'Air work' is Ricardo's term.
With a 4T he lumps all the mechanical friction with air work on the basis that that friction is encountered emptying and filling the cylinders.
Re HCCI.
Yes, very high rate of combustion creates mechanical problems. I've yet to hear of HCCI utilising more than 40% of the air. So substantially more air required for the same power output (which inevitably means boosted, bigger or faster) and if a symmetrically ported 2T then charge loss will be no better than with Lambda = unity.
Trapped charge in the crevices (behind valves, in ring lands, etc) can fail to combust as quenched in the crevices and when released has missed the main (fast) combustion so misses the cut and appears in the exhaust as UBHC. At higher levels of air utilisation HCCI creates NOx.
HCCI has (IMO) been put on a pedestal as the unobtainable holy grail with one single virtue. All it can do now is fall from that pedestal.
OK, got all that.
With HCCI the NOX was always on my mind (Elvis) too.
one problem that always exists in the background is inconsistency from one power stroke to the next, I see HCCI introducing a lot more of that and I can imagine there is back up spark to help solve it (always going to be a few revs behind though), BUT flame ignition can be very nearly as quick and a bit more progressive cylinder pressure and be consistent.
I wonder what the behaviour of the free radicals are during HCCI?
Hello Tommy Cookers.
You write:
"the exhaust thrust of aircraft engines substantially increases propulsive force (thrust) at conventional speeds
eg 20% at 400 mph and 30% at 500 mph"
Are there any links showing the 20% - 30% increase of the thrust force?
Hello Johnny comelately.
The kWh is a strange unit.
Multiplying the kW (wherein the time is in the denominator) by a time unit (hour), the time is completely gone. What is left is energy (like Joules).
Hello Pinger.
The VVA of VW of "your" video is the same with the VVA in the other video. The only difference is the shape of the camlobes. For the Atkinson cycle the one camlobe needs a long duration.
In both cases the VVA is a 2-step (two modes) VVA.
In comparison, the PatAir using one only camlobe per valve (or pair of valves) it achieves a continuous infinity of lifts and durations.
Compare the 2-mode hyrdaulic / mechanical VVA of VW with the pure mechanical DVVA (desmo VVA) at http://www.pattakon.com/pattakonDesmo.htm:
Rid of valve springs, rid of unnecessary loads (like the restoring force from the valve springs at medium and low revs), rid of heavy quick moving parts, rid of sliding friction etc, the reliable rev limit of the engine is no longer set by the valve train but by the underneath mechanism (crankshaft, connecting rods, pistons and block).
The DVVA varies continuously and independently the lift and the duration.
Most 2Ts have no poppet valves to compromise the combustion chamber shape/cause heat control issues.
I would argue that the lack of poppet valves in a 2T does not make up for the poor charge motion.
"You what?"
Do by all means, present your reasoning..
Swirl reduces trapping efficiency ( Pinger did a good job explaining the relationship between fluid forces and rate of change of momentum). It is very difficult to balance swirl, trapping efficiency and scavenge efficiency in a 2T.
In a 4T the tradeoff between the increased discharge coefficient of a swirl port and the benefits of swirl are much easier to optimize and the VE penalty is modest.
The limitations on breathing keep the 2T outputs low, despite variable timing/lift capabilities.
Most high-performance 2T's are primarily slanted towards a high power output - but check Manolis'
posts on the G2 Evinrude efficiency, which contradicts your assertion, as does the research findings
on sleeve-valve flow co-efficients/swirl-tumble effect advantages over poppets.
"Well, we knocked the bastard off!"
Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).
Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).
You write:
“Swirl reduces trapping efficiency ( Pinger did a good job explaining the relationship between fluid forces and rate of change of momentum). It is very difficult to balance swirl, trapping efficiency and scavenge efficiency in a 2T.”
With the HCCI combustion, the swirl is no longer a requirement (at least not in the strength it is required for the progressive combustion in the conventional spark ignition gasoline engines, and in the conventional compression ignition Diesel engines).
When the temperature gets above the threshold for auto-ignition, with or without swirl, all the fuel will react with its neighbouring air (and there is plenty of air always) instantaneously (actually in a few crankshaft degrees).
The problem is how to control the beginning of the HCCI combustion (as explains Mazda’s SkyActiv-X expert in the video a couple of pages ago).
