Relationship between vacuum and FE? New questions

[hat_man]

So I guess I'm still a bit confused. It sounds like an engine running at peak efficiency(or as near as possible) could possibly use more fuel than one running at less than peak efficiency? I'm believing you all just not quite following you all.

I now have two more questions. If peak efficiency is the engine running at its smoothest, is less than peak efficiency harder on the engine from a wear and tear point of view?

Secondly, if it isn't harder on the engine, how do I find that point or range that is using less fuel?

And finally, (hopefully) if that point or range of "less fuel" is found would it be better to run somewhere between peak efficiency and less fuel for the sake of engine life or strictly run at the less fuel end?

You all have been great "teachers". Sometimes I wish I could be a better student.

[freebeard]

Aren't we all stumbling around in the gloom?

I don't have or use a vacuum gauge, but it monitors a single variable. It is useful for comparisons and diagnostics, but doesn't directly give what you want. Even a new car with an instantaneous read-out is averaging fuel consumed (or so I understand), measuring the flow of small volumes of liquid is non-trivial. The solution I saw used small pins or columns in the flow and then measured the vortex street that results.

Look into Brake Mean Effective Pressure.

[Ecky]

Originally Posted by hat_man

So I guess I'm still a bit confused. It sounds like an engine running at peak efficiency(or as near as possible) could possibly use more fuel than one running at less than peak efficiency? I'm believing you all just not quite following you all.

I now have two more questions. If peak efficiency is the engine running at its smoothest, is less than peak efficiency harder on the engine from a wear and tear point of view?

Secondly, if it isn't harder on the engine, how do I find that point or range that is using less fuel?

And finally, (hopefully) if that point or range of "less fuel" is found would it be better to run somewhere between peak efficiency and less fuel for the sake of engine life or strictly run at the less fuel end?

You all have been great "teachers". Sometimes I wish I could be a better student.

1) Sounds like you have it. Peak efficiency in an engine which is "too big" or geared too short will burn more fuel and accelerate the car. In order to get to peak efficiency without burning more fuel or accelerating, it's often necessary to have taller gears or a smaller engine. Even my Honda Insight's 1L engine with stupidly tall gearing is only running at around 50% load in 5th gear on the highway. Most cars are considerably less. CVT transmissions are nice in that they can lower RPMs down and keep load up at just about any speed.

2) I would think not. Wear is hard to define, but I figure higher RPM and expecting a higher specific power output (e.g. wide open throttle at high RPM) would be what ages an engine faster. Friction goes up exponentially with RPM, so anything you can do to slow the engine down will help it last longer - to a point.

3) It's generally best to run the engine as slowly as possible. The C7 Corvette has a 6.2L V8 an nearly 500HP and is relatively portly at ~3400lbs, but is capable of 35-40mpg at lower highway speeds because at ~50mph the huge engine is turning at only 900rpm. I expect more manufacturers don't give tall gears like this, which save both fuel AND engine wear, because people don't like to change gears in their car and have the perception the car is "slow" if it can't pass easily or climb mountains in top gear.

[Hersbird]

On a car with a throttle plate, which is pretty much everything some diesels and direct injection don't use one, you will have losses at low rpm because of the motor having to suck hard that air around the throttle plate, that is vacuum. Even though that loss makes the motor run outside of peak efficiency because of the low rpm it's still using much less gallons per hour of fuel than it is as say 4000-6000 rpm with a high load where it is peak efficiency. So use that peak efficiency to accelerate, then settle into a low rpm cruise. If you need to climb a hill, get back into peak efficiency with a downshift, then go back to the low rpm cruise. There is a technique called "pulse and glide" you can read up on here for even more mpg but you need usually more room with light traffic for that. Otherwise you might save gas, but the 50 car accordion behind you will be using double.

[slowmover]

Originally Posted by Hersbird

Ironically I have an old "fuel economy" gauge that runs on vacuum. I think it's for a 64 Pontiac Bonneville console, it's really a pretty thing. Anyway it goes from 24 to zero with a secondary color bar code under that goes from economy to power, economy being the 24 side and power being zero. So on a 64 big block Pontiac they figure the more vacuum you have the more economy you are running, based on the throttle being closed, and the less vacuum the more power you are making. It doesn't really consider that the motor is operating with better thermal efficiency with the throttle open because with the throttle open on a big block carbed Pontiac you will be going 100mph in a 1/4 of a minute or so and you can probably watch the gas gauge go down. In that case high vacuum, throttle closed as much as possible is where you get the best mpgs.

What a beautiful gauge! Almost the first sight one encountered on opening the door to an early Grand Prix (its late enough at night I can recall the scent of new).

Vacuum inches (beyond diagnostic issues from article linked by freebeard) was how we’d travel the highway and determine downshift points on grades substantial enough to warrant a downshift. Especially, when towing.

One watched this gauge versus the tachometer: as vacuum declined, downshift early to keep HP high. But not so early as to burn fuel unnecessarily. (Too late and one was relegated to a significantly slower uphill pace; and at lower rpms where heat control was more difficult) It wasn’t (and still isn’t) about speed to the top of the grade. It’s about using power (fuel) to keep engine strain (load) at a peak of no more than 80%.

Long life trumps any momentary FE concern.

A Pontiac 389 or 428 had to work pretty hard. Just didn’t breathe so well up top and under a load. Except Buick, what it shared with other GM. Great off the line, though.

You sure could surprise a guy back then when you could out-drive him this way. Understanding engine (drivetrain) architecture predicted performance pretty well.

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