Does anybody know the relationship between fuel efficiency (milage, mpg), engine speed, throttle, and any other factors via an equation? Does gearing have any bearing on the system? Ignore factors such as air resistance or other forms of friction. I'm trying to form some sort of function that can predict instantaneous fuel efficiency for a 98 integra GSR.

What are the major factors involved?
How is throttle measured and used?

I'm thinking the end result of a fuel efficiency function is somewhat of an asymetrical parabola.

 

More throttle Less Rpm's= better volumetric efficiency

 

Yup. That's why some manufacturers (BMW as one example) are moving away from the use of a throttle completely. Instead they control the intake charge via variable valve lift and timing. If you slam the intake valves shut early the piston has to work against a vacuum, but it gets most of that energy back during the first part of the compression stroke where the vacuum does positive work. A throttle can only do negative work on a piston so you never recover any of the lost energy.

This contributes to the efficiency of diesels, too. They have no throttle and control load via fuel metering instead.

 

i was reading the other day about hondas new SOHC thats a 1.8L and has drive-by-wire throttle and more advanced I-VTEC so that at low rpm cruising the D-B-W opens the throttle fully to prevent pumping losses and just leaves one intake valve open for part of the compression stroke to keep engine output down i.e. rpm

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Satiagraha &raquo;</TD></TR><TR><TD CLASS="quote">Does anybody know the relationship between fuel efficiency (milage, mpg), engine speed, throttle, and any other factors via an equation? Does gearing have any bearing on the system? Ignore factors such as air resistance or other forms of friction. I'm trying to form some sort of function that can predict instantaneous fuel efficiency for a 98 integra GSR.

What are the major factors involved?
How is throttle measured and used?

I'm thinking the end result of a fuel efficiency function is somewhat of an asymetrical parabola.</TD></TR></TABLE>

Gearing will have effect in that it controls engine speed, but I don't think it has an effect beyond that. Other than that, what everyone else said, the more open the throttle and the lower the rpm the better the efficiency.

 

Now that I think about it, are we talking about designing a vehicle for greatest efuel efficiency or optimizing our understanding how to get the greatest efficiency out of the cars we already have? Because if we're talking about our own cars with set design I think it's acually a bit tougher. It would be much simpler if we could start by talking about one particular condition, like cruising at a constant 70mph.

From the standpoint of design, the open throttle thing definitely works to reduce pumping losses by increasing volumetric efficiency. But all things being equal, VE by definition increases airflow which means the engine has to burn more fuel to maintain the correct A/F mixture. You can compensate for that with higher gearing to reduce RPMs ---&gt; lower air flow requirements. You could also build a smaller displacement engine which would have the same effect. So to sum it up, designers can play with gearing and displacement to get maximum efficiency at a particular design condition. If not for the need to have more power to accelerate, we would be cruising around at full throttle on the highway with small engines and very tall gears.

Optimizing our own cars is tougher because there isn't a lot we can do to change the design of the engine. We can open up the intake but we will end up with more power instead of better fuel efficiency. The only thing we can really discuss is driving style. Hard acceleration is obviously bad. Stop and go driving is bad too because every time you accelerate you throw away your kinetic energy in the form of heat through the brakes. (Thats why hybrids use regenerative braking to get some of the energy back and store it in the battery.) So it would be better to coast down over a long distance rather than maintaining speed and then stopping quickly. Anything else? I'm runing out of ideas.

 

I can't wait till the camless engine...I know at least BMW is testing these designs. That will be the best, Infinite cam adjustability...you can have low overlap for good driveability etc. , then have full F1 overlap at high RPMs. Kinda what VTEC does, but it will be at any time. It will be interesting to see what BMW does to the throttle on these camless engines. I know they are using solenoids over the valves to open/close them but thats it.

 

hump.