Why no fast acceleration?

[skrotbil]

I know my hands are somewhat tied with a 6-speed automatic, but the best highway trip mileage I've achieved using ECO drive mode is 41.3 mpg on a vehicle that's EPA rated 31 highway. I do not use cruise control, as I seem to get better mileage when I feather the pedal myself, and traffic is too heavy for pulse and glide. I coast in gear on downhills, resulting in open loop and fuel cutoff, and try to get a running start on the uphill letting the speed bleed off gradually rather than forcing it. Any more suggestions on how I could improve mileage - other than coasting in neutral?

[UFO]

Originally Posted by skrotbil

I know my hands are somewhat tied with a 6-speed automatic, but the best highway trip mileage I've achieved using ECO drive mode is 41.3 mpg on a vehicle that's EPA rated 31 highway. I do not use cruise control, as I seem to get better mileage when I feather the pedal myself, and traffic is too heavy for pulse and glide. I coast in gear on downhills, resulting in open loop and fuel cutoff, and try to get a running start on the uphill letting the speed bleed off gradually rather than forcing it. Any more suggestions on how I could improve mileage - other than coasting in neutral?

Yes, don't do a running start before you get to the hill; accelerating downhill is not efficient. Save your engine power for going up hills and don't back off until you crest the hill, unless you know you will need to scrub speed on the downhill.

[skrotbil]

Originally Posted by UFO

Yes, don't do a running start before you get to the hill; accelerating downhill is not efficient. Save your engine power for going up hills and don't back off until you crest the hill, unless you know you will need to scrub speed on the downhill.

Thanks for that advice UFO. I will try that to see if it helps. Just found this post on another forum that seems to confirm what you said: "Building up momentum before the hill may or may not help. One thing I believe I observed is that going faster uphills is 'cheaper' than doing so on a level road (may be the better BSFC?). Especially if you can get a longer coast in return after the crest."

[Zach]

Originally Posted by UFO

Yes, don't do a running start before you get to the hill; accelerating downhill is not efficient. Save your engine power for going up hills and don't back off until you crest the hill, unless you know you will need to scrub speed on the downhill.

Sorry to bump a thread kinda old but isn't that the opposite of what the 100+ hypermiling tips states for DWL (56)? Accelerating on downhills right before an uphill climb since you burn less on the downhill? I am still trying to learn so some clarification woold be great.

[JRMichler]

Much of the advice in this thread is unclear to me. So here's my attempt to explain DWL. It's how I drive my truck.

When driving through rolling hills, observe how the cruise control works. Most cruise controls continually overshoot - they let the vehicle slow down uphill, then give it too much gas and top the hill going too fast. A reasonably skilled driver will match the accelerator to the hill and maintain constant speed up and down. Such a driver will normally get better gas mileage driving manually than by using the cruise control.

If that vehicle has a manual transmission and engine similar to mine, he will need full throttle (1-2 inches vacuum) uphill, and back completely off the throttle (20-25 inches vacuum) going downhill.

With some practice, it is found that partial throttle uphill, about 5 inches vacuum, allows getting up the hill while losing only 1 or 2 MPH. Similarly, careful throttle usage downhill allows the vehicle to pick up the speed lost uphill without completely backing off the throttle. The MPG improves some more.

The driver starts by maintaining constant speed by using manifold vacuum over the full range of 1-2 inches to about 25 inches. As the (s)he gets better at DWL, the range of manifold vacuum starts to decrease. First 1 to 25 inches, then 5 to 20 inches, then 7 to 17 inches, then to ????. As the range of manifold vacuum decreases, the speed at the tops of hills decreases, and speed at bottom decreases. DWL is minimizing the range of throttle opening, as shown by minimizing the range of manifold vacuum.

My own driving, after several years of practice, is almost all between 10 and 15 inches vacuum. Most of my driving is 55 MPH in 55 MPH speed limits. I regularly top hills at 50 MPH, and hit 60 MPH at the bottom. This technique is the largest single reason for the gas mileage that I get.

[user removed]

A 4 cylinder engine on a dyno at 1500 rpm.

Set the dyno load at 20hp, the amount of fuel burned to maintain that load is considered as 1 unit of consumption. Efficiency is low because you must restrict the airflow through the manifold.

This reduces the volume of air entering the cylinder which reduces compression and efficiency.

Now increase the load to 50 hp. Even though you are producing 150% (50hp versus 20), the fuel consumption only increases by 50%, or 1.5 parts of fuel for 50 hp versus 1part for 20.

50/1.5 = 33.33 hp per unit
20/1 = 20 hp per unit

Since each molecule of fuel is producing 33 units of work versus 20 units of work, lets understand why.

Increasing the load means reducing the manifold vacuum to produce the increased amount of work. This means more air, and more fuel for each combustion event.

The "all or nothing" tactic of pulse and glide (engine off or on) will give you the highest loads and efficiency, storing energy in the velocity of the vehicle while using 0 fuel in the glide (assuming engine off) or very small amounts (assuming engine on).

Doubling the thermal efficiency of the engine by not accepting inefficient operational parameters, can double your mileage.

Once that is understood then average speed becomes the critical factor. I could average 70 mpg instead of 55mpg if I choose to accept lower average speeds.

regards
mech

[user removed]

Minimal throttle restriction=maximum efficiency, as long as enrichment is not engaged.
Lower average speeds=maximum efficiency, as long as you can use top gear the most.

Higher speeds are not really less efficient, they require more work to overcome aero drag, that work increasing as the square of speed.

40 mph=1600 units of drag
80 mph=6400 units of drag, not 3600.

regards
mech

[ConnClark]

The biggest argument against fast acceleration is that excess pressure is left in the cylinder when you lay into the gas and it just ends up as noise when the exhaust valve opens. Its called blow down and with the otto cycle it is un avoidable. The more gas you give the engine the more that goes to waste.

[oldtamiyaphile]

Originally Posted by ConnClark

The biggest argument against fast acceleration is that excess pressure is left in the cylinder when you lay into the gas and it just ends up as noise when the exhaust valve opens.

Noise doesn't take a lot of energy to produce though, consider how much noise you can get from a typical 100w car stereo.

[ConnClark]

Originally Posted by oldtamiyaphile

Noise doesn't take a lot of energy to produce though, consider how much noise you can get from a typical 100w car stereo.

You will here the riceboy's fart can muffler long before you will here their stereo. Also a stereo is designed to make noise efficiently. A car engine does it as a by product.