Atkinsoning motorbike engines

[cRiPpLe_rOoStEr]

Sure, gears also have an important role. So, higher torque at lower RPM, even with less gears, end up more pleasurable for a daily commuter.

[renault_megane_dci]

I have been doing some thinking about cam timing and assuming we don't use came phasers (which I believe we realistically can't) I believe we need at least 295° cams :
20° advance + 180° + 95° retard

I am not very sharp on bike cams but it doesn't seem too wild (the hot factory cam for beemer airheads are 336 IIRC) at least for 2 valvers.

As for FE, on top of Atkinson timing on the inlet, we don't want a lot of overlap to promote low RPM efficiency as well as not too much duration on the exhaust (that's where we might need a regrind).

The timing of my particular bike is 260 on the inlet.
I think I will investigate 2 valver timing of the same base engine.
GN 400's and 125 are on the top of the list.


By the way, I could build a nice 12,2 to 1 CR engine with a DRZ 250 head and barrel on top of a DR 350 bottom.

Anybody can find the DRZ 250 (complete) head and piston + barrel in the US ?

[beatr911]

So to bringing this back from the dead, been doing more perusing.

Seems the modern "Atkinson" engines all use variable valve timing and even the "Mild Atkinson" engines too.

So, with the Prius retarding only the intake 33* during atkinson mode, I should be able to achieve a mild effect by retarding my cam one tooth which is 14.4*. If there is no piston contact, try for 28.8* and see what happens.

Also I found out that internal egr can occur if the exhaust is retarded as well, pulling exhaust back in during the intake stroke. Retarding up to 60* may be possible, however in a fixed cam engine idle egr can become excessive rather easily making idle quality really horrible. Hey, maybe try retarding both cams as as part of a second experiment.

[serialk11r]

Originally Posted by beatr911

So to bringing this back from the dead, been doing more perusing.

Seems the modern "Atkinson" engines all use variable valve timing and even the "Mild Atkinson" engines too.

So, with the Prius retarding only the intake 33* during atkinson mode, I should be able to achieve a mild effect by retarding my cam one tooth which is 14.4*. If there is no piston contact, try for 28.8* and see what happens.

Also I found out that internal egr can occur if the exhaust is retarded as well, pulling exhaust back in during the intake stroke. Retarding up to 60* may be possible, however in a fixed cam engine idle egr can become excessive rather easily making idle quality really horrible. Hey, maybe try retarding both cams as as part of a second experiment.

I wouldn't touch the exhaust cam. The last thing a motorbike engine with its massive exhaust cams needs is more internal EGR. When you have a giant intake cam to work with and a good high compression ratio, you should try to maximize the "Atkinson cycle effect" (reduced VE via late intake valve closure) before resorting to methods that compromise combustion stability in an engine that already has problems with combustion stability.

Internal EGR is the least desirable method of pumping loss reduction because it raises the temperature of the charge significantly (knock, reduced heat capacity ratio).

Remember that all car engines these days have variable valve timing. What makes the "Atkinson cycle" engines different is the large intake cam, which you already have on your bike (in fact, your bike's intake cam probably has quite a bit more duration than a Prius cam).

Valves hitting the piston shouldn't be a problem, as retarding the intake cam makes them open later, as the piston is already moving away from the valves Retarding the exhaust cam could be a problem there though.

[renault_megane_dci]

Originally Posted by beatr911

So to bringing this back from the dead, been doing more perusing.

Seems the modern "Atkinson" engines all use variable valve timing and even the "Mild Atkinson" engines too.

So, with the Prius retarding only the intake 33* during atkinson mode, I should be able to achieve a mild effect by retarding my cam one tooth which is 14.4*. If there is no piston contact, try for 28.8* and see what happens.

Also I found out that internal egr can occur if the exhaust is retarded as well, pulling exhaust back in during the intake stroke. Retarding up to 60* may be possible, however in a fixed cam engine idle egr can become excessive rather easily making idle quality really horrible. Hey, maybe try retarding both cams as as part of a second experiment.

Actually, I found out the prius inlet cam valve closure can be retarded as much as 95° after TDC.
33° does sound like a normal timing to me.

Also, newer motorbikes engine have a huge CR to make up for the overlap. The huge CR is mandatory in Atkinson engines.
This high CR, added to the reversion pulse in the inlet track wich I believe rules out the use of carburettors, asks for newer generation engines.

A few days ago, I had the opportunity to have a glance at a cam phaser from a car. Those are quite compact and I now believe they can be adapted to bike engines as long as the timing chain is on the side (not Suzuki's air/oil range)

[beatr911]

About carbies and late intake closing/massive duration.

