Compression, turbos, and efficiency

[abogart]

Originally Posted by cleanspeed1

Old Mechanic brought it up first, here's an article on the Mazda Sky Activ engine (s).


Mazda SKYACTIV-G 1.3 Engine Details | Motorward

The gasser is direct injected.

Yes, it's direct injected. But it is still spark-ignited, and therefore subject to the limits of how lean a mixture can actually be ignited by a spark. Although, if it is truly the lack of a homogeneous mixture - such as Old_Mechanic mentioned - which is the limiting factor, wouldn't spark-ignited engines running on gaseous fuels (CNG, propane, etc.) be able to run very lean mixtures without compromising ignition?

Also, I notice that they market the engine as having 14.0:1 compression. Is this geometric compression ratio derived from engine design, or actual maximum WOT compression of the intake charge? If the variable valve timing allows the intake valve to remain open during some of the compression stroke (not sure if this happens at WOT), the compression ratio would be lower due to reduction of the amount of cylinder area compressed. Sometimes I think that automakers are just developing extensive new technologies to compensate for the flaws in one type of engine, while basically ignoring that another type is just more efficient.

I'm not trying to trash the new, innovative engine; I'm simply being skeptical and objective.

Originally Posted by mort

Hi abogart,

Excellent post, but a clarification:

I don't know if you mean 75% of full throttle at high rpm or wot at 75% of red line or something else. It's very likely that the engine is most efficient near wot and very near the rpm for peak torque.

-mort

Sorry about that Mort!

Allow me to clarify... My definition of "load" was the manifold pressure (observed MAP as in. Hg on Scangauge) relative to atmospheric pressure (standard 29.9 in Hg). Regardless of throttle position, this most closely represents the percentage of maximum engine power being used at a given RPM. I rarely use more than 35% throttle (as observed TPS on SG) at any time. Under certain conditions (1400 RPM, TC locked), 29" MAP (100% load) can be obtained with as little at 25% throttle in my car.

Through actual experimentation, I found that accelerating at 22" MAP from 2000 to 2500 RPM gives less of a drop in average MPG than accelerating at any higher throttle setting in the same RPM range. I initially derived the 75% load value by figuring 22 / 29. However, I just realized that this would be inaccurate because the MAP does not go all the way to 0 at closed throttle, but closer to 7", depending on RPM. So I guess that I could reduce the scale to 22 and figure 15 / 22 which would be about 68% load... But I think there is some math involved here which is way beyond my current level of understanding .

Anyway, I found 22" to be the "sweet spot" which yields the best MPG for the way that I drive. Note that the car does have an automatic transmission (doh!) and these numbers may be construed by torque converter losses or any number of other factors. Not to mention that I really cannot do any RPM vs. throttle experimentation outside what the PCM will allow . You might very well be right though Mort, the auto trans. really complicates things.

[dcb]

Originally Posted by mort

Hi abogart,

Excellent post, but a clarification:



I don't know if you mean 75% of full throttle at high rpm or wot at 75% of red line or something else. It's very likely that the engine is most efficient near wot and very near the rpm for peak torque.

-mort

A reasonable explanation why WOT is not more efficient at making power than %75 throttle on a gasser may be that the cars computer goes into enrichment mode at WOT (the obd port MAY indicate open loop operation under these conditions as well).

[abogart]

Originally Posted by dcb

A reasonable explanation why WOT is not more efficient at making power than %75 throttle on a gasser may be that the cars computer goes into enrichment mode at WOT (the obd port MAY indicate open loop operation under these conditions as well).

I have observed this using Scangauge. Not sure of the actual TPS value, but it was definitely somewhere between 75% and 100% throttle opening. Loop goes to OPEN and AFR (air/fuel ratio) drops to 12.5 from 14.7.

Another reason that higher throttle settings may not necessarily be more efficient is fuel's resistance to detonation. I have observed while running regular 87 E10 fuel that the engine's knock sensor will detect detonation and start retarding ignition timing under as low as 23" Hg manifold pressure in certain cases.

Check out this thread for more on this subject.

