Ambitious newby

**brsanko** · 30-06-2012

Originally Posted by 2e0poz

BR i can see some issues with your design and position of TDC when firing happens. With everything being at opposing positions you will just end up with a pile of bits as it locks up upon detonation. I would look at a more traditional layout of the crankshaft myself.

Now this is the kind of advice I'm looking for. I currently have it set up so that the opposing pistons are basically one unit, so that each pistion assembly has 4 combustion chambers.(see pic) Would it work better if the piston assemblies were a few degrees off? More than a few?

**m_c** · 30-06-2012

Originally Posted by brsanko

I guess I assumed an opposed engine didn't really need counterbalance. If it does I have no idea how to design it

The opposing piston/rod assemblies will counter balance each other, however the crankshaft itself will still have offset masses flying around at great speed unbalancing things.

**m_c** · 30-06-2012

Originally Posted by brsanko

Now this is the kind of advice I'm looking for. I currently have it set up so that the opposing pistons are basically one unit, so that each pistion assembly has 4 combustion chambers.(see pic) Would it work better if the piston assemblies were a few degrees off? More than a few?

You need to consider power delivery. Each time a cyclinder fires, you get a high load followed by a power surge. This causes the crankshaft to deform, so it's got to be suitably designed to handle the peak torque. By having two cylinders firing at the same time, you'll be putting a lot more stress through the crankshaft.
Ideally you want power delivery at equal intervals, which would mean the two crank pins being 90degrees apart, however you'd have to take into consideration the crankshaft balance.

**blackburn mark** · 30-06-2012

Did you miss the 2 stroke part? There is no suck unless you do something really clever with the exhaust pipe geometry.

expansion chamber forms part of the exaust (the bulbus portion of a typical two stoke exaust) the inertia of the exaust gas (in the bulbus section) keeps its momentum forming somewhat of a vacume behind it drawing in the next charge

clacker valve :) reed valve we used to call them

"injecting at TDC" youll have to look into that one... im assuming that you would get better atomization at lower pressure (during induction or there abouts) at TDC the pressure would be quite high with it being a deisel

I currently have it set up so that the opposing pistons are basically one unit

i think your ok in principle but i would have thought a 45degree rotation on your crank timing so that your firing one charge at a time (higher frequency vibration but less amplitued than firing two charges simultainiously i would think)

crank balancing ..... errrm.... your on your own there :) im not sure if you just balance the crank or you take some of the piston and con-rod mass into consideration

**Robin Hewitt** · 01-07-2012

Originally Posted by blackburn mark

expansion chamber forms part of the exaust (the bulbus portion of a typical two stoke exaust) the inertia of the exaust gas (in the bulbus section) keeps its momentum forming somewhat of a vacume behind it drawing in the next charge

Traditionally a 2 stroke has the carburettor on the opposite side to the exhaust. The piston goes up producing a partial vacuum in the crank case, the bottom of the piston clears the inlet port and fuel air is sucked in . The piston descends and moments after the piston clears the exhaust it clears the fuel inlets which usually come up the sides connecting the crank case to the combustion chamber. You would think there would be enough heat to light the new fuel but I suppose the explosive decompression cools it somewhat and the mix doesn't really want to light until the pressure is up again. I'm actually rather surprised 2 strokes work at all, if they never existed and someone here came up with the design I'd probably pooh-pooh it.

I suppose you could use a supercharger to get the push, but you do need a push because the pressure must still be well high when the side inlets open.

Using exhaust to create a vacuum is steam locomotive technology but I'd say it could only ever be a helping hand in a 2 stroke, a bit of fine tuning, something to ease the back pressure, not enough to do the recharge.

In a steam train the exhaust comes up the blast pipe, into the smoke box, past the the ends of the fire tubes and up the funnel. What comes out the top is "steam" followed by smoke. What drives it is the valve gear shutting off the exhaust, the air simply doesn't want to stop so it carries on up the funnel drawing on the firebox. Unfortunately a funnel will only pull efficiently at one speed but the old boys knew that and filled the funnel with different size tubes to compensate.

**blackburn mark** · 01-07-2012

Originally Posted by Robin Hewitt

Traditionally a 2 stroke has the carburettor on the opposite side to the exhaust. The piston goes up producing a partial vacuum in the crank case, the bottom of the piston clears the inlet port and fuel air is sucked in . The piston descends and moments after the piston clears the exhaust it clears the fuel inlets which usually come up the sides connecting the crank case to the combustion chamber. You would think there would be enough heat to light the new fuel but I suppose the explosive decompression cools it somewhat and the mix doesn't really want to light until the pressure is up again. I'm actually rather surprised 2 strokes work at all, if they never existed and someone here came up with the design I'd probably pooh-pooh it.

I suppose you could use a supercharger to get the push, but you do need a push because the pressure must still be well high when the side inlets open.

Using exhaust to create a vacuum is steam locomotive technology but I'd say it could only ever be a helping hand in a 2 stroke, a bit of fine tuning, something to ease the back pressure, not enough to do the recharge.

In a steam train the exhaust comes up the blast pipe, into the smoke box, past the the ends of the fire tubes and up the funnel. What comes out the top is "steam" followed by smoke. What drives it is the valve gear shutting off the exhaust, the air simply doesn't want to stop so it carries on up the funnel drawing on the firebox. Unfortunately a funnel will only pull efficiently at one speed but the old boys knew that and filled the funnel with different size tubes to compensate.

ill buy that :)

**Swarfing** · 01-07-2012

Now this is the kind of advice I'm looking for. I currently have it set up so that the opposing pistons are basically one unit, so that each pistion assembly has 4 combustion chambers.(see pic) Would it work better if the piston assemblies were a few degrees off? More than a few?

Engine design is beyond my knowledge but i have rebuilt a few 1380's and couple of 1410 'A' series for group 'N' family saloon car racing. What I do know is that as soon as you start playing around with crankshaft timing on a week solution (stock crank) it all go horribly wrong

**blackburn mark** · 01-07-2012

im wrong on the "injecting during induction"
just read a document about a large two stoke deisel that is injecting into compressed air

**m_c** · 01-07-2012

A basic diesel engine will inject a good bit before TDC. Diesel is quite slow to ignite, so to get peak power and good air/fuel mixing, the fuel has to be ignited long before TDC (IIRC at typical slow idle it's about 18deg BTDC). This is why diesels knock so much, as the engine is pretty much on the verge of stalling at TDC due to the high pressure.

With modern common rail systems, fuel injection is now a multi-stage process. As the piston approaches TDC, a small amount of fuel is pre-injected, which results in an intial flame, then as TDC is approached main injection occurs as a further series of injections, which ignite quicker due to the already burning fuel.

As for diesel 2strokes, they have existed in the past. Some had conventional valve trains, some were ported, but pretty much all of them were supercharged.