If too early, the combustion completes several degrees before the TDC and the engine spends energy to compress at extreme temperature (which means, among others, increased thermal losses) an already burnt gas.
If too late, the efficiency drops (due to the lower expansion ratio) and the hammering torque may even cut the crankshaft (a similar effect to what Ricardo said for the Opposed Piston engines wherein a big phase difference between the two crankshafts over-stresses the late crankshaft and the engine).
For instance, in the following plot:
the curves EGR=0, 10 and 20 (combustion substantially before the TDC) cause a strong increase of the energy required for the compression of the already burnt gas (at the expansion stroke this energy is subtracted, i.e. it offers nothing but friction and significant thermal loss increase; the torque the crankshaft has to apply to compress the already burnt gas is quite strong because the eccentricity of the connecting rod long axis from the rotation axis of the crankshaft is big),
while the curve EGR=35 (combustion substantially after the TDC) is also bad because the expansion ratio is lower and because the impact increase of the pressure while the eccentricity of the connecting rod long axis from the rotation axis of the crankshaft overloads the crankshaft.
You can think of the HCCI combustion as pushing a rock / stone to the top of a hill / mountain (Sisyphus myth):
At some height, somehow (say, by burning the fuel in the cylinder), the weight of the stone (or the pressure in the cylinder) doubles.
Which is the ideal height to get the most energy?
If the weight doubles at the middle of the uphill, the rest pushing to the top of the mountain gets double harder and causes only problems and reduced overall energy.
If the weight doubles at the middle of the downhill, the effort to push the rock from the middle of the uphill to the top of the mountain was at least meaningless.
But if the weight of the rock doubles at the top of the mountain (TDC), it is the best case because the uphill was easy, and the net energy delivered by the proccess maximizes.
In the following image:
the combustion in the HCCI engine (center) is done (completely finished, no flame),
while the combustion in the spark ignition engine (at left) continues strongly (blue flames),
and while the combustion in the right cylinder (Diesel) also continues strongly (red flames (glowing particulates)).
Isn't this (the strict control over the timing of the HCCI combustion) what the PatBam geometry does?
A continuously variable VVA is better than a 2-step VVA (like VW’s) only because it can better adjust the lift and/or duration of the valves and/or the valve lift profile to the existing operational conditions.
For instance, in this plot it is shown the 2-step V-TEC of Honda versus the VVA-roller:
If neither the one mode (the set of green curves) is good enough for the existing operational conditions, nor the other mode (the red curves) is good enough, then what?
The blue curves of the VVA-roller are only few of the infinite available; depending on the operational conditions, a valve lift profile is quite near the optimum.
Depending on the control shafts angular position the valve lift profile varies continuously from zero (for valve deactivation if desirable), to tiny (for idling), to mild, to medium, to racing (top curve), all in the same engine, all instantly available:
Two-step compared to continuously variable systems:
"it is like having a shoe-shop offering just two sizes of shoes, one for children and one for adults. If the customer is lucky OK, if not..."
It is a pity “the” VW group compromised to such simple and inefficient VVA.
I hear you saying “But if it is adequate, why VW to go to more complicated solutions?”
On one hand it is not simpler (compare it to the continuously variable roller-VVA: count the parts and the machining they need),
on the other hand the mileage they achieved with the Atkinson cycle is ter, as the guy in the “Volkswagen's New Engine Cycle - The 'Budack' Cycle” video says: "the mileage is the same with or without it".
Have you got an HCCI test engine up & running yet, Manolis?
"Well, we knocked the bastard off!"
Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).
It is a pity “the” VW group compromised to such simple and inefficient VVA.
I hear you saying “But if it is adequate, why VW to go to more complicated solutions?”
On one hand it is not simpler (compare it to the continuously variable roller-VVA: count the parts and the machining they need),
on the other hand the mileage they achieved with the Atkinson cycle is ter, as the guy in the “Volkswagen's New Engine Cycle - The 'Budack' Cycle” video says: "the mileage is the same with or without it".
