2 stroke thread (with occasional F1 relevance!)

[Pinger]

A 2-stroke like the TPi of KTM is hurting at partial loads because it cannot help running with a high quantity of residual gas in the cylinder.

Yes, call it EGR and use it to curb NOx formation.

With a small quantity of air-fuel mixture “lost” into a big quantity of residual gas, the combustion in the 2-stroke cannot be complete / clean.

That manifests itself as a misfire (4-stroking) at very low loads but at loads above that level it isn't as problematic as all that. Anyway, the emission tests focus (through as weighting system) on mid loads, typically between 25 and 75% of full load.

Reasonably, the triple 750cc TPi, which will run at substantially lighter load than the single cylinder KTM250EXC TPi (which barely passes the euro-4 emissions regulations), cannot but being worse.

HC emissions are proportional with delivery ratio. Pumping losses are inversely proportional to delivery ratio. It's all in favour of the larger engine.
Test data I have here for a 2.31kW handheld 2T engine shows HC emissions at idle and full load of 13179 and 14220ppm respectively. At the lowest load (idle) it emits lower HC than at WOT showing that it is delivery ratio that causes the worst emissions of HC. This on an engine that will have a delivery ratio of no greater than 0.5. Up the delivery ratio and the full load figure will worsen. For this type of engine, the idle figure will assume a 15% relevance in the test, the full load, 85%. So even a lousy idle figure isn't disastrous for this engine (which can pass the EPA Phase 3 test at 49.4g/kWh, max permitted is 50g/kWh). Raise the delivery ratio though and it's stuffed.

The direct injection 2-strokes (like the E-TEC of Rotax / Evinrude / Bombardier) can use stratified charge (direct injection towards the spark plug) to improve the partial load operation of the 2-stroke. However at the areas of the cylinder wherein the mixture gets too lean, the flame outas out and the combustion cannot be complete / clean.

They introduce the additional problem of having the air and the fuel in different places and have to unite them in the vicinity of the spark plug. Homogeneous charge simplifies the task by having the fuel forever entrained in the air.

The lean mixture is not a limitation, any longer.

Yes it is as it will incur greater pumping losses with its greater air requirement.

The combustion is not progressive, any longer, but instantaneous, or too fast:

But will still be restricted by the strength of the engine and the desire for refinement the market expects.

At partial loads or idling the cylinder fills with air (no pumping loss during the intake, no sub-pressure into the crankcase).

Air does not move of its own volition. Energy will always be expended in moving it. The less air required, the lower the associated loss.

If Mazda is right about the 20% fuel consumption reduction with their lean burn HCCI, the guess is that for the 2-strokes (characterized by mechanical simplicity and lower friction losses) the improvement could be more than 40%.

Why? The 2T in even its dirtiest guise is already enjoying the benefit of reduced friction over a 4T.



A broader point. Unless you define the context of the engine ie, its end use, you cannot identify the test that has to be passed as they vary considerably and are specific to the engines intended use (and swept volume). Context is all.

[J.A.W.]

Also - a 3cyl unit, firing every 120`- can utilize that close order to synergistic gas-flow effect, unlike a single.

[FW17]

Lot has been discussed in this forum on the 2 stoke engine, very little on the pulse tuned runner pipe design used on V6 outboards which is a substitute for expansion chambers.

How well does these works is sealing the exhaust port? Has anyone tried it on road vehicles?

http://members.iinet.net.au/~pauldawson ... hausts.PDF

[Pinger]

Lot has been discussed in this forum on the 2 stoke engine, very little on the pulse tuned runner pipe design used on V6 outboards which is a substitute for expansion chambers.

How well does these works is sealing the exhaust port? Has anyone tried it on road vehicles?

http://members.iinet.net.au/~pauldawson ... hausts.PDF

There was some discussion on the merits of branch manifolds for cylinder groupings of 3 (and 4 ) cylinders earlier in the thread.
The 'compact branch manifold' as it's otherwise known has probably been utilised better on cars (eg Saab), sleds, and jetskis than outboards. With more physical room, the former can employ equal branch lengths to good effect. Outboards due to space constraints tend to have unequal branch lengths and, the lowest cylinder(s) has nowhere for escaped charge to reside such than it can be returned to the cylinder by a pulse before port closing. The cylinders above have something akin to branches which would permit this. The lowest cylinder(s) can only employ the pulse to prevent the loss in the first place.
Seems to work though despite the above compromises (Blair's book (Design & Sim') shows traces of the pulses in action on 3 and 4 cylinder engines). Beneath the outer exhaust cover of an outboard, there are some quite cunning manifolding arrangements. Over expansion chambers, they are more forgiving over the entire speed range and their compactness is a boon. An integral part of any multi (more than 2 cyls) 2T design - that isn't chasing absolute power - they'll be around for a while yet.


