2 Stroke motorcycle engines-why dont they use a spark advance curve like 4 strokes?

Why do 2 stroke motorcycle engines not have a spark advance like a 4 Stroke? the advance is to compensate for saturation of the coils at higher rpms right? so why is this not needed on a 2 stroke?

Advance is used to compensate for the flame propagation speed of the burning air-fuel mix. There’s no reason a 2 stroke can’t benefit from a spark advance curve but small engines tend to use a magneto triggered directly off of the crankshaft so it becomes impractical to do so.

Thankyou for response. The 2 stroke engines i am familiar with are much larger and are not magneto but points ,condensor type with coils. So i dont understand why the magneto system would elimante the need for advance

I didn’t say it eliminated the need, I said it was impractical to use advance with that kind of ignition.

Can you give a link describing the kind of engine you’re talking about?

A link ? Well lets use a very popular bike. 1973 Yamaha RD350. if you go to the yamaha website I’m sure you can still view service manuals for these. If not i can send you a scanned image from one of my manuals

Ah, the first motorcycle I ever rode. Fond memories…

As jz78817 points out, the need for spark advance is due to the time required for the flame front to propagate across the combustion chamber from the spark plug. Flame speed actually increases with engine RPM (due to the increased turbulence generated during the intake/compression process), but the correlation isn’t perfect: if you don’t advance the spark a bit as RPM’s increase, then combustion happens later than is ideal, and you lose some efficiency at high RPM’s. This is less of a problem in engines with smaller cylinders. To cite your example, the RD350 was a 2-cylinder engine, so each cylinder was approximately 175 cc’s, with a bore of 64mm (bore matters most here, since combustion happens when the piston is fairly close to TDC). Contrast that with, for example, a BMW R1200RT, on which each cylinder has a displacement of ~600cc’s and a bore of 101mm. In addition to computer-controlled spark advance, the R1200RT actually has two spark plugs per cylinder to help get combustion done faster and provide good efficiency/torque even at high RPM. Both engines (R1200RT and RD350) run up to about 7500 RPM.

Providing RPM-based spark advance on a vehicle like the RD350 would have added unacceptable cost, weight, and complexity in a situation where engineers were trying to minimize all three, all while adding only marginal benefit.

thank you for the reply . well i have had my rd350 up in the 13000 rpm many of times. aswell as the static timing spec is 2.0 mm btdc. rule of thumb everyone uses now because of pump gas octane ratings are lower is 1.8 mm btdc . i dont think it was a cost cutting situation . like in model airplane 2 strokes there is no ignition system only a glow plug. since every stroke for a 2 stroke is a power stroke i think as the revs go up there might not be as much of a problem for flame propagation since the piston top hasnet had 3 strokes to cool down what do u think?

Ok, why?

Except with glow engines, as the RPM/power increases, the wire in the plug stays hotter thereby “advancing” the ignition time in practice.

No. Burn rate is a property of the fuel and it’s mix with the air charge.

If the surfaces of the combustion chamber are hot enough to ignite the mixture, than runaway preignition (in which ignition occurs earlier and earlier on successive combustion events) is very likely, and the outcome is gonna be a holed piston.

The entire point of the ignition system (spark plug and related hardware) is that it (and nothing else) is supposed to dictate the timing of ignition. At the same time, the job of the cooling system (whether liquid or air cooling) is to keep engine parts cool enough so that

A) the oil can keep things lubricated, and
B) the ignition system can maintain control over when combustion starts.

You definitely don’t want combustion chamber surfaces hot enough to cause ignition. And as long as things are cool enough to avoid that happening, the mixture should burn at its usual rate.

Again, the RPM’s affect flame propagation in a big way. The laminar flame speed for most combustible gas mixtures is remarkably low. For gasoline in particular, the LFS is just 35 centimeters per second. Yeah, about a foot per second. You could outrun that! That assumes the mixture is completely quiescent and stays that way even after combustion starts. But even if you start with a completely quiescent mixture, it never stays that way. The burned gases are hot and occupy a larger volume than the initial reactants, so they expand and push things around. The flame front gets stretched/distorted, which adds surface area, which speeds the overall reaction rate. In a running engine, the mixture is never quiescent; during the intake event it comes roaring into the combustion chamber, gets compressed, and then gets ignited before it ever has a chance to stop swirling and tumbling. When the engine is spinning faster, the intake event is more violent, and so the mixture is swirling/tumbling faster, and so that greater turbulence results in a much higher flame speed, high enough to still get combustion completed relatively early in the expansion stroke, even without having adjusted the spark advance.

