question about aero

[Christ]

The engine doesn't spin any slower when you reduce load. An engine, at any point where it is not accelerating or decelerating, is idle. The way we tend to use the term idle is quite a misnomer. Idle means at rest, or not laboring. When you load the engine, once it stabilizes, it's effectively at idle again, even though it's at a higher speed than normal "idle".

Anyway, the point is that an engine only produces exactly enough power to satisfy it's current operating status. If you demand 30 mph, the engine will produce enough power to maintain 30 mph in addition to overcoming static and parasitic loads. Just because the RPM that coincides with 30mph happens to be capable of producing twice the required power, doesn't mean that it is producing that much.

[vskid3]

It takes less power to move at a given speed with better aerodynamics. The engine is still spinning, so to make less power, you give it less throttle, which gives it less air (and thus less fuel), so it will make less power at the same engine speed. Try going from a flat road to a slight downhill. In order to not speed up, you have to lift up on the throttle a bit. Its the same with better aerodynamics.

[Hubert Farnsworth]

Again this is where the part open throttle engine curves get really messy when talking about bsfc, its easiest to represent them as contour maps or islands of efficiency, speaking of which no one has located one for the 4a-fe either, but that's not relevant to this topic, however at reduced loads the engine makes less power but may not be operating at its most efficient state, other optimizations non withstanding. However does lower power output lead to lower fuel consumption if the combustion events are still occurring as frequently as ever? I guess as a mechanical engineering student who has only just begun to get into the exciting powertrain classes this is still a bit fresh theoretically vs physical experience.

Also my car probably has the C50 5-Speed Manual Transmission
Gear ratios for this transmission.
1st 2nd 3rd 4th 5th Reverse Final
3.545 1.904 1.310 0.969 0.815 3.250 3.722
At least it was used on the equivalent corollas and some of my research indicates that it may be the same as in my car. that 0.82 overdrive is why its still spinning quicker than is desired, peak torque is closer to 2-2.5k iirc.

[dcb]

Originally Posted by Hubert Farnsworth

Again this is where the part open throttle engine curves get really messy when talking about bsfc,

Even though aero with no gear change may move you lower on the load scale and farther from the bsfc island, you are still using less power, so that usually eclipses the decrease in gm/kwh.

aero + gear change can provide more than the sum of their independent changes.

aero + smaller engine is the next logical step (and just deal with slower acceleration)

[t vago]

Originally Posted by Hubert Farnsworth

I'm being serious, it doesn't seem logical that the engine would spin any slower even if the load is reduced due to the aerodynamics being improved

And the engine should not spin any more slowly at a given speed, if you improve the vehicle's aerodynamics.

However, the throttle controls the amount of engine power being produced, right? If you give the engine less throttle, it'll make less power. That means the engine requires less airflow through it, and less gasoline that must be mixed with that airflow.

[Hubert Farnsworth]

I suppose that indirectly the throttle controls the power output by regulating the air intake in a gasoline powered vehicle with a throttle plate, however in a modern common rail diesel or some direct injection gasoline engines with no throttle plate can control the fuel directly. If the reduced loading would allow for a lighter throttle application then yes the power output should be reduced to compensate at the same revs, meaning that on a throttled engine less air/fuel mix should be inducted, however with direct injection I'm not as certain how it is programmed.

[Christ]

Diesels are throttled by fuel, and only fuel. Purple who say they don't have a throttle are incorrect; They don't have an air restrictor, with a few exceptions. When you let off the pedal slightly, your changing the amount of fuel being injected. This in turn, throttles the engine down, asking for less pro out power at a given output speed.

[Hubert Farnsworth]

Controlling the fuel intake effectively throttles the power output of the diesel engine yes, but I was referring to the throttle plate on the intake air charge, which modern diesels typically don't require and are effectively at wot all the time on the air side ie the volumetric efficiency goes up, and the precise amount of fuel required can be injected just prior to the top of the compression stroke. This increase in volumetric efficiency coupled with the higher energy density of the diesel fuel helps lead to higher fuel efficiency/lower fuel consumption

[Christ]

Are you trolling?

If you already understand how this works, what is the point of your questions?

What is the end point of this thread?

[Hubert Farnsworth]

No I was not trolling, and therefore have no need to continue this conversation at present, however on an aerodynamically related question which would be more important, blocking the upper grille which is already narrow, and adding an air dam versus a belly pan and closing up the wheel arches.