What is the aim to design and how to assess the performance of restrictor ?

[Tilman]

Heyho,

@ ZAMR:

That is exactely what I was trying to say: There is a huge difference between an instantaneous consideration in which torque at a given rpm is the only thing that matters. But since a race is not instantaneous, there is no other way than (numerically) solving the differential equation to get some precise numbers.

By the way: There is a huge difference between accelerating a car from 0 kph until it went 75 m and accelerating a car from 50 kph until it went 75 m because you cannot neglect aerodynamic drag in the second example. You need some power/torque to overcome drag and this cannot be used to accelerate the vehicle.

@ Warpspeed:
You are comparing completely different engines! If I had an engine (one!) and two pairs of fuel and spark maps, one pair that gives me constant power over all rpms and one pair that gives me constant torque over all rpms with top power being equal to the power of the constant power pair than I would use the constant power pair for the reason you mentioned. But the point is: The constant torque pair is artificially throttling the engine! That is the reason why your example works, but it has nothing to do with the engines we are using.

However, I do not have an idea which condition was acceptable to declare a constant power engine and a constant torque engine as "equal" or comparable.

I do not see another way than defining some test conditions like the 75m acceleration from 0kph or some 50m acceleration on an autocross straight, taking measured power/torque curves and doing some numerical simulations.

[Warpspeed]

That is the reason why your example works, but it has nothing to do with the engines we are using.

My example was an EXTREME hypothetical case.

But something like a gas turbine car with only one fixed overall transmission ratio may come close to a constant power source.

But to drag us back to reality, consider two very different internal combustion engines pulling air through an identical FSAE restrictor.

One is a very highly tuned small capacity engine that roughly approximates the peaky power curve and constant torque ideal.

The other is an absolute monster capacity engine... with a positive displacement supercharger.... This engine produces no more top end power, simply because the restrictor strangles it.
But it develops a huge amount of low end and mid range torque, simply because the sheer volumetric displacement keeps the restrictor well into the choke region over the entire rpm operating range.

If such an engine could be built that was not excessively large or heavy, might it not be superior ?

Again we are talking hypothetical extremes here just to illustrate a concept.

Building an engine where the rules require a fixed diameter restrictor, is very different to building an engine of maximum allowed volumetric displacement, as is often the case in some racing classes.

And to take us right back to the thread topic, there may be more to be gained by re thinking the whole engine, rather than just trying to build some super efficient restrictor that is .001% more flow efficient.

As in many forms of competition, there are sometimes subtle ways of gaining a large advantage unrealized by other competitors, while still working strictly within the rules.

[Wesley]

Originally posted by mk e:
High rpm hp does not = peaky any more than more than low rpm power = board power delivery.

Your assertion that the usable rpm range is compressed by gear reduction is correct and needs to be considered, but the assertion that using gear reduction = slow acceleration and poor lap times is not.

Have you ever seen an F1 car accelerate?

I do agree with what I think were your under lying assertions though, balance is the key to success and in that balance focusing on easy of operation over absolute performance will probably produce a better competition result.

The impact that the intake and exhaust design on the power peaks is small compared to the impact the factory decisions regarding valve and port size and cam specifications have on the power curve. Given our small displacement, high revving engines, trying to create a power peak at low RPM using factory heads and cam specs is generally an exercise in futility. The cams, heads, and exhaust are designed from the factory to make power at high RPM and you are not going to "undo" this with a new intake and header and create a power peak with a very substantive change.

As a result, most teams optimize power levels from 9-12000RPM (on 600cc 4-cyls) because above that and you get closer to choking the restrictor, and below that, the cam and port geometry drops the motor on its face no matter how many millenia you spend simulating and dyno testing your intake and header.

Tuning for a power peak at any specific RPM results in a "peaky" engine whether low or high, but within the bounds of this competition, our power peaks are created at high RPM because that is the nature of the engines we start with; most teams do not do extensive port or cam work.

My assertion that lap times and acceleration times suffer is based on anecdotal assumptions:

- Shift times are in the 50-100ms range. Whether this is because the car has a manual shifter or because Johnny Fatfingers can't let go of the upshift button. Every shift you require adds that time proportionally to your acceleration score.

- Our cars are traction limited already, even with small displacement engines. Because of gearing and tire limitations, you can only front end load your acceleration by so much before you start reducing accel times, even with infintesimal shift durations. Tractive force diagrams will show you impressive numbers until you consider tire data in the critical launch region.

