7

I first heard this claim a few years ago on the Top Gear program from the BBC. Since then I keep hearing it whenever I hear the Lexus LFA discussed.

From this February 2012 article:

This necessitated the need for a digital rev counter, since an analogue system simply could not keep pace with the engine’s incredible ability to gain and lose revolutions.

Given the almost exact wording of all of these quotes is the same (see this google search), my guess is that this comes from a Lexus press release; although I cannot find it myself.

To me, this sounds like marketing fluff so that you can laugh at all the poor people who can only afford a BMW M3; but maybe not. From the same article:

So successful was this low-friction programme that the engine can rev from idle to its 9,000rpm redline in just six-tenths of a second

But is it possible that an analogue gauge can be too slow? Given that a lot of open-wheel racers can achieve massive acceleration with analogue gauges (although perhaps not in 6/10th of a second, but certainly sub-1.5 seconds), I don't see why the LFA would "require" a digital meter (although it is un-deniably cool).

  • Surely a (sport/racing) motorbike can rev that quickly? What about drag cars? Rotaries? This sounds like pure marketing. – John Lyon Aug 7 '12 at 4:47
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    Another thing is the speed of human perception. Would it really matter if you looked at the dial and saw data that's 150ms too old? – Tadeusz A. Kadłubowski Aug 7 '12 at 9:51
  • @TadeuszA.Kadłubowski - Probably not to a person but to the computer that is making constant adjustments based on the actual current value yes it could. That is not to say that it actually does. – Chad Aug 7 '12 at 16:54
  • @Chad: We're talking about max 9000rpm == 150Hz. That's quite slow by electronic standards. – Tadeusz A. Kadłubowski Aug 7 '12 at 19:06
  • @TadeuszA.Kadłubowski - In theory you could squeeze a bit more performance if you are able to make tiny electronic adjustments at a faster rate. How much of a difference it makes or if that faster speed is being utilized would be the question. – Chad Aug 7 '12 at 19:12
6

If you don't want to read the whole thing, my ultimate conclusion is

It is all digital anyway, but it could have equally well been displayed with an old fashioned mechanical needle. An LCD is cheaper and have fewer moving parts and is apparently sexier, so that is what they did.

Here is why.

The Lexus marketing speak for the LFA is at http://www.lexus.ie/range/lfa/features.aspx (make sure Features is selected and click on the performance cockpit item and look at the middle of the second paragraph after the picture.)

Lexus states:

"The central tachometer runs to 10,000rpm and features a fast reacting LCD needle designed to exactly replicate the V10 engine’s insatiable appetite for revs."

So, the display is driven by a computer that shows something that looks like a classic analog needle on an LCD display.

This display is most likely driven by the sensors in the engine that knows the speed of rotation of the crankshaft to some ridiculous level of precision. That is, ridiculous if its only purpose was to display the engines RPM. In fact, the RPM is an almost an incidental piece of data from the high resolution sensors measuring the angle of the crankshaft at any given moment in time so that the precise timing of the ignition system can be delivered.

For a V10 at 10,000 RPM, the need for exact and timely data is a real design challenge. A rough guess is that it measures the angle at least a thousand times per rotation, so my guess is that it knows the speed and position of the crankshaft around 170,000 times a second. I'm probably grossly underestimating the precision as I don't know the actual details for this car.

Now and then — maybe a few hundred times a second, maybe more — the RPM information is feed onto the main digital bus in the car (probably a standard CAN bus). The data is then received by the computer responsible for painting the LCD display of the tachometer, updating it at perhaps one or two hundred times a second. This is far faster than the human eye can follow, so much greater speed would be useless.

Could this be done with a mechanical needle looking just like similar displays have done for maybe a 100 years? Yes of course. The needle would probably be driven by a medium fast servo system and its own little micro controller. Even though it was a mechanical "analog" needle, it would still be digital in that it showed an approximation of the digital value from the CAN bus.

  • My first time here so excuse me if this is the wrong way of proposing improvements. What I have written above is probably too technical to be used as is. – johanges Aug 8 '12 at 23:08
  • Thanks for getting straight to the point about display of the data! I wouldn't have thought an LCD was cheaper, but in a car that expensive I doubt cost is much of an issue – Mark Henderson Aug 8 '12 at 23:47
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    The real cost is in reliability and engineering. LCD displays have no moving parts and are self contained modules. That means once you have made one, you can plug it in as a module in a bunch of products with minimal customization. Much faster to design and get into final shape while still being able to change the look until the last minute. – johanges Aug 9 '12 at 3:53
7

All modern engines are controlled with a microprocessor-based engine control units(ECU). OBD II has been mandatory in the USA since 1996.

Even the simplest microcontrollers have more than enough computing power to check all the probes on the engine 9000 times per minute. They run literally millions of operations per second.

So at least since 1996 cars have fully digital on-board computers that track engine performance in great detail. This data is fed via CAN bus to the dashboard and displayed in various ways to the driver (from revolution counter to "check engine" light).

The design of the different dashboard gauges, dials and control lights is a matter of aesthetic, usability and safety.

  • Could you please provide a reference to the idea that OBD has been mandatory since 1996. The Wikipedia reference links to a document saying 2006. – Oddthinking Aug 8 '12 at 12:59
  • @oddthinking: Basically it was Clean Air Act Ammendments of 1990 Title II section 205 that authorized EPA to regulate onboard diagnostic systems in cars. Here is a EPA document with a history overview of all rulemaking. I believe that 58 FR 9468 is the key regulation. – Tadeusz A. Kadłubowski Aug 8 '12 at 16:41
  • @Oddthinking: You linked to a rule that merely linked federal regulation of OBD II to the ISO standard about CAN bus communication (ISO 15765-4.3). – Tadeusz A. Kadłubowski Aug 8 '12 at 16:42

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