...a DIM LAMP question!

[gone-ot]

Q: if I know the maximum, aero-limited vehicle speed (124 mph) and the engine RPM at that speed (4,800 rpm), and the calculated horsepower output at that engine speed (135hp = 148lbft × 4800rpm/5252), shouldn't I be able to "back-into" other such variables as:

1) Engine efficiency (~0.23)
2) Drivetrain efficiency (~0.95)
3) Coefficient of Drag (Cd ~0.33)

...so as to obtain a "better" estimated MPG, using the EM calculator?

...I just plugged in the following numbers for our 2011 LTZ and got a calculated 110 hp value at 124 mph, which is too low:

• Vehicle weight = 3500 lbs (my guess: 3177 lbs curb + 323 lbs load)
• Crr = 0.009 (my guess)
• Cd = 0.33 (working backwards from 0.298/0.9 from GM)
• A - Frontal Area = 24 sg.ft. (from brochure dimensions)
• Fuel Density = 33557 (summer blend)
• Engine efficiency = 0.23 (a guess)
• Drivetrain efficiency = 0.95 (default value)
• Parasitic overhead = 0 (default value, what for A/C being "on"?)
• Air Density = 1.164 @ 86ºF (yes, this is the desert)

...both C&D and R&T magazines have published 124 mph as the 'tested' aero-limited top speed for the 2011 Cruze LTZ 1.4LT with 6-speed automatic.

...comments or suggestions or "±" critiques?


NOTE BENE -- the "DIM LAMP" reference is a tongue-in-cheek nod to "...light's ON, but only DIMMLY lit..." meaning I've got a half-baked idea, so do I keep cooking or turn the light full out?

[TheEnemy]

I'm pretty sure the drivetrain efficiency is too high. Try 85% for a stick, or 80% for auto.

Paracitic,
between 2000 and 5000 (guestimate) for A/C,
probably at least another 100+ for engine management
Mechanical fan? 1500+

[mort]

I'll add that your rolling resistance can only be that good on really good road with the best tires. I don't think a race track would qualify. Try .015 at least.
-mort

[gone-ot]

...the Goodyear RS-A's on my 2009 Vibe are listed as 0.00918 ~ 0.0092 in the 2005(?) NHSTA tire booklet.

...the Michelin Pilot's on our 2011 Cruze are 'supposed' to be better (?) than the Goodyear RS-A's, hence my "guesstimated" 0.009 value for the Pilot's (at 40 psi, not 30 psi).

[TheEnemy]

You might be pretty close, I was playing with it, and noticed that when I adjusted the drivetrain efficiency it only affected the FE calculation, so the HP figure given is at the wheels, not at the flywheel.

so 110hp/0.8 = 137hp at the flywheel

[gone-ot]

...Thanks! I should've thought of that (ie: flywheel HP vs. driven-wheel HP) but, obviously, didn't.