I came across this well written article over on the Renault Turbo Owners Club site and thought it would make some good reading to folks on here.
MEASURING ENGINE POWER
There is in fact no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula we saw in the previous articles - BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the cornerstone of all engine design and development work. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. We'll look at both of these separately.
ENGINE DYNAMOMETERS
If we want to know the power of the engine alone then an engine dynamometer (or dyno) is used. This is how nearly all manufacturers rate the output of car engines. The engine is bolted into a cradle and connected to the dyno with a prop shaft which bolts onto the back of the crankshaft (or the flywheel). The power figures measured in this way are therefore usually called "flywheel power". The dyno is essentially a "brake" which can apply a known torque (or "load") to the engine. When the engine is holding a steady speed under a given dyno load then the torque being applied by the dyno must be exactly equal to the torque being produced by the engine. If this were not so then the engine would either accelerate or decelerate. Let's say we want to know the engine torque at full throttle at 3,000 rpm. The throttle is gradually opened and at the same time the load applied by the dyno is increased - eventually by juggling the amount of load applied we get to the situation where the throttle is fully open and the rpm is steady at 3,000. The torque being applied is written down and then the operation would be repeated at say 4,000 rpm. Soon we get a complete chart of torque at all engine speeds. Of course we could also measure part throttle power if desired.
Modern dynos are computer controlled and can generate power and torque curves very rapidly without the operator having to manually adjust throttle and load controls. They can be programmed to measure every so many rpm, say in 250 or 500 rpm steps - or they can measure a continuous torque curve while the engine accelerates at a preset rate. This can be used to simulate how the engine would actually operate in a particular gear when installed in the car.
There are various ways in which the dyno load can be applied. Older dynos use a hydraulic system with a rotor inside a water filled cavity - rather similar to the torque convertor in an automatic gearbox. Modern dynos generate the load with large electric motors. Even a simple friction disk or drum brake will work fine and this is where the name "brake" in Brake Horsepower came from. The important thing is that the load is able to be measured accurately and that there are no frictional losses in the system that escape measurement.
In order for dyno results to be comparable and universally understood there are a number of things that need to be closely controlled during the measurement process:
Operating Conditions
Air temperature, pressure and humidity affect the amount of power an engine produces. Cold dense air means a greater mass of oxygen per power cycle and thus more power is generated (provided of course that air/fuel mixture is properly calibrated for the conditions prevailing). There are formulae that can be used to calculate how much the measured power would change if the test conditions were different. This enables dyno results to be "corrected" back to standard conditions to enable comparison with anyone else's test results. Sadly however there is no one universally accepted set of "standard" conditions because different automotive bodies in different countries use different standards to calibrate to. "SAE" power standards are used in the USA and sometimes in England. "DIN" standards are used on the continent and there are a few other oddball systems just to confuse the issue. So just because your car is rated at 100 bhp and a friends at 110 bhp doesn't necessarily mean that his engine is more powerful - it depends whether both measurements were corrected to the same standard conditions.
One of the tricks I've seen used to get bigger "corrected" bhp numbers is to use a very high ambient temperature reading for the dyno test. If the operator measures the temperature close to the engine rather than well away from it then obviously he will get a reading that is much higher than ambient. When the bhp numbers are corrected back to a lower standard ambient temperature they will increase. I saw an engine dyno sheet the other day where the ambient air temperature in February, in England was supposedly 37 degrees C. Now either that test was done with the temperature probe sat right on top of the engine or it's a part of country I don't yet know about where I would very much like to live !!
Engine Ancillaries
When installed in the car, the engine has to drive a number of items like the alternator and power steering pump which sap power. Also the exhaust and air filter systems will reduce power to some extent. If the engine is tested without any of these ancillaries fitted then it will show much higher power figures. The Americans used to rate their engines like this back in the fifties and sixties and often the installed power of the engine would only be 2/3 of the claimed figure in the sales blurb. This used to be called "gross" flywheel power and if the ancillaries were fitted the power was called "net" flywheel power. Nowadays the gross system, which was very misleading, is not used and all modern published data should be "net flywheel" power. Major manufacturers abide by rigorous standards which set out how the engine should be installed on the dyno to simulate closely the "in car" conditions.
