Atkinsoning motorbike engines

[beatr911]

This is very interesting and I beleive very applicable to motorcycle engines. Motorcycles typically have among the highest power densities of any vehicles on the road. Reducing the power density with atkinson or miller cam timing seems at least at first look a natural fit for improving fuel efficiency. Especially true when the motorcycle is overpowered for the demands of the rider (many of us). Using a small engine at nearly 100% capacity is fun but maybe isn't as efficient relative to BSFC as a slightly larger atkinson cycle.

As for fueling, very long intake valve duration creates real problems with carburetors, and this is why. First fuel is metered to the air that is travelling toward the cylinder, then again travelling back toward the airbox as the piston pushes it back out, then a third time traveling toward the cylinder again on the next intake stroke event. Maybe this could be tuned out with the right carb and jets, but EFI is the real answer. It just has more granularity for adjustment at particular engine speeds and loads and has the possibility of accomidating unusual vacuum pulses from wild cams.

For cam timing specification, it would be really interesting to compare a few "atkinson" cams to a similar engine without the "atkinson" cam. Oh, and also compare the engine dyno plots to look for power characteristics.

The NC700 engine is EFI and low speed, does it use this type of cam timing?

Apparently no one wants my old, very high mile Kawasaki Concours that still runs and rides flawlessly. Being the first to "Atkinsonize" this engine might be the reason it's still around. I don't need 108hp anymore. Derating to say 50hp would be plenty.

[beatr911]

A little searching found some info on the Mazda Millenia with a 2.3L V6 with actual Miller cycle engine.

Link:
Why hasn't the Miller cycle been more widely utilized? - Motor Trend The General Forum Forum

Quote:
"Miller Cycle is an interesting concept. Invented by American Ralph Miller rather than Mazda in 1940s, it changed the long-standing basic principle, Otto cycle. Conventional Otto cycle engines have 4 stages in each cycle - intake, compression, explosion (expansion) and exhaust. Each of them takes roughly equal time. Miller Cycle engine differs from it by delaying the inlet valves closing well into the compression stroke. What is the result of this ?
In Mazda's Miller Cycle V6 engine, inlet valves close at 47 degrees after BDC (bottom dead center, ie, the lowest position of piston during a cycle). This equals to 20% of the height of stroke. In other words, during the first 20% of the compression stroke, the intake valves remain opening, thus air flows out without compression. Real compression activated during the remaining 80% stroke. Therefore, the real effective capacity of the engine is only 80% of the volume of combustion chamber. Compression ratio is decreased from 10 : 1 to slightly under 8 : 1.

Valve timing of the Miller Cycle V6

Until now, you probably still don't understand its objective. Be patient, I am going to explain now.

Lower compression ratio means less energy loss in compressing air, i.e., the so-called "pumping loss". Moreover, lighter compression leads to lower temperature, thus reduces heat loss in cylinder wall and pistons. To compensate the reduction in real capacity, a supercharger is employed to increase the air density such that the engine actually resume 100% capacity. Of course, the supercharger must generates less pumping loss than those gain by reducing compression ratio. Otherwise Miller Cycle engine will be no more efficient than ordinary engines.

Note that the expansion stroke is the same as ordinary engines, it is not reduced like the compression stroke. As a result, power delivery and is as smooth as normally aspirated engines.

Disadvantage

Mazda's Miller Cycle engine burns 13% less fuel than its 3 litres conventional sister engine. It also generates more power and better torque curve. However, since its introduction in 1994 until now, no other car makers follow its trend. Even Mazda itself did not produce another Miller Cycle engine. Why ?

Think about it: although it is claimed to be a 2.3-litre engine, it is actually constructed as a 3-litre engine, no matter in dimensions and in material. Then, the supercharger and twin intercoolers (one per cylinder bank) will be extra cost compare with conventional 3-litre engine.

For a V6, this might be forgiveable, but those additional cost will be relatively expensive for a low cost four-cylinder engine. As a result, Miller Cycle concept can hardly be popular in the market.




My comments:

If I understand the article correctly, utilizing only 80% of the stroke of a 3.0 liter engine makes it a (3.0)(.80) = 2.4 liter engine. Manufacturing a dimensionally large engine to have a small displacement in combination with a supercharger does not appear to be advantageous from a cost benefit standpoint.


Read more: http://forums.motortrend.com/70/7352643/the-general-forum/why-hasnt-the-miller-cycle-been-more-widely-utiliz/index.html#ixzz296cYYy6x"

So the only cam timing information is the intake closes 47* after BDC. Hmm. A cam event diagram (see attachment) from a Suzuki GS500 shows the intake closes 62* after BDC theoretically making it even more of a Miller cycle than an actual one. Need more data on the rest of the cam events on the Mazda.

