Specific Question on Accelleration

Starting out from a normal stop
1600rpm or about 1-2mph per second

I also like to use EV when starting out from 0-10mph with my EV button. On the CV I try to keep my amps down around 30amp or lower depending on traffic.

Getting on the highway
I try to keep it at or below 3000rpm depending on traffic and merge conditions. I always get up to the speed limit before leaving the on ramp and merge with traffic. If you have a short on ramp you will most likely have to break the 3000rpm to get up to speed.

 

To quote john1701a; "Just Drive It"

I don't think there is any real point in trying to optimise your driving behaviour around the car's performance, because the car's synergy drive has already been optimised to suit typical driving behaviour.

I'm sure I'll get some crispy replies to this point of view 'cause there are so many people who enjoy taking their FE stats as high as possible, and keeping an eye on the MFD makes it possible to think about it the whole time you drive.

It IS fun, and I'm really glad that people can think about a stat other than time to reach their destination as a measure of their driving skill.
I'm just not convinced that moment-to-moment monitoring of one's driving behaviour will produce an appreciable increase in one's FE.

I have in mind an experiment to invite PC members to participate in to test this hypothesis. Let me see if I can approach one of the moderators and work some details out.

 

<div class='quotetop'>QUOTE(Winston @ Aug 15 2007, 03:44 PM) [snapback]496445[/snapback]</div>

That's a good question. I've often wondered the same thing. It's hard to relate in a post - i.e. what does "fairly briskly" mean? It would be nice if someone would quantify it. Like from 0 to 30 in x seconds.

I personally have slowed my acceleration a lot from the beginning. I used to blow people off the line all the time because that was my interpretation of "fairly briskly" but my mileage stank (mid 40s). Now I tend to accelerate pretty slowly - as slow as the traffic pressure behind me will allow - sometimes barely accelerating at all.

I get much better mileage now (mid 50s) - not only because of my slow acceleration mind you - I also went to 44/42 in my tires, and I drive farther because I moved out of town...

I suspect that my ICE is not at an optimal rpm a lot of the time, but since there is no Tach and I can't really justify spending money on a can-view..... I'm just going to stick with slow acceleration for now.

 

Hi Winston,

It depends on warmup and battery SOC (State of Charge). When you first start the car, it avoids using the engine at high load soas to prolong engine life. The heavy accelleration at this time comes more from the battery. As gasoline to transmission to inverter to battery is less efficient than battery to transmission to wheels, it best to go lite on the accelleration at this time. Very light, actually. This only lasts about a minute, however.

Now the car is into a warmup mode. I drive like other cars in this time. I try to maximize the cruise portions of the trip too. This will get you SOC up to 6 or 7 blue bars.

Once the engine is warm, and the battery is up to at least 5 bars, you can start pulse and gliding. This operation tends to run down the battery, if you have the oportunity to do allot of it. If not, the cruising inbetween will bring the battery back up.

All warmed up now, and sometime on the road, I may come to a stop light and be the lead car. Other Prius drivers may not have this opinion, but when I do not do this, I tend to loose mileage in the suburban light moderated secondary road. Might just be an anecdotal observation, but that is what seems to work for me. I get up to speed pretty quick. Not full throttle (about half actually, maybe a little more), but considerably quicker than the average cars. Then I lift throttle completely, and ease the throttle back up to a cruise position slowly, and drop it into cruise control if on the highway and faster than 40 mph. Otherwise, I try to go for a glide. And about at the end of the glide other cars catch up, and I will accellerate to a steady speed limit rate to avoid impeding traffic below the speed limit. At which time the other cars pass me. And a mile down the road I pass 2/3's of them in the fast lane at a stop light, and end up having to stop rather than glide through as the light turns green. Because the other 1/3 of the cars are in the slow lane ahead of me. The lights are usually timed to the speed limit.

Granny accelleration in a warmed up Prius, with a good SOC will definately hurt the fuel economy. The engine runs longer, and at a lower efficiency level. 35 mpg in the city is the common result. They take the car into the dealer, and take a ride with the mechanic, who demonstrates a 20 minute 50 mpg run, and sends them on their way.

