Can anyone with a technical background out there explain why the Camry hybrid gasoline engine has a very high compression ratio (12.5:1) and still only needs regular grade gasoline? Normally engines with a high compression ratio require high octane (premium) grade fuel to avoid engine knock. Most engines these days do however I know have a knock detector to retard the ignition if a lower grade fuel is used to avoid engine knock damage. Retarding the ignition of course reduces the engine performance. This begs the question as to whether using premium fuel would increase the horsepower output of the engine. This of course would only happen if the engine ignition was advanced by the computer to take advantage of the premium fuel. I've also noticed that the apparently similar engine in the regular four-cylinder Camry has a lower compression ratio (9.8:1) and again wonder why.

 

The engine in the TCH is a derivative of the Atkinson Cycle engine. Well - That's what Toyota calls it. What I think it really is - is either a modified Atkinson Cycle or a Miller Cycle engine minus the supercharger.

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

http://modifiedatkinsoncycleengine.blogspot.com/

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

In the Modified Atkinson / Miller Cycle engine, the intake valve is held open during part of the compression stroke. At lower RPM, this release of part of the intake charge reduces the effective compression of the engine. At higher RPM or under boost, the volume of lost intake charge is reduced and the effective compression ratio is raised considerably.
The higher compression is only a detriment in the lower RPM band under high loading. In the upper RPM where the loading is lighter, the detonation (or early detonation) can be controlled via Knock Sensor, Timing and fuel calibration.

Doncha just love engines??? I do...

 

Randy, that was an excellent answer. I was looking at the same websites as you when you posted (no, really...!).

Some of these pages discuss the use of variable valve timing, which apparently helps overcome the lower torque of Atkinson cycle engines at low RPM vs. the torque of Otto cycle (i.e., normal 4-stroke) engines. Does the VVT regime in the ECU take the knock sensor signal as an input?

 

Thank you Randy, that was a terrific explanation and has opened up a whole new world for me regarding the IC engine.

Regards, Paul

 

Paulgw and Big-Foot:

Your question and answer raise another interesting and related issue: Which does Toyota use to rate its Atkinson/Miller engines' displacement -- the intake stroke volume or the power stroke volume? The former is smaller than the latter because of the delayed intake valve closing, and it really ought to be the volume used for rating purposes, since the fuel consumed depends upon the intake stroke, not upon the longer power stroke (which is what gives the increased efficiency). I'd bet that it's the (longer) actual mechanical stroke volume (equal to the power stroke volume) that's actually used. Does anyone know the correct answer?

Stan

 

Thanks folks..

SPL - To answer your question about Displacement - Displacement calculations are done irrespective of camshaft profile / valve timing - even though effective displacement can be impacted by radical profiles such as the Atkinson / Miller design.
If you think about the displacement of an engine that is supercharged or Turbo-Charged, you'd know that the actual displacement changes with RPM as the power adder forces more and more air into the engine.

------

The formula for calculating displacement for one cylinder is:

Cubic Inches = Bore x Bore x Stroke x 0.7854

(Cubic Inches, convert from liters to CI by multiplying by 61.02, ex. 2.0 liters * 61.02 = 122 CI)


--------

I hope this helps!

 

I was reading this link and while it sounds interesting, I also got the feeling that it was reading in between the lines as "hybrids are crap, my idea is better so why hasn't anyone licensed my patent and made the world a better place (with me at the top of a money pile)".

 

Randy,

Does the crankshaft on an Atkinson/Miller cycle engine look like one in an Otto engine?

Check me on this. On an Otto cycle engine, stroke is the distance the piston moves from TDC to BDC. It is the same distance for each of the four cycles. I think this is the same distance as the measurement from the centerline of the main journals to the centerline of the rod journals. Right?

But on an Atkinson cycle engine, at least the BDC position varies with each of the four strokes. Does this come from moving the entire crankshaft on an eccentric (would seem inefficient) or by building the eccentrics for each piston into the crankshaft? Or something completely different?

 

Hi Bill,

I have to say that I was exceptionally disappointed when I learned that the Atkinson-Cycle engines used by both Toyota and Ford (Escape Hybrid) were not true Atkinson-Cycle in that they still have 4 distinct cycles and it takes two full revolutions of the crankshaft to complete one power cycle.
The True Atkinson Cycle engine takes 1 full revolution to accomplish all 4 strokes via it's linkage and intermediate shaft (a marvelous design).

The Engines we are dealing with here are (for all intents and purposes) standard Otto Cycle 4 stroke engines that have a special valve timing mechanism (VVT). I truly have no idea at this point how either Toyota or Ford can claim Atkinson-Cycle engine..

Atkinson:


Otto:

 

Randy,

So what we have in the TCH is a standard IC engine that we all are used to, modified through valve timing to create an expansion ratio that is higher than the compression ratio. Got it!

According to wikipedia, the hallmark of the Atkinson cycle is not necessarily that all four strokes occur in one rev of the crankshaft, but that the expansion ratio must be higher than the compression ratio:
"The Atkinson cycle may also refer to a four stroke piston engine in which the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. This reduces the effective compression ratio and, when combined with an increased stroke and/or reduced combustion chamber volume, allows the expansion ratio to exceed the compression ratio while retaining a normal compression pressure. This is desirable for improved fuel economy because the compression ratio in a spark ignition engine is limited by the octane rating of the fuel used. A high expansion ratio delivers a longer power stroke, allowing more expansion of the combustion gases and reducing the amount of heat wasted in the exhaust. This makes for a more efficient engine."