2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello all


In the following “drawing”:



(from the http://www.bernardhooperengineering.co.uk web site of Bernard Hooper, “former Chief Engineer of Norton Villiers, responsible for the highly successful Norton Commando”),

the skirt of the “compressor” piston appears more than short.


Looking for a good photo or drawing of Hooper’s step piston in his web site, nothing was found (!).

Searching in the US-PTO (at http://patft.uspto.gov/ ) for patents granted to Bernard Hooper, here is the first page of his first / basic patent:



and here is the fifth figure (at left) with the addition of the 1st claim (at right) in the same page:



Reasonably the wrist pin should be located underside the lower ring (the oil scraper ring) of the big diameter part of the step piston.



In order to have a “over-scavenging” of, say, 25%, the diameter of the compression end of the step piston of Hooper needs to be 1.5 times ((1,5^2) –1 = 1.25) the diameter of combustion end of the step piston: the combustion piston occupies a big part of the area of the compression piston.



These two photos (they are from the proposed power-point presentation for the Engine Expos of 2008 and 2009 at Stuttgart Germany, it is available at http://www.pattakon.com/opre1_files/pattakonOPRE.pps ) :





show a different kind of step piston.

The diameter of the combustion end is 80mm, while the diameter at the compressor end (the end at the wrist pin side) is 90mm.

And because (90/80)^2=1.265

The “geometrical” “overscavenging” ratio is 26.5%.

For the same combustion piston diameter (80mm) and the same “over-scavenging”, the piston rings at the compressor end of Hooper’s step-piston would be 120mm instead of the only 90mm used in the OPRE pistons.

Differently speaking, with 120mm “compression piston diameter”, the “over-scavenging” in the above OPRE would be: 125% (not 25%, as in Hooper’s engine, but 125%).



Bernard Hooper mentions, in his web site, a 200:1 fuel/oil ratio used in his prototype step-piston engines (against the typical 50:1 of the conventional 2-strokes of that era).

As the Achates with the side crankshafts, similarly the Step Piston architecture can reduce substantially the lubricant consumption of the 2-strokes.

The one (Achates Power) eliminates the thrust loads, the other takes the thrust loads away from the cylinder liner whereon the ports are.



In the link of Tommy Cookers


(i..e. the presentation at Engine Expo 2014, Stuttgart Germany, of the step piston technology by:

Dr Peter Hooper (son of Bernard Hooper?)
BEng (Hons), PhD, CEng, FIMechE, PgCert, FHEA

Senior Lecturer in Mechanical Engineering

School of Engineering, Auckland University of Technology,
City Campus, WS Building, St Paul Street, Auckland, New Zealand)


the big part (more than half of the slides) of the presentation deals with the reduced “torque fluctuation” of their multicylinder V-4 (and others) arrangement, which leads to better NVH properties and so to better Range Extender Modules (REM) for electric cars.



An OPRE having secured on each of its two crankshafts an electric generator solves the “torque fluctuation problem” in a different and absolute way: it cancels out, internally, any free inertia and combustion torque, leaving the basis of the OPRE REM (or the basis- i.e. whereon it is mounted, of the OPRE electric generator set) perfectly rid not only of inertia forces and inertia moments, but also from inertia and combustion torques.

It is not about a slight or substantial reduction of the “torque flactuation”, it is about the elimination of the “torque fluctuation”.



Did I mention any CFD?
Any time-area analysis?
Any Laser interferometry mapping?

No.
It is just the application of the basic physical laws, of geometry and maths etc.



Quote from Kelvin Cameron / CycleWorld link of J.A.W. regarding the unbelievable evolution of the 2-strokes through the years:

“What were their methods? CFD? Time-area analysis? Laser interferometry mapping? No, they did what we all did in those days; they tried stuff.”



If you take under account the “substantially longer piston dwell” characteristic of the OPRE engine and the rest advantages from its strange architecture . . .

(more at http://www.pattakon.com/pattakonOPRE.htm and http://www.pattakon.com/pattakonPatOP.htm and http://www.pattakon.com/pattakonPatPortLess.htm )

Thanks
Manolis Pattakos

[gruntguru]

Hello Gruntguru
The spark ignition 2.0 liter Boxer of Subaru (2013), with 86mm bore and 86mm stroke, seems to run reliably until 7,500rpm (peak power at 7,000rpm):

You write: “The steep drop you are referring to is due to reduced fuelling to avoid overspeed.”

But we talk for only 4,000rpm.
And the “same” engine can operate above 6,500rpm “for ever”.

