The cable news shows this morning were full of wide-eyed anchor larvae reporting that with gas prices high, people were driving less, and instead using more public transit! Oh, for a land in which the downward-sloping demand curve was not such a constant source of surprise and wonder to the broadcast media.
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Damn it! I missed parts 2 thru 997.
People are driving less, but are they using less fuel?
It's possible that people are not only driving less, as Keving Drum suggests, they might also be changing their driving habits. Given people's poor understanding of how their cars and traffic work, it's plausible that they haven't been changing their driving habits for the better.
Also, do larvae even have eyes to make wide? I think a source for this assertion would be appropriate.
Aaron,
What? Do you mean I can't save half my gas bill by driving to where I am going at twice the speed?
Unfortunately no, but you might save a little time without any noticable increase in your gas bill (maybe a slight decrease) by getting up to speed faster.
Did a rough calculation of when it cost effective to slow down (for an average american). Also did it for me: to slow down from 75mph, gas would need to cost $6.88, using my after tax income.
The decrease in average miles driven KD cites is much bigger than the decrease in gas production. Efficiency is going down?
Efficiency is going down?
People cutting discretionary highway driving to weekend destinations, while keeping city driving to work?
It was on the network news, too.
I suspect it is not a coincidence, and is part of a campaign by transit groups to make transit seem to be a cool thing white collar workers are doing, not just poor people who have no alternatives.
Not that I mind; it's be good for a number of reasons for more white collar people to use transit, not the least of which is that I think it would raise the bar for quality of service.
But I think the goal of these pieces wasn't so much to report an existing trend so much as to spur on a new one.
Let me put it this way -- if gas prices decline, I doubt you'll see a series of stories about people going back to their cars. In fact, during a brief respite in gas prices, my local paper ran a story (and almost interviewed me for) about people who started using transit when gas prices went up, and liked it so much that they stuck with it even when prices went back down.
Again, getting more people to ride the bus is probably a good thing, but I wish they were more honest about it.
Rob, that's plausible too. But aren't people flying less for vacation and aren't commutes usually farther than your weekend trips?
aren't commutes usually farther than your weekend trips?
The question isn't whether it's farther to the grocery store than to your work (probably not), it's whether your discretionary trips are longer and more highway-ish than your necessity trips.
It seems likely to me that an excursion intended to amuse is longer than one intended to acquire necessary staples or earn a living. At least, more of us live close to work and far from the beach than the other way around. Granted, the zoo is technically closer than my office, but the main reason I don't take long weekend drives is kids, not gas prices.
Given people's poor understanding of how their cars and traffic work, it's plausible that they haven't been changing their driving habits for the better.
The post you linked to has some confusion about it, noteably, the inferences drawn from the midpoint of the RPM band being the most efficient point. That is merely a statement regarding power production, which is the product of torque and RPM. The torque band generally peaks somewhere in the midpoint of the RPM band, so at that point, the engine will produce the maximum amount of power per unit of gasoline consumed (i.e., maximum efficiency).
However, that only matters if you NEED the power. To allow for the varied demands a car may face, as well as general salability in many cases, pretty much all conventional-engine cars in existence have far more power than what is required to maintain vehicle speed under average conditions. As such, running the engine at 3000-4000RPM in lower gear during level cruising will burn much more fuel than running the engine around 1500-2500RPM in overdrive, if the latter can maintain the vehicle speed without requiring the pedal to be buried.
The only way around this is to go to a hybrid biased strongly toward the batteries and electric motor. The engine is then designed to be much smaller and have less power, and can then be geared to run at a higher average speed, while the battery pack and motor compensate when temporary power bursts are required (hill, passing, merging, etc.). In the extreme version of this, the only engine onboard is a constant-RPM generator to charge the battery pack, and all of the propulsion is handled via the electrical system.
Driving below the posted limit is definitely not wise, though, as stop lights are timed according to the speed limit and deliberately driving slow will, as noted, put a driver out of synch with the light cycle. The gasoline savings will then be forfeited during idling.
