Are these Single point 'cooker-ring' mixers more snake-oil?

[Pinger]

I know the basics of how 2 strokes work lol, like you say though the process is more difficult to grasp or explain than 4T, the exhaust and intake kind of happen simultaneously. Could have 2 x 2T engines, both the same size / bore / stroke, both from the same manufacturer, but the porting can make a massive difference to all of torque, bhp and fuel economy (in different measures). Dad used to have a theory about 2T boat outboard engines that an engine with the biggest capacity to bhp ratio (specific power output) would generally be more low rpm torquey and run smoother than one with a higher specific output. At the time you could buy a 70hp or 85hp engine, they'd be the same engine except for a seemingly minor porting difference but the 70 would usually be a 'nicer' engine with a flatter torque curve and use less fuel even at port throttle than the 85.

Your father was in all probability correct. The porting (exhaust porting) has a further complication in that sometimes (when cylinders are in groups of three) it is more than needed for breathing but has to be open longer to get the benefit of exhaust tuning. Groups of four cylinders can run shorter timing and longer branch lengths but the trade-off is a narrower rpm range of effective pulse tuning.

I don't see much of a distinction between supercharger and blower, unless by blower you mean a setup that gives little boost pressure but delivers the volume of air to the intake port on the 2T? Just something to do away with the need for crank scavenging and maybe reed valves?

You are correct on all points. Usually, the exhaust port is open whenever the inlet port is (orifice engine) so no pressure build up in the cylinder is possible so supercharging isn't possible, Porting can be arranged to close the exhaust before inlet closing but this requires asymmetric timing for at least one set of ports. Opposed piston engines with phased crankshafts can achieve it. Rolls-Royce achieved it with the Crecy by employing sleeve valves. In all cases the actual supercharging is slight.
Rather than this arrangement being an alternative to crankcase scavenging, it was the original design and crankcase scavenging arrived later.

Different on the diesel 2T's, in theory a blower/supercharger is more beneficial to a direct injection 2T diesel than to a 2T carb petrol? In fact a direct injection supercharged 2T diesel (like some loco engines) is in many ways easier to understand than a conventional 2T petrol.

All true and when direct injection is possible then losing fresh air to the exhaust port can be beneficial to scavenging - but at a cost re the energy to pump the additional air. This has given the 2T a bad rep re 'air work'. The quantities of air a diesel 2T will consume per cycle is around twice that required by SI. That's a halving of air work right there.

If it's not compressing air an intercooler wouldn't be fitted anyway? Still is possible better blow through than draw through to prevent having explosive gas and air mixture inside the blower?

If the pressure ratio is low (circa 1.3 ie 5psi above atmospheric) then agreed, the need for an IC is obviated. Blow vs draw through - not so much in it perhaps. Draw and blow through have the same volume of mixture below the blower rotors. The draw through has charge above the rotors but should be shielded by the blowers ie, no flame path.

Where is the throttle on this 2T blowered petrol engine?

That is difficult to answer as I'm veering away from blower towards a combination of crankcase scavenged with turbocharger.

I still don't completely follow why you couldn't use a mixer between the blower and the intake port with the reducer referenced to pressure just before the mixer. I could see that working perhaps with a need to lengthen the intake to limit backpressure pulses from the intake port going back/forth through the mixer.

Everything there is spot on - including recognising the backpressure (can be limited with multi-cylinder design) on port closing.
Substitute turbo for blower and crankcase inlet port for direct-into-cylinder port and that's currently what I have in mind. Which is pretty straightforward but for the question of just how much boost pressure the reducer via its reference port can withstand both physically and without losing metering accuracy?

[LPGC]

I forget who on here runs a turbo'd car on a mixer system with turbo version of R90, is it @RobertX? Could be you lol. But you've said the idea is not to run boost pressure, won't be any boost at bdc in the cylinder anyway because intake air will just go out the exhaust...?

I would expect the blower setup to be simpler than the turbo setup on the 2T. Would starting a turbo'd 2T be more difficult than starting a conventional crank scavenged or blower 2T? I'm thinking you stick some fuel in, compress the mixture, try to ignite it but if it doesn't ignite there isn't much flow through the cylinder to shift the unburned fuel until the turbo starts to spool up, also if you fit a turbo small enough to push air at idle rpm won't it be too small for higher rpms / loads?

[Pinger]

I forget who on here runs a turbo'd car on a mixer system with turbo version of R90, is it @RobertX? Could be you lol. But you've said the idea is not to run boost pressure, won't be any boost at bdc in the cylinder anyway because intake air will just go out the exhaust...?

That would be the case if a blower was feeding directly into the cylinder and I've likely created confusion by both considering and discussing this arrangement. But, the design I currently have in mind is to use a turbo feeding the crankcase so a closed volume in which pressure can rise to boost level.

I would expect the blower setup to be simpler than the turbo setup on the 2T. Would starting a turbo'd 2T be more difficult than starting a conventional crank scavenged or blower 2T? I'm thinking you stick some fuel in, compress the mixture, try to ignite it but if it doesn't ignite there isn't much flow through the cylinder to shift the unburned fuel until the turbo starts to spool up, also if you fit a turbo small enough to push air at idle rpm won't it be too small for higher rpms / loads?

