Octane Rating

[hotrabbitsoup]

There is no program in the world that will increase the compression ratio.

Yeah, I know what you mean. Sounds like a noobish thing to think and i know enough to know that typically cr is limited mechanically however I thought I read somewhere where the skyactiv design is a variable cr design. Apparently this is possible from what little I can read on the internet about ice engine design. It employs a clever trick but I can't find the link where I spied that. It could all be BS.

Does anyone know how the Europe/Asian engine differs from the US engine? What's different, they both share the same displacement so if not for some clever mechanical trick to achieve variable compression what does Mazda do differently for the different markets? Possible different heads? I love my cx, I think, but 5-10 more HP and torque would seal the deal.

 

[Jcanracer]

Lemme see if I can help you out.
1. USDM compression ratio is 13:1 and is tuned for 87 octane because the market Mazda is going after here is concerned with fuel economy and keeping refueling costs low.
2. JDM and Euro markets get the 14:1 compression ratio (different pistons) and is tuned for 91 octane or better. This is for better performance and is treated as a bit more up-scale market because the economy-minded market overseas is already saturated by turbo diesels which yield better torque and fuel economy.

3. Perhaps you are confusing variable valve timing (possible) for variable compression ratio (not possible)?
Compression Ratio is fixed because it is determined by the piston (and piston to head clearance) which is fixed, see image:

 

[jaman_ca]

This is a bit tricky and can be confusing. A couple points to remember:

1. The total combustion volume is not included in the displacement calculation of an engine. That is, two engines with identical displacement of 2000 cc can have different combustion volumes. The displacement doesn't take into account the shape of the pistons, it's just based on bore and stroke.

2. Compression ratio on the other hand is all about the combustion volume. So, calculate the largest volume (taking into account the shape of the pistons and head) and calculate the smallest volume and take their ratio. Where it gets tricky is that this all assumes that the valves are closed the entire time. When an engine is operating, the valves are obviously opening/closing all the time. This means that the "true" or "transient" compression ratio is not necessarily exactly 13:1 all the time (lookup "dynamic compression ratio"). For example, if the intake valves are open during the compression cycle, then some of the air/fuel that would other wise be compressed is let out and the real compression ratio is actually less than the static 13:1. In other words, valve timing can have an appreciable impact on the compression ratio the engine experiences and we can't just go by the 13:1 static ratio.

 

[CX-SV]

Yes, the static CR is 13:1 in the US, 14:1 in Europe, it is what it is.

Is the Atkinson cycle effect on CX-5 done via valve timing?

 

[EricF]

Yes, the static CR is 13:1 in the US, 14:1 in Europe, it is what it is.

Is the Atkinson cycle effect on CX-5 done via valve timing?

Not 100%, but I'd be willing to bet the CX5 is set up like the hybrids that use the Atkinson style engine where the valve timing replaces the parts in the older style engines. I'd be interested to find out if mazda had to tweak anything to make it work with the higher compression ratio or not.

 

[cucamelsmd15]

2. Compression ratio on the other hand is all about the combustion volume. So, calculate the largest volume (taking into account the shape of the pistons and head) and calculate the smallest volume and take their ratio. Where it gets tricky is that this all assumes that the valves are closed the entire time. When an engine is operating, the valves are obviously opening/closing all the time. This means that the "true" or "transient" compression ratio is not necessarily exactly 13:1 all the time (lookup "dynamic compression ratio"). For example, if the intake valves are open during the compression cycle, then some of the air/fuel that would other wise be compressed is let out and the real compression ratio is actually less than the static 13:1. In other words, valve timing can have an appreciable impact on the compression ratio the engine experiences and we can't just go by the 13:1 static ratio.

I shall award you partial credit for your answer. (cheers) The only difference is that fuel injection is timed so that the valves are closed in Direct Injection systems. In port injection systems, the valves obviously have to be open. DI systems run at a much higher pressure and flow, but a shorter duration than common rail port injection systems, so the DI system pulse is actually much shorter than what you would commonly find on a port injection system.

 

[GAXIBM]

I have read that the variable intake valve timing is used under partial load so that only the air needed for the power demand is in the cylinder when the intake valve closes (late) so that you only compress the air and inject the fuel needed for the power demand. This allows you to control engine speed with the intake valve so the throttle can be open and you do not have a pumping against vacuume loss on the intake stroke. You get an effective longer expansion stroke as the effective compression stroke is shortened. Some fuel is injected early to cool the air for easier compression and less knock, the rest is injected at the completion of the compression stroke and fired.

 

[EricF]

For example, if the intake valves are open during the compression cycle

Why would the intake valve be open during the compression cycle? There wouldn't be anything to compress, as everything in the cylinder would be shoved back into the intake, like when the exhaust cycle comes up.

 

[Weeboom]

Hi all!

