Atkinson Engine???

An atkinson cycle engine is one that has the intake valves remain open for a portion of the compression stroke. I don't remember all of the big fancy details about the reasoning behind this, but the result is a lower compression ratio, while still maintaining a larger expansion ratio.

 

Over simplifying; the Atkinson engine permits the use of a 13 to 1 compression ratio without the need for the high test or premium fuel that would normally be required. Because of the valve closure delay pre-detonation is eliminated and greater fuel economy is achieved. Wonderful idea. Normally, the valve remaining open for a very brief period as in this case, would soon burn up, but it doesn't happen. Another advancement. Whatshisname. :idea:

 

Altho more efficient, IIRC Atkinson engines have lower low RPM torque than conventional engines. Luckily this is just where electric motors excel.

Brian

 

Atkinson wanted to improve on the internal combustion engine, and at the same time come up with a design that didn't infringe on the patents for the Otto engine (the standard four stroke engine). Atkinson came up with a design that had a different displacement for the intake stroke than for the power stroke. His idea was to make the exaust stroke longer to maximize the energy recovered. While improving efficiency, it did reduce the amount of torque available, and therefore, was not widely adopted. However, with the Synergy Drive, Toyota has coupled the highly efficient (but low torque) Atkinson engine, with the high torque of an electric motor (or two). An illustration of the ideal Atkinson engine can be found at:

http://www.keveney.com/Atkinson.html

Because of the complexity of mechanism to change the displacement of the intake and power strokes, a resonable compromise was achieved. A normal crankshaft is used, but the intake valve is left open during part of the compression stroke. This new stroke is called "backflow" because some of the air is pushed back into the intake manifold. The result is an engine with the efficiency of the one originally proposed by Atkinson, but without the mechanical complexity.

If you want to really get someone's attention, tell them that the Prius has a 5-stroke engine (intake, backflow, compression, power, and exhaust).

 

Have you ever visited Toyota THS-II site?
http://www.toyota.co.jp/en/tech/environmen...hs2/engine.html

Regards,
Ken

 

At low loads and therefore small throttle openings the pumping (filling the cylinders with air) losses are a somewhat high percentage of the net power produced on a typical Otto cycle engine. If you ever have the opportunity to look at the efficiency of an engine plotted as BSFC (Brake Specific Fuel Consumption, lbs fuel/hp-hr)VS RPM they'll sometimes plot a "family of curves" Each curve will represent a % throttle opening. The larger throttle openings typically have the lower BSFC values at any given RPM.

You could design a vehicle that had a very high throttle opening at say 65 mph to be better gas mileage but then that vehicle would have very poor acceleration. It would not have much more than the hp necessary to get it up to 65 mph, and nothing extra for acceleration. The Atkinson Cycle reduces pumping losses significantly and gets around this limitation somewhat. Due to it's reduction in torqe over the Otto cycle engine you either need a supercharger like Mazda did with the Millenium (then called Miller Cycle) or a large electric motor like the Prius to help out with lots of extra torque especially at slow speeds.

I remember talking to the guy the owned Crane Cams in the late 70's and he was experimenting with a cam on a big V-8 that he claimed was giving him 30+ mpg. But he was boosting compression ratios and using water injection to fine tune things and I don't think he ever really sorted things out. But I do remember that he was very negative about the innovation in Detroit at the time, lol. Some things never change do they?

 

True, but in addition I think if the intake valves closed near Bottom Dead Center the resulting compression ration would make it diesel. I estimate, but don't have the figures, that it would be around 16:1 compression. What you said about expansion ratio is true**. Getting rid if part of the compression stroke makes that possible. In addition, (given my recent trip to the top of Pike's Peak) I am theorizing that at higher altitudes the intake valves close sooner the keep the cylinder pressure more like sea level (around 150 psi maybe?) My Prime still performed beautifully all the way to 14,115'. I was expecting a pretty serious performance hit. Given it was all switchbacks up there I may just not have noticed, but the point is I did NOT notice any performance degradation. Perhaps someone knows the answer for sure, rather than just my surmise? I think the ceiling of most naturally aspirated* prop jobs is around 15K feet. *No turbo or supercharger **In most cars that lost power just goes to help warm the catalytic converter.

 

You have posted on a 13 year old thread. The likelihood is that none of the other participants are active As an example, wolfman has not posted in the last 5 years.

(batteries perform the same at 14,115 as sea level, so the prime shows less power loss due to elevation gain)

 

True, but in addition I think if the intake valves closed near Bottom Dead Center the resulting compression ration would make it diesel. I estimate, but don't have the figures, that it would be around 16:1 compression. What you said about expansion ratio is true**. Getting rid if part of the compression stroke makes that possible. In addition, (given my recent trip to the top of Pike's Peak) I am theorizing that at higher altitudes the intake valves close sooner the keep the cylinder pressure more like sea level (around 150 psi maybe?) My Prime still performed beautifully all the way to 14,115'. I was expecting a pretty serious performance hit. Given it was all switchbacks up there I may just not have noticed, but the point is I did NOT notice any performance degradation. Perhaps someone knows the answer for sure, rather than just my surmise? I think the ceiling of most naturally aspirated* prop jobs is around 15K feet. *No turbo or supercharger **In most cars that power just goes to help warm the catalytic converter.

Prius Prime Blizzard Pearl (for low heat gain)

 

With a conventional Otto cycle engine at the end of the power stroke when the exhaust valve opens there is still unused energy left in the exhaust gases - the Atkinson cycle extracts more energy by in essence having a longer power stroke than a normal engine.

The original Atkinson engine had a complicated, impractical arrangement of levers to accomplish this difference between compression and expansion (Atkinson cycle - Wikipedia).

All modern Atkinson cycle engines achieve the same effect but in a more practical manner using conventional arrangements of pistons and valves but with different timing.

In the case of the Prius the physical compression ratio of the engine is 13:1 which would be too high to use with regular gasoline. This is made to work by letting the cylinder fill with air-fuel mixture on the intake stroke but then leaving the inlet valve open so that about 1/3 of the mixture is pushed back into the inlet manifold. The inlet valve is then closed and the compression stroke completed as normal. (The Prius actually has two inlet and exhaust valves but that doesn't change the operation).

The effective compression ends up being about 9:1 while the power stroke is still the 13:1, extracting more energy from the combustion gases.

The torque is less than an engine of the same size would normally give as there is only about 2/3 of the air fuel mixture taken in.

As a result the Prius 1.8 litre engine only gives the same power and torque as a normal 1.3 liter engine - but it does that more efficiently as it can extract more energy from the gases.

kevin

 

... and why there are electric motors to fill in the power deficiency. The combined power gives an equivalent "feel" comparable to about 2.2 litres.