I was carefully observing my MPG this morning... and the numbers I saw confirmed something I've suspected for quite a while.

My home sits atop a 600' climb from the freeway (over a 1.4 mile distance)... a healthy climb, which generates about 3,000 rpm unless I crawl up. My current tank has about 240 miles on it. The drive from the freeway to my home dropped my displayed mileage for this tank from 51.4 to 51.1. It JUST went from 51.2 to 51.1 as I stopped the car, so it was "in the high ones".

This morning, I turned the car on and immediately left (no warm-ups, temp at 55 degrees, almost no throttle, rpms at 1,200). Within 15 seconds, it dropped to 51.0. Going downhill, it dropped to 50.9 and remained there to the freeway.

Here's where I have a problem. A cold engine on a downgrade at near-idle is consuming almost as much fuel as climbing an 8% grade at 3,000 rpm with a warm engine? This doesn't make sense.

I know a cold engine is far less efficient, but that seems extreme. I'd have to do some math to calculate the number of ounces of fuel consumed in both scenarios (engine volume, rpm, air / fuel ratio)... but a quick (and overly simplified!) calculation says that if two engines consume the same fuel over the same time period... and one is 3X the rpm of the other, then the air / fuel mixture of the slower engine is probably 3X that of the faster engine.

If I remember my shop classes correctly, a normal air/fuel ratio at sea level is 14.7 to 1 ("stoichiometric", as I recall!) . That means for 3X this ratio, it would have to be 4.9 to 1. I don't think an engine could even run at 4.9 to 1.

I'm wondering if the fuel isn't really being consumed at this rate... and perhaps this could be part of our 2 mpg error (displayed vs. calculated)?

Do others experience the same big hits to MPG on startup?

 

Jeff,

1 to 3 tenths tpyical for me.

I am about the same distance to the freeway (1.5 miles ) both leaving & coming home from work. Warm or cold, stop sign or lights. As much as I was light on the gas at first, I all but thrown in the towel as not expecting to be looseing something. Kinda makes sense as things just got get rollin', but still when one doing the crawl you think the numbers would not add-up the same. Guess it is just one of those prices one pays to play. Silly hybrids, lol.

Somewhat frustrating at best.

John

 

The engine runs rich until it comes up to temp. Great for us frosty northerners, not so much for you beachgoing types.

 

It's all relative, you see how much a non-hybrid uses during the exact same scenario.

 

Absolutely, cold start up is a terrible hit to FE... the colder it is the worse the hit.

Jeff, at least you don't have to deal with our freezing cold temps!

 

Jeff - What are you using the other Trip meter for again? Obviously one will be for the tank but what is the other one? If it is not being utilized then just reset that in the morning with the cold motor and head on down the hill. I did this yesterday and got an impressive 11.7 mpg at the end of the hill. I should test someday if letting the car idle for 10 minutes then driving or leaving straightaway has a better FE after the same 5 mile trip. =)

 

Flash, good point. I'll give it a try. I suspect I'll match your 11.7 MPG!!!

 

I have a very similar situation myself with my HCH-I as far as commute. From a cold start, I go up 150', then back downhill about 700' to the freeway, in slightly under two miles.

At one point, I actually tested what the idle fuel consumption was of the engine warmed up -- it appears to be around 0.1 gallons per hour. I figured this out by allowing the car to coast up to speed until the moment the instant MPG bar "maxed out" at 120 mpg, which occurred at around 12 miles per hour (1 gallon in 10 hours). Idle fuel consumption when the engine is just still warming up appears to be about three times as much. A 3000RPM, 8% climb is probably around 17mpg @ 30mph, or 1.8 gallons/hour.

I don't believe that the indicated numbers you just gave can possibly be right however, as I have never seen such a drastic fuel consumption from cold start. The total "downhill" mileage for me (including the initial 150' uphill immediately after startup, and the fuel I burn accelerating from stopsigns) ALWAYS brings up my tank average, even if only slightly.

 

Yes, you're right...the car would not run at a 4.9 AFR. It would be dumping so much fuel into the cylinders they'd be flooding and washing out the spark. I'm sure these hybrids run at 16.0-17.0 AFR while cruising and probably no more than 13.5-14.0 in your "cold" starts.

BTW, first post here...getting my HCH-II on wednesday!

 

Actually, on second thought, 4.9 would mean 1/5 of the volume of the intake would be fuel. This would end up as raw fuel in the cylinder and would probably end with hydrostatic lockup (can't compress a liquid!).

By the way, congrats on your purchase and your first post! Welcome!