Hi,

I've found a way to improve my evening commute mileage that works for my NHW11 and should work for the NHW20. It requires parking so you can reach a speed of 38 mph (60 km/h) within the 40-45 seconds it takes for the catalytic converter to light-off.

PROTOCOL

1. Work Parking - back in near the parking lot exit so you can see the on coming traffic in the nearest lane.
2. Time Start - watch the on coming traffic and don't start the engine until you can pull out and get on the higher speed, cross street without stopping. You want to minimize the amount of time idling in the parking lot and maximize the amount of 40-45 seconds accelerating before the catalytic converter can light-off.
3. Modest acceleration to 38 mph (60 km/h) - if you have a seconds counter on the dash, accelerate so it takes about 8 seconds per 10 mph (10 km/h every 5 seconds.) During this acceleration, the engine will turn over at a fairly constant speed and not race up.
4. Ends when engine races - as soon as the catalytic converter lights-off, the engine will race. Quickly look at your MPG.
5. Maximize "N" during ICE warm-up - as much as is safe and practical, use "N", the minimum fuel-burn, engine warm-up mode, until you enter the later stages.

I'm finding with this approach I can achieve 40+ MPG within 0.5-1.0 mile (1-1.6 km) of the office parking lot and maintain it until the coolant reaches 70 C.

BACKGROUND

This trick exploits a limit on engine fuel consumption found in the mass air flow rate before catalytic light-off:

• ~4.0 g/sec - idle mass flow rate in "N" before catalytic light-off
• ~6-7 g/sec - modest acceleration rate before catalytic light-off
• ~2-3 g/sec - idle rate in "N" after catalytic light-off
• ~18-20 g/sec - modest acceleration rate after catalytic light-off

I've got a Graham scanner to read out vehicle parameters. This limited duration, 40-45 sec. catalytic light-off window can be exploited if you can achieve "modest acceleration" to a speed that approaches 42 mph without forcing the engine to burn extra fuel. This especially helps as cold weather returns to North America.

Sad to say but I can't use this trick in my 25 mph (40 km/h) neighborhood. There I have to maximize use of "N" to let the ICE coolant(*) warm-up until it reaches 70 C.

Bob Wilson

* - the engine block heater adds 10 C and I have a thermistor hack that jumps the indicated coolant temperature from 40 to 70 C. But this driving trick works when there is no EBH plug or you don't have my thermistor hack.

 

*bows* Not worthy

 

bwilson4web — I find similar behavior in my TCH. Interestingly, I find that in above-freezing weather, the ICE goes closed-loop within a second or two after starting. (In sub-freezing weather, it can take 30 s or more before the ICE goes closed-loop.) This must mean that the fuel/air ratio is actually being feedback controlled almost immediately after start-up, although the quick-warm-up strategy (spark fully retarded, engine at fast idle, NiMH battery power assist to the wheels) that you refer to persists for another 30-45 seconds after the ICE goes closed-loop. If this is the time required for the catalytic converter to reach operating temperature, I guess this means that the catalytic converter isn't required to be in the feedback loop in order to control the fuel/air ratio stoichiometrically. Does this make sense?

Stan

 

Hi Stan,
I wonder how the Camry does it ... go closed loop early. In the Prius we have two O_{2} sensors with one at the front and the other at the back of the catalytic converter. Once they reach operational temperature, the closed-loop begins and you can trace the signal.

I have not idea how the Camry achieves early closed-loop without a working O_{2} sensor.

Bob Wilson

 

Bob,

EBH preheat and thermistor modification...

Wouldn't either or both of these techniques have the effect, HIGH potential for, foreshortening the life of the catalyst..??

Doesn't either of these technique have the effect of elongating the time for the catalyst to reach fully optimal operating temperature...??

It seems to me that the object of both techniques is to fool the ECU into going into closed loop A/F mixture control prematurely, before the catalyst has been optimized for this leaner, low emissions, mixture.

 

I don't know.

It changes the duration by spacing in 'not warming' while 'no running' times. Since no gas is flowing and the cat has yet to 'light off,' I don't see a risk. I liken it to heating something on the stove when it is too hot. You can take the pan on-and-off the burner and still warm the food without scalding it.

I'm under the impression that closed loop A/F occurs when the O_{2} sensors begin reporting the variation in mixture. What I don't know and isn't documented is whether the front O_{2} sensor is enough or whether it requires the rear sensor too. From a mixture control stand point, just the front is enough.

Bob Wilson

 

The TCH also has two sensors, like the Prius, one before and one after the catalytic converter. The first of these is called the air/fuel ratio sensor, while the second is called the heated oxygen sensor. See the attached PDF from the New Car Features Guide. Your speculation, that just the former of these (in conjunction with the mass air flow sensor in the intake manifold) is all that is required to control the air/fuel ratio, would be my guess for why my TCH goes closed-loop very rapidly (as measured by ScanGaugeII).

Stan

 

How does the scan gauge determine that it is in 'closed loop' mode?

I have a Graham scanner and can monitor both O_{2} sensor outputs and something called "Fuel Trim."

Bob Wilson

 

It's one of the standard signals carried on the CAN bus, that any ScanGauge can automatically read. No special coding is required to read this information.

Stan