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

[johnQ]

Hi, I am new here, I registered just for this interesting topic. I am a former physycist.
In principle it is possible to measure BSFC in not professional conditions.
You must have access to reliable fuel consumption measurment. Some newer cars display this quantity. If you have this together with rpm display, you can measure BSFC on standard dyna chassis. Just make several dyna tests with different throttle opening and film with camera (even callular phone camera) what is fuel consumption and rpm. Than you can combine dyna plots with filmed data and make bsfc chart. It will be even better then presented ones because it will show overall efficiency together with gearbox losses.
In principle it is also possible to measure it without dyna chassis. You must have a nice flat road and no wind at all (like sometimes in the evening). Then there should be a fixed relation between speed and power losses. If you know the mass of your car (together with a driver and fuel) then you can film how your car is losing speed at free gear. Of course couple of measurments are better then one. Then you can find this dependency. Then film how the car is accelerating at some throttle opening together with fuel consumption. (of course you can combine both measurements i.e. accelerate and then loose speed at no gear) That is just a method how to replace dyna chassis by a mass of the car. Such measurement is perhaps closest to what one wants to do in real-world economical driving. The point is in controlled conditions. If the road have some slight slope you must measure it driving in both directions but it must be flat. And if there is some wind it will ruin everything. The best is to measure everything couple times and combine results on one chart with errorbars. In fact analog (clock type) controls may have some inertia and lcd display of fuel consumption may be not very accurate. The best would be if one could directly connect to the computer of the car and record values on a laptop (on dyna chassis as well). Just the idea. When I buy a better car I will try to measure it.

[johnQ]

I forgot to say: dyna chasis plots are made by subtracting estimated transmission losess from real wheel measurement. One must have wheel power data, without subtraction.

[Daox]

Welcome to the site johnQ. Thanks for the info.

One of our members here did design a universal MPG gauge, the mpguino. With it, it is possible to get very accurate fuel consumption readings.

Now all we need is a nice long level road with no traffic.

[cfg83]

johnQ -

Welcome to EM! What Daox said, MPGuino all the way!

CarloSW2

[johnQ]

Great device. I think I must change my car and I plan to install LPG in it. (LPG is quite popular in Poland due to economical reasons) LPG controller usually has some display like mpguino, so maybe in autumn I'll be able to make some plots. I'll also talk to my friend who is a good electronics engineer because I looked at mpguino design and it seems that storing data on a laptop may be pretty easy.

[thorpie]

have just started a new thread with alternate views of BSFC charts

http://ecomodder.com/forum/showthrea...tml#post116493

[user removed]

I read through this whole thread this morning and would like to add my thoughts.

Best BSFC is a function of manifold vacuum, not throttle position. At 0 vacuum throttle position is irrelevant unless you go to WOT which is above best BSFC.

When I accelerate my Echo (53 MPG average) I just watch the vacuum gauge and try to stay within the range of 1 to 2 inches off 0 vacuum. I am in 5th gear at anything over 30MPH (car is geared poorly at 2550 RPM at 60 MPH-wish it was 2000)

This matches my power to the BSFC map, as long as I keep the RPM in the range where the map tells me the BSFC would be highest.

Load is measured by manifold vacuum. At the exact same throttle position as the load is increased the manifold vacuum will drop, until at max load the manifold vacuum will be 0.
0 manifold vacuum is the top of the map where the maximum power is produced but mixture is richer at WOT.

This is because the engine can produce no more power at that RPM. If you want to increase the acceleration you would have to use a lower gear.

So in conclusion there are two ways to determine if you are in the best BSFC range.
Stay in the RPM range of best BSFC which almost universally is between 1500 and 2500 RPM (good general rule-probably lower for diesels).

Choose the highest gear that will meet your acceleration requirements. Never use WOT unless it is a diesel.

Choose the vacuum level that comes as close as possible to the 1 to 2 inch range.

That will give you optimized BSFC.

An example;

You are climbing a hill in your highest gear with your THROTTLE POSITION FIXED. The hill becomes increasingly steep (more grade) as you continue climbing. Your RPM is in the best BSFC range.

At some point you can no longer maintain your speed (max load for throttle position). This would be where your manifold vacuum would be 0 and your load would be 100% for that throttle position and RPM.

Even though your throttle position might be only 25% of max, increasing the throttle position would not really give you a significant increase in power unless you went WOT and used fuel enrichment.

Lowest measured manifold vacuum without WOT enrichment is your best BSFC range.

To maintain your speed up the increasing steep grade you have two choices. Increase throttle position to WOT or use a lower gear. Choose the lower gear, because you do not want to use WOT and loose your ideal fuel mixture.

