2 stroke thread (with occasional F1 relevance!)

[Andres125sx]

JAW, I think the conversation would benefit enormously from you being right less than 100% of the time, refuting the others less than 100% of the time and acknowledging that your opinion is not the only valid one 100% of the time. It is OK to disagree without shooting the other poster down. You are not the only one guilty of these sins, though, and what you write is often most interesting, it is just that the tone makes a level-headed conversation almost impossible.

Since this is now an equivalence discussion (and a great one), I'd like to add an outsider's perspective. Not knowing much about engines, I can easily ignore how many cycles involve combustion, swept volumes, etc. The way I see it, the only fair comparison would involve not the mechanism of the engine, but its function. This way one can compare 4Ts with 2Ts, Wankels, turbines and anything else. And you guys know so many examples that surely there is a set of engines that can help here.
The way I see it, the fair comparison to do would be comparing engines of different types that have similar:
a) total weight
b) total volume of the engine (turbos included here, obviously, as is possibly cooling if external, which also has weight)
c) fuel consumption
And then comparing power, response characteristics, etc. Alternatively, if two of the three and power are similar, the third element would decide the comparison.
Anyone dares to come with examples under these comparison rules?

I think fuel use should be the parameters to compare. Or power output, that´s what has usually been done when comparing atmospheric engines with turbo ones, or petrol with diesel

Problem is when using these parameters, 4t always win, and some people love 2t so much they cannot accept it

I love 2t too, my nick should be the proof, but not so much to ignore reality.

To me the most fair comparison should be for same fuel consumption, give them same energy and let see who do it better. This way 4t is so much more efficient there´s no comparison, at least with MX bikes wich are the field I´ve more experience. My KX 250 2t 04 needed more fuel than any 450 4t and still was unable to keep up if track conditions were not perfect, much weaker bottom line, less traction... and I´ll repeat my 2t used more fuel, but to be fair maybe we should implement inyection on my 2t to do a really fair comparison, that´s true

[hollus]

Contradicting myself here
I know I suggested to compare for similar fuel use, but thinking a bit more about it, a motoGP bike uses 22Kg of fuel, the fuel becomes an almost irrelevant factor, with limited impact on the vehicle's total weight (driver included), the environment or the cost of the run. So maybe comparing a 4T with 22L (or make it free) and a 2T with 30L (or make it free) starts to make sense.
How much fuel would your 2T MX eat in a typical race?

[Andres125sx]

Why irrelevant?

Comparing means analyzing what´s better, wich in the end is what´s more efficient. 2t engines produce a lot more power for same engine size, but using a lot more fuel too

IMHO it´s the energy they use what ultimately measure efficiency, I don´t see how engine size may have some relevance. In that case rotary engines would win hands down as gruntguru posted, they produce a lot more power than any other engine of similar size

But who cares about engine size?

It´s power or fuel what should be equalized I think, IMO fuel. An engine use fuel to produce work, so the parameter to equalize different type engines to be compared should be the energy they use to see what of them do it better with same initial conditions. If some of them have more energy to use, the comparison is biased

[Andres125sx]

How much fuel would your 2T MX eat in a typical race?

Not sure, I think I could use a tank (7litres) for a whole day of practice, qualifying and two heats... maybe for the last heat would need a bit more, but I always fill the tank before each heat since a friend of mine broke a shoulder when the bike stopped just before a jump because it run out of fuel

But since MX bikes don´t have instrumentation I can´t say how much fuel they use with traditional mpg or l/100km measure. But 450´s could use a tank for the whole day and still have some remnant.... a friend of mine witha a crf 450 only fullfill the bike tank at the gas station, while I needed an aditional 5 litres tank for my kx250 2t

[wuzak]

JAW, I think the conversation would benefit enormously from you being right less than 100% of the time, refuting the others less than 100% of the time and acknowledging that your opinion is not the only valid one 100% of the time. It is OK to disagree without shooting the other poster down. You are not the only one guilty of these sins, though, and what you write is often most interesting, it is just that the tone makes a level-headed conversation almost impossible.

Since this is now an equivalence discussion (and a great one), I'd like to add an outsider's perspective. Not knowing much about engines, I can easily ignore how many cycles involve combustion, swept volumes, etc. The way I see it, the only fair comparison would involve not the mechanism of the engine, but its function. This way one can compare 4Ts with 2Ts, Wankels, turbines and anything else. And you guys know so many examples that surely there is a set of engines that can help here.
The way I see it, the fair comparison to do would be comparing engines of different types that have similar:
a) total weight
b) total volume of the engine (turbos included here, obviously, as is possibly cooling if external, which also has weight)
c) fuel consumption
And then comparing power, response characteristics, etc. Alternatively, if two of the three and power are similar, the third element would decide the comparison.
Anyone dares to come with examples under these comparison rules?

