'Cold Start' and use of "N"

This is an advanced, warm-up technique that is NOT for everyone. You'll never go wrong following @john1701a who posted,'Just drive the car, you'll do fine.' We'll go over how the Gen-3 warms up and then the technique.

Our Gen-3 Prius begins to cool as soon as it is parked but in the first 2-3 hours, there is significant warmth in the engine and transaxle that shortens the next warm-up. So if you have a lunch hour, driving a mile or more for a lunch break reuses the heat saved from the morning commute and recharges the engine warmth. The return from lunch is also a warm start and helps with the return commute warm-up. I've even taken a mid-afternoon 'expensive coffee break' to keep the heat. But after 8 hours, the car has cooled to ambient, a 'cold start', just like overnight, and has these phases:

• 45-55 seconds - heating the catalytic converter to 500-600C operating temperature. The car maximizes use of traction battery power while running the engine in a 'heat the catalytic converter mode.' Gentle acceleration can reach 25-35 mph while the engine spins just a little above idle rpm, 1300-1400 rpm, before it switches to a 'gas burning', warm-up mode.
  
• 1-1.5 miles - the car is trying to get the engine coolant warmed to a minimum of 40C, usable 55C range, and on to 88-92C:
  • below 40C - engine must run in inefficient 'must warm up the engine' mode
  • above 40C - if heater is OFF, engine will stop automatically at a stop
    
  • above 55C - engine will stop automatically even if heater is on, the car is in 'hybrid mode' and very efficient

This is a very advanced technique that people who have driven a manual transmission will find much easier to master. If you've always driven an automatic, probably not a good idea without two or three 'fair weather' practice sessions in a parking lot. Regardless, here is the technique:

1. Back-in for 'forward drive' when parking - this allows looking for approaching traffic so you can delay starting until traffic and pedestrians are clear. Looking out the front windows is a lot safer. So fiddle with the radio, seat belts, and take a stretch because you want to leave when you can avoid stopping in the first mile. The goal is a 'rolling idle to warm-up.' Turn off the heater just before stopping the car so you won't forget on the next start.
   
2. Choose a low-speed, slight downgrade, initial route - we are going to use "N" while rolling to minimize the 'warm-up' fuel burn while moving. So a neighborhood route posted at 25 mph is much better than straight to a 35-50 mph cross town route. At such slow speeds, stopping at a stop sign is no big deal so stop. A long, shallow downgrade helps but level is good too. Even an upgrade to a stop sign works because you can coast up to a stop without braking:
   1. READY car and accelerate keeping the engine in the 1300-1400 rpm range. If the engine 'spins up' lighten until you reach 25-30 mph and then sift to coast in "N". The engine will slow to idle warm-up speed, 1100-1200 rpm and minimum fuel burn. Keep the heater off.
      
   2. Shift into "D" when approaching a stop to see if engine stops.
      
   3. Once engine stops or 1-1.5 miles are driven, go to cross-town roads and drive to work normally.

Does this save a lot of gas: no. For example, I drove normally through my neighborhood in 35-40F weather and for the first mile, 32 MPG, burned about 0.03 gallons. The remaining nine miles measured 60 MPG, 0.15 gallons. So total fuel burn, 0.03+0.15=0.18 gallons for 10 miles or 55.6 MPG. In similar conditions in the past, I typically get 50 MPG for 10 miles, 0.20 gallons. Was it worth it?

On a longer than 10 mile commute, the warm-up cost becomes an insignificant fraction of the total fuel burn. But on a shorter commute, say 3-5 miles, the warm-up fuel burn becomes more significant. Caution as this is an unusual way to drive the Prius and not for everyone. Those who grew up with manual transmissions will find it easier to master but those who drove automatics, this is probably not the best approach.

Bob Wilson

 

I haven't studied A/C effects beyond the usual, it is a load on the car. Happily, not a load on the engine.

Bob Wilson

 

Using "N" to keep the engine from starting at a stop is something I've used too. I'm always happy to see folks experiment and improve a technique.

Bob Wilson