Brake Specific Fuel Consumption Question

Status
Not open for further replies.
H

Hokiefyd

Joined
Jun 24, 2004
Messages
14,505
Location
Top of Virginia
Hi all. I recently acquired a 2007 Toyota Corolla LE with the 1ZZ-FE engine and a 5-speed transmission. I'm trying to modify my driving style to achieve the best fuel economy. I ran across an SAE white paper written by Toyota about the development of this engine, and it included a BSFC curve. Shown below:

http://byfiles.storage.live.com/y1pbA3bRdcxsVNoz12SblprqLEnvpJDyR4vS9LxDCZnkdMxuWTCO9eojMjbqy9d1uNe

Now...I know/believe that's valid for WOT only. But what about at part and near-closed throttle? I know that to maintain a given speed (say 60 MPH), a certain and static amount of power is needed from the engine (assuming a flat road, etc). So no matter what gear I'm in (4th or 5th), I only need so much power to maintain that speed, so that's, in effect, going to determine me throttle angle (and ultimately the output of the engine).

If the BSFC curve has a similar shape (but obviously a different magnitude), at part throttle or near-closed throttle, the engine is going to be most efficient at around 3000-3200 RPM. Because BSFC is expressed in terms of fuel usage per unit power, the ultimate fuel consumption rate at 3000 RPM should be LOWER than the consumption rate at 2000 RPM, yes, since I'm maintaining a steady speed, and only generating x amount of power.

My question I'm getting to is this: should not I cruise in 4th gear at 3000 RPM to maintain 60 MPH, rather than in 5th gear at about 2200 RPM? To me, the answer has to be 'yes', if and only if the BSFC curve looks very similar at part throttle as it does at WOT, as is apparently graphed in the image above.

Thoughts?

Edit: apparently this forum cannot process an image without a 3-character extension (.jpg for example), so I had to make the
I]
 
Last edited by a moderator:
I think we are confusing maximum engine efficiency (gal/hp hr) with minimum consumption/hp hr. Your engine produces its maximum efficiency at ~ 3200 rpm (max hp/lb of fuel)( which is the maximum torque). Your minimum fuel consumptions will occur much lower usually. I understand your idea and it might be somewhat correct when testing an engine on a dyno in a test laboratory. Most engines I am familiar with produce the best fuel mileage at ~ 1/2 the maximum BMEP or in your case ~ 1600 rpm. The question is now, do you want to drive at 1600 rpm in 4th gear or 1600 rpm 5th gear? I hope I explained this somewhat clear. Ed
 
Thanks Ed.

The torque peak for this engine occurs a little higher in the RPM range (at least according to Toyota), 122 lb*ft at 4200 RPM.

"Your engine produces its maximum efficiency at ~ 3200 rpm (max hp/lb of fuel)( which is the maximum torque). Your minimum fuel consumptions will occur much lower usually."

Based on what you're saying, and indeed on my own experience, what I'm thinking is the BSFC (or "efficiency") curve for the engine cannot be the same at part-throttle as it is at full-throttle. I'm thinking that perhaps the fuel/spark mapping (and cam timing via the VVT in this particular engine) is adjusted during part-throttle cruise to get the best efficiency at a lower RPM compared to where it is at WOT. Plausible?

For what it's worth, Toyota recommends the following shift speeds for "best economy" in the owner's manual:

1-2: 15 MPH (~2800 RPM)
2-3: 23 MPH (~2800 RPM)
3-4: 40 MPH (~2800 RPM)
4-5: 45 MPH (~2400 RPM)

I started to drive this car like I drive V-8s -- that is, short-shifting whenever reasonable, but not to the point of lugging the engine. My shift points are typically moderately lower than Toyota's, especially the 3-4 shift. On my first tank, I was in 4th gear by 35 MPH. I netted 32 MPG on that tank, suburban commuting. On the second tank, I started driving it a little harder, getting rubber into 2nd gear, finding the redline fuel cut point...just generally getting "comfortable" with the car and its capabilities...and I got 33 MPG with that tank. So this tank, I'm driving with an easy foot, but I'm also letting the engine rev more...at least as high as Toyota's recommended shift points.

I'm sure there's a line beyond which revving the engine out wastes fuel, and before which you're actually using more fuel than necessary due to short-shifting and putting more load on the engine. I will follow Toyota's recommendations for the next few tanks to the letter, and see what I get. I still have an inquiring mind, though, and would love to understand the physics behind it.

Thanks again for your thoughts, and for any continuing discussion.
 
My understanding is you are going to be more fuel efficient with a lower rpm and a larger throttle plate opening. I think this equates to better volumetric effeiciency.
 
Originally Posted By: Junior
My understanding is you are going to be more fuel efficient with a lower rpm and a larger throttle plate opening. I think this equates to better volumetric efficiency.


Correct, if you get a BSFC fuel map superimposed on a power curve you can see that the most efficient rpm for steady speed level ground driving is lower rpm than most people are willing to run or have the gearing to run. It's typically around 80% to 90% of peak power (or torque) for a given rpm. If you geared a car that way you would be changing gears a lot and running near full throttle at very low rpm all the time. Not a very pleasant way to drive.

Short shifting and overdrive top gears help get you towarrds that goal without going overboard.


On example is my C6 Corvette. It runs about 30 mpg at a steady 70 mph and 1500 rpm in 6th gear. It takes about a 4 mpg hit if I run 5th at the same speed and the engine is still turning slower in 5th than a lot of economy cars. FWIW, that car develops peak torque at 4400 rpm which shows what a bogus wives tale the drive at peak torque rpm for economy mantra is.
 
