A 40-45 second window

I've stumbled across 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.

 

Tell me about it! I was in that camp too until I started to notice this pattern when the cold air returned.

I've not really looked at the SOC because once the engine start S1 warm-up, it burns more than enough fuel to handle the self-discharge as well as any EV usage. Although you are technically correct that the engine has to work to generate the charge, it is already burning fuel to warm-up the engine. I'm just getting some useful work from this excess fuel burn. I just don't see an overt increase over the waste already going on for warm-up.

Now I still maximize safe use of "N" to keep all engine warm-up run time at a minimum fuel burn. However, getting the car rolling before the cats light off seems to work.

Because my neighborhood speeds are modest, the morning run I have to baby the accelerator. I can't get to an area where I can go 38 mph before the cats light-off. Once they are working, baby the accelerator and/or maximize use of "N".

Bob Wilson

 

Correct. Divide by 14.7 and you get the fuel consumption in grams per second.

That is the only time it will work and only for 40-45 seconds. You'll need to gently accelerate but keep going up to ~38 mph and then hold. If you hear the engine 'race', the cats just lit off or you pushed a little too hard. If you can back off and then apply a little without the engine racing, the cats are still cold. Thereafter, I maximize safe use of "N" (easy to do in an NHW11 model) and wait until the coolant reaches 70 C to get into S4.

Bob Wilson