Driving Tips to Extend Mileage
#11
Re: Driving Tips to Extend Mileage
Originally Posted by kenny
I gotta disagree.
Getting 2 tons of metal from 0 to any speed more quickly has to take more energy than a more gradual change of speed.
Getting 2 tons of metal from 0 to any speed more quickly has to take more energy than a more gradual change of speed.
I accelerate like a snail and average 62.9 MPG in my 2006 Honda Civic Hybrid which has an EPA of 50 MPG.
Last edited by Double-Trinity; 09-09-2006 at 12:11 AM.
#12
Re: Driving Tips to Extend Mileage
Originally Posted by Double-Trinity
Well, the energy required to accelerate to any speed will be the same (1/2 mass*velocity^2).
For example, let's take that car into space, free from gravity and friction. Let's equip ourselves with jet packs. If you push on the rear bumper with 60 lbs of force and I push on the front bumber with 50 lbs of force, the car, of course will accelerate forward with a net force of 10 lbs. Let's say the car moves 100 ft. Energy here is of course, force x distance. So, the car has gained 10 * 100 = 1000 ft-lbs of energy. However, we have expended 60 * 100 = 6000 ft-lbs and 50*100 = 5000 ft-lbs of energy, totaling 11,000 ft-lbs. So, between the two of us, we have spent a total of 11,000 ft-lbs in order to give the car 1000 ft-lbs of energy.
Even here, we're grossly under-reporting the amount of energy that we expended. The jet packs delivered that force, presumably from some chemical reaction. Only a fraction of the energy that the chemical reaction produced is translated into mechanical work. Most of the energy is in the form of wasted heat. Let's say we're idealistically generous and give that energy conversion 40% efficiency. That means the corrected total amount of energy expended is 11,000 / 0.4 = 27,500 ft-lbs of energy.
If we turn our attention to the car's internal combustion engine, we know that even under the ideal conditions, the efficiency for turning all those exploding gas fumes into mechanical work driving the pistons is around 35%. This is in contrast to electric motors, which typically achieve efficiencies of 90%.
So, when comparing ICE with the electric motor, side-by-side, the electric motor wins hands down in the minimizing the amount of energy that needs to be expended to get a vehcicle up to speed.
Energy aside, what's really important is how this all affects our fuel efficiency. If we're only looking at the time it takes to accelerate that car up to cruising velocity, of course the less ICE you use... the less gas you use... which translates into.. higher fuel efficiency.
Well, not so fast. What's the overall FE? For that, of course, we need to look beyond that acceleration to cruising speed and look at... the cruising. If we are able to acceleration from stop only on electric and get to a cruising speed that results in major battery drainage and there are insufficient opportunites to restore that energy via regenerative braking... well, then the ICE needs to spend its energy recharging that battery. So, in this scenario, your effective FE for that acceleration is not near-infinite, you need to pay for it later.
The question then becomes, what burns less gas: 1) ICE-assisted acceleration or 2) ICE used to recharge the batteries. I don't think the answer to this is trivial. At the end of the day, it's about the total miles covered for some amount of gas. When you're using the ICE during acceleration, the amount of gas you're burning to travel say cover a 1/4-mile of acceleration is certainly greater than the gas required to cover that same 1/4-mile of distance when at cruising speed. However, even in this case, this difference depends on the cruising speed. The higher the cruising speed, the more the wind resistance, the more gas that needs to be burned.
In the hybrid scenario, this is even more complicated because we're talking about the gas that's burned to re-charge the batteries which in turn (in principle) delivers the energy to cover that 1/4-mile during cruising. So, of course, that amount of gas is greater because there's always efficiency losses converting from one form of energy to another.
And, to top that off, the factor that really drives how all these tradeoffs are made is the terrain that the car is covering, flat, hilly, traffic conditions, etc.
I think what we can say without controversy, especially given the hard facts of the super hypermilers, is that the highest FE is obtained if you're accelerating with minimal to no ICE assist and you're able to recover that energy through regenerative braking. The ideal conditions for that of course, is a perfectly flat road.
However, my experience at least with the HiHy (of surprisingly similar weight as the GS 450h) is that accelerating without the ICE is relatively rare unless you intend to get to a cruising speed of about 30-35 mph, and you have a nice flat road to accelerate on. The sweet spot, so far that I've found is to accelerate with light to moderate ICE-assist. If I try to "force" electric-only, I find that I end up draining my batteries too much and the ICE ends up charging the batteries before I have time (and opportunity that I'm willing to tolerate!) to recharge them through regenerative braking. Qualitatively, I don't think I have a net fuel efficiency win when my ICE is overly re-charging my batteries because of the cruising speeds that I typically drive at ~65-70 mph.
#13
Re: Driving Tips to Extend Mileage
Originally Posted by hsolo142
We should be careful here when we talk about energy. The energy that you have described is the total amount of kinetic energy that that car needs to gain in order to get to that velocity. It does not represent the total amount of energy expended (by whatever means) to get that car to that velocity.