[Dean in Milwaukee]

Higher compression adds mechanical advantage to the power stroke, independant of other also desirable factors.
Consider that an engines effective power stroke is mostly done barely 1/3 way down the stroke, and think about it this way:

Imagine a small firecracker being used to push a cannon ball out of a cannon. Would you get the best push on the cannon ball if the ball was right down on tight on top of the firecracker, ( high compression)or if it was held a bit away, giving some volume around the firecracker? ( lower compression)

Having a fixed and limited amount of firecracker produced gas available means that with more chamber volume the peak pressure realized will be reduced, reducing energy transfered to the cannonball even though you used the exact same firecracker, and the cannonball will go less far.

Hope this helped.

Dean in Milwaukee

[abogart]

Originally Posted by Dean in Milwaukee

Higher compression adds mechanical advantage to the power stroke, independant of other also desirable factors.

That's a nice analogy, Dean .

But isn't the added mechanical advantage of higher compression counteracted by increased energy loss during the compression stroke? It seems to me that if it takes more energy to compress the fuel/air charge further, but you get more energy out of it, shouldn't the two factors just cancel each other out?

[Dean in Milwaukee]

"But isn't the added mechanical advantage of higher compression counteracted by increased energy loss during the compression stroke? It seems to me that if it takes more energy to compress the fuel/air charge further, but you get more energy out of it, shouldn't the two factors just cancel each other out?"


Air is a perfect spring. 100% of the energy used to compress it is directly returned when it reexpands, at least thats how it would be in a perfect world.

The reality is that higher compression creates more piston friction, and part of the heat of compression and therefore its energy is lost to the cylinder. If this was'nt true, youd'e have a perpetual motion machine.

The way it works out though, a large % of the energy needed for compression is in fact simply returned to the piston on the downward stroke, and the extra friction and heat losses from bumping compression from say 9:1 to 11:1 is a fairly small increase, and far offset by the extra mechanical advantage gained by the increase.

This is especially true when an engine is throttled way back in cruise operation. The net effective cylinder pressure is reduced by the throttling, hurting system efficiency. An engine can actually handle much more compression at low throttle settings without knocking, but has compression ratio's chosen to handle heavy throttle application to optimize max output.

I've often wondered what sort of mpg results you could get by going sky high on compression to max out efficiency at cruise settings and just greatly retard timing for heavy throttle, ( making efficiency and max power at heavy throttle suffer) since the engine really spends very little time at heavy throttle anyway.

Dean in Milwaukee

[cleanspeed1]

Originally Posted by Dean in Milwaukee

"But isn't the added mechanical advantage of higher compression counteracted by increased energy loss during the compression stroke? It seems to me that if it takes more energy to compress the fuel/air charge further, but you get more energy out of it, shouldn't the two factors just cancel each other out?"


Air is a perfect spring. 100% of the energy used to compress it is directly returned when it reexpands, at least thats how it would be in a perfect world.

The reality is that higher compression creates more piston friction, and part of the heat of compression and therefore its energy is lost to the cylinder. If this was'nt true, youd'e have a perpetual motion machine.

The way it works out though, a large % of the energy needed for compression is in fact simply returned to the piston on the downward stroke, and the extra friction and heat losses from bumping compression from say 9:1 to 11:1 is a fairly small increase, and far offset by the extra mechanical advantage gained by the increase.

This is especially true when an engine is throttled way back in cruise operation. The net effective cylinder pressure is reduced by the throttling, hurting system efficiency. An engine can actually handle much more compression at low throttle settings without knocking, but has compression ratio's chosen to handle heavy throttle application to optimize max output.

I've often wondered what sort of mpg results you could get by going sky high on compression to max out efficiency at cruise settings and just greatly retard timing for heavy throttle, ( making efficiency and max power at heavy throttle suffer) since the engine really spends very little time at heavy throttle anyway.

Dean in Milwaukee

I have wondered the same thing, yet what would it take to get something like 30 to 1 or more static compression with what's out there now?

[Air-Hybrid]

Originally Posted by Dean in Milwaukee

I've often wondered what sort of mpg results you could get by going sky high on compression to max out efficiency at cruise settings and just greatly retard timing for heavy throttle, ( making efficiency and max power at heavy throttle suffer) since the engine really spends very little time at heavy throttle anyway.

That's where cooled-EGR comes in.
Adding a big chunk of cEGR at FOT and high loads can hold off knock quite effectively, without the need for traditional enrichment-charge-cooling.
Clearly you are going to loose a few % of peak HP but for a much better top-end FE.
http://www.swri.org/3pubs/ttoday/Sum...n-and-Cool.pdf

Thanks to ConnClark for link

In this way max. power suffers a little but not efficiency.