I think its 12:1 from the stuff I've seen. I think this is because its an atkinson cycle. From what I understand its the ratio of the volume of the cylinder with the piston retracted over the volume with it extended. Now in a standard Otto engine cycle, the fuel is injected with the piston retracted the valves are closed and the fuel air mixture is compressed 12x. That requires high octane fuel or the mixture will ingnite before the stroke is complete due to heat from compression.

HOWEVER... the FEH atkinson cycle pops the valves OPEN during part of the compression cycle. This is how they use a standard Otto engine linkage and still have a longer power stroke than a compression stroke. IN this case the fuel air mixture IS NOT compressed 12x. Some of it is pushed back into the manifold from what I've read and the presssure in the cylinder will not increase during that part of the stroke... lower pressure, less heat, no pre-ignition... no need for high octane fuel.

The advantage if I understand things correctly is that you get the full energy of the power stoke out and waste less of the energy compressing the fuel on the compression stroke more than is necessary. With less compression I guess you get a lot less of an OOMPH during the explosion so that might explain the lower torque at low RPM.

Now... I'm an electrical engineer.... I'm not an ICE expert. All info herein is from what I've read online, from asking my brothers and this forum. If there is an ICE expert out there that can fix any errors... I'll take no offense.

 

The compression ratio is normally the minimum cylinder volume (Piston up) divided by the maximum cylinder volume (Piston down). Our engines allow some of the fuel mixture drawn into the cylinder to be pushed back out. In this case, the compression ratio is the minimum cylinder volume (piston up) divided by the cylinder volume at the time the intake valve closes.


The real issue is head pressure before combustion. A higher compression ration causes a higher pressure before combustion. As does the following:

Altitude
Humidity
turbo/superchargers
high efficiency air intakes (K&N, etc.)

 

Wrong, compression ratio has always been maximum volume/ minimum volume. Our ICE has a compression ratio of approximately 12.7 to 1. This would normally cause to much compression and would result in preignition (pinging) but they hold the intake valve open MUCH longer to allow a large percentage of the intake charge to renter the intake to reduce the compression not the compression ratio. This results in a longer burn time and a large delta volume (like a diesel) To under stand this concept you must understand that gasoline has a much larger expansion rate than most ICEs can efficiently use. This is why allot of the fuel is turned into unused heat. If one could give the gasoline a larger delta volume more of the energy would be used to generate power. (again like a diesel) BTW this might be the reason E30 seems to work out the same as pure gasoline. This results in about 15% better fuel burn BUT lower horsepower esp at low rpm because of the reversion back into the intake disrupts the air flow.

 

Mark, is the volume you refer to in your comment the cylinder volume or the gas volume? If it is the gas volume, I agree with you. If it is the cylinder volume, compression ratio is very misleading when the valve timing is radically different than Otto Cycle engines.

Imagine the intake being left open for 90% of the compression stroke. Only 10% of the cylinder volume would be compressed into the combustion chamber!

 

You are confusing compression with compression ratio. They are different.

http://en.wikipedia.org/wiki/Compression_ratio

 

At: http://www.fordvehicles.com/suvs/escape/features/specs/

The plain I4 Escape has a 9.7:1 compression ratio.
The plain V6 Escape has a 10:1 compression ratio.

The FEH has a 12.3:1 compression ratio.

The FEH is an extra "low compression" engine.
The FEH does not knock under heavy acceleration with 85 octane, which is what I use whenever I can find it, and I use it at sea level sometimes.
( I fill up in the Mtns. and drive to Los Angeles, for Example )

Additional Info:

While the physical dimentions of the FEH may indicate a 12.3:1 ratio, due to the Atkinsen valve timing, the actual compression realized is something lower.

 

Wouldn't they hold the INTAKE valve open longer, not the exhaust? No one wants to be burping up fuel-air mixtures into the exhaust manifold... I'm guessing thats a mistype.

 

Found these two images from an old Ford Archive.
Maybe this will help???





I think the cam on the left is "normal" and the one on the right is FEH.

 

The 12.7 or 12.3 number for the FEH is the expansion ratio. For compression, there are both static compression ratio (SCR) and dynamic compression ratio (DCR). The DCR for the Miller/Atkinson is low compared to the expansion ratio. The SCR is fixed and determined by the chamber volume and swept volume of the cylinder. The DCR is additionally dependent on cam timing and air flow. A muscle car engine will also leave the intake open during the compression stroke, but at high rpm and airflow the charge will continue to pack instead of being pushed back into the intake like on a Miller/Atkinson. That is how a tuned port system can achieve 100% or even higher volumetric efficiency at the airflow it is tuned for.

 

Brain fade, intake is correct.

the one on the right has higher lift but shorter duration than the one on the left. Its really the cam timing not so much the lift. Most ICEs close the intake valve just after bottom dead center of the piston, Miller engines hold it much past BDC. Adkinson engines really use a strange arrangement to actually vary the piston stroke.