I'm a happy owner of a CX-5. I live in the states at the moment but I'm planning returning home to europe maybe after one year. I'm not really a techy car person and I'm wondering that will I ruin my car motor if I start using E95 or 98E10? At the moment I use 87. So, should I sell my car before moving?

Thanks guys!

 

[GAXIBM]

The only thing left in the cylinder is the amount of air you need for the current power demand (how hard your press the gas pedal). This happens only on light loads, under full load intake valve closes at end of intake stroke instead of staying open to let out the air you don't need when under light loads.

 

[jaman_ca]

Why would the intake valve be open during the compression cycle? There wouldn't be anything to compress, as everything in the cylinder would be shoved back into the intake, like when the exhaust cycle comes up.

We're talking milliseconds here, obviously it's not open the whole time.

 

[V8toilet]

Hi!

I purchased my 2013 CX-5 back in beginning of May. After the initial tank of gas that the dealer filled for me, I have been putting in two tanks of 91 from Chevron. But the last fill up i decided to put in 94. I know the manual says put in 87 octane and up with no more than 10% ethanol. At local gas stations in BC, Canada, only 94 is ethanol free. I thought because of the high compression of the skyactiv-g would perhaps benefit from the 94. Since putting in 91 and 94, I have noticed the exhaust note seems to have more of a "growl" to it compared to the 87 that the dealer put in. Just wondering have other people tried it as well or just the "mind trick" that my mind is playing with me(wow)

Thanks![/QUO

You may have noticed a difference not because the octane was higher but because the 94 didn't have ethanol in it. Most gas engines run better on 100% gasoline verses the 90/10 crap and also get more then 3% better mileage too. Ethanol has less BTU in it and is hygroscopic too.

 

[davborre]

Interesting thread.....what kind of mileage do you think we would get running 104+ Octane fuel(thought)

Obviously at some point you will have diminishing returns

 

[leedos]

I have read that the variable intake valve timing is used under partial load so that only the air needed for the power demand is in the cylinder when the intake valve closes (late) so that you only compress the air and inject the fuel needed for the power demand. This allows you to control engine speed with the intake valve so the throttle can be open and you do not have a pumping against vacuume loss on the intake stroke. You get an effective longer expansion stroke as the effective compression stroke is shortened. Some fuel is injected early to cool the air for easier compression and less knock, the rest is injected at the completion of the compression stroke and fired.

Here's a link that describes this:
http://www.sae.org/mags/aei/NEWS/10335

 

[CX-SV]

I see the "mind tricks" are over and most have reverted to using the correct octane fuel as recommended per Mazda CX-5 owners manual.

 

[CX-5 Rat]

I see the "mind tricks" are over and most have reverted to using the correct octane fuel as recommended per Mazda CX-5 owners manual.

These aren't the droids you're looking for.

 

[CX-5um]

I've been on 89 octane for the past 3 tanks now. Checking for any decrease in performance from 87.

I'm satisfied with the engine's performance in particular to driving with 2 other adults with AC full blast in the city. I suspect the computer eventually adjusted to the octane change plus I'm using quality gas (Quick Trip)t.

Recently filled up from QT again but this time with 87 octane. 2 adults full blast AC city driving. I can't tell a difference in performance from 89.

My conclusion for my application which is city driving is to use quality gas (Chevron, Shell, 76, QT, ect) and just select 87 octane. If you drive hard (redline a lot) then maybe yes maybe 91 octane would help performance wise but not fuel-saving wise due to driving style.

 

[CX-SV]

I'm consistently using only Chevron regular (6K miles), good economy and reasonable performance with no pinging. I redline the engine only occassionaly.

 

[EricF]

using a higher octane does nothing for performance and is a waste of money if your car doesn't need it. You're better off using quality gas than higher octane.

 

[leedos]

I'm averaging 27.5 nearly all city using 91 octane since new. I just cracked 2700 miles on my car and I can say it feels well broken in at this point. My fuel economy has also stabilized. I just filled up with 87 octane from QT to see what would happen. Maybe it's just me, my driving, my climate, the brand of gas, or the fact that I have a manual transmission, but I did notice a decrease in performance. Engine oil may make a minor difference, so note that I changed my oil at 2000 miles using Mobil 1 0w-20.

That said I don't think doing a back to back 87 vs 91 octane comparison is vaild. The ECU adjusts fueling and ignition timing to your car and climate. This correction process takes at least two full warmup cycles with varied RPMS's and load. If you change fuel grades, your engine tuning will be "off". After burning about 1/4 tank of 87, the engine seems to have adjusted itself to the different gas and performance has come back some, but not all the way. I'll go run 1 or 2 more full tanks of 87 from Chevron to see if performance returns. The 91 octane only costs me an extra $2-$3 per fillup. For even for a minor perfomance increase, l consider it money well spent with the way I drive. I'm actually quite pleased with my fuel economy so saving money is not really the issue for me. I use WOT to get up to speed on a regular basis, I refuse to impede traffic in the slightest degree.

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