RPM and speed

[Ogami musashi]

Hello dear members,

I'm a newbie in engine and transmission but recently was being asked on another forum about a slipstream condition in F1 (so involves aerodynamics, so this is my field).

The question was about the 19,000 RPM limiter on this year F1.

The person asked if, in slipstream condition because of the higher speed the engine could go above the 19 000 PRM, and if not, what was the type of limiter.

If any of you have a clue, feel free to answer.

But my question is not here, it is related to the fact this guy say that for a given gear shift, an RPM will always be related to the same speed, that is if in a slipstream , the drag lowering makes you accelerate the RPM will naturally follow even if you didn't push the accelerator more.

Inversaly they say if you are slowed down by higher drag, as long as you have the same speed, the same RPM will be produced.

is it true? and can you please tell me why?

thank you.

[johny]

i think ECU limitates rpm's always, if you are slipstreaming someone the rev limiter will be hit earlier but engine won't go over 19k

[C/\D]

Limiter is a passive limiting system, and if i am wrong correct me. They are using the throttle mechanism for the limiting action. An engine computer. Calculates the revs, depending on the speed, steering, acceleration, forces on the car, driver inputs, anything can change the rev ... etc using embeded telemetry. So in this situation. The micro computer computes the rev increasment in a very short range can be counted as miliseconds. And decreases throttle for very short ranges. So the system is similar to effect - reaction balance.

[mep]

This is a good question because if it's not allowed to
rev higher in slipstream then the effect of slipstream is
destroyed.
But I think the teams handle the problem in a simple
way by just setting the 7 gear a bit higher so that they
can't hit the rev limiter even in slipstream condition.

Anyway the rule is simple stupid like most of the attempts to
save money because the big companys simply infest the
money in other parts of the car. And with most of the smal
teams getting their engines from the big companys they have
the same engine but not the money to improve the parts the
big companys have improved.

[DaveKillens]

But my question is not here, it is related to the fact this guy say that for a given gear shift, an RPM will always be related to the same speed, that is if in a slipstream , the drag lowering makes you accelerate the RPM will naturally follow even if you didn't push the accelerator more.

Inversaly they say if you are slowed down by higher drag, as long as you have the same speed, the same RPM will be produced.

is it true? and can you please tell me why?

thank you.

The drivetrain, from the engine crankshaft to the rear tires are connected to each other at a fixed mechanical ratio. It's the different gears that can alter that ratio, but they are always connected to each other at a specific ratio. (barring the clutch opening). So if the engine speeds up 10%, the rear wheeels will spin 10% faster, and so on.

Now to discuss drafting, a separate subject. A car following closely behind another can sit in a zone where there is a lot less aero drag, and thus can allow the following car to run at reduced power. Mind you, the engine would be turning the same RPM at the same speed, regardless if it was in a draft or not. Now in the case of a Formula One engine limited to 19,000 RPM, if the ECU cut all power at 19K, then it just won't go any faster unless it was dropped out the back of a C130. The only way to raise terminal speed is to either raise the RPM limit, or change the gearing. As mentioned by mep, you just have 7th gear set higher.
For instance, let's assume that for a specific track the upper limit was 300kph. Then 7th gear could be set to attain 320 k at 19K RPM. Running alone the car would bog down and suffer with a gear too tall. But if it was drafting, then it is possible to catch a good tow, and pass the car ahead.

[Ogami musashi]

In case of drafting, what is the parameters that decrease with the decreased power needed to go at a certain speed?

[DaveKillens]

While in tow behind another vehicle, just the aerodynamic drag is reduced.
Basically, if you were going 300 kph alone, the car would see 300 kph's worth of drag. But while drafting, the car would see a much lower number, for example, 250 kph.
http://www.canosoarus.com/08LSRbicycle/LSR%20Bike01.htm

[Ogami musashi]

thank you dave but i do know the aerodynamics side of drafting, what i do not know is how the powertrain works (in details) and so i would like to know, because when drafting less power is needed, where this "less power" is found in the engine and transmission? what are the parameters that move to produce with less power the same speed?

[modbaraban]

thank you dave but i do know the aerodynamics side of drafting, what i do not know is how the powertrain works (in details) and so i would like to know, because when drafting less power is needed, where this "less power" is found in the engine and transmission? what are the parameters that move to produce with less power the same speed?

It’s very simple.

