This is a neat site for those interested in different types of ICEs - it has animations and descriptions of various engines, including the standard "Otto" engine and the "Atkinson" - which I know is used in the FEH. (Do any other hybrids use it?) It helped me understand why/how the Atkinson engine is more fuel efficient (a picture's worth a thousand words). (It also has the Wankel engine, two-stroke - I never knew how those things worked - and various steam engines.)

www.keveney.com/Engines.html

 

Hi Sweetbeet:

___Both the FEH and MM HEV’s use the Atkinson intake as well as the Prius II. The HH and RXh’s supposedly do not use an Atkinson intake cycle but when you look up their V6’s output, it is less then their non-hybrid Highlander and RX 330 - V6 counterparts. Toyota did something to detune those V6’s for lesser HP and if it is not Atkinsonized (Toyota says it is not), then it is something else?

___Good Luck

___Wayne R. Gerdes

 

Well none of them use an atkinson mechanism as illistrated on that page. On that website, it's clear to see they use multiple leverarms to get a physically longer expansion stroke than the compression stroke. Modern engines such as the Prius and Escape hybrid, the "miller cycle" engine from Mazda, and the low-speed operation of the 2006 Civic non-hybrid achieve an atkinson cycle by way of holding the intake valve open during part of the compression stroke.

Because the valve is open, intake mixture blows back into the intake manifold, which means it's not being compressed. If you close the intake valve after the piston is 30% of the way up the cylinder, then you've effectively shortened the intake length by that amount. Like I mentioned, the Mazda Millenia did this along with a supercharger to achieve easy 4-cylinder fuel economy with the power of a small V6 thanks to the supercharger.

The non-hybrid 1.8 liter 2006 Civic does it by means of using VTEC to have a cam lobe where the intake valves are left open into the intake stroke at low speed/load, but at higher RPM and when more power is needed it switches to a conventional cam profile suitable for high RPM power. This is obviously way cheaper than Mazda's solution of strapping on a thousand dollar supercharger to fill in the high RPM power.

 

The Miller Cycle evolved out of WWII and theories surrounding piston based aircraft engines and the effects of supercharging them. Miller theorized that you could balance the high RPM benefits of the Otto cycle that is used in the vast majority of vehicles today with the efficiencies of the Atkinson cycle by putting positive pressure on the intake by way of a supercharger. Efficiency at low RPM (reduced pumping loss, etc.) yet power at high RPM. This was especially important to aircraft because large wings that were launched for bombing runs could consume huge quantities of fuel on the ground waiting for take off and getting into formation. And yet the Atkinson cycle didn't perform well at high RPM, which is essential in aircraft because they run engines at high RPM for fixed durations at cruise. But with a different engine cycle and the addition of supercharges (which were already on the highest flying and fastest of allied aircraft by the middle of the way) both efficiency and power could be achieved.

The Millennia is a true Miller cycle engine, Atkinson cycle plus supercharger. However without the positive pressure placed on intake by a supercharger you do not have a Miller cycle engine, it is only Atkinson cycle.

The animations at http://www.keveney.com/Engines.html are only there to represent how the cycle works. On the page dedicated to the Atkinson, that is really a demonstration of one of the earliest Atkinson cycle concepts and the author has notes indicating an error in the illustration regarding piston travel at different parts of the combustion cycle. Atkinsonized engines today do this through variable valve timing and by blowing back intake gases into the manifold, as AZCivic mentioned earlier.

 

The disadvantage of the Atkinson cycle is HORRIBLY low efficiency at low throttle openings. The Atkinson cycle engine is only really efficient with WOT, or nearly so, when the cylinders recieve a FULL charge of A/F mixture.

Extending the combustion cycle does no good with a low charge in the cylinder as the explosive force will be expended, "exhausted" long before the piston reaches BDC.

Aircraft engines are either operating at WOT or near 75% power in cruise mode so the Atkinson cycle makes more sense for aircraft engines.

What is really needed is a 4 cylinder engine that has a "native" compression ratio of, say, 20:1 and varies that ratio, using (widely) variable intake valve closing delay, as a function of the desired charge level to be left in the combustion chamber. At WOT the effective compression ratio would drop to ~12.8:1 with a DFI engine.