2-stroke vs 4-stroke - power/efficiency differences???

[TheOtherLeft]

Bit of a noob question but what are the differences in power/efficiency/fuel consumption etc between 2 and 4 stroke engines?

I understand the mechanics theory behind 2 & 4 stroke engines and I always thought 4 strokes were better (cleaner burning, less noise etc) but a lot of the modern smaller engines are still 2 stroke (motorbikes etc) and even ship engines are as well (gigantor 2 stroke diesels).

What makes 2 strokes better in that regards? Given you get power from every 2 cycles as oppossed to every 4 cycles what are the other drawbacks/benefits of both systems?

Cheers,
Ben

[300WinMag]

2 stroke diesel is totaly different to petrol, ie it has a blower to force iduction air into the motor via a air jacket and ports in the cylinder block.
The blocks ten to be built heavier to acomodate this, compair to the engines actual capacity.
2 strokes in general produce more HP per cubic capacity than a 4 stroke, but a 4 stroke will have more touque as the power stroke is longer and the fuel air mix has no oil in it.
With new technology and emission laws 2 strokes are being fazed out, and four strokes are getting more and more powerful with lighter/stronger alloys for pistons, rods, cranks, valves and less friction in the drive chain assemblies.

[mangie]

2 stroke diesel is totaly different to petrol, ie it has a blower to force iduction air into the motor via a air jacket and ports in the cylinder block.
The blocks ten to be built heavier to acomodate this, compair to the engines actual capacity.
2 strokes in general produce more HP per cubic capacity than a 4 stroke, but a 4 stroke will have more touque as the power stroke is longer and the fuel air mix has no oil in it.
With new technology and emission laws 2 strokes are being fazed out, and four strokes are getting more and more powerful with lighter/stronger alloys for pistons, rods, cranks, valves and less friction in the drive chain assemblies.

Cube for cube, a two stroke eats a four stroke for hp and torque.

And they smell better!

[Slunnie]

Yep, 35hp from 100cc isn't a problem in a two stroke.

[300WinMag]

Why are all the MX bikes going four bangers then and GP bikes?

[cmarico]

To correct a previous poster,

You can get 2-strokes with oil injection rather than having it mixed in with the fuel. However this is rare as the injectors tend to clog up.

[ajsr]

Why are all the MX bikes going four bangers then and GP bikes?

emissions and politics.
they are the only reasons.
4t has a lot more maintance and cost per horsepower produced compared to 2t.
the big killer in 2 strokes is the overlap between the intake stroke and exhaust stroke it allows a precentage of raw fuel to exit unburn into the exhaust, hense the emission problems. Also the amount of oilly smoke doesn't help.

[300WinMag]

All true for petrol aplication. But Diesel is a different storey, even GM don't produce 2 stroke diesels anymore to the best of my knowledge. With common rail technolodgy 4 strokes produce way more tourque as they use the full length of the power stroke with their multi pulse injection.
Ships are a different storey altogether as weight isn't a issue.

[WRXZook]

Why are all the MX bikes going four bangers then and GP bikes?

Tougher emissions standards.

The move away from 2 strokes has involved concessions in some areas where it has been king ie snow mobiles, chainsaws etc to allow more time in the development of applicable technology. I believe cutoff dates have been set for motogp (125's and 250's) and Trials.

[T_Diesel]

Two strokes smell better

[pongo]

2 strokes tend to be lighter and need less servicing, etc. no need to worry about sump oil and filters either.

Less moving parts in a 2 stroke, theyll rev harder for longer and still come back for more abuse. Prob generaly cheaper and easier to rebuild too.

[pongo]

2 strokes tend to be lighter and need less servicing, etc. no need to worry about sump oil and filters either.

Less moving parts in a 2 stroke, theyll rev harder for longer and still come back for more abuse. Prob generaly cheaper and easier to rebuild too.

on bikes power band is easilly changeable with a swap of exhaust systems. Race bikes usually have a shed full of pipes to suit each type of track.


A cranky 2 stroke at full noise is a sweet sound

[nastytroll]

not all 2 strokes rev hard and depending on oil injection type and oil used can be very reliable.

Emmissions is a problem though.

[TheOtherLeft]

on bikes power band is easilly changeable with a swap of exhaust systems. Race bikes usually have a shed full of pipes to suit each type of track.

How so? Do the pipes increase/decrease back pressure so easily that it changes the power band?

[pongo]

on bikes power band is easilly changeable with a swap of exhaust systems. Race bikes usually have a shed full of pipes to suit each type of track.

How so? Do the pipes increase/decrease back pressure so easily that it changes the power band?

yep, its all in the expansion chamber. youll prob notice theat bikes have a large exhaust port and a lot smaller exhaust tip

[ajsr]

on bikes power band is easilly changeable with a swap of exhaust systems. Race bikes usually have a shed full of pipes to suit each type of track.

How so? Do the pipes increase/decrease back pressure so easily that it changes the power band?

it has to do with volume of the expasion chamber and how fast it draws exhaust gasses from the cylinder.

but what affects two stroke power delivery more is inlet and exhaust port heights

also power valve volumes and timing play a large part.

basicly the larger the exhaust/expasion chamber volume the better power in the lower to mid revs. to a point.

[KiwiBacon]

Cube for cube, a two stroke eats a four stroke for hp and torque.

And they smell better!

Yes they do, but they don't produce twice the power despite drawing twice the fuel and air.
Which is an indication of how poor the efficiency is.

