ICE loss vs. conversion loss - use or not use ICE from dead stop?

[2009Prius]

Someone suggested starting a new thread to continue a diversion from another thread. The question is for max MPG whether to use ICE from a dead stop (assuming car is warmed up and SOC is enough). It seems to me a compromise between ICE loss (ICE not most efficient at very low speed) and conversion loss (ICE -> battery -> motor is lossy process). We need more data. Thanks!

[donee]

Hi 2009P...,

Depends on traffic. If you cannot get the engine up above 1800 rpm for your accelleration, its best to use the electric lightly, until a big gap develops, then go to engine at 1800 to 2400 rpm.

It also depends on if you have allot of hills. Since you entered your location as "USA" its not possible to know if your location has allot of hills. If you have hills, then the energy to climb the hill goes into the battery on the way down. And so , your going to have the conversion loss whether you want it or not. If you do not use that energy up, then the car will free wheel the engine with Motor Generator 1 (MG1) until the battery SOC comes down.

The Prius is not an electric car. As such, the motors, cables and batteries are do not have the low resistance pure EV cars have. Heavy accelleration in EV mode is going to waste allot of energy due to the electric system high power inefficiency. I believe under such conditions the Prius only converts 60 some percent of the battery energy to car motion.

The best way to use stealth mode is when you want to keep going along at the same speed, but not start the engine.

[JimboK]

For background, here is the other thread.

Donee, with all respect, the problem with the less-than-1800 rule is that most won't know their RPM. Furthermore, I have come to believe that low RPM at low speed is not inefficient.

The subject of how much go-pedal to give during ICE-on conditions of course is a matter of substantial discussion. Low-RPM conditions are thought by many to be inefficient, and the theoretical power curves of the Prius seem to confirm this. But anecdotally, many hypermilers (me included) have reported good results with keeping RPM as low as 1200 or 1300 at low speeds.

I have done some preliminary testing comparing various acceleration rates during pulse and glide, and the results suggest rates in the range of 1300 are at least as efficient as those in higher ranges. The key word is preliminary, however; the test environment was less than ideal (traffic disrupted my glides on the two-lane rural road I was using), and I'd like to repeat the tests in a different environment.

Meanwhile, Hobbit recently has experimented using fuel injector timing as a surrogate indicator of engine load. The ICE really is inefficient when it's not running under load, and his view is that as long as injector timing has reached a certain operational plateau, the ICE is loaded. Quoting him:

"Even a fairly gentle startoff from the line yields injector times of high sixes or 7.x at ridiculously low RPMs like 1250, as high torque is produced to help takeoff."

My own recent experience applying this and using injector timing as my load criterion (monitored in CAN-View) suggests that in ICE-on conditions at low speeds it's actually difficult for the ICE not to be under load. Especially from a dead start, after "goosing" the go-pedal just enough for the ICE to light, I show injector timing levels well above the "loaded" threshold even with these low RPM.

So that has become my approach for "normal" acceleration purposes from a dead start. I agree that for short-duration, low-demand situations it is probably more efficient to run in EV mode; those are the "abnormal" situations: around a parking lot or driveway, or as you suggest, extending a glide.

For those without added instrumentation, I describe a rule of thumb for efficient ICE-on conditions here, (from which much of this post has been copied and pasted).

[usbseawolf2000]

I agree with JimboK with the ICE high efficiency at those low RPMs. If you plot hp from the graph below, 15 hp to 20 hp are around 1,200 RPM to 1,600 RPM. Prius ICE is 35-36% efficient at those low RPMs. Otto cycle ICE can not touch it even at their peak.



To answer OP's question, I accelerate with the battery 10-15 MPH before the ICE kicking in. The reasons are:
- A chance to use the power captured by the regen brake.
- Minimize conversion loss from ICE. Below 10 MPH, most of the ICE power goes to the generator.

At 0 MPH and if ICE runs at 1,500 RPM 100% of the power (~18hp) goes to the generator to power the MG2. If the ICE were to kick in at 15 MPH, 56% of the power goes through conversion and 44% goes through directly to the wheels.

Note: My style is adapted to fit traffic here in NY.

[tredstone]

Originally Posted by usbseawolf2000

Note: My style is adapted to fit traffic here in NY.

What works for you should work for me! What kind of mileage do you get?

[DaveinOlyWA]

wow... what a complicated way to say...

JUST DRIVE IT, BUT DRIVE IN CONSERVATIVELY!!

by considering the option of using EV, you have already conquered the beast. no sudden acceleration (since EV wont allow it) anticipate traffic flow by keeping a little extra following distance, timing lights, etc... and that is all you need.

as far as what mode to use during this?? that is not your job. let the millions of investment dollars behind the technology work for you!!

[daniel]

Originally Posted by 2009Prius

Someone suggested starting a new thread to continue a diversion from another thread. The question is for max MPG whether to use ICE from a dead stop (assuming car is warmed up and SOC is enough). It seems to me a compromise between ICE loss (ICE not most efficient at very low speed) and conversion loss (ICE -> battery -> motor is lossy process). We need more data. Thanks!

Your question is somewhat misleading, because at low vehicle speed the ICE is turning fast, and MG1 is generating electricity and sending it to MG2. The car is converting mechanical energy to electrical to mechanical, but most of that is bypassing the battery altogether and going directly from MG1 to MG2.

However, if you accelerate slowly enough to prevent the ICE from starting, then you are drawing from the battery and adding in battery losses.

Therefore your best bet for accelerating from a stop is to do so briskly enough that the ICE starts. Avoid jackrabbit starts, but also avoid crawling starts. In a pure EV, the more gently you accelerate the better. But the Prius is a horse of another color altogether because it's a gasoline car getting all its energy from gasoline via the ICE, but allowing the ICE to operate in a more efficient range since ICE speed is almost completely decoupled from vehicle speed: The ICE can turn at any speed the computer likes, regardless of vehicle speed. So much so that the ICE turns forward while the car goes backwards, without using a reverse gear.

[usbseawolf2000]

Originally Posted by tredstone

What works for you should work for me! What kind of mileage do you get?

I am averaging 52 MPG now.

Majority (80%) of my daily miles are @ 65-70 MPH in the HOV lane. Average at these speeds is probably 45 MPG. The remaining 25% are low speed local or highway traffic congestion and they bring up the overall MPG.

If I leave a little earlier from home, I take HOV lanes just to avoid a few jam exits. This way I can go slower (@55-60 MPH) in the right lane. I averaged 55 MPG in last summer this way. My best tank was 65 MPG with unusually heavy traffic jams on LIE.

My favorite part is the congestion section on LIE after HOV lane end. I get 75 - 99.9 MPG until I exit at 22B to home.

[usbseawolf2000]

Originally Posted by DaveinOlyWA

JUST DRIVE IT, BUT DRIVE IN CONSERVATIVELY!!... let the millions of investment dollars behind the technology work for you!!

Thanks Dave. You are right. Let the computer decide that question. The driver should just accelerate according to the traffic and forget about fine details.

[Ogo]

Well, the driver is still the boss and can influence greatly. Computer just takes care of other details.

Like others have observed. I see quite good fuel economy driving around 1300 rpm. Regarding acceleration, on lower speed ranges, accelerating in range between 1800 - 2300 rpm gives me best fuel economy results. On highway I have to push the rpm above 2500 to accelerate and keep it in range around 2800 - 3300 rpm to still be efficient. Above that rpm value, fuel economy is more or less getting quite efficiently killed.