Good cars with Atkins cycle mod'fd eng.s+Questions

Messages
702
Location
Portland, Oregon
Hi I'd like to know what some good cars are that have the Atkins-cycle modified Otto engine as well as a few questions about this engine. I'm amazed at the cleverness of simply changing the timing of the intake valve to change a regular gasoline engine (otto cycle) into a atkins cycle engine. It seems like a very minor change, and I'm wondering why this wasn't popularized decades and decades earlier. I believe Toyota was the first to mass market this technology, in their Prius hybrid cars. First, a brief three sentence review just for those who haven't wiki'ed modern Atkins-cycle modified gasoline engines: the idea is that far greater efficiencies can be achieved if you allow some intake air to backwash out of the cylinder back into the manifold early in the compression stroke, by leaving intake valves open just a bit longer at the very beginning of the compression stroke (right after the intake stroke). The effect is, that with less air taken in, the expansion/power stroke becomes longer than the effective intake stroke, and you get a lower pressure at the point where the power stroke is finished and the exhaust valves open--meaning that the waste energy at the end of the cycle is reduced. The method of reducing intake air (and corresponding fuel) with this valve timing method is much superior to the ubiquitous throttling off the intake air with the throttle plate, which was the method for about a century of automobile engines. In typical city driving, the engine spends so much time in minimal engine efficiency conditions where the throttle plate is choking the engine off air to near the point where the engine is doing zero net work (just overcoming friction to keep spinning). (Direct injection has improved idle condition consumption greatly, but my regular injection ~2l four-cylinder consumes almost a third a gallon per hour at 800 rpm idle--in driving conditions where I'm following traffic at constant speed with standby power, say at 2400 rpm, that's ~1 gallon per hour for producing no work! You're basically running somewhat of a vacuum pump not an angine; my manifold pressure is at about a third an atmosphere in idle conditions.) So, given this, why didn't this modification arrive decades earlier?? I hear one argument is that the horsepower is reduced, since less intake air+fuel per cycle is burned. However, wouldn't it be possible to make the engine be capable of switching on and off between otto and atkins cycle? It would seem you could use some type of adjustable cam to pick what ratio of ottoishness or atkinsishness are appropriate for the conditions, or alternatively, just some clutch system in the cams that would switch on and off between otto and atkins intake valve timing. Is this technology dependent on direct injection technology, or does it work well enough with standard fuel injection; i.e., is it a problem if you let fuel-air mix backwash rather than just air. For all model cars that I've browsed, the engine only seems to be available in hybrid models. Are there any non hybrid cars (or at least hybrid-lite) that use the Atkins cycle? Finally, what are your recc's for medium sized, or larger compact, models. The ford fusion seems pretty decent, as well as various toyotas, but I'm wondering what other brands are out there.
 
Messages
231
Location
Tucson, AZ
ueberooo, I'm guessing, given the variable valve technology that exists, and all manufacturers use, that it would be easy to adjust or delay the intake valve closing and "convert" an engine from Otto cycle to Atkins cycle. Now I would question whether all manufacturers are already doing it to some degree, and not publizing it, or whether there is a technical reason, whether it be engine response or increased emissions, for them not to calibrate their system this way, as clearly they could do it if they want to. I don't think things are as "cut and dry" as we sometimes make them out to be. I know for a fact that manufacturers retard the cam timing at light throttle to reduce emissions and increase part throttle fuel efficiency, which is counter to what I would have guessed. As the throttle opens, even at low rpms, the cam timing is advanced, only to be retarded again, later at high rpms. There is a lot going with engine design and tuning these days. Take care, Gary
 
Messages
3,566
Location
Somewhere
I think the Atkinson cycle, while more efficient creates less power than the Otto cycle. So you'd end up with a larger engine with around the same fuel economy. The reason they are used in Hybrids is because they are not the only powerplant - the electric motor helps out. That way the lower power is not noticed but the increased efficiency is.
 
Messages
17,344
Location
Silicon Valley
Originally Posted By: itguy08
I think the Atkinson cycle, while more efficient creates less power than the Otto cycle. So you'd end up with a larger engine with around the same fuel economy. The reason they are used in Hybrids is because they are not the only powerplant - the electric motor helps out. That way the lower power is not noticed but the increased efficiency is.
+1 You probably cut about 1/4 of the output if you run Atkinson vs Otto. I think Mazda's skyactiv is using Atkinson (or maybe partially Atkinson), but they can do that with DI and their 4-2-1 exhaust to compensate for the reduced power output.
 
Messages
11,369
Location
Florida, Cape Coral
The Mazda SkyActiv engine utilizes a modified Atkinson cycle and the output for their 2.5L is 184 HP (5800 rpm?) and 185 lb/ft torque at 3250 rpm. This output is as good or better than any 2.5 liter Otto cycle being presently used in a daily driver. Ed
 
Messages
15,863
Location
NE,Ohio
Quote:
the original Atkinson cycle piston engine allowed the intake, compression, power, and exhaust strokes of the four-stroke cycle to occur in a single turn of the crankshaft and was designed to avoid infringing certain patents covering Otto cycle engines.[1] Due to the unique crankshaft design of the Atkinson, its expansion ratio can differ from its compression ratio and, with a power stroke longer than its compression stroke, the engine can achieve greater thermal efficiency than a traditional piston engine. While Atkinson's original design is no more than a historical curiosity, many modern engines use unconventional valve timing to produce the effect of a shorter compression stroke/longer power stroke, thus realizing the fuel economy improvements the Atkinson cycle can provide.[2]
oh and the org. engines were odd too "opposed piston?" design
Quote:
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, the term "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.
seems pretty factual for an internet source
 
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Messages
415
Location
Slimy Mudhole
What you want is a MultiAir or a Valvetronic engine, these technologies go beyond anything the Atkinson cycle can offer. And that scuderi engine is just an investor bait, because that design is not feasible. They just take advantage of scientific illiterates that will invest money on things that they don't understand.
 
Messages
9,515
Location
Canuck living in California
Originally Posted By: OneEyeJack
I'm looking forward to the “Scuderi Split-Cycle Engine” that can use a compressed air regen system in place of heavier batteries.
Strap yourself in for a loooong ride. I found out the concept close to 10 years ago and it looks like it haven't moved forward an inch.
 
Messages
8,051
Location
Michigan
Originally Posted By: OneEyeJack
I'm looking forward to the “Scuderi Split-Cycle Engine” that can use a compressed air regen system in place of heavier batteries.
Don't hold your breath for the Scuderi engine.
 
Messages
8,051
Location
Michigan
Originally Posted By: Rand
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. seems pretty factual for an internet source
Except for the part about cylinder pressure at exhaust valve opening being equal to atmospheric pressure. I seriously doubt that. Even neglecting the fact that the engine still has an exhaust system that forces exhaust manifold pressure to be above atmospheric pressure, the gases in the cylinder would still be at ~60-80 psi (my SWAG) at exhaust valve opening.
 
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Messages
8,598
Location
Florida
I'm not sure how well this actually works. It seems like between 1995 and 2003, the Nissan Maxima and Mazda Millenia S/C had similar levels of power, and the Mazda didn't have an MPG improvement over the Nissan in real world testing. A Miller cycle engine is an Atkinson engine with a supercharger, so I am not sure how fair this example is.
 
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