The BSFC chart thread (post 'em if you got 'em)

[dcb]

At cruising speeds, messing up your aero will probably more than cancel out any efficiency gains from being in the "sweet spot". So I'll say yes, crazy

[tasdrouille]

Originally Posted by MazdaMatt

Yeah, thanks for that explanation... now... just to play devil's advocate here... would you not be even better off if you actually reduced your gearing so that you had to be at 1500rpm for that cruise speed, THEN mess up your aerodynamics to the point where you needed 124.8 torque so that you are on the centre island? Then you are at highway speed with the highest gear maintaining speed at the throttle position that results in the least possible fuel consumption... crazy? Adjust gearing and aero for optimal rpm and tp for most efficient load...

Is your goal efficiency or fuel economy? The numbers you see are fuel mass per used power unit per hour. The "cost per hp" benefit of being on an efficient area of the map rarely outweights the fuel cost of the additionnal hp themselves.

A bsfc map is usefull to figure how to P&G best. For steady state driving what's best is a dsfc map (distance sfc). Have a look on page 14 of this pdf

[Daox]

Originally Posted by MazdaMatt

mess up your aerodynamics... crazy?

Yeah, that would be crazy. You would increasing the horsepower needed to cruise and that leads directly to more fuel used.

[MazdaMatt]

Originally Posted by Daox

Yeah, that would be crazy. You would increasing the horsepower needed to cruise and that leads directly to more fuel used.

Not according to that graph. That graph says that at 1500 rpm and a higher thottle position than your 1000rpm point, you will be using less fuel. Otherwise, I need these charts better explained to me.


EDIT: OOOOHHHHH... nevermind!!! I didn't realize it was per kW.H. I thought it was per hour. Nevermind. I understand now why the idea was as stupid as i first though it was. So this tells me that hard accel in that sweet spot may not be the most efficient. I want to see fuel per hour islands for tp vs. tq.

[Daox]

I believe the dotted line on the top of the BSFC map is max engine load and the torque avaliable. The easiest way to translate use this chart is via engine load. But, since throttle position does not increase directly proportional to engine load there is no easy comparison. This is yet another benefit of the scangauge, because it will tell you engine load.

[jonr]

You definitely want to accelerate in the "250" island in the above graph. I call this moderate acceleration. But you can't cruise in the island because you would go too fast. So the best you could do is cruise at the leftmost red dot which is in the 300 island.

[pasadena_commut]

Originally Posted by jonr

You definitely want to accelerate in the "250" island in the above graph. I call this moderate acceleration. But you can't cruise in the island because you would go too fast. So the best you could do is cruise at the leftmost red dot which is in the 300 island.

Now we're coming to the heart of the issue.

In order to cruise at the leftmost dot one would need a crazy high gear for that motor to match the motor speed to the vehicle speed. Since 1000 RPM is barely over idle on most cars (not sure about this Saturn) my gut feeling is that this would not work, I'm thinking that the car would tend to stall in that gear. While it might be possible to actually maintain 65 mph on the flat on a windless day in that gear, I don't think you could actually accelerate to the cruising speed, and the slightest hill or headwind would stall the motor.

As you say, the car cannot cruise at maximum engine efficiency, but it can accelerate at it. There is a relationship between the rate of acceleration and the amount of HP coming out of the motor (a complex one though, involving mass and velocity dependent drag), so at at some (rapid and probably varying by speed) rate of acceleration the engine will be in the sweet spot. Drive the car with pulses of acceleration, shut the motor off the rest of the time, and presto bingo, maximum engine efficiency.

The down side is that PG pretty much sucks for passengers - for a lot of people it is a guaranteed recipe for motion sickness, and at the very least it is really annoying. It's also not very much fun for drivers of nearby cars. In my view PG is only an option for the solo driver on an empty road.

It would be a lot more human friendly if there was an energy storage device like a flywheel between the internal combustion motor and the drive train, so that the motor can be pulsed while the car itself is driven smoothly. This would also resolve the issue of no power steering, no power brakes, no power anything, when the motor is off. Sounds like a Prius, doesn't it? Unfortunately adding this capability to the car sitting in your driveway now is going to be very, very, VERY difficult - this is way past duct tape and coroplast!

[dcb]

You are wandering off into speculation land a little there. Around town you wouldn't know the difference between pulse and glide and "normal" driving. My passengers have never complained, they like the quiet parts.

[pasadena_commut]

Originally Posted by dcb

You are wandering off into speculation land a little there. Around town you wouldn't know the difference between pulse and glide and "normal" driving. My passengers have never complained, they like the quiet parts.

Not speculation, personal experience. When I was a kid one of the drivers in our carpool would PG on the highway section of the drive to school. Until I started reading about hypermiling techniques I never understood why the heck he drove that way. In retrospect he was probably pulsing up to 65 and then gliding down to 62 or so - his PG cycle was roughly 10 seconds long. I always felt sick as a dog by the time we reached our destination, which was only 15 minutes away, and I am not particularly prone to motion sickness.

You are probably right about it not being an issue around town - between close stop lights there might only be one pulse.

[jonr]

I agree - most cars do not have the gears to allow you to cruise at the leftmost red dot. So you may have to cruise at the 4th one. And lose ~25% of your MPG.

Regarding the driveability issues - the leftmost red dot is at 1/2 the power available at that rpm. So there is some room to generate more power before one has to down shift. Plus I don't mind downshifting and/or slowing down a little for the bigger hills.