Archives

In two months I had never engaged "B" mode in my Prius, until last night. I was headed down-hill from my apartment on a cold engine and thought "Huh, I wonder what this really does?" So I lifted off the throttle and did it. The car slowed down just as expected and was apparently using the engine to slow the car. But the unexpected part was seeing my analog tachometer drop to zero. It **appears** that it quit burning fuel (at least it wasn't igniting it) despite the fact that this was within 30 seconds of startup.

I don't know if I can actually use that for anything but it seems worth reporting.

Tim,

What was your SOC when you engaged "B"? What was your initial speed? Did you have a block heater plugged in before your start? Engine temp? What was the outside temp? Thanks.

Wayne

SoC was six bars, top of the blue zone. Travelling down-hill at 30mph or so, no block heater. No idea how warm the coolant itself was. Historically I have seen it pop up at 100F or so after exiting the reservoir and circulating through the cold metal after sitting for a few hours, but I didn't have the SG plugged in this time.

Your tach takes its input from the IGF line, ignitor feedback.
When the engine is being dragged around as an energy-dissipator,
no fuel or spark is applied, so no IGF pulses. Boom, zero RPM.
.
The only other rotational outputs at that point are the cam
and crank sensors, but at 3 and 34 pulses respectively per
engine rev and not evenly spaced, you'd have to do a bit of
extra hackery.
.
The same thing happens in warp-stealth on the highway, which you
may be able to use as a nice indicator that you haven't fallen
off the plateau yet...
.
_H*

Yeah, I didn't mean to imply that I thought the engine wasn't turning. I could hear it whirring away under the hood. I have been using the tach to tell me when I'm burning fuel and when I'm not and it does a great job. I was just surprised that the ECU would completely cut fuel even on a stone-cold engine.

Ah, but that was a common bug with carburetors, whose idle
circuits often tended to be set on the rich side ... gearing
down and dragging the engine fast would often cause backfires
in the exhaust system, cuz that "little bit" of fuel would
still go through the engine and eventually pile up as a
combustible mixture. I think most cars have gone full cut
on injection at throttle-close and high-RPM ever since there
were injectors... heck, I know my '89 Trooper did, even with
its relatively primitive brain-box. Even from stone cold.
.
_H*