2 stroke thread (with occasional F1 relevance!)

[wuzak]

& as noted, the commercial & sporting applications are currently restricted..
.. for what appear to be.. non-technological reasons..

What are the commercial restrictions?

None, apart from emissions.

The fact is that major manufacturers have chose to not change over to 2 strokes. That isn't through a commercial restriction, but because of those companies ' own choices.

[J.A.W.]

Not so W, KTM has had a couple of emissions compliant 2T DFI systems ( inc' the BMW developed Husqvarna).
.. ready to go for at least a couple of years.. ..as to why they are being held back, who knows.. ..ask them..

Certainly the lack of movement to market DFI 2Ts for motorcycles is a commercial decision,
..since the same tech is long proven in snow & watercraft mills - & which could be readily adapted for bikes..

Commercial... in as much as the manufacturers want to be certain of buyers, I guess..
& motorcycle buyers are known to be notoriously conservative & fickle..

The Bimota V-Due debacle is often cited as cruelling the chances of a 2T road-bike revival..

See here.. http://www.cycleworld.com/2012/08/13/tw ... urrection/


Sporting restrictions on 2Ts range from production based restrictions through calculated capacity handicaps..
..to outright bans, such as in Moto GP & F1..

[wuzak]

If the engine is emissions compliant then there are no commercial restrictions.

There is something holding 2 strokes back from automotive applications.

Note that the examples you list, as in water craft and snowmobiles, the emission standards are lower or non-existent.

Also note that the operating range for these two 200hp outboard motors is quite narrow - and at higher rpm (5000-5750 for the Mercury, 5000-6000 for the Honda) than would typically be seen in automotive applications.

http://www.mercury-marine.eu/mercury/en ... iMax%20200
http://marine.honda.com.au/High_Horsepo ... p_Outboard

Note that the Honda 4 stroke is ~440cc larger than the 2 stroke Mercury, but makes the same power. It is, however, about 40kg heavier. But that's for the whole outboard, not just the V6.

If 2 strokes are to compete with 4 strokes, there needs to be an equivalence formula. The same way that Diesels need an equivalence formula to compete with petrol engines. The aim of the equivalence formula is to get the outputs as near to identical as possible. Thus the 2 stroke will always be smaller nominal capacity.

[J.A.W.]

W, to update your knowledge on a few aspects..

You have to understand the motivations of manufacturers & the profit motive..

For decades the humble 2T was not seen as a viable competitor for 4T motorcycles,
either for sales or competition, with few exceptions ( such as the Scott, & pre war DKW),
but they were rated 1:1 on a cc-for-cc basis with 4Ts, in the showroom & for competition events..

By the early `60s German MZ 2T tech ( which went to Nippon) finally showed real 2T power..

By the end of the `70s 4Ts had been revealed to be expensive, overweight, underpowered,
& uncompetitive with 2Ts for top line racing motorcycles, on or off road, in any capacity class..

An exception was found for production based "Superbike" racing where the cc handicaps were 1st applied,
& then as 2T motorcycles were dropped from showroom production, they effectively vanished..

Honda, with their intense 4T ethos - used their clout within the sporting bodies to achieve the same
result in pure competition machines.. ..as a result 2Ts were also handicapped & then forced out..

The sport has since suffered from a grass-roots level up - due to the vastly increased costs involved in 4Ts..


As for your example of marine engines, obviously they are designed to be most useful for purpose,
& the drag of hulls in water necessitates a certain torque curve response..
..but as Uniflow has pointed out..
.. there is no technical reason 2Ts cannot be designed to run a fairly wide powerband too..

[J.A.W.]

As an historical reference, here is an interesting article from back when a big 2T was king of superbikes..

http://www.kawtriple.com/mraxl/articles ... bikes2.htm

No doubt - if a current maker was game, a similar ( or greater) relative superiority would also be available today..

[wuzak]

Here goes nothing....

Firstly, I wasn't saying that 2 strokes couldn't be tuned for a wider operating band. I was pointing out that marine or snowmobile engines may not be so simply adapted to automotive use, due to their designed operating ranges.

Most racing formulas have been air flow limited. This has been by use of capacity and rpm limits (such as in F1 in the past few years - prior to that without an rpm limit the technology acted as the restriction) or by an air resticter.

Take F3, for example. The engines are 2l (stock based, but we'll waive that for this example) restricted by a 28mm air inlet size. Power is about 200hp, Similar 2l production based engines in the BTCC and WTCC were capable of ~300hp.

So, for a 2 stroke to use that restriction it would either have to rev less or be of smaller nominal capacity.


In a fuel restricted formula, such as F1 now, the same applies. A 2 stroke would either have to be of smaller capacity or rev less. That is because of the simple fact that air consumption is related to fuel consumption. It may be that a 2 strok will be slightly more efficient, but any advantage would likely be quite small.

