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Wed Aug 26, 2009 4:04 pm

The Atkinson Cycle Engine works through complicated mechanics to make the intake stroke shorter than the power stroke. This provides for a lower compression ratio, a longer power stroke, and more thoroughly/efficiently burned fuel. Modern hybrid cars have adapted the Atkinson Cycle to work in a 4-stroke engine by leaving the intake valve open partway through the compression stroke. My understanding of our carburetted engines, this would cause vaporized fuel to spew into the engine compartment and cause a world of trouble.

I've heard of something called a "blow bottle", which sounds like it works with some valve trickery to give the excess air/fuel a place to go, but I've not heard of it outside of the one conversation. I've Googled it, and can't find quite what he was talking about. Anyone know what that guy was talking about?

The real question is what would happen if you were to grind the camshaft such that the intake valve closes 80% of the way through the intake stroke? This would have the effect of shortening the intake stroke as above, but without the hazards of gasoline mist throughout the engine compartment. My concern is that there might be enough suction to pull the valve open and negate the whole deal, or potentially even drain power from the engine as it tries to pull on the limited volume of air/fuel.

Thoughts? Thanks.

Wed Aug 26, 2009 5:45 pm

If you close the intake valve prematurely, you'll kill any power output you had because you'd be effectively choking the thing. Why not try a Miller cycle engine instead?

Wed Aug 26, 2009 7:18 pm

spectre6000 wrote: The Atkinson Cycle Engine works through complicated mechanics to make the intake stroke shorter than the power stroke.

It's not complicated mechanics. It's all cam timing.

Quote:
This provides for a lower compression ratio,

That's an understatement.

Quote:
a longer power stroke,

Relative to the compression stroke, which is where this strategy proposes it's merit: you've effectively shortened the compression stroke because the intake valve closes much later into the compression stroke than the conventional Otto cycle. By doing so, the engine spends less of this stroke compressing the air/fuel mixture, and therefore exerts less work to compress the air/fule mixture. The downside is that you now have less air/fuel to burn during the combustion stroke.

Quote:
and more thoroughly/efficiently burned fuel.

Can't say I necessarily buy this claim, at least not at the duty-cycle you'd be using your engine, which is way different than the duty cycle used by a hybrid car.

Quote:
Modern hybrid cars have adapted the Atkinson Cycle to work in a 4-stroke engine by leaving the intake valve open partway through the compression stroke.

All engines have the intake valve open partway through the compression stroke. The Atkinson cycle simply leaves the intake valve open way longer through the compression stroke than the Otto cycle.

Quote:
My understanding of our carburetted engines, this would cause vaporized fuel to spew into the engine compartment and cause a world of trouble.

Probably.

Quote:
I've heard of something called a "blow bottle", which sounds like it works with some valve trickery to give the excess air/fuel a place to go, but I've not heard of it outside of the one conversation. I've Googled it, and can't find quite what he was talking about. Anyone know what that guy was talking about?

Nope. Not even gonna trouble myself to investigate.

Quote:
The real question is what would happen if you were to grind the camshaft such that the intake valve closes 80% of the way through the intake stroke? This would have the effect of shortening the intake stroke as above, but without the hazards of gasoline mist throughout the engine compartment.

In theory, you can severely limit your volumetric efficiency by doing so, which may be similar to how the Atkinson cycle limits volumetric efficiency. And remember this point later on...the Atkinson cycle will make a large engine function like a small engine.

Quote:
My concern is that there might be enough suction to pull the valve open and negate the whole deal, or potentially even drain power from the engine as it tries to pull on the limited volume of air/fuel.

Shouldn't have an issue "sucking" the intake valve open; at least, probably nowhere close to the kind of pressure a turbocharger imposes on the backside of the intake valve when it's on its seat. You may lose a little energy going from cylinder vacuum at the end of the intake stroke to positive pressure partway through the compression stroke...probably not the deal-breaker.

However, this is the deal-breaker: you're effectively going to reduce a 2.3L engine to the capacity of a 1600 to make this work. Better yet, if you try this on a 1600, it will probably function like a 1L-1.2L engine. For example, the Prius has a 1.8L engine, 13:1 compression ratio (static), and it puts out 98 HP. That's a bit over 50 HP/liter. Granted, if you could pull 50 HP/liter out of a 1600, you'd be feeling pretty good. But hold your horses(power)...the Prius benefits from 4-valves per cylinder, variable valve-timing, and decades of engineering advances.

