Those who deal with engines, transmissions, fuel consumption optimization and the similar, may like to read the following “quote” from Mazda’s video / lecture at https://youtu.be/a82Wt53gu78
(the video in post #2358).
How is SkyActiv-X better?
Well, contrary to our intuition, in other words are trying to save the environment, we’re burning less gas, SkyActiv-X actually lets us have a better performance. The reason why it does that, is because of again compression ignition.
We’re plant work we’re anticipating on seeing about a 30% gain in torque
over our current generation 2-liter SkyActiv-G engines.
The reason why, is because compression ignition happens so fast, that all the pressure released at the same time pushes the piston down in a harder fashion; that force of the piston moving down literally translates to torque.
So, like a Diesel engine, using compression ignition we get a lot more torque gains especially at low rpm. Low rpm torque, especially on the street, is what increases drivability.
Everybody likes a big American V8, because it is very very drivable. Why? Because it has torque right off the bat, so this is the main benefit, performance benefit, of SkyActiv-X.
Is intently I do want to point out the car does is tuned for 87 octane, but it will run a whole gamut of fuels, all the way to 92 – 93 octane, just because of the SP CCI process and similar to our SkyActiv turbo engines the torque peak will move with the change in octane of gas and that just results in slightly different performance feel, but there’s no diminishing, there’s no reduction of torque.
Of course we expect to have great fuel economy as well.
In this graph
we see fuel economy for your engine geeks this is BSFC and then engine load. When you compare our old generation shared platform MZR engines to our current SkyActiv-G engines to the new 2-liter SkyActiv-G engines, you’ll see on average about a 20% gain in fuel economy.
That’s again of course to do the benefits of compression lean compression ignition.
More importantly if you at the profile of this graph it’s really broad and flat over this wide range of engine loads.
It doesn’t have just one little sweet spot that actually affects how real-world fuel economy gets better and that’s really important to us as a brand at Mazda.
Of course this just shows 2,000rpm.
If you want to get a bigger picture of how the car does over a wide range of operating conditions, we would have to put slices of different engine speeds on top of that and build out a 3D map.
So another way to look at that is let’s assign a color to each range of fuel efficiency, black up here, being really thirsty orange. Not so orange is actually very efficient, and we stack all the different slices for all the different engine speeds on top of that, and then we come up with this 3D map, rotate it on its side; here imagine this orange area as being the topmost layer of a very efficient; think of it as a cake each color represents it’s a layer stack them on top of each other, and you have a fuel economy cake for lack of a better term you have cakes of different shapes; the SkyActiv-G cake is a little shorter, a little peak here the SkyActiv-X is taller and broader.
Now the reason why this matters is that most engines design right now target this concept of downsized turbo charging right we use us less displacement which means less fuel while you’re just what were you’re just humming along but instant that you need torque or to accelerate; you have this very thirsty turbocharger that kicks in and then it uses a lot of fuel.
An engine like that it actually has a very peaky the fuel economy cake while it’s running on just as three cylinder there’s loes displacement it’s o its high peak, but the instant the turbo kicks in you fall off the cliffs on the side. That’s why most of these engines have to be coupled with a CVT transmission because it needs to be kept in the sweet pot right on its peak but as we know driving these things usually doesn’t result in a lot of driving pleasure for us.
So, SkyActiv-X allows us to not only make a taller cake but a broader cake. The yellow region now expands all the way out here; well the reason why I have to explain this strange concept of a fuel economy cake because I can’t call it an efficiency island which is more complicated to understand is at least to the next point and this is what you guys will really feel when you get into the cars and drive it right off the bat. OK.
Most transmissions are designed with fuel economy in mind.
We have to save gas by staying at a low rpm at highway speeds, right?.
So many factors just add a lot more gears onto it, so that at eighth gear, or ninth gear on the highway
you’re just sipping gas but the instant that you need to accelerate nothing happens
because you need to downshift because there’s no torque available in that gear.
SkyActiv-G also suffers from this problem not nearly as much but we had to set the final drive ratio so that it stayed on a higher level of the fuel economy cake/
Then let’s say, at 3,000rpm so our final drive ratio was set so that let’s say at 60mph we’re running at 2,000rpm instead of 3,000rpm which uses a lot more gas.
Now you might already figure this out on a SkyActiv cake we’re running at the same layer 2,000rpm at 3,000rpm suddenly we don’t have to compromise or this constrain to run low rpm as you guys all know our tpm typically means more power, better response.
So now we can gear SkyActiv-X car with the shorter final drive ratio so that we can run out 3,000rpm on the highway all day long with no fuel penalties, that means you have better drivability, you have the same great fuel economy. There’s no trade-off what\s that noise? Yeah um actually if you guys look at the cars there’s another entire realm of NVH packaging that we tool these cars to, in order to control the noise of both, compression ignition combustion and the higher rpm and all that if you guys have noticed driving some of out six generation as they’ve progressed they’ve gotten a lot quieter. We’ve taken NVH very seriously and we’ve also applied that and leaps and bounds above and beyond in the cementation cars and they will talk about that a lot more too.
So, that’s actually a great segue now we have a great, we have a fun driving car, you don’t have to match on the gas all the time so you actually become very smooth and controlled with your inputs. It’s very direct, fun.
End of “Quote”
The HCCI can be considered as a substantially more "constant volume combustion".
The combustion is "instantaneous"; it completes into a few crankshaft degrees (4-5 degrees at lower revs, 12-15 degree at higher revs).
All the fuel is burnt at high expansion ratios.
In comparison, the progressive combustion in the conventional spark ignition and compression ignition (Diesel) engines
ends up with a big percentage of the fuel being burnt at lower expansion ratios (in a spark ignition engine having 13:1 compression ratio, a good part of the fuel is burnt at a, say, 8:1, or even lower, expansion ratio (lower expansion ratio means lower BTE). The nominal compression ratio is not saying the whole truth. The "average expansion ratio" says a lot more for the efficiency of the engine.
In the above video the one series of playing cards represent the progressive combustion wherein there is a flame front (the falling playing card) separating the burnt gas (the already fallen playing cards) from the not yet burnt air-fuel mixture (the still standing playing cards).
The other series of playing cards represents the instantaneous HCCI combustion: when the pressure / temperature pass the threshold, all the air-fuel mixture burns simultaneously (all the playing cards fall simultaneously).
The HCCI fits with lean and extra lean air fuel mixtures.
The leaner the mixture, the lower the peak temperature in the cylinder (they call it: low temperature combustion) and the less the thermal loss to the cylinder walls.
Mazda's SkyActiv-X has to switch to conventional (progressive combustion) spark ignition at high revs because they cannot further delay the spark to successfully create the fire ball that triggers the HCCI combustion.
In comparison, the PatBam HCCI architecture (two-stage combustion, divided combustion chamber, no spark plug at all), is expected to run on "constant volume combustion" all the way: from the lower revs to the red line revs.