This might be where I saw it, but I have a feeling that it was somewhere else too. In the edmunds.com "2007 Hybrid Sedan Comparison Test" dated 2007-07-22:
http://www.edmunds.com/apps/vdpconta...5/pageNumber=1
the last paragraph contains the statement:
"Details like an Atkinson-cycle gasoline engine (a distinction that helps the Camry nip the Altima in fuel economy) and packaging designed to accommodate the hybrid batteries with minimal intrusion into the interior speak volumes for Toyota's commitment to hybrids."
I interpret this to mean that the Altima Hybrid does not have an Atkinson-cycle ICE.

Stan

 

Yeah, I saw that too. And I would read it the same way, but it is not as clear and authoritative as I would like.

The thing that makes me think that the Altima does have an Atkinson cycle engine is that both the Altima and the Camry have similar "tuning" differences relative to their conventional 4 cylinder brethren. If the Altima's engine is not retuned to the Atkinson cycle, I don't understand why it is rated at 158hp instead of the 175hp of the baseline Altima.

Of course, just because I don't know what else they might have done, that doesn't mean that my guess is correct. Nissan's website is remarkably mum on such technical details.

 

I'd check the Altima service manual. It would likely have information on the engine valve timing.

The TCH uses modified (from the std. ICE) valve timing to give the "atkinson" effect. The Prius uses both modified timing and an offset crankshaft!

Sorry, but Edmunds is the last place I'd trust for information.

 

The manual says it's an in-line 4 cylinder but then makes a point saying it fires 1,3,4,2. Is that Atkinson or Otto?

 

The firing order makes no difference in the "cycle" of the engine. A gas engine is most efficient when the "compression" stroke is 7 - 8 to one and the "expansion" (power) stroke is in the 13 - 14 to 1 range

A standard "Otto" engine has equal length compression and power stokes. In most engines this ranges from 8.0 to 1 to 10.5 to 1 compression and expansion ratios.

The true "Atkinson" cycle engine increases the length of the power stroke in relation to the compression stroke. A modern adaptation of this is the "Miller" method to achieve this variation. Although still called an "Atkinson" cycle, the static compression of the engine is increased to 13 to 1 or so and the intake valve is kept open later in the compression cycle.

This "bleeds off" some of the charge to be compressed giving the engine an effective 8.5 to 1 (roughly) compression ratio and a 13 to 1 (again roughly) expansion ratio. The engine will not produce as much power using this method (less air-fuel in the cylinder) but it is very efficient.

 

Look at page EG-3 (attached, from the TCH "New Car Features Guide"), which compares the intake and exhaust valve timings and opening angles for both the "Otto" and "Atkinson" versions of the Camry's ICE. (By the way, the "Atkinson" version also has an offset crankshaft, just like the Prius's.) In the hybrid version, the intake valves stay open for fully 280 degrees, compared with the 248-degree opening angle in the non-hybrid version. (In both versions, the exhaust valves stay open for 228 degrees.) Note the stated 12.5:1 "compression ratio" for the "Atkinson" engine. That's not true, of course; because of the delayed intake-valve closing, its effective compression ratio is actually ~10:1, just like the "Otto" version. They should really say "expansion ratio" not compression ratio in my opinion. Correspondingly, the "Atkinson" engine is effectively (for power purposes) not really a 2.4-liter displacement engine, but proportionately less, because it breathes in less fuel owing to the delayed intake-valve closing. The 2.4-L rating gives a misleading indication of its power.

Stan

 

Stan,

Where do you see reference to an offset crankshaft? I never remember seeing mention of that in the Camry info that I have.

 

n8kwx — Look closely at the lower diagram on page EG-2. You'll see the slight offset of the crankshaft towards the left, especially if you print the page and view the diagram at an angle. I'll have to look around to see if I can find an explicit reference to this offset. I was pretty sure that I had seen it somewhere.

Stan

 

n8kwx — I've now looked through everything I have, but can't find any explicit reference to a crankshaft offset in the Camry Hybrid. That diagram from the "New Car Features Guide" persuades me, however. It's also interesting that the non-hybrid "New Car Features Guide" shows a corresponding diagram for the non-hybrid's 2.4-L 2AZ-FE engine, and it also shows a similar crankshaft offset (no, it's not the same diagram!). I conclude that both Toyota engines (and perhaps all their new engines?) are using an offset as a means of improving fuel efficiency slightly by reducing frictional losses during their power stroke.

Stan

 

Stan,

Thanks for the double check. I was going through my service manual .pdfs last night and didn't see anything either.

There was even a chart detailing all of the differences between the hybrid 2.4 and the "regular" 2.4. It mentioned the valve timing changes and a few other things but nothing regarding the crank.

It may be correct that there is a bit of an offset, but I think it's the same as the regular 2.4 based on:

Toyota made all kinds of noise in the Prius literature regarding the offset crankshaft.
It would save manufacturing and logistics costs to keeps mods to a minimum.
Toyota has simplified some other things since the Prius with the TCH.

Marc