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Interesting graphs about brake specific fuel consumption

S

ShawnD1

Lifer
Brake specific fuel consumption is usable power divided by fuel. It's a way of measuring efficiency.


http://autospeed.com/cms/title_Brake-Specific-Fuel-Consumption/A_110216/article.html
image of 100% throttle efficiency, power, and torque for a V12 jag

Firstly, why should the SFC be [best] at middle revs? Or, to put this another way, what causes an increase in fuel used per kW at both low and high revs?

At low revs, SFC suffers because there’s increased time for the heat of combustion to escape through the walls of the cylinders and so not do useful work. At higher engine speeds, the frictional loses of the engine rise alarmingly (especially in this case with 12 cylinders!) and so the energy of combustion is again being wasted, this time in heating the oil.

There’s another reason that SFC is [best] at ‘middle’ rpm. Because the engine is tuned to develop best cylinder filling (ie to produce best torque) at middle revs, the engine’s breathing is at highest efficiency at these speeds. But don’t fall into the trap of saying that SFC is always at its best at peak torque – that’s not usually the case.



graph of BSFC at varying amounts of throttle

At 100 percent load (ie wide open throttle) this engine has a minimum [most efficient] SFC of 0.43 – see the bottom curve. As we by now expect, at both lower and higher revs that this, the SFC rises.

But have a look at what happens at 50 per cent load! The SFC results at half load and 1000 rpm (ie idle) doesn’t matter much (when would you be in that situation?) but at 2000 rpm, the SFC has gone up by 13 per cent. At 4000 rpm, it’s gone up by just under 30 per cent!
[up means less efficient]

And keep in mind that in normal use, even 50 per cent is a lot of throttle. A more frequently used load is 25 per cent. At 25 per cent load, the SFC at 2000 rpm has risen by a massive 117 per cent over that achieved at full load! You can also see from the shape of the 25 per cent load curve, BSFC is even more heavily influenced than ever by the rpm being used.



BSFC for a honda insight

The Honda Insight hybrid uses a 3 cylinder VTEC engine. This diagram, taken from a French engineering investigation of the car, shows measured SFC for the engine. The testing was of the CVT transmission Insight and was done on a chassis dyno.

Note how the blue/yellow island of best SFC is achieved at relative high revs and load, and how there’s a second area of low SFC at about half load and 1500-2000 rpm. I assume that this second area is achieved through the VTEC mechanism, that in this car, at low revs shuts off one of the two inlet valves for each cylinder, promoting better swirl.


Conclusion
For example, anything that allows you to keep the throttle open wider and the revs lower (like changing up to a tall gear and then holding it) will reduce fuel consumption because BSFC will be improved.
Click to expand...


It's a nicely illustrated summary of concepts discussed on this forum in the past.
-Open throttle is more efficient than limited throttle at almost any rpm.
-Constant throttle (25, 50, or 100%) is often most efficient in the lower-middle rpm range.

Basically you're supposed to drive the way I drive. Keep the rpm low without bogging, pedal all the way to the floor, reap the rewards of excellent gas mileage. warning: doesn't work with automatic transmissions.
 
No shit, that's why you cruise in the highest gear...

Oh, and it does work with auto transmissions. 😱
 
If you'll notice in my post (which you're referring to pretty directly here) I specifically mentioned in EVERY vehicle this is different, and this will change dramatically by scenario. I will show you engine logs if you want (as I said previously) showing my car's fuel injectors and fuel pressure at the same speed in each gear. Comparing this Jag's v12 to my turbo I4 is completely useless.
 
So I use less fuel stopped at a signal if I shift into neutral and rev the engine to 3500 instead of letting the engine idle? 😕
 
No shit, that's why you cruise in the highest gear...

Oh, and it does work with auto transmissions.
Click to expand...
Depends on the car. Instead of open throttle to accelerate a little bit, many of them have a strong bias toward dropping gears and maintaining half throttle. Example: my automatic Corolla on a 5 degree incline will drop gears instead of open the throttle. On something like a 20 degree incline, the transmission will drop 2 gears and run at 6000rpm instead of just giving more gas to the engine. On the same exact hill, my Civic could maintain speed in top gear with the pedal to the floor.
 
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Well BMW has stated that you get best gas mileage in stop and go by keeping RPM low with wide throttle opening, according to people on this forum. That's what I did with my 240sx and Miata.

Just don't let RPM drop too low... keep above 2000 or so.
 
ShawnD1 said:
Depends on the car. Instead of open throttle to accelerate a little bit, many of them have a strong bias toward dropping gears and maintaining half throttle. Example: my automatic Corolla on a 5 degree incline will drop gears instead of open the throttle. On something like a 20 degree incline, the transmission will drop 2 gears and run at 6000rpm instead of just giving more gas to the engine. On the same exact hill, my Civic could maintain speed in top gear with the pedal to the floor.
Click to expand...

Just a quick question; What makes an engine rev to 6000 rpm rather than 2000 rpm?
 
