Dyno power readings

[Alex Wong1697456877]

Can someone explain the different ways of measuring power output on a dyno? What's a coastdown test? Should the highest power reading be taken going up the revs or is there another way (and is it more misleading) ?

 

Cheers,

 

Alex

 

Edited by - Alex Wong on 15 Mar 2001 22:48:07

[Jason]

Alex

 

I believe that the coastdown is to measure the rolling losses such as wheels, gearbox Etc. you need to know this to estimate the flywheel figure.

 

Jason

 

Ps does this mean you will be at Cadwell

 

 

 

[Paul D Jones]

Not all dynos will read the same as they need calabrating and testing I saw Dennis Vessey local rolling road man do a Saxo then told the owner he had 110 bhp at the flywheel I asked Dennis how did he know that as the engine was in the car and he said he dipped the clutch to drop the transmition load and the computer worked out the differance.

Dennis is contactable on tel 01652 688904 [honest man for the motor trade]

[Alex Wong1697456877]

But what about on a dyno rather than a rolling road?

[Roger King]

 

 

Edited by - roger king on 15 Mar 2001 23:55:21

[Roger King]

Alex,

 

The normal practice on a dyno' is to test at steady speeds such as every 500rpm. If you take figures while the engine is accelerating the power reading will be lower because energy is lost (or at least stored) in accelerating the moving parts. Conversely if you take readings when decelerating you will get higher figures as the kinetic energy in the moving parts is released (I am making the assumption here that the engine is on full throttle and is being decelerated by the dyno' load). This is the reason that a lightened flywheel can improve the acceleration (and engine braking) of a car out of all proportion to the simple amount of mass removed.

 

Some more sophisticated dynos can do acceleration and deceleration testing which can be very useful for engine development. This is more akin to the real world situation that the engine will work in, but if you see a quoted power figure it will almost certainly be a steady speed figure.

[Nick M]

I'm not sure there would be a coast-down test on a dyno - it's able to measure (calculate) the power direct from the flywheel rather than having to work back from an "at the wheels" figure which is where the coast-down test comes into play.

 

I'm not sure of the procedure that's adopted to get the power run - I guess it's like a rolling road where you simply remove the load from the engine and watch the power curve until it starts to drop again.

 

I'd be interested to find out though as when (if !) my engine build is finished I'm going to get it mapped on a dyno rather than on rollers (primarily because it means I can run the engine in on the dyno rather than install it in the car and then have to remove it again to put it on the dyno).

 

Nick

 

[Alex Wong1697456877]

Roger,

 

Thanks - that's what I wanted to know. This was being explained to me but I didn't quite understand it...

 

My engine was dyno'd at QED today. It showed 234BHP max at 7500 rpm - measured whilst accelerating. see here for a copy of the chart

 

I kinda like the look of that torque curve. smile.gif Max torque was 180 ft/lbs at 6000 rpm.

 

Apparently the 4000rpm drop in power is a characteristic of the Q450 cam.

 

A quick power run when everything was cooler saw 241 BHP whilst accelerating.

 

Apparently they don't measure whilst holding rpm or decelerating. I was told that they could produce (meaningless) figures of 300bhp+ by measuring whilst decelerating suddenly!

 

Somehow, even if the numbers aren't as big as some others, I think the car is going to be very fast. teeth.gif - in a straight line anyway! - It is a Vx afterall wink.gif

 

 

 

Edited by - Alex Wong on 16 Mar 2001 00:53:30

[Alex Wong1697456877]

It was also a bloody nerve racking experience. Whilst I was watching the final power run, I suddenly started doubting all those big end and maincrank bearing bolts I torqued up.....

[R2D2]

Dyno's only really measure torque, they calculate power from a relatively simple formula and it is important that the zero of the dyno is set correctly.

One quick check that the dyno is set corectly is that the power and torque curves must cross at 5252 rpm. If they don't something is out fo adjustment.

 

The most common dyno uses a "swinging frame" and a load cell to measure force. Dyno manufacturers normally claim that these type of system are 0.25% accurate but in reality they are about 0.5%.

 

 

I think that QED use an Eddy Current Schenck machine, which is a fairly high quality unit.

I think that this type of dyno should be used at fairly constant throttle settings, I don't believe transient measurements will be accurate. The manner in which speed is being measured can also influence transient results. Modern digital encoders give pulse outputs and the signal has to be integrated to produce a speed measurement. This intergration has a time constant which influence the accuracy of the acel/decel ramps. under steady state conditions this time constant is relatively unimportant.