Exhaust flow is left to right.

http://i256.photobucket.com/albums/hh19 ... CF8264.jpg (Apologies, can't post image, only link).
This is a race version of the block the baffle plate would mate to (if used).
The (inline) sixes have less prounounced branches and the two groups of 3 pipes don't meet until they are in the mid section. The degree to which a negative pulse is utilised is open to question. The 'cones' have a very shallow taper and may be the length they are merely to fit into the midsection which has its own size (height) restriction.

[FW17]

Any pictures of a Mercury F1 motor exhaust manifold? they are sure to have developed the tech further along with the e-tech GDI

Since outboards pass their exhaust underwater, does that add to back pressure?

[Pinger]

Any pictures of a Mercury F1 motor exhaust manifold? they are sure to have developed the tech further along with the e-tech GDI

No, I've been concentrating on the older crossflow Mercurys as they are the engines I will be working on (3s and 6s).
I doubt that there's much change in the exhaust systems between carbed and GDI - space restrictions tend to dominate along with minimum branch length which is a function of cylinder spacing (the inline six Mercs couple even and odd cylinders meaning cylinder 1 and 5 (and 2 and 6) have to connect. Fortunately, the bores are relatively small and the crossflow porting (unlike looper) permits very close cylinder spacing (85mm bore centres).

Since outboards pass their exhaust underwater, does that add to back pressure?

Yes, listening to guys who know their OBs, they say that carb settings for idle must be set with the unit in the water, and that the idle speed is higher out of the water.
I have to check if the downpipe outlet is above or below the water line. I think it's above and the exhaust merely pressurises the mid section and finds its way through the water and out through the hub.

I also need to check the origin of any negative pulse. If generated at the end of each branch then the negative pulse goes back only to the exhausting cylinder (which created the pulse) and goes positive (cross charging) to the others. If there's a negative from the downpipe (or its end) then that would transmit a negative to all cylinders. That, at lower rpm (especially if the downpipe is short due to height restriction of mid section) could impinge on the cylinder that is just closing its exhaust port with unwelcome consequences. More research required!

As an aside, I see reference to a 'tuner' when OB guys discuss 2T exhausts. Possibly it is the downpipe they are referring to in which case its length and taper are of importance. Does anyone here know?

[FW17]

There's no doubting these expansion chamber systems work well, but as the photo shows they do take up a lot of
space. One and two cylinder engines are not much trouble, but what do you do with 3, 4 or even 6 cylinders? Most
vehicles just don't have the room for several expansion chambers, but there is a way to get the same benefits on
multi-cylinder engines without all the bulk - pulse tuning.
We still need a negative wave to arrive about BDC and then a positive to arrive just before port closing. The
divergent cone (megaphone) that provides the negative wave is not very big and several cylinders can share one
because the wave travel time is short. It’s the positive wave that's a problem because it usually travels much further
and can then arrive when another cylinders negative wave is present in the system, canceling the benefit. Instead,
pulse tuning gets it's positive, plugging pulse by using the opening of another cylinder's exhaust port, not the far end
of the exhaust system.
Twin expansion chambers fitted to a twin cylinder 950 cc Seadoo Rotax engine, by US speed
equipment maker Factory Pipe. The pipes are water jacketed and water injected by an electronic
controller to vary the gas temperature and get the sonic waves to arrive at the right time over a
wider RPM range. Engines with this setup can produce over 160 HP (120 kW).
On a 3 cylinder engine (or half a V6) the exhaust port timing is nearly ideal, with some overlap between exhaust
port open periods. When one cylinder's exhaust is opening, and radiating a strong positive wave into the system,
another cylinder's port is just closing. Therefore the positive or plugging pulse is present at the right time without
needing to travel a long distance. The sketch shows a typical 3 cylinder engine where the overlap between exhaust
port open periods is about 45 degrees. Only about 200 mm of travel distance is required for the positive wave to
arrive at the right time. That's why a 3 cylinder, or V6, outboard can have a quite sophisticated tuned system, but it
all hides neatly out of sight under the cooling water jacket



