Increasing fuel savings
- Thread starter epicstruggle
- Start date
Any freeway has debris on it. Rocks, random car parts and pieces, etc. Not to mention dump trucks that drop stuff...etc. etc.
All cars nowadays have computer controlled engines and fuel injection, so the amount of gasoline burned has no direct relation with the depression of the gas pedal. The computer determines the amount of power. When you depress the gas pedal fast, it will increase the amount much more than when you depress it slowly.
When the engine is idling, the amount of air and fuel injected are the lowest that are needed to make the engine rotate. But, there is a lowest (as well as a highest) bound to the percentage of fuel needed to get an explosion. So, when idling, you reduce the airflow as much as possible (creating low pressure in the cylinder) and inject the smallest amount of gasoline you can. And about the same happens when you cruise, although you allow more air and spend more fuel. The main difference between idling and cruising, is that the latter also needs an amount of power to counter air friction.
That's why companies like Ford experiment on-and-off with turning off a number of cylinders when idling or cruising on V8 or better engines. Shut all the valves, and use the air inside the cylinder as a spring. Like, disable between two and four cylinders when cruising, and up to five when idle. Another way is to have local combustion in the cylinder: you only inject a bit of fuel at the spark plug. This is the most common option nowadays.
Acceleration is different. While you want a small engine for cruising, to save fuel, you want the maximum acceleration possible when slamming down the gas. You push as much air in as will fit, and a bit more than the maximum amount of fuel allowed (as not all of it will mix well). And cramming in more air works just as well as using larger cylinders for that. It's about the mass, not the volume. A turbo or mechanical compressor doubles the mass of air in the cylinders.
For gasoline, you need between 1.3% (or a bit less) and 8% (or a bit more) fuel in the mixture. And considering that the amount of air is 4 to 12 times more, that means that you burn between 28 and 96 times as much fuel when accelerating as when cruising.
When the engine is idling, the amount of air and fuel injected are the lowest that are needed to make the engine rotate. But, there is a lowest (as well as a highest) bound to the percentage of fuel needed to get an explosion. So, when idling, you reduce the airflow as much as possible (creating low pressure in the cylinder) and inject the smallest amount of gasoline you can. And about the same happens when you cruise, although you allow more air and spend more fuel. The main difference between idling and cruising, is that the latter also needs an amount of power to counter air friction.
That's why companies like Ford experiment on-and-off with turning off a number of cylinders when idling or cruising on V8 or better engines. Shut all the valves, and use the air inside the cylinder as a spring. Like, disable between two and four cylinders when cruising, and up to five when idle. Another way is to have local combustion in the cylinder: you only inject a bit of fuel at the spark plug. This is the most common option nowadays.
Acceleration is different. While you want a small engine for cruising, to save fuel, you want the maximum acceleration possible when slamming down the gas. You push as much air in as will fit, and a bit more than the maximum amount of fuel allowed (as not all of it will mix well). And cramming in more air works just as well as using larger cylinders for that. It's about the mass, not the volume. A turbo or mechanical compressor doubles the mass of air in the cylinders.
For gasoline, you need between 1.3% (or a bit less) and 8% (or a bit more) fuel in the mixture. And considering that the amount of air is 4 to 12 times more, that means that you burn between 28 and 96 times as much fuel when accelerating as when cruising.
How much of this is due to just needing power, though, and how much is due to actual inefficiency? After all, when accelerating, you need to actually give it a fair amount of gas just to get the car up to speed, but once it's up to speed, you only need to give it enough to combat friction. Depending upon the aerodynamics of the car and the speed, this may be a little or a lot.DiGuru said:
Well, it's not really needed as such, but what people want. It's all about running that engine as efficiently as possible, for fuel economy when cruising, and for maximum power when accelerating. Both at the end of the scale allowed. Most engines got as efficient as they are over the years by doing the first, but people want the fastest acceleration they can get when they want it. Both of those sell cars.
Ty said:
I know you will destroy the transmission by towing an automatic in neutral for long distances. I had a friend with a large RV and he wanted to tow his car, and since it was an automatic he had to get a trailer type deal because a tow bar is intended for manuals. That is the world he got from his mechanic at least.
ShootMyMonkey
Veteran
Okay... but that is the difference between accelerating and coasting. You haven't really argued against what he's said. Although, you do bring up the point that gearing and the relative power demands does make a difference. For instance, you take my car, which is by design, underpowered (as that seems to be the only solution in people's minds for fuel efficiency) means that it can probably get better mileage with a more powerful (and specifically more torquey) drivetrain. It's fine for flat roads and normal highway driving, but uphill climbs or having 5 adults in the car is pretty much a lost cause. The difference there is load and rpm. If the engine is weak enough that I need to get into high rpms in order to make any power, that's bad for mileage, assuming I'm in a condition where power is needed. If I have to really press the pedal to get it to even rev up (e.g. load on the engine is high), then that also costs me in mileage, as the fuel mixture needs to be enriched.
