OdinGuru
03-29-2004, 01:17 AM
As most anyone who has been visiting this section of the forum knows, there is ample dyno and accelerometer based evidence that indicates that the Rensis is making significantly less than even the revised 238 HP Mazda crank spec. No one believes the six-speed manual transmission combined with the straight zero angle carbon fiber drive shaft should have losses of any more than the typical 15% (and probably should be significantly less). Thus we’d expect to see around 200 RWHP, but instead we’re seeing around 170 RWHP at most (down an additional 15% from what we’d expect). So where has our HP gone? Most will point the finger at Mazda and say even the revised spec is a lie. I’m a member of this camp, but it has never quite sat well with me that the engineers capable of putting together the INCREDIBLE feat of engineering (the RX8) I’ve come to LOVE could have been off by SO much on the capabilities of the Rensis. Granted, the spec is up to the marketing department, not the engineers, but they must have based it on something from the engineers (I don’t believe they just made it up out of “thin air”).
There has been one possibility that I’ve been kicking around for a while trying to figure out if it actually makes a difference. I’ve decided to write it up and post it here so that the rotary experts on the forum can either agree, or set me straight. Basically, the question comes down to this: what if there is a significant difference between STATIC HP and DYNAMIC HP? If so, perhaps there is some “truth” in Mazda’s specs, although they may still be misleading. This could be similar to the 1.3L displacement spec which has some “truth” but is also misleading in that there are good arguments for making the spec 2.6L or even 3.9L.
So Mazda, and any original manufacture, would be using engine dynos rather than chassis dynos to do their development and testing. As I understand it, on an engine dyno, an adjustable brake is used to measure the output of the engine. The pressure on the brake is increased (making more resistive force) and balanced against the engine output at some specific rpm. In this way, controlled repeatable measurements of output torque and power can be made at any part of the engine operational range. So as I see it, this would give them a measurement of static HP: one where the engine is operating at constant speed. The vast majority of techniques available to those of us who only have engines pre-installed in our cars, most of the techniques that are available are dynamic (engine is changing speeds during measurement). This includes the vast majority of chassis dynos as well as accelerometer based measurements (e.g. G-tech). Although the two techniques take different approaches to measuring power, obviously historically there haven’t been massive differences in the results, otherwise it would be common knowledge, and or different measurement techniques would be used. So for the difference to be significant, it would have to be due to something that was SPECIFIC to rotary engines and not found on cylinder based engines.
A chassis dyno will read a power less than an engine dyno because it takes into account several additional factors. The most well known factor is the frictional losses in the drive train, but also there are inertial losses due to the rotational inertia of the wheels, drive train, flywheel, and engine. Most of these factors are basically equivalent between normal front engine RWD cylinder based cars and the RX8. If anything we’d expect the RX8 to have a lower than normal loss due to the light weight carbon fiber drive shaft and the fact that there are only two zero angle universal joints. The only thing MAJORLY different is the engine. So the question becomes: is there a significant difference between the rotational inertia of a rotary and a cylinder engine? I believe there might be.
One of the main advantages of the rotary engine design is that there is SIGNIFICANTLY less reciprocating mass. In fact the only remaining reciprocation is a due to the eccentricity of the rotor movement. As a result, over 80% of the rotor momentum is conserved. In contrast, in a cylinder engine, most parts (cylinder heads, valves, rots, etc.) reciprocate and thus carry no momentum into the next cycle. Only a few parts like the crankshaft, camshafts, and pulleys rotate and conserve momentum. So depending on the weight of the rotor, could the rotational inertia of a rotary engine be greater than that of a typical cylinder engine (excluding flywheels)? If this is true, and the difference is significant, then we’d see a corresponding decrease in HP in dynamic vs. static measurements (because rotational inertia is not taken into account in static measurements). Thus it is POSSIBLE that this effect could be responsible for some of the gap between observed and published “specs.”
Am I barking up the wrong tree here? It seems to me that even if the engine itself had significant rotational inertia, then wouldn’t Mazda be able to get away with a significantly lighter flywheel then normal (perhaps this costs more). I know that there is a thread on installing a lighter flywheel, perhaps someone knows how the stock flywheel compares with typical ones used in this type of car. I don’t have an estimate of the extra rotational inertia that may be present, and thus don’t know how much of an impact it may have. I though it might be major considering the rotors are somewhat large, cast iron, and rotating at relatively high speed (in comparison with the wheels). For example at 90mph in 3rd gear the engine is at around 9000 rpm, but the wheels are only going about 1080 rpm. Nine times the rotational velocity results in 81 times the rotational kinetic energy for a given rotational inertia. Putting it another way, the wheels get accelerated better by the torque multiplication from the transmission, but the rotors don't so they are effectivly "heaver" by the overal gearing ratio (which is typicaly quite high in the RX8).
