2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello all.



HCCI Spontaneous Combustion vs Spark Ignition Progressive Combustion

The finger (bottom right) corresponds to the spark.

The hand (right middle) corresponds to the transfer port opening of the PatBam HCCI:



Thanks
Manolis Pattakos

[gruntguru]

The hand (right middle) corresponds to the transfer port opening of the PatBam HCCI:

Not the best analogy. The hot gas emerging from the transfer port cannot communicate with all sections of the chamber simultaneously. If the communication is by:
- conductive heat transfer - the propagation rate is the flame speed
- pressure increase - the rate is the speed of sound (as in knock)
- radiation heat transfer - the rate is the speed of light.

[manolis]

Hello Gruntguru

The dominant factors seem to be the speed of the sound and the speed of the burnt gas.


The following slides (from the youtube video animation):



piston at 10 degrees BTDC, above,

piston at the TDC, below.





With, more or less, all the air-fuel mixture of the main chamber concentrated into the bowl, the transfer ports open and the burnt (or still burning) gas (yellow) from the auxiliary combustion chamber bursts / explodes / rushes / plunges / dives into the bowl.


With the air-fuel mixture into the bowl being below, yet near, the threshold for auto-ignition,

with a maximum distance of 20mm from the transfer ports to the ends of the bowl,

and with, say, 500m/sec speed of the burnt gas exiting from the transfer ports (and, say, 500/5=100m/sec mean speed into the compact bowl),

the time required for the burnt gas to arrive to the ends of the bowl is:

0.02m / (100m/sec) = 0.0002 sec = 0.2msec.

This time interval corresponds to 7 crankshaft degrees at 6,000rpm.


During the initial part of the above mass transfer of hot / burnt gas to the ends of the bowl, another phenomenon takes place.

The compressed, but not yet burnt, air-fuel mixture into the main chamber undergoes a rapid / abrupt / sudden increase of its pressure.

The pressure wave from the opened transfer ports arrives to the ends of the bowl with sound velocity (the sound velocity at the specific temperature).

Before the opening of the “transfer ports”, the molecules / atoms of the air-fuel mixture in the main chamber are at equilibrium, yet at an “unstable equilibrium” because they are near the threshold for auto-ignition (similarly, the standing playing cards in the video are at an “unstable equilibrium”: a small displacement of the basis whereon they are standing, and they all fall).

The abrupt increase of the pressure after the opening of the “transfer ports” causes the local increase of the temperature and the spontaneous ignition of all the air-fuel mixture in the bowl (similarly, the slight displacement of the basis whereon the playing cards stands, video, causes the simultaneous fall of all the playing cards).

It seems that the two dominant factors (pressure wave and mass transfer) are supporting each other, are complementary.
What is left unburned by the pressure impact, will be burnt by the mass transfer.

Thanks
Manolis Pattakos

[gruntguru]

I like that concept - utilising the entire auxiliary chamber volume as the TDC chamber volume. Lots of squish turbulence and a very compact chamber. What you describe is not necessarily HCCI (except the initial ignition above the anvil) but should still be very fast and detonation free.

At the extreme case shown, you couldn't have any valve overlap without striking the piston however.

[uniflow]

Pinger, My sleeve valve engines sleeve does not rotate because of the two rods. The bearings I am using are standard thin section ball bearings, perhaps not the best choice for the job but seem to be working. Only 24mm stroke.
The engine is in bits at the moment, I have only one YZ250 gearbox and it has been transplanted into my 360, mark two TPI engine. The one with the dual variable rotary valve gibs, opening and closing adjustment. These also act as the throttle, ECU controlled.
Opps, sorry I've looked at the wrong page again, disregard this post.

[Pinger]

Pinger, My sleeve valve engines sleeve does not rotate because of the two rods. The bearings I am using are standard thin section ball bearings, perhaps not the best choice for the job but seem to be working. Only 24mm stroke.
The engine is in bits at the moment, I have only one YZ250 gearbox and it has been transplanted into my 360, mark two TPI engine. The one with the dual variable rotary valve gibs, opening and closing adjustment. These also act as the throttle, ECU controlled.
Opps, sorry I've looked at the wrong page again, disregard this post.

Thanks Uniflow, much appreciated.

[manolis]

Thanks Gruntguru.

The valves are shown as in a Diesel (near zero overlap, almost all the dead volume into the bowl).

In case of 2-stroke (say, for airplanes):





there are no poppet valves at all (no need for valve pockets): the dead volume is in the bowl.


You write:
“I like that concept - utilising the entire auxiliary chamber volume as the TDC chamber volume. Lots of squish turbulence and a very compact chamber. What you describe is not necessarily HCCI (except the initial ignition above the anvil) but should still be very fast and detonation free.”


The “very fast and detonation free” combustion (either it is a true HCCI, or not) is what brings the big difference.

The “fast combustion” is what Mahle is pursuing with their TJI.

According the following SAE / Mahle paper:



the average speed of the jet of the Mahle TJI is near 60m/sec:

This makes pessimistic my previous assumption for 100m/sec average jet speed (or "mass transfer" speed) when the transfer port opens in the compact bowl (~20mm maximum distance) of the above post PatBam HCCI.
On one hand the distance the jet of the burnt / burning gas covers in the PatBam bowl is short (say 20mm), on the other hand the pressure difference is way higher.
It is also a “converging” flow: at the first 10mm of their travel, the burnt gas molecules / radicals from the auxiliary chamber have reach the big majority of the mass of the unburned air-fuel mixture in the “main chamber” / bowl of the PatBam.

