2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello J.A.W.

According your plot, the RD350F2 ’86 makes, at its best point (9,000rpm) 47mN of torque from 350cc. This means a specific torque of 134mN/lt, i.e. only 22% above the specific torque provided by the 4-stroke Ducati Panigale.

This is too low.

In a first approach you can say that if the air / mixture trapped in the cylinder of the Ducati Panigale is X% of its cylinder capacity, in the Yamaha RD350F ’86 this percentage drops at only 0.6*X% .

In a second approach, you can say that if the Yamaha RD350F ’86 had the same "trapping efficiency" with the Panigale, then its peak torque would be 77mN (i.e. more than 60% higher than what it is now).


It seems the over-boosting by the exhaust is less efficient than the over-boosting by the intake (used in the Ducati Panigale).

The PatATE is not based on the exhaust to boost its cylinder, but on the intake as the 4-stroke Ducati Panigale.

And because the blow-down happens way faster in the PatATE than in the conventional 2-strokes, the PatATE appears even more promising at higher revs.



The conventional 2-stroke was good but can’t win the 4-strokes without major changes of its architecture.

The PatATE suggests a substantially different architecture for the 2-strokes which may cure the big problems (like emissions, lubrication, scuffing resistance etc) of the conventional 2-strokes.

Thanks
Manolis Pattakos

[Tommy Cookers]

all RD350s have a saggy part of the performance curve (unsurprisingly)
the unsmoothed truth is not what Yamaha published (unsurprisingly)
http://www.rd350lc.net/CurveBad4L0.jpg
that's why it's not an everyday machine (unsurprisingly)
ie it's not an example of a motor for the 'motorist'
even smoothed (J.A.W's plot shows) the clear change in characteristic (slope) ie the lack of speed stability needed in the real world
though the 27 bhp German-market only version might be ok
http://www.rd350lc.net/curves27-50.jpg

interestingly unsmoothed curves are found (the same source ??) for other machines eg Cycle World's old 750s review that J.A.W linked

no, I can't find a R5 (36 bhp 32mm ex port) vs RD350 aircooled (39 bhp 29.5mm) comparison

[Pinger]

Hello Pinger.

According your plot, the 2-stroke Evinrude E-TEC 150 has less than 20% higher torque at 4,000rpm than the same capacity 4-stroke F150 of Yamaha, and the same torque with the 4-stroke F150 above 5,000rpm.

Not good values for a two-stroke.

At 3000rpm where the motor is expected to push the boat out of the water the 2T makes one third more torque than the 4T. Exactly suited to intended role.

If Evinrude E-TEC was filling its cylinders as Ducati does, Evinrude would make the double torque and power than what it makes now.

You miss the point entirely.
To attempt complete cylinder filling on a 2T is to lose too much charge to the exhaust port (unless elaborate measures are taken with the exhaust which likely as not will be effective only over a narrow speed range and simply not viable where space is at a premium).
Instead, accept lower volumetric efficiency (per cycle) and make the torque and power from the multiplicity of firing events. Take the simplicity, compactness, and light weight as bonuses.

Why on earth a state of the art race 4T (Panigale) is being held as a benchmark as to what utility 2Ts should do escapes me.

[manolis]

Hello Pinger.

You write:
“You miss the point entirely.
To attempt complete cylinder filling on a 2T is to lose too much charge to the exhaust port (unless elaborate measures are taken with the exhaust which likely as not will be effective only over a narrow speed range and simply not viable where space is at a premium).”


This is the point:

To achieve complete cylinder filling on the 2-stroke without losing charge to the exhaust port, and without narrowing the useful rev range.



You also write:
“Instead, accept lower volumetric efficiency (per cycle) and make the torque and power from the multiplicity of firing events. Take the simplicity, compactness, and light weight as bonuses.”


For an outboard engine this may be OK.

But think of an aeroplane engine or of a motorcycle engine.



You also write:
“Why on earth a state of the art race 4T (Panigale) is being held as a benchmark as to what utility 2Ts should do escapes me.”


The Panigale was mentioned as a reference 4-stroke engine for comparison with sport and racing 2-stroke engines (like the Yamaha RD350LC and the Yamaha RD500).

Also as an engine that, based on its intake system, achieves an excellent filling of its cylinders.

If a 2-stroke (like, say, the PatATE) can achieve the cylinder filling of the Panigale, then . . .



Here is the porting of the RZ350 (more at http://erlenbachracing.co/rz350intakeporting.htm ) :



Spot on the short transfer ports and on the “tall” exhaust port that limits the useful “expansion” stroke.

Thanks
Manolis Pattakos

[Pinger]

This is the point:

To achieve complete cylinder filling on the 2-stroke without losing charge to the exhaust port, and without narrowing the useful rev range.