 

The latest generation of dynamic dyno, which can both absorb and motor an engine use in-line torque measuring and are much more accurate. These system not only measure steady state mean torque but they can plot the transisent behaviour of the engines in terms of rpm rise rates. It is this type of system that is being used for the "throttle mapping" that F1 cars have used instead of traction control.

 

 

[Jason]

Alex

 

Your torque and power curve look very similar shape to mine (I've mailed a copy to you) I'll look forded to our swap at the ring .

 

Jason

R500FUN

 

 

 

[Alex Wong1697456877]

Chris,

 

Thanks - I like the 5252 rpm nugget - I assume thats a result of the calculation of power from torque. How is it that one engine can show more power and less torque than another then?

See you at the brook when I finally get to go there again! (It's been over 6 months since I've been to my area meet! blush.gif

 

Jason,

 

They are quite similar aren't they. A bit less BHP on mine, a bit less torque on yours. Just what you'd expect comparing a Vx with a K! I look forward to the run at the ring (provided i can fit in your race seat!!)

 

Cheers,

 

Alex

 

[stevefoster]

Alex, I bet it was nerve racking! Part of me would want to go away and let them 'abuse' the car and then to see the results and part of me would want to stay and

watch the process. I would esp want to stay is rejetting or mapping was going to take place to see the method used.

 

[Slipper man]

So, Alex. The only slow bit left is the driver.

 

wink.gif

[Alex Wong1697456877]

Slipperman,

 

Pots and kettles.....

 

cool.gif

 

Alex

 

BTW, I asked our esteemed leader who out of the 2 of us had a better chance at Curborough. After careful consideration of all factors, he said it was too close to call. What do you think?

 

Edited by - Alex Wong on 16 Mar 2001 13:30:01

[Tony Martyr]

Alex,

get the book Engine Testing Theory and Practice by M.Plint and Tony Martyr out of your local library for the full story.

First you are talking specifically about a chassis dynamometer test (sometimes known as a rolling road).

Every vehicle can be charactorised by its Road Load Equation which is a 4 term function that adds, rolling losses, friction losses, air resistance and gradient terms. The dominating term is the air resistance which can be considered as altering with the square of speed.

Many cheap rolling roads (most tuners machines) use flywheels plus the roller inertia to aproximate the inertia of the vehicle being tested then add the resitance of a power absorber (such as an eddy-current device)

This allows the vehicle on test to aproximately experience real life resitance to power at the wheels over its speed range.

The key word is approximately. There are many systematic, calibration and control errors in the types of machine used by tuning shops. However this is only a big problem if you are interested in absolute numbers. Most work is comparitive, that is you are interested in power gains from state A to state B. It takes a much more expensive and clever machine to get acurate figures. I spend a lot of my life dealing with 48inch emission homologation chassis dyns. These cost £250k and are out of the range of most tuning shops. My car was mapped by John Noble after modification and I am happy that the power increase was 15HP at the wheels at a known speed but I have NO faith in the actual figure.

So to answer your question!

Coast down tests are meant to be used to check the Road Load equation setting in the chassis dyno. If the RLE is known you can calculate the rate of deacceleration of that car on a flat road with no wind. So you take the car up to 100kph and allow a free coast down and compare it with the theory, if the curves line up you have a realistic setting for any test. If a car has a too rapid coast down it is indicative of high friction somewhere, brake drag of uneven tyre diameter with a LSD etc

Most Power tests are done by putting the dyno in speed control and flooring the car's throttle. You then measure the resisting torque the rolls are having to apply.

The two major problems with small chassis dynos is that the small double rolls give a totally false tyre friction and that they inherently can not be calibrated well.

This is a big subject. If anybody thinks it is worth an article in Low Flying I could do it.

[Alex Wong1697456877]

Hi Tony,

 

Yes it's definitely worth an article, but I was actually asking about engines being dyno'd out of the car although I got some of the terms mixed up.

 

Cheers,

 

Alex

[Tony Martyr]

Alex,

I've just read subsequent exchanges and realise that although you talk of coast down (which is a standard vehicle test) you were in fact talking about an engine dyno test.

The true way of taking a power curve is the put the dyno in speed control mode, the trottle actuator in position mode at 100% then go up the engine speed range in 500rpm(or smaller increments) The torque figure has to settle for usually 15 secs before the next increase in speed is made. Going up should be a mirror of stepping down. In a primitive or physically small cell the air temperature ingested by the engine often goes up as the power goes up so you get an increasing torque error. proper cells have temperature control of combustion air.

Taking torque figures during acceleration is totally misleading and a common con trick.

I think you ought to buy the book, it means almost £1 per copy to me after tax and it will make you understand the strange world test engineers occupy

[Paul Drawmer]

After reading the above worthy stuff, I hesitate to add anything.

