Atkinsoning motorbike engines

[mechman600]

Originally Posted by beatr911

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?

What I mean is having a 14:1 compression requires a low effective compression ratio in order to avoid pre-ignition (aka holes in pistons). Another way to look at a low effective compression ratio is a partial cylinder fill. The Atkinson cycle does this by delaying intake valve closing, pushing some air back out, as we all know. But custom grinding camshafts is beyond the ability of the average DIYer.

Another way to achieve a partial cylinder fill is to not let intake manifold vacuum drop below a certain threshold by restricting intake airflow with a throttle. 15"HG of intake vacuum is theoretically equals a half filled cylinder. My idea of a secondary throttle valve (in conjunction with a 14-16:1 compression ratio) is simply a means of automatically maintaining a certain vacuum level instead of having to stare at a vacuum gauge and modulate the throttle accordingly. Maintaining 15"HG of vaccum at lower RPM will require much more restriction than at higher RPM. And trust me, if you let the intake vacuum drop much below that with a 14:1 compression ratio on regular gas, bad bad things will happen quickly.

The ultimate question is whether a high expansion ratio will offset BSFC losses of a throttled partial fill.

[renault_megane_dci]

I have been toying with the idea of getting a Kawasaki GPZ 500 (known as EX in the US I believe)
It has 58 hp engine with 290° cams and a 10.8 CR.
The engine has a 50 hp derivative with 262° cams and 9.8 CR.
The dynamic CR on both these engine is 7.3 give or take .05.

The idea was to retard the 290° cam to the 262 inlet opening timing (wich would give a 85° inlet closing and be close to the Prius's) and use the 262° exhaust cam.
Then, aiming for the previous dynamic CR, the head (or jug) need to be skimmed 0.72 (58hp Dynamic CR) or 0.75 mm (50hp DCR) for a 12.8 static CR where the valves cutouts in the pistons don't even need to be touched (inlet has been delayed).

Perfect plan !

Until I realised the potential of improvement on a 500cc bike is never gonna match the actual number of an unmodded 250, let alone an atkinsonned 250.

My 250 is already plenty powerful for my ride (to work) to the point I was thinking of getting a 125.
Ho, crap ...

The Kawi engine also exist in 250 form, maybe I need to start on that one ...

[serialk11r]

Originally Posted by renault_megane_dci

Until I realised the potential of improvement on a 500cc bike is never gonna match the actual number of an unmodded 250, let alone an atkinsonned 250.

My 250 is already plenty powerful for my ride (to work) to the point I was thinking of getting a 125.
Ho, crap ...

The Kawi engine also exist in 250 form, maybe I need to start on that one ...

I dunno...the 500 with modified cams could do better on the highway thanks to lower revs and better balance (less friction), but yea 500 is a bit big.

Personally I like 4 cylinder bikes for the sound and I'm okay with losing a little fuel efficiency for that absolutely beautiful scream, and I think the reasonable 4 cylinders start at 400cc (while the CBR250 was really cool, it's a bit stupid to constantly be using 10000rpm on the street from a practical point of view). 600cc is just crazy, they almost match the power of my car, which is fast enough as is.

If I ever go abroad to a country where it's harder to get killed riding a motorbike, I might have to get myself a CBR400RR and retrofit it with fuel injection.

[beatr911]

I guess I'm still not getting it. It seems to me the restricted intake is the same as the throttle not being opened much relative to load. If I remember right, for the same engine output, the Atkinson cam timing has fewer losses than a more closed throttle.

For me, altering cam timing by re-indexing the existing cam would be much easier than increasing compression. But really, both need to be done to get the most benefit, or maybe any benefit at all. I'm hoping to get some benefit from the reduced compression by using operating conditions that high compression won't tolerate. Things like hot air intake, more advanced ignition timing, higher load, lean fuel mixtures and a denser cheaper lower octane fuel blend.

For efficiency, we need to match the power output of the engine to what is required. So the engine output for you depends on your driving cycle. We also must consider that the engine may not be most fuel efficient at 100 output, but something less. Injected closed loop motors obviously do better in closed loop mode than open loop, so about 75% or so of maximum. Carbureted bikes tend to be a little rich at full carburetor flow to keep the engine at a safe temperature, so not running 100% is probably a safe bet here as well to keep engine longevity reasonable.

My commute will have high speeds 75+mph for over an hour, temperatures in excess of 100*F at times, some steep climbs, elevation between 2500 and 3500 feet. It will be an unusually high power demand commute. Still, my 108hp bike is over powered. Any reduced output through delayed intake closing will not be missed provided an increase in fuel economy is realized. I could probably tolerate up to 50% reduction in power output.

Here is a somewhat confusing explanation and diagram of the intake timing modes of the prius. What do you make of it?
Prius tech training

So maybe running atkinson timing is not good at near maximum output?

[RedDevil]

Originally Posted by beatr911

I guess I'm still not getting it. It seems to me the restricted intake is the same as the throttle not being opened much relative to load. If I remember right, for the same engine output, the Atkinson cam timing has fewer losses than a more closed throttle.

Let's put 2 examples of that head to head.

One is an ordinary Otto engine on half throttle, meaning the intake pressure is half the barometric pressure
The other an Atkinsoned Otto engine where the inlet valves close halfway the compression stroke, on full barometric intake pressure.
Both engines draw the same amount of air.

