2 stroke thread (with occasional F1 relevance!)

[J.A.W.]

Again - Mudflap, you are not actually directly addressing - your assertion.

Both cases cited in fact note 2T merit advantages over 4Ts - nor does the uniflow factor directly apply*.
& the academic 'on paper' flow study - which proposes exhaust poppets, has yet to offer proof, in metal..

*See the Garrett CI uniflow 2T turbo-compound helo mill proposal for NASA, that I linked, bottom of P.5 in this thread.

[63l8qrrfy6]

Let me reiterate:

2Ts cannot achieve the same levels of SR (and all the benefits associated with it) as a 4T without significant negative effects on pumping losses, TE and in some instances SE.

This argument started from johnny's statement that the 2T combustion is superior - I disagree with this. Its merit is that it happens twice as often.

I am not disputing the mechanical efficiency advantage of 2Ts over 4Ts nor am I comparing different ports/valves etc.

[johnny comelately]

Again - Mudflap, you are not actually directly addressing - your assertion.

Both cases cited in fact note 2T merit advantages over 4Ts - nor does the uniflow factor directly apply*.
& the academic 'on paper' flow study - which proposes exhaust poppets, has yet to offer proof, in metal..

*See the Garrett CI uniflow 2T turbo-compound helo mill proposal for NASA, that I linked, bottom of P.5 in this thread.

I got so excited then, only to realise I had imagined the C, thought it was for NASCAR

[johnny comelately]

Stepping aside from the theoretical and looking at a unique characteristic of the highly tuned "conventional" two stoke is an unusual thermodynamic occurrence regarding the stinger diameter, where if a little too small, depending on fuel and configuration the reversals within can "blowtorch" the piston (crown). This is distinct from any detonation effects.

[johnny comelately]

Another "thermodynamic" anomaly, we put a 98cc stihl chainsaw on nitro for a fellow who races them (he beat the 250cc's but not the 500's.
Fuelling being the issue (30% - 50% nitro) left the jets out altogether, still too lean so drilled the body.
at one stage it was so rich on idle that they made him cut at the end of the row down wind as he was unsettling the other competitors with fumes.
eventually though HCCI, hot bulb, autoignition (and with its own little supply of oxygen)was discovered and with the pipe and plug glowing it couldnt be turned off until the choke choked it (air)

[J.A.W.]

Let me reiterate:

2Ts cannot achieve the same levels of SR (and all the benefits associated with it) as a 4T without significant negative effects on pumping losses, TE and in some instances SE.

This argument started from johnny's statement that the 2T combustion is superior - I disagree with this. Its merit is that it happens twice as often.

I am not disputing the mechanical efficiency advantage of 2Ts over 4Ts nor am I comparing different ports/valves etc.

Mudflap, your claim was that the 4T - despite compromised combustion chamber shape, offers superior combustion, & you wrote that 2Ts suffer from "poor charge motion" - which just aint so..

If anything, its the 4T with its laboured ( only one power stroke in every 4) method, & low output,( unless
expensively built - to spin up at real high rpm, &/or be 'boosted'), which has "poor charge motion"..

[gruntguru]

yes, something like that ...or ...
when is a pumping loss not a pumping loss ?

ie we might think the Wartsila's breathing is a struggle
but they don't care how much compressor work is needed as they are not compounding (significantly) from the turbine

Tommy.
I'm sure they do care. You don't get to 50+% TE without scrounging every last penny.

All.
On pumping loss. Losses only occur when there is a pressure drop in the flow path. The energy lost is pressure drop x volume flow. (For non-steady flow, integrate.)

Hence large volumes of air can be moved with little energy if the pressure difference is small and large pressures can be generated using little energy if the volume flow or delta is small.

The main source of pumping loss in SI 4T is across the throttle plate at part load. Engines using enleanment to regulate power, have lower pumping losses than throttled stoich' engines at part load even though they are flowing more air.

[johnny comelately]

Quote "The main source of pumping loss in SI 4T is across the throttle plate at part load. Engines using enleanment to regulate power, have lower pumping losses than throttled stoich' engines at part load even though they are flowing more air.

I look on those as throttling losses leading to efficiency losses, as the throttling lowers the effective compression taking the engine out of its efficiency zone, hence, as you say, the advantage of diesels.

