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Atkinsoning motorbike engines

Hi folks,

With a broken GN 250 engine (shopping list not done so far), I am starting to think about my options.

I had the DR 350 bottom end project under the blanket wich would add a 6th speed and boost capacity throught stroke to 280 and added 10% torque increase but now I am starting to fancy this setup with an atkinson ground cam à la PRIUS.

Unlucky for me, I only have a single cam so it means the cam needs to be reground rather than simply retarded on twin cam engines (but those might require shorter exhausts cam anyway)

Anyway, I would like to discuss the topic with fellow motorbike ecomodders because lets face it, our engines have either barely enough power (125) or too much (250 and up).

From what I have red, the Prius cam timing is extended inlet timing for at least a third of the stroke up.
I couldn't find the actual numbers, does anybody has it ?

I am worried about the inlet charge point of view with carbs, what is gonna go out and how will it affect accuracy of the mixture (I am fearing it will richen it) ?
Maybe it is a case of switching to injection.

I am worried about the compression ratio boost required : not usually easy on bikes (the longer stroke of my particular setup should help beautifully here)

I am worried about the violent sidetrack from the original setup : motorbike top speed is made through high rpm but how will atkinson engines perform in this domain and will the new lower output of a smaller engine (say a 250) end up in a much slower top speed wich streamlining can not bring back from the dead ?
My top speed is already a very modest 83 mph @ 8K ish rpm.
A prius peak power is 4.5 rpm.
I can't deal with a 60 mph top speed in my trafic conditions.

I couldnt tell you the specs either, but it makes the vehicle feel like you got a signaficant vacuum leak. On the vacuum side with a max of 14, I see between 5-9 when the Insight is in its "eco" or using its vtec to make the atkins cycle. Also the piston pin and rod and or crank are offset too to lessen friction and the piston skirt is layered in teflon. :eek:

Quote:

Originally Posted by Cobb (Post 331877)

piston pin and rod and or crank are offset too to lessen friction

That is now a common feature in the automotive business.

Not a lot of offset is required too (thinking 6 to 12 mm, must be dependant on stroke) so this sounds like one more new project (like I did need one more ...)

I think what your describing is actually called the Miller cycle rather than the Atikinson Cycle. Both have the same basic goal: expansion ration > compression ratio, however, my understanding is that the Atkinson cycle actually uses a different conrod/crank mechanism, whereas the Miller cycle uses the conventional conrod/crank and just adjusts the intake valve closing (IVC). Changing just the IVC, however, won't give you any benefit. The benefit would come from running a high compression ratio piston that would otherwise knock.

On another note, I've toyed with the idea of finding a motorcycle with a blown engine and putting a small diesel on it. Just a thought...

Quote:

Originally Posted by Diesel_Dave (Post 332021)

I think what your describing is actually called the Miller cycle rather than the Atikinson Cycle. Both have the same basic goal: expansion ration > compression ratio,

Modern Atkinson cycle engines
A small engine with Atkinson-style linkages between the piston and flywheel. Modern Atkinson cycle engines do away with this complex energy path.Recently Atkinson cycle has been used to describe a modified Otto cycle engine in which the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. The effective compression ratio is reduced (for a time the air is escaping the cylinder freely rather than being compressed) but the expansion ratio is unchanged. This means the compression ratio is smaller than the expansion ratio. Heat gained from burning fuel increases the pressure, thereby forcing the piston to move, expanding the air volume beyond the volume when compression began. The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio allows more energy to be converted from heat to useful mechanical energy meaning the engine is more efficient.

The disadvantage of the four-stroke Atkinson cycle engine versus the more common Otto cycle engine is reduced power density. Due to a smaller portion of the compression stroke being devoted to compressing the intake air, an Atkinson cycle engine does not take in as much air as would a similarly designed and sized Otto cycle engine.

Four-stroke engines of this type with this same type of intake valve motion but with a supercharger to make up for the loss of power density are known as Miller cycle engines.

(Atkinson cycle - Wikipedia, the free encyclopedia)

Quote:

Originally Posted by Diesel_Dave (Post 332021)

Changing just the IVC, however, won't give you any benefit. The benefit would come from running a high compression ratio piston that would otherwise knock.

The compression ratio increase is one of the difficulties I raised but one more that springs to mind is the intake sizing (valves and carb) is gonna be too big on a velocity point of view.

Quote:

Originally Posted by Diesel_Dave (Post 332021)

On another note, I've toyed with the idea of finding a motorcycle with a blown engine and putting a small diesel on it. Just a thought...

Engine swap is very difficult in France (homologation required)

Quote:

Originally Posted by renault_megane_dci (Post 332167)

Okay, I did some more digging and I think we just differed over semantics.

When I was referring to the Atkinson cycle, I was thinking of the "original" Atkison engine (1882) like this:
workaround ideas to discuss among friends: Atkinson cycle: a nice marketing strategy to fool the hybrid car buyers
that uses a different mechanical mechanism, rather than different valve timing.

In 1957, the so-called Miller cycle was introduced which accomplishes the same goal as the Atkinson cycle through IVC adjustment and, yes, sometimes includes a supercharger.

All production engines that I'm aware of that are called "Atkinson cycle" do not have the non-traditional mechanism, and use IVC adjustment.

So whether it's called Atkinson or Miller, we're talking about the same thing.

Sorry for the confusion.

Sorry about me insisting, I should have started with a proper explanation of what I had in mind.

Quote:

Originally Posted by Diesel_Dave (Post 332202)

When I was referring to the Atkinson cycle, I was thinking of the "original" Atkison engine (1882) like this:
workaround ideas to discuss among friends: Atkinson cycle: a nice marketing strategy to fool the hybrid car buyers
that uses a different mechanical mechanism, rather than different valve timing.

Yes, the original Atkinson cycle would not be very DIY friendly on whatever engine.

Anyway, the modified IVC does not seem to raise much interest from the ecomodded motorbike crew so I'll try and see if I can get more input from car modders ...

I, for one, am interested in the atkinson cycle. I was thinking copying the Geo Metro XFI cam profile, that is a type of atkinson cycle. It closes early instead of Late. I also like the Idea of adding a turbo to compensate for the power loss.

Keep us updated on what you find.
GIl

Quote:

Originally Posted by gil (Post 332847)

A supercharger is better to neutralize the backpressure generated at the intake manifold due to the valve closing delay. But then nothing prevents you to try a twincharger (turbo + supercharger).

Quote:

Originally Posted by gil (Post 332847)

Geo Metro XFI cam profile, that is a type of atkinson cycle. It closes early instead of Late. I also like the Idea of adding a turbo to compensate for the power loss.

Atkinson cycle is with an expansion ratio superior to the compression ratio.
It is not do-able with a shorter cam.

FE is doable with a shorter cam : you trade power for efficiency.

Making up a loss of power with a turbo is gonna also "compensate" the FE improvement unless the gearing is altered in such a way that friction improvement (less rpm) makes up for the extra power.

The best way towards FE is less weight / aero penalty hence less power required, power being fuel and air.

The atkinsoning of my engine would be mostly a way to reduce its power while improving its efficiency (I am sure you can imagine ways to reduce power without improving FE ...)

Anyway, I didn't find the Prius cam timing so far ...