So, this technology that otherwise enables a Toyota 1.8L to achieve 100 HP/liter on the Otto cycle is now reduced to ~50 HP/liter on the Atkinson cycle. And, given the technology that allows our meager 1600 to put out 35 HP/liter on the Otto cycle, you may be reduced to less than 20 HP/liter on the Atkinson cycle. So, unless you want an anemic engine that is reminiscent of 1940's era VW performance, you'll have to build a larger displacement engine...which can turn this into an expensive and, likely, disappointing experiment.

I'm not saying it can't be done, but you're making a lot of compromises in the pursuit of "efficiency". The Prius and other hybrids are EFFICIENT...from their aerodynamics to their engine dynamics. Our cars are aero-bricks with dinosaur engine technology. What little efficiency you gain through this exercise may be overshadowed by the ratio of "effort you put into designing and building this engine" to "how little power you actually get out of it", which is not unlike the mechanics that underlie the Atkinson cycle.

If you've got the stones to pursue this, I'd be interested to see how it turns out.

Wed Aug 26, 2009 7:37 pm

Are you suggesting this for VW motors?
It is really kind of a silly thing to do unless the motor is designed for steady state operation and you want to get fuel economy when not under load.
But compression release is not a new thing. Many 2 stroke motorcycles have it to aid in starting and if you go overseas you will find it on old Mitsubishi 2 stroke cars. Some came with a manual valve that you could actuate from the dash that would reduce pressure on the compression stroke for greater fuel economy.

Wed Aug 26, 2009 8:24 pm

Stripped66 wrote: Spectre6000 wrote: The Atkinson Cycle Engine works through complicated mechanics to make the intake stroke shorter than the power stroke.
It's not complicated mechanics. It's all cam timing.
The Atkinson Cycle Engine had a lot of crazy stuff going on to physically alter the actual piston stroke. The modern application to our familiar piston engines is purely cam timing.

Stipped66 wrote: Spectre6000 wrote:
This provides for a lower compression ratio,

That's an understatement.

Could you elaborate?

Stripped66 wrote: Spectre6000 wrote:
a longer power stroke,

Relative to the compression stroke, which is where this strategy proposes it's merit: you've effectively shortened the compression stroke because the intake valve closes much later into the compression stroke than the conventional Otto cycle. By doing so, the engine spends less of this stroke compressing the air/fuel mixture, and therefore exerts less work to compress the air/fule mixture. The downside is that you now have less air/fuel to burn during the combustion stroke.

Right, the nugget of the theory.

Stripped66 wrote: Spectre6000 wrote:
and more thoroughly/efficiently burned fuel.

Can't say I necessarily buy this claim, at least not at the duty-cycle you'd be using your engine, which is way different than the duty cycle used by a hybrid car.

The idea is that the same air/fuel mixture has more space (and relative time) to expand into and burn more fully/efficiently. As is, there are quite a few unburned hydrocarbons that are effectively wasted energy. By burning the fuel more fully, you release more of its energy and realize efficiency.

Stripped66 wrote: Spectre6000 wrote:
Modern hybrid cars have adapted the Atkinson Cycle to work in a 4-stroke engine by leaving the intake valve open partway through the compression stroke.

All engines have the intake valve open partway through the compression stroke. The Atkinson cycle simply leaves the intake valve open way longer through the compression stroke than the Otto cycle.

Quote:
My understanding of our carburetted engines, this would cause vaporized fuel to spew into the engine compartment and cause a world of trouble.

Probably.
Is the blow back a concern then, or is there not enough in a regularly-timed engine to make it a concern? What percentage of the compression stroke would the intake valve remain open in a regularly tuned engine? I had figured keeping it open 20% of the way through the compression stroke which may or may not be enough to "spew fuel" into the engine compartment...

Stripped66 wrote: Spectre6000 wrote:
I've heard of something called a "blow bottle", which sounds like it works with some valve trickery to give the excess air/fuel a place to go, but I've not heard of it outside of the one conversation. I've Googled it, and can't find quite what he was talking about. Anyone know what that guy was talking about?