Some vehicles have a little more room and display a more obviously tuned system. The photo shows a Bombardier
Ski-Doo snowmobile powered by a 110HP, 3 cylinder 700 cc Rotax engine. This vehicle allows room for a larger
more complex system. The exhaust manifold still allows for the short distance between exhaust ports for a pulse
wave, but adds a much larger expansion cone to a provide stronger negative wave, and very possibly it also
augments the positive wave at certain speeds.
The CVT transmissions used in this type of vehicle is also part of the equation. This type of exhaust system can
make some engines "peaky", that is the benefit of exhaust tuning arrives fairly suddenly at some point in the rpm
range and with conventional fixed gearing would make the machine difficult to drive.

[Pinger]

In the above example picture notice that the branch length is 200mm. But, cylinders 1 and 3 have to communicate so the length becomes 400mm for one of the 3 cylinders. This shows on Blair's pressure traces.

Also, the divergent cone is shown and its workings elaborated on - in conjunction with the branch manifold. Some OBs have less a branch manifold than a log type manifold or even more extreme, just an exhaust box - akin to a plenum chamber. For those, I'm more inclined to believe that a negative pulse is generated much sooner than is the case with a divergent cone but that doesn't preclude an additional negative pulse from the cone/downpipe/tuner(?). Making sense of it all isn't at all easy!

[J.A.W.]

No reason some technical sophistication cannot be added to the exhaust to gain a wider power-spread
for road machines..

A internal cone, able to be moved fore & aft - within the fat part of the pipe - in tune with rpm change,
(controlled as exhaust 'power valves' & the like have been - for the past few decades)..

[johnny comelately]

No reason some technical sophistication cannot be added to the exhaust to gain a wider power-spread
for road machines..

A internal cone, able to be moved fore & aft - within the fat part of the pipe - in tune with rpm change,
(controlled as exhaust 'power valves' & the like have been - for the past few decades)..

http://www.rpdinc.com/10435-thickbox/plastic-funnel.jpg

we did have that where the distance between the two cones was varied via a rack and pinion system according to rpm, eventually banned

[J.A.W.]

For kart-racing Johnny, ( & it was 'manually operated', like a trombone, AFAIR)- but def' not - road machines..

[johnny comelately]

For kart-racing Johnny, ( & it was 'manually operated', like a trombone, AFAIR)- but def' not - road machines..

Ok right, i suppose I should have said it was on a road racing bike; 19something

[J.A.W.]

@ Pinger, recent 2T developments in (highly constrained rules-wise) Kart racing have included (AFAIK)
exhaust port-nozzle-pipe research into both liquid cooling ( to reduce temp of the returning fuel-air charge),
& 'sonic' nozzle design shapes - to obviate 'shock-choke' - gas-flow issues at high rpm..

[Pinger]

@ Pinger, recent 2T developments in (highly constrained rules-wise) Kart racing have included (AFAIK)
exhaust port-nozzle-pipe research into both liquid cooling ( to reduce temp of the returning fuel-air charge),
& 'sonic' nozzle design shapes - to obviate 'shock-choke' - gas-flow issues at high rpm..

Cooling the 'stored' charge is pretty cunning. Highlights an inherent problem with branch manifolds which is that the piston at or around TDC gets a positive pulse (which it has no use for) which can only add heat to its skirt. The pulse will be of lower pressure than with a full fat expansion chamber and will impact only the skirt - not the rings. A minor problem given the lower state of tune, but any unnecessary heat flow to a piston is unwelcome.

Re 'adjustability' of tuned pipes to rpm. Power valves do a good job there as well as their primary task of masking the port. The change in timing neatly re-synchs the pipes to the rpm. High maintenance though by what I can ascertain, requiring regular cleaning of carbon if they are not to stick.

[Pinger]

The world's first looper?

https://oldmachinepress.com/articles/page/23/

Scroll about 1/4 way down.