PLEASE chime in if you think this makes any sense/no-sense.
- David
There has been one possibility that I’ve been kicking around for a while trying to figure out if it actually makes a difference. I’ve decided to write it up and post it here so that the rotary experts on the forum can either agree, or set me straight. Basically, the question comes down to this: what if there is a significant difference between STATIC HP and DYNAMIC HP? If so, perhaps there is some “truth” in Mazda’s specs, although they may still be misleading. This could be similar to the 1.3L displacement spec which has some “truth” but is also misleading in that there are good arguments for making the spec 2.6L or even 3.9L.
So Mazda, and any original manufacture, would be using engine dynos rather than chassis dynos to do their development and testing. As I understand it, on an engine dyno, an adjustable brake is used to measure the output of the engine. The pressure on the brake is increased (making more resistive force) and balanced against the engine output at some specific rpm. In this way, controlled repeatable measurements of output torque and power can be made at any part of the engine operational range. So as I see it, this would give them a measurement of static HP: one where the engine is operating at constant speed. The vast majority of techniques available to those of us who only have engines pre-installed in our cars, most of the techniques that are available are dynamic (engine is changing speeds during measurement). This includes the vast majority of chassis dynos as well as accelerometer based measurements (e.g. G-tech). Although the two techniques take different approaches to measuring power, obviously historically there haven’t been massive differences in the results, otherwise it would be common knowledge, and or different measurement techniques would be used. So for the difference to be significant, it would have to be due to something that was SPECIFIC to rotary engines and not found on cylinder based engines.
A chassis dyno will read a power less than an engine dyno because it takes into account several additional factors. The most well known factor is the frictional losses in the drive train, but also there are inertial losses due to the rotational inertia of the wheels, drive train, flywheel, and engine. Most of these factors are basically equivalent between normal front engine RWD cylinder based cars and the RX8. If anything we’d expect the RX8 to have a lower than normal loss due to the light weight carbon fiber drive shaft and the fact that there are only two zero angle universal joints. The only thing MAJORLY different is the engine. So the question becomes: is there a significant difference between the rotational inertia of a rotary and a cylinder engine? I believe there might be.
One of the main advantages of the rotary engine design is that there is SIGNIFICANTLY less reciprocating mass. In fact the only remaining reciprocation is a due to the eccentricity of the rotor movement. As a result, over 80% of the rotor momentum is conserved. In contrast, in a cylinder engine, most parts (cylinder heads, valves, rots, etc.) reciprocate and thus carry no momentum into the next cycle. Only a few parts like the crankshaft, camshafts, and pulleys rotate and conserve momentum. So depending on the weight of the rotor, could the rotational inertia of a rotary engine be greater than that of a typical cylinder engine (excluding flywheels)? If this is true, and the difference is significant, then we’d see a corresponding decrease in HP in dynamic vs. static measurements (because rotational inertia is not taken into account in static measurements). Thus it is POSSIBLE that this effect could be responsible for some of the gap between observed and published “specs.”
Am I barking up the wrong tree here? It seems to me that even if the engine itself had significant rotational inertia, then wouldn’t Mazda be able to get away with a significantly lighter flywheel then normal (perhaps this costs more). I know that there is a thread on installing a lighter flywheel, perhaps someone knows how the stock flywheel compares with typical ones used in this type of car. I don’t have an estimate of the extra rotational inertia that may be present, and thus don’t know how much of an impact it may have. I though it might be major considering the rotors are somewhat large, cast iron, and rotating at relatively high speed (in comparison with the wheels). For example at 90mph in 3rd gear the engine is at around 9000 rpm, but the wheels are only going about 1080 rpm. Nine times the rotational velocity results in 81 times the rotational kinetic energy for a given rotational inertia. Putting it another way, the wheels get accelerated better by the torque multiplication from the transmission, but the rotors don't so they are effectivly "heaver" by the overal gearing ratio (which is typicaly quite high in the RX8).
PLEASE chime in if you think this makes any sense/no-sense.
- David