jd4840 wrote:Would engine misfiring also explain the noises the engine makes as the car is sitting still at the end of pit lane prior to a launch from a stand-still? Is extra fuel sitting in the cylinders from leaving the pits, or idling too long?
The driver throttles it up, but the car just sits there popping and belching.
It makes a guttral sound as though it were a Harley Davidson motorcycle engine on steroids.
No, it does not explain that noises. There is a thread on valve overlap
here. That's the origin of
that noise (when idling).
When on a wet track or accelerating on the exit of a curve, you hear the misfiring of traction control, as Lurch explained. That's a different thing.
I quote that thread:
Now, picture for a moment an engine on the first stroke: the cylinder is going down, the intake valve is open and the air-gas mix is entering.
If you close the intake valve just when the piston reaches the bottom of its movement (what is called "BDC" or "Bottom Dead Center"), you are going to stop the inrush of air right when is entering the cylinder at its highest speed.
The air has inertia, like almost anything in this world. So, you left the valve open a little more time, with the final effect that, even while the cylinder is moving up, on the compression stroke, the air is still entering the cylinder because of that inertia, giving you a little extra mixture inside the cylinder.
The opposite is true on the exhaust stroke: even when the piston has moved away from the TDC (top dead center) and it's starting to move down, (theoretically it has started the intake stroke), the air is exiting at top speed and it "pulls" a little extra air out.
Finally, when you overlap the exit and intake of air, the exiting air helps to "pull" in the intake air.
This effect of the "inertia of the air" is more noticeable at high rpms, simply because the air is moving faster. This is why the overlap of the valves is greater in race engines, that develop ultra-high rpms.
The "magic" of valve design and manufacturing resides in closing the damn thing right when the air stops moving across it, not when the cylinder is at an arbitrary "theoretical" position at TDC or BDC. This theoretical position is good only for extremely low rpm engines.
Incidently, this is the reason why it is so hard to keep a racing engine at low rpms (just hear a drag car when it is idle: the engine sound is extremely uneven) and it is so easy to stall a F1 car on the grid or on the pits.
This is also the reason why it is forbidden to have variable timing camshafts, like the hydraulic or electric systems devised for "normal" cars. Actually, I don't understand the reasoning behind it (NOTE: behind forbidding variable valve timing), but if you had variable camshafts, F-1 engines would develop even higher accelerations from a standing start: the engine valve overlapping would be optimum at any engine speed.
Here you have the two images taken from
the animation at "How Stuff Works", comparing the valve overlap on a regular engine and on a racing (or higher rpm) engine:
Regular engine, small overlap of valves
Racing engine, huge overlap of valves
So, the irregular sound of idling engines in racing (for example, have you heard a dragster idling?) is because the valve overlap is less than optimum and the engines grunts, rattles, misfires, backfires, rocks and rolls.
It
needs to be at high rpm to perform well (like some stressed executives at the office...
).
About the noise when shifting pointed out by G-Rock, I have no idea. It
could be that the engine is adjusting its rpm to sinchronize the gearbox (to make the input axle from the engine run at the "same" rpms as the output axle to the wheels). Please, don't trust much this last explanation that I took from the "Department of Ideas You Take Out of Your Hat".