Hi Manolis
I am sure the 4,000 rpm limit is not mechanical - more likely, as you say, it relates to insufficient time for combustion. I only wanted to make the point that the drop in torque is not directly due to reduced efficiency but to deliberately reduced fuelling in order to avoid the low efficiency operation.

Incidentally (and I need to find some research to confirm this), one problem is the common rail system and its fuel delivery characteristic - constant flow rate while injector is open. The traditional mechanical injection system delivers a flow rate proportional to crankshaft rotation ie injection rate increases with rpm. This would present a limitation (for CRD) to the requirement of high injection rate at high load. At low rpm and load (low turbulence, swirl etc) the injection rate needs to be reduced in line with the lower combustion rate possible, along with improved resolution on small fuel delivery events. To some extent this can be overcome by varying fuel rail pressure but this has limits due to high particulate emissions at lower pressures.

[Pinger]

Hi Manolis
I am sure the 4,000 rpm limit is not mechanical - more likely, as you say, it relates to insufficient time for combustion. I only wanted to make the point that the drop in torque is not directly due to reduced efficiency but to deliberately reduced fuelling in order to avoid the low efficiency operation.

Current auto CI engines require multiple turbos to cover the entire rpm range. With only one, the choice is to starve low rpm operation of air (unacceptable for particulate emissions and end user satisfaction) or a cap on torque at higher rpm. Obviously, in the latter, the fueling will be turned down at higher rpm.

Incidentally (and I need to find some research to confirm this), one problem is the common rail system and its fuel delivery characteristic - constant flow rate while injector is open. The traditional mechanical injection system delivers a flow rate proportional to crankshaft rotation ie injection rate increases with rpm. This would present a limitation (for CRD) to the requirement of high injection rate at high load. At low rpm and load (low turbulence, swirl etc) the injection rate needs to be reduced in line with the lower combustion rate possible, along with improved resolution on small fuel delivery events. To some extent this can be overcome by varying fuel rail pressure but this has limits due to high particulate emissions at lower pressures.

CRD load control is via rail pressure. The duration and multiplicity of injections (pre, main, post(s))controls combustion quality. Control of air motion (including swirl flaps) is the tool for air and fuel mixing - as I understand it.

There is a Bosch document available online that usefully demonstrates CRD control.

edit PS.

OK, not Bosch, here >> http://www.yildiz.edu.tr/~sandalci/ders ... aining.pdf

[Tommy Cookers]

[quote=J.A.W.]
......Fact is - the 2T advantage of every piston downstroke being a powerstroke is why they are utilized in the
largest recip' settings, by powering huge cargo vessels efficiently - enabling direct drive - & thus allowing
the propeller & crankshaft to both turn at the most efficacious rates for traversing long oceanic routes..........[/quote]

afaik ......
Dr Diesel patented CI adding heat notionally at constant volume but could never make this work
he then patented CI adding heat notionally at constant pressure (though Ackroyd Stuart had already made such an engine)
ie all 'true' Diesels notionally add heat at constant volume ie a Diesel is slow-speed CI engine
the first 20+ years of Diesels were slow-speed trunk piston (ie conventional rods) mainly 4 strokes
then for much greater powers (ideas from marine steam) came crosshead double-acting 2 strokes (and 4 strokes)
sealing problems with the lower combustion chamber led to today's longer or very long stroke single-acting low/very low rpm 2 strokes
clearly the 2 stroke here gives downsizing and cost benefits relative to any notional 4 stroke equivalent
and presumably the heat rate is higher than in Dr Diesel's day

lost of ships are powered by medium or high speed trunk piston CI 4 strokes that we shouldn't call Diesels or even diesels,
usually without mechanical reduction ie using independent steerable electric motor pods or mechanical internal jet thrusters
and this (saying that mechanical reduction and reversing are the modern way) seems a useful summary
https://sites.google.com/site/pakistanm ... sel-engine


btw - stepped piston engined motorcycles were in production pre-WW2 eg the Dunelt in the UK

[manolis]

Hello Gruntguru.

It seems that after a rev limit the shortage of time is such that the “injection” strategies cannot help, any longer.

For instance, the pre-injection used at the lower revs, can improve the conditions the fuel finds in the cylinder during the main fuel injection.

But as the revs increase, a pre injection becomes more a problem, than an advantage (it must start too early, when the pressure and temperature are not yet high enough).


It reminds the case of the MultiAir / TwinAir of FIAT / Chrysler / Schaeffler INA wherein there is a multi-lift mode (Fig 5):



At low revs it may give some advantages, at medium – high revs it is meaningless / useless: the intake valve follows a conventional valve lift profile because there is shortage of time for multi-lifts.
However it gives a significant advantage over the rest engine makers when it is advertised like: “the rest engines cannot do this, we can”.



