'16 real MPG

It depends on the circumstance. If you take a short trip when the car is already warmed up, you will get very pleasing numbers. If you take a short trip when the car is cold, you will be extremely disappointed. Once you get past the cold start penalty, the rest of the time while "warmed up" can be easily in the 80's. The longer the trip, the higher your numbers.

It takes me 20 miles to overcome a cold start and get the average from start over 70. 2 to 4 mile trips won't get you fully warmed up unless you have very warm weather. For a 2 mile trip, I walk. For 4 miles, I walk or bike. Don't forget your umbrella.

 

Do you think it is also possible that the short trips started with engine already warmed up, battery fully charged, and at low enough speed that the car ran EV for a significant portion of the trip?

 

Isn't there's something at startup, "closed loop" vs "open loop"? Both loops are over my head, but maybe there's less "open loop" with 2016?

There was a thread here: a guy was trying to justify leaving his car on while gassing up, to avoid the closed loop. Maybe the ultimate penny wise, pound foolish.

 

Ok, here's the REAL short trip numbers for you Hyper dudes. (I actually paid attention to it.) 2.5 miles. Is this supposed to be important??? Mean something? I got bigger fish to fry. I'm not that bored yet!

 

There is something to that. With the Gen 2 & 3, after powering down, powering up again always resulted in the engine starting and idling for around a minute. With Gen 4, that does not happen. In fact, even after 24 hours, if the temps are above 60F, I can power up and drive thru the neighborhood without the engine starting. The engine starts only when your battery is low or your power demand is too high.

Sure. At least once a week, on the way home from work, I stop at the grocery store just outside the entrance to my neighborhood. From there to my house it is about 0.7 miles on a downward slope. I mainly coast with some regen before the speed bumps. The engine never runs and the battery level stays the same. Trip from start shows 199.9 MPG.

 

In my gen 2, I would always leave the car "running" when I went into the convenience store to get a soda or when I stopped at the ATM and waited in line to avoid the restart penalty. I don't need to do that anymore...

 

You've given me an idea:

• Use A/B trip meters - to measure cold start overhead
• Reset B at work and leave car running - to measure idle overhead
• Compare idle fuel burn versus cold-start - there should be a break-even point where the cost of a restart equals the fuel burned sitting in idle

Actually I will use a more sophisticated, recording OBD. This will let me accurately document what happens during the test.

Bob Wilson

 

I know in VW parlance "closed loop" is when the engine bases fueling on the MAP sensor readings, before the oxygen sensor(s) and catalytic converter are warmed up. Once the O2 sensor and catalytic converter are up to operating temp, fueling duties switch to the MAF sensor for "open loop" operation. As you might imagine, "open loop" operation is more efficient.

 

Closed loop will provide more accurate/efficient fueling. During open loop operation the computer's requested AFR is not adjusted/trimmed based on O2 sensor signal.

Good youtube video explaining open/closed loop operation

Very good explanation of fuel trims by Paul Danner, quite long, but lots of great info.

 

Sounds like the gen4 doesn't automatically turn on the ICE at start up above a certain temperature and SOC level. Instead, it waits until there is a higher power demand before starting the warm up cycle. Those engine off miles can drive up the average MPG, but the electric used will ultimately be replaced by burning gasoline.

The gen4 has reduced the warm up penalty, but it is still there.

Oxygen sensors need to be hot in order to work. Testing an old one involved looking for a current change while applying a propane torch to it.

While cold, the oxygen sensor isn't sending a signal to the ECU. Without that signal, the ECU doesn't know if unburnt oxygen is getting past the cylinders or not. Without that info, it can't make adjustments to the air:fuel mixture without possibly going too lean, and producing NOX. So it uses a default fuel trim map that tends to be on the rich side. Using that default is when the car is in open loop.

When the oxygen sensor is working, the car is in closed loop because it is now getting feedback about the outgoing exhaust. The ECU can then lean out the fuel mix until the sensor reports oxygen in the exhaust, and then the ECU backs off a bit on the leaning. So less gas is used while in closed versus open loop.

