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Old 05-11-2008, 10:18 PM   #1
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Why would improving aerodynamics/drag/weight/etc. affect mileage?

When I say these things, I'm saying this in a very specific manner. I'm under the presumption that the engine speed and fuel consumed is at a perfect ratio to that of the transmission. If you're in 5th gear and the engine is spinning at 2000rpm, it should consume X amount of fuel and be moving at a specified speed, always. I'm aware of load sensing and whatnot but I'm still confused about what I'm going to ask.

If you were to improve the aerodynamics, the drag, the weight, etc, making the car more "optimal" but you left everything else intact, when running at a constant speed, wouldn't your mileage be the same? Doesn't the engine consume the same amount of fuel regardless of the load?
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Old 05-11-2008, 10:25 PM   #2
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Doesn't the engine consume the same amount of fuel regardless of the load?
No..

More load requires a greater throttle opening, which means more air enters the engine.. At a constant fuel/air ratio then you must add more fuel so that the engine does not run lean..

This is assuming an otto cycle engine (gasoline or alcohol), diesels are different, they always ingest the same amount of air since they have no throttle.. In a diesel cycle engine more torque at a given rpm is achieved by adding more fuel.. Diesels run lean at everything except full power, the lower the load, the leaner the mixture, one of the reasons they are more efficient than otto engines.
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Old 05-11-2008, 11:01 PM   #3
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Originally Posted by fumesucker View Post
No..

More load requires a greater throttle opening, which means more air enters the engine.. At a constant fuel/air ratio then you must add more fuel so that the engine does not run lean..

This is assuming an otto cycle engine (gasoline or alcohol), diesels are different, they always ingest the same amount of air since they have no throttle.. In a diesel cycle engine more torque at a given rpm is achieved by adding more fuel.. Diesels run lean at everything except full power, the lower the load, the leaner the mixture, one of the reasons they are more efficient than otto engines.
At a given RPM, how does the engine regulate how much energy is produced? At higher loads, does it become more efficient in terms of volume by advancing spark timing?

What is the typical spark timing of an engine, then when can and wants to improve performance, the spark timing is advanced? I remember hearing the spark timing is retarded under circumstances where "improving performance" would cause the engine to ping because it would run too hot or something. Can somebody clarify this?

Is the statement I made only true for carbureted engines? Remember, the RPM can't be obtained by driving down a hill, i.e. no engine braking. On engines that don't support any sort of lean burning, how would running at greater than stoich yeild more performance? I can't see how adjusting the throttle while maintaining the exact same engine speed would use more fuel. I don't believe I can think of a situation where the load on the engine is say 40%, the throttle is at 10% and the engine maintains a given speed, then in another scenario the load on the engine is 80%, the throttle is at 100% and the engine is able to maintain its speed and not increase its speed.

Generally, I've found that the only reason why you get poorer mileage going up a hill is because you can't be in the highest gear while going up a hill.
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Old 05-11-2008, 11:33 PM   #4
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At a given RPM, how does the engine regulate how much energy is produced? At higher loads, does it become more efficient in terms of volume by advancing spark timing?
The energy produced in an otto cycle engine is determined by how much air is ingested. An engine is basically just an air pump, fuel is added and burned to heat the air and make it expand.. The more air you have expanding, the more power is produced.

Spark timing varies based on a lot of factors, engine load, rpm, temperature, fuel mixture and combustion chamber design are the most important I think.

Quote:
What is the typical spark timing of an engine, then when can and wants to improve performance, the spark timing is advanced? I remember hearing the spark timing is retarded under circumstances where "improving performance" would cause the engine to ping because it would run too hot or something. Can somebody clarify this?
The spark is generally advanced with higher rpm and also with lower loads.. High speed cruise at small throttle openings is where the timing is most advanced and low rpm, high load is where the spark is retarded the most.

The flame propagation speed through the mixture in the cylinder is dependent on load to some extent.. When the mixture is dense (high load) flame speed is high, when the mixture is more nearly a vacuum the flame speed is less. You basically want all the mixture burned by or shortly after top dead center so if the flame speed is lower at a given rpm you have to advance the spark.

As the engine turns faster, flame speed does not increase at a given load, you have to advance the spark so that burning is complete by or shortly after TDC.. There is simply less time at higher rpms for the flame to burn so you advance the spark to give more time.