Worth to note: in the Mahle TJI the small chamber is permanently “open” to the main chamber (through the nozzle orifices):



while in the PatBam HCCI the auxiliary chamber opens after the combustion, which means substantially higher pressure differences.

The fast HCCI combustion (i.e. the near “constant volume combustion) has to happen at, or near, the TDC, otherwise the almost impact torque increase can generate strong tortional loads on the crankshaft / casing:



At the right plot, red curve for Ti=130Celsius / AFR=2.2, the impact increase of the torque on the crankshaft (there is a significant eccentricity of the force applied on the crankpin from the rotation axis of the crankshaft, while the force/pressure increases explosively) may cut the crankshaft.

Thanks
Manolis Pattakos

[gruntguru]

the average speed of the jet of the Mahle TJI is near 60m/sec:
This makes pessimistic my previous assumption for 100m/sec average jet speed (or "mass transfer" speed) when the transfer port opens in the compact bowl (~20mm maximum distance) of the above post PatBam HCCI. . . .

. . . .Worth to note: in the Mahle TJI the small chamber is permanently “open” to the main chamber (through the nozzle orifices) . . . . while in the PatBam HCCI the auxiliary chamber opens after the combustion, which means substantially higher pressure differences.

So 100m/s might not be so pessimistic? Besides, you have a much shorter path than the pent-roof TJI.

[manolis]

Hello Gruntguru.

You write:
“So 100m/s might not be so pessimistic? Besides, you have a much shorter path than the pent-roof TJI.”


Supposing 150m/sec average jet speed and 20mm maximum distance, the time required in order the burnt gas (and the radicals it contains) to go from the transfer port to the ends of the bowl is:

0.02m / 150m/sec = 0.00013sec=0.13msec

At 6,000rpm this time interval corresponds to less than 5 degrees.



The question is whether such high speed mass transfer / convection is advantageous. Mahle may know.

Thanks
Manolis Pattakos

[J.A.W.]

See link for a fancy-pants new Italian V-twin 2T motorcycle, (reportedly built with engineering input from ex-Ferrari blokes).

On offer is a 250cc road bike, & a dedicated 288cc track/race bike. The smooth, bone-like 'organic' con-rods look interesting.

http://www.vinsmotors.com/modelli/dueci ... e/?lang=en

[manolis]

Hello J.A.W.

You write:

"The smooth, bone-like 'organic' con-rods look interesting."




It seems they are not "the" connecting rods of the engine (wherein the axes of the small and big ends cannot help being parallel to each other), but some connecting rods of the suspension.



Thanks
Manolis Pattakos

[J.A.W.]

Indeed Manolis, but by contrast to those on a (ok, a budget built) Yamaha RZ, which are 3 steel tubes welded together,
they are an aesthetically pleasing, elegantly engineered, light (if expensive) production solution - to a like stuctural item.

[Pinger]

I hope it is clear now that a 2-stroke can be turbocharged and supercharged heavily, at a degree similar to that of the 4-strokes.



You also write:
“Until the pistons melt.....”

A Diesel runs from lean to extremely lean.

The temperature of the piston and of the exhaust gas are substantially lower than in a spark ignition / stoichiometric engines.

With the intake cycle pressure and the exhaust cycle pressure shifted by some 20%-30%, the temperatures are not for melting the pistons.

Reviving an earlier post here.
Other than the 125cc bike on the dyno (in a subsequent post - which appears to be a one-off run to prove a point rather than a serious proposition for road use) are there any documented cases of turbocharged SI 2Ts?
The only others I can think of are from some time ago where sled engines had a turbo attached to the (far) end of their exhausts. I've not seen any follow up as to reliability etc, etc. Any info anyone?

[J.A.W.]

Here's one - of several - that a quick search brings up, Pinger;
http://viper.mountainperformance.com/SK ... _p_92.html

I'd guess that apart from altitude compensation, the idea of increasing the complexity/cost/weight of a sports machine
tends to run counter to the essential 2T ethos - (except perhaps in extreme performance/racing applications).

Since current 2T sporting engines are already carefully designed to offer optimum power outputs which can only be matched by 4T's of significantly greater capacity/cost/complexity/mass, the need for a factory forced induction 2T - is somewhat moot.

Obviously a dedicated/clean-sheet forced induction 2T engine - incorporating at design stage, the required ability to
robustly/reliably cope with extra power & concomitant added pressures/heat stress/packaging issues - is not beyond the technical capabilities of companies like BRP/Rotax ( & they have a patent on just such a unit), but whether or not there is a compelling marketing case for production/sale of such a machine, is another matter - altogether.

(Meeting lucrative contracts for particular military requirements - aside, of course).

[Pinger]

Thanks J.A.W.

What I'm trying to get numbers on is exhaust backpressure - which on a turbo 4T is apparently at least twice (often higher) than inlet manifold pressure.

Any exhaust backpressure figures for turbo 2T anyone? Similar to 4T?