CX-5 Turbo 87 vs 91 octane

erhayes

Contributor
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2022CX5 PP
I'm interested to understand how MAZDA achieves engine control when switching to 87 and 91 octane fuel. The first thing that comes to mind is that they relies on knock sensors but, that doesn't seem to be reasonable to me. Does anyone know how MAZDA does this and other engines like Acura RDX doesn't? Ed
 
I'm interested to understand how MAZDA achieves engine control when switching to 87 and 91 octane fuel. The first thing that comes to mind is that they relies on knock sensors but, that doesn't seem to be reasonable to me. Does anyone know how MAZDA does this and other engines like Acura RDX doesn't? Ed
2.5T is getting more power over 4,000 rpm with 93 octane premium fuel. Don't have any technical info from Mazda, but we can assume the PCM will advance more timing over 4,000 rpm with 93-octane than 87-octane to get more horse power. Knock sensor would still be doing its job to retard the timing whenever it's necessary.

Here's the torque curve:

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source: https://www.thetruthaboutcars.com/2018/12/2019-mazda-cx-5-turbo-first-drive-your-italian-alternative/
 
I'm interested to understand how MAZDA achieves engine control when switching to 87 and 91 octane fuel. The first thing that comes to mind is that they relies on knock sensors but, that doesn't seem to be reasonable to me. Does anyone know how MAZDA does this and other engines like Acura RDX doesn't? Ed
Yes knock sensors, they operate at a very high rate.

https://www.youtube.com/watch?v=rXaZIsVD17A

https://www.mazda.com/en/innovation/technology/skyactiv/skyactiv-g/
 
2.5T is getting more power over 4,000 rpm with 93 octane premium fuel. Don't have any technical info from Mazda, but we can assume the PCM will advance more timing over 4,000 rpm with 93-octane than 87-octane to get more horse power. Knock sensor would still be doing its job to retard the timing whenever it's necessary.

But how is the car sensing what the octane in the fuel tank to know whether to advance the timing or not over 4000?
 
But how is the car sensing what the octane in the fuel tank to know whether to advance the timing or not over 4000?
Because the ECU is continuously adjusting the timing to the verge of knock. It does whatever the mix of fuel in the tank allows it to do.

at 4000 rpm, each individual cylinder is firing 16.67 times in just one second of time. The knock sensors have a sampling rate that is much higher than that...
 
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But how is the car sensing what the octane in the fuel tank to know whether to advance the timing or not over 4000?

It's not sensing the octane, it's sensing the result of the combustion.

It may not be advancing at all, but just retarding when sensing knock. Just a guess.
 
So will the 2.5T produce the 250HP if using 91 octane? We don't have 93 octane here.

Not likely, I imagine it would fall somewhere in-between the 87 and 93 horsepower numbers.

However you can buy some octane boost additive at the auto parts store and add to your fuel.
 
I'd like to add that back in 2014 (my CX5), there was a discussion concerning feed back concerning the spark plugs and how important it was to use the "MAZDA" spark plug because of feed back properties. I am wondering if the control might be by sensing combustion through feed-back from the spark plugs rather than 100% from know sensors? Ed
 
There it is folks.

But the question is still out there: how does it know when to change timing for higher octane fuel?

If it adjusts the timing for 87 octane and I start running 93 in it, what specifically tells it to adjust the timing? I can see how it would sense knock and then retard timing, but what specifically tells it to adjust timing for the current high-octane fuel?

The only thing that sort of makes sense is that the engine (at some rate of frequency) changes the timing to induce knock, and then adjusts it until the knock stops...and it does this periodically.
 
I'm interested to understand how MAZDA achieves engine control when switching to 87 and 91 octane fuel. The first thing that comes to mind is that they relies on knock sensors but, that doesn't seem to be reasonable to me. Does anyone know how MAZDA does this and other engines like Acura RDX doesn't? Ed

They told me the O2 sensors detect octane levels.

That sounds bunk to me, but that's what Mazda told me when I called Mazda corporate.
 
I'm interested to understand how MAZDA achieves engine control when switching to 87 and 91 octane fuel. The first thing that comes to mind is that they relies on knock sensors but, that doesn't seem to be reasonable to me. Does anyone know how MAZDA does this and other engines like Acura RDX doesn't? Ed

If you read the manual, every turbo 2.0 and 2.5 out there will have reduced performance on 87 octane. In the WRX, it adds a full second to the 0-60 nearly. Mazda is just one of the few that quantifies it in the manual.
 
They told me the O2 sensors detect octane levels.

That sounds bunk to me, but that's what Mazda told me when I called Mazda corporate.

