2 vs 4

[DJ99X]

OK. I've been sucked into an argument over on the Vital forums. I said that 4 strokes are much more efficient than two-strokes.

I claimed that a 125 two-stroke should be able to match a 250 four-stroke, as at any RPM, they will (theoretically) be combusting the same volume of fuel/air. But the 250F has more power at almost every RPM, meaning they are more efficient.

All I got was attacks from two-stroke nuthuggers, and people saying that I don't know what I'm talking about. It was like I suggested the earth was round, but they thought it was flat. Not one of them could give me a decent answer.

Can some intelligent people here please agree with me, or at least explain to me where my assumption has gone wrong?

Say a 125 engine and 250f engine are revving at 5000 RPM. The 125 would fire 5000 times a minute: 5000 x 125 = 625000 cc fuel/air combusted
The 250f engine would fire 2500 times a minute: 2500 x 250 = 625000 cc fuel/air combusted

Thus, as they are processing the same amount of fuel, the one with the higher power would be more efficient would it not? The efficiency is essentially work energy/chemical energy isn't it?

That's why I can't justify why people think equal displacement is fair...

[yzmxer608]

Sounds right to me, but it would only apply to 125s and 250f's since they are exactly half/twice the displacement. 250's and 450f's would be off a little (wouldn't the 450 still be more efficient though, since it processed less air+fuel while still having more power?).

5000 * 250 = 1,250,000
2500 * 450 = 1,125,000

But just looking at the engine at equal displacement (which obviously makes more sense to do) that seems right.

[DJD]

You should already know that vital is a cesspool of ignorant fools who wont acknowledge anything other than their own point of view.

[DJ99X]

That is true. I normally keep out of those threads. But once I made my point, I couldn't keep out of it

[ddmx]

This little bit of knowledge might help you out. Two strokes are generally less efficient than four stroke engines due to incomplete expulsion of exhaust gases and partial expulsion of fresh air-fuel mixture with exhaust gases. If we're talking ideal otto cycle then it can be confused that two strokes are more efficient. Ideal otto cycle sums it up.

[ddmx]

I might add to that, and I think we all know this, but a well designed two stroke comparable in weight and power to a four stroke CAN provide significantly more power just because every other stroke is a power stroke unlike the 1 in 4 with a four stroke.

[ddmx]

meant to say weight and displacement

[Phathry25]

DJ, this is exactly my stance on the argument. I've gotten in trouble all over the internet for stating it the way you did as well. On the plus side, I've finally found someone who see's it like I do.

[DJ99X]

Thank god you agree with me Phathry. That means JLV does as well

This little bit of knowledge might help you out. Two strokes are generally less efficient than four stroke engines due to incomplete expulsion of exhaust gases and partial expulsion of fresh air-fuel mixture with exhaust gases. If we're talking ideal otto cycle then it can be confused that two strokes are more efficient. Ideal otto cycle sums it up.

I'm talking about complete combustion, and the engines ability to convert the same amount of chemical energy into work energy

[jlv]

The two stroke's compression stroke only goes from the top of the exhaust port to TDC. The four stoke gets the full displacement on the compression stroke. So half is unfair to the two stroke. If you only counted the 2 stroke displacement from the top of the exhaust port it would be unfair to the four stroke since the expansion chamber has the two stroke around 1.2 times atmospheric* when the exhaust closes.

The argument's stupid anyway though. If someone made a 10 lb 40 hp turbine powered CVT bike would you want the rule book to render it uncompetitive because it wastes too much air?

* 1.2 times is something I remember from a 15 year old Mototech article so I'm probably way off.

[DJ99X]

Finally, that's what I was after

Yeah, turbines use crazy amounts of air. The one I designed last year used somewhere in the order of 20 m3/s for like 3 MW. Imagine standing next to that thing

[rgaede]

a 125 two stroke makes just as much power, just not as much torque. I would have to find my engines book to calculate it, but the four-stroke outputs the power in a way that allows much more of the power to be put to the ground. I know that in '05, the kx125 was putting out more power than all but one 250f. It was just a pain to ride as fast which is why Decoster has been quoted saying that a two-stroke takes more skill to ride.

[DJ99X]

I don't like when people say it like that. Power is just proportional to torque x rpm. So for high power, you still need high torque. Peak power is usually after peak torque though, as the torque curve sorta flattens off, but the increase in RPM is enough to increase the power output to a point. You could have a 125 with a similar shape torque curve to a 250f, only much lower.

[DJ99X]

The two stroke's compression stroke only goes from the top of the exhaust port to TDC. The four stoke gets the full displacement on the compression stroke. So half is unfair to the two stroke. If you only counted the 2 stroke displacement from the top of the exhaust port it would be unfair to the four stroke since the expansion chamber has the two stroke around 1.2 times atmospheric* when the exhaust closes.

The argument's stupid anyway though. If someone made a 10 lb 40 hp turbine powered CVT bike would you want the rule book to render it uncompetitive because it wastes too much air?

* 1.2 times is something I remember from a 15 year old Mototech article so I'm probably way off.

So is it still burning 125cc of mixture? I know two-strokes have a compression ratio of about 8.5:1 whilst the new four strokes are more like 13.5:1. But are the two strokes actually getting the full 8.5:1, or is that just a comparison of the volume at the lowest point to the volume at the highest point (i.e not the true amount of compression)

[jasper125]

Totally agree DJ don't get why people won't agree its that simple, but this is only engine whize if you would look at horsepower per kilo the diffrence won't be that big i think