The difference is that a torque/HP curve measures the highest power produced by the engine, and at what RPM that happens. Regardless of how much fuel is burnt.

The BSFC chart measures how much fuel is used to produce the various power levels. So if it takes 20 horsepower to steady state cruise at 70 mph, you would use the chart to determine what RPM to run at for maximum efficiency.
 
Quote:
If you geared a car that way you would be changing gears a lot and running near full throttle at very low rpm all the time. Not a very pleasant way to drive.


I had this debate that you didn't chime in on. I think it was Eddie arguing with me. I asserted that 1-3-5 shifting had proven more economical in acceleration due to using wider throttle openings over going through the gears. It's not always "practical" ..but
21.gif
 
Originally Posted By: Gary Allan
Quote:
If you geared a car that way you would be changing gears a lot and running near full throttle at very low rpm all the time. Not a very pleasant way to drive.


I had this debate that you didn't chime in on. I think it was Eddie arguing with me. I asserted that 1-3-5 shifting had proven more economical in acceleration due to using wider throttle openings over going through the gears. It's not always "practical" ..but
21.gif



I doubt that it would make much if any difference if you actually short shifted at the optimum rpm...except, if you hit every gear on a 5 or 6 speed, you will be shifting so quickly and frequently that you will give up on it. So in the real world, skipping gears is better.

1-3-5-6 works well in my Corvette for driving in boring traffic. My VW TDI doesn't like 1-3 because 1-2 is a big jump, so I go 1-2 then 4 or 5 for driving in traffic depending on the condition. My 4.3 L GMC was happy with 1-3-5, it varies by vehicle.
 
This is kinda related, does anyone know if this is true? I found it on another site.

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

For the corolla that is around 45 mph.
 
The 1ZZ-FE engine has a stroke of 3.60".

RPM Piston Speed
1500 15 ft/s
2000 20 ft/s
2500 25 ft/s
etc. etc.

I have read that statement elsewhere as well -- that piston speeds in the 16.4 to 19.8 ft/s range are "ideal" as far as economy goes. In that case, by 2000 RPM, I'm already out of the ideal range. Plus, there are certainly other factors at play, especially in modern engines, like fuel injection maps, cam phasing schedules, etc.

This is a very interesting discussion, but I've pretty much given up on trying to find the technical info behind the answer, in my cases. There are too many unknowns that Toyota's engineering department would never let loose of (like variables I mentioned above). Plus, variable valve timing is a new thing to me and, while I understand how it works and its general benefits, I haven't even begun to wrap my head around how that can affect and "tune" the efficiency curve of the engine.
 
Mike,

It looks like someone looked at some data without understanding it then came to the wrong conclusion.

On my Corvette 16.4 ft/s would be 1620 rpm which is 76 mph. Best economy on a C6 Corvette occurs around 40 mph at 850 rpm in 6th which is 8.6 ft/s. That was verified by a Corvette owner with a lot more patience than I have in the flatlands of central Florida under windless conditions.

Best economy in the vast majority of cars occurs at someplace around 35 to 45 mph in the highest gear they will run smoothly in at that speed.
 
Your engine may be more efficient per rpm, but the fact remains that it has to turn 800 times more per minute to cruise at the same speed.

If fifth gear did not provide any additional fuel savings, Toyota would not have provided it.
 
Anduril, not really....why do cars come with wasteful six speed manuals these days, where you spend all day "rowing". Nissan with their 5 speeds in the 80s basically filled in the 4 speed spread with 5 "close ratio" style.
 
The absolute best thing to do rather than trying to figure all the numbers out is buy a scangauge to get real life mpgs. Real on the road readouts that the charts will never give you.
 
Originally Posted By: XS650
.... Best economy on a C6 Corvette occurs around 40 mph at 850 rpm in 6th which is 8.6 ft/s......


Amazing what modern engines can accomplish, eh? Driving in top gear at 850 rpm is quite a feat in any vehicle; my 81 F-150 w/300 six, T-18 and 2.73 gears can barely manage that and the 300 is a well-known lugger. Kinda embarrassing to be out-lugged by a Corvette.
Joe
 
Originally Posted By: Lazy JW
Originally Posted By: XS650
.... Best economy on a C6 Corvette occurs around 40 mph at 850 rpm in 6th which is 8.6 ft/s......


Amazing what modern engines can accomplish, eh? Driving in top gear at 850 rpm is quite a feat in any vehicle; my 81 F-150 w/300 six, T-18 and 2.73 gears can barely manage that and the 300 is a well-known lugger. Kinda embarrassing to be out-lugged by a Corvette.
Joe


There is nothing to be embarrassed about. At 40 mph a Corvette takes so little power to move that it's not doing much more than idle.
11.gif
Those 300 sixes are nice engines.
 
Status
Not open for further replies.

Similar threads

D
Towing and gear selection
Replies
12
Views
445
BrendanC
T
taught badly at driving a manual
2
Replies
39
Views
2K
JTK
buster
GM 8L45 and Octane
Replies
3
Views
872
ragtoplvr
C
First tow/with 2023 Silverado 5.3 - Total trip 539 miles
2
Replies
25
Views
3K
CKN
F
Modern Automatics, Low RPM, and Engine Longevity
2 3
Replies
45
Views
3K
Guisnu
Back
Top Bottom