In the spark ignition engines the rev limiter cuts instantly the power.
The fall of the power till the rev limiter is a warning for the driver to shift gear.

They should do the same in the compression-ignition engines (Diesels), but it seems they cannot: the torque is too picky in comparison to the spark ignition engines. And the main reason seems to be the lack of time.



By the way, in my previous post I wrote:

“An OPRE having secured on each of its two crankshafts an electric generator solves the “torque fluctuation problem” in a different and absolute way: it cancels out, internally, any free inertia and combustion torque, leaving the basis of the OPRE REM (or the basis- i.e. whereon it is mounted, of the OPRE electric generator set) perfectly rid not only of inertia forces and inertia moments, but also from inertia and combustion torques.
It is not about a slight or substantial reduction of the “torque flactuation”, it is about the elimination of the “torque fluctuation”

Here is an explanatory photo:



Replacing the two big diameter flywheels (they are used, among others, to manually start the engine by two ropes) by two electric generators, the resulting REM has zero torque fluctuation on its basis.

Thanks
Manolis Pattakos

[manolis]

Hello Tommy Cookers

You write:
“btw - stepped piston engined motorcycles were in production pre-WW2 eg the Dunelt in the UK”


Here is a Dunelt “step-piston” 2-stroke engine



The step piston is used not to lower the specific lubricant consumption but to overscavenge the engine.

The crankcase is used in the conventional way for the scavenging.
With the substantially bigger diameter of the lower end of the piston, the scavenging pump (crankcase) “displacement” gets substantially larger than the displacement of the engine.

Spot on the size and shape of the piston crown “deflector” (mentioned in the article of Kevin Cameron / Cycleworld as the cause of several problems in the old two strokes)

Thanks
Manolis Pattakos

[gruntguru]

I notice the lack of any attempts to minimise crankcase volume. There is a huge volume in the underside of that piston. The "over-scavenge" benefit is no doubt lost as revs increase.

[J.A.W.]

It seems that after a rev limit the shortage of time is such that the “injection” strategies cannot help, any longer.

For instance, the pre-injection used at the lower revs, can improve the conditions the fuel finds in the cylinder during the main fuel injection.

But as the revs increase, a pre injection becomes more a problem, than an advantage (it must start too early, when the pressure and temperature are not yet high enough)...


In the spark ignition engines the rev limiter cuts instantly the power.
The fall of the power till the rev limiter is a warning for the driver to shift gear.

They should do the same in the compression-ignition engines (Diesels), but it seems they cannot: the torque is too picky in comparison to the spark ignition engines. And the main reason seems to be the lack of time.

Thanks
Manolis Pattakos

& yet Manolis, the BRP E-TEC voice-coil injector system is rated up to 10,000 rpm for 2Ts..
..perhaps it is not so much the injector time - those Evinrude 2Ts are sold in 'heavy fuel' form too ( as mil-spec)..

What did the Audi & Peugeot Le Mans race CI engines employ, injection, & rpm-wise?
The factory sites seem coy about the figures..
.. although other sources claim "34,800 psi" injection pressure, & "5000 rpm" as a limit..

[Pinger]

The BRP E-TEC voice-coil injector system's load on its ECU requires that the ECU be cooled by circulating fuel through it. Cleaning that out after being left with fuel in it isn't a prospect I relish.

At least that is why they say fuel is circulated. I wonder if it isn't to pre-heat the fuel to aid atomisation - which was always the bogey man for DI on 2T. In fairness, one of the key markets is sleds - where they tend not to be used in anything other than very cold environments.

[J.A.W.]

The BRP E-TEC voice-coil injector system's load on its ECU requires that the ECU be cooled by circulating fuel through it. Cleaning that out after being left with fuel in it isn't a prospect I relish.

At least that is why they say fuel is circulated. I wonder if it isn't to pre-heat the fuel to aid atomisation - which was always the bogey man for DI on 2T. In fairness, one of the key markets is sleds - where they tend not to be used in anything other than very cold environments.

I doubt that the cold is too much of a problem for starting a cold crankcase scavenged 2T, with no heavy oil to churn,
& so long as the ECU can deliver a rich mixture.. Evinrude outboards obviously have to be cleared for tropical duties..

Doubtless, P - the well proven ECU coolant function - is via a dedicated liquid-to-air heat-sink..

