 4 stroke engines that run blowers too, so really it is not that different to a schnurle ported 2 stroke

Can't remember who, but a well known engine designer once called a racing 2 stroke " a mobile petrol cooled exaust system " Sorry to be a bit off topic.

Dean

I am sure you have looked into this in great detail but I don't believe all blown four stroke engines have large valve overlaps. Pressure is far more effective at scavenging than a working exhaust system so the need for, let's say, 60 degrees of overlap disappears. In fact with an external fuel system, carburettor or injection, all you would be doing is blowing fuel down the exhaust system. Not what you want with a petrol engine and if you are interested in fuel consumption. However if you are burning methanol, with its low calorific value and, particularly, high latent heat of evaporation, large overlaps can be a major advantage due to the evaporative cooling from the fuel. This comes at the expense of fuel consumption. Alfa Romeo used this system in their early 1950s GP engines, they had a fuel consumption of something like 3/4 m.p.g.

JA

I was under the impression that forced induction (super/turbo charging) was intended to increase the volumetric efficiency of an engine - two or four stroke. All you are trying to do is fill the cylinder each time instead of only part filling it due to gas flow restrictions through valves/ports and inlet tracts etc.

Obviously, once you can fill the cylinder 100% and that's good - a little more would be better and clearly a LOT more would probably be just about right - but then we discover that we need stronger engine bits, special fuel, better cooling and all sorts of other problems to fix until we end up with ....... an F1 engine

Mark

Valve overlap is not the same thing as having the exhaust open after the transfer port closes. With overlap, the exhaust opens first, then the inlet opens, then the exhaust closes, then the inlet closes. The timing won't generally be as extreme on an engine designed for supercharge. This arrangement allows the mixture being blown in to scavenge the cylinder effectively, then after the exhaust closes it can build up pressure in the cylinder until it is higher than would usually be the case with a normally aspirated engine, giving higher power. Yes, this does also lead to higher temperatures, so there is a limit to what any given engine will stand.

With a standard two stroke arrangement, it is inherently impossible for the transfer to remain open after the exhaust closes, so you can't build up a positive pressure in the cylinder, apart from tricks with the exhaust system. Blowing will improve the scavenging and so reduce the dilution with exhaust gas that usually occurs, but at the expense of fuel economy.

John

SAAB tuned their 3 Cyl. 2 strokes by the exhaust method and by putting a carb. on each cylinder. I was lucky enough to see Carlson in Kenya with his tuned SAAB 95, the engine was a lovely thing to see. Three very large carburetters took up a lot of space in the engine compartment.

I watched as he revved the engine and the Tacho reached 9000 rpm. My SAAB 95 would do just 87mph flat to the floor. Carlson was balked on a hill where a Mercedes had become stuck in the mud, all he did was jump out and with help tipped the SAAB over and back on its wheels and went on his way.

I ever regret selling both my SAAB 95 and the SAAB 96 that I had. For a new FORD Escort.

Clive

Ok, so i think it will help if i explain what i mean by super charged. the object of super/turbo charging any engine is to increase its ability to intake the air fuel mix over and above what its naturally capable of, right? So that being said you can super charge it by forcing more mix in. weather or not you achieve positive pressure in the combustion chamber is not the be all and end all (ideal but not the only end result) there are a number of efficiency losses in a model 2 stroke engine but by adding intake pressure you can remove these and gain a large power increase but still without reaching positive combustion chamber pressure. using the increased transfer pressure to help remove the leftover exhaust gases is also another plus to supercharging this engine, ( without the excessive fuel consumption), also the desgin of this v12 is with 2 con rods to 1 big end journal. this gives a 90 deg lag over tdc at number 1 to number 2 cyl. this equals a significant loss of crankcase suck effort, hence the addition of the super charger. So all this equals a super charged 2 stroke V12.

Dean

p.s. scavenging pumps are recovery pumps and strictly speaking not the correct label for intake pressure pumps. plus "super charged" sounds way more "heavy metal" don't you think? LOL i will keep you all updated with progress

Ok, so i have finished the front housing now, yay 1 more part down. this holds 2 21 x 12 x 5 ball races to carriy the front of the crankshaft. the rear end being held on a single ball race of the same dimensions. here are also the quad carb bodies i have roughed out along with the manifold.

Blower case is the next challenge, will post these photos as soon as i have done it.

cheers for now

dean

ok, so i had quite a bit of success over the weekend but also a somewhat frustrating disaster. got the supercharger case all but finished and both end caps machined up , made the offset turning fixture to machine the bearing holes for the blower rotor. this turned out to be not quite as straight forward as i thought as you will see in the latest photo updates.

having made the fixture i inadvertantly fitted the cover up 180 deg out and machined the wrong side, OOPS, bugger. oh well start that part again................

dean

Hey guys , just thought i'd give you all an "assembled so far" bunch of photos of whats been done on this project up to date. i only have the blower rotor and vanes left to complete, then onto the crank.

cheers

dean

will keep posting progress if anyone's interested in seeing it?

dean

Lots of armchair watchers methinks.

People in here tend not to clutter a good build thread with fluff until after its completion

We are watching, and are impressed

Count me in 1000% Dean, antique clocks and watches are my bread and butter, but engines are my lifelong passion, I was manufacturing and installing automotive turbocharging systems for a living way back in 1979!
I'm in awe of your progress with this project 'Superb' is all I can say!

John.

Well started the crank tonight, here are a couple of photos of the first two stages of machining

1st is bar stock cut to length and faced and centered both ends

2nd shot shows the front main fournel roughed out to go into a collet chuck.

3rd photo shows machining the flywheel diameter.

cheers for now

dean

So heres the photos, LOL

dean

so the sequence goes something like this, make the blank then machine the flywheel dia. then maching the front bearing to 1mm oversize and mark out all the flywheel, bigend and center main bearings. next its off to the mill for indexing all the bigend location marks, mill out the bulk of material for the be to leave a small oversize square. this reduces the turning in the lathe for the b.e. journel. once all the b.e. and main journels have been roughed out i grind the b.ends to final size followed by the mains. Big end journels are turned with the crank mounted in a special fixture. finally cut all the chamferes on the flywheel faces and deburr and polish.

dean

OK so another fine day in paradise and we saw the completion of stage two machining of the V12 crank. all B.E pins and main bearings have been roughed out to oversize by 1 mm ready for final machining to size.

Hopefully i will be able to finish machine and grind the crank this week, before christmas. Next will be the center main split bearings, backplate cover and blower drive.

cheers for now

dean

Dean

Apart from a general sense of inadequacy the last sequence of shots was an "I've just learned something useful" moment.

Keep them coming

Pete

The vane type rotor, what ratio drive are you going to use? How does it equate in volume passed to give an adequate supercharge pressure.

Clive

All these fancy words like blower and scavenge pump and supercharger stirred the family braincell into remembering a book I once read

I was Tubal Cains "Diesel Engine Design" and it contains a huge amount of detail on different routes used for different engines and their advantages/problems

This is part of the crankshaft section

supercharger section, etc etc

Edited By Ady1 on 21/12/2013 12:33:17