ROLLING ROAD DYNAMOMETERS
Also called chassis dynamometers, these are used to measure power at the driving wheels. This avoids the inconvenience of having to remove the engine to test it if a tuning modification has been made. However, it means that the power figures obtained will be lower than the flywheel power because of the frictional losses in the drivetrain and tyres. This leads to one of the biggest sources of confusion, error and plain misinformation in the tuning industry. You see, as discussed above, all major manufacturers quote flywheel power so it is understandable that people want to know if the hard earned cash they spent on tuning mods increased the power of their engine and by how much. To know this for certain means knowing how much the transmission losses are. There is enormous pressure on rolling road operators to be able to quote flywheel bhp rather than wheel bhp and most operators now run proprietary software systems which "supposedly" print out flywheel power.
PROBLEM !! - THESE SOFTWARE SYSTEMS DO NOT AND CANNOT WORK !!
Yes - I know - the whole chassis dyno tuning industry quotes flywheel figures and here's me saying none of it works. So I'd better explain some more and then you can make your own mind up.
First, let's look at how a chassis dyno works. The car is driven onto a rig so that the driving tyres are resting between two steel rollers. The torque is measured at different speeds in exactly the same way as an engine dyno works except that it is torque at the rollers rather than torque at the flywheel. The braking load is applied to one of the rollers by either a hydraulic (water brake) or electrical system again in just the same way as the engine dyno would apply a torque to the crankshaft of the engine. The same universal equation at the top of the page can then be used to calculate bhp at the rollers by knowing the torque and the rpm of the rollers (NOT the rpm of the engine at this stage) - but if the engine rpm is measured simultaneously then we can know roller bhp at a particular engine rpm. The BIG problem with all this is if any tyre slip is taking place. Remember these are smooth steel rollers which over time get quite polished. How much grip do you think you would get if roads were made of polished steel rather than tarmac? The effects of tyre slip are complex (i.e. I don't pretend to fully understand them myself!) but what I do know is that you can get some really strange bhp figures from highly tuned engines on narrow tyres and the readings are invariably too high not too low.
MEASURING ENGINE POWER
There is in fact no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula we saw in the previous articles - BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the cornerstone of all engine design and development work. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. We'll look at both of these separately.
ENGINE DYNAMOMETERS
If we want to know the power of the engine alone then an engine dynamometer (or dyno) is used. This is how nearly all manufacturers rate the output of car engines. The engine is bolted into a cradle and connected to the dyno with a prop shaft which bolts onto the back of the crankshaft (or the flywheel). The power figures measured in this way are therefore usually called "flywheel power". The dyno is essentially a "brake" which can apply a known torque (or "load") to the engine. When the engine is holding a steady speed under a given dyno load then the torque being applied by the dyno must be exactly equal to the torque being produced by the engine. If this were not so then the engine would either accelerate or decelerate. Let's say we want to know the engine torque at full throttle at 3,000 rpm. The throttle is gradually opened and at the same time the load applied by the dyno is increased - eventually by juggling the amount of load applied we get to the situation where the throttle is fully open and the rpm is steady at 3,000. The torque being applied is written down and then the operation would be repeated at say 4,000 rpm. Soon we get a complete chart of torque at all engine speeds. Of course we could also measure part throttle power if desired.
Modern dynos are computer controlled and can generate power and torque curves very rapidly without the operator having to manually adjust throttle and load controls. They can be programmed to measure every so many rpm, say in 250 or 500 rpm steps - or they can measure a continuous torque curve while the engine accelerates at a preset rate. This can be used to simulate how the engine would actually operate in a particular gear when installed in the car.
There are various ways in which the dyno load can be applied. Older dynos use a hydraulic system with a rotor inside a water filled cavity - rather similar to the torque convertor in an automatic gearbox. Modern dynos generate the load with large electric motors. Even a simple friction disk or drum brake will work fine and this is where the name "brake" in Brake Horsepower came from. The important thing is that the load is able to be measured accurately and that there are no frictional losses in the system that escape measurement.