Basic oxygen sensors relied on the hot exhaust gases to heat them up, which took time. Modern ones have their own heating element, and closed loop can be reached in under a minute. There are also wide range oxygen sensors that give a more accurate reading of the oxygen level. The gen4 now uses one, but they have been around since the 1992 Civic VX, possibly before that.

There is a second bank of oxygen sensors downstream of the catalytic convertor, but they are for monitoring the cat's operation, and aren't used by the ECU for running the engine.

 

I watched the whole thing. The good stuff is the second half when he shows the scanner and how the STFT and LTFT react to a vacuum leak and adding propane to the intake. LTFT reacts in less than a minute. I thought it would take much longer. It makes me re-think what happens when switching from E10 to E0 or vice versa. You will run rich or lean only during open loop when the STFT is not yet enabled. So any change in fuel economy due to ethanol or lack thereof could be immediate, or at least after the next time you go into closed loop.

 

Thanks, Weasel. I had it backwards!

 

I had a 92 Civic VX and I remember how expensive it was to replace the heated O2 sensor. I also had a digital voltmeter monitoring the O2 output so I could tell when I was in lean burn mode. I had another DVM hooked up to the fuel tank level sensor, measuring ohms so I knew the fuel level because the gauge in the dash went bad. Those were fun times.

 

It wasn't just heated, the oxygen sensor was also wide range, and likely not a regularly stocked part for the times. Hopefully, the part costs have dropped for modern applications.

 

My Prius Two (not Eco) came with the Enasave 01 A/S tires.

 

Open loop fueling will typically be rich of stoichiometric (14.7:1). Also O2 sensor temperature isn't the only parameter used to go into closed loop, coolant temperature is another. A thermostat failed open can cause an EFI engine to take longer to go into closed loop or maybe not at all. Depends on how it's programmed.

Remember that STFT is trying to keep the O2 sensor signal at stoich (or requested/commanded AFR) and LTFT is trying to keep STFT close to 0.

Ethanol content in gasoline will effect fuel trim because alcohol has less BTU (and hydro carbons) per unit of volume than 100% gas. Google ethanol stoichiometric air fuel ratio, its 9:1. Considerably richer than 14.7:1 of gasoline. Therefore once in closed loop operation the O2 sensor will be sending a leaner signal to the computer. When the computer compares this to the requested/commanded AFR the STFT will begin to increase. The more positive the fuel trim (both short and long term) the more fuel is injected and burned, why you see a lower MPG. The higher the ethanol content the more impact to MPG.

The Prius manual states not to use higher than E15 (Gen 4 pg 737, Specifications). Most likely because the injectors and fuel pump are not capable of providing enough fuel to maintain stoich even at maximum flow rate. Other reasons are that ethanol can reek havoc on fuel system components not designed for it.

E85, even though it has less BTU per unit of volume than 100% gasoline, is used by tuners/modders as a "poor man's race fuel" due to the gasoline having >100 octane rating. But that's a discussion that requires many cold beverages.

 

I believe all new gasoline cars have components in the fuel system that is safe for ethanol. I have heard of some Volt owners using a
30% blend of Ethanol with no problems or engine light activation.

I also had a 1994 Civic VX, the rare one, EPA rated at the time 55 MPG hwy, I only had to replace to O2 sensor once under warranty. This was a dependable car, 45-50 mpg real world, and lasted over 300,000 miles and nearly our entire family used it during its life....

 

I did this experiment on our 2003 and 2010 a while back:

• Up to E50 - no problem other than the ethanol effects on MPG
• Above E50 - check engine light and a fuel trim fault
• E85 - works fine except on cool morning, ~40F, when the engine can stall on first start

Unfortunately the retail price of E85 does not make anything above E10 worthwhile.

Bob Wilson

 

4th fill up. Best mileage yet!
575 miles. 10.1 gallons. 56.9 MPG calculated. Display 59 MPG.