How Fast to Accelerate

Ideally you would accelerate entirely on ICE just under when enrichment circuits would open in the fuel injection. (I would pay good money if this was the cruise control acceleration default) In practice if you get no arrrows to or from the battery you are not getting any conversion losses while accelerating.

 

I use the HSI display 99% of the time. If you have no one around you, accelerate under the "pulse zone". Note that this isn't enough acceleration for all situations.

My philosophy when accelerating off the line is that you only have to keep up with the car next to you. After getting up to speed, I let the lead car in the lane next to mine gain a few car lengths, so anyone behind me can pass if they want to.

I do still normally find it necessary to go into the power band when anyone is around me, when starting from a signal light.

The goal is to avoid using the battery. If you accelerate hard, you'll notice you not only have the ICE running, but you are also drawing battery power. Like I mentioned earlier, sometimes it's just unavoidable.

On surface streets, the things that will achieve the best fuel economy is:
1) anticipating red lights & traffic 2) maximize P&G 3) minimize excessive acceleration 4) minimize regen braking 5) minimize friction braking

On on-ramps, I just accelerate as quickly as possible. There's no point to consider fuel economy when you have to merge into traffic. Safety is #1.

 

I understand the idea of losses and energy transfer - but if you're not using the battery to accelerate, what's the point of HSD?

-Iain

 

I have been struggling with this same question lately. It seems odd that we should pay all this money for a system we should try not to use. The simple truth is, since all energy ultimately comes from the gas tank, the most efficient thing to do is run the ICE in its most efficient range and send that energy directly to the wheels. If you use energy from the battery, remember that it ultimately came from the ICE so you have both generation and motor losses in addition to the normal losses associated with the ICE.

So, if you could pull out of your driveway and P&G all the way to work, you wouldn't benefit from the HSD.

But that's not how real driving works. If you are forced to "waste" energy by braking, you can save some of it by regenerating. Now that it's in the battery, how do you best use it? It comes in useful when you do have to accelerate in the power range. This allows use of a smaller ICE that can be supplemented when you need it.

Also, the ICE has to warm up to run efficiently. While it's running, might as well try to re coop some of that wasted energy in the battery. You can use some of that energy putzing around a parking lot or creeping along in stop and go traffic. Not ideal, but better than not having that option.

 

I've found that EV use a little more than what would be considered judicious is essential for slower drives. This is contrary to what most here consider ideal. You'll see most posts clearly advocating against EV use at the start of acceleration, for example.

While not highway acceleration like OP is asking, Wayne Gerdes has done between 97.1 and 101.1 mpg on a 40-mile route with 28 stop signs and getting stopped three traffic lights in different generation Prii. He said very clearly regarding this route that he accelerates to 15-17 mph using EV only before keeping instant mpg on Scangauge 2 to 5 mpg higher than current speed or around 1500 rpm.

Of course, we can't drive like this all the time, but what it tells me is that there are techniques for best acceleration that are counter to what are *technically* the best, which I think could be relevant for this topic. I'm getting a Scangauge soon, and will be doing some acceleration tests in various situations, including highway acceleration from zero, because I've been exceedingly curious about this, and it seems without specific numbers these acceleration threads always leave a little to speculation at the end...

 

The point of HSD is that you can't always control driving conditions. HSD allows for decent acceleration while using a small, efficient ICE. Just because you sometimes need to use the battery doesn't mean that you should try to use the battery. It's like that big gas engine in your conventional car. Just it can generate 300 hp, but you don't need to use all 300 every time you take off from a stop.

As a bonus, HSD also allows for regenerative braking. It's just a bonus. Obviously you would get better mileage if you didn't have to brake at all, but that's not going to happen in the real world.

So why have HSD? Because we drive in the real world.

Tom

 

Two points:
1) You're forgetting that the batteries are needed to spin the engine up to speed before gas is injected to the ICE when it is turned on, they also top off the 12-volt battery as needed, allowing the A/C and heat to run under electric power (without the ICE, like most cars). So the big battery pack is needed to do the pulse and glide - to turn the engine back on, and to power the other things when the engine is off.

2) Beyond that, with the battery to provide extra power as needed, then the engine can use the more efficient Atkinson cycle type of engine design instead of the standard otto cycle (most engines convert about 30% of the energy in gasoline into actual horsepower on the axle, the Prius engine supposedly gets another 12% on top of that). The Atkinson cycle however doesn't provide as much torque, meaning the real-world acceleration is not acceptable. But with the battery ready to step in and provide extra oomph when it's needed, then the Atkinson cycle engine is feasible.

(dkelly - it's 'recoup')

 

In normal conditions, battery power (EV) is used exclusively during startup for the first minute, while the engine heats up. This allows the Prius to have ultra low emissions (lowest of any gas burning vehicle, as far as I know).

