Is the gas engine really on?

The centre console or MFD shows you are not using any petrol, it doesn't show engine revs. If you are doing more than 42mph the smaller electric motor spins the engine to prevent the smaller electric motor over-speeding.

In a conventional ICE car it is possible to have a zero fuel situation because the injectors sometimes don't inject any fuel in a high manifold vacuum situation like going down a hill on a closed throttle.

 

Since I first drove the Prius I noticed that on stopping at a stoplight at on a multi-mile downhill, the car gave a little shake a few seconds after stopping that seemed very like the likely feel of the gasoline engine stopping. This despite the fact that the engine icon had not been on for many seconds.

Sure enough, my Scangauge 2 shows the rpm dropping from somewhere near 1000 to zero just on that shake.

So on this point, as on several others, I think the SG2 is telling the truth and the Prius display is false (or, to be more charitable, oversimplified).

I'll watch fuel flow rate and rpm through this sequence to verify that the engine is using fuel during at least the last few seconds of this sequence and report, but I'm quite confident that it is.

 

I think you're mistaking the car trying to burn off extra electricity from the long downhill with the ICE actually running.

It's not running. the Car tends to "seat" the ice into a proper startup point.. and sometimes idles it with electric motors only to burn off excess electricity.

From everything i've read on here.. there are only a few cases where the ICE idles... heavy A/C use and the Defroster... along with engine warmups.

 

Above ~41mph the ICE will continue to turn at ~980 rpm. The injectors and the ignition are off, as it is only serving to govern the electric motor. When the car "shakes" at a light for example, the ICE is stopping completely, confirmed by the scan gauge. The MFD is only over simplified when it shows the ICE not running when it is still turning, but using no fuel.

Icarus

 

V8Cobra Kid is right, if your battery is full, or displaying all green bars which can happen at the end of a long coast down hill then any excess electricity needs to be dumped. This is no worse than conventional brakes dumping heat as they always do. To dump excess electricity to avoid charging the battery beyond it's comfort zone of 80% the HSD uses the small electric motor to spin the petrol engine with no fuel injected then when it senses the battery is on 80% charged it stops causing a slight but noticeable shudder. It isn't burning any petrol.

Sometimes you might notice withing a few minutes of a cold start after you stop at a traffic light you feel the petrol engine give a similar shudder, this is actually the petrol engine shutting down as it finishes its warm up phase and enters its most economical phase. (stage 5 I believe) There is about a 1 second delay in the MFD but I believe it is accurate within that 1 second delay. At the end of the day it is the amount of fuel you use to travel whatever distance which really matters in a Prius. The Prius will return better numbers than any other mass produced car currently on the market.

 

Your traveling at speeds above 41mph (or below 35mph) OR your not in stage 4 mode.

 

The MFD is not false. The screen you describe shows energy flow, not engine RPMs. The engine on the Prius often spins without burning fuel.

Tom

 

Conversely, I've noticed sometimes during start up that the MFD shows me moving on battery, with no arrows from the ICE, while at the same time showing 9 mpg rather than 99.9 mpg.

 

Related subject is "Stealth mode", Generally after 5 min warm-up and blue bar condition, Prius goes easily into stealth mode, at about 30-40mph. BUT, after a green charge is attained or faster speeds, (45-70mph), stealth is all but impossible to get. What is the, obviously very strict criteria governing Stealth mode???? AND if in many cases the ICE is only spinning and using no petrol, what is the advantage of stealth???

 

One I figured this out the MFD made a lot more sense. The MFD simply shows if the ICE is providing power to the wheels, or battery, or both. The MFD DOES NOT show if the ICE is running or not.

Again, think energy flow, not on or off.

Bob

 

stealth is benificial under 43mph in order to "cross out" the minimal help that the artificial drag provides.

So.. it's neutral without neutral.. as long as the gears can free spin without the engine spinning... which is maxed by the size of the gear and the rpm limit of MG2... i think it's MG2... *shrug*

if you don't use Stealth.. the computer will slow you down without generating anything (just about nothing)... so save it on mild declines and gain a little speed so you can go farther.. or.. generate more electricity from the higher speed braking.. since braking is much more efficient than coasting without stealth.