I’ll try t explain it this way as I like simracing. (all figures are rough, not calculated and brought just for example)

When I’m setting up a car for a certain circuit and I already have the aero set up (this means I already know of current top speed ability). In Silverstone the top-speed would be about 310km/h. I tune the 7th gear in order to reach about 18.0-18.5k rpm at the end of the longest straight (the Hangar straight).
Why not tune the 7th gear to reach full 19krpm to the limit at the same speed of 310lm/h (and thus improve acceleration using ‘shorter’ gears)?
Because during the race in a slipstream (draft) i.e. following another car in a reduced drag zone I can make use of the remaining of the 7th gear and reach more than 310km/h, say about 320km/h hitting the 19krpm limit... and hopefully overtake

Hope I made myself clear.

[riff_raff]

In theory you could exceed 19,000 rpm on the rev limited engine if the slipstream itself was powerful enough to drag the car along without any help from the engine. Thus the rear wheels would back drive the engine through the transmission. The rev limiter only limits engine power and speed output, not how much power (and at what speed) the engine can absorb during braking.

But that would have to be one powerful slipstream pulling a car with huge drag. So in reality it wouldn't happen.

[Ogami musashi]

I think maybe i'm not clear enough:

Let's take an example, the same car hit 300km/h a 17k RPM (we don't care about the gear ratio here, we suppose it is constant).

According to Dave and some other persons, you being in slipstream (less drag) or not 300km/h will always corresponds to 17K RPM.

As power is torque*RPM, because in slipstream you need less power to achieve the same speed and considering your RPM will stay the same, it is necessary that the torque is decreased.

My question was: where (in the powertrain chain) this power is reduced? what are the members of the powertrain that are affected by this power reduction?

Take an analogy: a plane flying at low altitude with the same thrust will fly slower than at high altitude because of decrease density (less drag).
So to fly at the same speed a plane requires less thrust at high altitude.
This is pretty linear, the RPM of the engine will lower too, but in cars this is not the case, i want to know why and what member of the equation Power needed=mass*speed are decrease, I.E what in the engine or transmission decrease its performance to produce less power.


Hope i was clearer sorry if not.

[johny]

if you're in a slipstream and rpm's aren't reduced you'll have more power.

The easiest way to achieve that limit in rpm's would be cutting fuel injection

[pRo]

My question was: where (in the powertrain chain) this power is reduced? what are the members of the powertrain that are affected by this power reduction?

You're basically using power for two things. To overcome all resistances and to accelerate.

Say you have reached your top speed of 300kmh. That means all the power is going into overcoming resistances. The biggest one being air resistance. That's the reason you can't accelerate any more.

Now say you catch another car and draft behind it. The air resistance is reduced, so you have to use less power to keep that speed and you can use the remaining power to accelerate. Or keep the speed with less power, if you wish.


Think about normal driving. You're on a level straight, keeping the throttle at certain position, which equals certain power. You come to a downhill. You no longer have to output the same power to keep your speed. You use less power to keep the same speed and rpm. Or if you keep your throttle where it was, you will accelerate. Downhill equals drafting.

[DaveKillens]

Let's assume a steady state drafting scenario, such as NASCAR at Talladega, with just two cars. Let's leave out such factors as acceleration and consideration for braking.
The lead car is running flat out, top gear. The car directly in the draft would run identical RPM, but under full throttle would close on the lead car, until they collide. To stay in station, the car in the rear would have to ease off the gas (lessen the power produced) just to maintain the same distance.
I hope that helps.

[Ogami musashi]

i think i'm cursed to misunderstanding today!

I know what draft is, what i want to know is the relation of Power to RPM and torque.

Dave used an example that if i understood is revelant of the problem of the question:

The lead car is running flat out, top gear. The car directly in the draft would run identical RPM, but under full throttle would close on the lead car, until they collide.

You suggest here that the following car at identical RPM would be at greater speed than the leading car right?

If yes, then this is here the problem, i've been told that no, if you're following a car in draft/slipstream condition, you accelerate because of less drag yes, but that also make the RPM increase, so one RPM=one speed whatever the drag and roll resistance levels are, so i want to know WHY wheels have to turn the same speed (wheels RPM are proportional to engine ones) to achieve a speed where the power needed is less.


So i repeat my problem is to know why one RPM=one speed (assuming we are on the same gear shift and gear ratio).