SG Fuel cut off value

[R.I.D.E.]

Lets use my old VX as an example.

At idle fuel consumption is about .2 gallons per hour.

At 55 MPH fuel consumption is 1 gallon per hour (close approximate).

What would fuel consumption be at 2000 RPM with no load?

Compare this to the fuel consumption at 55 MPH=1 GPH.

I woudl appreciate it if someone with a scan guage could do this calculation, especially the fuel consumption at 3 times the idle speed, compared to the idle speed fuel consumption.

This would answer a question that has perplexed me for some time, which is whether fuel consumption is a linear or exponential function of engine speed?

It may be that in reality it is a combination of both.

regards
gary

[theholycow]

It has been said that internal engine friction is an exponential function of RPM, so I would assume that no-load fuel usage is not linear (even if it does not follow RPM perfectly).

[palemelanesian]

Maybe we can extrapolate something from this: http://metrompg.com/posts/rpm-mpg.htm



Using a model of MPG = 1/rpm (x 120,000 to normalize the numbers into the same range), we get the following:

 rpm    mpg   model
2080   55.9   57.7
2500   47.0   48.0
3495   41.5   34.3
5175   25.9   23.2

That's a 97% correlation, according to Excel.

[theholycow]

Unfortunately, there are too many variables involved there to help us with the no-load RPM question. For example, aerodynamic drag...

[palemelanesian]

No, the test is run with the same speed each time, eliminating aero drag. It is, however, loaded and not unloaded, but loaded is where most of us drive anyway.

[theholycow]

Oh....in that case, it's probably pretty good, the only non-engine variable would be drag in the transmission which is probably minimal.

[palemelanesian]

Wow.
Using that same ratio, on my car, to calculate the value of a transmission swap.

My car currently runs 2750 rpm at 60 mph. Changing to a CX or VX transmission would drop that to 2200 rpm at 60 mph. Using this calculation, it would raise my steady-state mpg at 60 from 43 to 54 mpg!

[theholycow]

Actually, does it work? Does that chart need to be GPH instead of MPG? Also, there is another variable...pumping loss from throttle position.

Maybe I'll try in neutral with my meter and see what I can learn.

[R.I.D.E.]

The greatest variable in engine efficiency is how much of the available atmospheric pressure is being introduced into the combustion chamber.

Best efficiency will always be at moderate RPM with the lowest percentage of throttle position that allows each cylinder filling event to be as close to 100% of atmospheric pressure as possible.

The reason is:

An engine is basically a lever. It compresses a fuel air mixture, then ignites the mixture, which expands and applies pressure to the top of the piston.
Your best leverage is when you have the maximum possible pressure differential between compression and combustion (cylinder filling event).

The longest lever if you catch my drift.

Under higher loads you can get maximum atmospheric induction at relatively low throttle positions.

Using a vacuum guage, if you apply just enough throttle to get the lowest vacuum reading on your guage (while under some load) you have found the most efficient "sweet spot" of the engine. This will be true at relaitively low RPM, where the losses due to friction and reciprocation are at their lowest amounts.

A BSFC map clearly demonstrates this phonemon as well as illustrating that WOT is not the most efficient, when you consider throttle position. Throttle position can be as low as 30% up to 80% depending on the load applied.

This is why I always tried to stay in higher gears in my VX. You could easily check by adding more throttle, with no significant increase in power.

There may be specific examples where this does not apply perfectly in every situation, but as a general rule it's hard to beat.

In most cases, to achieve higher mileages, you have to make adjustments that are directly related to you specific driving environment. The best example in my case is the timing of the traffic lights on my normal route, which numbered over 50 in a 32 mile round trip.

If I accelerated too slowly I would catch almost every light changing yellow. This situation required me to accelerate more aggressively to avoid getting caught by the numerous lights.

I would still be interested if anyone could do a quick test with their scan guage to get an idea of what the unloaded fuel consumption of their engine would be at 2000 RPM compared to idle speed.

It would also be nice to see what horsepower was required to maintain both idle and 2000 RPM speeds using the same engine as the example.

My engine design eliminates almost 100% of reciprocation losses. It would be nice to be able to quantify the percentage of those losses compared to the cumulative losses of the same example engine, but the calculations include so many offsetting variables that precision may be impossible. This would allow basic efficiency calculations for a design that has never been built.

regards
gary

[civic lover]

Originally Posted by VetteOwner

yea did you think you were using all 140+ hp all the time? lol i hope not!

at idle there's virtually no load on the engine hence why in a manual tranny car if you dont press the gas some when trying to take of from a dead start its gonna stall(generally speaking).

Of course I didn't think this. But when I am driving it says it is only like 2 hp. This just isn't correct.