The engineers that designed the RD350 probably said[sup]*[/sup] “yeah, we can get more power/efficiency at high RPM if we include a mechanism to provide increased spark advance, but even without it, we will get enough power/efficiency, and the bike will be cheaper/more reliable.”

[sub]*Of course they said all that in Japanese.[/sub]

According to my RZ350 service manual, (the later generation of the RD) timing is indeed variable (thru the use of the CDI I’m assuming).

17 degrees at 1200 rpm, up to 27 @ 3500, then dropping slowly and steadily to around 15 @ 9000, then a sharp drop to around 10 @ 10000. I don’t recall what the redline off the top of my head. Somewhere around 12-13K.

I had a Suzuki from the early 70’s, a Yamaha from the early 80’s and a barn-load of the RZ’s. All have CDI units, so maybe they have variable timing as well. I’ve never had an RD350, but my brother had an RD400 Daytona Special, and I recall it had a CDI as well.

Just tossin’ this out there.

Missed edit:

The RZ red-lines at 10, tach goes to 12.

'82 Yamaha 465 single- No indication of variable timing, even with the CDI unit. Stays at 16.5 degrees.

My '78 Yamaha DT400 had a CDI and I’m guessing variable timing. I’m sure the technology was implemented when it became economically feasible to do so. You wouldn’t add a $200 component to a $500 bike to gain a couple of horsepower unless you were a racer with a lot of money to spend. Once the electronics became cheaper, it was implemented on factory bikes.

Motorcycle technology has always lagged behind automobile technology. Fuel injection became common in automobiles about 30 years ago and in bikes about 10 years ago even though it would’ve given the bikes more power back then. It’s easier to justify adding a $1,000 cost to a $10,000 car than a $2,000 bike. (I’m just coming up with all these numbers off the top of my head by the way.)

ok in regards to cost and horsepower gains take a honda cl350 1969 it had spark advance also a parellel twin with close to the same bore

thank you for bringing this up. so very easy in a cdi ignition to have a spark curve. so 2 strokes indeed need spark advance . it still strikes me funny cuz the japanese were producing 4 stroke twins with point ignition with mechanical advance at the same time but not 2 strokes?

The chief benefits of a two-stroke are lower cost/complexity/weight. If you try to add variable timing, you chip away at those advantages.

That’s a four-stroke.

Keep in mind that a 2 stroke engine fires once every rotation, unlike a 4 stroke which fires every other revolution. Advancing the spark would increase heat and the piston would not have the extra stroke to cool. That could be catastrophic. I know more 2 stroke tuning is done with the expansion chamber than with timing. It’s volume is designed to fill in relation to intake timing, so that less raw gas is just passed through the engine and out the exhaust. Expansion chambers are an art. I can’t imagine how they would work of there was advance kicking in as the RPM increased.

Not an art, a science - it’s just that the math is a bitch. But once you get a computer to tackle the math, you can model the shock wave propagation in the pipe and choose the cone dimensions so that you pull a lot of mixture through the cylinder while both ports are open, and then cram it all back into the cylinder before the exhaust port closes, all at whatever RPM you want your power band to be at.

yes i understand each stroke is a power stroke. As someone added before their rz350 having an advance curve. so indeed 2 strokes are using advance curves . I guess what i was getting at is how come the four strokes that i mentioned had advance units but not the 2 strokes of the same era. The concern of over whelming costs is hard to believe just a some weights and springs. so they were on the four strokes how come not the strokes untill cdi untis became main stay?

cost comlexity and weight do not seem to be issues involving a simple weighted advance unit . on a weed eater maybe but on a full size motorcycle i beg to differ . so once again is spark advance needed on a 2 stroke and if not why not