I just went on the web to see if there's a way to test octane levels in gas. The first You Tube was a good ol' boy who put some in a small cup and sipped it! He must be related to the guy at Mazda corporate. ;)
 
They told me the O2 sensors detect octane levels.

That sounds bunk to me, but that's what Mazda told me when I called Mazda corporate.

Totally bunk. No car knows what octane you put in it specifically. It monitors combustion, as you already know to determine what adjustments to make.
 
Totally bunk. No car knows what octane you put in it specifically. It monitors combustion, as you already know to determine what adjustments to make.

Yep. Mazda corporate is literally worthless. I asked them a ton of stuff, ranging from simple to complex. They fell apart on "simple". "Complex" got "we are not engineering and no, you cannot speak to them as they are in Japan."
 
As mentioned, there's no sensor or switch in the car that says "ooh, he just gave it the good stuff, we got 250HP now"

The power adjustment is mainly a result of timing adjustments. With higher octane fuel, the engine can run more aggressive timing without knocking. Knocking is very bad so this sensor talks to the computer and the computer lets the engine push it as far as it can go without damaging itself. If the knock sensor says everything is good it will throw timing at it until it gets to the point where it says "whoah, that's all we can do" and it stops. As computing power has increased car computers are quicker than they used to be and if the tuning and programming has been put into the car the computers can change the engine tune to make better use of available qualities of fuel. This allows the driver to decide what they want or need out of the engine by what they put into the tank.

Where traditional vehicles are rated for 1 recommended AKI rating, anything above that is a waste as the computer isn't capable of adjusting the engine's performance to take advantage of the additional capabilities of the fuel itself. The engine will just see everything is good and let it run at it's low octane tune even if high octane is in it.

They told me the O2 sensors detect octane levels.

That sounds bunk to me, but that's what Mazda told me when I called Mazda corporate.

There are typically 2 x 02 sensors (minimum) in cars these days: a front one and a rear one. There may be a set of these on each bank if you're running a V or H configuration engine, one pair for left bank and one pair for right bank. Typically only 1 pair for an inline engine.

The front O2 sensor is the one that helps control the engine. It measures the gases exiting the combustion chamber before it hits the catalytic converter. The measured gas mixture tells the computer the air / fuel mixture ratio: it tells the computer if the car is running lean, rich, or optimal. If it's lean it gives the engine more fuel to bring the mixture to optimal. If it's rich it will pull fuel to bring the mixture down to optimal. It doesn't sense the fuel type.

To keep it simple, the knock sensor controls spark timing and the O2 sensor controls the air/fuel mixture. The cam and crank position sensors tells the computer if the valves and crank shaft are in the correct positions in relation to each other to accept a spark / combustion. If the valves and pistons aren't in the right spot the computer will cut the spark or fuel to avoid the engine damaging itself.

The rear O2 sensor typically doesn't control the engine. It simply measures the exhaust gases after the catalytic converter and compared them to the readings from the 1st O2 sensor to ensure that your catalytic converter is doing it's job
 
This is one way that MAZDA may be controlling combustion when using 87 or 91 octane fuel. I did read that MAZDA was researching this type of feed back using the spark plugs. Ed
ABSTRACT The cycle-to-cycle and cylinder-to-cylinder variations that occur in a spark ignited engine create the opportunity for monitoring combustion in real time to provide useful benefits for engine control. Reduction of variation and operation of the engine at closer-to-optimum conditions is possible if real time feedback of the combustion process is available. An in-cylinder pressure sensor with pressure-based control algorithms is one method of monitoring the combustion process. However, such a solution presents new challenges of an additional cylinder penetration location, sensor packaging and added cost. A substitute for the in-cylinder pressure sensor is a device which measures the flame conductivity, commonly known as an ionization current sensor. It can be integrated with the spark plug in the case of SI engines, or with the glow plug in the case of compression ignition engines. This paper presents a methodology of data acquisition and processing which leads to the extraction, in real time, of the following parameters critical for closed loop engine control: combustion phasing, knock detection and combustion stability. The algorithms utilize a combination of various methods including crank-angle-aligned data extraction, digital filtering of the ion sense signal and neural-network-based ion signal interpretation. A Delphi Ion Sense Development Controller (ISDC) is used for high-speed data acquisition and subsequent real time combustion parameter computation. Our paper demonstrates key features and reports the performance of ionization-current-based combustion sensing using engine data. Key combustion parameters, generated by the ISDC on a combustion event basis, are sent to the engine controller via a Controller Area Network (CAN) for closed loop individual-cylinder combustion control.
 
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