A more "ideal" Atkinson cycle engine, 4 cylinder engine, would have a "native" compression ratio of, say, 25:1 and then moderate that using delayed intake valve closing to as low as 10:1. 25:1 for low power cruising and 10:1 for WOT, putting a FULL CHARGE in each cylinder.

 

Because the intake valve is open during part of the compression stroke, there is no constant compression ratio. The maximum would be 12.3 to 1 if the intake valve closed when the piston was at bottom dead center. The higher the RPM, there is less time for the fuel mixture to escape back into the intake manifold and the higher the compression ratio becomes. This is why at low RPM's, the FEH has very little torque. At low RPM's the compression ratio gets as low as 4.0 to 1 and why ethanol with less BTU's drops MPG so much. The pinging I'm getting at 6,000 RPM's at high speeds on the freeway is because of the loss of BTU's with E-10 and the higher compression ratio. In this case, higher ethanol percentages might prevent the pinging, but would reduce power even more.

Wayne Gerdes posted the specs on the compression ratio awhile back:
http://www.cleanmpg.com/forums/showthread.php?t=353

GaryG

 

"The higher the RPM, there is less time for the fuel mixture to escape...."

Hmmmm....

And here all this while I've been thinking that the inertia of a mass doesn't change except due to volume and motion.

If the mixture is lower getting out of the cylinder at high RPM wouldn't it be just as slow getting in, given the same time limit..??

 

"At low RPM's the compression ratio gets as low as 4.0 to 1...??

Methinks you are confusing physical compression ratio with EFFECTIVE compression ratio and rather than low RPM's my guess would be you meant to say low throttle openings.

If the throttle is WIDE OPEN, even at low RPM's, the cylinder can intake a FULL CHARGE of A/F mixture and thus the "effective" ratio will be close to equal the actual cylinder ratio.

The "effective" compression ratio only gets as low as 4:1 with a partially open throttle wherein the cylinder is mostly "filled" with a vacuum to begin with.

 

WW, read before you reply please.

I was comparing the feh to the otto cycle, not saying it has one....

 

not a gasoline engine on pump gas tho...

 

Wrong. You are conveniently forgetting an important parameter called volumetric efficiency. The curve for VE usually closely follows the torque curve. At low RPM it is usually quite low, meaning substantially less charge is being ingested relative to what is theoretically possible, given by displacement*RPM/2. This is due to various things including the low kinetic energy of the charge and the pulse effect of a 4 cylinder engine. For an Atkinsonized engine the VE is much worse at low RPM than for an Otto. Hence, the effective DCR becomes very low.

 

The only modern DFI engine that uses "pump gas" and doesn't have a ~12:1 compression ratio is the CX-7 (9.8:1) and it makes up for the difference via turbo BOOST.

 

Not "conveniently" forgeting anything.

At low engine RPM, say 1000RPM, and WOT (Wide Open Throttle) then there is virtually NOTHING preventing a FULL A/F mixture charge from entering each cylinder. The rise in HP/torque as RPM rises is the result of more "firings" per second until a point is reached wherein there is not enough intake valve opening time for a full charge to enter and then the HP/Torque numbers begin to decline.

Now, on the other hand, if you hold the throttle partially closed thereby artificially restricting volumetric efficiency, limiting an engine under load to say, 1000RPM, then your statement would be correct.

 

Look Rick, I got first place with E-10 in the MPG Challenge. That was all there was available to the owner Debbie Katz vehicle I drove. I would have made a 70 plus MPG with straight gas. It's not a lot of work and I talked and played music the whole time with Debbie Katz.

With my '09 FEHL, I will set new records on this crap of E-10 in my conditions.

GaryG