Unfortunately no, but you might save a little time without any noticable increase in your gas bill (maybe a slight decrease) by getting up to speed faster.
I'd go a step further than this and say "don't slow down as much"
Think like a racing driver. When you're in a race, you have an X horsepower engine. Your goal is to get as much speed as possible, given fixed power output. The solution is to carry as much speed as possible through a corner, accelerate as early as possible, and and as hard as possible.
On the mechanical side, as a general rule of thumb an ICE operates most efficiently at low RPM and high manifold pressure. For equivalent power output, high revs mean high friction losses and low MP means high pumping losses. It's better to floor it in 5th than to give it half throttle in 3rd. Much, much better.
If you are accelerating, you should always have the throttle wide open. If you don't want to accelerate hard, wide open throttle in a high gear is very efficient.
The same applies on the street. Carry as much speed as you can in an on-ramp. Don't sit around at red lights, lag them on the opposite yellow.
Hmmm. I'm also doubting that long, leisurly were ever a big part of American culture, and to make up 5% of miles driven...
Oh, for a land in which the downward-sloping demand curve was not such a constant source of surprise and wonder to the broadcast media.
If they act surprised, it is probably because they expect their viewers to be.
Thanks SAM and Mouse. I've gone that step further too.
[Confussion is necessary to make the point simple.]
The only way around this is to go to a hybrid biased strongly toward the batteries and electric motor. The engine is then designed to be much smaller and have less power, and can then be geared to run at a higher average speed, while the battery pack and motor compensate when temporary power bursts are required (hill, passing, merging, etc.).
You actually want the exact opposite. You do not want a small engine running at high average RPM, you want a large motor running at low RPM. The goal is to fill the cylinder with as little pumping loss, flow reversion, or bearing drag as possible.
Look at any engine designed for constant output - say a light aircraft piston engine. These engines are not tiny, high-RPM engines. The IO-360 in a little tiny Cessna 172 is a near-six-liter four cylinder that turns 2300 rpm in cruise.
Or, if you're looking at engines that drive traction motors, consider the diesel-electric locomotive with dynamic braking (which is pretty much what a hybrid is, minus a battery). Most such engines are enormous, slow-turning machines.
Similar examples exist in ultralarge prime movers such as mining shovels and ships.
Look at any engine designed for constant output - say a light aircraft piston engine. These engines are not tiny, high-RPM engines. The IO-360 in a little tiny Cessna 172 is a near-six-liter four cylinder that turns 2300 rpm in cruise.
That's not the description of an ideal constant-output engine; it's the description of an ideal aircraft engine, where you want an unconditionally long service life and very high reliability, and frequent starting and stopping is not required. A slow-moving engine with a lot of low-end torque and a large amount of moving mass, fits the bill.
Your other examples seem to be describing engines with a comparable application. They are not describing a suitable reduced-output engine for a hybrid car, which has a substantially different duty cycle.
Or, stated better, the 6L 4-cylinder engine you cite probably IS running near the maximum efficiency point of its plausible RPM band. It's also rated to produce 180hp at 2700RPM and has fuel economy commesurate with those ratings, and frequent starting and stopping would place enormous wear and stress on the engine and the starting mechanism, since a large amount of rotating mass has to be brought up to starting speed.
This would not serve as a replacement for, e.g., the engine in the Toyota Prius, which derives 76hp at 5000RPM from a 1.5L 4-cylinder engine that is far lighter and more fuel conservative, and is subject to frequent starting and stopping.
A slow-moving engine with a lot of low-end torque and a large amount of moving mass, fits the bill.
Your other examples seem to be describing engines with a comparable application. They are not describing a suitable reduced-output engine for a hybrid car, which has a substantially different duty cycle.
This is false. Let me quote an authority you are likely to respect, yourself:
In the extreme version of this, the only engine onboard is a constant-RPM generator to charge the battery pack, and all of the propulsion is handled via the electrical system.
This would be a series hybrid, where the ICE is used only for constant-load electrical charging. Now let's examine constant-load ICE's driving generators here - diesel-electric locomotives, home generators, mining excavators, electric marine propulsion.
