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Active member
Sep 5, 2005
So, many modern car engines use this cycle to run more efficient and get good mileage. I suppose they use variable cam timing for this.
I was wondering, could you setup the cam timing on a conventional NA engine like an 8valve redblock to do the same?
Sure, it will have less power but what if that wouldn't bother you?
Is the single cam a deal breaker? If you change cam timing on a single cam you change intake and exhaust at the same time. Does the engine have to have a twin cam setup to use pseudo-Atkinson-cycle?
(i have a Jag XJ6 that has a twin cam head. maybe that engine would be a better candidate?)

This cycle is used in a Prius engine. It also has about a 14:1 compression ratio. I suppose that is needed to take advantage of the PAC?
So i suppose i would have to raise the CR of the XK engine.

Any thoughts? or insights?

142 guy

May 31, 2014
Saskatchewan, Canada
The Atkinson cycle is about improving thermal efficiency. If you are willing to accept some pretty crude generalizations, they get that by having a low dynamic compression ratio (time from intake valve close to firing) and a high expansion ratio (time from firing to the exhaust valve opens). To get the high expansion ratio, you would have to alter the engines static compression ratio. That may not be so simple because how the volume expands is a function of the rod length and what is good for an Otto cycle may not be ideal for an Atkinson cycle.

You can diddle with cam timing which delays intake valve timing (lowers compression ratio ?) and delays exhaust opening (increased expansion ratio). Sort of like an Atkinson cycle? You can find lots of references discussing the impact of cam timing. However, those discussions are with fixed cam profiles. I expect that the lobe center separation on a cam meant for an Otto cycle engine is not going to work very well or achieve the objectives of the Atkinson cycle. I am guessing that the lobe centers on an Atkinson cycle are probably far apart compared to an Otto cycle cam profile.

I am not particularly familiar with the Prius engine. I am sort of familiar with the Audi class 1 (B cycle) engine
B cycle is Audi's name for their version of a sort of Miller like cycle (got to avoid those patent infringements) which is an Atkinson like cycle which uses forced induction to to deal with the poor torque characteristics of a pure Atkinson cycle engine.

Most of these modern Atkinson-like / high expansion cycle engines use separate intake and exhaust valve control and alter the phasing of the cams to move between Otto like and Atkinson like combustion cycles to switch between power delivery and fuel economy. The Prius can probably happily live as a pure Atkinson cycle engine designed for efficiency because it has a relatively big ass electric motor to make up on the power side. The Audi B cycle and similar non hybrid power train engines have to waffle back and forth between Otto like and Atkinson like cycles.

Your XK engine might allow you to alter the cam timing to increase the lobe centers and delay intake closing and exhaust opening. However, when you fiddle your cams to get the lobe separation you may find that your valve timing events are all screwy. Depending on the duration of the lobes and how much you separate the centers you might start to get overlap in the wrong place. So, in addition to increasing the mechanical compression ratio to get a higher expansion ratio and timing the cams to move the lobe centers you probably need different durations on the cams. I have no intuitive feel for what you might need to do with lift and then you have the whole question of whether your rod length is optimum for an Atkinson cycle. If you could figure all that out, you would still be stuck permanently in Atkinson cycle mode so you would take a permanent power hit.

Interesting idea; but, likely to be a relatively expensive experiment with the need to fabricate special cam profiles.
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