There's the idea of granny-accelerating, and there's the whole granny-shifting thing. Ideally, in addition to accelerating not-so-hard, you also don't want to stay in low gears for very long. And if you shift early and accelerate to those shift-point rpms quickly, you can do that, as the fuel consumption rate grows pretty linearly with rpm (assuming all else is equal). As long as the speed you're moving at grows less quickly than the rpms, you'll do worse on fuel economy as the rpm goes up. Since lower gears means taller ratio, the problem is magnified.
However, if your engine is too weak to move the car well at low rpms, you have a double problem as the engine won't be able to run efficiently. This is why diesels are so much more efficient because they make power at lower rpms, and the power they make is pretty much controlled by the amount of fuel injected.
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Well, I'm not trying toShootMyMonkey said:
I'm trying to find out more about certain aspects of how cars work that I've never seen described.Well, I don't know if I buy the first point, but the second point is clearly wrong: as long as you are in one gear, speed and revolution rate are directly proportional (given by the gearing ratio). The thing that hurts you most with higher speeds is air friction.
As for the rpm's affecting fuel consumption, it's more interesting how higher rpm's affect the ratio of fuel used/power output, a ratio which I don't believe increases monotonically with rpm's for all levels of power output.
I mean, I'm sure there's some lost efficiency that comes in due to friction, but if you want X amount of power (say, when going uphill), then if the rpm's are high enough, you won't need to enrich the fuel mixture. But too high and friction within the engine will start cutting into fuel economy. With fuel injection engines, there may also be issues with incomplete mixing of gas/air at higher rpm's leading to lowered fuel economy. Surely there's an optimal point somewhere in the middle.
I know that when I press down on the gas pedal, there's a point beyond which further pressing doesn't change the power output much (this is on a manual, so further pressing on the gas doesn't result in downshifting). I typically try to ride the car so that I never press the gas pedal beyond this point, which varies depending upon the rpm's.
Yes, but to increase the rpm in the first place, you need higher pressures all the time, while the mass of the air that is sucked into the cylinders decreases (fixed by the air inlet and valves). So you need to make the mixture richer and richer, and the fuel efficiency less and less. Or the volume bigger.
As ShootMyMonkey said, that's why diesel engines are so populair in most of the world: they have spontaneous combustion according to the pressure and heat in the cylinder, and so can always use the same amount of air, only increasing the amount of fuel when you go faster.
In short: they're proportional efficient to the power asked, not the rpm (within bounds). And that's why they are more efficient at lower rpm.
As ShootMyMonkey said, that's why diesel engines are so populair in most of the world: they have spontaneous combustion according to the pressure and heat in the cylinder, and so can always use the same amount of air, only increasing the amount of fuel when you go faster.
In short: they're proportional efficient to the power asked, not the rpm (within bounds). And that's why they are more efficient at lower rpm.
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That description doesn't make any sense. But it may be correct in the result. A slightly different argument that makes more physical sense would be:DiGuru said:
The pressure differential between an "empty" cylinder and the air intake is always about the same, so with a fixed-size valve, if you're running at higher rpm's, the air doesn't have as much time to enter the cylinder, and so there is less air there when the gas is exploded.
That said, the amount of air that would enter the cylinder shouldn't be linear with rpm's. As long as the valve is large enough compared to the cylinder size, the amount of air that is able to enter the engine should be nearly constant at low rpm's, and decreasing per explosion at higher rpm's.
In this way, while at lower rpm's you'll be able to explode more gas per revolution before your air/gas mixture is too rich, at higher rpm's you can explode more gas total per unit time. This will occur because most of the air that enters the cylinder will do so in the first bit of time that the valve is open.
Of course, at very high rpm's (set by the size of the valve and cylinder), there will never be enough time to start to fill the cylinder, and the amount of air that makes it into the cylinder will be approximately linear with time allowed (which decreases at higher rpms).
Unfortunately, I have no true sense as to the time scales involved here. But at least now I think I have a better understanding as to the purpose of more than two valves per cylinder: it increases the intake/exhaust area as compared to the volume of the cylinder (my little Chevy Aveo has 4 valves/cylinder).
This isn't actually explaining anything to me. You still have the fundamental problem of how to oxygenate the fuel as much as possible. What's the difference in how the air gets to the cylinder in a diesel engine vs. a gasoline engine?
ShootMyMonkey
Veteran
I made a typo actually... "As long as the speed you're moving at grows less quickly than the consumption...."
At lower gears, the growth of fuel consumption with rpm is not so linear (because not all else is equal through the rpm range -- load is a factor in low gear). First and second gear in most vehicles is rarely intended to be an "efficiency" gear and more of a "torque multiplying" gear in order to get you moving even at idle rpm. You think about the fuel consumption at 3000 rpm in first gear, you're likely only going 15 mph, vs. the same rpm in top gear, and you're likely at highway speed, and you will use less fuel at top gear at the same rpm
I'd certainly agree with that. Part of the problem with so many fuel mileage measuring models is that it pretty much assumes a flat A/F ratio and simply bases its estimates on intake vacuum pressure rather than fuel flow rate through the injectors.
