Gas engine start up speed?

[dave schulte]

My '07 gets about 50 mpg on the highway but only low 30's around town. I have noticed that the gas engine always kicks in at 8 mph no matter the weather conditions or if the AC and/or heat is on or off. I accellerate very gently. It often fails to shut down at stop lights and sometimes seems to be racing rather than idling. At what speed does you gas engine kick in? Does it usually shut down when cruising to a stop? Mine never does. Thanks!

[efusco]

You're not driving in B mode are you?
Are you driving with the defrost on?
What AC temp are you set to? If too cool/hot it can cause the ICE to run more.

The ICE should shut down at a stop if you keep your foot firmly on the brake for 5-6 seconds once you're fully warmed up. In colder weather, esp. if you're not using a block heater it can take up to 10-15 minutes for the car to get fully warmed up in cold weather, esp. in stop & go driving. If your commute is particularly short (<4-5 miles) you may not be getting fully warmed up. That would account for the low FE and conditions you describe.

[qbee42]

This topic is much more complex than your question suggests. There is no one threshold speed after which the ICE starts to run. The turn-on speed is a function of engine temperature, load, speed, climate control settings, current engine state, and maybe some other factors that I have missed (phase of the moon?). There are some links on this forum that point to excellent technical descriptions in full gory detail, but I suspect that is more information than you want. So in short, here are some of the factors that affect engine shut-off:

1) The internal combustion engine (ICE) must be warm and keep the emission controls warm. The amount of time this takes depends on a number of factors, including outside temperature and whether the thermos still contains hot engine coolant.

2) The main electric motor (MG2) can power the Prius only for short periods of time before battery levels get too low. Also, the lubrication system is not designed for extended electric-only operation.

3) The control system tries to run in the cleanest, most efficient manner. Most of the time this means running the ICE along with the motors. Running from battery power is not very efficient. All of the power comes from gasoline, so running from electricity involves running the ICE to charge the battery, then pulling the charge from the battery to send to the motors. There are a lot of losses in this cycle, so if possible, the control system tries to avoid it.

4) Cabin heat is generated by the ICE (both by heat and electricity). In cold weather this means that the ICE has to run more to keep itself and the cabin warm.

You will find that short trips are mileage killers. This is probably the main reason that you see lower mileage around town. Try to combine several short trips for greater efficiency. That way the ICE only has to heat up once instead of several times.

Tom

[Bob64]

<div class='quotetop'>QUOTE(dave schulte @ Nov 12 2007, 11:06 AM) [snapback]538406[/snapback]</div>

I have noticed that the gas engine always kicks in at 8 mph no matter the weather conditions or if the AC and/or heat is on or off.
[/b]

Let me guess, your basing this on your instantaneous MPG readings. If so, then it is not kicking it, but it just isn't showing that it is on (because the MPG readings are off the chart). So the answer would be: your not in stage 4 yet.