 

However, just as an aside. I was at John Wilcox the other day..(a manufacturer of replacement air filters was using my BMW325 as a sample) and I had a long and fascinating chat with JW about rolling roads, dynos etc. He has 3 dyno test cells! And a real toy-shop for power junkies.

 

The important thing he said, was that whilst some closed cell dynos could be regarded as reasonably accurate, none of these things should be used for comparisons. They are really only good for comparing differences measured ON THE SAME MACHINE. Certainly very few rolling roads are calibrated, and the figs from one should not be compared with another. All you could really do is compare the shape of the curves - no NOT on page 3!

 

BTW - he said that any VX with more than a *real* 230 was doing very well.

 

So, I stood there and watched my car do 6500 on the rollers. Result: std air box and panel filter is a wee bit better than a replacement cone!

[R2D2]

 

 

[R2D2]

 

 

Horsepower = (torque lbsft x rpm)/5252

 

or

 

Kilowatts = (torque Nm x rpm)/9549

 

These are the basic equations for converting torque to power. The constants 5252 and

9549 are obtained by analysing the dimensions of the equations.

 

The arguement about torque vs horsepower has been going on for ever but still seems misunderstood and I an not sure that I completely understand.

 

Torque is the basic force that allows work to be done. Power is the speed at which work is being done.

 

The type of engine that you choes for a specific application depends on the things that you are trying to achieve.

 

F1 Engines produce about 800 HP at between 18/20000 rpm depending on the manufacturer.

This is equivalent to a torque of about 220ftlbs at peak rpm. (Finding a curve of a current F1 engine is about impossible)

 

A Ford Focus World Rally car produces a peak torque of about 450 ftlbs but only has a claimed 300HP but at 5000rpm

 

Caterpillar diggers produce about 400HP at 1700rpm.

 

The designs take into account reliability, life, fuel consumption etc. etc. and basic charateristics obviouly take BMEP, capacity and duty into account.

 

Anotherthought is that the weight of any rotating component depends almost directly on the amount of torque it needs to transmit, so high speed engines and transmissions can be much lighter than slow revving units.

 

The ability to push a car through the air does really depend on torque so when the force needed to displace a given volume of air is equal to the torque available at the wheels the car will not accelerate any more. Whether this torque is acheieved by a high revving engine with a large step down gear ratio or a low revving engine with different gearing is irrelevant. The important issue is to match the gearing to the engine characteristics, weight, drag and application of the car.

 

Caterhams obviously need a significant force to push them through the air at high speed because of the poor aerodynamics (Cd typically 0.65 ?) but and a bike engined car of 160bhp should have an identical top speed to a 160bhp Ford engined car if the final drive gearing is adjusted so that the torque at the rear wheels is identical.

 

If the bike engine gives 160bhp at 12000rpm and the Ford 160bhp at 8000rpm then the final drives would need to be 50% different. The differences would then be related to acceleration and handling. It may also be important to adjust intermediate gear ratios to suit individual power curves.

 

I know this is simplistic but it may start an interesting debate about torque, horsepower and gearing.

[Tony Martyr]

Paul,

John Wilcox is absolutely correct. For many years large companies have tried to correlate the results of engine test cells with the same sealed engine. Ford tried in the 1980s by using a Koln engine sent to Brazil, they forgot that the gravitational constant is significantly different in the two locations so failed to understand the basic physics.

Each generation of test engineers tries it and learns that the variability of engine output and test system defeats them. If you can get several cells within a span of 4% the standard of calibration and similarity of systems is of a very high order. The concept of accuracy is the most subtle and difficult thing that any engineer has to come to terms with.

These numbers that folk quote for vehicle power are just part of a pissing contest it is rarely part of real engineering. I have been in cells where Winston Cup race engines were being tested and know it was just exciting bull****.

The best comparitive tests are the 'sum of the parts' tests such as 0-100mph-0 track tests. The dyno test results from modern 4 quadrant transient cells are usefull to development engineers because we can simulate real world life within a software vcehicle but such systems cost upwards of £75ok per cell. QED type cells, which are compentantly run are great only for checking engine viability and comparing original state with modified state. They are not and do not pretend to be engine test laboratories.

[DonkeyDave]

234 BHP, another reason not to go to Cadwell then :-)

[Alex Wong1697456877]

Another reason?

 

I shouldn't worry. I haven't uprated the nut behind the wheel! teeth.gif

 

Alex

[Slipper man]

My point exactly wink.gif

 

Who will win at Curborough? If you continue to cheat on the tyre front, you should. If the tyres are fair, who knows.....