I wasn't aware the compression ratio of 12.3 to 1 on the hybrids was done with differen't linkages/bores, etc. I understood the bore and stroke were the same... did I miss a post somewhere that explained differently?

 

The bore and stroke are the same on all the 2.3 engines. However, since the hybrid has much higher compression ratio, that means the compression chamber volume when the piston is at top dead center is much smaller. The simple way to do that is to shave material off the top of the block or the bottom of the head. However, with modern engines it isn't that simple because that doesn't leave enough room for valves, etc. or maintain an efficient combustion chamber shape.

My guess is that the hybrid has different pistons and a different cylinder head as a minimum. Beyond that, there are many things that could also need to be changed. When you reshape pistons that often changes the optimum location for the piston pin and requires a slightly longer or shorter rod. Rod design changes could require a crankshaft modification although that is less likely. It's also possible that there are some minor changes in the block since the hybrid has a completely different transmission etc.

With the computer control of machining equipment it is much easier to make small changes for a specific application than it used to be.

 

Wouldn't that change the displacement from 2.3L? It sounds like its reducing the cylinder length....

I'm completely confused on what they mean by compression ratio on an engine where the intake valve is left open during the compression stroke.

See what I THOUGHT (and apparently was wrong about) is that say you have the normal 2.3L and the compression is 4:1 (I'm not sure what it is to be honest). I'm an EE so correct the mistakes in the following please. I though that meant than when the piston was fully retracted (down) there was .575L of space above that piston in the cylinder (2.3L / 4cyl)... when the piston comes up it compresses 4:1 meaning the cylinder volume is now .144L (1/4 the original volume). The compression ratio is 4:1 because the volume was compressed the whole way.

In a 12 to 1 compression engine you'd have .048L at the end. If you used a different linkage and such....

But once you pop open that intake valve during the compression stroke.... How is it 12:1 anymore?... You only end up compressing when the valve finally closes and it would seem the real ratio would be the volume at time the valve closes until the cylinder fires.

Anyone have a tech manual or a parts list that can compare the parts for the cylinder head between the models and illuminate EEs like me?

 

TeeSter, shaving the head doesn't change the displacement because displacement measures the volume sweep of the piston as it moves up and down.

You're kinda right about the compression not being 12.3:1 when the intake valve finally closes. The MECHANICAL compression ratio is 12.3:1, but the effective compression ratio (the pressure of the air/fuel mixture) is probably very similar to that of the standard 2.3L engine.

-J

 

REALLY! Thats how they measure displacement? I guess that makes sense sort of it is "displacement" after all...not "total volume".

Learn something new all the time. Thanks for clearing that up.

 

I'm not an EE or an other type of engineer so please excuse my ignorance on this... but do I have it right?

While I understand that the stroke is the measure of the piston movement from the bottom of its travel (BDC) to the top of its travel (TDC). I can also see how in a normal engine that the intake valve(s) are closed at (or slightly after) the BDC point. Since this is the case the compression ratio should be near a 1:1 ratio to the bore, stroke and remaining combustion chamber volume.

But in the Atkinson cycle engine, the intake closes after the piston has started upward from BDC. Isn't the effect of this the same as having a shorter compression stroke (in a non-Atkinson engine) since compression doesn't start until later in the piston's movement? If so, doesn't this result in a compression ratio that is not a 1:1 ratio to bore and stroke?

If I'm correct on all of the above, what does the 12.3:1 compression ratio on the FEH 2.3L refer to? The standard compression ratio with the 1:1 ratio of bore and stroke or the effective ratio with the Atkinson's reduced effective stroke?


Different question: Can you use a compression gauge to measure the difference between these two motors since the reported bore and stroke are the same? Maybe sample all 4 cylinders (since there may be some variation between cylinders), get an average and then compare the overall average between the two motors?

 

There is the physical displacement, the displacement as-built, which is ( I assume ) the 2.3L in all the publications. This 2.3L should be the same in both the Hybrid, and regular engine. The 2.3L is how much air all 4 cyl. draw in combined.

Like stated before, some of that air is allowed to escape before the valves close.
Thus the effective compression ratio is lower than the physical dimentions otherwise indicate. In other words, 12.3:1 is physically possible, but more like 10:1 is actually used.

The term for this is "Dynamic Compression Ratio".

From Wikipedia:

"The calculated compression ratio, presumes that the cylinder is sealed at the bottom of the stroke (bottom dead center - BDC), and that the volume compressed is the actual volume.

However: intake valve closure (sealing the cylinder) always takes place after BDC, which causes some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. This "corrected" compression ratio is commonly called the "dynamic compression ratio".

This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio.

The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)

The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure."

Our Ford Hybrids are like that last sentence.

-John

 

Thanks, John.

So the 12.3:1 doesn't really tell us much about the FEH engine other than if it had a standard cam it would need high octane gas, a different tune and would probably make relatively big power numbers as compared to the standard Escape 2.3L. Instead with the Atkinson cam it's about the same or a little less DCR?

 

It's actually needed to make up for all the lost CP due to the valve timing... If not the engine would be about useless....

 

John and Sean add some good explanation. Just to complete the loop, the reason the Atkinson cycle is so much more fuel efficient is that it allows the cycle to exhibit what is called asymmetric compression. Since the piston is not compressing the mixture during the whole compression stroke but it is being pushed for the entire power stroke, the losses during the compression stroke are reduced. The only downfall is the loss of low speed torque that the electric motor makes up for.