For example, let's take that car into space, free from gravity and friction. Let's equip ourselves with jet packs. If you push on the rear bumper with 60 lbs of force and I push on the front bumber with 50 lbs of force, the car, of course will accelerate forward with a net force of 10 lbs. Let's say the car moves 100 ft. Energy here is of course, force x distance. So, the car has gained 10 * 100 = 1000 ft-lbs of energy. However, we have expended 60 * 100 = 6000 ft-lbs and 50*100 = 5000 ft-lbs of energy, totaling 11,000 ft-lbs. So, between the two of us, we have spent a total of 11,000 ft-lbs in order to give the car 1000 ft-lbs of energy.
For example, let's take that car into space, free from gravity and friction. Let's equip ourselves with jet packs. If you push on the rear bumper with 60 lbs of force and I push on the front bumber with 50 lbs of force, the car, of course will accelerate forward with a net force of 10 lbs. Let's say the car moves 100 ft. Energy here is of course, force x distance. So, the car has gained 10 * 100 = 1000 ft-lbs of energy. However, we have expended 60 * 100 = 6000 ft-lbs and 50*100 = 5000 ft-lbs of energy, totaling 11,000 ft-lbs. So, between the two of us, we have spent a total of 11,000 ft-lbs in order to give the car 1000 ft-lbs of energy.
If we turn our attention to the car's internal combustion engine, we know that even under the ideal conditions, the efficiency for turning all those exploding gas fumes into mechanical work driving the pistons is around 35%. This is in contrast to electric motors, which typically achieve efficiencies of 90%.
So, when comparing ICE with the electric motor, side-by-side, the electric motor wins hands down in the minimizing the amount of energy that needs to be expended to get a vehcicle up to speed.
Energy aside, what's really important is how this all affects our fuel efficiency. If we're only looking at the time it takes to accelerate that car up to cruising velocity, of course the less ICE you use... the less gas you use... which translates into.. higher fuel efficiency.
So, when comparing ICE with the electric motor, side-by-side, the electric motor wins hands down in the minimizing the amount of energy that needs to be expended to get a vehcicle up to speed.
Energy aside, what's really important is how this all affects our fuel efficiency. If we're only looking at the time it takes to accelerate that car up to cruising velocity, of course the less ICE you use... the less gas you use... which translates into.. higher fuel efficiency.
In situations of moderte demand, such as an acceleration, it would be best to run the engine at its most efficient output, along with just as much elecrtic assist as could be recovered by braking later.
Determining this all experimentally in terms of driving/FE, as you went on to say in the rest of your post is going to be extremely tedious. What I will say is that in my Honda, in which the engine always run, is that it's best to accelerate at throttle position at or slightly above where elecrtic assist first kicks in. The system logically would not "assist" the engine if it would be more efficient to apply more load, so the point where elecrtic assist first kicks in (on the Honda) should mark the beginning of the engine's most efficient operating range.
This tends to produce a relatively slow acceleration, as my car has a very small engine. However, in the case of the 450h, with an engine producing about three times as much horsepower, at the same RPM/throttle position, the larger engine should produce much more power, and thus a relatively fast acceleration at its most efficient output.
The question then becomes, what burns less gas: 1) ICE-assisted acceleration or 2) ICE used to recharge the batteries. I don't think the answer to this is trivial. At the end of the day, it's about the total miles covered for some amount of gas. When you're using the ICE during acceleration, the amount of gas you're burning to travel say cover a 1/4-mile of acceleration is certainly greater than the gas required to cover that same 1/4-mile of distance when at cruising speed. However, even in this case, this difference depends on the cruising speed. The higher the cruising speed, the more the wind resistance, the more gas that needs to be burned.
Qualitatively, I don't think I have a net fuel efficiency win when my ICE is overly re-charging my batteries because of the cruising speeds that I typically drive at ~65-70 mph.
Last edited by Double-Trinity; 09-10-2006 at 06:15 PM.
#14
Re: Driving Tips to Extend Mileage
Originally Posted by Double-Trinity
This tends to produce a relatively slow acceleration, as my car has a very small engine. However, in the case of the 450h, with an engine producing about three times as much horsepower, at the same RPM/throttle position, the larger engine should produce much more power, and thus a relatively fast acceleration at its most efficient output.
I also found this PDF off of the Lexus website:
http://www.lexus.com/assets/models/p...ance_guide.pdf
On page 8, they list the Top 10 tips for improving fuel economy for the GS 450h:
Their #2 tip is: Accelerate slowly.
Interesting enough, the #1 tip is: Plan ahead to combine short trips in order to minimize cold starts.
#16
Re: Driving Tips to Extend Mileage
Have you tried 0W-20 engine oil - especially now that the weather is getting colder?
Check your owner's manual and confirm that the dealership is using the recommended oil.
Most delaerships still use 5W-30 as their BULK fluid because it is cheap!
For a fully-synthetic 0W-20 engine oil, consider ENEOS - manufactured by Nippon Oil.
www.eneos.us
Check your owner's manual and confirm that the dealership is using the recommended oil.
Most delaerships still use 5W-30 as their BULK fluid because it is cheap!
For a fully-synthetic 0W-20 engine oil, consider ENEOS - manufactured by Nippon Oil.
www.eneos.us
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