When Accelerating, how far right should I go on the Hybrid System Indicator?

In the dim recesses of time, when cars had carburetors, dinosaurs roamed the earth, and I was a teenager, accelerating hard caused the carburetor to pump in all the gas it had available and hope for the best. Acceleration was bad for gas mileage.

After 1990, no car has a carburetor, they are all fuel injected, so the manufacturers have MUCH better control of the air/fuel mixture. But old wife's tales die hard, just look at the folks panicked by 10,000 mile oil changes, their granddaddy told them 'every 3,000 miles' and they are not just about to let 50 years of engineering improvement change their mind.

Barring wheel spin (and your Prius won't let you spin your tires) it is going to cost the same amount of gas to get to 60 slowly as to get to 60 briskly.

Your Prius cruises very efficiently compared to 'normal' cars, it decelerates VERY efficiently compared to 'normal' cars. It accelerates pretty much as efficiently as any other car.

You want to be done accelerating and moved on to cruising as soon as possible.

If I could program my 'ideal' acceleration, I would want to use all the power the ICE had to offer before it asked for any power from the battery. But I have wiser things to be watching than all the little gauges, so I watch other drivers, not my MFD.

The old
Accelerator Pump
The new (the third graph shows how constant the air/fuel ratio is even when you race the engine)

 

I pay pretty close attention to the HSI when cruising or decelerating but not so much when accelerating. If I'm increasing speed more than 30 MPH or so I usually have it over in the red.

BMW did some pretty good studies some years back that indicated accelerating decisively (I think they said 2/3 throttle or so) used less gas that creeping up to highway speed. Of course that was using their cars but I suspect those studies would apply to most cars including ours. It would be difficult to prove without the use of a dynamometer though.

 

That question makes the assumption there's a level on the Eco-Meter you should avoid.

There isn't.

What causes a big impact to efficiency is remaining in the PWR (red) zone too long. Being in there briefly is fine. The system takes advantage of having both a traction motor and a combustion engine, along with the ability to send power in several directions all at the same time.

From my perspective (having an aftermarket tachometer available), it's brisk acceleration for just a few seconds... seeing it just briefly tap 3500 RPM before falling back down as quickly as it came up. That gets the vehicle up to speed faster than typical traffic, yet still keeps Prius green. Gotta love the synergy.
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With traditional Otto cycle engines, i.e. virtually all gasoline non-hybrids, that finding pretty well lines up with the common wisdom of hypermilers, and with the proven results of the Pulse & Glide technique. Otto engines are not very efficient at the low throttle conditions generally used for cruising or gradually creeping up to speed, though the point of best engine efficiency does vary among engine designs. This is where a BSFC chart of a specific engine is very helpful.

The Atkinson cycle engine in the Prius, combined with the HSD management, appears to produce near-maximum efficiency over a much broader range of RPM and load. So I've generally had the impression that acceleration rate is pretty not a first-order issue from an efficiency viewpoint, so pick whatever rate fits the situation. Don't floor it, don't let it poke along at just idle RPM, but most of the range in between should produce fairly similar results.

 

All very true, but I'd like to add that...

It's not so much how fast you accelerate but what is your top speed that you accelerate to. Accelerating to a lower top speed (e.g. 40 mph) will be more fuel efficient that accelerating to a higher top speed (e.g. 70 mph). In addition, what matters even more wrt to fuel efficiency on a Prius is whether it can continue at that top speed and conserve its momentum or whether it must give up energy/momentum to stop or slow down for traffic. A vehicle tends to be more energy efficient if it is allow to maintain a speed range, say of 40mph to 50mph, and not have to do a full stop. It's more energy efficient to accelerated going downhill and less energy efficient to accelerate going uphill ( because the powerplant is going against uphill gravitational forces as well as friction and its own inertia). That is to say a vehicle's momentum (mass*velocity) is the same whether its created while going downhill or uphill but its more fuel efficiency for the Prius to accelerate (build up momentum) going downhill than uphill. Lab results show that the 2010 Toyota Prius ICE best power efficiency is from about 1200 RPMs to 2200 RPMs [1]. This corresponds on the HSI display to being slightly past the middle of the ECO bar to the far leftside of the PWR bar.

Walter

[1]
http://www1.eere.energy.gov/vehicle...eview_2011/veh_sys_sim/vss031_rask_2011_o.pdf
The Argonne National Laboratory(ANL) Advance Technology Vehicle Lab Benchmarking Level 2: 2011 DOE Hydrogen Program and Vehicle Technlogies Annual Merit REview. May 10,2011. Eric Rask, ANL.
Project ID #VSS031. See page 8 to 17 Chart [Gen 3 best Power/Torque to RPM efficiency (33% to 36%) about between 1200 RPMs to 2200 RPMs]

 

I try to keep it in ECO right on switch to pwr for economy if possible. It keeps ICE in most efficient range.

 

My experience is the opposite -- spin up the RPM going uphill and store some gravitational potential energy while the engine is operating at fairly high efficiency. Then coast or glide downhill on that stored gravitational energy with the engine shut completely off, rather than operating in the low RPM / lower efficiency portion of the graph on page 8 of your link below.

Huh? On what page do you get this best ICE efficiency band ending at 2200 RPM? I see:

33% at 1200 RPM
34% at 1400 RPM
35% at 2000 RPM
36% at 2500 RPM
No visible downtrend > 2500 RPM.

On what page do you see this? I don't see it anywhere in the slide deck, and my own experience is that HSI position does not correspond well to RPM. The mapping changes considerably with speed and possibly other factors.

I still don't see the 2200 RPM band edge you claim, nor anything relevant to your statements on any slide other other than page 8.