What do we have in common here? Low RPM, high MAP.
Let's back up some more and look at constant-load systems such as diesel trucks and marine engines:
Same thing. Low RPM, high MAP.
There are very solid engineering reasons behind this.
On the combustion side: Given a specific fuel and cylinder bore, the time it takes for the flame front to propagate (causing full cylinder pressure) is fixed. You want combustion to be done much before 90 past TDC so there is an optimum RPM.
On the mechanical side: Reciprocating losses go up with the square of speed, and friction loss goes up similarly.
On the pneumatic side: The higher your RPMs, the more often and faster you have to accelerate/decelerate the air in the intake tracts.
On the thermal side: The cube/square relationship between combustion chamber volume and surface area means that the larger the combustion chamber, the less heat is lost through the chamber walls.
For engines on the 100HP order of magnitude, we're talking about cylinder bores in the 100mm magnitude. The magic number here seems to be a hair more than 2k rpm.
We'll get back to this.
frequent starting and stopping would place enormous wear and stress on the engine and the starting mechanism, since a large amount of rotating mass has to be brought up to starting speed.
Which shows the heart of our disagreement. I believe you are confusing a parallel hybrid with a serial hybrid.
A p-hybrid needs to start and stop continually, and worse yet, to vary RPM greatly. An s-hybrid, which you previously advocated, does not. It starts up, runs for a while at constant RPM, and shuts down. Since this startup is done with a running electrical system, the oil pump is already running and the motor is up to temp.
A p-hybrid, on the other hand, runs like a normal car engine, which means it has a dynamic load. A good car engine is optimized to run well at freeway cruise and normal acceleration, which is around 30hp. It runs inefficiently at WOT/redline, but this is a good tradeoff because it spends vanishingly small time periods there.
This would not serve as a replacement for, e.g., the engine in the Toyota Prius, which derives 76hp at 5000RPM from a 1.5L 4-cylinder engine that is far lighter and more fuel conservative, and is subject to frequent starting and stopping.
And here you'd be misinformed.
On point 1: An 80hp Jabiru 2200 is around 120lbs, all up. This is significantly lighter than any liquid-cooled auto engine plus radiator/coolant.
Google and prime-mover.org will show you that the BSFC of a 1960's Lycoming is slightly lower than that of a modern 4-cylinder auto engine. The modern auto engine has the advantage of maintaining efficiency and flexibility over a large RPM/MAP range. The Lycoming does not.
There's a reason Toyota engineers put that 1.5L engine in there, and it's not because it's efficient at 76hp/5k RPM. It's horribly inefficient there, but as discussed before, it's designed like any other car engine to run well at standard load. A Prius has weight and drag that's not too far off that of Camrys and Corollas. This means that most of the time it runs at around 30hp.
At 30hp, that Prius engine is turning, wait for it.....
.... slightly over 2k RPM.
Which is to say that Toyota's engineers agree with, Caterpillar, GE Evolution, Volvo, Mercury Marine, Kawasaki, Honda (generators division) and me.
Of course, if you find a way to make an extremely small, high-rpm engine put up the same upper-30/low-40's BSFC numbers that a large/slow engine can make, you need to stop posting here immediately, patent that engine, and go out and make fistfuls of money.
secret asian man wins the thread.
Unfortunately, high-load, low-rpm driving is difficult with normal cars. Automatic cars shift when you try to apply high load, resulting in a sudden and very fuel-inefficient transition to high rpm. Manual cars don't have that problem, but most car engines are unhappy running at rpm that low with very high load -- a lot of stress gets put on the bearings. (This is why it sounds graunchy when you floor it at 1500 rpm in fifth or sixth gear.)
The series hybrid combined with a large battery and a wall plug, so the IC engine can simply not run for the first forty miles or so of driving after being charged, is the way to go. Chevy has created just such a concept with the Volt, but I'm very skeptical they actually mean to bring it into production.
Hmmm...
2800 cubic inches, 90 inches of manifold pressure, 1800 RPM = ?
Of course, the damn thing ran on 130 octane gas - try buying that at your local Arco.