That, is nigh on impossible.
Exhaust tuning will always be compromised by the variety of rpm in which it must operate and any mechanical valving must achieve complete blow-through - not one fraction more or one fraction less - across the entire speed and load ranges while avoiding any diffusion with the residual products from the previous cycle.

You also write:
“Instead, accept lower volumetric efficiency (per cycle) and make the torque and power from the multiplicity of firing events. Take the simplicity, compactness, and light weight as bonuses.”


For an outboard engine this may be OK.

But think of an aeroplane engine or of a motorcycle engine.

Small aircraft prefer low rpm 4T to obviate the need for reduction gearing for the prop, and the power delivery characteristics imposed on a 2T by the inevitable expansion chambers is either loved or loathed - mainly loathed. No mechanically operated valving has been found that provides the required level of performance - and for reasons already stated, isn't likely to be.

You also write:
“Why on earth a state of the art race 4T (Panigale) is being held as a benchmark as to what utility 2Ts should do escapes me.”


The Panigale was mentioned as a reference 4-stroke engine for comparison with sport and racing 2-stroke engines (like the Yamaha RD350LC and the Yamaha RD500).

Also as an engine that, based on its intake system, achieves an excellent filling of its cylinders.

If a 2-stroke (like, say, the PatATE) can achieve the cylinder filling of the Panigale, then . . .

I've yet to see how you are going to harness intake effects to the degree that the Panigale does. Intake tuning on a 2T is even more restrictive regarding rpm range than exhaust tuning.

Spot on the short transfer ports and on the “tall” exhaust port that limits the useful “expansion” stroke.

Does it though?
When 2T runs the same compression ratio as 4T then it runs with the same expansion ratio. Only the start and end points are different.
View the 2T from a different perspective. It is an engine that uses half of its cylinder for gas exchange and the upper, other half for power production. It uses the same compression and expansion ratios as its 4T competitor (of the same capacity and rpm range) but each cycle utilises half the mass charge of the 4T - but does so twice as often. Really, there is no loss of expansion - just fewer pumping strokes.

[manolis]

Hello Pinger.

You write:
“When 2T runs the same compression ratio as 4T then it runs with the same expansion ratio. Only the start and end points are different.
View the 2T from a different perspective. It is an engine that uses half of its cylinder for gas exchange and the upper, other half for power production. It uses the same compression and expansion ratios as its 4T competitor (of the same capacity and rpm range) but each cycle utilises half the mass charge of the 4T - but does so twice as often. Really, there is no loss of expansion - just fewer pumping strokes.”


Here is the valve lift versus the crankshaft angle of several older 4-stroke Ducati engines:



For each of the above 4-stroke Ducati engines, the exhaust valve opens at 90, or so, degrees before the BDC.
Similarly for their intake valves: they close at 90 degrees, or so, after the BDC.


According your reasoning / understanding of the 2-stroke engines,

i.e. that the stroke is divided into an upper half for power production and into a lower half for gas exchange,

each of the above 4-stroke Ducati engines “uses half of its cylinder for gas exchange and the upper, other, half for power production”.


On this reasoning, these engines should provide half power and torque as compared to 4-strokes having exhaust valve(s) opening at the BDC and intake valve(s) closing at the next BDC.

In practice the opposite happens. The above Ducati 4-stroke engines had top specific power and torque.


The theory of the two cylinder halves needs reconsideration / amendment.


Back to the 2-stroke engines:


Adding to the previous graph the curves from the following plot:



it results this diagram:




Compare the pair of the red and blue curves (drawn by bold line) at left (2-stroke PatATE, exhaust and transfer opening versus the crankshaft angle) with, say, the pair of curves “G Corsa EX” (thin cyan line at left) and “G CORSA IN” (thin dark-cyan line at right) of the Ducati “G-Corsa”.


Then spot on the left pair of curves for the Yamaha RD350LC conventional 2-stroke: dark-green bold line for the exhaust, yellow bold line for the transfer, with the exhaust closing more than 30 crankshaft degrees after the transfer.

Aren’t the PatATE curves quite different than those of the conventional RD350LC 2-stroke?


Aren’t the PatATE curves similar to those of the 4-stroke Ducati G-Corsa?

This is what the PatATE brings to the 2-stroke engines: a completely different (and relatively similar to 4-stroke) way for the gas exchange.

Thanks
Manolis Pattakos

[J.A.W.]

Hello J.A.W.

According your plot, the RD350F2 ’86 makes, at its best point (9,000rpm) 47mN of torque from 350cc. This means a specific torque of 134mN/lt, i.e. only 22% above the specific torque provided by the 4-stroke Ducati Panigale.

This is too low...



The conventional 2-stroke was good but can’t win the 4-strokes without major changes of its architecture.