 

Thanks for the answer, by keeping all these factors in mind, and trying to drive, the cell phone police may get after me. I hope they don't find out that Prius driver brains may be in overdrive.

 

The (multiple) posters who assert that the ICE is always burning zero fuel when its Icon on the display is off disagree with the data the Prius is reporting on the CAN bus as displayed by Scanguage2.

I monitored displayed horsepower, fuel consumption (gph), rpm, and water temperature on this morning's driving. While at initial cold the values are higher, a typical set of values when the ICE is actually powered despite showing as an turned-off icon was about 980 rpm, 2.7 horsepower, and .18 gallons/hour. I think my water temperature only made it up to about 160F at the end. On my final stop at my destination, I got the famous rough shut-down, about five seconds after coming to a full stop. After stopping the very slight shake of the running ICE was obvious, along with the much bigger shake when it stopped, just when the displays on my SG2 agreed it did.

I think that the "rough shutdown" is actually no rougher than any other ICE shutdown, but that we notice it because of the lack of road vibration and shaking when the car is already stopped. I think that under other operating conditions the ICE does not usually go from running to stopped while the car is at rest.

The Prius manual makes no assertion as to the meaning of the ICE icon being turned on, but does say that the arrows represent power flow. I suspect that the majority of Prius drivers who have an opinion think that when the icon is turned on the engine is consuming fuel, and that when off it is not. Tain't so.

 

The ICE on the Prius can consume fuel without delivering power. It does this routinely during the warm up phase.

As for rough ICE shutdowns, it is true that they are more noticeable in the Prius because they happen at random times, but there is another factor. The Prius parks the ICE at a particular position to make starting cleaner and easier, and also burns off fuel during the shutdown. Sometimes this process causes a little gallop in the ICE during the shutdown, where it lurches around for another spin.

Tom

 

In a related observation, I can move through my neighborhood (25 mph speed limit) 1 mile downhill on a glide (no arrows of any kind on the MFD).
As I glide into my garage, I can hear that the ICE is running (reverb off the garage walls and still no arrows of any kind) and I can feel it shut down. This happens after a 50 mile commute (warm up phase way over) and I "typically" have 50% charge (blue bars)
IF the ICE is running, not just turning, it has to be consuming fuel. It is running AFTER I stop in the garage for 5-10 seconds and the MFD has shown no ICE power usage for a 1 mile downhill glide. It is not warming up or bleeding off excess charge. Any ideas?

 

Scan gauge will tell mpg up to 9999mpg. Lots of times the scan will show ~900rpm, fuel mileage ~250mpg. So even when the mfd shows no arrows, you can indeed burn gas. A bigger question for me, is when the ICE is running at ~900 rpm burning no fuel, why doesn't the compression of the engine act a a brake. Has anything been done with the cam timing so that there is no compression stroke? Seems like a simple idea (although complicated to build) that would save even more fuel.

Icarus

 

Actually (and rather curiously), completing the warmup sequence is not dependent on time, distance, or temperature, except to the extent the ICE has to be at least 70C. From there stage 4 (the final stage) is not reached until after the car comes to a complete stop, is allowed or induced to run the ICE for 7+ seconds, and shuts down on its own while still stopped. What you are describing sounds just like stage 3a behavior. See this for more. (Admittedly a little different issue from that raised by the OP.)

 

I'm guessing, as Jimbo states, that the ICE is doing its idle check. It has to do this before going into stage 4. The previous generation of Prius lacked the idle check, and as a result, required more frequent cleaning of the throttle body.

Tom

 

This website (you experts) is an AMAZING source of info.
I know non-hybrid ICE's. Need to rebuild an old 327...I'm your guy. I decided shortly after getting this car that I would except the "quirks" as normal and just drive it. I've have done so and been very happy. I didn't stop noticing, I just stopped worrying!
Thanks again for the help!