Crankcase will take care of starting and lower load range with turbo coming in for higher loads. Blowers are relatively simple but the power they absorb is hard to tolerate. The 2T I am envisaging has to be fuel efficient. Not quite diesel efficient but definitely has to match or exceed 4T SI efficiency. A blower absorbs power at all loads. The crankcase on the other hand is there to be used and, at part loads precisely because it is a lesser pump than a 4T cylinder. doesn't suffer the same throttling losses as a 4T at part load.
Comparing the two's air work, the 4T is very good at WOT and poor when throttled. The converse is true for 2T hence the need to get the most efficient (turbo using exhaust energy otherwise going to waste) for high load WOT operation.

[LPGC]

OK, I thought we were talking about a 2T that didn't have crankcase scavenging. Are you thinking a part crank scavenged part blown induction system will be less air work than a fully crank scavenge setup?

Seems a tough call to make a fuel efficient 2T. Is fuel efficiency the main thing you're interested in?

Air work is one thing but even air work could be extended to include exhaust gasses. Don't some 2 strokes have exhausts with varying width to pulse tune return shock waves to help better trap cylinder charge tuned for a specific rpm and the further away from that rpm the less efficient because shock waves impact on compression and keeping fuel in the cylinder? As you implied, change one aspect of a 2T it affects other aspects more so than on a 4 stroke.

Would think direct injection would be one of the biggest benefits to a 2T, if you don't put any fuel in until the upstroke after the exhaust port is closed there won't be as much wasted HC? This for a long time the benefit of blown diesel 2T's for applications such as diesel loco engines over petrol 2T's.

[Pinger]

OK, I thought we were talking about a 2T that didn't have crankcase scavenging. Are you thinking a part crank scavenged part blown induction system will be less air work than a fully crank scavenge setup?

What I'm aiming for is reduced air delivery by crankcase pumping augmented with turbo boost to compensate for that and, tight transfer porting.

Seems a tough call to make a fuel efficient 2T. Is fuel efficiency the main thing you're interested in?

The most fuel efficient piston engines bar none are the big 2T diesels in ships. They experienced a step change in efficiency when blowers for scavenging were replaced with turbochargers. (They use stored compressed air for starting).

Air work is one thing but even air work could be extended to include exhaust gasses. Don't some 2 strokes have exhausts with varying width to pulse tune return shock waves to help better trap cylinder charge tuned for a specific rpm and the further away from that rpm the less efficient because shock waves impact on compression and keeping fuel in the cylinder? As you implied, change one aspect of a 2T it affects other aspects more so than on a 4 stroke.

Yes. If we look at the two types of exhaust used for pulse tuning the single expansion chamber type employed with one per cylinder is capable of phenomenal specific power outputs - 400+ hp/litre (closer to 900 if we measure as 4Ts are measured). The downside is a very narrow rpm operating range.
The other is compact branch manifolds as utilised in outboards which, while not so effective as expansion chamber types can be made to work over a broader rpm range - the more so when cylinders are grouped in threes. The two critical parameters are duration of exhaust opening (set by port height) and the branch length of the manifold - sometimes difficult to get short enough for moderate port timings with groups of three. Loopers tend to have slightly longer branch lengths, crossflows on account of reduced ex durations (as they employ wider ports) require shorter. Sometimes the exhaust duration will be extended beyond what is required for gas flow to better exploit this tuning with groups of three cylinders.

Would think direct injection would be one of the biggest benefits to a 2T, if you don't put any fuel in until the upstroke after the exhaust port is closed there won't be as much wasted HC? This for a long time the benefit of blown diesel 2T's for applications such as diesel loco engines over petrol 2T's.

In the early 90s it looked as if this holy grail had been achieved. However, with engines of reasonable rpm capability (circa 5-6000 rpm) there is a woefully short time frame for the injected fuel to be admitted to the cylinder and more crucially - evaporated in time for combustion. To overcome that, at higher rpm and loads the fuel injection must begin before the exhaust port has closed (often before BDC has been reached) leading to fuel lost to the exhaust. Injecting later incurs the lack of mixing/evaporation time as mentioned and, complicates the metering as as the cylinder pressure is rising during the compression stroke, the fuel quantity admitted reduces.
It never worked out as promised unfortunately. Crankcase scavenging with homogenous air/fuel is therefor worth reconsidering (provided an excellent - not looper or crossflow but uniflow - scavenging system is employed). This has the additional benefit of providing excellent evaporation of non-volatile fuels such as methanol/ethanol - or indeed any synthetic fuel developed - which need not have volatility as a high priority.

[Pinger]

I got a bit more done on my project over the weekend. It seems that to make it work I need an inlet pressure ratio of around 4. That is, circa 3 bar of boost. Can a (pressure referenced) reducer (eg, R90) cope with that level of boost?

[hitman2]

Manual here for it. Might help you

http://lpginjectors.nl/en/omvl-reducers ... er-d8.html