Thanks
Manolis Pattakos

Hi Manolis..

Do bear in mind that the 30 year old Yamaha RD/RZ 350 - while marketed as a exciting sporty 2T..
..is a basic, inexpensive, & softly tuned machine - sold with a warranty as suitable for a long TBO,
& quite capable of reliable economical operation - while undertaking mundane commuting tasks..

This essential 2T simplicity means that owner riders can undertake most mechanical maintenance tasks,
& the tuning potential means that skilled tuners (such as ' Wobbly' on the 'Kiwi Rider' forum) can virtually
double the output of that engine in race-tune, using fairly inexpensive* methods, while keeping the power delivery progressive for the rider & include a reasonable TBO/reliability factor as well..

* An 'order of magnitude' less costly than running a Ducati Panigale, on road, let alone in race-trim..

These things are proven.. & Manolis, while your designs seem to hold promise, only an 'in metal' showing will do..
( We are still waiting for real proof from the 'Ryger' - over 18 month since LucF last claimed "real soon").

[Pinger]

each of the above 4-stroke Ducati engines “uses half of its cylinder for gas exchange and the upper, other, half for power production”.


On this reasoning, these engines should provide half power and torque as compared to 4-strokes having exhaust valve(s) opening at the BDC and intake valve(s) closing at the next BDC.

In practice the opposite happens. The above Ducati 4-stroke engines had top specific power and torque.

You know very well how the above functions. The Ducati in question uses all its cylinder and for a significant time to generate flows that once having velocity imparted to them continue to flow due to inertia. The engine that has valve events opening and closing at BDC can do nothing but produce what torque it can at very low rpm - hence very low power.
Early exhaust valve opening on a 4T is acknowledgement of the fact that the exhaust stroke is a wasteful drag and made easier by first lowering cylinder pressure as much as possible before commencement of it. Clearly here, the extra expansion gained by opening the valve later does not compensate for the increased energy lost in emptying the cylinder during the exhaust stroke. Here, the 4T can be said to be mimicking 2T blow-down.

Aren’t the PatATE curves similar to those of the 4-stroke Ducati G-Corsa?


This is what the PatATE brings to the 2-stroke engines: a completely different (and relatively similar to 4-stroke) way for the gas exchange.

Yep, and all you're missing is the additional two pumping strokes to generate the pressure differentials to initiate and sustain flow.
Where are you sourcing the pressure to induce the fresh charge into the cylinder to displace the exhaust products during blow-through?

[manolis]

Hello Pinger

You write:

“You know very well how the above functions.”



I wish I knew.
Nobody does.




You also write:

“The Ducati in question uses all its cylinder and for a significant time to generate flows that once having velocity imparted to them continue to flow due to inertia. The engine that has valve events opening and closing at BDC can do nothing but produce what torque it can at very low rpm - hence very low power.”



However, the 2-stroke PatATE has a crankcase full of slightly compressed fresh air or air-fuel mixture.

So it does not need a piston to create a vacuum in the cylinder for the induction of the fresh air or air fuel mixture.

This process was done previously: as the piston was moving towards the TDC, the vacuum in the crankcase caused fresh air or air-fuel mixture to enter. The only difference between the 2-stroke PatATE and the 4-stroke Ducati is that a part of the cycle is realized underside the piston (crankcase).

Several degrees after the TDC the flow towards the crankcase weakens and the rotary valve closes the intake port.


As in the four stroke Ducatis, similarly the 2-stroke PatATE consumes a part of the time of the cycle to generate flows that once having velocity imparted to them continue to flow due to inertia.

It doesn’t matter that the one (Ducati) is a four-stroke and the other (PatATE) is a 2-stroke; they are based on the same working medium and on the same principle:



In both cases the engine consumes time to create gas flow, and then exploits the inertia of the flowing gas to overfill the cylinder.



Differently speaking, the two engines perform the same sequence of works.
In the 4-stroke all of them take place above the piston crown while in the 2-stroke some of them take place underside the piston, in the crankcase.

Thanks
Manolis Pattakos

[gruntguru]

Pinger, I think you are missing the clear advantages of a system which can achieve what Manolis is trying to do.

1. The ability to separate the phasing of the exhaust and transfer events can only be beneficial. Remember that the degree of such separation in the PatATE is a design variable which can be optimised to suit the intended application. The conventional two stroke is limited to zero separation - should we assume that this is optimal for every application? Many of the characteristics (exhaust emissions in particular) that make two strokes unsuitable for the applications where four strokes dominate, could be suppressed in the PatATE.
2. A large increase in port width permits larger ports for better flow and faster open/close rates.
3. Shared exhaust/transfer function of the hybrid port provides cooling for the otherwise problematic exhaust port.
4. Cooler exhaust port should allow reduced lube rate.
5. Exhaust reversion could be dramatically reduced - again less stress on the lube and reduced lube consumption.

