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It looks like Mazda sees high compression-ratio engines as the way forward to boost fuel economy and specific power on their midsize gasoline engine lineup (13:1 is an impressive achievement).

According to their promotional material they reduce the risk of detonation by reducing the temperature of the intake charge. This is done by designing longer exhaust runners in an attempt to eliminate the possibility of hot exhaust gases reaching the intake valves of other cylinders.

Does this mean that exhaust gas re-circulation is avoided altogether?

If so, what implications would this have on the design of the catalytic converters?

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  • Here and here is some more information about Skyactiv you may find interesting. The first video starts out (and is silent) about the diesel model engine, but the info about the trans is great. The second is the first in a four part series. Impressive, even after the rhetoric.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jun 21, 2015 at 12:29

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I haven't found anything which definitively states one way or the other on the EGR valve for SKYACTIV-G technology. What I can tell you is the purpose of the EGR and because of that why the technology itself would not cause the demise of the EGR.

EGR or Exhaust Gas Recirculation is a technology which was introduced to limit temperatures during combustion. The way it does this is by introducing metered amounts of spent exhaust gases (6-10%) back into the intake stream where it replaces oxygen. Since spent exhaust gases cannot be burnt again, it limits how hot the combustion gases can get. A major part of the intake air charge coming into the engine is nitrogen (~75%). If combustion flame front temperatures extend to 2500°F range and above, nitrogen oxides (NOx) formation increases dramatically (NOTE: do not confuse nitrogen oxide with nitrous oxide), which in effect are burnt nitrogen molecules. Any NOx molecules which do form are reduced in the catalytic converter which has a section specifically designed for such use.

The main purpose of the SKYACTIV-G technology is to reduce the chance of detonation (pinging or pre-ignition) at the higher compression ratio, the use of EGR will help in this endeavor by making the air/fuel mixture a little harder to detonate.

Another tidbit I found was that EGR is required on all OBD-II controlled engines. Because of this, I would bet Mazda could not eliminate the EGR at all, at least for vehicles which are for sale inside the US.

Another tidbit I found is this thread on the xtremesystems.org forum where the guy is saying removing the EGR on the SKYACTIV engine won't help much, indicating to me it actually has one.

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  • Well, based on the limited research I conducted it seems that Mazda employs a "cool" EGR system which helps keep the temperature of the intake charge lower than the norm. It also looks like Mazda 3 owners are suffering from carbon build-up issues as a result which is why they seem to prefer deleting the system altogether.
    – Zaid
    Commented Jul 12, 2015 at 1:53
  • @Zaid - Haha! Just keep plenty of Seafoam onhand for Seafoam treatments and should all be good!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 12, 2015 at 3:01
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Vehicles with variable cam timing on both cams don't need EGR in the traditional sense. They vary cam timing to increase overlap during cruise to allow exhaust gasses to partially fill the intake stroke as a traditional EGR would.

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