Case Study: 45% above EPA?

[IamIan]

Originally Posted by Piwoslaw

Actually, most BSFC maps suggest that IC engines are most efficient around 80% load, with rpms around max torque.
The BSFC chart thread

Just two things to keep in mind about peak efficiency point on a BSFC.

#1> It is just about engine efficiency on a stand , does not take into account a moving vehicle with factors like wind resistance that increase exponentially with higher speeds ... ie , if you have to increase your speed by 20% to get the ICE RPMs up to see a 1% increase in ICE efficiency ... it might be a net loss of MPG even with the 1% increase in ICE efficiency due to the higher losses from the faster vehicle speed.

#2> The BSFC assumes you need and / or want exactly 100% of all the power generated at any point on the BSFC ... no more , no less ... If at that peak efficiency point the ICE would be producing more power than you wanted or needed ... it might just be wasted ... more power than you wanted / needed would give you acceleration you didn't want ( and wastes fuel energy to give it to you ) ... even if the ICE was more efficient in the conversion of fuel chemical energy to shaft mechanical energy.

ie ... peak ICE efficiency does not necessarily = peak vehicle efficiency.

[RedDevil]

...but it does make clear why pulse and glide works.
Short bursts of power using the engine efficiently combined with long coasts with the engine off instead of running the engine all the time, wasting fuel on pumping losses from having the throttle all but closed.

The alternative is having a final drive ratio that allows for low RPM as highway speed and an engine that runs efficiently at those low revs. The lower the revs, the higher the load on the engine and the efficiency.

[IamIan]

Originally Posted by RedDevil

...but it does make clear why pulse and glide works.
Short bursts of power using the engine efficiently combined with long coasts with the engine off instead of running the engine all the time, wasting fuel on pumping losses from having the throttle all but closed.

The alternative is having a final drive ratio that allows for low RPM as highway speed and an engine that runs efficiently at those low revs. The lower the revs, the higher the load on the engine and the efficiency.

Pulse and glide on the ICE ... to avoid running at low efficiency conditions ... yes.

Not so about the vehicle speed itself changing.

The two are in competition ... changing vehicle speed will always need more joules of energy than a steady state vehicle traveling at the same average speed , for the same distance , and same conditions... that is always a negative of Vehicle P&G.

Sometimes the additional joules of P&G vehicle are countered by enough increase in ICE P&G efficiency ... sometimes not ... like many things it depends on the specifics.

Another alternative ... instead of an ICE that can work as you describe ... would be an efficient transmission that compensates for the difference between Tire RPM and Load vs ideal ICE RPM and load.

[RedDevil]

Originally Posted by IamIan

Pulse and glide on the ICE ... to avoid running at low efficiency conditions ... yes.

Not so about the vehicle speed itself changing.

The two are in competition ... changing vehicle speed will always need more joules of energy than a steady state vehicle traveling at the same average speed , for the same distance , and same conditions... that is always a negative of Vehicle P&G.

Sometimes the additional joules of P&G vehicle are countered by enough increase in ICE P&G efficiency ... sometimes not ... like many things it depends on the specifics.

Another alternative ... instead of an ICE that can work as you describe ... would be an efficient transmission that compensates for the difference between Tire RPM and Load vs ideal ICE RPM and load.

Wow. First, joule is a measure of energy in itself. A certain mass moving at a certain speed has energy in the form of momentum that can be expressed in joules.
Raising the speed means increasing the momentum and needs additional energy.
Lowering the speed releases energy. The friction with the air, tires and mechanical parts generates heat. That friction is always there when moving, but when coasting there are no other forces involved.
The energy you have to invest to accelerate is gained back when coasting as the engine would have to run when maintaining speed.
Changing speed does not change the total amount of energy, just the state of that energy.

Tire RPM is just a function of speed and tire size, should not matter much and there is not much you can change about the size anyway unless you adapt the speedo.
The relation between RPM on the engine and the speed is what matters. Changing the final drive is way easier than changing the gearbox in most cases

[wobombat]

Iamian you pose a good point. In order to maintain the same average speed with p&g as in steady state driving, you need more energy. This is because air resistance increases exponentially with speed.
Another detraction from p&g, older engines suffer from compression loss. I haven't confirmed this and I haven't seen this on this site, but when you p&g you are trying to hold larger pressures in leaky cylinders, so more pressure is lost, therefore more gas used

[RedDevil]

When you P&G the difference between highest and lowest speed does not need to be so big that air resistance plays a mayor role. The speed would be low anyway. It pretty much evens out.
Below 55 mph the air resistance is way less than the other factors combined.
It is not my invention. P&G is a well-known and proven way to save gas. Check the hypermiling tips.