Also the HSD allows the ICE to be 98 hp while peak hp is 134. So it allows the use of a smaller more efficient engine, while allowing you to have enough available power to pass other vehicles, merge into traffic on on-ramps, and accelerate on steep inclines.

HSD also allows the ICE to shut off during idle or low power requirements. Otherwise the ICE would always idle at 1000 RPM just to keep itself from stalling.

Also during sustained lower power conditions, such as moving across a parking lot, the ICE is inefficient. Running the ICE less than 15-20 hp is very inefficient. The Prius would reduce intake (increasing pumping losses) and/or use EGR to reduce power. This probably impacts efficiency a lot. In this instance, EV is the best choice.

The useful range of a full battery provides about 27 horsepower for 100 seconds sustained. This is what Toyota decided was necessary to buffer the Atkinson engine in Gen II and Gen III.

 

I just take off just like everyone else in traffic, i'm almost always amost full red in power area. I've tried doing it lighter but i don't see even a 1-3% gain in mpg so i don't waste my time.

 

I guess I am doing something wrong. I will accelerate to speed, < 40 mph, then take my foot off the accelerator and let the instant mpg max out and have the bar to the left center side of the HSI using the battery to keep speed, then repeat. I am getting around 55mpg

 

Dear all, I don't think anyone's doing it wrong after this morning. Some numbers to help us out:

Mike with a Prius: Acceleration to highway speeds

edit: note this is with a Gen II, but I imagine this is uniform across the generations and other cars as well.

 

I've had my 2011 Prius 2 for a month now. Morning temps in Wisconsin have been in the teens, 20's and sometimes 30's all month. Except for some 60 degree days where I got 55-59 on my 24 mile commute at highway speeds in traffic. I hadn't read many forums before the purchase. I saw a you tube video that said to accelerate gently with the hybrid system indicator screen showing the power still in the Eco range. I did that for a few weeks and my first 5 minutes of driving were always under 25. Sometimes under 20. The next 5 minutes I got 35-45 sometimes over 50.

One day I had to get my son to school fast. I accelerated quickly then once up to speed on the 25-35-40 mph roads, backed off and drove a tad over the speed limit, but held that in a glide-like mode. On that very cold morning my first 5 minutes were well over 25mpg. Probably 33-35 or so. It didnt' register with me so I continued babying it for the last 2 weeks of this month until a few days ago. Now I've been pushing down around 1/3 of the way on the gas pedal at least. The engine is in it's sweet spot, the car gets up to speed quickly allowing me to very shortly back way off on the pedal and coast. My first 5 minutes of driving are again over 30 mpg in below freezing temps, every time. Given the amount of fuel needed to just keep the engine idling when cold means you can't get decent mileage going slow.
Once warm and at highway speeds, fast vs. slow acceleration hasn't seemed to make a huge difference. I'll try what the article said to do, slow acceleration down hill, more quicker uphill.

 

I'm glad other folks have had the same thoughts and questions I did...

It's been about three solid weeks of driving for me, mostly highway miles. The first week I was so conscious about running in ECO mode and doing the slow acceleration and P&G as I'd read about. 52.2mpg

The second week I would occasionally hit the power mode to get out a bit quicker after red lights (and to avoid tractor trailers). 51.9mpg

This week I said screw it and switched from ECO to Power fairly often as traffic dictated, dropped the hammer frequently, and just generally drove with less concern on miles per gallon and more on getting to where I wanted to get to. 52.1mpg

Moral of the story? I'm not seeing dramatic swings or massive pros or cons for either driving super consciously, or like an nice person Are there people out there that are driving "properly" and hitting the upper 50s and pushing 60mpg? Sure. But at the end of the day, I'm still seeing three times the mpg I had in my truck, and I'm enjoying the ride. YMMV.

 

I've been doing some more experimenting too. had the car since late feb. with a cold engine on cold mornings, quicker accleration then backing off and gliding with a light touch still works best. Once the car is warm however, or if accelerating from a stop to highway speeds, things seem different. If I have to get from 0 to 55-70, doing it gently seems to be the best. If I hit it, it takes much longer to make up the MPG drop once cruising. cold engine, things are different. I may be nuts too. Warm engine in town, it still works well to accelerate quicker. I read elsewhere to accelerate using the MPG guage, keeping it high as you go. That equates to a rate of acceleration that won't work with people behind you that need to get somewhere, such as a job. I'm in the same boat but I'll do it if I'm alone, and it works. I admit, I'm a newbie so experienced drivers may have found better techniques. Holding up traffic is one thing I won't do. It traps too many people at stoplights and that means their engines are idling longer, lower MPG, and they take longer to get where they are going.