2 stroke thread (with occasional F1 relevance!)

[J.A.W.]

As opposed to the 4T's 3 non-power strokes?
L.O.L...

Seriously though W-F1, if you want a 1T mill, look up the Maxim gun, its yet another 19th century invention..

[Tommy Cookers]

Why does the power fall drastically in a 2 stroke? Is it because the expanding gases leave through the exhaust port

They run out of blow down time, time for the gas pressure to drop below the trasnsfer pressure. At this point transfer gasses get pushed back down into the crank case for a time before transfer can occur. ........

the usual 2 stroke early opening of the exhaust port is not a problem eg in the present electrically turbocompounded F1
because this wasted energy is recovered by the turbine
this is the Crecy argument, but it makes more sense in F1

[FW17]

this is the Crecy argument, but it makes more sense in F1

Or any hybrid

[wuzak]

So if the 4 stroke is at 10000 rpm and 2 stoke is at 5000 rpm both will have same piston speed and same power and efficiency?

No, the $T's piston speed will be double (assuming teh same crank throw).

The 2T will have slightly less air throughput. Power and efficiency depend on a lot of things, but I wouldn't expect them to be wildly different.

I assumed a case were

4t = B x S
2t = B x 2S

Not sure what you are trying to work out there.

The swept capacity calculation is the same for bit 4T and 2T. C = pi/4 * B^2 * S * n. (n = number of cylinders).

Piston speed is a function of rpm and stroke. So double the rpm with the same stroke means twice the piston speed.

The difference between 4T and 2T os that the former takes two crankshaft revolutions to process its nominal capacity of air, while the latter only takes one revolution, assuming 100% volumetric efficiency.

Naturally aspirated 4T engines can get VE >100%. Not sure about 2Ts.

[wuzak]

Why does the power fall drastically in a 2 stroke? Is it because the expanding gases leave through the exhaust port

They run out of blow down time, time for the gas pressure to drop below the trasnsfer pressure. At this point transfer gasses get pushed back down into the crank case for a time before transfer can occur. ........

the usual 2 stroke early opening of the exhaust port is not a problem eg in the present electrically turbocompounded F1
because this wasted energy is recovered by the turbine
this is the Crecy argument, but it makes more sense in F1

Except that you never get back as much energy from the scavenge air as was taken to compress it in the first place.

Also, in the time of the Crecy the excess air was great for turbo-compound solutions, as it lowered the exhaust gas temperature to a level which could easily be handled by the turbine materials of the time.

In contrast, the Allison turbo-compound's exhaust gas temperature was above what the best turbine blade materials could handle (IIRC it was about twice the Crecy's).

Lower temperature means that less energy can be extracted from the turbine.

[Tommy Cookers]

the usual 2 stroke early opening of the exhaust port is not a problem eg in the present electrically turbocompounded F1
because this wasted energy is recovered by the turbine
this is the Crecy argument, but it makes more sense in F1

Except that you never get back as much energy from the scavenge air as was taken to compress it in the first place.
Also, in the time of the Crecy the excess air was great for turbo-compound solutions, as it lowered the exhaust gas temperature to a level which could easily be handled by the turbine materials of the time.
In contrast, the Allison turbo-compound's exhaust gas temperature was above what the best turbine blade materials could handle (IIRC it was about twice the Crecy's).
Lower temperature means that less energy can be extracted from the turbine.

(I had found it unprofitable to say that the energy used in pumping extra air is not fully 'recoverable' ie compensatable somehow, and likewise to say that the benefits of raised exhaust pressure are in principle rather independent of the use of extra air)

yes, afaik early opening of the exhaust port was an inherent Crecy feature rather independent of the amount of extra air used
(this amount grew during development)
and so would today eg in F1 be independent
ie the temperature dropped across the turbine could be higher than in WW2, making the turbine cycle more efficient and powerful
the ideal compound argument is that if the different expansions are suitably proportioned the combined efficiency is greater
though our mgu-k etc limits also affect the proportioning

and turbocompounding would inevitably sacrifice some of the benefits of the established race 2 stroke 'tuned exhaust' design ?
with fixed geometry anyway

[uniflow]

A single cylinder 360 cc 2T with vairable rotary valve induction ( and exhaust power valve ) will punch out 70HP now with a wide flat power curve, add to that ECU control ( traction )and you have got an MX bike that will eat a 450 4T. Lighter, more power, less fuel burn ( per HP ), cheaper to maintain. BUT not allowed to race with the 450's.
This not using FOS tech, I would imagine with this FOS cylinder configuration a 250cc would be similar in output, a real race contender.
I think that is the problem with alowing 2T to race with 4T, twostrokes still have a lot of development to come ( and easily, read cheaply, compared to 4T ) so the rules would have to keep changing as 4T's keep getting swamped.

[riff_raff]

A 2T F1 engine would only be better if it was a uniflow design and an electric turbo-compound system was used for scavenging/boosting.

[J.A.W.]

Not all r-r, since..
-an N/aspirated 2T should/would readily beat an N/A 4T mill of equal cc..
& the same goes for forced induction too.. ..its a physical matter of work done/thermodynamics..

[uniflow]

http://dirtbikemagazine.com/home-featur ... re-you-die

[wuzak]

-an N/aspirated 2T should/would readily beat an N/A 4T mill of equal cc..

And use more fuel doing so....

Seriously JAW, 2 strokes and 4 strokes are not directly comparable by their nominal capacity.

The 2 stroke makes more power than the same nominal capacity 4 stroke because it moves more air and uses more fuel. Simple, really.

[Carlos]

This 2013 DOT paper may be of interest It discusses both loop scavenging and uniflow schemes; NA, supercharged and turbocharged that may be applicable to petrol, spark ignition engines>

Advances in The Design of Two-Stroke, High Speed, Compression Ignition Engines
By Enrico Mattarelli, Giuseppe Cantore and Carlo Alberto Rinaldini
DOI: 10.5772/54204
http://www.intechopen.com/books/advance ... on-engines

[J.A.W.]

-an N/aspirated 2T should/would readily beat an N/A 4T mill of equal cc..

And use more fuel doing so....

Seriously JAW, 2 strokes and 4 strokes are not directly comparable by their nominal capacity.

The 2 stroke makes more power than the same nominal capacity 4 stroke because it moves more air and uses more fuel. Simple, really.

Actually W, what is "simple really" is the physical fact that 2Ts do more work in the 4th dimension (time)..
& in fact -if you had read the links showing research findings for DFI 2T showing BSFC efficiency you'd realize..
that the - long assumed to be inherent - 4T advantages in that area are actually illusory too..

Only in fairly recent years ( after nearly a century of direct competition) was the 2T mill artificially deemed..
.. by sporting bodies.. to be..

"...not directly comparable by their nominal capacity." - as a sop to 4T makers/runners..

This was followed by explicit bans, since as Uniflow (& Carlos, stat above ) have recently pointed out,
-there is still significant development potential for 2Ts & naturally in racing performance, too..

[Blanchimont]

So, what's currently the most efficient 2 stroke engine(petrol and diesel, NA and forced induction) available with a capacity below 5000cc? How do emissions look if compared with a similar 4 stroke engine regarding power?

Any papers on this?

[J.A.W.]

Feel free to read the numerous recently linked research papers - on this thread - for that data, B..
( Such as on page 30 - for example).

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