So, there are pumping losses and pumping losses, I was referring to the ones that effect piston work, compression itself being a major one coupled with ignition point which can be a huge loss with lots of advance. with conventional two strokes the primary compression is another one, exacerbated by some who up that ratio by stuffing the crankcase creating bigger pumping losses especially if the transfer ports are the limiting factor (nitro neatly sidesteps a lot of this )

2T Gas exchange potential being at BDC is very limited by duration availability which then brings into question the bore stroke ratio to enable the longest duration. And here we have the great assistance of pulses , at least in the smaller engines.
Then, of course , all this must relate, particularly regarding EVO, to the number of cylinders and crankshaft layout. This being one of the problems with single cylinder testing extrapolating to multi cylinders.
But all this is very much assisted by modern simulation ability.
Now the gas exchange process is the great undiscovered, where is the inlet and where is the exhaust located and how big are they. an other impact of this is speed, where the slow revving marine engines have that advantage in having time on their side to exchange. another side effect in the smaller engines is the power band created by a version of optimisation. Part1.

[manolis]

Hello Pinger

You write:
“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.”



The “Budack” cycle of VW is a simplified, one step MultiAir / Twin Air:



(i.e. from the infinite continuously variable modes of the "old" MultiAir of FIAT, "the" VW can operate with their "Budack" VVA at only two modes, a normal and an EIVC (early intake valve closing)).

The VW Budack "cycle" needs a throttle valve to control the load at partial / light loads (pumping loss), while the MultiAir needs not throttle valve: the intake valves is the only throttle valve of the MultiAir.


Take the case of the partial load of the MultiAir (central slide):



The intake valves close early (say 40-50 degrees after the TDC).

Then, the trapped in the cylinder fresh and cool air-fuel mixture expands (and cools down as it expands) until the BDC.
Its volume increases several times.

The crankshaft provides mechanical energy in order to expand the air-fuel mixture.
If adiabatic, this expansion would cause a significant temperature drop.
But the fresh air-fuel mixture is heated by the surrounding hot cylinder walls.

During the following compression stroke, the already heated air-fuel mixture resists and the crankshaft has to provide more energy for its compression (i.e. the crankshaft pays energy for the expansion of the air-fuel mixture, and then (because the air fuel mixture is heated by the walls) only a part of this energy is recovered as mechanical energy on the crankshaft).

The more partial the load, the higher the pumping loss.


Take now the PatAir running on its unlimited true Atkinson – Miller cycle (LIVC):



At partial load / low-medium revs,
the intake valves remain wide open during the intake stroke and during an initial big part of the compression stroke.
The intake valves finally close some 40 degrees before the TDC.
Most of the air-fuel mixture entered during the intake stroke, returns back into the intake manifold during the compression stroke almost without spending energy (with the intake valves wide open at low – medium revs, the gas flows so easily that the quantity of energy required is too small).


At the TDC the air-fuel mixture in the MultiAir (and in the Budack) is more (because it has to cover the increased pumping losses) and is substantially hotter than the compressed mixture in the PatAir.

The temperatures in the following combustion are lower in the PatAir (which means less thermal losses (and NOx emissions), which means more energy on the crankshaft during the following expansion).

As compaired to the MultiAir / TwinAir of FIAT / Alfa Romeo / Chrysler (and to the VW Budack VVA), the PatAir:





which mechanically is nothing more than a MultiAir having different intake cam lobe profiles on its camshafts,
not only consumes less mechanical energy during the intake and the compression cycles, but it also provides more mechanical energy during the expansion cycle (due to the lower thermal losses).


They above described differences may seem small / insignificant, however they dramatically change the way an internal combustion operates, its BTE, its BSFC, its emissions etc.

Thanks
Manolis Pattakos

[manolis]

Hello Gruntguru.

You write:
“The main source of pumping loss in SI 4T is across the throttle plate at part load. Engines using enleanment to regulate power, have lower pumping losses than throttled stoich' engines at part load even though they are flowing more air.”


As for the 2-stroke engines, they have more significant problems than the pumping loss in the partial loads / low medium revs.

A 4-stroke stoichiometric spark ignition suffers from low BTE at partial loads.
From, say, near 40% BTE at full load / medium revs, the BTE of a Mazda SkyActiv-G can easily drop below 20% BTE at partial loads.

With their SkyActiv-X (HCCI) Mazda not only achieves a 20% higher peak BTE than their current high-tech engines (SkyActiv-G) but, more importantly, it keeps the Brake Thermal Efficiency high and more or less constant at the partial loads.
Until now, the lower revs – higher load was the recipe for higher mileage.
The SkyActiv-X expert of Mazda recommends (in the video a few pages ago) to use shorter gear ratios (i.e. higher revs) and lighter load!
(the increased noise is a drawback, but the increase mileage is more compelling; the way better response is a big advantage, too).

This is a big change in the 4-stroke gasoline engines.