In hot weather, with the AC on, and traffic, requires different accelleration. While the car can definately accellerate in these conditions, not having belt power drag (due to the electric powered AC compressor), doing so will put allot of load on the battery. With the continous AC load, the battery SOC might get down into the 2 or 3 bar range, and this changes the automatic aspect of the cars operation. It tries to get that SOC back up. Which makes it difficult to glide, or warp stealth (use electric only drive near 40 mph). So, in this situation, I accellerate a little slower than average, so that I can get charging during the accelleration. Because in traffic there is preciously little cruise time, and lots of stopped time. So, the battery SOC needs to be coddled for best fuel economy. If you do not do this, the car will be just fine, but it will just use more gas.

 

For a car with a conventional drivetrain, better mileage is usually achieved with higher acceleration for shorter periods of time. The reason is that a conventional gasoline engine has higher pumping losses at lower rpm, so it's actually more efficient (in terms of turning gasoline into work) to run it at higher rpm.

I don't know which technique results in the best mileage for the Prius. The Atkinson/Miller (my definition of Miller does not require supercharging) cycle engine in the Prius, along with the rest of the hybrid drivetrain make the car sufficiently different that I don't believe you can draw the same conclusion as for a car with a conventional drivetrain.

 

<div class='quotetop'>QUOTE(samiam @ Aug 15 2007, 07:22 PM) [snapback]496480[/snapback]</div>

Oboviously you do not need to be posting in the FE category and keep your -.02 to yourself. Why even post in this field if you don't really care about FE? I see that you have a 2004 so you have driven a few miles and you have a great many post >300......I just hope all of your post don't look like this completely useless post.....and if you look at the beat the epa thread you will realize that you are wrong. Do you have any idea what your current lifetime FE is in your 2004? I believe there is a direct correlation betweeen acceleration/SOC and TPS. I have been viewing the TPS on my ScanguageII for a few days now to track instantaneous FE vs. TPS and SOC. I try not to accelerate past 40MPH as beyond that the ICE will stay running. When I have chances for Pulse and glide I pulse to 40 if flat terrain(about 37 if slightly down hill to prevent exceeding 40mph) then let go of the accelerator then just feather into it(accelerator) so I have a no arrows condition which is near 15-17 TPS on my SGII for a glide down to 32MPH. At all cost in the city I try not to exceed 40 MPH. To those of you reading this post that do not have a SGII.....when you are coasting greater than 40 MPH the ICE runs at about 960 RPM to keep MG1 from over speeding. Your engine icon on your energy screen might show that the engine is off but greater than 40MPH it is in all reality at 960 RPM. So the true P&G is up to about 39-40 MPH not to exceed 40 for a full clarification. I have been experimenting with slow and fast acceleration up the hill I need to climb to get out of my neighborhood but I do not have enough data to post for now.

 

Hi Priusenvy,

Regular cars with Otto engines do not have a battery to fill in the natural reaction time of the engine. A quick throttle depression will cause the control system to squirt extra fuel to overcome this limitation. The extra gas does not burn completely, unfortunately . At least in the common BMW I see around here. Some other cars do better. JackRabit starts will eat allot of extra gas in these cars. A slow steadily increasing peddle depression gets the throttle open the maximum amount, without an extra squirt of gas. Getting close to full throttle before the speed limit, then lifting to the cruise level (not all the way off like in a Prius), is best.

The Prius applies the battery power quickly, and the engine comes along later. There is only a little squirt of gas to start the engine, so its much smaller than a regular car. And the car will do that little squirt whenever it starts the engine, slow or fast throttle opening.

The Atkinson engine still has a throttle. The Prius does not have variability lift of the valves. So, there still is throttle loss. The Atkinson engine extracts more energy by having a larger expansion ratio than standard engines. The throttle is controlled non-linearly. Unless the car has a good amount of load its not going to pop the throttle open all the way. The engine RPM is then controlled by by the transmission ratio. At least, that is my guess why it seems to work. Lifting accellerator then closes the engine throttle quick, without any part throttle operation. The engine time is miminized in this kinda of cycle.