Throckmorton said:
Just don't let RPM drop too low... keep above 2000 or so.
Click to expand...
Just make sure the engine doesn't start bogging and shaking the car 😀
(varies from car to car. My Honda didn't like operating below 2000rpm)


et Another ShawnD1 Troll Thread.
Click to expand...
We should make a laundry list of groups you think are some kind of conspiracy. EPA's fuel rating department = conspiracy, autospeed.com = conspiracy, ecomodder = conspiracy, people who get better than EPA rated mileage (me) = conspiracy.
 
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kornphlake said:
So I use less fuel stopped at a signal if I shift into neutral and rev the engine to 3500 instead of letting the engine idle? 😕
Click to expand...

You'll use less fuel per power produced, but since you're producing more power, ultimately you're using more fuel. In other words, no.
 
Throckmorton said:
It's not fucking up. It's doing what it thinks you want it to do, downshift so more power is available.
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It will drop into second when the cruise control is on. It doesn't need to do that because the engine is more than capable of maintaining speed (or accelerating) while in third gear. This is why I said in the OP that driving for max efficiency cannot be done in an automatic.
 
ShawnD1 said:
It will drop into second when the cruise control is on. It doesn't need to do that because the engine is more than capable of maintaining speed (or accelerating) while in third gear. This is why I said in the OP that driving for max efficiency cannot be done in an automatic.
Click to expand...


Um, duh. All cruise control does is stick an electric motor on the throttle cable and changes how open/closed it is based on vehicle speed. Cruise on non high end luxury vehicles doesn't engage some sort of computer program for max efficiency.
 
Demo24 said:
Um, duh. All cruise control does is stick an electric motor on the throttle cable and changes how open/closed it is based on vehicle speed. Cruise on non high end luxury vehicles doesn't engage some sort of computer program for max efficiency.
Click to expand...

In a car with a manual transmission it does, because it stays in top gear the whole time. High gearing with wide open throttle is very efficient.

There's no way to tell an automatic to do the same thing. It won't apply full throttle unless the gas pedal is all the way to the floor, and it will drop gears at the same as open the throttle. The result is that you're either 50% throttle in gear 4, 75% throttle in gear 3, or 100% throttle in gear 2. All of those are painfully inefficient.
 
ShawnD1 said:
In a car with a manual transmission it does, because it stays in top gear the whole time. High gearing with wide open throttle is very efficient.

There's no way to tell an automatic to do the same thing. It won't apply full throttle unless the gas pedal is all the way to the floor, and it will drop gears at the same as open the throttle. The result is that you're either 50% throttle in gear 4, 75% throttle in gear 3, or 100% throttle in gear 2. All of those are painfully inefficient.
Click to expand...
You still talking about cruise control?
 
Howard said:
You still talking about cruise control?
Click to expand...
Sort of. I was saying that cruise control in a manual does what you would normally do if you wanted maximum efficiency. It stays in top gear and hammers on the gas.

In an automatic, cruise control or not, you can't tell the car to do that.
 
ShawnD1 said:
In a car with a manual transmission it does, because it stays in top gear the whole time. High gearing with wide open throttle is very efficient.

There's no way to tell an automatic to do the same thing. It won't apply full throttle unless the gas pedal is all the way to the floor, and it will drop gears at the same as open the throttle. The result is that you're either 50% throttle in gear 4, 75% throttle in gear 3, or 100% throttle in gear 2. All of those are painfully inefficient.
Click to expand...

It is (according to the link you provided) more efficient at making power, but what you're fucking missing is that with the transmission in the wrong gear that power doesn't get to the ground nearly as well as it does when downshifting.

The automatic transmission is supposed to downshift, because there is no reason to go WOT at top gear. It downshifts to put more power to the ground so you can actually accelerate. Jesus you're an idiot.
 
ShawnD1 said:
Sort of. I was saying that cruise control in a manual does what you would normally do if you wanted maximum efficiency. It stays in top gear and hammers on the gas.

In an automatic, cruise control or not, you can't tell the car to do that.
Click to expand...
It doesn't seem as though an automatic would need to drop a gear simply to maintain a highway speed.
 
It downshifts to put more power to the ground so you can actually accelerate. Jesus you're an idiot.
Click to expand...
You don't NEED to drop gears though. If your car needs to drop gears to climb highway hills then your car is a piece of shit. Even my tiny little Civic could do that, and that's a Honda. It's basically a motorcycle engine with a car strapped to it and it never had a problem going up steep hills in top gear.


My trip to the Canadian rockies was interesting. Manual tranny Honda Civic had no problem getting fairly close to rated mileage when going up hill the whole time because it stayed in top gear. My crayola? Oh shit. Not even close. It stayed in third gear for probably about half of the drive up. Do you have any idea how much gas an engine burns when it's running at 4000rpm for half an hour straight? Fucking lots. What's pathetic is considering how small an incline that drive really is. Calgary is 3438ft above sea level, Banff is 4537ft above sea level, and the drive is about 80 miles (130km). It's such a small incline that any car in the world should be able to do that going full speed in top gear. A corolla is a pile of shit, so it ends up dropping gears instead of just giving it more gas. Fuel efficiency? We don't need no stinking fuel efficiency.
 
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