The goal is not a simple engine to compete in the weed-whacker market - it is a more sophisticated two stroke with all the advantages (other simplicity) without the disadvantages.

[Tommy Cookers]

to meet the (road) mass market requirement we need a so-called 'flat' torque curve
ie certainly no RD350 curve other than the 27 hp German market or 27 hp Rajtoot versions
consider eg Villiers made (falling torque curve) trials engines (7hp@2500 - 12hp@6000) and (rising curve) scrambles versions (7hp@2500 - 19@6000) etc

small cylinder size can avoid compromise between port timing and port area, less small sizes eg RD350 can't
(the reason why 440 hp/litre is available only from 125 cc race cylinders)
consider eg Suzuki used their 'motorcycle' 500 engine in cars because they hadn't gone for RD levels of power/litre

for efficiency is a small capacity high rpm (2 stroke) engine any better than a larger, slower-running one ?
both being primarily air pumps and port-limited

[J.A.W.]

to meet the (road) mass market requirement we need a so-called 'flat' torque curve
ie certainly no RD350 curve other than the 27 hp German market or 27 hp Rajtoot versions
consider eg Villiers made (falling torque curve) trials engines (7hp@2500 - 12hp@6000) and (rising curve) scrambles versions (7hp@2500 - 19@6000) etc

Actually T-C, that rising torque curve is exactly what provides the desired 'peppy-snappy-lively' feel,
thus giving the 'power hit' excitement factor 2T sports fans really like..

The advent of the 'YPVS' variable exhaust timing valve did 'flatten' that 'power hit' significantly..
..albeit offering improvement in low rpm tractability/economy, & yet also - in max hp too.
(viz.. check the `85/86 RD/RZ 350 dyno charts posted back on page 80, it shows the inlet port up-tune)

The rising torque curve feature inherent to sporty feeling 2Ts is also useful in marine applications,
to 'pop' the hull 'out of the hole' & 'up on the plane' - Honda has to use its complex 'V-TEC' system on
its 4T outboard mills to try & emulate that functional capability..

[Pinger]

“You know very well how the above functions.”

I wish I knew.
Nobody does.

Crack exhaust valve open early to drop cylinder pressure and to achieve maximum valve lift as soon as possible and maximise time available for closing to prevent cam lobe and valve losing contact. Elementary.

This process was done previously: as the piston was moving towards the TDC, the vacuum in the crankcase caused fresh air or air-fuel mixture to enter.

And that same ability to draw vacuum will be competing with....

In both cases the engine consumes time to create gas flow, and then exploits the inertia of the flowing gas to overfill the cylinder.

....the inertia that you believe will be propelling fresh charge into a cylinder whose pressure is rising due to a rising piston.

Two counts of pressure differentials opposing flow, one count of inertia through a convoluted duct. And inertia will win?

[Tommy Cookers]

[quote=J.A.W.]...... The rising torque curve feature inherent to sporty feeling 2Ts is also useful in marine applications,
to 'pop' the hull 'out of the hole' & 'up on the plane' - Honda has to use its complex 'V-TEC' system on
its 4T outboard mills to try & emulate that functional capability..[/quote]

my point was that the mass ie car market needs what we call a 'flat' torque curve (and a new 2 stroke design should provide it)
yes, a boat (like an aircraft) doesn't need this

I wrote yesterday (then deleted) that 'proper' aircraft engines have valve and port sizes intentionally large to give lowish gas speed
avoiding the torque peaking at lowish rpm (as needed in cars etc - the so-called 'flat' torque curve)
so a step towards the truly flat torque curve that would be ideal for helicopter transmissions (they have slipper clutches limiting torque in run-up)
the reason why the Franklin was fine in the Bell 47 but not in the Tucker Torpedo car
(helicopters have complex systems to give constant motor rpm and power variation for flight entirely by varying torque to match load)

'proper' aviation really wants engines ie CI etc that run on kerosene (Avtur) because the taxman makes that cheap
the others should want something that's cheap to buy, light, and simple etc

[J.A.W.]

...'proper' aviation really wants engines ie CI etc that run on kerosene (Avtur) because the taxman makes that cheap
the others should want something that's cheap to buy, light, and simple etc

Yeah, T-C - as you & I well know, ' politics' of course, introduces all kinds of 'artifice/fictions'..

'Milspec' units are not bound by such B.S. blather though:
http://marineenginedigest.com/profiles/ ... udemfe.htm

'Liability' issues as relate to civil aviation, no doubt impede the introduction of such 2T tech into G.A. usage..
..otherwise a suitably adapted avtur fuelled 3.4 litre V6 E-TEC mill - ought to be available for flight duty..