Your efficiency is poor when you have significant manifold vacuum, because the vacuum is robbing your engine of it's highest effective compression by reducing the available atmospheric pressure to the cylinders.

If you have the ability to read manifold vacuum. then you can easily get best BSFC by using the RPM limitations and maintaining the manifold vacuum at 1 to 2 inches any time you are accelerating.

That's it.

regards
Mech

[thorpie]

Hello Mech
I would just like to reword and clarify what you have indicated about the drop off in efficiency with Wide Open Throttle (WOT).
You state that carburettion (presumably direct injection and carburettors) increases the mixture richness at WOT, and this provides needed extra power but reduces efficiency. You also indicate that at some point below WOT there is no vacuum, and that this is the point of maximum efficiency.
So if I have this correct it seems that:

• the narrowest point in the inlet flow is the valve(s);
• as long as upstream there is no smaller choke area than this valve choke point, then there is zero vacuum;
• the throttle is open to provide this valve choke area at say about ¾ throttle, which gives zero vacuum for this and any larger throttle opening;
• opening the throttle over this ¾ position does not result in additional airflow, but the fuel supply system does increase the amount fuel added to the air.

If this is correct then it indicates that a relatively simple mod to ensure that efficiency is not compromised by using WOT would be to restrict the throttle so that it opens to a maximum of this ¾ location. This would reduce optional performance but would result in a simpler overall regime, ie if accelerating put your foot to the floor (to now give ¾ throttle).
Would this be a correct interpretation or have I messed something up?
Thanks

[user removed]

Basically you are correct.

WOT is wide open throttle-floored-butterfly at its maximum open position.

Fuel injection systems are very precise mixture delivery systems.

Carburetors are always a compromise.

You can have 0 manifold vacuum and 0 valve restriction (or at least very close to 0) if you are in your highest gear accelerating from 1000 RPM. Throttle position could be as low as 15% of maximum and would not affect your manifold vacuum (easily confirmed with vacuum gauge readings).

The complete induction system is designed for significantly more air delivery than any engine requirement at 1000, 2000, or even 3000 RPM. WOT airflow will not produce significant manifold vacuum until you get close to max RPM.

Air pressure at sea level is a nominal 29.92 inches of mercury.

In a strong hurricane that pressure can drop below 26 inches of mercury.

At 18,000 feet altitude atmospheric pressure is half of the 29.92 sea level amount.

Another factor is density. Air density drops about 25% from 32 to 200 degrees fahrenheit.

This is not pressure but density, even if the pressure reads the same amount the density can be lower as the ambient temperature rises.

Carburetors can respond to density and pressure, but the ventures portion of the carburetor will always create some pressure drop. This is not true with fuel injection because no venturi is required to create suction to draw the fuel from the float chamber as it is in a carburetor. The old SU carburetors used a variable venturi to maintain a somewhat constant velocity of the air passing over the needle and seat, even when the volume of air was increasing or decreasing, the velocity was relatively constant.

Most BSFC maps for engines are fairly similar as long as you separate the diesels from the gasoline engines that are throttled. Unthrottled diesels have no manifold vacuum unless they are near their maximum RPM. Even then it would be a very small amount.

The reason most engines produce best BSFC at 80% of max load and in RPM ranges of 1500-3000 is because higher loads would require WOT and enrichment.

Below that sweet spot you would see manifold vacuum and reduced air incoming to the cylinder on the intake stroke, which reduces the actual compression ratio of the engine when combustion occurs.

Basically you control the speed of the engine by choking off its air supply, which is effectively the same thing as moving it to a much higher altitude than sea level. Its the same reason why engines loose much of their power at high altitudes and require supercharging or turbocharging in high flying aircraft to increase the effective pressure to the cylinders for more power.

In easy to understand terms, you can find your BSFC with no instrumentation by accelerating using only enough throttle to get your best rate of acceleration in the highest gear you can use. Increasing the throttle position will not increase your rate of acceleration by any significant degree.

Never use WOT unless you absolutely have to have that power to merge into traffic or to avoid an emergency situation.

While blocking the throttle to eliminate WOT altogether would accomplish the objective, it would also eliminate the availability of max power for collision avoidance or any other potentially dangerous situation.

I am not saying BSFC maps are identical in all engines, and I am not trying to say there is absolutely no residual manifold vacuum at lower throttle settings.

As a general rule the restrictions at low RPM are very small, to the point of insignificance.
BSFC sweet spots are generally similar in passenger car engines that are designed for EFFICIENCY.


regards
Mech

[roflwaffle]

Not sure if I posted this already, but here's the chart from VW's old 1.5L diesel.
vw15d.png - File Shared from Box.net - Free Online File Storage