 

<div class='quotetop'>QUOTE(donee @ Aug 15 2007, 06:32 PM) [snapback]496489[/snapback]</div>

That is too general a statement, be careful. In the heart of urban in-city traffic, not suburban/exurban, slow acceleration is not going to hurt if you glide properly. Since most traffic lights are 1/2-1/4 mile apart, you'd brisk up to the next light so you have to be careful.

It is easier to come up to some speed and glide the remaining distance. 1/2 mile to the next light, accel 1/8 mile and glide 3/8 miles, doesn't matter how you get to the 1/8 mile point. Having said that, I find that due to traffic, speed limits and timing of lights an acceleration of 1300 to 1500 RPM works quite well. Put another way, however you get to 15MPH is up to you, after that try to match MPG with MPH +/- 2 until at the speed you desire. Results shown in the "I beat the EPA thread" including last tank of 80.1MPG for 931 miles.

Now to wait for JimboK to respond from his similar thread.

Wayne

 

<div class='quotetop'>QUOTE(Winston @ Aug 15 2007, 05:44 PM) [snapback]496445[/snapback]</div>

According to the marathon team that accomplished 110 mpg for one full tank in a stock Prius on public access roads. . . .

http://priuschat.com/index.php?s=&show...st&p=114922
"I have learned a lot as a result of participating in the marathon.

The first thing that we could all agree on is that there seems to be no correct way to accellerate. The Prius is very forgiving as long as you keep acceleraton within the 15 - 50 MPG range. Maybe the range is even wider but we did not try anything outside. Rick used 15-20 for his 115 MPG shift, I used 25-35 for my 115 MPG shift and Wayne used 40-50 for his 120MPG shift. The extra 5 MPG was more likely due to gliding down to much lower speeds than Rick or myself."

<div class='quotetop'>QUOTE(samiam @ Aug 15 2007, 06:22 PM) [snapback]496480[/snapback]</div>

Unless you are achieving anywhere near 100 mpg without monitoring your driving behaviour, I think it is safe to say that this comment has been repeatedly proven inaccurate.

<div class='quotetop'>QUOTE(Fibb222 @ Aug 15 2007, 06:25 PM) [snapback]496483[/snapback]</div>

I suspect that the change in typical route and distance had more to do with it than your slow acceleration, but I may be mistaken.

<div class='quotetop'>QUOTE(donee @ Aug 15 2007, 06:32 PM) [snapback]496489[/snapback]</div>

Not according to the marathon team that did pulse and glide continuously for 48 hours for 110 mpg.

http://priuschat.com/index.php?s=&show...st&p=114922
"What I found best for my style was not deadband at all. It was what I call "accelerate with charge". I found that during the night the headlights made it difficult to maintain the HV battery charge so I had to revert to "accelerate with charge", pink arrows from ICE to wheels, green arrows from MG to battery, no orange arrows at all. This kept the battery level steady. I continued this technique on my second shift in the daytime and found that I had an abundance of charge (no headlights) and I could actually use stealth to help me over small hills instead of using the ICE. This is how I went from 105MPG to 112 MPG. So much for my deadband habit."

<div class='quotetop'>QUOTE(donee @ Aug 15 2007, 06:32 PM) [snapback]496489[/snapback]</div>

And again this seems inaccurate according to the 110 mpg marathon team. . . .

http://priuschat.com/index.php?s=&show...st&p=115298
"Slow acceleration uses fuel at a lower rate, but takes longer to get to speed, while moderate acceleration uses fuel at a higher rate, but takes less time to get to speed. It appears to be a wash. "

 

<div class='quotetop'>QUOTE(priusenvy @ Aug 15 2007, 05:27 PM) [snapback]496541[/snapback]</div>

A conventional engine has lower pumping losses at larger throttle openings so it is more efficient to run the engine at low RPM with a higher throttle opening for a given output power.