P&G with an automatic can be quite hard, if you cannot get the tranny in neutral during coasting (may damage the tranny so check the manual). Also you don't want the kickdown on the pulse.
Maybe you can mod the tranny to shift up at lower revs, allowing for slower cruising in top gear and better acceleration without downshift.

[IamIan]

Originally Posted by RedDevil

Wow. First, joule is a measure of energy in itself. A certain mass moving at a certain speed has energy in the form of momentum that can be expressed in joules.

Joules are a measure of energy yes.

Momentum however is not the same thing as Kinetic Energy ... Momentum is not measured in joules ... Kinetic Energy is... although both relate to a moving body , they are not the same thing.

Originally Posted by RedDevil

The energy you have to invest to accelerate is gained back when coasting as the engine would have to run when maintaining speed.

Nope.

It is not a 100% efficient system.

Weather you input joules of energy to maintain a given average speed constantly or you input joules of energy in pulses it net the same average speed ... either way you must input energy... but the varying speed always costs more energy to the same average speed and otherwise same conditions.

Originally Posted by RedDevil

Changing speed does not change the total amount of energy, just the state of that energy.

nope.

Going faster takes more energy to cover the same distance.
A fluctuating speed takes more energy to cover the same distance at the same average speed and conditions.

Originally Posted by RedDevil

When you P&G the difference between highest and lowest speed does not need to be so big that air resistance plays a mayor role. The speed would be low anyway. It pretty much evens out.

Any variation in speed results in more joules of energy being used.... even 0.000001% variation.

Originally Posted by RedDevil

It is not my invention. P&G is a well-known and proven way to save gas. Check the hypermiling tips.

As I said above ... I'll repeat again here:

Originally Posted by IamIan

Sometimes the additional joules of P&G vehicle are countered by enough increase in ICE P&G efficiency ... sometimes not ... like many things it depends on the specifics.

I think it is more useful to understand how it works ... both the good and the bad ... then one is more able to maximize the good aspects , and minimize the bad ... pretending the bad doesn't exist is not useful.

[wobombat]

Lets say you've taken steady speed to its limit and are traveling at the bottom of top gear, let's say at 40 MPH. Two questions.

1.How do you DWL when you reach a hill? If you decrease speed you'll be forced to downshift, which I think is obvious would waste more gas.

2. How do you get better Fuel economy from P&G? The only way you can extend the bracket is to faster speeds. I'd say 10 MPH is the minimum speed change to realistically take when doing a P&G, so in this case you'd pulse to 50, then back down to 40, then back up to 50. Given that the average speed in this case is 45, that increases air resistance by 26% over the steady speed. That, along with all the reasons listed above, to me would mean that P&G would not actually be a bigger gas saver. It would require much more effort and clutch wear, though.

Oh and by the way, concerning the compression losses, a while back I tested my car's 0-60, and found approximately a 20% loss of power over it's original rating, at full throttle. True there's a lot of factors, but I would say we could roughly estimate that that is approximately the loss of power encountered by having really high loads using that engine. That to me is a really significant factor too when considering P&G.

[jeff88]

Why 45%? Or are you trying to increase your FE by 10mpg and the percent is not your concern?

In terms of maintenance, definitely check your battery. If you have even a less then perfect battery, your alternator will be working harder, negatively effecting your FE. Make sure your oil and transmission fluid are not only "fresh", but not too full, it will make the bottom end of your engine and transmission work harder.

Do you have LEDs?

In terms of aero, I would do a rear diffuser. It is relatively simple, easy to take off, depending on how you design it and can be fairly well hidden if you want it to be.

What part of Oakland? I assume you are staying well clear of the "Maze".

[101Volts]

Here's a link to a page about Pulse and Gliding.

Driving technique: exploring 'Pulse and Glide' - MetroMPG.com

I believe I was becoming confused earlier when speaking of P&Ging on page 1. I've done it but to a lesser extent, And to some decent effect; On highways, I accelerated to 65 on down slopes and slowly went down to 55 on up slopes and then held 55ish.

Also, I must again mention proper tire pressure; If you exceed the most fuel-efficient pressure (Which may be lower than you think,) Your FE is going to go down a lot more than you may think.