Thanks
Manolis Pattakos
The point I was making was adoption by VW of the Miller/Atkinson cycle via the early as opposed to late inlet valve closing the latter being the more conventional of the two - rather than a critique on VVT mechanisms. Presumably VW felt its objectives had been met with its mechanism even if the fuel consumption isn't as good as at least one reviewer expects. But not necessarily a like with like comparison - how much thirstier would the Tiguan be without the new strategy?
Talking of the Miller/Atkinson cycle, with a 2T, when attempts are made to close the exhaust port early (ie, closing fewer crank angle degrees ABDC than the crank angle degrees BBDC of its opening) it moves the cycle in the opposite direction of the Miller/Atkinson cycle.
And, while much is made of a 2T sacrificing much of its (lower) stroke to gas exchange, nothing is ever said of the 4T terminating its expansion stroke some 60-70 deg BBDC.
the simple-minded question is whether it justifies the added complication.
Some more torque at the lower rev range (in expense of worse emissions) . . .
Regarding the R-R Merlin 130 series and its multi-ejector exhaust system that significantly increases the thrust (and the noise):
In the case of a Portable Flyer powered by PatBam engines:
the cruising speed is lower than 200mph (the pilot will suffer at higher speed).
Also, the PatBam engine is to run from lean to extremely lean mixtures (which means that the exhaust gas is substantially cooler than in the conventional engines, and that there is no unburned fuel in the exhaust); i.e. there is not significant energy in the exhaust gas to exploit.
By the way, are there any data for the lambda used in the R-R Merlin 130 engines?
Supposing it is around 0.8 (for maximum power), a 20% of the fuel arrives unburnt at the exhaust; its after-burning can offer a lot energy to the backwards moving gas (and forward thrust to the airplane . . .
You write:
“Have you got an HCCI test engine up & running yet, Manolis?”
Isn't this (the strict control over the timing of the HCCI combustion) what the PatBam geometry does?
We don't know as it is as yet untested.
I have concerns that it will not provide the required control because the required pressure rise in the sub-chamber may not be as desired due to leakage as there is no sealing and the clearance between 'anvil' and the sub-chamber must be greater than the piston clearance (plus anticipated piston/bore wear).
This leakage path may deprive the sub-chamber of reaching a pressure high enough relative to the main chamber and that (required) pressure may be higher than anticipated due the unfavourable surface to volume ratio leading to heat loss.
The clearance issue will be doubly worse for the opposed piston design.And that would be with the engine brand new. I see structural issues emerging over prolonged use impinging on the above also (an exhaust valve relies on being seated to cool, the 'anvil' gets no such respite while presenting a large surface area to absorb heat and a very limited heat path to lose the heat).
HCCI control is so fickle as to go beyond theory. A working practical example is required.
Exhaust noise level reduction, as a functional concern for fighter aircraft, has never been a priority..
(It was however an issue, for the civil-Merlin when powering airliners, esp' when in competition
with the notably quieter Bristol Hercules radial, a sleeve-valve engine - able to run at much lower boost).
& the BRP/Evinrude G2 Team might well suggest that the Ricardo 2/4 test unit actually proves the opposite..
..it is the ultra-complex needs of accomodating needless 4T function - which 'spoils the pot' - as such.
"Well, we knocked the bastard off!"
Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).
I thank J.A.W.for the exhaust thrust reference (continuing onto page 2 is also useful)
(and for the Repco etc link in another thread)
wuzak might have been poster (maybe in this thread 3 years ago ??) of the 20% at 400 mph Corsair NACA reference
my previous computer is stuffed with these and others and disorganised - but sorry I can't access it right now
altitude is a factor of course - there might even have been a question of exhaust restriction to increase jet velocity
air racers (modified WW2 stuff) iirc might use this and high boost where a prop can't use all possible crankshaft power
late Merlins eg 130 would eg be run at 60% rich according to Sam Heron
(only for cooling - he says RR and Allison both treated 115/145 fuel as 145/145 ie high boost was useable with any mixture)
eg the Pierce and Walsh paper on the Wright Turbo-Compound gives their rich and lean AFR energy balance plots
shown on this site maybe the 'big thread' on F1 engines 3ish years ago or the TERS thread eg P22 has gg's recovery calculator
lean 0.056 FAR shows 4.6% of the fuel energy remaining in the exhaust 3.1% as CO and 1.5% as methane
rich 0.100 FAR shows 47.8% of fuel energy remaining in the exhaust 31.8% as CO 1.8% as methane and 14.2% as hydrogen
this is nearly 50% rich - what impresses me is that the Avgas has been dismantled and its exhaust is still a good fuel
remember this is 115/145 whose stoichiometric FAR is several % higher than traditional road gasoline's
(btw my best bsfc for the later TC iirc rendered into trad road gas equivalent possible in cruise was 0.36 lb/hp/hr)
the burning exhaust (single plume) of the TC in cruise was notoriously conspicuous to airline passengers
with a rich mixture there is presumably no burning until the exhaust gas has emerged into the atmosphere
all aircraft SI piston engines are run rich at high power
the Merlin and DB engines had sporty valve timing but EVO seems rather early even on big engines with modest timing
the CR is low to allow high boost for maximum takeoff etc power so the exhaust has high energy
re the 4 stroke terminating its expansion stroke at 60 or 70 deg bbdc .....
