Hello Uniflow.
You write:
“The oil problem is a real problem for twostrokes but the raw hydrocarbons out the exhaust is the first big problem to overcome”.
The port injection softens, but can’t really solve, “the raw hydrocarbons out the exhaust” problem of the 2-strokes because, no matter how late the fuel is injected into the transfer ports, the fuel cannot help entering into the cylinder several crankshaft degrees (say 60? 50? 40?) before the closing of the exhaust port.
A reasonable solution of the “first big problem” of the twostrokes is the direct injection of gasoline (as in the Rotax E-TEC 850).
However, this solution adds its own limitations to the system: if the injection starts early, unburned fuel can escape to the exhaust; if the injection starts late enough to not allow fuel to escape to the exhaust, the remaining time before the ignition may not be adequate for complete, clean and efficient combustion.
The pair of the secondary “indirect” injectors used by the Rotax 850 E-TEC for the higher rpm proves, beyond doubt, the “issue”: the engine at high revs scavenges the cylinder with air – fuel mixture as the old 2-strokes, making the loss of unburned fuel to the exhaust inevitable.
Another reasonable solution of the “first big problem” of the twostrokes is the compression ignition with direct injection (like the Wartsila X92 giant marine 2-stroke Diesel, the Junkers Opposed Piston Diesel, the Achates Power Diesel, the Rootes Diesel).
Even better with the PatOP Diesel (post at
https://www.f1technical.net/forum/viewt ... start=1297 )
the architecture of which:
provides some 30% additional piston dwell near the combustion dead center (direct injection diesels with 30% higher revs at their peak power),
receives the thrust loads away from the “ported” cylinder liner,
enables 4-stroke-like lubrication, etc, etc.
Another reasonable solution of the “first big problem” of the twostrokes is the early closing of the exhaust.
Easy to apply (but with some side effects) in the case of Opposed-Piston through-scavenged spark ignition engines.
Easy to apply (again with some side effects, like the added complexity and cost, the limited rev limit etc) in case of 2-strokes spark ignition engines having poppet valves (say, as in the Detroit Diesels, or as in the pattakon PatPortLess).
Another reasonable solution of the “first big problem” of the twostrokes is the use of asymmetric transfer in combination with an unconventional “stratified charge”:
Rich air-fuel mixture (say, from a carburettor) feeds the space underside the piston crown, inside the piston.
The scavenging starts with compressed clean air from the crankcase.
At the end of the exhaust (or even after the closing of the exhaust) the rich mixture passes to the cylinder through the asymmetric transfer ports.
Instead of waiting, with the fuel in the cylinder, till the exhaust port to eventually close, now the fuel entering into the cylinder finds the exhaust ports closed.
Regarding the other real problem of the 2-strokes, the lubrication:
With the thrust loads removed from the piston skirt, as in the Harmonic PatTwo (
http://www.pattakon.com/pattakonPatTwo.htm ) :
and with all the bearings in a separate / sealed / isolated compartment,
only the piston rings need lubricant to avoid metal-to-metal contact. This can bring the specific lube consumtion to the level of the good 4-strokes.
Alternatively, with 4-stroke crankcase (plain bearings with lubricant provided under pressure, splashed lubrication of the cylinder liner whereon the thrust loads are taken, oil scraper ring just above the wrist pin, cleaning and recirculation of the lubricant etc):
and lower compression ring that, at BDC, abuts on the cylinder liner whereon 180 degrees earlier was abutting the oil scrapper ring, the lubrication is quite simple, efficient and controllable.
With loop scavenging, as above, spherical combustion chamber and a lot of squeeze, interesting spark ignition 2-strokes can result (4-stroke like lubrication, no unburned fuel to the exhaust, perfectly rid of inertia vibrations, etc).
And if the Opposed Piston arrangement is too long, slice the engine in the middle.
Thanks
Manolis Pattakos
. Same injector positioning aiming toward the incoming air in the B ports, same same. Actually I call it DTPI delayed transfer port injection as this is the trick. The fuel charge is delayed for a time before it is introduced into the air stream making it hard for the fuel to escape the cylinder ( runs out of time ) . Also the positioning of the injectors in the B ports helps stall short circuiting, off the pipe where short circuiting is worst.