-Working within the gearing limitations of motorcycle gearboxes, the split in ratios is very unfavorable for our 3-4x reduced speed range from original design intent. Because of this ratio split, a broader torque curve and larger axle ratio bandaids this design deficiency. This is of the largest impact on the auto-x and endurance, and a lot of corner exit power is lost to this lack of variability (combined with drivers that don't always hit that downshift just right.)

No number of gear shifts can put you in gear 3.5 unless you've had a really bad day. This is clear on a tractive force diagram of any typical FSAE car, with long stretches in 2nd and 3rd gear and almost complete disuse of 5-6th gears.

All of my posts are based under the assumptions of an OEM motorcycle-engine powered FSAE car, and do not apply to purpose built race cars not working within the design boundaries of OEM consumer products.

[Wesley]

Originally posted by Warpspeed:
I would beat you, at any speed, simply because I had more torque everywhere.

Bingo. Has nothing to do with power. It is important to note that power is a calculated number from torque itself. Torque is measured. Cars are unique in that they operate across a wide speed range, but with sufficient gear adjustment and manipulation (with transmissions, ratios, etc) that can be overcome.

[Buckingham]

When can we finally stop arguing 'power vs torque' or 'which really accelerates the vehicle'???

Torque [J] = Work [J] = Energy [J}

Power by definition is the time-derivative of Energy.

POWER [J/s] = dENERGY [J] / dTIME [s] !!!!!!

so...

Bingo. Has nothing to do with [the change in energy over time]. It is important to note that [the change in energy over time] is a calculated number from [the change in energy] itself. [the change in energy] is measured.

sounds kinda funny when you say it like that


POWER [J/s] = dENERGY [J] / dTIME [s]

If we want a known change in speed (dENERGY) to occur in less time (dTIME) we need more POWER.
It's really as simple as that.

"But what if I increase X Torque at this RPM for a sacrifice in Y Power at that RPM, is that better?"

Plot both on Power vs RPM curves and calculate which has more average power over the operating RPM range. Whichever has more, is better.

[Wesley]

Originally posted by Buckingham:
Plot both on Power vs RPM curves and calculate which has more average power over the operating RPM range. Whichever has more, is better.

Absolutely. You can't have power without work. We're arguing about rate dependent lap times (power) but there are a lot more considerations than that (torque and tractive force) that actually make you go fast.

[Mbirt]

Originally posted by Buckingham:
When can we finally stop arguing 'power vs torque' or 'which really accelerates the vehicle'???

Torque [J] = Work [J] = Energy [J}

Power by definition is the time-derivative of Energy.

POWER [J/s] = dENERGY [J] / dTIME [s] !!!!!!

so...

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Bingo. Has nothing to do with [the change in energy over time]. It is important to note that [the change in energy over time] is a calculated number from [the change in energy] itself. [the change in energy] is measured.

sounds kinda funny when you say it like that


POWER [J/s] = dENERGY [J] / dTIME [s]

If we want a known change in speed (dENERGY) to occur in less time (dTIME) we need more POWER.
It's really as simple as that.

"But what if I increase X Torque at this RPM for a sacrifice in Y Power at that RPM, is that better?"

Plot both on Power vs RPM curves and calculate which has more average power over the operating RPM range. Whichever has more, is better. </div></BLOCKQUOTE>Forreal!

This isn't that hard for us to grasp, but the description of torque and power as seperate entities in automotive literature and among gearheads in general definitely breeds confusion among the impressionable. Correcting this should be the next crusade after squashing AGW theory and draconian CAFE standards.

[Z]

Originally posted by Buckingham:
Torque [J] = Work [J] = Energy [J}

To All Above,

Firstly, good to see this sort of discussion of fundamental principles.

Secondly, I must be a bit of a PITA and point out that Torque is NOT EQUAL to Work or Energy.

Work, and so also mechanical energy, is loosely defined as a "force by a distance moved". But, more accurately, it is the SCALAR PRODUCT of the force and distance vectors, hence giving a scalar quantity (work or energy have no obvious direction).

On the other hand, Torque (I prefer "couple") is the VECTOR PRODUCT of a force vector and distance (or displacement) vector. Torques thus have very definite directions in 3-D space (although they are "free" vectors).

So to find the amount of mechanical work, or energy, coming out of a crankshaft, we have to scalar (or "dot") product its torque vector with its angular displacement vector (magnitude measured in radians), thus getting a scalar quantity. But note that "angular displacements" are "pseudo"-vectors, so have to be treated cautiously (non commutative).

Or we can get the power by scalar producting the torque vector with the angular velocity vector (radians/second). Angular velocity and acceleration vectors are well behaved vectors in that they commute.

(Gotta get back to work ...)

Z