In order for dyno results to be comparable and universally understood there are a number of things that need to be closely controlled during the measurement process:
Operating Conditions
Air temperature, pressure and humidity affect the amount of power an engine produces. Cold dense air means a greater mass of oxygen per power cycle and thus more power is generated (provided of course that air/fuel mixture is properly calibrated for the conditions prevailing). There are formulae that can be used to calculate how much the measured power would change if the test conditions were different. This enables dyno results to be "corrected" back to standard conditions to enable comparison with anyone else's test results. Sadly however there is no one universally accepted set of "standard" conditions because different automotive bodies in different countries use different standards to calibrate to. "SAE" power standards are used in the USA and sometimes in England. "DIN" standards are used on the continent and there are a few other oddball systems just to confuse the issue. So just because your car is rated at 100 bhp and a friends at 110 bhp doesn't necessarily mean that his engine is more powerful - it depends whether both measurements were corrected to the same standard conditions.
One of the tricks I've seen used to get bigger "corrected" bhp numbers is to use a very high ambient temperature reading for the dyno test. If the operator measures the temperature close to the engine rather than well away from it then obviously he will get a reading that is much higher than ambient. When the bhp numbers are corrected back to a lower standard ambient temperature they will increase. I saw an engine dyno sheet the other day where the ambient air temperature in February, in England was supposedly 37 degrees C. Now either that test was done with the temperature probe sat right on top of the engine or it's a part of country I don't yet know about where I would very much like to live !!
Engine Ancillaries
When installed in the car, the engine has to drive a number of items like the alternator and power steering pump which sap power. Also the exhaust and air filter systems will reduce power to some extent. If the engine is tested without any of these ancillaries fitted then it will show much higher power figures. The Americans used to rate their engines like this back in the fifties and sixties and often the installed power of the engine would only be 2/3 of the claimed figure in the sales blurb. This used to be called "gross" flywheel power and if the ancillaries were fitted the power was called "net" flywheel power. Nowadays the gross system, which was very misleading, is not used and all modern published data should be "net flywheel" power. Major manufacturers abide by rigorous standards which set out how the engine should be installed on the dyno to simulate closely the "in car" conditions.
ROLLING ROAD DYNAMOMETERS
Also called chassis dynamometers, these are used to measure power at the driving wheels. This avoids the inconvenience of having to remove the engine to test it if a tuning modification has been made. However, it means that the power figures obtained will be lower than the flywheel power because of the frictional losses in the drivetrain and tyres. This leads to one of the biggest sources of confusion, error and plain misinformation in the tuning industry. You see, as discussed above, all major manufacturers quote flywheel power so it is understandable that people want to know if the hard earned cash they spent on tuning mods increased the power of their engine and by how much. To know this for certain means knowing how much the transmission losses are. There is enormous pressure on rolling road operators to be able to quote flywheel bhp rather than wheel bhp and most operators now run proprietary software systems which "supposedly" print out flywheel power.
PROBLEM !! - THESE SOFTWARE SYSTEMS DO NOT AND CANNOT WORK !!
Yes - I know - the whole chassis dyno tuning industry quotes flywheel figures and here's me saying none of it works. So I'd better explain some more and then you can make your own mind up.
First, let's look at how a chassis dyno works. The car is driven onto a rig so that the driving tyres are resting between two steel rollers. The torque is measured at different speeds in exactly the same way as an engine dyno works except that it is torque at the rollers rather than torque at the flywheel. The braking load is applied to one of the rollers by either a hydraulic (water brake) or electrical system again in just the same way as the engine dyno would apply a torque to the crankshaft of the engine. The same universal equation at the top of the page can then be used to calculate bhp at the rollers by knowing the torque and the rpm of the rollers (NOT the rpm of the engine at this stage) - but if the engine rpm is measured simultaneously then we can know roller bhp at a particular engine rpm. The BIG problem with all this is if any tyre slip is taking place. Remember these are smooth steel rollers which over time get quite polished. How much grip do you think you would get if roads were made of polished steel rather than tarmac? The effects of tyre slip are complex (i.e. I don't pretend to fully understand them myself!) but what I do know is that you can get some really strange bhp figures from highly tuned engines on narrow tyres and the readings are invariably too high not too low.
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