Just re-capping. I took a Petter AC1 single cylinder engine base and designed a completely different head to operate the rig as a Two-stroke. Indeed it looked a bit like a VW Golf set-up, but it was designed entirely from first principles.
The reason for the two-stroke idea is three-fold: 1) Two bangs per two cycles therefore smoother than a four-stroke - aircraft propellers hate vibration; (2) Slightly less "power" per bang, again promoting smoothness; (3) two-strokes are very efficient at a particular narrow rev range, and aircraft mostly fly at Cruise or (occasionally) at Full Power.
The air from the blower drops straight down but, at entry, is angled by the back of the valve head to promote loop scavenging.
The Combustion Chamber for this diesel is tent shaped - quasi flat bottom (=piston top) (don't panic), two opposing walls made of valve faces and blank surface.
The injector (Delphi; ex-Clio) is at the apex of the tent, 7 mm off true cylinder centre. After much interesting analysis, I understand the injector, ie inductive & impedance properties, current change during action and voltage kick-back. All this is OK.
There are 6 holes (170 mu dia) at 72 deg intervals, squirting down at 18 degrees from horizontal. The pressure is about 1000 bar. The fuel is about 80 deg C (from pressure applied to its viscosity) on entry to the cylinder.
The best previous piston top had 6 flutes which "reflected' the jet (see earlier post & foto).
The current top is, I think OK (or we wouldn't have done it) and has a ring machined as a groove just where the jets would impinge. Its centre is centred on the axis of the injector tip. This ring, in section, has a vertical outer wall 2 mm deep but a there is a slope up and in towards the flat of the piston. There are 2 minor supplementary rings concentric with this, of smaller dia.
We had found that at these delivery pressures, the rings (like the flutes) provide walls which very finely atomise the fuel.
Usually, with these injectors, the fuel jets catch fire before they hit the piston (classically the chamber wall; in our case, the wall of the groove)
Next (the famous) Bump Space (BS) at TDC. I didn't know what this was - just hadn't heard the phrase. But anyway, Petter have their acceptable range.
Owing to a miscalculation, I have a BS a tiny bit less than Petters limit (40 mu less as I recall). Since I know there is totally free movement (ie piston does not strike exh valve face), I was happy.
But could this affect anything? I don't believe so. Of course the CR goes up a fraction - but squish air movement? As the piston approaches TDC the BS is millimetres and the piston is slowing to walking space. You don't expect that, do you? But squish effects must have occurred much before TDC.
We believed the air flow in our chamber would be like a horizontal rolling wave around a horizontal axis, compared to the horizontal rotation around a vertical axis in a toroidal chamber, ie it should be fine (& there's that VW four-stroke head!).
We put the design on the Lotus Engine Simulation software and it met expectations big time which was encouraging. But that assumes perfect combustion so the design is as of this date, still unproven.
Problem we had: Difficulty starting. We blow in warm air (multiple gloplug system) and there's a gloplug in the combustion chamber, tho not ideally situated because of lack of room. The starter turns it fast. Brown smoke appeared in puffs but there's no "catching on".
Sometimes we thought the firing throws the starter pinion off but then doesn't continue on its own so the pinion re-engages unpleasantly.
It's as though the scavenging is failing and the cylinder fills up with burnt gases so cannot combust. Could this be a total design flaw? Remember, this is my design, not a modified existing item.
We alter valve timing and injection timing but its much the same story. In general, inlet opens 20 deg after exh and closes 10 deg after exh closes. We have altered injection from between 18 and 5 deg BTDC.
We tend to go EVO at 95 ATDC, or recently 120 ATDC. Compression is high so we lose a little by closing IV late (eg 250 ATDC, ie 70 ABDC.) but it should not affect things.
We think of every possibility and still it wouldn't start. Could the exhaust be blocked (remember there is the turbine there)? I blow into it, no probs, except dirty face. Blow air in; air flows thro' exh valves (set open) and out of the inlet. Seems OK.
Then I decided to take the exh manifold off and peer into the exh port (where I can see into the cylinder during "EV open". Will I see a flame momentarily?
My God, It fires up beautifully straight away. After a year of struggling. Not even smoky. Within minutes the problem is clear. Not our engine, its the turbocharger!! Its leaking engine oil (which supply its bearings) onto the exh turbine, which has formed a tarry deposit on the blades. The turbine spins OK but has been restricting, but not blocking, passage of gases between the blades.
We will rebuild it and look forward to telling you about the future of the Light Aircraft Industry!!!!