Hi Jeff - I can't find the listed compression online right now either. I know that the Prius is somewhere around 13:1 and the Ford Escape is around 12-12.5:1 - I thought I recalled reading a review of the Camry that it was 12:1 compared to the 9:1 CR of the standard Camry 2.4l engine..

Yes - the Atkinson Cycle component of the TCH uses the valve timing to bleed off compression back into the intake tract at lower RPMs. The Variable Valve timing can then cause the dynamic compression ratio to increase by closing the intake valve earlier..

 

Perhaps one of the tech types out there could help me understand the logic behind the knock sensor operation a little better?

I understand that as soon as knock is detected the computer backs off the ignition advance followed by if necessary enriching the fuel mixture.

How does this work in reverse?

I am guessing that the system must be continuously be trying to "lean out" the fuel mixture and advance the ignition until knocking reoccurs to take advantage of for example of a fill up with higher octane fuel. As knocking only occurs under heavy load this is presumably the only time that this readjustment could happen? Many car operating manuals these days that recommend premium fuel say that a regular grade can be used with reduced performance.

If my assumption is correct it would seem that all engines (especially Atkinson cycle type which could dynamically change the effective compression ratio) would take advantage of a higher octane fuel? I assume it is possible however for an engine that is rated for 87 octane fuel is limited by the manufacturer to what they deem that grade of fuel can handle.

Comments please!

 

Found this on another Prius chatsite:

Also:
Seems like to me the evidence is clear. Higher octane = not only a waste of money but possible maintenance issues.

 

True enough and relatively simple.

This is actually the most complex part of the ECU.
Please note that I have dramatically simplified the process in the attempt to make it easier to understand.

We need to start somewhere so we'll start with the Air / Fuel (AF) mixture.
When in Closed-Loop mode (reading all sensors - o2 in particular) the system will first off endeavor to maintain a Stoich AF ratio while reading exhaust temperature in addition to air charge temp and coolant temp.
Bringing this to the next level - the ECU reads the loading of the system by comparing barometric absolute pressure (BAP) to manifold absolute pressure (MAP) in addition to throttle position sensor (TPS) and current RPM. In the process of parsing all of this information, the ECU will richen or lean out the fuel mixture by changing two different parameters;
1) Injection pulse timing
2) Injection pulse duration (width)

(now we get to the ignition timing)

Like fuel mixture programming, the spark timing is controlled on multiple levels.
When at an idle and low/no load (BAP/MAP delta), the ignition will typically be advanced to relatively high value in the advance tables (an array of advance values in degrees - offset from TDC Compression on #1 cylinder) which gives the engine the best idle quality by giving the spark a much longer duration to get it's job done. The spark table is the basis (sort of like a road map) for adjusting the spark advance. The ECU will typically read the parameters from literally all of it's sensors and advance the timing to the value in the map and beyond. The Knock Sensor is the safety valve in the process. As the ECU does his job in adjusting the fuel mixture and spark advance - the Knock sensor is on a priority interupt to the CPU in the ECU itself. In other words, the ECU will typically just go on it's merry way manipulating the spark, fuel, turning on/off coolant fans etc until suddenly a detonation event interupts the process. When the ECU is interupted like this, the events start to unfold pretty much as you stated..

This is a sample spark table as used in the MegaSquirt project:


Absolutely...

It is possible that the ECU programming limits the maximum values to a greater degree than I have seen. What I have seen in the past is that the ECU can advance the timing from 2 to 3 degrees beyond the value in the spark advance table. If the programmer wanted to keep the maximum spark absolute - he would not let the system advance beyond the values on the table. The spark table is also typically used when the system is in open-loop mode and the ECU would not add any additional advance to the table values while in open loop.

I hope this is of some help..

Thanks!

 

Hi Larry,

I read the same post on hybridcars.com - all I can say is that this is the opinion of the individual that posted it and like my opinions posted here - are not corroborated or validated as known scientific fact. You can, however, corroborate either opinion / statements if you so chose.
My statements can be backed by reviewing the logic in engine control systems that are well documented on the www as well as your local public library.

