40+ mpg highway Mazda CX-5

[California98Civic]

Originally Posted by kafer65

... I figure about 4% difference in circumference ...

Is that possible when most tread depth on all season car tires is maybe a little under a third of one inch when new?

[kafer65]

Yeah, your right! I took too much off the radius before I squared it. It should be more like 2.5% assuming loss of 6/32 of tread (IF I got it right this time).

[matt36415]

Originally Posted by kafer65

The skyactive engines use piston and timing tech to run 87 octane and vary timing atkinson style to vary compression from 6-13:1 in USA.

I know engines vary ignition timing based on knock sensors, I didnt think they could change the compression ratio though. That would be a good trick if it can.

Is this real or just a mistake?

[Mustang Dave]

Static compression ratio is set by piston displacement and combustion chamber volume. It can't be changed without replacing or modifying parts.
However....
Varying the valve timing can vary the effective compression (and expansion) ratios considerably.

[serialk11r]

Originally Posted by matt36415

I know engines vary ignition timing based on knock sensors, I didnt think they could change the compression ratio though. That would be a good trick if it can.

Is this real or just a mistake?

All engines nowadays have 40+ degrees of adjustment on the cams so you can vary volumetric efficiency by quite a lot, the compression ratio does not change.

However this technique is limited, you cannot dial the intake cam back past a certain point without incurring losses from the piston pulling down against a closed valve. It's highly unlikely that this engine is capable of achieving 50% volumetric efficiency, because its compression ratio is too high to use internal EGR to allow this.

[kafer65]

l just managed to get dash indicated 43 mpg on our weekend trip to Dollywood. That was nearly the same conditions as the 42 mpg run. Same station filled up and 60 miles of city driving before the highway run with whole family aboard. Clear 75-85 degree temps but with long deep local hills. DWL and pulsing AC and using AC to help with engine braking. That was 350 miles total and now I'm trying to hold on to that to finish out the tank on my commutes. Mazda notes that it uses 4-2-1 exhaust header to keep hot EGR from pulsing back into the engine as one strategy. Mazda used an Adkinson/Miller cycle engine 2.5 V6 in the Millinea about a decade ago. Using injected air at the last instance to achieve compression. Couldn't you vary the amount you inject to get VCR?

[kafer65]

I just bested myself again! I broke 47 mpg yesterday on the first 155 miles on a fresh tank. Firstly, I'm getting better at DWL and I seem to be able to do a reset on my mileage by letting off of the throttle and gently reapplying to attain a target mpg as I'm driving. Second, I got lost in a congested area with lots of short steep hills which limited my top speed average to a max of 55 mph but mostly 30-40. I still used AC pulsing.

[whatmaycome14]

Originally Posted by kafer65

I just bested myself again! I broke 47 mpg yesterday on the first 155 miles on a fresh tank. Firstly, I'm getting better at DWL and I seem to be able to do a reset on my mileage by letting off of the throttle and gently reapplying to attain a target mpg as I'm driving. Second, I got lost in a congested area with lots of short steep hills which limited my top speed average to a max of 55 mph but mostly 30-40. I still used AC pulsing.

WHAT?!! That's awesome! Thats basically the milage that I've been getting in my civic. I'm so pissed off now. Thanks a lot!

[kafer65]

Well, if it helps you feel better, just think of it as a civic with a lift kit and belly pan I'm encouraged that if I kammed the back I could hit 45 mpg at higher speeds due to longer glides. Off with the side mirror! Just need to find the will to start fiddling with the door panels on a new car.

[akennyd]

Thanks for posting, kafer! I am slowly learning to drive my Mazda better and creeping up on 40mpg, car is now sitting with 40.1 average after a fill up yesterday on the way home from work and back this morning. My Mazda 6 has the 2.5L in it with 6 speed manual. I LOVE this car so far!!!! Have a little over 2,000 miles on it.