500cc GP bikes. The last of the strokers made 200hp, maybe 220hp. Lets say the former. A 4 stroke 500cc GP bike may have made ~150hp. So the 2 stroke gives 33% more power. That's not to say that is because the 2 stroke is 33% more efficient - but is because of using more air and fuel. Today the 2 stroke may be of equivalent efficiency, or slightly better according to you, but back then it was not the case. The fuel use would, probably, have been more than 133% of the 4 stroke's fuel use.

2 strokes and 4 strokes cannot compete today with the same nominal capacity. There has to be an equivalence factor - which historically has been hard to achieve (for example, turbo/supercharged engines vs N/A engine). So the logical way for them to compete fairly is with either an air restriction or fuel restriction. The latter is the better option, and that's why it has been adopted for F1 and the WEC.

Again, I go back to swept capacity. It is a good comparison between engines of the same type, but not so much between 2 strokes and 4 strokes.

It also depends on your definition of capacity. A compressor is defined by how much air it moves. If we use the same metric for engines, the 2 stroke's capacity should be (roughly speaking) twice the capacity of the 4 stroke with the same bore and stroke. A 1l 4 stroke engine will pump 500cc of air (100% VE) per revolution, a 1l 2 stroke will pump 1l (100% VE).

[gruntguru]

There is certainly an emissions issue for two strokes - even the modern DI types (which can have impressive "engine out" emissions). The scavenge process makes it very difficult to avoid excess air in the exhaust, so cleaning up NOx with a 3-way cat is not an option.

[gruntguru]

The topic of allowing 2-strokes under the current F1 rules with fuel-flow-limiting is an interesting one. In theory a 1.6 litre two stroke will only need half the MAP to see the same airflow and AFR. The lower PR would mean lower turbine and compressor powers - certainly there would be less power at the MGUH assuming a "pressure" turbine. The answer would be blowdown energy however i doubt more than 2 cylinders could share a turbine nozzle without exhaust events overlapping. I suppose paired, simultaneous firing is one option - restricting the exhaust events to 3 per revolution like the 4 stroke V6. (EDIT - that would still need 3 turbine entries.) The sound would be way better with even firing of course.

A 3 cyl 800cc 2 stroke would of course blow the current PUs away!

[J.A.W.]

Quite right G-G,
& as noted in the Italian research paper posted by Carlos on the previous page ( & noted by Orbital in their studies too),
the evenly disposed 3 cylinder 2T has a natural 120` synergistic/harmonic blowdown thrust-torrent to be exploited..

As you have also previously noted F1 is not too concerned with emission control either..
& the whole emissions control output criteria was politically manipulated anyhow ( diesel smoke particulates anyone?)..

@ W, the final 2T GP bikes (125cc, but effectively 1/4 of a 500cc mill) were at ~440hp/litre & still improving..
AFAIK, the best 4T Moto GP has managed is ~250hp/litre, or about what impoverished MZ 2Ts were doing ~50 years ago..
(& still not yet a match for the showroom stock production Aprilia road-bike 2Ts @ ~280hp/litre).

The mass/power density/cost advantages accruing to the 2T, still stand, of course..
Unless the 2T is artificially constrained via some handicapping ( or banned),
- the 4T is simply too slow ( by ~1/2) to compete..

[Andres125sx]

What a obsession with hp/litre figures....

If that´s the parameter to look at in your opinion, what´s the reason you do not defend rotary engines instead of 2 strokes?

[Richard]

Forgive my ignorance, but I thought hp per kg of fuel (or mpg or km/l or l/100km) is the only metric that counts for commercial application?

[J.A.W.]

What a obsession with hp/litre figures....

If that´s the parameter to look at in your opinion, what´s the reason you do not defend rotary engines instead of 2 strokes?

Because - that would properly belong on a dedicated Rotary piston thread?

[J.A.W.]

Forgive my ignorance, but I thought hp per kg of fuel (or mpg or km/l or l/100km) is the only metric that counts for commercial application?

That certainly applies for container & other bulk shipping mills that utilize 2Ts..

But for sporty/competition/fun machines for discretionary sale to private users,
as opposed to purely business-use purposes..
..other parameters are perhaps also too valuable/important to be ignored.. ( or we'd only have Diesel/CI mills?)

Corporate decision makers may over-rule engineers - in what goes on sale - to maximise profit, on the units sold..
Yet there are also political/compliance/ideological issues to be dealt with..

[J.A.W.]

What a obsession with hp/litre figures.

They are a fair unit indicator of work done, for a high performance racing mill..
..so when obtained with moderate cost in dollars/complexity/weight/mass..
& proven to be reliable/tractable.. ..represent a solid benchmark achievement..

Why do you imagine 2Ts are now specifically banned from F1 & Moto GP competition?

[gruntguru]

Most racing categories world wide specify a maximum engine displacement. Engines with the most kW/litre will have the most kW, hence the obsession with kW/litre.