There is a limit to how low the engine speed should be allowed to drop - probably below 1200-1500 RPM is inefficient.

In the Prius the throttle is controlled so it is almost wide open for any RPM above 1200-1400 RPM to reduce pumping losses.

kevin


<div class='quotetop'>QUOTE(donee @ Aug 15 2007, 06:58 PM) [snapback]496607[/snapback]</div>

The Prius engine operates much of the time at virtually full throttle any time the engine is running above 1200 RPM, then as you say the power is controlled by changing the RPM with the transmission ratio.

kevin

 

<div class='quotetop'>QUOTE(kevin17 @ Aug 15 2007, 10:59 PM) [snapback]496684[/snapback]</div>

I assume you phrased it this way so that readers who still think in terms of a traditional transmission would understand the concept, but just to make sure. . . .

You do realize that the Prius has a fixed transmission ratio, right?

 

To summarize, low RPM, high torque. The Prius tries to do that
automatically; plenty of non-hybrid hypermilers are experts at
doing it manually.
.
_H*

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Aug 15 2007, 09:58 PM) [snapback]496716[/snapback]</div>

Yes - I was just using the original poster's language.

kevin

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Aug 15 2007, 08:42 PM) [snapback]496675[/snapback]</div>

Wow if this is correct that would be very interesting indeed. My improvement from mid 40s to mid 50s could possibly be explained by my longer trips, more air in the tires, warmer season, and less deceleration (anticipating lights) not so much slower acceleration. I hope you are right. If you are then I can occasionally take some hot shot off the line for amusement.

For those of you that think 1700 to 2300 rpm is ideal, what would that translate into in terms of mph/s? - for those of us without can/scan-view.

Thanks

 

<div class='quotetop'>QUOTE(Winston @ Aug 15 2007, 06:44 PM) [snapback]496445[/snapback]</div>

I have a ScanGauge that goes between our cars. When it's in the Prius, I focus on the fuel rate in GPH (gallons per hour). I try to keep that in the 1.2-2.0 gph range, close to 1.7gph, while pulsing (accelerating) using pulse and glide.

When I don't have the ScanGauge in the Prius, I estimate the ratio between the mph on the speedometer and the mpg on the MFD. I'm not quite a calculator, so I usually try to keep the mpg a little more than half the mph.

My objective is to try to keep the engine operating in its most efficient range. Using a target RPM, MAP (Manifold Absolute Pressure), engine load, or TPS (throttle position sensor) using a ScanGauge should give approximately the same results.

I believe that focusing on accelerating x mph per second is a mistake, if the range of speed is broad. Accelerating 1 mph requires much less energy when your speed is load. By this measure, optimal fuel efficiency requires faster acceleration at slow speeds and lower acceleration at high speeds.

I believe that trying to target a mpg is also a mistake, if the speed range is broad. Accelerating with 25mpg when traveling at 5 mph puts an inefficiently low load on the engine, using only 1/5 gallon per hour. This is such a low engine load that the Prius will try to wisely rely solely on the battery rather than the engine at such an inefficiently low load. Accelerating with 25mpg when going 75mph would be placing an inefficiently high load on the engine, using 3GPH.

 

I don't have hypermiler stature, but before I changed my OEM tyres to a very poor choice for great mpg results I was consistently just scraping 70 mpg tank after tank in all suburban type driving during spring and fall gentle climate driving.

My advice: put your effort into 1. anticipating traffic 2. gliding below 43 mph on appropriate roads 3. Picking routes that let you time signals (or even better -- don't have them ! 4. Stay off highways if traffic makes you go over 65 mph.

I personally am willing to be uncomfortable but not miserable in the car, and so use AC sparingly.

As for acceleration from stop: Don't floor the pedal, and don't crawl. Everything in-between is pretty ok. Not good enough for ya ? Shoot for 1/3 to 1/2 gas pedal depression.