Nope. Not even gonna trouble myself to investigate.

Didn't think so...

Stripped66 wrote: Spectre6000 wrote:
The real question is what would happen if you were to grind the camshaft such that the intake valve closes 80% of the way through the intake stroke? This would have the effect of shortening the intake stroke as above, but without the hazards of gasoline mist throughout the engine compartment.

In theory, you can severely limit your volumetric efficiency by doing so, which may be similar to how the Atkinson cycle limits volumetric efficiency. And remember this point later on...the Atkinson cycle will make a large engine function like a small engine.

Quote:
My concern is that there might be enough suction to pull the valve open and negate the whole deal, or potentially even drain power from the engine as it tries to pull on the limited volume of air/fuel.

Shouldn't have an issue "sucking" the intake valve open; at least, probably nowhere close to the kind of pressure a turbocharger imposes on the backside of the intake valve when it's on its seat. You may lose a little energy going from cylinder vacuum at the end of the intake stroke to positive pressure partway through the compression stroke...probably not the deal-breaker.

However, this is the deal-breaker: you're effectively going to reduce a 2.3L engine to the capacity of a 1600 to make this work. Better yet, if you try this on a 1600, it will probably function like a 1L-1.2L engine.

The floor plan at this point is stoke bore, long stroke. This should help counter the volumetric differences. On the surface, a 20% increase in stroke combined with a 20% timing difference in regard to the intake valve timing would negate itself in terms of decreased performance characteristics due solely to displacement. The losses due to the "energy going from cylinder vacuum at the end of the intake stroke to positive pressure partway through the compression stroke" will still affect the end result, and I have a feeling I'm probably still missing a few things as well. If I can figure out the details, they can be addressed. Disappointing results come from expecting unrealistic outcomes (see Seneca "On Anger"); if I can decrease the number of unknowns I can adjust my approach or my expectations accordingly. The research and engineering has all already been done when it comes to this sort of thing, I just have to find who knows what and where to find it.

Stripped66 wrote:
If you've got the stones to pursue this, I'd be interested to see how it turns out.

I've got stones to spare, I just need dialog like this to help point them in the right direction. Whatever VW magazine it was that did the mileage motor article got almost 40 mpg in a naturally aspirated, carbureted Beetle by just throwing parts at it. I'm certain beyond the shadow of a doubt that I can get more than that with a more aerodynamic Ghia and some engineering direction.

Wed Aug 26, 2009 8:29 pm

Jimmy111 wrote: Are you suggesting this for VW motors?
Yup.
Quote:
Some came with a manual valve that you could actuate from the dash that would reduce pressure on the compression stroke for greater fuel economy.
Please explain.

Mon Oct 05, 2009 6:53 am

http://www.animatedengines.com/atkinson.shtml

The Atkinson cycle is a different means of connecting the crankshaft, the miller cycle is what you are referring to.

http://www.gizmohighway.com/autos/miller_engine.htm

The Miller cycle requires a turbo charger or a super charger so the volume of the air in the cylinder is the same as a normally aspirated Otto cycle engine of the same displacement after the excess is pushed back into the intake manifold.

The goal is to increase the efficiency of the crankshaft by reducing its load at the point when a normal Otto cycle is at its most inefficient, between 90deg btdc and 30deg btdc. By using forced induction and modified valve timing a miller cycle gains 15% efficiency above a comparable Otto cycle.

Most people say what a waste of a super charger/turbo charger, why spend the extra $1500 for 15% better fuel economy without any power gain when I can spend the $1500 for a more powerful engine instead.

So to answer your question: I believe it can be done on any normal Otto cycle engine (including the air cooled VW) with a special long duration intake valve cam and a super charger or a turbo charger. The question is if the 15% is worth it.

Mon Oct 05, 2009 7:35 am

I am building a twin plug engine based on the Atkinson principle now.. Its in the design stage..

More as it unfolds and as the Customer completes his particulars for the application.

Mon Oct 05, 2009 9:07 am

Jake, It's very validating to know that not only is someone else working on this idea, but someone with your level of knowledge, experience, and resources. I can't wait to see how it turns out.