[tweak'e]

......But Diesel is a different storey, even GM don't produce 2 stroke diesels anymore to the best of my knowledge. With common rail technolodgy 4 strokes produce way more tourque as they use the full length of the power stroke with their multi pulse injection.

its not really the multi pulse injection that does that but rather the large amount of boost they run allows for more fuel which means more gas to push the piston for longer.

with the large amount of EGR modern vehicles use for emission control its a wonder they haven't brought back the 2 stroke. down side of 2 stroke (especially when turboed) is how much exhaust is left in the cylinder when air is drawn/blown in. poor for performance but great if you want EGR for emission control.
if Nox filtering tech comes in and removes the need for EGR then that would kill the idea fairly quick.

the added cost of a supercharger increases the cost of a 2 stroke over a 4 stroke, tho i wonder if variable turbo's would produce enough boost low down to do the same job.

the other thing that comes to mind is if the pistons of a 2 stroke would handle the hotter running of modern motors. the old Detroit 2 stroke ran fairly cool, it even had rubber o rings as a head gasket.

[WRXZook]

Nothing like the sound of a GM 2 stroke under load at full noise. Better still if it's a V12 or V16 with 4 turbos feeding 2 superchargers.

[mangie]

Cube for cube, a two stroke eats a four stroke for hp and torque.

And they smell better!

Yes they do, but they don't produce twice the power despite drawing twice the fuel and air.
Which is an indication of how poor the efficiency is.

What's this, defending four strokes? You Sir, are a girlie man.

[KiwiBacon]

What's this, defending four strokes? You Sir, are a girlie man.

If two strokes are enough, you should probably see these guys.
link

[superslowlux]

Cube for cube, a two stroke eats a four stroke for hp and torque.

And they smell better!

Yes they do, but they don't produce twice the power despite drawing twice the fuel and air.
Which is an indication of how poor the efficiency is.

What's this, defending four strokes? You Sir, are a girlie man.

Have you ever ridden a four stroke? 2 stroke is dead, fun to ride though

[3 cyl]

2 strokes run better on their side, and when you roll them you can start them as soon as it's back on it's wheels.

[mangie]

Cube for cube, a two stroke eats a four stroke for hp and torque.

And they smell better!

Yes they do, but they don't produce twice the power despite drawing twice the fuel and air.
Which is an indication of how poor the efficiency is.

What's this, defending four strokes? You Sir, are a girlie man.

Have you ever ridden a four stroke? 2 stroke is dead, fun to ride though

I've ridden a few, but I ask you this, have YOU ever plopped your arse on a RG 500?? Sweet Jesus

[ajsr]

Cube for cube, a two stroke eats a four stroke for hp and torque.

And they smell better!

Yes they do, but they don't produce twice the power despite drawing twice the fuel and air.
Which is an indication of how poor the efficiency is.

What's this, defending four strokes? You Sir, are a girlie man.

Have you ever ridden a four stroke? 2 stroke is dead, fun to ride though

I've ridden a few, but I ask you this, have YOU ever plopped your arse on a RG 500?? Sweet Jesus

mmm rg 500 now that was a bike.

or a kx500 to finke

[Jeeps]

Aside from the road/enduro/motorcross stuff, a lot of guys in motorcycle trials prefer 2 strokes because 4 strokes generally have too much low down torque and it comes on too quickly cause you to lose traction. I remember testing a new honda 4 stroke back in the early 90's at a qld titles and thought it was great but it never took off due to the engine characterisitcs. Only in recent years have they started to become popular again because they've managed to control the torque curve a lot better. Some of the newer ones have fully pogrammable ECU's like this new Montesa Cota4RT Repsol 280, bloody nice bike but it would want to be for $17,500. The same/similar bike but 2 stroke is thousands less




cheers

[Slunnie]

on bikes power band is easilly changeable with a swap of exhaust systems. Race bikes usually have a shed full of pipes to suit each type of track.

How so? Do the pipes increase/decrease back pressure so easily that it changes the power band?

it has to do with volume of the expasion chamber and how fast it draws exhaust gasses from the cylinder.

but what affects two stroke power delivery more is inlet and exhaust port heights

also power valve volumes and timing play a large part.

basicly the larger the exhaust/expasion chamber volume the better power in the lower to mid revs. to a point.

Port duration is probable more accurate, but with the expansion chamber it is a lot more complex than this. It works on pressure waves that travel up and down the expansion chamber. When the exhaust port cracks the pressure wave travels the header and expansion chamber and this rebounds the pressure wave back again to the exhaust port. This in effect allows the motor to pressurise the combustion chamber again before the exhaust port closes with fuel/air mix that has been drawn into the CC and out through the exhaust without being burnt. Almost like a passive low pressure supercharge in some respects, but absolutely fascinating to see on the 2 stroke engine building simulators on the computer. We used to play more with the lengths rather than the expansion chamber as such. Shorten it for top end, lengthen it for grunt

[TheOtherLeft]

cheers

Looks like a postie bike on roids.

[big_fyt]

well i don't stick around anywere long enough on my rm 250

i am yet to be beaten by any 4 stroke on mine


it is all in the set up tuneing of the bike and the rider to how fast it can go.

however for trail riding you want a 4 stroke for sure.


there are heaps of 2 strokes out there that don't mix oil into fuel pretty much every road registerable 2 stroke has it how is that rare?

[mickbeny]

Hi all...Apparently one of the Outboard companies have got an injection system that works and makes their 2 stroke engines meet the stringent emmission laws.So we may yet see a resurgence of 2 stroke engines yet.