The Otto cycle engine draws air at half the atmospheric pressure, but regains a bit (about 25%?) of the lost energy during the first half of the compression stroke.
The Atkinson style engine draws air at atmospheric pressure, so no loss there and no gain through the first half of the compression stroke. After that the rest of the cycle is the same.

Both engines take the same amount of air but the Otto engine has to pull the intake air from a vacuum, while the Atkinson engine does not.

The Atkinson engine is therefore more efficient than a same sized normal engine on partial load. But where the Atkinson engine is at its peak, the Otto engine can produce more power. If you don't need that you could also use a smaller normal engine with the same peak power as the Atkinson engine, at roughly the same efficiency.

Atkinson engines however profit from the fact that at higher revs the intake air's momentum (flowing in) prevents it from flowing out again before the valve shuts; the higher the revs the less chance it will have to flow back. So the actual air intake volume on higher revs may be bigger than you'd expect.

As air intake momentum plays a role the air needs to have as much speed as possible, meaning narrow intake channels ending at single valves. And those reduce top revs.

(This all I searched and deducted after wondering why my Insight has only 8 valves while my '85 Civic with practically the same engine size had 12)

[mechman600]

Atkinson type compression ratios would destroy the engine quickly at WOT without late intake valve closing. Atkinson's real advantage is its compression ratio. My point was that you can achieve the same partial fill as the Atkinson cycle by throttling. Less efficient but much easier.

[serialk11r]

I wouldn't worry about not increasing compression ratio. Bikes have a good compression ratio already, and by the time you get to 13:1 the law of diminishing returns is really setting in (practical limit is thought to be about 14:1, you can easily lose power and efficiency by going up to 14:1 if you don't design the combustion chamber properly, and little to nothing is gained from going up further to 15:1). Think of it like this, by Atkinsoning your 12.x compression ratio bike engine you're lowering effective compression and burn temperatures to what a normal car engine from the late 90s burning 87 gas is at. Could it be better? Yea, some car engines have lower bore/stroke ratio and high compression as well, but your bike engine has less friction thanks to the shorter stroke and can hit 10000rpm or something and make fantastic power. Car engines don't have that flexibility and almost all of them have lower compression ratio anyways, so you're in pretty good shape.

[nemesis]

Very intersting read. I have a 06 kawasaki zx10r and I have some experience with cam timing, different static compression, and different cam profiles, but have very little data for mpg, since I drag race the bike mainly.

My bike started its life with 12.7 static compression, 300 duration with 9.1mm lift intake cam and 292 duration 8.5mm lift exhaust cam and 17/40 gear ratio. It ran pretty good stock and I was getting around 30mpg or so. Intake valve was closing at 74deg abdc. I then redid my fuel map and the bike was getting 43mpg. My next step I put a 9.7mm lift and 296 duration intake cam and raised my static compression ratio to 14to1 and my intake cam was set to close intake valve at around 80deg abdc. The bike still ran on pump gas but peak hp went up past rev limit of 12.5k rpm so I had to advance the camshaft 5 degrees so it would peak before rev limiter and improve power #'s everywhere else which it did. I haven't gotten much data for mpg though due to my gear ration that 15/40 right now. Here is the dyno #'s between 12.7 comp and 74 deg intake closing time and 14 comp and 80deg intake closing time.

[nemesis]

I've played with different cam timing on both intake and exhaust side. There was little to be gain on the exhaust side, but the intake cam advance always gained midrange and low end power with lowering the rpms where peak hp/tq was made. And retarding intake cam always opposite. Retarding the cam will produce higher peak hp #s at higher rpms, unless you were hitting rev limiter or to rev to the higher rpms would requier stiffer springs which would cause some power loss. From everything I've read and did to my bike, it gave me better mpg with smaller intake cam that was advanced since the bike would stay at 60mph or so on the highway with less throttle open than engine with longer duration cam that was retarded which would requier more throttle to stay at that same speed. Retarded intake cam would also produce higher peak hp but peaky power band, and advanced intake cam would produce lower peak #s at lower rpm but broader powerband. Also in many books that I read it says that anything over 14 static comp would return very little power gains probably due to pumping loses, but by retarding the cam and lowering dynamic compression you could still use pump gas with stock ignition timing table.

BTW my next step it to turbocharge 14to1 engine, but use e85 since I don't think the engine would live on pump gas unless the ignition timing wlll get retarded to the point of very little power gain. But we will see.

[beatr911]

[QUOTE=RedDevil;355820] ...The Atkinson engine is therefore more efficient than a same sized normal engine on partial load. But where the Atkinson engine is at its peak, the Otto engine can produce more power. If you don't need that you could also use a smaller normal engine with the same peak power as the Atkinson engine, at roughly the same efficiency.
QUOTE]

OK thanks, that's what I'm shooting for. Derating Big Red for more efficiency.

Nemesis, those are some well detailed experiences. Some good meat to chew on.
So it sounds like you are thinking that increasing dynamic compression by advancing the intake cam will have a larger MPG benefit than reducing intake friction by retarding the cam to gain some atkinson effect. Correct?

Wow, lots of variables in the engine equation here. I think to get a good answer some engine modeling software is in order. Or more fun yet just run some experiments.