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

[Daox]

Sounds like a Volt. An onboard generator charges a battery (so it can be setup for optimal output at a said rpm) once the plug in battery has been run low on charge. The car is propelled by an electric motor.


Gearing a motor down to 1000 rpm would be completely doable. In a hilly area you'd have to deal with fluctuating speeds is all. The engine wouldn't stall, it simply wouldn't be able to keep you up to speed.

[dcb]

And you can always downshift if it gets too steep. But those red dots are pretty good evidence that slower engine speeds = better mpg, now that I finally figured out what they were

[jonr]

Only at cruise. Mid range RPMs are best for acceleration. Many people don't understand this point.

[dremd]

could anyone explain the TDI BSFC chart to me?

[dcb]

When accelerating, give it almost full throttle and center your shifts on 1750 RPM.

When cruising it is business as usual, highest gear you got with the slowest speed you can stand.

The red dot curve that Daox put together looks suspiciously similar to the red dash curves on that tdi graph. My uneducated guess is that those red dash curves are the torque required for three different speeds (not labeled). And they come tantalizingly close to the "sweet spot" but I don't know if the stock gearing will get you there or not.

[aerohead]

Originally Posted by MetroMPG

cfg, do you know what the line is at the top of your chart with the points marked on it? The original Geo engine chart appears to have a similar line.

Top line is Full-Throttle/Max. BMEP

[aerohead]

Originally Posted by jonr

Exactly.

It would be nice if the Saturn BSFC chart had one more island - there might be a 225 around 2500 rpm/125 Nm.

The lowest I've seen is about 255,with the Orbital Engine 2-stroke,used in the 1992.GM Ultralite.Anything below that is in the domain of Diesels.

[aerohead]

Originally Posted by pasadena_commut

Ok, let's say you can open the gearbox and replace the gears with any ratios you want. How do you choose those gears for optimal efficiency?

Say you want to optimize for efficient cruising at 65 mph. So for the top gear you look at the BSFC and pick the RPM centered on the lowest fuel use, then 65 mph and that RPM (and tire size) determine the gear ratio. That does not determine the load though, so there is nothing much you can do to pick the location on the BSFC chart along that vertical line for your cruising speed. Is that optimal? Probably not, since a higher gear will increase the load on the motor but move the RPM to the left of the center of the most efficient point, and could lead to better cruising efficiency. Now you want to add a gear for accelerating at that same optimally efficient RPM. How do you choose that acceleration gear? Seems like an even more difficult problem since for a fixed gear, and a given RPM, there will be a fixed speed. It would help if we knew what the path of RPM vs Torque looks like as a function of RPM for a fixed gear ratio.

The best solution they can think up so far,is a twelve-speed tranny.And as you've noticed,they're not available yet.Mercedes is up to 7-speeds,and Toyota has over-taken them with a Lexus 8-speed.Ricardo,in England was putting stacked transmissions in cars so he'd have the "perfect" ratio for any contingency.For us right now,its a matter of getting through the transient environment,up to speed,then conserve momentum.

[dremd]

what does "bmap (bar)" stand for on the TDI bsfc chart?

I assume that lower numbers (labeled on chart) stand for higher efficiency?

[sickpuppy318]

Red dashes??? HAH!!
Behold the power of blue lines!!

[jonr]

I'd be happy with just higher gear ratios. I'm generally in 6th at 45 mph.

How hard is it to change the final drive ratio(s)?

[sickpuppy318]

BMEP represents is Brake Mean Effective (cylinder) Pressure. Basically this pressure is the result of the amount of fuel in the cylinder times combustion efficiency. Any given load on the engine (accel and aero drag) will require a certain pressure for the RPM at that load. Lower RPM means pressures have to be higher to compensate, thus the advantage of taller gearing.

[OldGuy]

Here is a link to an article which is very helpful in understanding BSFC charts:

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Enjoy.

[dremd]

Thank you guys VERY MUCH.

I'm assuming that the Blue lined BSFC chart is also ALH TDI.

[jonr]

Looks like the Insight manages to hit 200 g/(kW·h) around 4000 rpm.
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[tasdrouille]

This one is for the VW 5 cylinders 2.0 diesel that was in the Audi 5000

[thorpie]

Originally Posted by Daox

Its really not that difficult to pick an optimal gear ratio if you have a BSFC chart for your engine.

etc, etc

When I look at the chart I see that the motor is 4 times larger than it needs to be!
If we adjust the chart for a motor 23% of the original size the torque figures drop by close to 23% (give or take), but the usage (g/kwh) figures stay much the same (give or take).
We then end up with this chart.

And then the gear ratios need changing so that 6th gear is slap bang in the lowest bsfc location (it becomes what fourth gear now is) and there are two lower gears to give some acceleration (which would still be absolutely piddling!)

[jonr]

Yes, but when pulling out into fast traffic, you are going to wish the engine was 4x bigger.

Turning off cylinders (or two engines under the hood) tries to do this.

[superchow]

The 07 Civic (R18A) engine opens the throttle butterfly completely under low load conditions and pumps some air back out during the compression stroke (effectively making it a 1.5L instead of 1.8).

The system is so fine tuned that I can't even guess the theoretical best engine speed/load combination. In theory in low revs and heavy load it shifts to the performance cam, thus negating the "low-load" efficiency.

Has anybody seen a BSFC chart for the R18A engine? Or anybody know a Honda engineer who has the keys to the company vault?

[Daox]

Unfortunately, BSFC maps are pretty rare. The ones found here are really just from people digging and finding them on the net. I have yet to find a good source for them.