fwiw I have repeatedly speculated......
eg EVO timing is a compromise and so could usefully be variable
blowdown at power is choked ie supersonic or sonic and thermodynamically wasteful ie partial non-reversiblity
(the value of high exhaust pressure in F1 etc is to reduce this waste by increase of exhaust density ie load on blowdown)
EDIT
P15 of TERS thread (my 2nd post April 27 2014) has a link to Wright TC brochure incl energy balance mentioned above
and a link rotaryeng.net conspicuously has the energy balance plots with FAR
Last edited by Tommy Cookers on 27 Mar 2018, 23:48, edited 12 times in total.
Re exhaust thrust. Didn't Crecy development eventually finalise exhaust thrust as a main objective? Aided by the rapidity of port opening (though compromised by the sleeve's velocity w.r.t. a conventional 2T uncovering its port at piston velocity)?
The limitations on breathing keep the 2T outputs low, despite variable timing/lift capabilities.
Most high-performance 2T's are primarily slanted towards a high power output - but check Manolis'
posts on the G2 Evinrude efficiency, which contradicts your assertion, as does the research findings
on sleeve-valve flow co-efficients/swirl-tumble effect advantages over poppets.
The link you have shared only deals with heat transfer as far as I can see - which portion is relevant to what I have stated ?
Here's two references I found after 5 minutes of searching supporting my statement:
Enrico Mattarelli, Giuseppe Cantore and Carlo Alberto Rinaldini - Advances in The Design of Two-Stroke,High Speed, Compression Ignition Engines:
From the scavenging quality point of view, uniflow scavenging is generally better than loop,
even if the necessity of imparting a swirling motion to the inlet flow can spoil the advantage
a little bit. Since the swirl requirement is more stringent for direct injection, DI Uniflow scavenging
configurations generally yield lower trapping and scavenging efficiency than Uniflow
IDI designs. Another advantage of the IDI design is the cost of the injection system,
that can be of the mechanical type. The downsides are the low thermal efficiency and the
limitation on power rating due to smoke emissions at high speed and load.
The optimization of a DI combustion system without swirl is far from trivial and it requires a strong support
by simulation and specific experiments, with ensuing rise of the engineering costs. The same
problem may be faced in the development of an opposed piston design, because of the lack
of reference in recent projects.
XinyanWang, Jun Ma and Hua Zhao - Analysis of scavenge port designs and exhaust valve profiles on the in-cylinder flow and scavenging performance in a two-stroke boosted uniflow scavenged direct injection gasoline engine
This paper shows correlations between SR, TE and SE amongst other things and perfectly illustrates the tradeoffs.
LE-found this too:
TULWIN, T., WENDEKER, M., CZYŻ, Z. The swirl ratio influence on combustion process and heat transfer in the opposed piston compression-ignition engine. Combustion Engines. 2017, 170(3), 3-7. DOI: 10.19206/CE-2017-301
It specifically mentions pumping losses associated with high swirl. It is also interesting that the highest SR is about 4. A 4T DI diesel typically sees SR in excess of 10 (https://pdfs.semanticscholar.org/15fd/3 ... 6ad019.pdf)
Last edited by 63l8qrrfy6 on 27 Mar 2018, 22:52, edited 1 time in total.
- https://www.youtube.com/watch?v=XpouJWlRBdY
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