I think fuel use should be the parameters to compare. Or power output, that´s what has usually been done when comparing atmospheric engines with turbo ones, or petrol with diesel

Problem is when using these parameters, 4t always win, and some people love 2t so much they cannot accept it

I love 2t too, my nick should be the proof, but not so much to ignore reality.

To me the most fair comparison should be for same fuel consumption, give them same energy and let see who do it better. This way 4t is so much more efficient there´s no comparison, at least with MX bikes wich are the field I´ve more experience. My KX 250 2t 04 needed more fuel than any 450 4t and still was unable to keep up if track conditions were not perfect, much weaker bottom line, less traction... and I´ll repeat my 2t used more fuel, but to be fair maybe we should implement inyection on my 2t to do a really fair comparison, that´s true

Brake Specific Fuel Consumption is the best way to compare the different engines. It is a measure of efficiency of the engine, combining fuel consumption and power.

Traditioanlly 4 strokes have been stronger in this regard, though direct injection 2 strokes are more efficient than older units. Though there aren't too many manufacturers using them, and none of the major car manufacturers are either - so that's whay they aren't being used in F1.

Power to weight is also a good metric. This is a strong point for traditional 2 strokes, with their design simplicity. Though complexity and weight can easily be added to 2 strokes, just look at the sleeve valve Crecy.

Size for power the 2 stroke also wins. I'm not sure about cooling, but if the 2 stroke is less efficient (as in older style units) it would imply that more heat is rejected through thr exhauist or cooling system or both. More heat rejection through the coolant means bigger radiators are required, meaning a bigger impact on aero.

The thing that has killed 2 strokes in production cars and road bikes is the emissions. The simplest of the 2 strokes have crankcase induction, which means oil has to be added to the fuel-air mix to help with lubrication, some of which, inevitably, gets burned in the combustion chamber.

A 2 stroke with Direct Injection and forced induction, allowing for a sealed bottom end, would probably be very competitive with 4 strokes in car use. In fact, Ralph Sarich demonstrated such potential many years ago and received some interest from major manufacturers, but they have not adopted the technology.

At the time the DI 4 stroke was just starting to appear, and I wonder if that tipped the balance back in favour of the 4 stroke, also considering that the manufacturers had a lot of investment in 4 stroke design and manufacture.

[J.A.W.]

Ah, W.. ..excellent..
You have done a bit more in-depth investigation of the subject..

& Kevin Cameron concurs..

http://www.cycleworld.com/2014/02/07/as ... reetbikes/


Here is a site showing research findings for 2T DFI, note the fuel consumption benefits..

http://www.provenion.de/65-1-FFI.html


@ Hollus - this is NOT about opinion, the 2T advantage - is in accordance with thermodynamic law..

[J.A.W.]

Applied research has shown that the 2T thermodynamic advantage re: BSFC - is achievable..

"Power & BSFC were superior to that of the latest 4T engine..."

http://www.fisita.com/education/congres ... s/sc30.pdf

http://www.webpages.uidaho.edu/niatt/re ... N09-02.pdf

Commercialisation & racing homologation are another matter, of course..

[J.A.W.]

2-stroke F1 would actually make a huge amount of sense now.

The game changer is 500bar gasoline direct injection pressures now available (as used on current F1 engines). That means that high speed DI 2-stroke can have efficiency comparable to 4-strokes as they no longer blow un-burnt fuel through the exhaust. (In my previous engine development job I've seen DI 2-strokes that had higher efficiency than competing 4-strokes)

A 1.5-2L 2-stoke V6 with DI and 15000rpm max speed would sound awesome - more like the old 3L V10's while costing probably only 20-30% of even the old 2.4 V8's. It would also cut their size and weight dramatically with hugely reduced complexity and component count. Not to mention elimination of lots of external systems.

Could likely be made even cheaper and maybe more efficient/cleaner by mandating a specified roots or fixed ratio belt driven centrifugal scavenging supercharger + intercooler rather than crankcase scavenging to allow better scavenging, higher compression ratios and reduced need for tricksy lubrication and air flow solutions in the crankcase and reed valves etc.

Emissions would be no worse than 2.4L V8's. Efficiency could be made much higher if capacity was not limited, but rather fuel-flow restricted (say max fuel flow at 13000-15000rpm).

