25MPG w/2000 Mustang

[MadisonMPG]

Originally Posted by 90crxHF

I do understand what you're saying here, and I'll to it with this...


Now, for a 4 cylinder powered car to theoretically get twice the gas mileage, the power to weight ratio's will need to be similar. The very second the engine is struggling, FE goes out the window. However, even in a large V8, most V8's these days are reaching very high effeciency levels.... I mean, I can go get a 360 Cube motor that makes over it's displacement now, stock?? Wow, what an efficient engine, or is it over efficient??

Engines ALWAYS struggle.

[Christ]

By definition, an engine is struggling. It's a working piece of machinery. If it's not working, it's wasting (or off).

If we're going to have an actual discussion here, we need to straighten out a few definitions. Firstly -

Efficiency has nothing to do with fuel economy. Efficiency literally refers to the amount of energy extracted from a unit of fuel, nothing more.

An engine CANNOT be over efficient. There is no such beast, I'm afraid. Efficiency's goal is to reach 100%. Over efficiency would be considered overunity. Not possible, at least in today's science.

Fuel economy is a completely different beast from Engine efficiency. Fuel economy isn't the amount of work you make per unit of fuel. It's the distance you travel per unit of fuel.

They are only related insofar as if you increase engine efficiency, you could also have increased fuel economy for the range which has the increased efficiency.

In other words, if your engine is 20% efficient at 2,000 RPM (cruising), and you're getting 10 MPG at that engine speed, you could (theoretically) make the engine more efficient, by 50%, so you're at 30% efficiency, giving you the option to use less fuel (not 50% less) to do the same work (cruise), which would give you better fuel economy equal to the amount of fuel you are no longer using distributed over the miles you've traveled.

To clarify -

20% efficient means you're only using 20% of the fuel's heat value (BTU) for work. The rest is wasted.

Increasing from 20% to 30% efficiency means that instead of wasting 80% of your fuel's heat energy, you're only wasting 70% now. The gain is 50%, but you're only using 10% less fuel (I know, that makes it seem not worth it, but it's the way math works.)

Since you're using 10% less fuel to make the power you need to cruise at 2,000 RPM, you're also using 10% less fuel to go the distance/time that you normally go at 2,000 RPM.

If that distance/time were 100 MPH at 2,000 RPM, and you were originally getting 10MPG at 100MPH at 2,000 RPM, but you're now using 10% less fuel to go 100 MPH at 2,000 RPM, you're now capable of getting 11 MPG at 100 MPH at 2,000 RPM, an increase of 10% fuel economy.

Your theory about low-load is also scientifically inaccurate, because an engine is less efficient with less load. The BSFC chart for an engine assumes 100% load and 100% throttle. It changes with throttle angle, and it changes with load variances. The most efficient engines in the world are constantly under load, they don't operate outside of being loaded.

If your theory were true, you'd be getting better gas mileage idling in your driveway than driving down the road.

[comptiger5000]

Yep. Engines like to work. If I putt around and keep pace with others, or accelerate like I did in a 6 cylinder Jeep, I get about 11mpg in my normal drive cycle. If I pick up faster, and leave everyone well behind me at traffic lights, I can get 12 - 12.5 in my normal cycle.

[nemesis]

I have a 2000 mustang as well, automatic too, can't wait to switch to 5 speed and get 2mpg+ over what I am getting now.

[suspectnumber961]

If you check with the EPA...and looking a many different vehicles you almost always see that the fewer the cylinders the better the mpg...and it's also better for a manual vs an automatic....same vehicle...more or less.

And I doubt that the higher load idea really works out for most vehicles...in other words...less load = better mpg...in most cases?

Which is why they gave me a patent on my throttle detent spring.

[comptiger5000]

The better for fewer cylinders, however, is usually because 4 cylinders usually come in smaller, more efficient cars. Put a 4 cylinder in an H2, the gas mileage won't get any better.

Also, the throttle detent spring helps primarily at cruise, so you can maintain a steady, light throttle to hold speed. The load theory works, that's why P&G works better than steady state. You can spend less time accelerating, but at a faster rate, keeping the engine in a more efficient part of the BSFC map. Then, you can coast and repeat. This is better than running a lightly loaded engine constantly.

With diesels, however, the fuel consumption is far more linear with load, so while more load (around 75-85%) is still more efficient, the difference is less.