There is a point, though, where it does tend towards monotonicity. It's just that that happens to be at higher speeds, where the A/F ratio only needs to be rich enough to overcome friction (drag included).
The thing with a diesel is that the charge of fuel is stratified and injected just after TDC where it spontaneously combusts. Since the fuel quantity controls the power output, you can run the engine extremely lean if the extra power is not needed. If you run it over-rich, you still get more power (not quite as fast a growth, though), but you get incomplete combustion which causes the black smoke we see with diesels so often. Of course, you can put a turbo or a supercharger or propane/nitrous injection or something else to help things along so that you can run more fuel and still get complete combustion.
The problem the American EPA has with diesels are due to the fact that rather than rating emissions against distance traveled, the US rates emissions by unit volume of exhaust, which shows the NOx emissions of diesels to be horrible. If you measured by kilometer like the EU, while the NOx emissions would still be higher than that of unleaded gas, they wouldn't be as high as it might seem, and the overall emissions would still be better than any comparable gas engine will ever be able to achieve.
Do you mean that the fuel/air is mixed prior to entering the cylinder? I know that gasoline cars used to do this, back in the days of the carburator. I wonder why, in the move to fuel injection, they moved away from mixing the fuel outside the cylinder? I would typically think that would result in higher fuel economy, after all.ShootMyMonkey said:
And besides, it would seem to me that you have more control over the timing of the combustion in a gasoline engine than a diesel engine. So I'm really wondering why diesels are more efficient, as I would naiively expect that for the same explosive power, you could obtain better mechanical efficiency with gasoline (due to the ability to time the explosion).
Yes, that is stupid.
OpenGL guy
Veteran
I think it's the opposite. Fuel injection allows more precision when mixing the fuel and air, thus improving power and efficiency.Chalnoth said:
ShootMyMonkey
Veteran
In a diesel, the air and fuel do not mix before entering the cylinder. The fuel-injector is in the cylinder head itself, and only air alone is drawn in on the intake stroke. The same is true of gasoline engines which are called "Direct Injection" engines (though there are some like the Lexus IS series which use a combined EFI + Direct Injection). Most gasoline engines, however, the air and fuel mix before or during entering the cylinder in order to give you a more homogeneous mixture of air and fuel. Fuel injection gives you very precise control over the fuel delivery since it's computer-controlled.
In the early days of fuel injection, you had TBI, which put a single high-flow injector into just next to the throttle body before the intake manifold. While this gave you some efficiency improvement over carbeuration since you could precisely control fuel flow, it was really effectively a drop-in-replacement for a carbeurator and could work just fine on any wet manifold. The biggest problem with TBI of course is that since there's fuel moving around in the manifold, you end up putting fuel all over the place including cylinders that aren't really asking for it. Nowadays, you have port EFI, which puts a lower-flow injector near the end of the manifold right next to the intake valve of each cylinder, so you still get air and fuel mixing on the intake stroke, but the appropriate amount of fuel is delivered to each cylinder exactly when it's needed.
There are forced induction engines which use an extra injector early in the manifold even today, but those are serving dual-duty because they're usually engines that were NA to begin with and got modified to add on turbos or superchargers. The extra air demands extra fuel and the stock injectors may not be able to provide enough (that's job one). The other thing is that a spray of evaporating fuel with all that compressed air helps cool it down.
Direct Injection moves the injector into the cylinder head so fuel can be delivered late or early depending on load conditions. You get some of the benefits of diesel like being able to control the power through fuel delivery quantity and timing, but it can still run like a normal gas engine at WOT.
Nah, the two are the same when it comes to control over timing. With a gas engine, you get combustion when the spark goes off. With a diesel, you get combustion the instant fuel is injected. Both things are computer-controlled.
And it's always a lot easier to get more explosive energy out of diesel since the compression ratios are so much higher.
OpenGL guy
Veteran
Diesels gain efficiency because detonation happens everywhere simultaneous, but, with a regular gas engine, detonation begins at the spark plug.ShootMyMonkey said:
ShootMyMonkey
Veteran
Yes, that too. Rather, combustion happens throughout the front of fresh fuel, and since diesel injects at such high pressures, you effectively get combustion everywhere. I was just commenting on his thought that the timing of combustion is more precise in gas.
The whole thing with HCCI engines is interesting, but those have a lot of the issues that Chalnoth believed were the case with diesels. And that's what makes them so hard to implement.
The whole thing with HCCI engines is interesting, but those have a lot of the issues that Chalnoth believed were the case with diesels. And that's what makes them so hard to implement.