The engine designs would be simple enough that we wouldn't need the participation of big auto companies as development is orders of magnitude cheaper than with high speed 4-stroke valve trains. They would be cheap and easy to rebuild, and would greatly reduce necessary head count both at races and at home bases. Eliminating arguments for long life engines.

All in all it would be hugely beneficial to getting F1 costs down, and simplifying the cars, while taking us back to the much loved high frequency exhaust noise.

The same arguments applies to other forms of motorsport. DI 2-stroke should be the standard for high performance and low cost with good efficiency. It also has more relevance to a world that wants hybrids and range-extenders.

Foyle's most excellently cogent post bears reiteration here, IMO, for new-comers to the thread, who may've missed it..

[J.A.W.]

&, for those who find 450 4Ts boring.. & "...want the ultimate racing weapon.." ..KTM/KX 500..

Those who find a 450 4t boring have never ridden a 450 4t

They never said that, please do not invent absurd argumentation

"absurd argumentation"?
Be serious A-125.. the quote is accurate, so kindly - do not resort to emotive put-downs/misrepresentation..

[Andres125sx]

Only you said "those who find a 450 boring", nobody else said that BS, what proves you´ve never ridden a 450

Do you know Cairoli? He competed with a 350 fighting with 450´s because 450´s are simply too powerful, and we´re talking about a World Champion, so if you say a 450 is boring you simply don´t know what you´re talking about, period

[J.A.W.]

A-125..
How can you possibly make the call - on what anyone else but yourself - finds boring?

As I noted, my last dirt bike was a KTM 380 2T, & so no, aint no 4T 450 gonna impress me much, power-wise..

Here's an example of the kind of fun a bigger cc 2T provides.. https://www.youtube.com/watch?v=DxqPMR9MCx4

I did take the opportunity to have a ride on a 250 4T yesterday though, & it was - IMO - most def' "boring"..

[J.A.W.]

According to this comparison test of KTM 250 MX machines..

http://www.motocrossactionmag.com/bike- ... -ktm-250sx

Power-wise the 4T has to rev right out to nearly 11,000rpm to make its peak of 35.5hp..
..whereas the 2T makes almost 50hp at just 8,300rpm..

The 2T is..
"...lighter, faster, cheaper, better handling & easier to maintain..."

&, after that - why they rate the 2T best..

"The 2T's greatest strength isn't in its obvious list of superlatives, but rather the simple fact is,
that it heightens the intensity of racing more than the same displacement 4T."

[riff_raff]

Applied research has shown that the 2T thermodynamic advantage re: BSFC - is achievable..

"Power & BSFC were superior to that of the latest 4T engine..."

http://www.fisita.com/education/congres ... s/sc30.pdf

http://www.webpages.uidaho.edu/niatt/re ... N09-02.pdf

Commercialisation & racing homologation are another matter, of course..

Great point. The recip engines having the highest BTE and BSFC are the massive 2T CI engines used on large ships. But they have no relevance to race engine applications.

However, with the use of electric compounded turbo systems in F1, I can imagine a turbo-scavenged high-speed uniflow 2T race engine configuration using an overhead rotary exhaust valve. It would have good throttle response and would be extremely compact and lightweight for the power it produced.

[J.A.W.]

Aye r-r, the fundamental Carnot advantage of the 2T over the 4T is in fact - a given..

It is simply a matter of realization, & technically as shown - it is practicably achievable..

But.. ..politics, esp' manufacturer/sporting body-wise.. ..sadly.. that's quite another matter..

[uniflow]

How much fuel would your 2T MX eat in a typical race?

Not sure, I think I could use a tank (7litres) for a whole day of practice, qualifying and two heats... maybe for the last heat would need a bit more, but I always fill the tank before each heat since a friend of mine broke a shoulder when the bike stopped just before a jump because it run out of fuel

But since MX bikes don´t have instrumentation I can´t say how much fuel they use with traditional mpg or l/100km measure. But 450´s could use a tank for the whole day and still have some remnant.... a friend of mine witha a crf 450 only fullfill the bike tank at the gas station, while I needed an aditional 5 litres tank for my kx250 2t

Our transfer port injected YZ250 ( 2T ) uses between 14% and 21% less fuel than a standard YZ250 under the same conditions. Same track, same riders, swapping bikes every lap, enduro / trail riding. Numerus track tests, real world.
Don't be too sure on the 2T not having the same traction as a 4T, ECU driven electronics in our EFI YZ250 are capable of missed injection cycles ( and other tricks ), making the 2T as good at hooking up, if not better than any 4T.