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SUVs Mazda CX-7 Mazda CX-7: Bring the diesel version to the USA!
22 messages, Last post on Mar 01, 2009 at 11:14 AM
You are in the Mazda CX-7 Forum. Your Hosts are steve_ & tidester
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Replying to: carlitos92 (Mar 21, 2008 6:35 pm) The turbocharger is the primary reason the CX-7's I4 engine gets such HORRID FE. In order to provide for the "rise" in effective compression ratio as the boost comes up the engine MUST be derated in normal off-boost mode. Otherwise the DISI (DFI, Direct Fuel Injection) engine could have a native compression ratio of 12:1 and give you more decent FE when cruising along at relatively constant speeds. The real answer would be to extend the range of the current VVT system such that the native compression ratio could be raised to 12:1 as in other DFI applications and then use delayed intake valve closing to gradually reduce the compression ratio as/when the turbo spools up and provides more and more boost. Even better yet, make the native cylinder compression ratio 15-16:1 and then use the valve timing to bring in down to 12:1 under normal driving conditions. Then gradually transition to <10:1 as boost rises. High FE Atkinson cycle mode off-boost and then transition to Miller cycle mode as boost rises. Even eliminate the wastegate this way, the delayed intake valve closing could be used to limit the turbine speed. |
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I went by the Kirkland Mazda dealer today to look over the CX-7 and collected a brochure. Before I actually buy one to experiment with/on I was wondering if anyone in the Puget Sound region might be willing to participate in an experiment to improve the FE of the CX-7, possibly substantially so. What I would like to do is mill the head of a CX-7 engine to get a compression ratio of 12:1. 12:1 has become typical for a DFI, Direct Fuel Injection, engine, but since the CX-7 is turbo charged the factory's "native" compression ratio is, as it must be, quite a bit lower in order to provide for the additional "compression" from the turbocharger as it spools up. During the experiment the wastegate would have to be "wired" open so the effective compression ratio could not rise (much) beyond 12:1. If anyone owning a CX-7 is interested I would be will to pay the costs purchase and install a new factory head once the experiment is complete. I suspect the off-boost FE improvement might be in the range of 50%
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Replying to: wwest (Feb 26, 2009 5:08 pm) What would be the basis for your suspicion? Is it just a hunch or are there real world numbers behind it? tidester, host SUVs and Smart Shopper
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Replying to: tidester (Feb 26, 2009 10:14 pm) So "off-boost" this engine is quite heavily derated/detuned and runs in that mode 95-98% of the time. Cadillac just happens to have two V6s in the market at the moment, one with DFI and one without. Compare the two and you get my theory.
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Replying to: wwest (Feb 28, 2009 2:17 pm) tidester, host SUVs and Smart Shopper
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Replying to: tidester (Feb 28, 2009 9:59 pm) The turbo wastegate could be held open for low to moderate levels of acceleration. |
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