A 2-stroke stoichiometric, on the other hand, has to scavenge the cylinder at the partial loads, without leaving fuel to pass unburned to the exhaust.
The amount of the residual gas in the cylinder is extreme and deteriorates the combustion much more than the sub-pressure in the 4-stroke SI.

And according the videos from Mazda (SkyActiv-X) and Toyota (with their dynamic engine), the best recipe to improve combustion is to complete it at (or right after) the TDC.

A 2-stroke has a better and more compact combustion chamber, but has also way more residual gas to delay the combustion at the partial loads (throttled).

A delayed combustion cannot be an efficient combustion.

The instantaneous lean cold combustion of the Mazda-X can improve the 2-strokes more that it improves their 4-strokes.

The same for the PatBam HCCI:



The most important in an internal combustion engine is the efficiency of the combustion. All the rest are secondary.

Thanks
Manolis Pattakos

[J.A.W.]

"Most important" Manolis?

Important, to be sure, but really only an important parameter among many in the internal combustion mill..

Gas turbines for example, have had their combustion process worked out for decades, but the gas-pumping
/fluid work part of their function - has been a real struggle to achieve overall "efficiency" in fuel use terms.

The 'simple' 2T has, for over 60 years, been seen as an inexpensive method to harness a pulse-jet to shaft work.

& it is the overall cost-benefit ( notwithstanding the dreaded 'marketing' dept) which keeps interest going.

After all, automatic firearms have, for well over a century now, been an example of practicable 2T function.
A firing stroke does the 'work' & vents waste - while the recoil ( recip') recharges on the upstroke..
No one would now consider having an extra two non-productive strokes in a gun - to be anything but a waste.

[johnny comelately]

No, it's not an "exercise in fluid dynamics with a happy ending" engine, it is an (internal) combustion engine
But seriously, its no use getting in the equivalent of hurricane Katrina (the only one i can remember) to burn half of it , like nitro engines as we haven't got solar powered turbos (yet) and its no use creating perfect combustion if its in a thimble. it needs everything to come together, soon. e engines are the enemy and we must fight them.

[manolis]

Hello J.A.W.

Mazda changing only the combustion achieved a 20% reduction of the fuel consumption.



The SkyActiv-G at left has changed to the SkyActiv-X at middle only by improving the combustion.

Is there something else that can do such a difference?


And I anticipate that the difference for the 2-strokes will be even greater.

Thanks
Manolis Pattakos

[Pinger]

Hello J.A.W.

Mazda changing only the combustion achieved a 20% reduction of the fuel consumption.

So Mazda say. What of the real world?
Your feted Multi Air was credited with fantastic fuel efficiency but in real world terms it is little better than the 8-valve 4-cylinder engine that Fiat have used for decades and costs buttons to make (and most likely uses a fraction of the resources to manufacture if ecology is the arbiter of good and bad).

Facebook has shown what happens to share price when the news is bad. Good news - true or false - props up share price. Mazda are in this game to make money. When Mazda market its HCCI and the fuel efficiency is confirmed, fine, I'll believe it then. Until then it's just so much hype.

Is it true that Honda are using HCCI in F1? Or is it merely incapable of matching the others using the same technology as them?

[63l8qrrfy6]

Let me reiterate:

2Ts cannot achieve the same levels of SR (and all the benefits associated with it) as a 4T without significant negative effects on pumping losses, TE and in some instances SE.

This argument started from johnny's statement that the 2T combustion is superior - I disagree with this. Its merit is that it happens twice as often.

I am not disputing the mechanical efficiency advantage of 2Ts over 4Ts nor am I comparing different ports/valves etc.

Mudflap, your claim was that the 4T - despite compromised combustion chamber shape, offers superior combustion, & you wrote that 2Ts suffer from "poor charge motion" - which just aint so..

If anything, its the 4T with its laboured ( only one power stroke in every 4) method, & low output,( unless
expensively built - to spin up at real high rpm, &/or be 'boosted'), which has "poor charge motion"..

Manolis has kindly explained my point regarding combustion efficiency:

A 2-stroke stoichiometric, on the other hand, has to scavenge the cylinder at the partial loads, without leaving fuel to pass unburned to the exhaust.
The amount of the residual gas in the cylinder is extreme and deteriorates the combustion much more than the sub-pressure in the 4-stroke SI.

A 2-stroke has a better and more compact combustion chamber, but has also way more residual gas to delay the combustion at the partial loads (throttled).

A delayed combustion cannot be an efficient combustion.

The only thing to add is that there are quite a few 2Ts with poppet valves (including the RTA96C) - what is the point of adding that much complexity, friction, ruining the combustion chamber shape and increasing the discharge coefficients ?