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Is the statement I made only true for carbureted engines? Remember, the RPM can't be obtained by driving down a hill, i.e. no engine braking. On engines that don't support any sort of lean burning, how would running at greater than stoich yeild more performance?
There are a couple of reasons for running richer than stoich.. The first is to make absolutely sure that every fuel oxygen molecule is combined with a fuel molecule to produce energy.. The second reason is for cooling the intake charge to make it a bit denser, fuel vaporization cools the intake charge by taking heat from the air.

Quote:
I can't see how adjusting the throttle while maintaining the exact same engine speed would use more fuel. I don't believe I can think of a situation where the load on the engine is say 40%, the throttle is at 10% and the engine maintains a given speed, then in another scenario the load on the engine is 80%, the throttle is at 100% and the engine is able to maintain its speed and not increase its speed.
I'm not quite sure what you are trying to say here..

Quote:
Generally, I've found that the only reason why you get poorer mileage going up a hill is because you can't be in the highest gear while going up a hill.
You get poorer mileage because it takes more energy to go uphill than down.. Think of pedaling a bicycle, you have to pedal hard to go uphill, you can coast going downhill.

If you use an instantaneous mpg gauge you will see that even slight changes in incline make a surprisingly large change in mpg..
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Old 05-12-2008, 05:55 AM   #5
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Engines produce more power for less fuel per unit of power under load. Is actually more efficient climbing a hill, but only if you can coast down the other side of the hill with the engine off or idling.

Fuel consumption on a small 4 cylinder can vary from .25 gallon per hour to over 3 gallons per hour depending on the sustained load.

If your assumption about consumption as only a function of speed was correct. That same engine would have to run at 3/.25 or 12 times idle speed, which works out to 8400 RPM.

Its the total volume of air the engine ingests and combusts that determines to consumption assuming the mixture is constant (not always). The engine doing more work at any given speed consumes more fuel (your climbing a hill observation) but consider this. In essence climbing a hill is propelling the car at a certain speed and actually lifting the car vertically at a certain speed depending on the grade of the hill. Like a roller coaster the car slows down climbing and accelerates descending.

regards
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Old 05-12-2008, 06:12 AM   #6
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Quote:
Originally Posted by ************* View Post
At a given RPM, how does the engine regulate how much energy is produced? At higher loads, does it become more efficient in terms of volume by advancing spark timing?

What is the typical spark timing of an engine, then when can and wants to improve performance, the spark timing is advanced? I remember hearing the spark timing is retarded under circumstances where "improving performance" would cause the engine to ping because it would run too hot or something. Can somebody clarify this?

Is the statement I made only true for carbureted engines? Remember, the RPM can't be obtained by driving down a hill, i.e. no engine braking. On engines that don't support any sort of lean burning, how would running at greater than stoich yeild more performance? I can't see how adjusting the throttle while maintaining the exact same engine speed would use more fuel. I don't believe I can think of a situation where the load on the engine is say 40%, the throttle is at 10% and the engine maintains a given speed, then in another scenario the load on the engine is 80%, the throttle is at 100% and the engine is able to maintain its speed and not increase its speed.

Generally, I've found that the only reason why you get poorer mileage going up a hill is because you can't be in the highest gear while going up a hill.
Let me put this in simple language. If God put a throttle in your throat, you would definitely put out less power when the throttle was closed.
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Old 05-12-2008, 07:48 AM   #7
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Quote:
Originally Posted by ************* View Post
When I say these things, I'm saying this in a very specific manner. I'm under the presumption that the engine speed and fuel consumed is at a perfect ratio to that of the transmission. If you're in 5th gear and the engine is spinning at 2000rpm, it should consume X amount of fuel and be moving at a specified speed, always. I'm aware of load sensing and whatnot but I'm still confused about what I'm going to ask.

If you were to improve the aerodynamics, the drag, the weight, etc, making the car more "optimal" but you left everything else intact, when running at a constant speed, wouldn't your mileage be the same? Doesn't the engine consume the same amount of fuel regardless of the load?
The first time I drove in a car with instant MPG readout, I was blow away by how much the mileage fluctuates with pedal input. Like you, I thought that at given engine speeds there was very little difference how much fuel the engine consumed. When I saw the mileage in a Chrysler Town and Country go from 83mpg to 16mpg just from pressing on the gas pedal, my understanding of how the engine consumes fuel totally changed. The harder you press on the throttle, regardless of your RPM, the more fuel it uses. It's called levels of enrichment. Having a light touch on the pedal results in low enrichment, even if you are at 2000RPM -- and at the same RPM -stomp on the pedal--you will enter high enrichment mode (lots of gas pumped into the cylinders) and your mileage will go from 32mpg to 16mpg in the blink of an eye.
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