If you read T-C's recent post on the compound steam unit used by Titanic & her sisters in the 'Crank...' thread P,
you'd see the double acting piston in 'gif' motion,& AFAIR, the voice-coil injector is capable of similar double-duty..

[Tommy Cookers]

[quote=manolis]
.......Spot on the size and shape of the piston crown “deflector” (mentioned in the article of Kevin Cameron / Cycleworld as the cause of several problems in the old two strokes......[/quote]

in motorcycles etc loop scavenge replaced the old 'deflector piston' porting layout almost overnight immediately post WW2
but LS dictates much increased cylinder spacing and so weight and bulk of multi-cylinder engines
so haven't outboard makers avoided the LS layout for most (or all ?) of the 70 years since ?

engine weight and bulk and fuel efficiency seem to be very important in F1, and fuel efficiency in Moto GP

[manolis]

Hello J.A.W.

With the direct injection of the BRP / Rotax spark ignition 2-strokes rated up to 10.000rpm, a reasonable question is whether (and how efficiently) the air fuel mixing and the combustion can really cope with such rhythms.



Hello Pinger.

To heat the fuel before the injection (for the sake of atomization) doesn’t sound as a problem (there is a red-hot exhaust that can “cook”, for “free”, anything).

And it seems easy the ECU to be cooled differently (without the circulation of the fuel).





The real question is:

Why didn’t they put, so far, one of their 2-stroke engines (say, the Evinrude E-TEC 250 G2 HO) in a car to pass all tests (emissions, fuel efficiency, durability, etc) and prove its superiority at all conditions?



Also:


Having a lightweight, fuel-efficient, green and reliable 2-stroke engine, why they keep offering 4-stroke boxers for airplanes?


Thanks
Manolis Pattakos

[Pinger]

in motorcycles etc loop scavenge replaced the old 'deflector piston' porting layout almost overnight immediately post WW2
but LS dictates much increased cylinder spacing and so weight and bulk of multi-cylinder engines
so haven't outboard makers avoided the LS layout for most (or all ?) of the 70 years since ?

Cross flow (aka deflector/baffle pistons) survived in marine outboards as they (as you say) package well (though looper cylinders can (and are) twisted on their axis to achieve the same) and because they are marginally better at low (trolling) speeds. Superior scavenging at low delivery ratios is their advantage. At higher delivery ratios loopers scavenge better and for high specific outputs the looper is the choice (for the reduced heatflow to the piston - as discussed).
Cross flow motors are usually easier and cheaper to produce and by better utilising the cylinders circumference they can employ lower port heights. They aren't entirely without merit!

[J.A.W.]

Hello J.A.W.

With the direct injection of the BRP / Rotax spark ignition 2-strokes rated up to 10.000rpm, a reasonable question is whether (and how efficiently) the air fuel mixing and the combustion can really cope with such rhythms.

The real question is:

Why didn’t they put, so far, one of their 2-stroke engines (say, the Evinrude E-TEC 250 G2 HO) in a car to pass all tests (emissions, fuel efficiency, durability, etc) and prove its superiority at all conditions?



Having a lightweight, fuel-efficient, green and reliable 2-stroke engine, why they keep offering 4-stroke boxers for airplanes?

Thanks
Manolis Pattakos

Hi Manolis, the BSFC & emissions ratings published as obtained by the G2 Evinrude demonstrate efficacy, no?
I understand that the latest Evinrude HO ( hi-output) will use the long-proven exhaust port valve control system, too.

But as discussed earlier in this thread, marketing considerations ( often not rationally based) trump engineering..
The perceived ( by potential buyers ) 'dirty-cheapo-nasty' history of 2T engines - is seen as a stymie point for car sales..
..& this does not apply in 'traditional' markets such as marine, & snowcraft usage..

As for flight, this area is highly conservative & fraught with legal liability factors.. & BRP/Rotax are very cautious..
..even Porsche had their 'fingers burnt' & promptly withdrew, when they offered their well proven flat 6 as an aero-mill..

[Pinger]

Water cooled ECU for outboards and fuel cooled for the sleds - seems to be the way of things.
That the ECU requires cooling (the one on my car is one side of the airbox and finned) is no surprise and in OBs and sleds I guess they use as cooling medium what is at hand (air would add bulk on OBs and sleds). But, atomisation was what held DI back as back in the 1980s when first tried using mechanical injector pumps the required (small) droplet size was unobtainable. Sophisticated injectors amplifying the pressure and/or air blast is what made DI possible.
Still it is hampered by short available injection time at high loads (if charge loss to the exhaust port is to be avoided) and the technology is complex - relative to IDI or carbs.