Does anyone have actual experience with constant velocity carburetors and very long duration cams where the intake closes, say 90* ABDC? Interested in the peculiar effect on the carbs at various speeds/loads. My understanding is the reversion would cause air to be metered three times causing an extreme rich ratio, but only at some engine speeds. I'm guessing idle and low speeds to be affected most.

As manufactured, he 997cc Concours has a 275* duration, and intake closes about 54*ABDC. Though this is what is usually considered a long duration cam and closing is pretty late, the four individual carbs meter just fine.

Just wondering if there is some knowledge as to the upper limit of intake closing ABDC that individual constant velocity carbs can tolerate.

[renault_megane_dci]

It is interesting to figure out what is happening at various engine speed.

The Atkinson cycle aims to maximise expansion to compression ratio.
If I understand it right, at lower throttle openings, the cylinder is already not filled up to its max because of the throttle.
At this opening, there is not gonna be a great deal of reversion because there is a throttle plate in the way and the balance between cylinder and inlet pipe is not that great.
It is also not a spot in the rpm range where Atkinson cycle is gonna improve much on Otto since most of your loss is pumping loss from the throttle plate.

When you hit peak torque, the story changes, there is air flowing backwards.
Now, what about we tune the inlet length (from valve to throttle plate) to have a spare volume fairly higher to what is gonna be dumped out of the cylinder ?

The air might have a tendency to compress before it goes back being refuelled again.

With a longer inlet we could have more "stop and go" pulses but without any issue carb wise ...

Let's not forget we want to increase static compression ratio to match the previous dynamic compression ratio to get the most out of this mod.

[mechman600]

Originally Posted by renault_megane_dci

Let's not forget we want to increase static compression ratio to match the previous dynamic compression ratio to get the most out of this mod.

Yes - I don't think 12.5:1 (static) mentioned above is nearly enough to make the Atkinson cycle "profitable". 12.5:1 is already the realm of many motorcycle engines. I am thinking that 14-16:1 is more like it.

Since DIY cam grinding is a daunting task (at least for me), I wonder if a partial cylinder fill could be achieved by a second throttle valve that restricts intake to achieve a certain vacuum at all times. It would have to be self adjusting with a manifold absolute pressure sensor and control system of some sort...or with the use of a vacuum gauge and a very careful throttle hand.

I realize this would cause more pumping losses compared to an actual Atkinson cycle, but I wonder if these losses would be more than offset by a very high compression ratio.

[beatr911]

To keep perspective, for at least my experiment, I will not be re-engineering this bike much at all. Probably just changing valve and ignition timing and possibly some jetting changes with O2 sensor information.

The manual says static compression is 10.2. Dynamic compression at totally stock timing is about 8.4, retarding one tooth on the intake cam drops it to ~7.3 (if I remember right, my data is at home). From what I've read, dynamic compression of about 8.5 is about the max on pump gas. My 8.4 runs well on regular grade gasoline.

So, to take advantage of the lower compression, I'll use advanced ignition timing, lean carburetion, and the hottest intake air I can muster.

The key for this experiment is to find the balance between reducing dynamic compression and intake reversion. Gain in one, lose the other. Finding the correct balance that is biased toward fuel economy is the goal. As much fun as it sounds to tinker endless nights optimizing this engine, I do not want to incur too much work. I would like this to be repeatable by others, or have others at least use this to some sort of advantage.

Oh, yeah, no big hurry as I ride a vanpool to work and am not on the bike as much anymore. Please be patient if you are eating popcorn while reading.

[beatr911]

RMD, Do you mean to say that Atkinson cam timing really will have no effect at light loads? Good thought. Maybe this is just folly as this engine runs nowhere near even 30% nearly all the time. I now see that Atkinson may probably be best used when an engine is in a high load situation where throttle restriction is at a minimum allowing reversion. Maybe this engine is just too powerful and a smaller, say 500cc engine would be better.

Though I do not know if the Honda NC700 series motor uses an Atkinson mode, I can see that it could be a good candidate. It has a shared intake plenum, single point fuel injection, NOT designed for high RPM and is loaded reasonably high at freeway speeds (50-ish HP). It is probably too powerful yet for maximum mpg but needed to have enough to satisfy most riders.

Mechman, yes 14:1 would be better. 10.2 static might not be enough to do anything. The losses of dynamic compression outweighing the less restrictive throttle. Lacking enough information or engine modeling tools to know if this whole exercise will yield an increase in BSFC for the power demands of how I ride the bike, I'm left to simply experiment.

Not sure what you're after with the throttle. It already has one that maintains vacuum in the intake. Would you say to increase the spring in the slide of the CV carb?