[beatr911]

Hey, just found the cam events from the Mazda miller engine. See attachment.

So to compare:
Mazda miller. Note: supercharger must play in to the timing.
Exhaust duration - 232*
Intake duration - 252*

Suzuki GS500, 10,000 rpm max I believe
Exhaust duration - 280*
Intake duration - 276*

Ford Focus 2.3L Cosworth performance cams, up to 7200rpm
Exhaust duration - 255*
Intake duration - 255*

So in this case duration is long on the sample GS500 cams so a regrind is needed. Also phasing will need significant rework.

Still need to find cam specs on a naturally aspirated miller or "atkinson" engine without a variable valvetrain - if it even exists.

[janvos39]

Megane

I think it is possible to make the Atkinson approach like the Prius by making variabel timing on the inlet cam on a single camshaft.
See drawing and page 11 on my thread of the Burgman mods.



Inlet timing rotation of the Prius is almost 60 degrees. Closing as late as 95 degree after bottom dead center.
So the Prius is using the 13 to 1 compression ratio to have good efficiency in that working condition.

[cRiPpLe_rOoStEr]

In the original Miller cycle, the supercharger was essential to overcome issues related to the backfire thru the intake manifold. It had some popularity in marine and stationary setups.

BTW Nissan developed a Miller-cycle version of its 1.2L 3-banger that fits the current generation of the Nissan Micra, which is also made in Mexico with a 1.0L 4-banger (AFAIK only for the Brazilian market) and the same 1.6L used in the current Versa sedan.

[renault_megane_dci]

Achievieng 95° inlet retard on a SOHC camshaft looks almost impossible (would need a lot of regrind on the exhaust) certainly more so on a milder cam engine (mine is 260 / 260).

On a twin cam engine on the other hand you can alter the phasing of the cam so it is principaly a matter of delaying inlet events.

The main issue is raising the compression ratio this much.
And obviously converting to EFI wich is not that big a deal on a single I presume.


I am still on the market for a DR 350 bottom end (extra stroke => more CR)

Unfortunately for me, I can never get hold of a complete DRZ 250 head since they are not sold in Europe as far as I can tell (did some searching but no luck so far).
DRZ 250 does have the twin cam setup I am looking for.

At least I now see some interest in the post, I was becoming to be a desperate ecomodder !

Let's agree on a short list :
We want a DOHC engine,
It needs to have some very hi comp piston available or to be able to be hybrided with a smaller bore setup,
It needs to be converted to EFI,
the bike needs to get a massive gear reduction.

[beatr911]

I think I recall reading somewhere that a natually aspirated fixed cam Miller cycle engine had a very narrow power band. Maybe thats why the cam phasers are used on the Prius and others, to widen the useable rpm to a more useful span.

I think it would be a good idea to look at an existing sample engine to verify this.

Maybe engines with cam phasers should be added to the requirements. What motorcycles have variable valve trains? All I can think of are the Kawasaki Concours 14, and (I think) the Honda VFR 800. Both have EFI. Any others?

[renault_megane_dci]

Benchmarking a car engine is definitely a good idea but benchmarking a bike engine no so much (in my opinion) since they alwas fall for the power appeal.

Fitting a cam phaser sounds good but DIY it on a bike engine, not so much.
Also rules out all the central cam sprocket engine (Suzuki oil cooled engine à la GSX-R)

Maybe the very low weight of the bike can outcome the narrower power band ?

[cRiPpLe_rOoStEr]

Originally Posted by renault_megane_dci

Maybe the very low weight of the bike can outcome the narrower power band ?

Motorcycles usually have a higher specific power, but their specific torque is not so much impressive at all. If I were going to mess with the cams, I'd also try to decrease the RPM band to not sacrifice so much torque and also getting it at a lower RPM band, leading to a better throttle response than only "atkinsonizing" it

[Michael Moore]

My experience over the last 30 years of riding big 4 cylinder street bikes borrowed from friends is that they usually have such broad power bands and amounts of torque that they'd be just fine for street riding if they came with 4 speed gearboxes instead of 5 or 6 speeds. For real world riding (not the race track) a bike that can do 0-70 in first gear with an honest 7-9000 RPM power band really doesn't need another 5 gears to row through. Some friends of mine who had GS1100/1150 Suzukis put on sprockets with about 3-4 less teeth, and even then it was easy to start off in first, skip over second to third, and then skip up to top gear and just leave it there.

Riding a narrow power band 125 with a lot of gears and trying to keep it on the pipe can be great fun -- for a while. For day to day use a bigger engine with a wide and friendly power band is much more pleasant.

cheers,
Michael