SkyActiv Diesel: The Technology of Diesels

[Sassimac]

Hello All...

Now, never having owned a diesel prior to my CX5, I have to admit to being completely unfamiliar with diesel technology. I am quite OK with four stroke theory, and turbo technology. My old CX7 was tuned by myself and ran extremely well.

This being the case, I have installed my old Dashhawk into my CX5, to get an idea on things like Load, Throttle Position, Boost Pressures etc. But in order to make some degree of sense with the readings, I need to understand better diesel technology.

For example, when idling, a petrol engine may sit around 12inHg, but my CX5 Diesel sits at 3inHg. This seems rather odd initially. I am not sure if it is a scaling issue with the Dashhawk interpreting the OBD data in relation to atmospheric pressure. Or possibly the fact that as I believe, diesels engines do not really use throttle plates to alter power. In the case there is no throttle plate, I can understand the idle "vacuum" would be very close to zero. Remember, that what we see as "boost pressure" is relative to atmospheric pressure.

The other issue I am trying to understand is the boost pressure specification for the motor. Unless I am going decidedly mad, I could swear I saw over 24PSI boost..!! In fact, the engine spent most of it's time in boost. Again, I'm not sure if there is a scaling issue relating to atomspheroic pressure. As in, deduct ~14.7...

Looking at the CX5 Diesel manual, there is a "Shutter Intake Valve." On first glance, it maybe typical a throttle plate assembly. But somehow I don't think it is.

Any ideas here...????

 

[V8toilet]

Most diesels do not have a throttle plate and so they do not regulate the air input into the engine like in a gas engine. They operate wide open at all RPM. Gas engines try to keep the air fuel ratio at stoichiometric (14.7 to 1) by regulating the air and fuel at the same time and go rich (12-1) under load. Diesels regulate the RPM by varying the fuel ratio wildly from 15-1 to over 100-1. The turbo for a diesel I'm not sure about but since diesels operate at very low RPM I suspect that the turbocharger generates boost right off idle for this reason.

 

[Sassimac]

Hi There...

Thanks for that.

I perfectly understand AFR's for petrol engines. I spent about 4 years playing with tuning my CX7. Only fun part about that was the software I used was in Lambda..!!!

Not having a throttle plate makes the readings more plausible. The MAP sensor would always be exposed to atmospheric pressure, therefore the Dashhawk would likely display ~zero. Which it does.

That being the case, I can focus more on understanding what boost pressures are developed. As it is a twin turbo, I would agree that it is possible to be in low boost most of the time. And when high load is required, slip in the big turbo. And it is actually fairly sizeable..!!!

Maybe I did see 24PSI........

Now I need to understand the relationship between diesel fuel and oxygen.

 

[Sassimac]

Hello All...

Yes, it's really interesting to watch the data logger in real time.

The boost does hit 22-24PSI under load..!!!!

 

[Sassimac]

Hello all...

I just came across a link that states:

"But despite their extraordinarily wide-ranging torque and impressive performance, SKYACTIV-D engines are exceptionally clean and economical diesels. In fact, both versions meet Euro 6 emissions criteria (almost three years before they take effect) without pricey exhaust aftertreatment systems. The secret is their ultra-low compression ratio, lightweight design and variable twin turbocharger (providing maximum boost pressure of 2.7 bar), to name just a few of the clever concepts that went into these engines."

Ok...

So, 2.7 Bar is 39PSI. Now, I doubt that the Dashhawk would show me 39PSI, but it does show 24PSI. If you consider the MAP (Manifold Absolute Pressure) sensor does see 39PSI, but in relative (to atmosphere) terms, this would be 24-25PSI. Bear in mind, you need to deduct 14.7PSI to allow for atmospheric pressure.

Starting to make sense now.

 

[V8toilet]

Makes sense to me too. I know that the low (for a diesel) compression ratio of Mazda's Skyactiv diesel helps reduce NOx emissions. NOx emissions are a result of high combustion temperatures and high compression greatly contributes to that. Those after treatment systems reduce diesel efficiency and so MPG.

 

[Sassimac]

Hello All...

More observations.

It's interesting to watch the Relative Throttle readings. As discussed earlier, the suggestion is that diesels do not really require throttle plates. However, there is a "Shutter Intake Valve." At idle or very low loads, the reading is around 10-20%. Vacuum reads about 0.4inHg. So, a slight vacuum. When driving normally, this reading leaps to 82% and pretty much stays there. So, this does seem to add credence to the theory that the engine really acts as expected. Throttle always wide open.

Hopefully I can find the PID for fuel pressure.

 

[onebklyn]

Interesting reading.... I am interested in this topic because i am looking at purchasing gauges for my CX-5 and need to find the right gauge limit for my boost.
onebklyn

 

[HyFlyer]

Are you adding physical gauges or an app on a phone or tablet such as torque or Forscan?

I use Forscan with my 2019 North American diesel. Great app. Inexpensive powerful tool.

What are you trying to accomplish?

 

[onebklyn]

Physical gauges… I’m into modifications so I want to create a gauge pod for three gauges. Boost pressure, water temp, oil temp. I’m been using the OcTech app but want to mod my CX-5

 

[HyFlyer]

@onebklyn I can pull up a pid for boost pressures on mine and record it I'd you like (if you're just looking for limits/normal range)

If I recall, it shows absolute pressure (including atmospheric)