In regard to the content of the owners manual - I believe that this is a "minimum" octane requirement.
Per page 371 of my 2007 Toyota Camry Hybrid manual
Thanks!

 

Randy:

Thanks very much for taking the time to answer my questions, I am really glad you gave me the "dramatically simplified" version...!! It's going to take me a while to digest, maybe once I've slept on it I will have one or two more questions to try your patience a little more!

Best regards, Paul

 

Randy,

On the manual saying "or higher"

All that means is that the car can RUN on higher octane if 87 is not available. It's not a recommendation or an invitation. If Toyota wanted the TCH owners to run premium fuel, they would have tuned the engine for that fuel and told us what octane to use.

Why do you guys keep on talking about it like higher octane is some kind of helpful option when I have posted several links saying it's wasteful and totally not helpful to the Atkinson cycle engine?

There are hundreds of thousands of Priuses on the road, using the same HSD system as the TCH. Don't you think that BY NOW if higher octane was a help for these engines that it would be common knowledge on websites like this one?

How many pieces of knowledgeable advice and science can you ignore?
How about this one, from the world-renowned experts at Car Talk:

Higher Octane is a waste of money

 

Hi Larry,

You quote an article that was written almost 10 years ago by a couple of guys that I have a lot of respect for. This particular question was in regard to the octane required to run a 1995 Buick with the 3.8L V6 engine. I'm actually pretty familiar with this engine. 9.4:1 compression and dual knock sensors. Nice mill that carried GM through some rough times. Still - it's not current day technology. The ECU of that 95 was a 16bit system that is almost prehistoric in comparison to today's technology with 32bit ECU's..

We should compare apples to apples here...

I can show you a hundred links that say that hydrazine and liquid oxygen are the only fuels that are capable of propelling a spacecraft to space.
However - it's a good thing that Burt Rutan and his engineers at Scaled Composites didn't take this as gospel.
Instead they used a combination of Nitrous Oxide and HTPB (rubber) to propel the world's first private spacecraft into space to win the X-Prize..

I won't spend anymore time trying to convince you or anyone else of something you just simply don't want to believe.
It's quite alright to disagree with me or anyone else on their opinion. That's your right.
Just remember, it's the person that thinks outside the box that helps drive progress.
If staying inside the box is your "bag" - enjoy...

Thank you..

 

Randy,

I have a few questions just for my curiosity.

1. What is your goal for trying higher octane fuels? Are you trying to get better mileage? Are you trying to get better economics or miles per dollar? Are you trying for better performance as in 0-60 times? Are you trying to get better reliability from the engine? The only one of those I can see as a benifit to hybrid owners would be the reliability, but even that should not be a big issue as I have not heard of any prius owner having problems with their engines. Better 0-60 times in a hybrid, just kinda sounds silly. If I wanted a corvette, I would have bought one. Better fuel mileage, if for the sole purpose of finding out if you can squeeze a couple more MPG out, then fine, but from an economics point it does not seem feasable.

2. Emissions - someone pointed out that premium might case worse pollution. Personally, I don't know but I have been told that premium has less energy density, and burns slower. You staed that you have personally dyno'd many engines and have not seen a difference in emissions when switching from regular to premium. How did you measure this? Did you get a fuel ratio reading, or the full boat of emmissions readings such as oxides of NOx, CO, CO2, etc...?

3 Can you run the experiment and let us know how it turns out?

 

I have no "goal" here in regard to trying higher octane fuels. re-read my posts and you'll see that I merely state the following;


I had also stated;

Indeed the sole purpose of this experiment on my part is to see if there is any increase in fuel economy when using the higher octane fuels.


The Dyno we use is a SuperFlow-902. In addition to the standard WinDyn metrics - we also have a 5-gas analyzer where we can measure and capture CO O2 NO NO2 NOx.

While I won't be pulling the engine and electronics out of my driver to test this on a Dyno, I will be collecting statistical information and will offer it to members of this forum for the asking.

Thank you..